High-Field Growth of Semiconducting Anodic Oxide Films on … · 2019. 7. 30. · rate in terms of...

Review ArticleHigh-Field Growth of Semiconducting Anodic Oxide Films onMetal Surfaces for Photocatalytic Application

Ronald Vargas 1 David Carvajal1 Brunella Galavis2 Alberto Maimone23

Lorean Madriz 1 and Benjamiacuten R Scharifker 14

1Departamento de Quiacutemica Universidad Simoacuten Boliacutevar Apartado 89000 Caracas 1080A Venezuela2Departamento de Ciencia de los Materiales Universidad Simoacuten Boliacutevar Apartado 89000 Caracas 1080A Venezuela3Centro de Tecnologiacutea de Materiales Fundacioacuten Instituto de Ingenieriacutea Apartado 40200 Caracas 1040-A Venezuela4Rectorado Universidad Metropolitana Apartado 76819 Caracas 1070A Venezuela

Correspondence should be addressed to Ronald Vargas ronaldvargasusbve and Lorean Madriz lmadrizusbve

Received 12 September 2018 Revised 21 November 2018 Accepted 2 December 2018 Published 25 February 2019

Academic Editor Adel A Ismail

Copyright copy 2019 Ronald Vargas et al This is an open access article distributed under the Creative Commons Attribution Licensewhich permits unrestricted use distribution and reproduction in any medium provided the original work is properly cited

This work summarizes progresses achieved in the physical chemistry aspects of the growth of anodic oxides under high-fieldconditions for the synthesis of semiconducting thin solid films and their implementation as photocatalytic materials We discussthe scope and mechanisms for anodic oxide growth describing the development of kinetic models and the correlations betweentheory and kinetic data leading to fundamental information to characterize the primary processes occurring during theanodization of valve metals under high fields The main features related to the widely used self-assembly of nanostructures byvalve metal anodization are highlighted and briefly discussed This is followed by general considerations of heterogeneousphotocatalysis on these functional materials considering the kinetics of the heterogeneous catalytic processes involved and theoverall photoelectrochemical performance High control of the characteristics of the materials obtained with the methoddescribed combined with the possibility of electrochemically assisting photocatalysis allows application of this technology tothe treatment of wastewaters energy conversion and related fields

1 Introduction

Numerous investigations in recent years have focused onthe development of heterogeneous photocatalysts activatedby sunlight and their applications to the environmentalpurification of wastewater conversion of solar energy andproduction of H2 [1ndash22] Electrochemically assisted photo-catalysis shows superior performance in terms of purifica-tion of water from organic matter [12] polarizationbetween two electrodes during the photocatalytic processassisting in separating the charge carriers and catalyzingthe redox reactions thus enhancing the efficiency of miner-alization of the pollutants [12 15 16 23] Moreover effortshave been directed to developing nanostructured photoca-talysts of different nature [24ndash27] Most applications

involve colloidal semiconductors [8 24 28] but the useof large area nanostructured thin solid films has been rap-idly growing in the past few years [12 16] and methodsto obtain films made of stable materials andor compositesunder controlled conditions with the possibility of modu-lating the morphology of the nanostructured films areneeded High-field anodic growth of metal oxide and itsalloys offers these possibilities but to achieve these goalsfundamental understanding of the dynamics of the electro-chemical processes in the solid state during metal anodiza-tion must be obtained leading to the rational design ofsuitable synthetic methods for functional nanostructuredfilms of interest On the other hand application of thesematerials to photocatalytic systems needs the estimationof the kinetic parameters of the heterogeneous reactions

HindawiInternational Journal of PhotoenergyVolume 2019 Article ID 2571906 15 pageshttpsdoiorg10115520192571906

and the kinetics of the photochemical processes Further-more optimization of the solar to chemical energy conver-sion during the photocatalytic process requires the designof efficient photochemical reactors with appropriate com-parison of their performance in relation to different photo-catalytic systems

The aim of this work is to summarize the salient physico-chemical concepts involved in the growth of anodic oxidefilms on metal surfaces under high fields to rationalize theprecise control of the structural and morphological charac-teristics of the semiconductor materials obtained that thissynthetic method allows We also discuss the heterogeneousphotocatalysis based on these functional materials consider-ing their photoelectrochemical performance as well as thekinetics of the heterogeneous catalytic processes involvedin relation to their use in the environmental problem of treat-ing wastewaters

2 Anodic Oxide Growth on Metal Surfaces

The passivation of iron-based materials was described in1836 by Schoumlnbein and Faraday who identified the phenom-enon as a chemical process involving the formation of anoxide thin layer on the metal surface [29]

In this case it is usually observed that metals becomepassive by formation of dense oxide thin films exhibitinglow ionic conductivity thus inhibiting the active dissolu-tion MrarrMz+ + zeminus or corrosion of the underlying metalwith the oxides representing barriers to the flow of ionsand electrons [30 31] This leads to valve-like behavior withcurrent rectification upon reversal of the electrical potentialand film thickness proportional to the electric field Thepassive thin film is then formed by the anodic reactiondepicted by [30 31]

M +z2

H2OrarrMOz2 + zH+ + zeminus 1

The anodization reaction (1) leads to the synthesis ofhighly reproducible thin solid films of valve metal oxidesThe anodization process may also occur from lower oxides

MOy2 +z minus y2

H2OrarrMOz2 + z minus y H+ + z minus y eminus 2

and an oxide film may also be formed by electrocrystalli-zation of ions from solution

My+ + zH2OrarrMOz2 + zH+ + z minus y eminus 3

Anodic potentials enhance passivation according toequations (1)-(3) but transpassivity and anodic breakdowncause the opposite effect [31] The opposite process impliesreduction reactions that destabilize the passive films leadingto cathodic breakdown

Figure 1 shows a simple scheme of the several ion-and electron-transfer reactions (ITRs and ETRs respec-tively) that may occur during anodization processes thesechemical process may stabilize or dissolve the passivefilm [30 31]

The most important processes are summarized asfollows [30 31]

(1) Electron-transfer reactions (iredox) eg oxygenevolution

(2) Growth of the thin oxide film (iox) by the transfer ofoxygen ions from the electrolyte into the oxide

(3) Transfer of metal ions from the oxide to solution andoxidation of water (iox)

(4) Reduction by the reverse reactions (ired)

(5) Corrosion with charge transfer by metal ionscompensated by that of oxygen ions (iox + ired = 0)

(6) Capacitive charging of the interface (iC) upon poten-tial changes

The total current flowing through the interface is thendescribed by

i = iredox + iox + ired + iC 4

Following Young [32] a classification based on theexperimental observations indicates that the chemical ele-ments with a complete valve effect are Al Bi Sb Ta Tiand Nb and those with an incomplete valve effect are AgCd Fe Mg Si Sn W Zn and Zr the behavior depending

ETR

ITR

iredox

iox

iox

ired

ired + iox = 0

ETR + ITR

Metal Electrolyte

H2O

H2OO2minus

O

R

H+

Mz+

MOz2

MOz2

MOz2Hx

2H+

Mz+

Mz+

MOz2

minus

minus

Figure 1 Scheme of the several ion-transfer reactions (ITRs) andelectron-transfer reactions (ETRs) during the anodizationprocesses

2 International Journal of Photoenergy

on the composition of the solution used during the anodiza-tion process [32]

Figure 2 shows a typical experimental setup to obtain thinsolid films by the anodization process The preparation ofpassive films is affected by the experimental conditions[30 31] The following aspects are usually important (i)effects due to the presence of a native passive film and pre-treatment of the metal surface (ii) potential-time programfor anodization (iii) chemistry of the solution pH possibilityof corrosion nature of supporting electrolyte presence ofions as possible doping agents and additives for the promo-tion of nanostructures (iv) hydrodynamics (bubble controlin the presence of parallel gas evolution reactions possibilityof erosion of the formed passive film and enhanced masstransport of ions in the case of synthesis of doped thin films)and (v) posttreatment of the metal oxide film as drying mod-ification or thermal treatment

3 High-Field Anodization

Oxide films of valve metals can be formed after anodic polar-ization under high fields higher than 1 times 106 Vcm Undersuch conditions oxide growth occurs by ion hopping betweenregular sites or interstitial positions in the lattice The hop-ping mechanism requires an activation energy W whichincreases exponentially with the jump distance a thus it isonly possible between neighboring sites [30] and the processis catalyzed by imposing increasingly positive potentials

In general application of an appropriate anodic polariza-tion implies the formation of an oxide layer involving oxida-tion of the metal at the metalndashoxide interface by reaction withthe flux of oxygen ions migrating from the solution across theoxide film assisted by the electric field The final result ismetal consumption with the oxide layer growing in the solidstate A representation of this process is shown in Figure 3

The electrochemical potential of ions and the energies ofelectrons in the electron bands depend on the local Galvanipotential which is a function of the electronic and ionic con-ductivities of the metal oxide the thickness of the film andthe pH of the solution [30 31] The excess charges at theinterface generate a space charge region within the oxidetherefore local potential changes result in a linear field thatcontrols the migration of charge carriers ie ions and elec-trons Metal oxides usually show semiconducting behaviorthus implying capacitive charge accumulation in the solid

state A potential drop occurs across the film but charge accu-mulation at the solidndashelectrolyte interface is only possible forvery thin oxide films in which case the capacitance of theoxide film becomes comparable to the inner Helmholtz planecapacitance at the interface [31] If the potential drop occurswithin the film then migration of charge carriers within thefilm controls the anodic growth

Due to several reasons films may suffer local enhance-ment of the conductivity during the anodization processThe oxide lattice may not withstand the large ion or electronfluxes arising at very high fields or else high reaction ratesof corrosion due to the presence of aggressive chemical species(like chloride or fluoride ions) may locally increase the con-ductivity leading to breakdown of the passive film This maybe indicated by several effects such as irregular current peaksvisible sparks potential fluctuations increasing electricalnoise or even audible noise depending on the oxide bandgapand the nature and concentration of ions in solution [30ndash48]

4 Kinetic Model

In order to represent the high-field anodization phenome-non Figure 4 shows two lattice planes corresponding to stablesites at the positions ldquoxrdquo and ldquox + ardquo then the ion flux dndt atthe metalndashoxide interface can be obtained from the reactionrate in terms of the mole balance for the transference [30]

dndt

=dnrarrdt

minusdnlarrdt

= n x prarr minus n x+a plarr 5

where nrarr and nlarr stand for the ion movements in eitherdirection across the film and prarr and plarr are the probabilitiesof the atomic hopping processes equivalent to kinetic con-stants for the microscopic balance Their values can be esti-mated with an Arrhenius type relationship

p = ϑ exp minusWRT

6

where ϑ is the attempt frequency for the hopping ions andWis the activation energy of this process In Figure 4 the ionsare centered in stable positions in the lattice these stable posi-tions imply minima in the potential energy of the crystal

