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Photo-electrochemical properties of WO3 particulate layers
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Photo-electrochemical properties of WO3 particulate layers Martin Zlamal ∗ , Josef Krysa Department of Inorganic Technology, Institute of Chemical Technology Prague, Technicka 5, 16628, Prague, Czech Republic
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Article history: Available online xxx Keywords: Monoclinic tungsten trioxide WO3 particulate layers Photo-electrochemical properties Photoelectrochemical water splitting
a b s t r a c t Monoclinic tungsten trioxide particle layers were prepared on FTO glass substrates by the sedimentation method and further annealing at different temperatures to improve the adhesion of WO3 particles to substrate. Linear voltammetry of these layers was measured within the periodically chopped light illumination [very narrow single peaks at 314, 365 and 404 nm and the standard solar illumination (AM1.5G)]. An optimal amount of WO3 in the layer was found for each light source. Back side illumination is more advantageous only for thick layers and visible light source. Better adhesion at higher annealing temperatures resulted in more stable layers with the higher photocurrent. The increase of annealing temperature above 500 ◦ C caused the formation of undesirable crystal phases (produced by the reaction of WO3 and FTO layer) and the significant decrease in photocurrent. © 2014 Elsevier B.V. All rights reserved.
1. Introduction
2. Experimental
Tungsten trioxide is widely used for the photocatalytic experiments, especially for those using visible light illumination. It is an n-type semiconductor with an indirect band gap (Eg ∼ 2.6 eV [1]) that can capture approximately 12% of the solar spectrum and it can absorb light in the visible spectrum up to 500 nm [2]. It is also convenient material to be used for hydrogen production via photoelectrochemical water splitting [3]. A wide range of WO3 layer preparation methods and their combinations are used including sol-gel process [4], hydrothermal synthesis [5], anodization of wolfram [6], sputtering techniques [7], chemical vapor deposition [8], spray pyrolysis [9], drop-casting [10] and electro-deposition [11]. Layers formed by the particles of material have an advantage of relatively high specific surface area usable for the surface reactions. However, the mechanical stability and the conductivity of these layers are very low due to the weak adhesion of the particles. Both stability and conductivity can be improved by the layer calcination. The aim of this work was to evaluate the effect of annealing temperature, amount of WO3 in the layer, light wavelength and irradiation direction with respect to the utilization of WO3 particulate layers as photoanodes in a photoelectrochemical cell for water splitting. WO3 films were fabricated from the suspension of WO3 particles by drying and consequent thermal annealing.
2.1. Layer preparation
∗ Corresponding author. Tel.: +420220444112. E-mail addresses: [email protected] (M. Zlamal), [email protected] (J. Krysa).
Tungsten trioxide films were prepared from commercial powder (99.9% WO3 , Fluka) by the sedimentation method. This material has relatively big aggregates and it was not possible to produce a stable water suspension, so it was ground for 23 min in ball mill (Pulverisette 23, Fritsch, 25 balls). Milled material was mixed with distilled water to obtain a suspension with a known concentration of WO3 . Before application of the suspension on FTO glass (TCO2215, Solaronix), the suspension was ultrasonicated for 5 min (= 15 kJ) by ultrasound probe (UW3100, probe MS73, Bandelin). Substrate slides had dimensions of 1 × 5 cm2 and after application of constant volume of the suspension (1 ml) they were dried at 60 ◦ C. Then the part of formed layer (1 cm2 ) was removed to provide contact area for electric circuit at one end of the substrate. Prepared layers were thermally annealed at 300, 400, 450, 500 and 600 ◦ C for 2 h. The different amount of WO3 in the layer was achieved by varying concentration of WO3 suspension (using the suspension with concentration of 0.5 g WO3 /dm3 produces layer containing 0.1 mg WO3 /cm2 ). 2.2. Film characterization The structural and morphological properties of the deposited films were determined using X-ray diffraction (X’Pert PRO, Panalytical) and scanning electron microscopy (S4700, Hitachi). Photo-electrochemical measurements were performed in a glass cell with quartz window (Fig. 1) using three-electrode
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Fig. 2. XRD of WO3 powder and layers prepared on FTO substrate annealed at different temperatures. Major peaks of Na2 W3 O10 (*), SnWO4 (◦ ), SnO2 (+) and monoclinic WO3 (x) XRD patterns are showed.
