Nanoscale

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View Article OnlineView Journal | View Issue

aDepartament de Fısica, Facultat de Cienc

E-08193 Bellaterra, Spain. E-mail: Miguel.GbInstitute of Robotics and Intelligent Sys

Technology (ETH) Zurich, CH-8092 Zurich,cServei de Microscopia, Universitat AutonomdInstitucio Catalana de Recerca i Estudis A

Universitat Autonoma de Barcelona, E-0819

uab.cat

† Electronic supplementary informa10.1039/c3nr03491g

Cite this: Nanoscale, 2013, 5, 12542

Received 8th July 2013Accepted 8th October 2013

DOI: 10.1039/c3nr03491g

www.rsc.org/nanoscale

12542 | Nanoscale, 2013, 5, 12542–125

3D hierarchically porous Cu–BiOCl nanocomposite films:one-step electrochemical synthesis, structuralcharacterization and nanomechanical andphotoluminescent properties†

Miguel Guerrero,*a Salvador Pane,b Bradley J. Nelson,b Maria Dolors Baro,a

Monica Roldan,c Jordi Sort*d and Eva Pellicer*a

Three-dimensional (3D) hierarchically porous composite Cu–BiOCl films have been prepared by a facile one-

step galvanostatic electrodeposition process from acidic electrolytic solutions containing Cu(II) and Bi(III)

chloride salts and Triton X-100. The films show spherical, micron-sized pores that spread over the whole

film thickness. In turn, the pore walls are made of randomly packed BiOCl nanoplates that are

assembled leaving micro–nanopore voids beneath. It is believed that Cu grows within the interstitial

spaces between the hydrogen bubbles produced from the reduction of H+ ions. Then, the BiOCl sheets

accommodate in the porous network defined by the Cu building blocks. The presence of Cu tends to

enhance the mechanical stability of the composite material. The resulting porous Cu–BiOCl films exhibit

homogeneous and stable-in-time photoluminescent response arising from the BiOCl component that

spreads over the entire 3D porous structure, as demonstrated by confocal scanning laser microscopy. A

broad-band emission covering the entire visible range, in the wavelength interval 450–750 nm, is

obtained. The present work paves the way for the facile and controlled preparation of a new

generation of photoluminescent membranes.

1. Introduction

Three-dimensional (3D) porous lms have become increasinglypopular in the last few years as they offer a number of appli-cations in electrocatalysis,1 separation and concentration of gasmolecules,2 batteries,3 sensors4 and electronic devices,5 amongothers. Several strategies are currently being pursued for thepreparation of porous lms like de-alloying,6 galvanic replace-ment,7 or templating (e.g. using colloidal crystals as templates).8

Electrodeposition has recently emerged as a powerful andcheap technique to prepare 3D porous lms of metals andmetallic alloys through different strategies: deposition in rigidporous templates (e.g. polystyrene opal templates),9 usinghydrogen bubbles as a dynamic template10 or from electrolytesolutions containing suitable surfactants.11 The utilization of

ies, Universitat Autonoma de Barcelona,

uerrero@uab.cat; Eva.Pellicer@uab.cat

tems (IRIS), Swiss Federal Institute of

Switzerland

a de Barcelona, E-08193 Bellaterra, Spain

vançats (ICREA), Departament de Fısica,3 Bellaterra, Spain. E-mail: Jordi.Sort@

tion (ESI) available. See DOI:

50

hydrogen co-evolution as a source of porosity during electro-deposition is a very convenient and fast way to prepare networkporous structures of a number of metals (e.g. Cu,12 Ni,13 Pd,14

Pt,15 bimetallic Cu/Pd16) and alloys (e.g. Cu6Sn5).17 Among them,Cu is perhaps one of the most studied. Different electrolyteshave been tested for the preparation of porous Cu lms withdistinct pore morphologies, pore sizes and interconnectivities.Macroporous Cu lms constructed by nanodendritic walls havebeen obtained from acid sulphate solutions containing cetyl-trimethylammonium bromide (CTAB).12 Cu foams with 3Dinterconnected spherical pore networks have been prepared inacid sulphate solutions containing HCl, polyethylene glycol(PEG) and 3-mercapto-1-propane sulfonic acid (MPSA) as addi-tives.18 Remarkably, very few studies have reported the utiliza-tion of the hydrogen bubble template approach for thepreparation of porous composite lms. A smart combination ofmetals with other compounds in a single 3D porous networkstructure could signicantly broaden the range of applicationsof porous lms.