In the absence of an electric field the activation energyand the hopping probabilities are equal for both hopping

Electrochemicalcell

Electrolyte

Workingelectrode

Referenceelectrode

Auxiliarelectrode

Figure 2 Scheme of experimental setup for obtained thin solidfilms by the anodization

Metal Oxide Electrolyte

Figure 3 Scheme of ion transport during high-field anodization

3International Journal of Photoenergy

directions An electric field contributes to the potentialenergy and catalyzes the hopping mechanism due to theasymmetry generated in the energyndashposition coordinateprofile (see Figure 4) the potential providing free energy todecrease the effective energy barrier

Considering the fundamental chemical kinetics theoryfor electrochemical processes by Butler and Volmer (seedetails and other considerations in eg [30 33]) the activa-tion energy can be rewritten as

Wrarr =W minus αazFEf 7

and

Wlarr =W minus 1 minus α azFEf 8

where α is the transfer coefficient that describes the symme-try of the activation barrier α defines the distance betweenpositions of minimal energy z is the charge number of theelectroactive species F is the Faraday constant Ef is the fieldstrength and Ef = E minus Edeg d where E is the applied poten-tial Edeg is the equilibrium potential of the oxide electrodeand d is the thickness of the film

Then the reaction rate can be expressed in terms of thecurrent flux at the interface after the consideration of theFaraday second law i = zF dndt and substitution of (6)(7) and (8) in (5) yields

i = aϑρCexp minusWRT

expαazFEf

RTminus exp

1 minus α azFEf

RT

9

where ρC = na zF is the concentration of mobile chargesFurthermore the presence of a high electric field implies

ion movement in the growth direction the opposite move-ment is improbable thus the high-field reaction rate becomes


expαazFEf

RT10

After recognizing that the exchange current is i0 = aϑρCexp minus WRT and the Tafel slope is β = αazFRT andsubstituting Ef = E minus E0 d (10) simplifies into (11) asfollows

i = i0 exp βE minus E0d

11

This is a simple relation that indicates the exponen-tial dependence of the reaction rate with the applied poten-tial The correspondence of the currentndashtime experimentaldata obtained at constant potential with (11) thus indicatesthat the rate-determining step for the oxide growth is themovement of the ions within the oxide Additionally if thepotential drop occurs only within the film with 100 Faradicefficiency for film growth ie no oxide dissolution no oxy-gen evolution nor capacitive charging then the film thicknesscan be estimated as [34]

d = d0 +M

zFA ρox

t

0i t dt 12

where d0 is the oxide film thickness initially present ρox theoxide density A is the electrode surface area M the oxidemolecular weight and t

0i t dt is the charge transferred foroxide growth

This result indicates the possibility of describing theoxide thickness during valve metal oxide growth as a linearfunction of the charge passed during anodic polarizationand it is an important feature for controlling the synthesisof thin films of metal oxide semiconductors Another sig-nificant feature of this methodology is that the potentialdetermines the kinetics of the phenomena that occur atthe interface

5 Metal Oxide Nanotubes Synthesized byHigh-Field Anodization

In 1999 Zwilling and coworkers reported a simple methodol-ogy for the synthesis of self-organized metal oxide nanotubesbased on the high-field electrochemical anodization tech-nique in HF electrolytes [49] TiO2 nanotubes were the firstnanostructures obtained but the approach was extended toother valve metals as tungsten zirconium hafnium thalliumtitanium alloys and diverse valve metals alloys [22 27 50] Ingeneral highly ordered arrays of vertically aligned nanotubesor nanopores result from this electrochemical synthesis in aclosely packed structure and with a wide range of character-istics for functional applications such as

(i) high photocatalytic activity

(ii) high resistance to the photocorrosion process

(iii) good biocompatibility for drug release systems

(iv) good electrode materials for diverse sensingapplications

(v) electrodes suitable for Li ion batteries and fuel cells

Pote

ntia

l ene

rgy

Positionnx nx+a

WW120576 = 0

120576 gt 0

120572zFa120601(1-120572)zFa120601

WW

Figure 4 Effect of the electric potential on the activation barrierfor the hopping mechanism between two adjacent planes in theoxide lattice


(vi) good materials for solar energy conversion insolar cells

(vii) self-cleaning materials

(viii) materials for new optical devices

As discussed below whether oxidation leads to theanodic growth of a compact oxide on the metal surface orthe formation of an array of nanotubes is determined bythe competition of two chemical processes the anodic oxideformation at high fields defined by (1) on the one hand andthe chemical dissolution of the oxide through the formationof fluoride complexes

MOz2 + z + 2 Fminus + zH+ rarr MF z+22minus+z2H2O 13

on the other The formation of complexes may occur also bydirect reaction of fluoride with the metal cation transportedby the high field at the oxide-electrolyte interface

Mz+ + z + 2 Fminus rarr MF z+22minus

14

Figure 5(a) shows a simple scheme to visualize the micro-scopic transport phenomena that occur during the high-fieldanodization of titanium in aqueous electrolyte In the pres-ence of fluoride ions Figure 5(b) the situation changes dra-matically by dissolution of TiO2 at the electrodendashelectrolyteinterface At least two new phenomena need to be consid-ered (i) the ability to form water-soluble TiF6

2- complexesand (ii) the incorporation of fluoride into the growing filmdue to its small ionic radius implying the field-assisted trans-port of this ion through the oxide film and thus competingwith the transport of oxygen ionsO2- in the solid state The ini-tial nanopores formed at the surface develop in a nanotubulararray due to the competition of the high-field electrochemicalformation of TiO2 and the formation of Ti-F complexes by thechemical attack of fluoride to the formed TiO2 [22 50]

Figure 6(a) depicts the current-time curve registeredfrom high-field anodization with formation of a nanotubearray film In this case the electrolyte is an aqueous solu-tion containing fluoride ions Three stages are observed inphase I an initial exponential decay of the current thenthe current increases during phase II after a time delaydependent on the fluoride concentration with shorterdelays at higher fluoride concentrations In phase III the

Electrochemicalcell

F =

Electrolyte

Workingelectrode

Referenceelectrode

Auxiliarelectrode

ΔUt

t

Ti

Ti2O

Ti(OH)xOy

Ti

Ti2O

[TiF6]2minus

Electrolyte

H+H2O

O2minus

O2minusFminus

Fminus

Ti4+

Ti4+

Electrolyte

(a) (b)

Etching

Oxidation

Figure 5 Anodization process representation in (a) absence of fluoride electrolyte and (b) presence of fluoride electrolyte

WRA

t

i

Compact TiO2 layer

(a) (b)

TiTi Ti

(c)

Compact TiO2 layerInitialporous

structureSelf-organizednanotube layer

H2SO4

H2SO4 + HF

I II III

Figure 6 Key steps during the self-organized nanotube layer formation based on anodization process in fluoride electrolyte


current reaches a steady state at a value directly proportionalto the fluoride concentration in solution Figure 6(b) depictsthe steps involved in the formation of the nanotube arrayfilm A barrier oxide is formed during phase I leading tothe decay of the current during phase II the surface is locallyactivated with pores which start to grow randomly increas-ing the active area and originating the rising current In phaseIII the pores interfere with each other as the chemical pro-cess of film formation and dissolution far from equilibriumis maintained leading to a steady-state dissipative structurewith minimal entropy production [51] in this stage thecurrent flow is equally shared by the available pores andself-ordering conditions are established A rigorous treat-ment of self-ordering in the formation of nanotubes is stilllacking theoretical and experimental studies are needed toelucidate it and contribute to the rational optimization ofthe synthesis of nanotubes Notwithstanding the evidenceavailable makes it clear that according to the schemepresented in Figure 6 self-ordered nanotubular films ofvalve metals can be formed by high-field anodization influoride-rich electrolytes Figure 6(c) shows the ideal nano-tubular array film correlating its formation to the compet-ing rates of the two major phenomena the electrochemicalgrowth of the metal oxide and its chemical dissolution byformation of the metal-fluoride complex [22 50]

According to the discussion above the factors to considerfor the synthesis of nanostructured films are (i) a high-fieldanodization protocol with a valve metal or an alloy of thesemetals (ii) a potential-time program for anodization (iii)an electrolyte containing fluorides and (iv) the effects of anative passive film and pretreatment of the metal surfaceCurrently self-assembled nanotube films of several semicon-ductors such as TiO2 WO3 ZrO2 HfO2 Ta2O5 Nb2O5doped materials binary alloys such as TiAl TiNb TiWand TiZr and biomedical alloys such as Ti6Al7Nb andTi29Nb13Ta46Zr have been reported [22 50]

According to the literature [22] nanotube arrays with thefollowing characteristics have been synthesized

(1) Directly attached to the metal

(2) Annealed to an appropriate crystal phase

(3) Typical dimensions (TiO2 nanotubes)

(a) Length 100 nm to 100μm (related to the anodi-zation time)

(b) Diameter 10 nm to500nm(related to the voltage)

(c) Wall thickness 2 to 80 nm

The shape of the nanostructures can be controlled withthe anodization voltage and the fluoride content or thenature of the fluoride chemical compound used in the elec-trolyte Figure 7 shows examples of shapes obtained in thecase of TiO2 nanotubes

6 Photocatalysis Based on Metal OxidesSynthesized under High Fields

61 General Considerations The metal oxides synthesized byanodization methods typically result in a thin film with thesemiconductor material attached to the metal substrate Thismay represent an advantage for the collection of electrons gen-erated during the light activation resulting in good efficienciesAdditionally these films present good mechanical propertiessuch as high resistance to erosion and corrosion

The crystallinity of the semiconductor films may beimproved after synthesis subjecting them to thermal treat-ments to promote phase transitions The crystal phase deter-mines the possibility of using the semiconductor material insome applications the transformation depending on thetemperature and pressure employed in a muffle reactor andthe use of an inert or an oxygen-rich atmosphere promotingstoichiometric or nonstoichiometric growth of the newphases In certain occasions mixed phases are of interestand in order to synthesize these materials an initial phaseis at first formed with new conditions imposed later in accor-dance to the phase diagram of the material with the kineticsof the phase transition determining the rate of synthesis ofthe new material

Doping the thin films formed by anodization is possibleeither by incorporation of foreign ionic chemical species in asecond anodization bath by surface modification during athermal treatment with a vapor compound or by equilibrium

400 nmHFW166 120583m

WD99 mm

mag 998682250 000 times

vac modeHigh vacuum

HV3000 kV

detETD

622017121949 PM M4

(a)

mag 998682100 000 times

WD100 mm

modeSE

HV3000 kV

1 120583mIVIC M4

(b)

