Photoelectrochemical and photocatalytic systems based on titanates for hydrogen peroxide formation

Tomasz Baran,a Szymon Wojtyła,b Alberto Vertova,a Alessandro Minguzzi,a Sandra Rondininia,*

a Department of Chemistry, University of Milan, via Golgi 19, 20133 Milan, Italy b SajTom Light Future LTD, Wezerow, Poland

AbstractThe development of efficient, sustainable, cheap and safe methods for the synthesis of hydrogen peroxide is a very important challenge from the environmental viewpoint, since H2O2 can be used as a solar fuel or for degradation of organic, inorganic and biologic pollutants from wastewater. Efficient photocatalytic or photoelectrochemical formation of hydrogen peroxide from water and/or oxygen in an acidic medium has been obtained using nickel or cobalt titanates under visible light illumination. The concentration of H2O2 achieved in the photoreaction reached as high as 2.5 mM when nickel titanate was irradiated for 1 h in oxygen saturated suspension. In such conditions, NiTiO3 provide active sides for both, oxygen reduction and water oxidation leading to H2O2 formation.

IntroductionConversion of solar energy into easily storable chemicals has recently attracted considerable attention as strategic technology to provide a clean energy. Photoelectrochemical hydrogen production from water or photocatalytic carbon dioxide reduction to methane are considered as some of the most promising approaches for solar energy utilization [1,2]. The use of hydrogen peroxide (H2O2) as an alternative solar fuel is also very promising since the reduction of oxygen to H2O2 is thermodynamically more favorable than reduction of H+ towards formation of hydrogen [3]. Moreover, H2O2 presents several great advantages in comparison with H2 as a fuel [4]. It is liquid at room temperature and atmospheric pressure thus it has higher energy density and can be stored, transported and used without a high-pressure tank. The theoretical open circuit potential of the H2O2 fuel cell is 1.09 V [5], by consuming hydrogen peroxide as both an oxidant at an anode and a reductant at a cathode [6] according to:

H 2O2→O2+2 H+¿+2 e−¿¿¿ (0.68 V vs NHE) (1)

H 2O2+2 H+¿+ 2e−¿ →H 2O ¿ ¿ (1.77 V vs NHE) (2)The open circuit potential is thus just slightly lower than that of a hydrogen/oxygen fuel cell (1.23 V [6] ).

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Moreover, H2O2 is a clean oxidant, with water as the only byproduct. In industry, it is commonly used for pulp bleaching as well as for disinfection due to its antimicrobial activity [7]. Reactive oxygen species such as hydrogen peroxide, hydroxyl radical, singlet oxygen and superoxide anion radical are the main mediators in photocatalytic wastewater treatment and air purification [8,9]. The use of H2O2, mainly produced by electrochemistry and photochemistry, is a suitable approach to the degradation of many types of pollutants, including organic contaminants of low volatility or poor adsorption properties, which are difficult to remove using conventional methods [10,11].

Several potential routes for formation of H2O2 as an effect of irradiation of semiconducting materials are considered in the literature [12–14]. First of all, as illustrated in reactions 3−6, it can be formed via reduction of oxygen by photogenerated electrons [12,15,16].

O2+eCB−¿→ O2

∙−¿¿ ¿ (0.33 V vs NHE) (3)

O2∙−¿+H +¿ →HO 2

∙ ¿¿ (4)

HO2∙ +HO2

∙ → H 2O2+O2 (5)

2O2∙−¿+2 H 2 O→ H 2 O2+O2+2O H−¿¿ ¿ (6)

The second possible pathway is based on water oxidation to hydroxyl radical by a photogenerated hole followed by radical recombination (eq. 7-8).

H2O+hVB+¿→ OH∙+H +¿ ¿¿ (2.7 V vs NHE) (7)

OH ∙+OH ∙ →H 2 O2 (8)Obviously, formation of H2O2 via two-electrons oxygen reduction (eq. 1) must be also considered [17].