Bismuth oxychloride (BiOCl), a V–VI–VII group wide bandgap semiconductor, is considered one of the most importantbismuth oxyhalides. BiOCl has attracted an enormous amountof interest within the scientic community because of itsexcellent catalytic, optical, electric, magnetic and luminescentproperties. As a result, BiOCl has found numerous uses as a

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photocatalyst,19–21 or ferroelectric material,22,23 for many years.Moreover, it is widely used as a pigment in cosmetics, buttonsand jewellery because it confers a semi-transparent appearanceto the objects. Until now, several strategies have been reportedfor the preparation of BiOCl micro/nanostructures mostly in theform of powders: electrospinning,24 hydrothermal approach,25

reverse microemulsions26 or sonochemistry.27 However, formany industrial applications immobilized BiOCl lms are moreadequate than powders. For example, problems encountered innanopowder photocatalysts such as catalyst recovering (oncethe solution has been decontaminated) and particle aggrega-tion can be circumvented if the active material can be properlyimmobilized without compromising its specic surface area.Nevertheless, studies on the preparation of nanostructuredBiOCl lms are still scarce compared to the vast literature onBiOCl nanopowders. Cao and co-workers fabricated a BiOCllm with ower-like hierarchical structure by reacting a Bi lmwith H2O2/HCl solution.28 Very recently the preparation ofBiOCl lms by electrodeposition of Bi and subsequent anodicoxidation has been reported.29 In all these studies the BiOCllm is used for photocatalytic purposes. Other applicationswould also benet from having immobilized BiOCl lms likeelectrochemical sensing or optics.

Herein, we present a facile, efficient and environmentallyfriendly approach for fabricating three-dimensional (3D)porous composite lms made of Cu and BiOCl components inone-step cathodic electrodeposition. An optimized bathformulation containing suitable concentrations of bismuth(III)and copper(II) chloride salts and Triton X-100 as an additiveyields a crack-free porous composite structure. Also, themechanical properties of the composite lms have been studiedby nanoindentation in order to correlate the obtained data withboth the porosity degree and the presence of Cu in the lms.Finally, the hierarchically porous Cu–BiOCl lms exhibit astrong and stable photoluminescence response in the entirevisible range.

2. Experimental section2.1 Preparation of Cu-free and Cu–BiOCl composite lms

Cu-free and Cu–BiOCl composites were prepared by directcurrent electrodeposition (ED) in a thermostatized one-compartment three-electrode cell using a PGSTAT302N Autolabpotentiostat/galvanostat (Ecochemie). The working electrodewas positioned vertically within the electrolyte and consisted of5 mm� 6 mm Si chips (crystal orientation 100), on top of whicha Ti adhesion layer of 50 nm and a Au seed-layer of 500 nm hadbeen successively deposited through e-beam evaporation.The working area was 0.25 cm2. A double junction AgrAgCl (E ¼+0.210 V/SHE) reference electrode (Metrohm AG) was used with3 M potassium chloride (KCl) inner solution and an inter-changeable outer solution. The outer solution was made of 1 Msodium chloride. A platinum sheet served as the counterelectrode. For Cu–BiOCl composite electrodeposition, the elec-trolyte contained 1.2 M HCl, 8 � 10�3 M BiCl3, 8 � 10�3 MCuCl2, 2 � 10�2 M NaCl, and variable amounts of Triton X-100,saccharine and CTAB. The Cu-free BiOCl lms were prepared

This journal is ª The Royal Society of Chemistry 2013

from an electrolyte free of CuCl2 salt. Analytical grade reagentsand ultrapure water (18 MU cm) were used to prepare theelectrolyte. The cell was lled with 50 mL electrolyte. All thesamples were obtained galvanostatically by applying a constantcurrent density of �1 A cm�2. Deposition was conducted atroom temperature under vigorous stirring (5000 rpm) using amagnetic stirrer bar. A similar electrical charge owed in allexperiments, yielding lm thicknesses between 75 and 100 mm.Prior to deposition, the Au surface was degreased with acetoneand dried with a so tissue. The Si backside was insulated bypainting it with a nonconductive ink to ensure only the Ausurface was conductive. Aer deposition, the coated substrateswere rst dipped in 0.12 M HCl, aerwards dipped in Milli-Qwater, and nally dried in air.