Figure 7 Experimental TiO2 nanotubes obtained after the anodization of Ti foil synthetized according to the methodology reported in [52]


adsorption of organic or inorganic compounds from aqueoussolution [16]

In general characterization of the anodic thin films isneeded and diverse experimental techniques have beendeveloped for these purposes optical electronic andoratomic microscopy [50] electrochemical measures control-ling voltage current densities or charges [30ndash48] electro-chemical impedance spectroscopy [53 54] gravimetricmeasurements with the electrochemical quartz microbalance[55 56] infrared absorption Raman laser UV-vis UVndashvisreflectance luminescence acoustic X-rays resonanceellipsometry and neutron-based spectroscopies [15 44ndash4750 57ndash62] and dynamic characterization based on photo-electrochemical methods [23 63 64]

62 Photocatalytic Considerations Under conditions ofMOxsolar light photocatalysis an electron from the valenceband is promoted to the conduction band [65]

MOx + hνrarrMOx eminus + h+ 15

generating a hole h+ in the valence band (VB) and an electroneminus in the conduction band (CB) The holes can react withorganic compounds to generate free radicals

h+ + Rrarr Rbull+ + e‐ 16

They can also be trapped by water to form hydroxyl rad-icals on the surface of the photocatalyst

h+ + H2OrarrHObull +H+ + e‐ 17

Charge balance is preserved by reaction of electrons inthe CB with acceptor species dissolved in the aqueous solu-tion Thermodynamically the reduction potential of thischemical species must be equal or more positive than thepotential corresponding to the edge of the CB of the semicon-ductor another necessary condition for reactivity is that the

electron density of the redox couple in the electrolyte mustoverlap with the density of states of the CB Frequently over-all charge balance is completed by the oxygen reduction reac-tion under solar light irradiation

e‐ + O2 rarrObull‐2 18

or by reduction of an oxidized species Ox in solution capableof being reduced by an electron from the conduction band

e‐ + Oxrarr Red 19

Additionally reduction can be electrochemically assistedthis implies separating the anodic and cathodic reactionsand after polarization the electron at the CB can beextracted to the external circuit enhancing charge separa-tion as shown in Figure 8 Consequently electronndashholerecombination diminishes and general improvement of thelight-induced redox reactions obtain

63 Reaction Kinetics of the Photocatalytic Process There areseveral parameters that influence the photocatalytic process[65 66] for instance the intensity of the radiation thatreaches the surface of the photocatalyst This variable is par-ticularly important when the process is carried out on a pilotscale with an irradiance affected by the weather When aphotocatalytic reaction is carried out in a pilot-plant reactorthe reaction time needs normalization with respect to theintensity of incident radiation otherwise when consideringtime as an independent variable its variation throughoutthe process by cloud cover and the distribution of the radiantflux in the reactor should be taken into account [67] Thisproblem has been addressed introducing a standardizedlighting time tIpW n accounting for the average radiationintensity

tIpWn = tIpWnminus1 + ΔtnIexp tn

IpV iVT

emspΔtn = tn minus tnminus1 20

Semiconductorhv

Recombination

H2OOH

E

endashendash

R

R

2 H2O

H2 + OH minus

endash

ndash+

Ox

Red

VBh+Ox

Red

CBendash

Figure 8 Scheme of electrochemical assisted photocatalysis


where Ipexp tn is the average of solar irradiation intensitymeasured during an experimental time interval Δtn and V iand VT represent the irradiate volume in contact with thephotocatalyst and the total volume of the reactor respec-tively For photocatalysts that absorb UV light as TiO2 thetypical UV power Ip = 30 mWcm2 of a perfectly sunnyday is usually used In the case of using photocatalysts thatabsorb visible radiation (20) can be used consideringthe average solar radiation of the spectrum of a sunny dayIp = 1000-1500Wm2 or some value that can be determinedor considered representative for the experimental setupAdvanced considerations for the design and operation ofphotocatalytic reactors have been reviewed [68]

In the case of photocatalysis based on thin solid filmsobtained by the high-field anodization method the natureof the film material the disposition of the film with respectto the illumination and the possibility of the electrochemicalassisted photocatalysis influence the photocatalytic process[12 15] The morphology crystal phase and compositionof the nanocatalyst affect the performance [12 69] Also inphotocatalytic reactions the pH of the solution determinesthe charge of the catalyst surface For example changes inthe pH can result in an improvement of the efficiency inthe photocatalytic removal of pollutants in the presence ofTiO2 due to the impact of the adsorbed states on the reactionrate on the photocatalyst [70ndash73]

Another significant factor in the study of the chemicalkinetics of photocatalysis is the nature and concentration ofthe substrate [28 74] Studies of concentration effects at other-wise constant conditions allow estimating the kinetic con-stants that characterize the reaction The concentration ofthe organic substrate influences the saturation of active siteson the catalyst surface deactivating them at high concentra-tions [75] At typical radiation intensities at the surface ofthe Earth photocatalytic reactions for the decomposition oforganic compounds dissolved in water can be describedaccording to the kineticmodel of Langmuir andHinshelwoodBasically it involves fast establishment of adsorption-desorption equilibrium of the organic compound on thesurface of the photocatalyst with subsequent surface reactionof the adsorbed species with photogenerated hydroxyl radi-cals The rate law describing such behavior is expressed by

r = kKc1 + Kc

21

where r is the reaction rate c the concentration of the organiccompound k is the rate constant of the surface reactionbetween hydroxyl radicals and organic compound and Kis the equilibrium constant of adsorption-desorption ofthe organic compound on the surface of the photocatalyst[5 65] From this model it can be seen that the kineticsof the reaction changes from first order to zero order as theconcentration of the substrate increases since with Kcltlt1the reaction rate becomes proportional to the concentrationand with Kcgtgt1 the kinetics is independent of the concen-tration Thus the observed reaction constant turns out tobe inversely proportional to the concentration of theorganic compound kobs = kK 1 + Kc This is difficult to

detect from ln c vs t plots [76] However upon plottingthe inverse of the initial reaction rate as a function of theinverse of the initial concentration a linear response isobtained and the kinetic constants characterizing the reac-tion on the specific photocatalyst can be determined as k= 1intercept and K = interceptslope [5 28 65 77] Whenconsidering a multicomponent system or when reactionintermediaries accumulate significantly the Langmuir-Hinshelwood model takes the following form

r =kKc

1 + Kc + ΣKici 22

where sumKici represents the contribution of the i-th compo-nents of the system thus the photocatalytic reaction involvesa thermodynamic contribution the adsorption of the adsor-bate on the catalyst surface characterized by K and a kineticcontribution due to the reaction of the adsorbate with the oxi-dant agents formed by electron transfer to the hole at the VBof the semiconductor characterized by k

7 Application of High-Field-GrownSemiconducting Anodic Oxide Films toPhotocatalytic Processes

71 Photo(electro)catalysis Photocatalysts obtained byhigh-field anodization have been reported [22 78 79 82ndash86]with growing interest on self-assembled nanostructures suchas nanotubes of TiO2 and other materials [16 22 26 27 50]Additionally mineralization has been observed with elec-trochemically assisted photocatalysis [12 22] and photo-catalytic oxidations without polarization have been alsoreported [9 50] but in general these reports have not beensupported by kinetic studies to determine oxidationparameters hindering identification of conditions for effi-cient mineralization The simplest kinetic model to inter-pret heterogeneous photocatalysis as discussed above isthat of Langmuir and Hinshelwood and some reportsexplore the degradation of pollutants on semiconductormaterials synthesized by anodization in terms of this kineticmodel [9 28 75 79ndash81] Better understanding of the chem-ical kinetic principles of photocatalytic processes with appli-cation of bias potentials is also of interest [75 82 83]

In recent years efforts have been oriented in the struc-tural modification of nanometric materials obtained byhigh-field anodization The nonmetallic doping by thermaltreatments in controlled atmosphere is a common strategyto obtain nanotubes of TiO2 with high oxygen vacancy den-sity and Ti3+ in the structure This results in new energylevels below the conduction band increasing the density ofcharge carriers improving the separation of charges by cap-ture of electrons and extending absorption towards the visi-ble range [84 85] In addition nanotube materials have beenmodified by decoration with metallic nanoparticles or semi-conductors improving the generation and separation ofcharges [86 87] These electrodes are chemically stable andare presented as good candidates for the treatment of waste-water both by their high capacity to generate hydroxyl


radicals [88] as well as photoanodes in photoelectrochemicalcells for the production of H2 through the water-splittingreaction [88ndash90]

The properties of the photocatalyst may be tuned chang-ing the synthesis conditions For example mixed oxidenanotube layers of TiNb TiMo and TiW have been testedand it has been found that under visible light-inducedphotocatalysis the TiW oxide tubes show very high effi-ciency for methyl orange degradation [91ndash95] A commonapproach has been to decorate the film with nanoparticlesof Ag Au and alloys other oxide materials or by adsorptionof dye sensitizers with application of external bias Withthese increased photocatalytic activities have been invari-ably researched [78 96 97] The many reports on the degra-dation of aliphatic alcohols [98ndash100] aliphatic carboxylacids [98ndash101] aromatic alcohols [99 102 103] aromaticcarboxyl acids [99 100 104] chloroaromatic compounds[99 105] aromatic nitro compounds [106 107] amino acidsand derivates [99 108] aromatic amines [109] surfactants[110] herbicides [111] and dyes [112ndash114] indicate thatthe materials obtained by the high-field anodization methodare able to solve with high efficiency the environmentalproblem of wastewaters

72 Analytical Chemistry and Sensing Taking advantage ofthe good photocatalytic and structural characteristics ofmaterials synthetized under high-field anodization variousapplications as active elements for chemical sensors havebeen developed

TiO2 nanostructures have been used as photoelectrodesfor the determination of chemical oxygen demand (COD)[115 116] The results demonstrated the possibility toachieve total oxidation of organic matter composed ofdiverse organic compounds The COD study covered therange between 0 and 850mg O2L with good results in thepresence of chlorides between 0 and 2000 ppm and pHbetween 4 and 10 In addition the time of the determinationwas reduced to intervals between 1 and 5min As an addedvalue of these investigations the design construction andcommercialization of the COD detection equipment wereobtained The technology based on TiO2 nanostructuresformed by high-field anodization results in the principalcomponent of the commercial analyzer COD PeCODtrade fromManTech Inc The equipment is robust and requires the useof a source of ultraviolet radiation to activate the electrodethe time of average sampling is around 15min Recentreports improve reaction conditions for the use of modifiedTiO2 nanotubes [117]