3. Results and discussion 3.1. Film characterization
Fig. 1. Glass electrochemical cell with quartz window used for photoelectrochemical measurements of prepared WO3 layers.
arrangement in 0.1 M sodium sulfate electrolyte. The irradiated surface area of each sample was set to 1 cm2 by a Teflon tape, a Pt plate was used as counter electrode and Ag/AgCl (+207 mV vs. SHE) as reference electrode. The cell was connected to the potentiostat (PGZ100, VoltaLab) and it was situated at the end of an optical bench consisting of light source, appropriate filter and PCcontrolled shutter. Two types of light sources were used: a) 500 W Hg arc lamp (Lot-Oriel) with 314, 365 and 404 nm bandpass filters and infrared water filter and b) solar simulator – 150 W Xe arc lamp (Newport) with AM1.5G filter. The irradiance of used lights entering the cell was 1 mW/cm2 for all three wavelengths 314, 365 and 404 nm, and 1 sun (100 mW/cm2 ) that is simulating the standard solar illumination conditions AM1.5G. Irradiances were set by the distance of electrochemical cell from the light source. Irradiance was measured by a) photodiode (S1337-BQ, Hamamatsu) for 314, 365 and 404 nm lights and b) calibrated reference cell (91150 V, Newport) for AM1.5G solar simulator light. Linear voltammetry of prepared WO3 layers was measured with sweep rate 5 mV/s while periodically illuminated (5 s light/5 s dark). Samples were irradiated from the electrolyte/electrode (EE) interface or from the substrate/electrode (SE) interface.
The powder of WO3 used for layer preparation was identified by XRD as monoclinic. Thermal annealing of such powder up to 600 ◦ C has no effect on its crystal structure. The XRD patterns of WO3 powder and particulate layers deposited on FTO substrate and annealed at different temperature are shown in Fig. 1. The crystallite size calculated by Scherrer formula is approximately 33 nm. Particulate WO3 layers (0.1 mg/cm2 ) on FTO substrate annealed up to 500 ◦ C have also monoclinic structure. Strong background corresponding to SnO2 in FTO film (diffraction lines at 2 = 26.7, 37.8, 51.8, 61.7 and 65.7◦ ) makes it difficult to detect monoclinic WO3 phase. Only diffraction lines at 2 = 23.1, 23.6, 24.4 and 49.9◦ are distinguishable. WO3 layers annealed at 600 ◦ C contain significant amount of Na2 W3 O10 and SnWO4 phases (Fig. 2) and peaks of WO3 almost disappear. These products of reaction between WO3 and FTO glass were not detected previously even for temperature up to 700 ◦ C [12]. The reason is probably in the four times higher thickness and nanorod form of the reported layers. According to SEM analysis (Fig. 3), WO3 layers consist of agglomerates with diameter of 50–300 nm. Prepared layers were not well uniform in thickness. This means that some uncovered area of support glass can be found for layers with low amount of WO3 in the layer. The particulate layers containing 2 mg WO3 /cm2 have average thickness about 5 m (inset in Fig. 3). From WO3 density 7.16 g/cm3 [13] and amount of WO3 in the layer, theoretical thickness of compact film can be calculated as 2.8 m. Using previously reported approach [Eq. (1)] [14], porosity of the deposited layers can be estimated as 44%. This value is in good agreement with the porosity of particulate TiO2 films prepared by the same technique [15].
2.3. Water splitting
3.2. Photoelectrochemical properties
For water splitting, solar light simulator described previously was used in combination with two chambers glass cell with 0.1 M NaSO4 water solution as electrolyte. Anode and cathode chambers were separated by the glass frit to prevent mixing of electrolytes. Potential of +1 V vs. Ag/AgCl reference electrode was applied to the sample (working electrode). Platinum sheet was used as counter electrode. Both chambers were bubbled by argon gas (1 ml/min) separately and chambers were open to air by glass capillary to prevent mixing with air. Output gas from the cathode chamber (counter electrode) was analyzed by gas chromatograph (Master, Dani) equipped with plot column (Rt®-Msieve 5A, Restek) and TCD detector (Vici).