Several photocatalytic and photoelectrocatalytic systems for hydrogen peroxide formation have been recently reported, for example based on titanium dioxide [12,18–23], nitrogen or sulphur doped TiO2 [24], TiO2 loaded with Au−Ag bimetallic alloy [13], Bi2WO6

[25], Au nanoparticles supported on BiVO4 [26], ZnO [7,14,22,27], graphitic carbon nitride [17,28,29]. Most of the systems use water as solvent and electron donor, however non aqueous environments (acetonitrile, dimethylformamide, benzotrifluoride) or water with sacrificial electron donors (e.g. ethanol, propanol, hexanol, benzyl alcohol) have been also reported [13,21].

The present work presents the comparison of photoelectrochemical and photocatalytic approaches based on titanates (NiTiO3 and CoTiO3) as photoactive materials towards hydrogen peroxide formation.

The choice of titanates comes from their reported energy configuration, that make them suitable to reduce oxygen to H2O2 but not to water or to reduce water to H2 [30]. In addition, they are made of low cost, abundant elements.

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ExperimentalIn all procedures, MilliQ grade water was used.Synthesis of materials

Nickel titanate (NiTiO3) nanoparticles were prepared as a follows. A TiCl4 solution was prepared by mixing TiCl4 (1.1ml, equivalent 0.01 mol) and 1.1 ml of 37% HCl. Then, this solution, nickel nitrate (0.01 mol) and urea (0.5 mol) were dissolved in 250 mL deionized water under vigorous stirring. The resulting suspension was stirred for 10 h at reflux temperature (110°C) and then filtered, washed twice with water and once with anhydrous ethanol and dried at 60°C in air. The filtrate was light green. The solid was heated in air to 800°C at a temperature-programmed rate of 5°C/min and sintered at 800°C for 5 h.

Cobalt titanate (CoTiO3) was obtained according to the following method: 0.3 ml of titanium(IV) isopropoxide and 0.25 g of nickel acetate Ni(CH3CO2)2·2H2O were added under stirring at room temperature to a solution of 0.57 g of citric acid in 9.2 ml of deionized water. The solution was stirred for 1h, then 6.84 ml of ethylene glycol were added and the resulting solution was stirred and heated in air at 90°C for 5h and at 210°C for 5h. The obtained precipitate was filtered, washed twice with water and once with ethanol, dried at 60°C in air for 5 h and subsequently calcined at 1000°C in air for 2 h.

Characterisation of materialsDiffuse reflectance spectra (DRS) in UV-Vis range were recorded using a UV-2600

spectrophotometer (Shimadzu) equipped with an integrating sphere (15 cm dia.). Previously dried BaSO4 (puriss, Sigma-Aldrich) was used to dilute the sample in 1:50 wt. ratio (samples were ground with BaSO4). BaSO4 was used also as a reference material.

Materials’ structures were identified using X-ray diffraction (XRD) at X'Pert PRO (X-ray (Cu) 40 kV).

Electrochemical impedance spectroscopy (EIS) measurements were performed in 0.1 M K2HPO4 + KH2PO4, pH = 7 previously saturated with nitrogen. Saturated calomel electrode (SCE) and a platinum plate were used as reference (RE) and counter electrode (CE), respectively. Working electrodes (WE) have been prepared by dropcasting of a semiconductor suspension in ethanol (1 g dm-3, sonicated for 5 min) onto FTO (Fluorine doped Tin Oxide 7 Ω/sq, 1 cm x 1.3 cm). Each electrode was dried at 80°C for 1h. Impedance spectra were recorded at constant potential in the frequency range 100 kHz to 0.1 Hz using a 1260A Impedance/Gain-Phase Analyzer (Solartron). Amplitude was set to 5 mV. The potential was stepped between 0.2 V and 0.8 V vs SCE (0.84 V and -0.16 vs RHE) 100 mV steps, with a waiting time of 100 s before each spectrum acquisition to guarantee the steady state. The equivalent circuit used to fit the impedance spectra contains the solution resistance, the polarization resistance and the space charge capacitance (vide infra).