An additional widely documented application is thedetection of H2 [15 22 118 119] The interaction of a gaswith a metal oxide semiconductor is primarily a surface phe-nomenon Therefore nanoporous metal oxides offer theadvantage of providing large sensing surface areas It isknown that the electrical resistance of materials such asTiO2 nanotubes is sensitive to H2 and the detection processis reversible In general the H2 sensor based on nanotubesdemonstrated good sensitivity for the wide-range detectionof dilute hydrogen atmospheres and high concentrationsFor example typical measurements ranging from 50ppm to

2 H2 were reported Another interesting feature is the pos-sibility of performing the detection in a wide temperaturerange from 20degC to 300degC [15 118 119]

73 Self-Cleaning Surfaces Anodization by high fields can beused to obtain self-cleaning surfaces [120] with adjustableandor switchable water adhesion [121] and superhydropho-bic properties for the corrosion resistance of the base material[122 123] and with antimicrobial properties based on thephotocatalysis principle too [124]

Liu et al [121] reported the use of two polymersresponding to different physicochemical stimuli (i) poly(N-isopropylacrylamide) and (ii) poly (dimethylamino)ethyl methacrylate These compounds were used to modifythe surface of previously anodized alumina substrates Thefinal composite material changes the adhesion of waterdroplets as a function of response conditions such as pHtemperature and electrolyte composition This type of sur-faces will find applications in microfluids generation ofmicrodroplets smart coatings and self-cleaning surfaces

Similarly Li and coworkers [120] have commented onthe synthesis of alumina surfaces with macronanohierarchi-cal structures made on aluminum substrates by an easy andfast anodization method By means of a modification basedon stearic acid a superhydrophobic surface with a contactangle of 158deg and an ultralow slip angle of about 0deg wasachieved In addition the superhydrophobic surface showedself-cleaning properties and corrosion resistance

Also the antimicrobial properties of the TiO2 nanotubesprepared by the breakdown anodization process were studiedAs a main result the TiO2 nanotubes showed excellent bacte-rial inhibition percentages of 9753 for E coli and 9994for S aureus after 24 h of UV irradiation Moreover theauthors demonstrated in the same study that the commercialand control samples did not show any antimicrobial propertyunder the same conditions [124] Other studies [122 123]also showed good self-cleaning performance of the anodiza-tion process performed on valve metals and by appropriatechemical modification in a second step high resistance tocorrosion with interesting wettability and anti-icing proper-ties were obtained In general these surfaces show promisingapplications including outdoor sports equipment transpor-tation facilities and industrial machinery

74 Photocatalytic Fuel Cell The photocatalysis process is thelight-induced combustion reaction of organic matter there-fore if carried out in a separated compartment it allows oxi-dation or organic compounds yielding the correspondingelectron flux as an added value To achieve this the oxygenreduction reaction must be coupled in a cell using an appro-priate electrocatalytic material and determining factors arethe control of electric losses during operation of the cell aswell as selection of appropriate electrodes [125 126]

Liu et al [127 128] reported aTiO2-nanotube-array-basedphotocatalytic fuel cell system using visible light First anarrow bandgap semiconductor such as Cu2O and CdSwas combined with TiO2 nanotubes Second the genera-tion of electrical currents from the photocatalytic oxida-tion of various refractory organic compounds with


oxygen reduction at the secondary electrode was evaluatedThe studied model compounds included aromatics azodyes pharmaceutical and personal care products andendocrine-disrupting compounds The approach demon-strated the possibility of obtaining energy from various refrac-tory organic compounds with simultaneous water cleaning

Recently Ye and coworkers [129] evaluated the applica-tion of a membrane-free photocatalytic fuel cell composedof a TiO2 nanotube array photoanode and a Cu cathode formicropollutant removal from water They reported the mostimportant operation conditions to obtain high performanceincluding pH pollutant concentration the oxygen reactivespecies that were formed the presence of chemical sub-stances as inorganic ions in the electrolyte and the hydrody-namic conditions Also significantly enhanced removal of acommonly present aqueous micropollutant 4-chloro-2methylphenoxyacetic acid was obtained

In general it was found that the cell performancedepended critically on the good conductivity of the high-fieldanode as well as on the efficient photocatalytic degradationof the organic compounds However the use of appropriatematerials the quantum efficiency and the design of reac-tors are aspects that still need to be improved by modernresearch [125 126]

75 H2 Production Thermodynamically H2 production dur-ing the photocatalytic process is possible when the photoa-node has a more negative conduction band potential thanthe redox potential required to form H2 from water In gen-eral the reaction rates of photocatalytic processes on numer-ous semiconducting materials are limited by the kinetics ofthe charge transfer process to a suitable redox species There-fore modifications of the electrodes with cocatalysts such asPt Rh Ru Ag Au and its alloys have been used to promoteH2 evolution [22] Photoanodes based on TiO2 nanotubelayers have been reported to be more promising than nano-particle layers due to their well-defined geometry on theone hand but especially because it is more feasible to incor-porate cocatalysts for example by electrocrystallizationandor simple chemical reduction Additionally after thegeneration of charge carriers by sunlight the electrons go tothe back contact of the photoanode then electron lifetimeand conductivity become determinant factors for the overallefficiency [17 22 26 130]

Spanu et al [17] investigated a well-defined charge sepa-ration platform for photocatalytic H2 evolution based on aPt-WO3-TiO2 ldquostackedrdquo structure constructed on anodicallygrown TiO2 nanotube arrays These structures show stronglyimproved photocatalytic H2 evolution compared to anyother single cocatalyst system such as Pt-TiO2 WO3-TiO2and pristine TiO2 nanotubes The photocatalytic activity isascribed to the enhanced charge carrier separation mecha-nism enabled by the well-defined TiO2-WO3-Pt architecturethat provides swift electron transfer through WO3 andtowards Pt for H2 evolution

Additionally Spanu et al [130] remarked that the photo-catalytic H2 evolution reaction on pristine TiO2 occurredwith low efficiencies due to (i) trapping and recombinationof charge carriers and (ii) sluggish electron transfer kinetics

Recently this group introduced an approach to fabricate anefficient noble metal-free photocatalytic platform for H2 evo-lution By dewetting NiCu bilayers into alloyed NiCu cocata-lytic nanoparticles at the surface of TiO2 nanotube arraysthey found improvements in H2 production especially whenthe metals were in equimolar proportion The alloyed NiCucocatalyst on TiO2 nanotubes allowed them to reach H2 gen-eration rates comparable to those delivered by conventionaldecoration of TiO2 with noble metals such as platinum

Finally we have considered the advantages of usinganodic oxide thin films and the possibility of precisely con-trolling the characteristics of the final semiconductor mate-rials by growing them under high fields By enhancingcharge separation through external application of electricpotentials their performance can be improved to facilitatethe technological implementation of industrial photocataly-sis This advantage has been employed in several reactors ofdifferent design [12 16 22 26 80] but the industrial produc-tion of thin oxide films and photocatalytic reactors based onthem is yet to be developed [12 26]

8 Conclusions

After reviewing the high-field growth of anodic oxide filmsfor photocatalytic application three important conclusionscan be highlighted (i) the synthesis of nanostructured metaloxides through high-field anodization allows to control thecharacteristics of the obtained materials (ii) the electrochem-ical assistance of the photocatalytic processes on these mate-rials yields efficient wastewater treatment and (iii) themeasure of reaction rates is important to determine andcompare the physicochemical parameters describing thechemical interactions during photoelectrolysis and toapply this understanding to the design operation andcontrol of reactive systems

Conflicts of Interest

The authors declare no conflicts of interest

Acknowledgments

We are grateful to the members of the ElectrochemistryGroup at Simoacuten Boliacutevar University for many stimulating dis-cussions about the subject of this paper

References

[1] V Etacheri C Di Valentin J Schneider D Bahnemann andS C Pillai ldquoVisible-light activation of TiO2 photocatalystsadvances in theory and experimentsrdquo Journal of Photochem-istry and Photobiology C Photochemistry Reviews vol 25pp 1ndash29 2015

[2] Y Lv W Yao R Zong and Y Zhu ldquoFabrication of wide ndashrange - visible photocatalyst Bi2WO6-x nanoplates via surfaceoxygen vacanciesrdquo Scientific Reports vol 6 no 1 article19347 pp 1ndash6 2016

[3] L Madriz J Tataacute and R Vargas ldquoThe photocatalytic oxida-tion of 4-chlorophenol using Bi2WO6 under solar light


irradiationrdquo International Journal of Photochemistryvol 2014 Article ID 387536 6 pages 2014

[4] L Santos-Juanes F S Garciacutea Einschlag A M Amat andA Arques ldquoCombining ZVI reduction with photo-Fentonprocess for the removal of persistent pollutantsrdquo ChemicalEngineering Journal vol 310 no 2 pp 484ndash490 2017

[5] R Vargas and O Nuacutentildeez ldquoPhotocatalytic degradation of oilindustry hydrocarbons models at laboratory and atpilot-plant scalerdquo Solar Energy vol 84 no 2 pp 345ndash3512010

[6] J Schneider M Matsuoka M Takeuchi et al ldquoUnderstand-ing TiO2 photocatalysis mechanisms and materialsrdquo Chemi-cal Reviews vol 114 no 19 pp 9919ndash9986 2014

[7] D Friedmann C Mendive and D Bahnemann ldquoTiO2 forwater treatment parameters affecting the kinetics and mech-anisms of photocatalysisrdquo Applied Catalysis B Environmen-tal vol 99 no 3-4 pp 398ndash406 2010

[8] M R Hoffmann S T Martin W Choi and D WBahnemann ldquoEnvironmental applications of semiconduc-tor photocatalysisrdquo Chemical Reviews vol 95 no 1pp 69ndash96 1995

[9] D Loacutepez W Lozada S Blanco L Madriz G Duraacuten andR Vargas ldquoFotocataacutelisis de p-nitrofenol sobre peliacuteculas deTiO2 nanoestructuradordquo Avances en Ciencia e Ingenieriacuteavol 2 no 4 pp 47ndash58 2011

[10] L Madriz H Carrero J Herrera A Cabrera N Canudasand L Fernaacutendez ldquoPhotocatalytic activity of metallopor-phyrinndashtitanium mixtures in microemulsionsrdquo Topics inCatalysis vol 54 no 1-4 pp 236ndash243 2011

[11] L Madriz H Carrero O Nuacutentildeez R Vargas and J HerreraldquoMechanistic aspects of photocatalytic activity of metallopor-phyrin ndash titanium mixtures in microemulsionsrdquo QuiacutemicaNova vol 39 no 8 pp 944ndash950 2016