2.2 Morphological and structural characterization

The morphology and structure of the deposits was studied byscanning electron microscopy (SEM), high resolution trans-mission electron microscopy (HRTEM) and X-ray diffraction(XRD). SEM imaging was performed on a Zeiss Merlin micro-scope. TEM characterisation was carried out on a JEOL JEM-2011 microscope operated at 200 kV. For TEM observations, asmall area of sample surface was scratched-off and the powderreleased was dispersed in ethanol. Aerwards, one or two dropsof the suspension were placed dropwise onto a holey carbon-coated Cu TEM grid. The chemical composition of the lms wasdetermined by energy dispersive X-ray spectroscopy (EDX). Thepore size distributions were determined by manual analysis ofenlarged micrographs by measuring ca. 100 pores on a givengrid to obtain a statistical size distribution and a mean porediameter.

XRD patterns were recorded on a Philips X'Pert diffractom-eter using Cu Ka radiation, in the 20–90� 2q range (0.03� stepsize, 8 s holding time). The global structural parameters, suchas phase percentages, crystallite sizes, hDi (dened here as theaverage coherently diffracting domain sizes), and microstrainsor atomic level deformations, h32i1/2, were evaluated by ttingthe full XRD patterns using the “Materials Analysis UsingDiffraction” (MAUD) Rietveld renement program.30–32 Fouriertransform infrared spectroscopy (FT-IR) was carried out on aBruker TENSOR27 instrument.

2.3 Mechanical characterization

The mechanical properties (hardness, reduced elastic modulusand elastic recovery) were evaluated by means of nano-indentation, using a UMIS device from Fischer-Cripps Labora-tories equipped with a Berkovich pyramidal-shaped diamondtip, operating in the load control mode. The maximum value ofapplied load was 0.6 mN to ensure that the lateral size of theindentation imprint was much smaller than the lm thickness.The thermal dri during nanoindentation was lower than0.05 nm s�1. Proper corrections for the contact area (calibratedwith a fused quartz specimen), instrument compliance, andinitial penetration depth (a pre-contact load of 0.2 mN wasused) were applied. The hardness (H) and reduced elasticmodulus (Er) values were derived from the load–displacement

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curves using the method of Oliver and Pharr.33 From the initialunloading slope, the contact stiffness, S, was determined as:

S ¼ dP

dh(1)

where P and h denote the applied force and the penetrationdepth during nanoindentation, respectively. The reducedYoung's modulus was evaluated based on its relationship withthe contact area, A, and contact stiffness:

S ¼ b2ffiffiffiffip

p Er

ffiffiffiffiA

p(2)

where b is King's factor that depends on the geometry of theindenter (b ¼ 1.034 for a Berkovich indenter).34 The reducedmodulus takes into account the elastic displacements thatoccur in both the specimen, with Young's modulus E andPoisson's ratio n, and the diamond indenter, with elasticconstants Ei and ni (for diamond, Ei ¼ 1140 GPa and ni ¼ 0.07):

1

Er

¼ 1� n2

Eþ 1� ni

2

Ei

(3)

The hardness was calculated from the following expression:

H ¼ PMax

A(4)

where PMax is the maximum load applied during nano-indentation. Finally, the elastic recovery was evaluated as theratio between the elastic and total (plastic + elastic) energiesduring nanoindentation, Wel/Wtot. The values of Wel werecalculated as the area between the unloading curve and thedisplacement axis. In turn, Wtot is the area between the loadingcurve and the displacement axis.34,35 To extract semi-quantita-tive information on the porosity level of the electrodepositedlms, the mechanical properties of non-porous bulk commer-cial BiOCl (Sigma-Aldrich, 98% purity) were also measured bynanoindentation. The results presented in this work are thestatistical average of a set of 75 indentations for each sample.