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Photocurrent analysis has been performed using a 3-electrodes setup. The WE was prepared as for EIS measurements. SCE and platinum plate were used as a reference and counter electrodes, respectively. 0.1 M potassium hydrogen phthalate (pH 3.9), 0.1 M K2HPO4 + KH2PO4

(pH 7) or 0.1 M K2HPO4 (pH 9) were used as supporting electrolytes. The FTO-supported powder was irradiated from backside to eliminate the influence of film thickness, using LED (= 350 nm, 400 nm or 565 nm purchased by Huey Jann Electronics Industry). The light intensity of LED measured at the sample level using an Ophir Nova II Radiometer was: LED 350 nm – 450 W m-2, LED 400 nm – 780 W m-2, LED 565 nm – 270 W m-2. Electrochemical measurements were controlled by Autolab potentiostat (pgstat204).

Photocatalytic and photoelectrochemical testPhotocatalytic tests towards H2O2 formation were performed in a quartz cuvette. The

photocatalyst (1 g dm–3) was suspended in aqueous potassium hydrogen phthalate solution (50 mL, 0.1 M, pH = 3.9). The suspension was purged with O2 or N2 for 15 minutes prior to experiment and then during all time of irradiation. The suspension was irradiated using a LED (= 400 nm or 565 nm).

Photoelectrochemical tests towards H2O2 formation were performed in a cuvette made of optical glass in potassium hydrogen phthalate solution (15 mL, 0.1 M, pH = 3.9). The working electrode was prepared as described above, while SCE and platinum wire were used as reference and counter electrodes, respectively. Constant potential (see detailed information in Results) has been applied to the WE using an Autolab potentiostat (pgstat204).

H2O2 detectionTwo methods were used to determinate H2O2:

1.Electrochemical, based on the use of a Au disc (1.8 mm diameter) as the working electrode ( in situ measurements). The Au disc was polished with alumina powder 0.5 m for 3 min and then sonicated in distilled water for 10 s. Cyclic voltammetry (CV, in range from 0.47 to 1.77 V vs RHE, scan rate 10 mV s-1) analyses were performed in a conventional three-electrode cell, namely: WE: Au disc, CE: Pt plate and RE: SCE, driven by an Autolab potentiostat (pgstat204). H2O2 concentrations were determined through a calibration straight line, described by the following equation:

c [ M ]=I [ A ]+6.024 ∙ 10−7

0.0274 (9)

where I is the current of the anodic peak at about 1.1 V vs SCE. The calibration curve (see fig. S1) was obtained by measurements on solutions containing known amounts of hydrogen peroxide (concentration range 0.001 – 10 mM). 3 measurements were performed for each concentration. 2. Indirect UV-Vis spectroscopic method, based on literature reports with minors modifications [31]. A vanadium pentoxide solution was prepared by dissolving 0.5 g of V2O5 (ICIS, 98%) in sulfuric acid (5.61 mL, 17.8 M, Sigma Aldrich) and adjusting the final volume of solution to 200 mL with water (miliQ). The solution was then mixed with samples of solutions from

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photoelectrochemical (photocatalytic) experiments in vol. ratio 1:1. Spectra were measured using a UV-3600 Plus UV-VIS-NIR spectrophotometer (Shimadzu). A calibration curve (see fig. S1) was obtained adding samples with different contents of standard hydrogen peroxide solution (30%) to the V2O5 solution. The concentration of H2O2 in the sample can be calculated according to the Beer–Lambert law, where the determined molar absorption coefficient is equal to 168.43 M−1 cm−1 ± 12.61. The method can be considered reproducible: according to the literature, the relative standard deviations (RSD) in five measurements were less than 0.3% [31].