[12] S Garcia-Segura and E Brillas ldquoApplied photoelectrocataly-sis on the degradation of organic pollutants in wastewatersrdquoJournal of Photochemistry and Photobiology C Photochemis-try Reviews vol 31 pp 1ndash35 2017

[13] T Hisatomi K Takanabe and K Domen ldquoPhotocatalyticwater-splitting reaction from catalytic and kinetic perspec-tivesrdquo Catalysis Letters vol 145 no 1 pp 95ndash108 2015

[14] N Bao X Feng and C A Grimes ldquoSelf-organizedone-dimensional TiO2 nanotubenanowire array films foruse in excitonic solar cells a reviewrdquo Journal of Nanotechnol-ogy vol 2012 Article ID 645931 27 pages 2012

[15] G K Mor O K Varghese M Paulose K Shankar and C AGrimes ldquoA review on highly ordered vertically oriented TiO2nanotube arrays fabrication material properties and solarenergy applicationsrdquo Solar Energy Materials amp Solar Cellsvol 90 no 14 pp 2011ndash2075 2006

[16] Y-C Nah I Paramasivam and P Schmuki ldquoDoped TiO2and TiO2 nanotubes synthesis and applicationsrdquo Chem-PhysChem vol 11 no 13 pp 2698ndash2713 2010

[17] D Spanu S Recchia S Mohajernia P Schmuki andM Altomare ldquoSite-selective Pt dewetting on WO3-coatedTiO2 nanotube arrays an electron transfer cascade-basedH2 evolution photocatalystrdquo Applied Catalysis B Environ-mental vol 237 pp 198ndash205 2018

[18] V C Anitha R Zazpe M Krbal et al ldquoAnodic TiO2 nano-tubes decorated by Pt nanoparticles using ALD an efficientelectrocatalyst for methanol oxidationrdquo Journal of Catalysisvol 365 pp 86ndash93 2018

[19] M Zubair H Kim A Razzaq C A Grimes and S I InldquoSolar spectrum photocatalytic conversion of CO2 to CH4utilizing TiO2 nanotube arrays embedded with graphenequantum dotsrdquo Journal of CO2 Utilization vol 26 pp 70ndash79 2018

[20] P Enciso Jndash D Decoppet M Graumltzel M Woumlrner F MCabrerizo and M F Cerdaacute ldquoA cockspur for the DSS cellsErythrina crista-galli sensitizersrdquo Spectrochimica Acta PartA Molecular and Biomolecular Spectroscopy vol 176pp 91ndash98 2017

[21] M Hojamberdiev Y Cai J J M Vequizo et al ldquoBinaryflux-promoted formation of trigonal ZnIn2S4 layered crystalsusing ZnS-containing industrial waste and their photocata-lytic performance for H2 productionrdquo Green Chemistryvol 20 no 16 pp 3845ndash3856 2018

[22] K Lee A Mazare and P Schmuki ldquoOne-dimensional tita-nium dioxide nanomaterials nanotubesrdquo Chemical Reviewsvol 114 no 19 pp 9385ndash9454 2014

[23] L M Peter ldquoPhotoelectrochemistry from basic principles tophotocatalysisrdquo in Photocatalysis Fundamentals and Per-spectives J Schneider D Bahnemann J Ye G Li Pumaand D Dionysiou Eds pp 1ndash28 RSC Energy and Environ-mental Series UK 2016

[24] X Chen and S S Mao ldquoTitanium dioxide nanomaterialssynthesis properties modifications and applicationsrdquoChemical Reviews vol 107 no 7 pp 2891ndash2959 2007

[25] S Ozkan A Mazare and P Schmuki ldquoCritical parametersand factors in the formation of spaced TiO2 nanotubes byself-organizing anodizationrdquo Electrochimica Acta vol 268pp 435ndash447 2018

[26] T Berger D Monllor-Satoca M JankulovskaT Lana-Villareal and R Goacutemez ldquoThe electrochemistry ofnanostructure titania dioxide electrodesrdquo Chem Phys Chemvol 13 no 12 pp 2824ndash2875 2012

[27] J M Macak H Hildebrant U Marten-Jahns andP Schmuki ldquoMechanistic aspects and growth of large diam-eter self-assembly TiO2 nanotubesrdquo Journal of Electroanalyt-ical Chemistry vol 621 no 2 pp 254ndash266 2008

[28] U Gaya Heterogeneous Photocatalysis Using InorganicSemiconductor Solids Springer Science + Business MediaDordrecht 2014

[29] C Schoumlnbein and M Faraday ldquoOn peculiar voltaic conditionof ironrdquo Philosophical Magazine vol 9 pp 2499ndash2513 1836

[30] M M Lohrengel ldquoThin anodic oxide layers on aluminiumand other valve metals high field regimerdquo Materials Scienceand Engineering R Reports vol 11 no 6 pp 243ndash294 1993

[31] J W Schultze and M M Lohrengel ldquoStability reactivity andbreakdown of passive films Problems of recent and futureresearchrdquo Electrochimica Acta vol 45 no 15-16 pp 2499ndash2513 2000

[32] L Young Anodic Oxide Films Academic Press London1961

[33] M J Dignam ldquoThe kinetics of growth of oxidesrdquo in Compre-hensive Treatise of Electrochemistry J O Bockris B E Con-way E Yeager and R E White Eds vol 4 ofElectrochemical Material Science Springer Boston MAUSA 1981

[34] O Linares-Peacuterez V Fuertes M Peacuterez and M Loacutepez-TeijeloldquoCharacterization of the anodic growth and dissolution ofoxide films on valve metalsrdquo Electrochemistry Communica-tions vol 10 no 3 pp 433ndash437 2008


[35] P Acevedo-Pentildea G Vaacutezquez D Laverde J E Pedraza-Rosas and I Gonzaacutelez ldquoInfluence of structural transforma-tions over the electrochemical behavior of Ti anodic filmsgrown in 01 M NaOHrdquo Journal of Solid State Electrochemis-try vol 14 no 5 pp 757ndash767 2010

[36] P Acevedo-Pentildea J Vazquez-Arenas R Cabrera-SierraL Lartundo-Rojas and I Gonzalez ldquoTi anodization in alka-line electrolyte the relationship between transport of defectsfilm hydration and compositionrdquo Journal of the Electrochem-ical Society vol 160 no 6 pp C277ndashC284 2013

[37] C E B Marino E M de Oliveira R C Rocha-Filho andS R Biaggio ldquoOn the stability of thin-anodic-oxide films oftitanium in acid phosphoric mediardquo Corrosion Sciencevol 43 no 8 pp 1465ndash1476 2001

[38] J L Trompette L Massot L Arurault and S FontorbesldquoInfluence of the anion specificity on the anodic polarizationof titaniumrdquo Corrosion Science vol 53 no 4 pp 1262ndash12682011

[39] E M Patrito R M Torresi E P M Leiva and V AMacagno ldquoPotentiodynamic and AC impedance investiga-tion of anodic zirconium oxide filmsrdquo Journal of the Electro-chemical Society vol 137 no 2 pp 524ndash530 1990

[40] M E Sibert ldquoElectrochemical oxidation of titanium sur-facesrdquo Journal of the Electrochemical Society vol 110 no 1pp 65ndash72 1963

[41] V Brunetti H M Villullas and M Loacutepez Teijelo ldquoAnodicfilm formation on silver in solutions containing chromaterdquoElectrochimica Acta vol 44 no 17 pp 2843ndash2851 1999

[42] V Brunetti and M Loacutepez Teijelo ldquoOxidehydroxide films ontin Part I kinetic aspects of the electroformation and electro-reduction of the filmsrdquo Journal of Electroanalytical Chemis-try vol 613 no 1 pp 9ndash15 2008

[43] V Brunetti and M Loacutepez Teijelo ldquoOxidehydroxide films ontin II characterization of the anodic growth in alkaline solu-tionsrdquo Journal of Electroanalytical Chemistry vol 613 no 1pp 16ndash22 2008

[44] F A Filippin O E Linarez Peacuterez M Loacutepez Teijelo R DBonetto J Trincavelli and L B Avalle ldquoThickness determi-nation of electrochemical titanium oxide (TiTiO2) formedin HClO4 solutionsrdquo Electrochimica Acta vol 129 pp 266ndash275 2014

[45] M A Peacuterez and M Loacutepez Teijelo ldquoEllipsometric study ofWO3 films dissolution in aqueous solutionsrdquo Thin SolidFilms vol 449 no 1-2 pp 138ndash146 2004

[46] M A Peacuterez and M Loacutepez Teijelo ldquoCathodic behavior ofbismuth I Ellipsometric study of the electroreduction of thinBi2O3 filmsrdquo Journal of Electroanalytical Chemistry vol 583no 2 pp 212ndash220 2005

[47] M A Peacuterez O E Linarez Peacuterez and M Loacutepez TeijeloldquoCathodic behavior of bismuth II Electrochemical and ellip-sometric study of the hydrogen insertion into bulk bismuthrdquoJournal of Electroanalytical Chemistry vol 596 no 2pp 149ndash156 2006

[48] A Aladjem ldquoAnodic oxidation of titanium and its alloysrdquoJournal of Materials Science vol 8 no 5 pp 688ndash704 1973

[49] V Zwilling E Darque-Ceretti A Boutry-Forveille D Perrinand M Aucouturier ldquoStructure and physicochemistry ofanodic oxide films on titanium and TA6V alloyrdquo Surfaceand Interface Analysis vol 27 no 7 pp 629ndash637 1999

[50] J M Macak H Tsuchiya A Ghicov et al ldquoTiO2 nanotubesself-organized electrochemical formation properties and

applicationsrdquo Current Opinion in Solid State and MaterialsScience vol 11 no 1-2 pp 3ndash18 2007

[51] G Nicolis and I Prigogine Self-Organization in Nonequilib-rium Systems from Dissipative Structures to Order throughFluctuations Wiley New York NY USA 1977

[52] A Maimone S Camero and S Blanco ldquoCaracterizacioacuten deloacutexido de titanio obtenido mediante tratamiento teacutermico yanodizado electroquiacutemicordquo Revista de la Facultad de Inge-nieriacutea Universidad Central de Venezuela vol 30 no 1pp 189ndash200 2015

[53] F Fabregat-Santiago G Garcia-Belmonte I Mora-Seroacute andJ Bisquert ldquoCharacterization of nanostructured hybrid andorganic solar cells by impedance spectroscopyrdquo PhysicalChemistry Chemical Physics vol 13 no 20 pp 9083ndash91182011

[54] M E Orazem and B Tribollet Electrochemical ImpedanceSpectroscopy Wiley New York NY USA 2008