Fig. 1 On-top SEM images of the Cu–BiOCl composite obtained in the electro-lyte with 4 � 10�4 M Triton X-100 at different magnifications.

2.4 Photoluminescence measurements

The photoluminescence (PL) properties of the lms werestudied by Confocal Scanning Laser Microscopy (CSLM) anal-ysis. Samples were mounted on Ibidi culture dishes (IbidiGmbH, Martinsried, Germany) and imaged with a TCS-SP5CSLMmicroscope (Leica Microsystems CMS GmbHMannheim,Germany) using a Plan Apochromat 10�/0.4 and 20�/0.7 (dry)objectives. Both Cu-free and Cu–BiOCl composites were excitedwith an argon laser (488 nm) and the autouorescence wasdetected in the 500–785 nm range. The three-dimensional(3D) structure of lms was imaged using the same confocalmicroscope in reection mode with a 488 nm argon laser anddetected in the 480–495 nm range. To determine the 3Dmicrostructure, stacks of 50 to 80 sections were collected every1 mm along the material's thickness. Three-dimensional modelswere generated from the xyz series using the Imaris X64 v. 6.2.0soware (Bitplane; Zurich, Switzerland).

A series of images (xyl) were taken to determine the spectralemissions of the samples and to establish their maxima.

12544 | Nanoscale, 2013, 5, 12542–12550

Intrinsic uorescence was excited with a 405 nm line of a bluediode laser. Fluorescence emission was collected with a hybriddetector in 20 nm bandwidth increments (lambda step size ¼7 nm) in the range from 450 nm to 750 nm. A set of 30 regions ofinterest (ROIs) of 100 mm2 was used to analyze the mean uo-rescence intensity (MFI) of the samples in relation to theemission wavelength.

Finally, photostability experiments were performed tomonitor long time-lapse experiments. A 50 mW blue diode 405nm laser was used at a fairly high output (Acousto-OpticTunable Filter, AOTF¼ 90%) in a eld of 1.5 mm� 1.5 mm andthe uorescence signal was detected in the range from 450 nmto 750 nm. Images were taken every 1 s for 10 min. A set of 20ROIs of 100 mm2 were selected to show the MFI in the region inrelation to time. Data from all studies were analyzed using theLAS AF soware ver. 2.4.1. (Leica Microsystems).

3. Results and discussion3.1 Synthesis, morphological and structuralcharacterization of Cu-free and Cu–BiOCl composite lms

Representative on-top SEM images of the Cu–BiOCl compositelms grown in the electrolyte containing 4 � 10�4 M of TritonX-100 are shown in Fig. 1. At low magnication, a uniformdistribution of spherical pores of 5–10 mm in diameter (macro-porosity) is observed over the entire surface of the lm (Fig. 1a andb). Moreover, the pore walls are not dense but inherently porous,with voids having dimensions from tenths of nanometers to a fewmicrons (Fig. 1c). A close-up observation of the pore walls (Fig. 1d)reveals that they are in fact built from numerous randomly packednanoplates, and that the nanoplates are stacked together eitherthrough plane-to-plane or edge-to-edge conjunctions, giving aower-like appearance. Hence, the lms are hierarchically porous.The porosity spreads over the whole thickness as demonstrated bythe cross-section image of the composite layer (Fig. 2). It is worthmentioning that the Triton X-100 additive suppresses the forma-tion of cracks. With other additives like saccharine and CTABcracked porous lms were obtained (ESI, Fig. S1†). As a result, thesamples were so powdery and brittle that good contact between

This journal is ª The Royal Society of Chemistry 2013

Fig. 2 Cross-section SEM image of the Cu–BiOCl composite obtained in theelectrolyte with 4 � 10�4 M Triton X-100.

Fig. 3 (a–c) On-top SEM images of the Cu-free BiOCl film.