ResultsCharacterization of materials

Structures of NiTiO3 and CoTiO3 were investigated using X-ray diffraction (XRD). The observed XRD patterns of NiTiO3 particles prior and after calcination are reported in figure 1A. The calcined sample shows characteristic XRD peaks, which can be attributed to the reflections of rhombohedral NiTiO3 (space group R- 3 (148)). The pattern of CoTiO3 is presented in figure 1B. All diffraction lines are well defined and compared to JCPDS card No. 77-1373, which indicates the presence of CoTiO3 (main peaks – 2 theta: 24.04, 32.9, 35.6, 40.7, 49.2, 53.9, 57.0, 62, 63.7, 71.1 degree) in the powder as the only crystalline phase. The obtained cobalt titanate has an imielite structure (space group R- 3 - rhombohedral with hexagonal axes) [32]. Particle sizes were calculated by Scherrer’s equation: CoTiO3 - 32 nm ± 7 nm and NiTiO3 - 22 nm ± 3 nm. These values are in good agreement with SEM images (see fig. S2), that clearly indicate that both materials are composed of small particles (from several to tens nanometers) aggregated in bigger irregular blocks.

Figure 1. XRD analysis of NiTiO3 and its precursor before calcination (A), as well as CoTiO3

(B).

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DRS spectra were measured to evaluate light absorption range and band gap energy, as shown in Figure 3. The measured reflectance (R) was transformed to the Kubelka-Munk function (KM), by considering the absorption coefficient (α) and the scattering coefficient (S), according to the following equation:

KM=( 1−R∞ )2

2R∞=

αS

, (10)

where R∞=R sample

R reference. Light absorption onset of CoTiO3 (figure 2A) about 500 nm because of the

O2→Ti4+ charge transfer (CT) transition [33]. Moreover, the Co2+→Ti4+ CT can be observed at about 550 nm, and the strong absorption band with maximum at 650 nm can be assigned to the Co2+ spin-allowed crystal-field transition [33,34]. In turn, as demonstrated in fig. 2B, light absorption edge of NiTiO3 (related with a O2→Ti4+ CT transition) is shifted to higher energies in comparison to CoTiO3. Well visible absorption bands around 450 and 510 nm due to the Ni2+→Ti4+ CT [33]. The differences between the energy of CT transitions (lower energy for Co2+

than for Ni2+) can be explained based on differences between the 3rd ionization energy of these ions (Ni: 3395 kJ mol-1, Co: 3232 kJ mol-1) [35].

The bandgap energy (EBG) was estimated using Tauc’s plot, since KM(hν−EBG )n /2

hν,

where n = 1 or 4, for direct and indirect semiconductors, respectively. Both materials are direct semiconductors and the calculated band gap energies (see figures 2C-D) are equal 3.0 eV and 2.4 eV for NiTiO3 and CoTiO3, respectively, with good agreement with data reported in the literature [33].

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Figure 2. Diffuse reflectance spectra of the prepared materials transformed to the Kubelka-Munk function (A - CoTiO3, B - NiTiO3) and the method of band gap energy estimation from Tauc plots (C - CoTiO3, D - NiTiO3). Arrows (A and B) indicate the wavelength of incident light used for the photoelectrochemical characterization.

Mott–Schottky analysis (from EIS) was used for the estimation of the conduction band edge potential, VCB. To do this, the space charge capacitance (Csc) of the photoelectrode/solution interface was measured in a selected range of frequencies and various applied potentials in the range 0.855 to 0.145 V vs RHE at pH 7. The results are shown in Figure 3 as a Mott-Schottky plot, where the linear part is described by the following equation:

1/CSC2 =2/eε ε 0 N (V−V FB−kT /e ) (11)

where ε is the relative permittivity of the sample, ε0 is the vacuum permittivity, e is the elementary charge, N is the donor concentration, VFB is the flat band potential, k is the Boltzmann constant, T is the temperature [36]. Mott-Schottky plots of both NiTiO3 and CoTiO3 exhibit a positive slope, indicating the n-type semiconducting character. The conduction band potentials estimated by straight line extrapolation are 0.43 V vs RHE for CoTiO3 and 0.28 V vs RHE for NiTiO3. These data are in accordance with literature values, for instance VCB(NiTiO3) = 0.2 V [30]. Considering band gap energies calculated from DRS it is possible to estimate the valence band edge potentials: 2.83 V vs RHE for CoTiO3 and 3.28 V vs RHE for NiTiO3. Figure 3 presents also Nyquist plots obtained for NiTiO3 and the equivalent circuit used to fit impedance data.