[55] D A Buttry and M D Ward ldquoMeasurement of interfacialprocesses at electrode surfaces with the electrochemicalquartz crystal microbalancerdquo Chemical Reviews vol 92no 6 pp 1355ndash1379 1992

[56] NWayneAccelerating Testing Statistical Models Test Plantsand Data Analysis Wiley-Interscience New Jersey USA1990

[57] A Cantarero ldquoRaman scattering applied to materials sci-encerdquo Procedia Materials Science vol 9 pp 113ndash122 2015

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[59] O S Heavens Optical Properties of Thin Solid Films DoverPublications INC New York NY USA 1991

[60] J I Pankove Optical Processes in Semiconductors DoverPublications INC New York NY USA 1975

[61] A Knoks J Kleperis and L Grinberga ldquoRaman spectralidentification of phase distribution in anodic titaniumdioxide coatingrdquo Functional Materials vol 66 no 4pp 422ndash429 2017

[62] H Vaškovaacute ldquoA powerful tool for material identificationRaman spectroscopyrdquo International Journal of MathematicalModels and Methods in Applied Sciences vol 7 no 5pp 1205ndash1212 2011

[63] L M Peter ldquoDynamic aspects of semiconductor photoelec-trochemistryrdquo Chemical Reviews vol 90 no 5 pp 753ndash769 1990

[64] N Sato Electrochemistry at Metal and Semiconductor Elec-trodes Elsevier Amsterdam 1998

[65] A Hakki J Schneider and D Bahnemann ldquoUnderstandingthe chemistry of photocatalytic processesrdquo in PhotocatalysisFundamentals and Perspectives J Schneider D BahnemannJ Ye G Li Puma and D Dionysiou Eds pp 29ndash50 RSCEnergy and Environmental Series UK 2016

[66] U Gaya and A Abdullah ldquoHeterogeneous photocatalyticdegradation of organic contaminants over titanium dioxidea review of fundamentals progress and problemsrdquo Journalof Photochemistry and Photobiology C PhotochemistryReviews vol 9 no 1 pp 1ndash12 2008

[67] S Malato P Fernaacutendez-Ibaacutentildeez M Maldonado J BlancoandW Gernjak ldquoDecontamination and disinfection of waterby solar photocatalysis recent overview and trendsrdquo Cataly-sis Today vol 147 no 1 pp 1ndash59 2009

[68] F Machuca-Martiacutenez M A Mueses J Colina-Maacuterquez andG Li Puma ldquoPhotocatalytic reactor modelingrdquo in


Photocatalysis Fundamentals and Perspectives J SchneiderD Bahnemann J Ye G Li Puma and D Dionysiou Edspp 29ndash50 RSC Energy and Environmental Series UK 2016

[69] D I Naranjo S J Garciacutea-Vergara and S Blanco ldquoScanningelectron microscopy of heat treated TiO2 nanotubes arraysobtained by anodic oxidationrdquo Journal of Physics ConferenceSeries vol 935 article 012025 2017

[70] R Vargas and O Nuacutentildeez ldquoHydrogen bond interactions at theTiO2 surface their contribution to the pH dependentphoto-catalytic degradation of p-nitrophenolrdquo Journal ofMolecular Catalysis A Chemical vol 300 no 1-2 pp 65ndash71 2009

[71] W Wang ldquoEffect of solution pH on the adsorption and pho-tocatalytic reaction behaviors of dyes using TiO2 andNafion-coated TiO2rdquo Colloids and Surfaces A Physicochemi-cal and Engineering Aspects vol 302 no 1-3 pp 261ndash2682007

[72] M Mrowetz and E Selli ldquoPhotocatalytic degradation of for-mic and benzoic acids and hydrogen peroxide evolution inTiO2 and ZnO water suspensionsrdquo Journal of Photochemistryand Photobiology A Chemistry vol 180 no 1-2 pp 15ndash222006

[73] H Mansilla C Bravo R Ferreyra et al ldquoPhotocatalyticEDTA degradation on suspended and immobilized TiO2rdquoJournal of Photochemistry and Photobiology A Chemistryvol 181 no 2-3 pp 188ndash194 2006

[74] M Tariq M Faisal M Muneer and D Bahnemann ldquoPhoto-chemical reactions of a few selected pesticide derivatives andother priority organic pollutants in aqueous suspensions oftitanium dioxiderdquo Journal of Molecular Catalysis A Chemi-cal vol 265 no 1-2 pp 231ndash236 2007

[75] D Carvajal R Vargas C Borraacutes S Blanco J Mostany andB R Scharifker ldquoPhoto (electro) oxidation of organic com-pounds with strong adsorption properties on TiO2 kineticmodelrdquo Catalisis vol 5 pp 89ndash96 2016

[76] G Pardo R Vargas and O Nuacutentildeez ldquoPhotocatalytic TiO2-as-sisted decomposition of Triton X-100 inhibition of p-nitro-phenol degradationrdquo Journal of Physical Organic Chemistryvol 21 no 12 pp 1072ndash1078 2008

[77] L Madriz M Parra R Vargas B R Scharifker O Nuacutentildeezand D Carvajal ldquoFotocataacutelisis heterogeacutenea bajo luz solarbasada en TiO2 y Bi2WO6 aplicaciones ambientalesrdquo Revistade la Universidad del Zulia vol 7 no 18 pp 11ndash54 2016

[78] M Zlamal J M Macak P Schmuki and J Kryacutesa ldquoElectro-chemically assisted photocatalysis on self-organized TiO2nanotubesrdquo Electrochemistry Communications vol 9no 12 pp 2822ndash2826 2007

[79] A G Kontos A I Kontos D S Tsoukleris et al ldquoPhoto-in-duced effects on self-organized TiO2 nanotube arrays theinfluence of surface morphologyrdquo Nanotechnology vol 20no 4 article 045603 2009

[80] M A Lazar S Varghese and S S Nair ldquoPhotocatalytic watertreatment by titanium dioxide recent updatesrdquo Catalystsvol 2 no 4 pp 572ndash601 2012

[81] H Tang Y Xu and Q Zhang ldquoPreparation of Ag nanopar-ticle surface modified TiO2 nanotube arrays and establish-ment of a catalytic kinetic modelrdquo Advances in EnergyScience and Environment Engineering vol 1829pp 0200401ndash0200405 2017

[82] P A Mandelbaum A E Regazzoni M A Blesa and S ABilmes ldquoPhoto-electro-oxidation of alcohols on titanium

dioxide thin film electrodesrdquo The Journal of Physical Chemis-try B vol 103 no 26 pp 5505ndash5511 1999

[83] M E Calvo R J Candal and S A Bilmes ldquoPhotooxidationof organic mixtures on biased TiO2 filmsrdquo EnvironmentalScience and Technology vol 35 no 20 pp 4132ndash4138 2001

[84] N Wang Y Ma J Chen et al ldquoDefect-induced betavoltaicenhancement in black titania nanotube arraysrdquo Nanoscalevol 10 no 27 pp 13028ndash13036 2018

[85] L Yu C H MingLi Y Zhang J He X Zhou and H ZhuldquoPhotoelectrochemical properties of N doped black TiO2nanotube arraysrdquo Materials Letters vol 216 pp 239ndash2422018

[86] M Plodinec I Grcic M G Willinger et al ldquoBlack TiO2nanotube arrays decorated with Ag nanoparticles forenhanced visible-light photocatalytic oxidation of salicylicacidrdquo Journal of Alloys and Compounds vol 776 pp 883ndash896 2019

[87] K Du G Liu X Chen and K Wang ldquoFast charge separationand photocurrent enhancement on black TiO2 nanotubesco-sensitized with Au nanoparticles and PbS quantum dotsrdquoElectrochimica Acta vol 277 pp 244ndash254 2018

[88] Y Yang L C Kao Y Liu et al ldquoCobalt-doped black TiO2nanotube array as a stable anode for oxygen evolution andelectrochemical wastewater treatmentrdquo ACS Catalysisvol 8 no 5 pp 4278ndash4287 2018

[89] H Cui W Zhao C Yang et al ldquoBlack TiO2 nanotube arraysfor high-efficiency photoelectrochemical water-splittingrdquoJournal of Materials Chemistry A vol 2 no 23 pp 8612ndash8616 2014

[90] E Liu P Xue J Jia et al ldquoCdSe modified TiO2 nanotubearrays with Ag nanoparticles as electron transfer channeland plasmonic photosensitizer for enhanced photoelectro-chemical water splittingrdquo Journal of Physics D Applied Phys-ics vol 51 no 30 article 305106 2018

[91] P Roy S Berger and P Schmuki ldquoTiO2 nanotubes synthesisand applicationsrdquo Angewandte Chemie International Editionvol 50 no 13 pp 2904ndash2939 2011

[92] S Berger H Tsuchiya A Ghicov and P Schmuki ldquoHighphotocurrent conversion efficiency in self-organized porousWO3rdquo Applied Physics Letters vol 88 no 20 article203119 2006

[93] A Ghicov S Aldabergenova H Tsuchyia and P SchmukildquoTiO2ndashNb2O5 nanotubes with electrochemically tunablemorphologiesrdquo Angewandte Chemie International Editionvol 45 no 42 pp 6993ndash6996 2006

[94] I Paramasivam Y-C Nah C Das N K Shrestha andP Schmuki ldquoWO3TiO2 nanotubes with strongly enhancedphotocatalytic activityrdquo Chemistry ndash A European Journalvol 16 no 30 pp 8993ndash8997 2010

[95] P Agarwal I Paramasivam N K Shrestha and P SchmukildquoMoO3 in self-organized TiO2 nanotubes for enhanced pho-tocatalytic activityrdquo Chemistry-An Asian Journal vol 5no 1 pp 66ndash69 2010

[96] I Paramasivam J M Macak A Ghicov and P SchmukildquoEnhanced photochromism of Ag loaded self-organizedTiO2 nanotube layerrdquo Chemical Physics Letters vol 445no 4ndash6 pp 233ndash237 2007

[97] Yndash Y Song P Roy I Paramasivam and P SchmukildquoVoltage-induced payload release and wettability controlon TiO2 and TiO2 nanotubesrdquo Angewandte Chemie Inter-national Edition vol 49 no 2 pp 351ndash354 2010


[98] I Mora-Seroacute T Lana-Villarreal J Bisquert Aacute PitarchR Goacutemez and P Salvador ldquoPhotoelectrochemical behaviorof nanostructured TiO2 thin-film electrodes in contact withaqueous electrolytes containing dissolved pollutants a modelfor distinguishing between direct and indirect interfacial holetransfer from photocurrent measurementsrdquo The Journal ofPhysical Chemistry B vol 109 no 8 pp 3371ndash3380 2005