Polarization curves of the prepared WO3 particulate layer under the chopped light (Fig. 4) showed increasing photocurrents at the potential interval from 0 to 0.5 V vs. Ag/AgCl and a photocurrent plateau from 0.5 to 1.5 V vs. Ag/AgCl. Further potential increase leads to the steep increase of dark current, which indicates electrochemical evolution of oxygen. The thermal annealing of WO3 layer at 600 ◦ C has negative effect on measured photocurrent values (compared to layer annealed at 500 ◦ C) (see Fig. 4). Increasing temperature also shifts the end of photocurrent plateau to more positive potentials. This can be explained by the increase of oxygen overpotential on WO3 layer with increasing annealing temperature. Photocurrents measured at 1 V (vs. Ag/AgCl) and
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Fig. 3. SEM images of 2 mg WO3 /cm2 layer on FTO top view and cross-sectional view (inset).
corresponding values of IPCE (incident photon to current efficiency) were used for direct comparison of the prepared layers. IPCE was calculated using Eq. (1): j j · h · c · NA · 100 [%] · 100 = F ·I F ·P·
that IPCE for AM1.5G light is generally very low and is not relevant even it can be calculated [16].
where j is photocurrent density (A/cm2 ), F is Faraday’s constant (96485 C/mol) and I is photon flux density for specific wavelength (mol/cm2 s), which can be calculated from irradiance P (radiant flux density) (W/m2 ) and corresponding wavelength (m), Planck’s constant h (6.6261 × 10–34 J s), speed of light c (299.79 × 106 m/s) and Avogadro constant NA (6.0221 × 1023 /mol). The initial irradiances were recalculated from initial values entering the cell (1 mW/cm2 for 314 and 365 nm and 10 mW/cm2 for 404 nm) to irradiance incident on the sample through known transmittance of glass cell wall (0.368, 0.945 and 0.956 for 314, 365 and 404 nm, respectively). Transmittance of the electrolyte was assumed to be 1 for all wavelengths. The total number of photons of AM1.5G light (for 280–4000 nm) is 7.16 × 10–3 mol/m2 s. Large amount of them has wavelengths which cannot be absorbed by WO3 . This means
3.2.1. Influence of annealing temperature The influence of annealing temperature on IPCE of particulate layers (0.1 mg WO3 /cm2 ) is shown in Fig. 5. IPCEs at 365 nm (and photocurrents) increase with annealing temperature up to 500 ◦ C. Strong decrease of photocurrent and IPCE is observed for annealing temperature at 600 ◦ C. This could be explained by the formation of new undesirable crystal phases of SnWO4 and Na2 W3 O10 at the interface of the FTO layer and WO3 layer, which were detected by XRD (see Section 3.1). These phases probably act as insulating material and they block migration of the electrons from WO3 to FTO layer. The dependence of photocurrent and IPCE on annealing temperature has a similar trend for all three wavelengths. Annealing at 500 ◦ C results in the highest photocurrents and IPCE; therefore, this annealing temperature was used for all further experiments. This improvement could be explained by better adhesion of particles to the FTO glass support and by particle sintering which results in enhanced electron transfer.
Fig. 4. Polarization curve of 0.1 mg WO3 /cm2 layer annealed at three temperatures, 365 nm, 1 mW/cm2 and EE interface.
Fig. 5. IPCEs for 0.1 mg WO3 /cm2 particulate layers annealed at different temperature, EE interface.
IPCE =
(1)
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Fig. 6. Influence of sample irradiation direction to photocurrent for different amount of WO3 in the layer for a) 314 nm, b) 365 nm, c) 404 nm and d) AM1.5G.