Figure 3. Nyquist plots for NiTiO3 - each curve represents data obtained for one selected potential applied to WE (A) and Mott-Schottky plots for CoTiO3 and NiTiO3 in electrolyte of pH 7 (B).

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CV measurements (in dark) of electrodes made of CoTiO3 and NiTiO3 under oxygen or nitrogen atmosphere are shown in figure S3. Preliminary photoelectrochemical characterization (photocurrent measurements) were performed in order to investigate the materials’ photoactivity. At pH = 7, the NiTiO3 on FTO photoelectrode generates anodic photocurrents up to 200 A∙cm-2

under illumination with 350 nm, up to 80 A∙cm-2 under 400 nm and up to 10 A∙cm-2 under 565 nm, as shown in figure 4. These results are in good agreement with the reflectance spectrum shown in figure 2.

In alkaline or acidic electrolytes, the above trend is confirmed (figures S4 and S5 in Supporting Information). The highest current density has been obtained upon irradiation at 350 nm, but the differences between 350 nm and 400 nm are significantly lower than at pH 7. Photocurrent densities in acidic electrolyte are significantly higher at 400 nm than in neutral or alkaline solutions. In turn, at 350 nm, the highest photocurrents were observed in neutral pH.

Figure 4. Photocurrent analysis on NiTiO3 – influence of incident light wavelength. Linear voltammetry of FTO/ NiTiO3 in N2-saturated electrolyte of pH 7 under chopped illumination with LED =350 nm, =400 nm or =565 nm as a light source. Scan rate 10 mV∙s-1.

Lower photocurrent generation was obtained for the photoelectrode prepared by drop-casting a CoTiO3 suspension on FTO (figure 5). At pH 7, between 0.7 and 1.4 V vs RHE and under illumination at 400 nm the material generates anodic photocurrents up to 1 A cm-2.

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Figure 5. Photocurrent analysis on CoTiO3. Linear voltammetry of FTO/CoTiO3 in electrolyte of pH 7 under chopped illumination with LED =400 nm as a light source. Scan rate 10 mV∙s-1.

The photocurrent density strongly depends on the wavelength of incident light. The Incident Photon to Current Efficiency (IPCE) of both photoelectrodes was measured at 0.87 V vs RHE (chronoamperometric measurements for 3 various wavelengths) at pH = 7 and it was calculated using the following equation

IPCE (%)=( 1240 ∙ JI ph ∙ λ )∙ 100 (12)

where is the wavelength of the incident light, Iph and J are the measured photocurrent density and the measured irradiance at the selected wavelength, respectively. As shown in table 1, IPCE

Table 1. Influence of incident light wavelength on the incident photon to current efficiency of NiTiO3 and CoTiO3.

wavelengthIPCE

NiTiO3 CoTiO3

= 350 nm 12,8 % 0,0488 %= 400 nm 2,07 % 0,0207 %= 565 nm 0,455 % 0,00325 %

Photocatalytic activity towards H2O2 formation

The photocatalytic performance of NiTiO3 toward hydrogen peroxide formation in various conditions was investigated in an aqueous solution at pH 3.9. The concentration of H2O2

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during irradiation was determined by CV, as shown in Figure 6 - inset. Figure 6 shows the time-dependence of hydrogen peroxide concentration during irradiation of a NiTiO3 suspension: i) saturated with oxygen, ii) saturated with nitrogen or iii) saturated with nitrogen and in the presence of 2.47 M methanol. The highest yield of H2O2 was observed in the presence of oxygen. In the absence of the latter, a noticeable decrease of H2O2 is observed. Moreover, addition of methanol as sacrificial hole scavenger to the solution saturated with N2 further decreased the yield. These results suggest that photoexcited NiTiO3 promotes both water oxidation and O2

reduction towards the production of H2O2. In all cases it is evident that the rate of H2O2

production on NiTiO3 is almost constant. This indicates that the photocatalyst stably produces H2O2 without loss of activity and with no appreciable decomposition of the produced hydrogen peroxide.