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the samples and the indenter tip could not be attained and thenanoindentation data became noisy and unreliable. For thisreason, samples prepared using saccharine or CTAB as additiveswere not considered for in-depth mechanical and photo-luminescence studies. Saccharine is typically used as a stress-reliever and grain-rener in electrodeposited metals and alloys,35

but does not work for all systems and it has little effect onporosity. CTAB is a cationic surfactant with a positively chargedhead group that interacts electrostatically with the electrodesurface. It gets adsorbed on the electroactive sites and, as aconsequence, changes crystal growth mode and, in turn, grainmorphology and size.36,37 Triton X-100 is recognized as an excel-lent wetting agent, which means that it favours H2 bubbledetachment from the substrate,38 which likely promotes theevolution of porosity in the coating. At the same time, it wouldinhibit the formation of cracks in the lm as it builds. It isbelieved that such a synergetic effect is not achieved withsaccharine and CTAB additives.

Signicantly, Cu-free BiOCl lms exhibit different porearchitecture (Fig. 3a). The lm surface is still very rough but thespherical macropores are no longer evident (Fig. 3b). A rose-likemorphology is observed at high magnication (Fig. 3c). Thissuggests that the reduction of H+ (R1) that accompanies Cudischarge (R2) is responsible for the formation of the sphericalmacropores. The BiOCl nanoplates are then accommodatedwithin the macropore walls dened by the Cu building blocks.The electrochemical reactions involved in the formation of themacropore skeleton are thus:

H+ + 1e� / ½H2 (R1)

Cu2+ + 2e� / Cu (R2)

On the other hand, the formation of BiOCl nanoplates wouldmainly proceed via the reaction of Bi(III) with an electro-generated base (OH�) in the presence of Cl� (R3):

Bi3+ + Cl� + 2OH� / BiOCl + H2O (R3)

The formation of the BiOCl component does not take placeby simply dipping the substrate in the electrolyte if no current isowing through the system (i.e., by hydrolysis). Hence, theformation of the BiOCl nanoplates necessarily involves a reac-tion with any species generated during the electrolysis (could be

This journal is ª The Royal Society of Chemistry 2013

H2O2 that reacts with freshly reduced Bi, as well). Moreover, theelectroreduction of Bi(III) toward metallic Bi would also takeplace:

Bi3+ + 3e� / Bi (R4)

It has been reported that metallic bismuth electrodepositedat hydrogen evolution potentials exhibits a non-hierarchicalporous dendritic structure.39 The potential–time (E–t) transientsrecorded during Cu-free and Cu–BiOCl lms are shown in theESI, Fig. S2.† The E–t curves show an important decrease ofthe deposition potential toward more positive values during therst few seconds (nucleation events) until stabilization around�3 V vs. Ag/AgCl is achieved. The stabilized potential is similarfor both Cu-free and Cu–BiOCl lms and there is not anysignicant decrease/increase of the average potential with time,which suggests that the surface area of the deposition frontremains almost constant. The uctuations are due to theintense hydrogen evolution.

Both the macropore size and size distribution of the Cu–BiOClhierarchically layered composites can be greatly tuned by varyingthe Triton X-100 concentration in the electrolyte (ESI, Fig. S3†).Namely, both values decrease as the concentration of Triton X-100is increased from 1 � 10�4 M (mean macropore size 24.7 mm) to4 � 10�4 M (mean macropore size 9.1 mm). Energy-dispersiveX-ray spectroscopy (EDX) analysis shows the presence of Bi, O andCl (coming from the BiOCl layer) as well as Cu homogeneouslydistributed within the nanocomposite (Fig. 4).

Nanoscale, 2013, 5, 12542–12550 | 12545

Fig. 4 EDX mapping distribution of Cu, Bi, O and Cl elements in the Cu–BiOCl composite film.