Similar observation concerning the optimal annealing temperature at 500 ◦ C for WO3 on FTO glass was made by Kalanur et al. [12], and it was assigned to the change of crystal phase to monoclinic and to the change of morphology and the decrease in the surface area at higher temperature. Also Hong et al. [17], who prepared monoclinic WO3 particles by hydrothermal method, reported annealing temperature 450 ◦ C for the preparation of stable thick particulate layer from particle suspension by ‘doctor blade’ technique. Santato et al. [18] observed an increase in photocurrents with annealing temperature in the range 400–550 ◦ C and explained this by an increase in porosity, crystallinity and particle size but further increase of annealing temperature to 600 ◦ C has no positive effect. Unfortunately, photocurrents for annealing temperature 600 ◦ C were not reported. Thus our explanation of the negative influence of annealing temperature 600 ◦ C due to the reaction of WO3 particles with Sn and Na from the substrate is new and not yet published. Fig. 5 shows also the influence of light wavelength on IPCE for each annealing temperature. The energy of photons decreases with increasing wavelength from 314 to 404 nm and at the same time the number of photon increases. Together with increasing wavelength, the amount of absorbed light by WO3 film decreases [19, 20] which results in higher IPCE for shorter wavelengths. 3.2.2. Influence of WO3 amount, light source and direction of irradiation The evaluation based on measured photocurrents is more practical and we can guess the performance of photoelectrochemical water splitting. Therefore, in the next step we investigated the influence of WO3 amount, light source and side of irradiation of the layers. Fig. 6 shows the dependences of photocurrent on the mass of WO3 layer for different light sources and for two irradiation directions. The mass of the layer (mg WO3 /cm2 ) can be recalculated
to hypothetic layer thickness (assuming compact nonporous WO3 film). In this study the hypothetic layer thickness increases from 140 (for 0.1 mg WO3 /cm2 ) to 2800 nm (for 2 mg WO3 /cm2 ). The optimal mass of WO3 layer for which the photocurrent is maximal for sufficiently powerful light source is shown in Table 1. Maximal photocurrent at 314 nm was observed for layer containing 0.2 mg WO3 /cm2 (280 nm). With increasing layer mass photocurrent decreases. This can be explained by the value of absorption coefficient (˛) which can be recalculated to the light penetration depth (1/˛). Penetration depth of light with wavelength 314 nm in WO3 films was determined to 100 [19] or 70 nm [20]. For EE irradiation, it means that for layer thickness higher than 280 nm the part of the WO3 layer close to SE interface is not irradiated sufficiently and this results in the lower transport of electrons towards the FTO support, e.g. smaller photocurrent. Illumination from the SE interface by 314 nm light produces very low photocurrents. This is because of the low transmittance of FTO glass support which is equal to 0.081. When photocurrent is corrected on the light absorbed by FTO (divided by 0.081), we obtain hypothetical photocurrent around 61 A which is about 30% higher than for EE irradiation (≈40 A). The measured value of photocurrent for SE irradiation (often convenient geometry for application) can be improved by using a FTO quartz support, but at an incomparably higher cost. For illumination by 365 nm light, there is a shift of the optimal layer mass of WO3 to 0.5 mg/cm2 (700 nm). This shift is caused by smaller absorption coefficient/longer light penetration depth for 365 nm in comparison with 314 nm, e.g. 1/˛ of WO3 for 365 nm equal to 500 [19] and 250 nm [20], respectively. In the case of 404 nm, there is an increase of photocurrent with WO3 layer mass up to 0.5 mg/cm2 (700 nm) followed by plateau up to 2 mg WO3 /cm2 (2800 nm). This means that the optimal film thickness is between 700 and 2800 nm which is in agreement with
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Table 1 Optimal film thickness and relation to light penetration depth. Irradiation wavelength (nm)
Optimal film mass (mg WO3 /cm2 )
Theoretical optimal film thickness (m)
0.2 0.5 0.5–2
0.28 0.70 0.7–2.8
Light penetration depth (m) 1/˛ [19]
314 365 404
the lowest reported absorption coefficient of WO3 film (in relation to 365 and 314 nm) that gives the light penetration depth equal to 1100 [19] and 500 nm [20], respectively. In the case of SE illumination, photocurrents increase for both 365 and 404 nm with WO3 layer mass. For layer mass 0.1–0.5 mg/cm2 , photocurrents are lower than those for EE illumination because of FTO glass transmittance (0.72 and 0.81 for 365 and 404 nm, respectively). When the photocurrent is corrected on the light absorbed by FTO (divided by 0.72), we obtain for 365 irradiation a hypothetical photocurrent around 55 A which is about 20% higher than for EE irradiation (≈45 A). But for layer mass 2 mg/cm2 , photocurrent for SE illumination is for both 365 and 404 nm higher than for EE illumination. This can be explained by the fact that the film thickness is significantly higher than the light penetration depth and the negative effect of FTO glass absorbance is eliminated by the positive effect of SE irradiation. This is more significant for 404 nm where the photocurrent for EE irradiation is 32 A/cm2 while that for SE irradiation corrected on light absorbed by FTO (divided by 0.81) is equal to ≈45 A/cm2 (about 40% higher). For irradiation by AM1.5G light source, the situation is similar to the irradiation wavelength 404 nm. Only values of photocurrents are nearly 10 times higher because of the significantly different light intensity and emission spectra.