Figure 6. Kinetic curves of photocatalytic H2O2 formation at NiTiO3 in suspension. Irradiation with light = 400 nm. Solution (pH 3.9) bubbled with O2, N2 and solution with methanol (2.47 M) saturated with N2. Each point is an average value of concentration obtained from 3 tests. Error bars represent standard deviations. Concentration of H2O2 estimated based on electrochemical detection, see cyclic voltammogram – inset.

In parallel to electroanalytical detection, the formation of hydrogen peroxide was confirmed using the spectroscopic method based on vanadium pentoxide as sensor. H2O2 reacts with V2O5 in acidic solution leading to a peroxovanadate complex. In contrast to V2O5, the peroxovanadate complex has a red-brown color and it absorbs in the visible range with a maximum absorptivity at 454 nm [31]. Spectra presented in figure 7 represent the increase of H2O2 concentration during irradiation of NiTiO3 suspension saturated with oxygen. The resulting H2O2 concentrations (inset) are well correlated with those obtained from the electroanalytical method (compare with figure 6).

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Figure 7. Spectra of peroxovanadate complex formed due to reaction of V2O5 with H2O2. Spectral changes represent the photocatalytic test towards H2O2 formation at NiTiO3 in O2

saturated suspension. Inset: photocatalytic H2O2 formation at NiTiO3 in suspension bubbled with O2 under 400 nm irradiation.

NiTiO3 and CoTiO3 were tested towards photocatalytic H2O2 formation upon irradiated with incident light wavelength of = 350 nm, 400 nm or 565 nm. As shown in figure 8, both materials are significantly more active under higher energy light (350 nm or 400 nm) than under lower energy light (565 nm). The results agree with conclusion from diffuse reflectance spectra: for both materials, absorbance at 400 nm is higher than at 565 nm (figure 2). Moreover, the differences between activity under 350 nm and 400 nm are smaller than expected from photocurrent analysis. A comparison of the two materials clearly shows that NiTiO3 is more efficient than CoTiO3, as shown in figure 8, upper graphs. However, for 565 nm light the activity of NiTiO3 and CoTiO3 are more similar. It should be highlighted that the used light sources have not only different energy of light (nm) but also light power (W/m2) – both parameters can affect the reaction rate.

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Figure 8. Kinetic curves of photocatalytic H2O2 formation at NiTiO3 and CoTiO3, in suspension. Irradiation with light = 350 nm (upper graph), = 400 nm (middle graph) or = 565 nm (lower graph). Electrolyte (pH 3.9) bubbled with O2 prior and during irradiation. Concentration of hydrogen peroxide estimated based on electrochemical detection using gold electrode.

The differences between hydrogen peroxide concentration determined from spectroscopic and electroanalytical methods are acceptably small. For instance – photocatalytic test under 400 nm irradiation, in O2 atmosphere at NiTiO3, time 60 min – CH2O2 was 2.47 mM from the spectroscopic method (figure 7, inset) and 2.39 mM form the electrochemical onen (figure 8, middle graph). The difference is 3.2%.

Photoelectrochemical activity towards H2O2 formation

Photoelectrochemical test towards hydrogen peroxide production are the second group of experiments, carried out to completely characterize the system. Electrodes prepared by drop casting on FTO glass were biased at constant potential (1.24 V vs RHE or 0.34 V) under

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irradiation. These two potentials represent two different conditions: 0.34 V – the applied potential is close to the conduction band edge potential and 1.24 V – the potential is significantly higher, but lower than the valence band edge potential. An exemplary chronoamperometry waveform (first 15 min) are presented in the supporting information (fig. S6). Figure 9 shows the results obtained for NiTiO3 photoelectrodes. The rate of H2O2 formation is noticeable higher for electrodes polarized at 1.24 V than at 0.34 V. Obtained results confirmed the trend suggested by the analysis of photocurrent: photocurrent density increases with increasing applied potential, as demonstrated in figure 4. On the other hand, no significant differences are observed between tests performed in the presence or absence of oxygen. Experiment performed under short circuit showed significantly lower activity of NiTiO3, as shown in figure S7.