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The crystallographic structure of the as-prepared Cu-free andCu–BiOCl coatings was studied using XRD (Fig. 5). Apart fromthe reections coming out from the substrates (Si and Au), someof the diffraction peaks can be assigned to the tetragonal BiOClphase (S.G., P4/nmm; JCPDS no. 06-0249, a¼ b¼ 0.3891 nm andc¼ 0.7369 nm). In addition, a narrower reection at 2q¼ 43.30�

is identied in the XRD pattern of the Cu–BiOCl composite lm,which matches the position of the (111) plane of the face-centered cubic (fcc) Cu metal. Additional reections assigned tometallic Bi are also detected in both patterns. The detection ofmetallic Bi would be in agreement with (R4). The patterns wereRietveld tted and the obtained volume phase percentages,crystallite sizes and microstrains are listed in Table 1.Remarkably, all phases exhibit sub-100 nm hDi values,

Fig. 5 XRD pattern of the (a) Cu-free BiOCl film and (b) Cu–BiOCl composite film.

Table 1 Phase volume percentages, crystallite sizes (hDi) and microstrains (h31/2i2)BiOCl composite films

Film

% by vol hDi/nm (�5)

BiOCl Bi Cu BiOCl Bi

BiOCl 59 41 — 83 28Cu–BiOCl 43 22 35 65 20

12546 | Nanoscale, 2013, 5, 12542–12550

evidencing the nanocrystalline character of the compositeelectrodeposited lms. In turn, the values of microstrains(which are related to the density of dislocations and otherstructural defects) are relatively low, much lower than thosetypically obtained in heavily deformed metals,40 but still higherthan the microstrain values in some electrodeposited nano-crystalline metallic alloys.35

To gain further information on Cu–BiOCl hierarchicallyporous lms, the product was further investigated by HRTEM.Fig. 6a shows a blend of nanosheets corresponding to BiOCl.The size of the sheets is well in the sub-100 nm range, inagreement with XRD results. Fig. 6b is a lattice-resolved HRTEMimage taken on a single nanosheet. The corresponding latticefringes have an interplanar distance of 0.27 nm, which matchesthe d-spacing of the (110) reection of tetragonal BiOCl (d110 ¼0.2753 nm). This is in accordance with the selected-area elec-tron diffraction (SAED), indexed to a pure tetragonal phase(inset of Fig. 6a). This result indicates that the preponderantgrowth direction of the BiOCl layer is the [001] orientation,which is parallel to the (110) orientation. Metallic Bi on theBiOCl nanosheets is also detected, in agreement with XRD data(Fig. 6c). The inset shows the FFT of the area enclosed in redthat belongs to the rhombohedral Bi phase (JCPDS no.44-1246).39 The IR spectra of the sample show the typical bandsof BiOCl in three different regions (2854 cm�1, 1463 cm�1 and612 cm�1) (ESI, Fig. S4†), again conrming the presence of theBiOCl component.

3.2 Mechanical properties

The mechanical properties of the electrodeposited layers wereassessed by nanoindentation and compared to those ofcommercial bulk BiOCl. Representative load–displacement

determined from the Rietveld fitting of the full XRD patterns for Cu-free and Cu–

h31/2i2

Cu BiOCl Bi Cu

— 8.2 � 10�4 2.0 � 10�4 —105 2.0 � 10�3 5.0 � 10�4 8.0 � 10�4

This journal is ª The Royal Society of Chemistry 2013

Fig. 6 (a) TEM and (b and c) HRTEM images of the Cu–BiOCl composite. Theinset in (a) shows the FFT of the region shown in (b), for which the spot of the(110) plane of tetragonal BiOCl has been indicated. The inset in (c) shows the FFTof the area enclosed in the red square, for which the (012) plane of rhombohedralmetallic Bi has been indicated.

Fig. 7 Representative load (P)–displacement (h) nanoindentation curves of (a)BiOCl and (b) Cu–BiOCl composite films. A schematic picture of the indentationprocess is shown as an inset.

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nanoindentation curves of the Cu-free and Cu–BiOCl depositsare shown in Fig. 7. The maximum indentation displacementattained in the porous BiOCl (curve a) is clearly larger than forthe Cu–BiOCl composite (curve b), indicating that the latter ismechanically harder. Remarkably, not only the hardness, butalso the reduced Young's modulus, the ratio H/Er (which isrelated to the wear resistance), and the ratio We/Wtot (indicativeof the elastic recovery) are larger for the Cu-containing lm (seeTable 2), thus evidencing the key role of Cu in enhancing themechanical integrity of these electrodeposited materials.