3.2.3. Stability of photoelectrochemical performance Chopped light polarization curves of particulate layers (0.1 mg WO3 /cm2 ) annealed at different temperature were measured repeatedly (six times) to see the performance stability. Fig. 7 shows the decrease of the photocurrents (after six repetitions). Only the sample dried at 60◦ and the sample annealed at 300 ◦ C show very low stability of photocurrent which can be due to the loss of particles. The minimal decrease of photocurrent was observed for annealing temperatures 450 and 500 ◦ C where the loss of particles from layer is minimal due to particle sintering. Further decrease in stability for 600 ◦ C could be caused by the presence of undesirable phases produced by annealing (see Section 3.1).
0.1 0.5 1.1
1/˛ [20] 0.07 0.25 0.5
Fig. 8. Time evolution of H2 and measured photocurrents, 0.5 mg WO3 , 1 cm2 , AM1.5G, 1 sun.
3.3. Water splitting The time evolution of hydrogen formation on Pt counter electrode (while using prepared WO3 particle layer calcined at 500 ◦ C as working electrode for oxygen evolution) and corresponding photocurrents are shown in Fig. 8. After 2 h of irradiation the concentration of hydrogen in output gas (1 ml/min, Ar as carrier gas) is stabilized at 300 ppm (i.e. 0.3 l H2 /min). Interestingly, the current corresponding to the production of hydrogen is only 60% of the measured photocurrent. It means that around 40% of produced current is consumed at the cathode for another reduction reaction, e.g. oxygen reduction. Even the hydrogen concentration remains constant, photocurrent further decreases. Possible explanation for the current decrease can be in the loss of particles which can be higher than in the case of repeated short time photocurrent measurements (Fig. 7). The expected decrease in hydrogen concentration (due to photocurrent decrease) was not observed due to the decreased concentration of dissolved oxygen resulting in the increase in hydrogen current efficiency (from 60% at 2 h to 70% at 4 h).
4. Conclusions
Fig. 7. Relative decrease of the photocurrents after repeated use of particulate WO3 (0.1 mg WO3 /cm2 ), 365 nm, 1 mW/cm2 , EE interface.
The annealing of deposited particulate WO3 layers at temperatures 450–500 ◦ C results in better adhesion of particles to the FTO substrate and significant increase in photocurrent. The annealing at 600 ◦ C caused the formation of undesirable crystal phases (confirmed by XRD) and the significant decrease of photocurrent. For WO3 , mass of layer, corresponding to the maximal photocurrent, increases with the increasing wavelength of irradiation light in the range from 314 to 404 nm. The substrate/electrode side of illumination is advantageous for the films with higher mass of the photocatalyst (2 mg WO3 /cm2 ) and for the light source containing only visible or significant part of visible light.