[99] D Jiang S Zhang and H Zhao ldquoPhotocatalytic degradationcharacteristics of different organic compounds at TiO2 nano-porous film electrodes with mixed anataserutile phasesrdquoEnvironmental Science amp Technology vol 41 no 1pp 303ndash308 2007

[100] H Zhao D Jiang S Zhang and W Wen ldquoPhotoelectrocata-lytic oxidation of organic compounds at nanoporous TiO2electrodes in a thin-layer photoelectrochemical cellrdquo Journalof Catalysis vol 250 no 1 pp 102ndash109 2007

[101] D Jiang H Zhao S Zhang and R John ldquoKinetic study ofphotocatalytic oxidation of adsorbed carboxylic acids atTiO2 porous films by photoelectrolysisrdquo Journal of Catalysisvol 223 no 1 pp 212ndash220 2004

[102] D Monllor-Satoca and R Goacutemez ldquoA photoelectrochemicaland spectroscopic study of phenol and catechol oxidationon titanium dioxide nanoporous electrodesrdquo ElectrochimicaActa vol 55 no 15 pp 4661ndash4668 2010

[103] Y Xie ldquoPhotoelectrochemical application of nanotubulartitania photoanoderdquo Electrochimica Acta vol 51 no 17pp 3399ndash3406 2006

[104] H Liu S Cheng M Wu et al ldquoPhotoelectrocatalytic degra-dation of sulfosalicylic acid and its electrochemical imped-ance spectroscopy investigationrdquo The Journal of PhysicalChemistry A vol 104 no 30 pp 7016ndash7020 2000

[105] T A Egerton ldquoDoes photoelectrocatalysis by TiO2 workrdquoJournal of Chemical Technology amp Biotechnology vol 86no 8 pp 1024ndash1031 2011

[106] M Tian G Wu B Adams J Wen and A Chen ldquoKinetics ofphotoelectrocatalytic degradation of nitrophenols on nano-structured TiO2 electrodesrdquo The Journal of Physical Chem-istry C vol 112 no 3 pp 825ndash831 2008

[107] B Su Y Ma Y Du and C Wang ldquoStudy of photoelectro-catalytic degradation behavior of p-nitrophenol withnano-TiO2 modified film at a rotating ringndashdisk electroderdquoElectrochemistry Communications vol 11 no 6 pp 1154ndash1157 2009

[108] H Hidaka T Shimura K Ajisaka S Horikoshi J Zhao andN Serpone ldquoPhotoelectrochemical decomposition of aminoacids on a TiO2OTE particulate film electroderdquo Journal ofPhotochemistry and Photobiology A Chemistry vol 109no 2 pp 165ndash170 1997

[109] J Carvalho Cardoso T Mescoloto Lizier and M V BoldrinZanoni ldquoHighly ordered TiO2 nanotube arrays and photo-electrocatalytic oxidation of aromatic aminerdquo AppliedCatalysis B Environmental vol 99 no 1-2 pp 96ndash1022010

[110] H Hidaka K Ajisaka S Horikoshi et al ldquoComparativeassessment of the efficiency of TiO2OTE thin film electrodesfabricated by three deposition methods photoelectrochem-ical degradation of the DBS anionic surfactantrdquo Journal ofPhotochemistry and Photobiology A Chemistry vol 138no 2 pp 185ndash192 2001

[111] Y Xin H Liu L Han and Y Zhou ldquoComparative study ofphotocatalytic and photoelectrocatalytic properties of ala-chlor using different morphology TiO2Ti photoelectrodesrdquo

Journal of Hazardous Materials vol 192 no 3 pp 1812ndash1818 2011

[112] K Vinodgopal and P V Kamat ldquoEnhanced rates of photo-catalytic degradation of an azo dye using SnO2TiO2 coupledsemiconductor thin filmsrdquo Environmental Science amp Tech-nology vol 29 no 3 pp 841ndash845 1995

[113] A Turolla M Fumagalli M Bestetti and M AntonellildquoElectrophotocatalytic decolorization of an azo dye on TiO2self-organized nanotubes in a laboratory scale reactorrdquo Desa-lination vol 285 pp 377ndash382 2012

[114] K Vinodgopal I Bedja and P V Kamat ldquoNanostructuredsemiconductor films for photocatalysis Photoelectrochem-ical behavior of SnO2TiO2 composite systems and its rolein photocatalytic degradation of a textile azo dyerdquo Chemistryof Materials vol 8 no 8 pp 2180ndash2187 1996

[115] J Zhang B Zhou Q Zheng et al ldquoPhotoelectrocatalyticCOD determination method using highly ordered TiO2nanotube arrayrdquo Water Research vol 43 no 7 pp 1986ndash1992 2009

[116] J Qiu S Zhang and H Zhao ldquoNanostructured TiO2 photo-catalysts for the determination of organic pollutantsrdquo Journalof Hazardous Materials vol 211-212 pp 381ndash388 2012

[117] J Zhang X Chan and A Chen ldquoDetermination of chemicaloxygen demand based on photoelectrocatalysis of nanopor-ous TiO2 electrodesrdquo Sensors and Actuators B Chemicalvol 223 pp 664ndash670 2016

[118] Z Li D Ding Q Liu C Ning and XWang ldquoNi-doped TiO2nanotubes for wide-range hydrogen sensingrdquo NanoscaleResearch Letters vol 9 no 1 pp 118ndash126 2014

[119] Z Chen M Cong J Hu Z Yang and Z Chen ldquoPreparationof functionalized TiO2 nanotube arrays and their applica-tionsrdquo Science of Advanced Materials vol 8 no 6pp 1231ndash1241 2016

[120] S Y Li J Wang Y Li and C W Wang ldquoSuperhydrophobicsurface based on self-aggregated alumina nanowire clustersfabricated by anodizationrdquo Microelectronic Engineeringvol 142 pp 70ndash76 2015

[121] X Liu Q Ye B Yu Y Liang W Liu and F Zhou ldquoSwitch-ing water droplet adhesion using responsive polymerbrushesrdquo Langmuir vol 26 no 14 pp 12377ndash12382 2010

[122] S Zheng C Li Q Fu et al ldquoFabrication of self-cleaningsuperhydrophobic surface on aluminum alloys with excellentcorrosion resistancerdquo Surface and Coatings Technologyvol 276 pp 341ndash348 2015

[123] S Zheng C Li Q Fu et al ldquoDevelopment of stablesuperhydrophobic coatings on aluminum surface for cor-rosion-resistant self-cleaning and anti-icing applicationsrdquoMaterials amp Design vol 93 pp 261ndash270 2016

[124] J Podporska-Carroll E Panaitescu B Quilty L WangL Menon and S C Pillai ldquoAntimicrobial properties of highlyefficient photocatalytic TiO2 nanotubesrdquo Applied Catalysis BEnvironmental vol 176-177 pp 70ndash75 2015

[125] P Lianos ldquoProduction of electricity and hydrogen by photo-catalytic degradation of organic wastes in a photoelectro-chemical cell the concept of the photofuelcell a review of are-emerging research fieldrdquo Journal of Hazardous Materialsvol 185 no 2-3 pp 575ndash590 2011

[126] A Sfaelou and P Lianos ldquoPhotoactivated fuel cells (Photo-FuelCells) An alternative source of renewable energy withenvironmental benefitsrdquo AIMS Materials Science vol 3no 1 pp 270ndash288 2016


[127] Y Liu J Li B Zhou H Chen Z Wang and W Cai ldquoATiO2-nanotube-array-based photocatalytic fuel cell usingrefractory organic compounds as substrates for electricitygenerationrdquo Chemical Communications vol 47 no 37pp 10314ndash10316 2011

[128] Y Liu J Li B Zhou et al ldquoEfficient electricity productionand simultaneously wastewater treatment via ahigh-performance photocatalytic fuel cellrdquo Water Researchvol 45 no 13 pp 3991ndash3998 2011

[129] Y Ye H Bruning X Li D Yntema and H H M RijnaartsldquoSignificant enhancement of micropollutant photocatalyticdegradation using a TiO2 nanotube array photoanode basedphotocatalytic fuel cellrdquo Chemical Engineering Journalvol 354 pp 553ndash562 2018

[130] D Spanu S Recchia S Mohajernia et al ldquoTemplateddewettingndashalloying of NiCu bilayers on TiO2 nanotubesenables efficient noble-metal-free photocatalytic H2 evolu-tionrdquo ACS Catalysis vol 8 no 6 pp 5298ndash5305 2018


TribologyAdvances in

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ria

ls


Journal ofNanomaterials

Submit your manuscripts atwwwhindawicom

and the kinetics of the photochemical processes Further-more optimization of the solar to chemical energy conver-sion during the photocatalytic process requires the designof efficient photochemical reactors with appropriate com-parison of their performance in relation to different photo-catalytic systems

The aim of this work is to summarize the salient physico-chemical concepts involved in the growth of anodic oxidefilms on metal surfaces under high fields to rationalize theprecise control of the structural and morphological charac-teristics of the semiconductor materials obtained that thissynthetic method allows We also discuss the heterogeneousphotocatalysis based on these functional materials consider-ing their photoelectrochemical performance as well as thekinetics of the heterogeneous catalytic processes involvedin relation to their use in the environmental problem of treat-ing wastewaters

2 Anodic Oxide Growth on Metal Surfaces

The passivation of iron-based materials was described in1836 by Schoumlnbein and Faraday who identified the phenom-enon as a chemical process involving the formation of anoxide thin layer on the metal surface [29]

In this case it is usually observed that metals becomepassive by formation of dense oxide thin films exhibitinglow ionic conductivity thus inhibiting the active dissolu-tion MrarrMz+ + zeminus or corrosion of the underlying metalwith the oxides representing barriers to the flow of ionsand electrons [30 31] This leads to valve-like behavior withcurrent rectification upon reversal of the electrical potentialand film thickness proportional to the electric field Thepassive thin film is then formed by the anodic reactiondepicted by [30 31]

M +z2

H2OrarrMOz2 + zH+ + zeminus 1

The anodization reaction (1) leads to the synthesis ofhighly reproducible thin solid films of valve metal oxidesThe anodization process may also occur from lower oxides

MOy2 +z minus y2

H2OrarrMOz2 + z minus y H+ + z minus y eminus 2

and an oxide film may also be formed by electrocrystalli-zation of ions from solution

My+ + zH2OrarrMOz2 + zH+ + z minus y eminus 3

Anodic potentials enhance passivation according toequations (1)-(3) but transpassivity and anodic breakdowncause the opposite effect [31] The opposite process impliesreduction reactions that destabilize the passive films leadingto cathodic breakdown

Figure 1 shows a simple scheme of the several ion-and electron-transfer reactions (ITRs and ETRs respec-tively) that may occur during anodization processes thesechemical process may stabilize or dissolve the passivefilm [30 31]