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The porosity level is known to have a strong inuence on theelastic constants of metallic and ceramic materials. In a rstapproximation, it has been shown that:41–43

Eporous

Ebulk

¼�rporous

rbulk

�n

(5)

where rporous/rbulk is the relative density. The exponent n isequal to 2 for open-celled foams or sponge-like structures,43,44

whereas n ¼ 3 for materials exhibiting an array of poresarranged forming a honeycomb hexagonal array normal to thesurface.45 Given the porous nature of the electrodepositedlayers, n � 2 can be assumed in our case. Moreover, since theYoung's modulus of diamond is very high, the true Young'smoduli of Cu–BiOCl and BiOCl are very similar to the measuredEr values (3). The relative density is in turn related to theporosity volume fraction:

rporous

rbulk¼ 1� P (6)

In a rst approximation, assuming Er z E, eqn (5) can beused to roughly estimate the porosity level of the electro-deposited BiOCl, taking the measured mechanical properties ofcommercial BiOCl as those of the bulk BiOCl. Considering abulk density for BiOCl of 7.78 g cm�3,46 using n ¼ 2, and Ebulk ¼0.41 � EBi + 0.59 � EBiOCl (in agreement with the rule ofmixtures for the E value of a composite material),47 the densityof the porous BiOCl lm is calculated to be 0.84 g cm�3 (i.e.,porosity level P ¼ 89%). This high porosity level explains thedrastic decrease in the Young's modulus of the electroplatedBiOCl lm with respect to the commercial material. If eqn (5) isused for Cu–BiOCl (and the bulk Young's modulus of Cu–BiOClis assumed to be 0.35 � ECu + 0.22 � EBi + 0.43 � EBiOCl) whereEBiOCl is that of the commercial, then the porosity level for theCu–BiOCl lm electrodeposited with the Triton X-100 concen-tration of 4 � 10�4 M would be P ¼ 0.87, which is similar to thevalue obtained in other porous electrodeposited systems, likelocalized electrodeposited Cu–Ni porous microcolumns.44 The

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Table 2 Summary of the mechanical properties, evaluated by nanoindentation, of the electrodeposited Cu–BiOCl and BiOCl films, as well as commercial non-porousBiOCl

Sample H (GPa) Er (GPa) H/Er We/Wtot

Cu–BiOCl 0.072 � 0.004 1.18 � 0.09 0.061 � 0.005 0.598 � 0.068BiOCl 0.020 � 0.001 0.42 � 0.06 0.047 � 0.007 0.179 � 0.018Commercial BiOCl (non-porous) 2.90 � 0.05 39.0 � 0.5 0.074 � 0.002 0.797 � 0.050

Fig. 8 3D CSLM images of the Cu–BiOCl composite film in (a) reflection modeand (b) photoluminescence response.

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samemethod can be used to determine the porosity level of Cu–BiOCl lms electrodeposited using different Triton X-100concentrations from the values of reduced Young's modulus(see Table S1†). The porosity level tends to increase as the TritonX-100 concentration decreases (i.e., P ¼ 88% for a Triton X-100concentration of 2 � 10�4 M and P ¼ 92% for a Triton X-100concentration of 1 � 10�4 M). It should be pointed out that thismethod of indirect assessment of the porosity degree frommechanical measurements has been corroborated in othermaterials by means of complementary techniques, such asBrunauer–Emmett–Teller (BET) analysis.48

Concerning the hardness, the value obtained in the porousBiOCl (H¼ 20 MPa) is also signicantly lower than the hardnessof commercial BiOCl (H ¼ 2.9 GPa, which is similar to thereported value for BiOCl from the literature).46 The decrease ofhardness (or compressive yield stress) with porosity is also awell-documented effect42,49,50 and has been modeled using niteelement simulations of nanoindentation curves.50 An equationanalogous to eqn (5) may be used to correlate the yield stress ofthe porous structure with that of the bulk solid material:

sporous ¼ C2sbulk

�rporous

rbulk

�m

(7)

where C2 ¼ 0.3 and m ¼ 1.5 for an open-cell foam.42 Althoughthe relationship between hardness and yield stress in bulk oxideceramic is oen taken as Hbulk z 1.6sbulk,51 a quantitativerelationship between hardness and yield stress in porousmaterials is yet to be established. Some authors consider thatthe indenter in porous structures is not constrained by thesurrounding material regardless of eventual densication.Then, the nanoindentation would be equivalent to a uniaxialcompression test and Hporous ¼ sporous.42 In our case and usingthe aforementioned assumptions, eqn (7) would give m z 1.27for porous BiOCl, which is a quite reasonable value.