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Acknowledgements This work was supported by the Grant Agency of the Czech Republic (P108/12/2104). References [1] N.S. Gaikwad, G. Waldner, A. Bruger, A. Belaidi, S.M. Chaqour, M. NeumannSpallart, Photoelectrochemical characterization of semitransparent WO3 films, J. Electrochem. Soc. 152 (2005) G411–G416. [2] Y. Sun, C.J. Murphy, K.R. Reyes-Gil, E.A. Reyes-Garcia, J.M. Thornton, N.A. Morris, D. Raftery, Photoelectrochemical and structural characterization of carbondoped WO3 films prepared via spray pyrolysis, Int. J. Hydrogen Energy 34 (2009) 8476–8484. [3] X. Liu, F.Y. Wang, Q. Wang, Nanostructure-based WO3 photoanodes for photoelectrochemical water splitting, Phys. Chem. Chem. Phys. 14 (2012) 7894–7911. [4] T. Caruso, M. Castriota, A. Policicchio, A. Fasanella, M.P. De Santo, F. Ciuchi, G. Desiderio, S. La Rosa, P. Rudolf, R.G. Agostino, E. Cazzanelli, Thermally induced evolution of sol-gel grown WO3 films on ITO/glass substrates, Appl. Surf. Sci. 297 (2014) 195–204. [5] F. Zheng, M. Zhang, M. Guo, Controllable preparation of WO3 nanorod arrays by hydrothermal method, Thin Solid Films 534 (2013) 45–53. [6] W. Wei, S. Shaw, K. Lee, P. Schmuki, Rapid anodic formation of high aspect ratio WO3 layers with self-ordered nanochannel geometry and use in photocatalysis, Chem. Eur. J. 18 (2012) 14622–14626. [7] S.L. Liew, W. Wang, S. Bin Dolmanan, G.T.W. Goh, D.Z. Chi, Solar water oxidation in sputter-deposited nanocrystalline WO3 photoanodes via tuning of Ar:O-2 flow rate combinations, Mater. Chem. Phys. 143 (2014) 1171–1177. [8] C.M. White, D.T. Gillaspie, E. Whitney, S.H. Lee, A.C. Dillon, Flexible electrochromic devices based on crystalline WO3 nanostructures produced with hot-wire chemical vapor deposition, Thin Solid Films 517 (2009) 3596–3599. [9] O. Arutanti, T. Ogi, A.B.D. Nandiyanto, F. Iskandar, K. Okuyama, Controllable crystallite and particle sizes of WO3 particles prepared by a spray-pyrolysis method and their photocatalytic activity, Aiche J. 60 (2014) 41–49.
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Photo-electrochemical properties of WO3 particulate layers Martin Zlamal ∗ , Josef Krysa Department of Inorganic Technology, Institute of Chemical Technology Prague, Technicka 5, 16628, Prague, Czech Republic
a r t i c l e
i n f o
Article history: Available online xxx Keywords: Monoclinic tungsten trioxide WO3 particulate layers Photo-electrochemical properties Photoelectrochemical water splitting
a b s t r a c t Monoclinic tungsten trioxide particle layers were prepared on FTO glass substrates by the sedimentation method and further annealing at different temperatures to improve the adhesion of WO3 particles to substrate. Linear voltammetry of these layers was measured within the periodically chopped light illumination [very narrow single peaks at 314, 365 and 404 nm and the standard solar illumination (AM1.5G)]. An optimal amount of WO3 in the layer was found for each light source. Back side illumination is more advantageous only for thick layers and visible light source. Better adhesion at higher annealing temperatures resulted in more stable layers with the higher photocurrent. The increase of annealing temperature above 500 ◦ C caused the formation of undesirable crystal phases (produced by the reaction of WO3 and FTO layer) and the significant decrease in photocurrent. © 2014 Elsevier B.V. All rights reserved.
1. Introduction
2. Experimental
Tungsten trioxide is widely used for the photocatalytic experiments, especially for those using visible light illumination. It is an n-type semiconductor with an indirect band gap (Eg ∼ 2.6 eV [1]) that can capture approximately 12% of the solar spectrum and it can absorb light in the visible spectrum up to 500 nm [2]. It is also convenient material to be used for hydrogen production via photoelectrochemical water splitting [3]. A wide range of WO3 layer preparation methods and their combinations are used including sol-gel process [4], hydrothermal synthesis [5], anodization of wolfram [6], sputtering techniques [7], chemical vapor deposition [8], spray pyrolysis [9], drop-casting [10] and electro-deposition [11]. Layers formed by the particles of material have an advantage of relatively high specific surface area usable for the surface reactions. However, the mechanical stability and the conductivity of these layers are very low due to the weak adhesion of the particles. Both stability and conductivity can be improved by the layer calcination. The aim of this work was to evaluate the effect of annealing temperature, amount of WO3 in the layer, light wavelength and irradiation direction with respect to the utilization of WO3 particulate layers as photoanodes in a photoelectrochemical cell for water splitting. WO3 films were fabricated from the suspension of WO3 particles by drying and consequent thermal annealing.
2.1. Layer preparation
∗ Corresponding author. Tel.: +420220444112. E-mail addresses: [email protected] (M. Zlamal), [email protected] (J. Krysa).