Similar as in case of photocatalytic experiments (figure 7), electrodes prepared based on CoTiO3 showed significantly lower activity towards hydrogen peroxide formation (data not shown). Nonetheless, the dark control tests (without irradiation – electrode biased at 1.24 V or 0.34 V) showed a significantly lower H2O2 production rate than in case of tests under light.

Figure 9. Kinetic curves of photocatalytic H2O2 formation at NiTiO3 photoelectrode polarized with 0.34 V or 1.24 V vs RHE in electrolyte of pH 3.9, saturated with N2 or O2. Irradiation with light = 400 nm. Concentration of hydrogen peroxide estimated based on electrochemical detection using gold electrode.

Although concentrations of hydrogen peroxide in the solution during photoelectrochemical reactions are significantly lower than in the case of photocatalytic tests, it is hard to compare these two experiments mainly because different amounts of photoactive material (in 50 ml: 50mg in photocatalytic and 0.1 mg in photoelectrochemical tests) and different area of

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material under illumination are considered. At the same time, different mechanisms can be considered for the two cases.

The activity of various heterogeneous photosystem can be described using the apparent quantum yield, obtained by replacing the absorbed light intensity (as typically used for quantum efficiency calculation) by the incident intensity [37]. In heterogeneous photosystems, the incident intensity is much easier to calculate as a number of photons of a given wavelength per time and volume arriving at the inside of the flat front window of the photoreactor [37].

The apparent quantum efficiency (AQE) was calculated according as to

QE (%)=number of reacted electronsnumber of incident photons

∙ 100=2 ∙number of H 2O2 moleculesnumber of incident photons

∙100

The number of incident photons (=400 nm) is 1.57⋅1016 photons/s as calculated in SI. Calculated AQE for NiTiO3 in PEC are: 0.47% (at 1.24 V vs RHE in O2), 0.14% (at 1.24 V vs RHE in N2), 0.39% (at 0.34 V vs RHE in O2), 0.13% (at 0.34 V vs RHE in N2).

As indicated in the Introduction, hydrogen peroxide can be formed on various routes. Reaction involving water oxidation to hydroxyl radicals can be given as a highly probable example. The formation of HO• radicals was studied using spectroscopic methods based on the HO• sensor - terephtalic acid (TA). TA is a well-known hydroxyl radical scavenger, which does not react with other reactive oxygen species such as O2

• or H2O2 [38]. As presented in figure S8, the oxidation of TA with HO• leads to 2-hydroxyterephthalic acid, that can be easily detected by fluorescence measurement (exc = 315 nm, emm = 425 nm). Irradiation of the NiTiO3 suspension (photocatalytic process) in the presence of TA leads to 2-hydroxyterephthalic acid formation, as demonstrated by fluorescence spectra shown in figure S8 that confirms the formation of hydroxyl radicals. As expected (vide infra), in the case of CoTiO3 hydroxyl radical are not formed.

Considering band gap energies and potentials of conduction and valence bands of both materials it can be concluded that hydroxyl radicals can be formed only at the NiTiO3 surface (see fig. 10). In the case of CoTiO3, such process is thermodynamically forbidden due to the too low potential of the valence band edge and in turn of photogenerated holes.

Figure 10 shows the schematic diagram of the band structure of both materials and the simplified mechanism of the redox reactions in case of photocatalytic process. In photocatalytic conditions (using suspended photocatalysts powder) absorption of light leads to a one electron reduction of molecular oxygen to a superoxide anion radical and subsequent reaction towards the formation of H2O2 (eq. 3-6) or other reactive oxygen species. Moreover, a two electron oxygen reduction to H2O2 is also a highly probable process (eq. 1). In parallel, water (or surface –OH group) can be oxidized by photogenerated hole with formation of a •OH radical, leading again to H2O2 (eq. 7-8). The differences between reaction in the presence and absence of oxygen as well as reaction in the presence of sacrificial hole scavenger supports this mechanism that is based on the formation of H2O2 via both oxygen reduction and water oxidation.

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Figure 10. The energy band structure of NiTiO3 and CoTiO3 in pH = 7. Data taken from DRS and Mott-Schottky analysis. Primary redox processes are relevant to the reactions described by equations 3 - 8. For the sake of image clarity, all reactions are denoted by their redox couple only.