The most important processes are summarized asfollows [30 31]

(1) Electron-transfer reactions (iredox) eg oxygenevolution

(2) Growth of the thin oxide film (iox) by the transfer ofoxygen ions from the electrolyte into the oxide

(3) Transfer of metal ions from the oxide to solution andoxidation of water (iox)

(4) Reduction by the reverse reactions (ired)

(5) Corrosion with charge transfer by metal ionscompensated by that of oxygen ions (iox + ired = 0)

(6) Capacitive charging of the interface (iC) upon poten-tial changes

The total current flowing through the interface is thendescribed by

i = iredox + iox + ired + iC 4

Following Young [32] a classification based on theexperimental observations indicates that the chemical ele-ments with a complete valve effect are Al Bi Sb Ta Tiand Nb and those with an incomplete valve effect are AgCd Fe Mg Si Sn W Zn and Zr the behavior depending

ETR

ITR

iredox

iox

iox

ired

ired + iox = 0

ETR + ITR

Metal Electrolyte

H2O

H2OO2minus

O

R

H+

Mz+

MOz2

MOz2

MOz2Hx

2H+

Mz+

Mz+

MOz2

minus

minus

Figure 1 Scheme of the several ion-transfer reactions (ITRs) andelectron-transfer reactions (ETRs) during the anodizationprocesses










4 Kinetic Model


dndt

=dnrarrdt

minusdnlarrdt



p = ϑ exp minusWRT

6



Electrochemicalcell

Electrolyte

Workingelectrode

Referenceelectrode

Auxiliarelectrode








and





expαazFEf

RTminus exp

1 minus α azFEf

RT

9




expαazFEf

RT10



11


d = d0 +M

zFA ρox

t

0i t dt 12











Pote

ntia

l ene

rgy

Positionnx nx+a

WW120576 = 0

120576 gt 0

120572zFa120601(1-120572)zFa120601

WW










14




Electrochemicalcell

F =

Electrolyte

Workingelectrode

Referenceelectrode

Auxiliarelectrode

ΔUt

t

Ti

Ti2O

Ti(OH)xOy

Ti

Ti2O

[TiF6]2minus

Electrolyte

H+H2O

O2minus

O2minusFminus

Fminus

Ti4+

Ti4+

Electrolyte

(a) (b)

Etching

Oxidation


WRA

t

i

Compact TiO2 layer

(a) (b)

TiTi Ti

(c)



H2SO4

H2SO4 + HF

I II III

















400 nmHFW166 120583m

WD99 mm

mag 998682250 000 times

vac modeHigh vacuum

HV3000 kV

detETD

622017121949 PM M4

(a)

mag 998682100 000 times

WD100 mm

modeSE

HV3000 kV

1 120583mIVIC M4

(b)



















IpV iVT


Semiconductorhv

Recombination

H2OOH

E

endashendash

R

R

2 H2O

H2 + OH minus

endash

ndash+

Ox

Red

VBh+Ox

Red

CBendash






r = kKc1 + Kc

21



r =kKc

1 + Kc + ΣKici 22



























8 Conclusions




Acknowledgments


References



















































































































































Journal of

Chemistry





Journal of

Volume 2018






Volume 2018




Journal of






Enzyme Research


Journal of



MaterialsJournal of






Na

nom

ate

ria

ls












4 Kinetic Model


dndt

=dnrarrdt

minusdnlarrdt



p = ϑ exp minusWRT

6



Electrochemicalcell

Electrolyte

Workingelectrode

Referenceelectrode

Auxiliarelectrode








and





expαazFEf

RTminus exp

1 minus α azFEf

RT

9




expαazFEf

RT10



11


d = d0 +M

zFA ρox

t

0i t dt 12











Pote

ntia

l ene

rgy

Positionnx nx+a

WW120576 = 0

120576 gt 0

120572zFa120601(1-120572)zFa120601

WW










14




Electrochemicalcell

F =

Electrolyte

Workingelectrode

Referenceelectrode

Auxiliarelectrode

ΔUt

t

Ti

Ti2O

Ti(OH)xOy

Ti

Ti2O

[TiF6]2minus

Electrolyte

H+H2O

O2minus

O2minusFminus

Fminus

Ti4+

Ti4+

Electrolyte

(a) (b)

Etching

Oxidation


WRA

t

i

Compact TiO2 layer

(a) (b)

TiTi Ti

(c)



H2SO4

H2SO4 + HF

I II III

















400 nmHFW166 120583m

WD99 mm

mag 998682250 000 times

vac modeHigh vacuum

HV3000 kV

detETD

622017121949 PM M4

(a)

mag 998682100 000 times

WD100 mm

modeSE

HV3000 kV

1 120583mIVIC M4

(b)



















IpV iVT


Semiconductorhv

Recombination

H2OOH

E

endashendash

R

R

2 H2O

H2 + OH minus

endash

ndash+

Ox

Red

VBh+Ox

Red

CBendash






r = kKc1 + Kc

21



r =kKc

1 + Kc + ΣKici 22



























8 Conclusions




Acknowledgments


References



















































































































































Journal of

Chemistry





Journal of

Volume 2018






Volume 2018




Journal of






Enzyme Research


Journal of



MaterialsJournal of






Na

nom

ate

ria

ls







and





expαazFEf

RTminus exp

1 minus α azFEf

RT

9




expαazFEf

RT10



11


d = d0 +M

zFA ρox

t

0i t dt 12











Pote

ntia

l ene

rgy

Positionnx nx+a

WW120576 = 0

120576 gt 0

120572zFa120601(1-120572)zFa120601

WW










14




Electrochemicalcell

F =

Electrolyte

Workingelectrode

Referenceelectrode

Auxiliarelectrode

ΔUt

t

Ti

Ti2O

Ti(OH)xOy

Ti

Ti2O

[TiF6]2minus

Electrolyte

H+H2O

O2minus

O2minusFminus

Fminus

Ti4+

Ti4+

Electrolyte

(a) (b)

Etching

Oxidation


WRA

t

i

Compact TiO2 layer

(a) (b)

TiTi Ti

(c)



H2SO4

H2SO4 + HF

I II III

















400 nmHFW166 120583m

WD99 mm

mag 998682250 000 times

vac modeHigh vacuum

HV3000 kV

detETD

622017121949 PM M4

(a)

mag 998682100 000 times

WD100 mm

modeSE

HV3000 kV

1 120583mIVIC M4

(b)



















IpV iVT


Semiconductorhv

Recombination

H2OOH

E

endashendash

R

R

2 H2O

H2 + OH minus

endash

ndash+

Ox

Red

VBh+Ox

Red

CBendash






r = kKc1 + Kc

21



r =kKc

1 + Kc + ΣKici 22



























8 Conclusions




Acknowledgments


References



















































































































































Journal of

Chemistry





Journal of

Volume 2018






Volume 2018




Journal of






Enzyme Research


Journal of



MaterialsJournal of






Na

nom

ate

ria

ls











14




Electrochemicalcell

F =

Electrolyte

Workingelectrode

Referenceelectrode

Auxiliarelectrode

ΔUt

t

Ti

Ti2O

Ti(OH)xOy

Ti

Ti2O

[TiF6]2minus

Electrolyte

H+H2O

O2minus

O2minusFminus

Fminus

Ti4+

Ti4+

Electrolyte

(a) (b)

Etching

Oxidation


WRA

t

i

Compact TiO2 layer

(a) (b)

TiTi Ti

(c)



H2SO4

H2SO4 + HF

I II III

















400 nmHFW166 120583m

WD99 mm

mag 998682250 000 times

vac modeHigh vacuum

HV3000 kV

detETD

622017121949 PM M4

(a)

mag 998682100 000 times

WD100 mm

modeSE

HV3000 kV

1 120583mIVIC M4

(b)



















IpV iVT


Semiconductorhv

Recombination

H2OOH

E

endashendash

R

R

2 H2O

H2 + OH minus

endash

ndash+

Ox

Red

VBh+Ox

Red

CBendash






r = kKc1 + Kc

21



r =kKc

1 + Kc + ΣKici 22



























8 Conclusions




Acknowledgments


References



















































































































































Journal of

Chemistry





Journal of

Volume 2018






Volume 2018




Journal of






Enzyme Research


Journal of



MaterialsJournal of






Na

nom

ate

ria

ls


















400 nmHFW166 120583m

WD99 mm

mag 998682250 000 times

vac modeHigh vacuum

HV3000 kV

detETD

622017121949 PM M4

(a)

mag 998682100 000 times

WD100 mm

modeSE

HV3000 kV

1 120583mIVIC M4

(b)



















IpV iVT


Semiconductorhv

Recombination

H2OOH

E

endashendash

R

R

2 H2O

H2 + OH minus

endash

ndash+

Ox

Red

VBh+Ox

Red

CBendash






r = kKc1 + Kc

21



r =kKc

1 + Kc + ΣKici 22



























8 Conclusions




Acknowledgments


References



















































































































































Journal of

Chemistry





Journal of

Volume 2018






Volume 2018




Journal of






Enzyme Research


Journal of



MaterialsJournal of






Na

nom

ate

ria

ls




















IpV iVT


Semiconductorhv

Recombination

H2OOH

E

endashendash

R

R

2 H2O

H2 + OH minus

endash

ndash+

Ox

Red

VBh+Ox

Red

CBendash






r = kKc1 + Kc

21



r =kKc

1 + Kc + ΣKici 22



























8 Conclusions




Acknowledgments


References



















































































































































Journal of

Chemistry





Journal of

Volume 2018






Volume 2018




Journal of






Enzyme Research


Journal of



MaterialsJournal of






Na

nom

ate

ria

ls







r = kKc1 + Kc

21



r =kKc

1 + Kc + ΣKici 22



























8 Conclusions




Acknowledgments


References



















































































































































Journal of

Chemistry





Journal of

Volume 2018






Volume 2018




Journal of






Enzyme Research


Journal of



MaterialsJournal of






Na

nom

ate

ria

ls

























8 Conclusions




Acknowledgments


References



















































































































































Journal of

Chemistry





Journal of

Volume 2018






Volume 2018




Journal of






Enzyme Research


Journal of



MaterialsJournal of






Na

nom

ate

ria

ls


















































































































































Journal of

Chemistry





Journal of

Volume 2018






Volume 2018




Journal of






Enzyme Research


Journal of



MaterialsJournal of






Na

nom

ate

ria

ls




High-Field Growth of Semiconducting Anodic Oxide Films on … · 2019. 7. 30. · rate in terms of...

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Transcript of High-Field Growth of Semiconducting Anodic Oxide Films on … · 2019. 7. 30. · rate in terms of...