Tungsten trioxide films were prepared from commercial powder (99.9% WO3 , Fluka) by the sedimentation method. This material has relatively big aggregates and it was not possible to produce a stable water suspension, so it was ground for 23 min in ball mill (Pulverisette 23, Fritsch, 25 balls). Milled material was mixed with distilled water to obtain a suspension with a known concentration of WO3 . Before application of the suspension on FTO glass (TCO2215, Solaronix), the suspension was ultrasonicated for 5 min (= 15 kJ) by ultrasound probe (UW3100, probe MS73, Bandelin). Substrate slides had dimensions of 1 × 5 cm2 and after application of constant volume of the suspension (1 ml) they were dried at 60 ◦ C. Then the part of formed layer (1 cm2 ) was removed to provide contact area for electric circuit at one end of the substrate. Prepared layers were thermally annealed at 300, 400, 450, 500 and 600 ◦ C for 2 h. The different amount of WO3 in the layer was achieved by varying concentration of WO3 suspension (using the suspension with concentration of 0.5 g WO3 /dm3 produces layer containing 0.1 mg WO3 /cm2 ). 2.2. Film characterization The structural and morphological properties of the deposited films were determined using X-ray diffraction (X’Pert PRO, Panalytical) and scanning electron microscopy (S4700, Hitachi). Photo-electrochemical measurements were performed in a glass cell with quartz window (Fig. 1) using three-electrode
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Fig. 2. XRD of WO3 powder and layers prepared on FTO substrate annealed at different temperatures. Major peaks of Na2 W3 O10 (*), SnWO4 (◦ ), SnO2 (+) and monoclinic WO3 (x) XRD patterns are showed.
3. Results and discussion 3.1. Film characterization
Fig. 1. Glass electrochemical cell with quartz window used for photoelectrochemical measurements of prepared WO3 layers.
arrangement in 0.1 M sodium sulfate electrolyte. The irradiated surface area of each sample was set to 1 cm2 by a Teflon tape, a Pt plate was used as counter electrode and Ag/AgCl (+207 mV vs. SHE) as reference electrode. The cell was connected to the potentiostat (PGZ100, VoltaLab) and it was situated at the end of an optical bench consisting of light source, appropriate filter and PCcontrolled shutter. Two types of light sources were used: a) 500 W Hg arc lamp (Lot-Oriel) with 314, 365 and 404 nm bandpass filters and infrared water filter and b) solar simulator – 150 W Xe arc lamp (Newport) with AM1.5G filter. The irradiance of used lights entering the cell was 1 mW/cm2 for all three wavelengths 314, 365 and 404 nm, and 1 sun (100 mW/cm2 ) that is simulating the standard solar illumination conditions AM1.5G. Irradiances were set by the distance of electrochemical cell from the light source. Irradiance was measured by a) photodiode (S1337-BQ, Hamamatsu) for 314, 365 and 404 nm lights and b) calibrated reference cell (91150 V, Newport) for AM1.5G solar simulator light. Linear voltammetry of prepared WO3 layers was measured with sweep rate 5 mV/s while periodically illuminated (5 s light/5 s dark). Samples were irradiated from the electrolyte/electrode (EE) interface or from the substrate/electrode (SE) interface.
The powder of WO3 used for layer preparation was identified by XRD as monoclinic. Thermal annealing of such powder up to 600 ◦ C has no effect on its crystal structure. The XRD patterns of WO3 powder and particulate layers deposited on FTO substrate and annealed at different temperature are shown in Fig. 1. The crystallite size calculated by Scherrer formula is approximately 33 nm. Particulate WO3 layers (0.1 mg/cm2 ) on FTO substrate annealed up to 500 ◦ C have also monoclinic structure. Strong background corresponding to SnO2 in FTO film (diffraction lines at 2 = 26.7, 37.8, 51.8, 61.7 and 65.7◦ ) makes it difficult to detect monoclinic WO3 phase. Only diffraction lines at 2 = 23.1, 23.6, 24.4 and 49.9◦ are distinguishable. WO3 layers annealed at 600 ◦ C contain significant amount of Na2 W3 O10 and SnWO4 phases (Fig. 2) and peaks of WO3 almost disappear. These products of reaction between WO3 and FTO glass were not detected previously even for temperature up to 700 ◦ C [12]. The reason is probably in the four times higher thickness and nanorod form of the reported layers. According to SEM analysis (Fig. 3), WO3 layers consist of agglomerates with diameter of 50–300 nm. Prepared layers were not well uniform in thickness. This means that some uncovered area of support glass can be found for layers with low amount of WO3 in the layer. The particulate layers containing 2 mg WO3 /cm2 have average thickness about 5 m (inset in Fig. 3). From WO3 density 7.16 g/cm3 [13] and amount of WO3 in the layer, theoretical thickness of compact film can be calculated as 2.8 m. Using previously reported approach [Eq. (1)] [14], porosity of the deposited layers can be estimated as 44%. This value is in good agreement with the porosity of particulate TiO2 films prepared by the same technique [15].