Finally, the elastic recovery for porous Cu–BiOCl, Wel/Wtot ¼0.598, is not far from that of commercial BiOCl. This parameterindicates how much energy is released from the material aerbeing loaded and it could be of particular interest in applica-tions subject to impact loading.52

Fig. 9 Spectral profiles representing the mean fluorescence intensity versusemission wavelength for Cu–BiOCl films in the 450 nm to 750 nm range. Data arethe mean from n ¼ 10 regions of interest from three fields examined.

3.3 Optical emission properties/photoluminescence

Three-dimensional photoluminescence (PL) and structure(reection mode) images of the porous Cu–BiOCl compositelm are shown in Fig. 8 (see also ESI, video le†). Remarkably,the PL response consists of broad-band emission which coversthe entire visible range (in the wavelength interval 450–750 nm,as shown in Fig. 9) and has a maximum at around 503 nm (i.e.,

12548 | Nanoscale, 2013, 5, 12542–12550

green region). The observed distribution of PL is homogeneousin depth. Unfortunately, the PL intensity of the Cu-free BiOCllms was much lower, probably due to the presence of largeamounts of Bi, which acts absorbing part of the incident light,

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thus attenuating the PL response. The broad green emissionobserved in Cu–BiOCl is in agreement with previous studies onphotoluminescence of ower-like BiOCl structures.53 Suchgreen emission is shied to lower energies (around 2.5 eV), ascompared to the energy bandgap of bulk BiOCl (3.46 eV).54

Therefore, the observed PL signal is mainly attributed to deeptrapped uorescence emission arising from surface states, i.e.light emitted when a photogenerated hole, trapped in a midgapstate, encounters an electron before it can relax non-radiativelyto the ground state.55 As in many other photoluminescentmaterials there is a large Stokes shi between the absorbanceand emission peaks, i.e., the emitted light is of lower energythan the excitation light. Such a difference stems from the non-radiative relaxation being lost as thermal decay.

Also worthmentioning is that contrary to organic uorescentdots, where a pronounced decrease in the PL intensity isobserved during illumination with UV or visible light(a phenomenon referred to as photobleaching56), here only adecrease of 20% of PL intensity was observed aer continuousirradiation for 10 min. Due to this stability in the PL signal andthe relatively large width of the PL peak, the investigatedmaterial could be interesting for a variety of opto-electronicdevices.

4. Conclusions

3D hierarchically porous crack-free Cu–BiOCl composite lmshave been fabricated by one-step electrodeposition in acidicelectrolytes containing Triton X-100. The lms exhibit sphericalmacropores surrounded by nanoporous walls in which theBiOCl nanoplates can be clearly identied. It has beendemonstrated that the electrodeposited Cu particles act asbuilding blocks of the porous structure since less porous lmsare obtained from Cu(II)-free solutions. The nanomechanicalproperties of the BiOCl and Cu–BiOCl lms correlate with theporous character of the lms. 3D CSLM images demonstrate ahomogeneous distribution and stable photoluminescencewithin the 3D structure arising from the BiOCl compound. It isenvisaged that this synthetic approach could be easily scaled todifferent combinations of metals and oxyhalide compounds torender 3D hierarchically porous structures for a variety ofapplications ranging from photonics, photocatalysis, sensing orelectrocatalysis.

Acknowledgements

Financial support from the MAT2011-27380-C02-01 researchproject from the Spanish MINECO and the 2009SGR-1292 fromthe Generalitat de Catalunya is acknowledged. The Serveid0Analisi Quımica and the Servei de Difraccio de la UAB areacknowledged. M.D.B. acknowledges funding support from anICREA-Academia award.

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