A quite similar mechanism could be suggested for the photoelectrochemical process, but for the behavior of the photoelectrochemical system that leads to significantly different results. In titanates, being n-type semiconductors, under irradiation holes transfer to the semiconductor surface to oxidize water to hydroxyl radicals. In the meantime, photoexcited electrons transfer to the counter platinum electrode, where oxygen is reduced to water, since Pt is well known catalyst of oxygen reduction reaction with a 4-electrons mechanism. For this reason, H2O2 formation rate is not affected by the presence or absence of O2 (see fig. 9). Moreover, increasing the bias means increasing band bending and favors charge transport within the semiconductor, thus reducing the probability of recombination. This well explains the influence of applied bias as shown in fig. 9. On these bases we calculated, in the case of NiTiO3 (biased at 1.24 V, N2 atmosphere), the theoretical concentration of H2O2 from the integrated quantity of charge (fig. S6), and according to Faraday’s law, is 0.25 mM. As demonstrated in fig. 9, the 15 min irradiation resulted in 0.19 mM of H2O2, with a photocurrent efficiency of 76%.

These strong evidences for the validity of the model for the photoelectrochemical system further supports also the mechanism proposed for the photocatalytic way.

It should be noted that hydrogen peroxide can be decomposed in a photocatalytic reaction. Reaction of H2O2 with HO• radicals can lead to the formation of hydrogen superoxide, as demonstrated by equations 12-13, but hydrogen peroxide can be formed also as an effect of recombination of as obtained radicals (eq. 14).

H 2O 2+OH ∙ → H2 O+H O2∙ (12)

H 2O 2+eCB−¿ →OH ∙+OH−¿¿ ¿ (13)

H O2∙ +H O2

∙ → H2 O2+O2 (14)According to the obtained results (linear increase of H2O2 concentration with time) this problem doesn’t give appreciable effects, in contrast to literature reports. For example, Diesen et al. [12]

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described TiO2-based system in which the H2O2 amount increases with time for about 20 min before reaching a constant level, that corresponds to the steady-state where the yield of H2O2

production is equal to the yield of consumption.

ConclusionNickel and cobalt titanates were obtained via an hydrothermal route. Both materials

showed absorption of light in the visible range, as demonstrated by diffuse reflectance spectra. NiTiO3 and CoTiO3 were tested towards hydrogen peroxide formation on photocatalytic (suspension of materials under irradiation) and photoelectrochemical ways (electrodes under irradiation). We found that NiTiO3 successfully produces a high concentration of H2O2, in particular when used as photocatalyst (up to ca. 2.5 mM), which is higher than those of many early reported photocatalytic systems [12,17,24]. In contrast, the activity of CoTiO2 is significantly lower than that of NiTiO3.The collected results suggest that H2O2 is produced both from oxygen reduction (by photogenerated electrons) and from water oxidation (from photogenerated holes). This is proven by the fact that (i) in the absence of oxygen, H2O2 formation rate is significantly decreased to about one halve; (ii) the formation of HO• radicals is demonstrated; (iii) if a photoelectrochemical arrangement is adopted and a Pt counter electrode is used, only the anodic reaction persists (on Pt, H2O reduction occurs with a 4-electrons mechanism) and no effect for the presence/absence of oxygen is recorded.

AcknowledgementsWe thank Dr. Claudio Carrara and Dr. Emanuele Instuli from Industrie De Nora S.p.A. for assistance with XRD measurements. The research was supported by Fondazione De Nora within an Fondazione Oronzio e Niccolò De Nora Fellowship in Applied Electrochemistry . The use of instrumentation purchased through the Regione Lombardia – Fondazione Cariplo joint SmartMatLab Project (Fondazione Cariplo grant 2013-1766) is gratefully acknowledged. S.W is thankful for NCN within 2014/15/N/ST5/00694 Preludium project. The research was performed in part using the equipment purchased through supported SONATA project UMO-2016/21/D/ST4/00221 founded by National Science Centre, Poland.

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