2.3. Water splitting
3.2. Photoelectrochemical properties
For water splitting, solar light simulator described previously was used in combination with two chambers glass cell with 0.1 M NaSO4 water solution as electrolyte. Anode and cathode chambers were separated by the glass frit to prevent mixing of electrolytes. Potential of +1 V vs. Ag/AgCl reference electrode was applied to the sample (working electrode). Platinum sheet was used as counter electrode. Both chambers were bubbled by argon gas (1 ml/min) separately and chambers were open to air by glass capillary to prevent mixing with air. Output gas from the cathode chamber (counter electrode) was analyzed by gas chromatograph (Master, Dani) equipped with plot column (Rt®-Msieve 5A, Restek) and TCD detector (Vici).
Polarization curves of the prepared WO3 particulate layer under the chopped light (Fig. 4) showed increasing photocurrents at the potential interval from 0 to 0.5 V vs. Ag/AgCl and a photocurrent plateau from 0.5 to 1.5 V vs. Ag/AgCl. Further potential increase leads to the steep increase of dark current, which indicates electrochemical evolution of oxygen. The thermal annealing of WO3 layer at 600 ◦ C has negative effect on measured photocurrent values (compared to layer annealed at 500 ◦ C) (see Fig. 4). Increasing temperature also shifts the end of photocurrent plateau to more positive potentials. This can be explained by the increase of oxygen overpotential on WO3 layer with increasing annealing temperature. Photocurrents measured at 1 V (vs. Ag/AgCl) and
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Fig. 3. SEM images of 2 mg WO3 /cm2 layer on FTO top view and cross-sectional view (inset).
corresponding values of IPCE (incident photon to current efficiency) were used for direct comparison of the prepared layers. IPCE was calculated using Eq. (1): j j · h · c · NA · 100 [%] · 100 = F ·I F ·P·
that IPCE for AM1.5G light is generally very low and is not relevant even it can be calculated [16].
where j is photocurrent density (A/cm2 ), F is Faraday’s constant (96485 C/mol) and I is photon flux density for specific wavelength (mol/cm2 s), which can be calculated from irradiance P (radiant flux density) (W/m2 ) and corresponding wavelength (m), Planck’s constant h (6.6261 × 10–34 J s), speed of light c (299.79 × 106 m/s) and Avogadro constant NA (6.0221 × 1023 /mol). The initial irradiances were recalculated from initial values entering the cell (1 mW/cm2 for 314 and 365 nm and 10 mW/cm2 for 404 nm) to irradiance incident on the sample through known transmittance of glass cell wall (0.368, 0.945 and 0.956 for 314, 365 and 404 nm, respectively). Transmittance of the electrolyte was assumed to be 1 for all wavelengths. The total number of photons of AM1.5G light (for 280–4000 nm) is 7.16 × 10–3 mol/m2 s. Large amount of them has wavelengths which cannot be absorbed by WO3 . This means
3.2.1. Influence of annealing temperature The influence of annealing temperature on IPCE of particulate layers (0.1 mg WO3 /cm2 ) is shown in Fig. 5. IPCEs at 365 nm (and photocurrents) increase with annealing temperature up to 500 ◦ C. Strong decrease of photocurrent and IPCE is observed for annealing temperature at 600 ◦ C. This could be explained by the formation of new undesirable crystal phases of SnWO4 and Na2 W3 O10 at the interface of the FTO layer and WO3 layer, which were detected by XRD (see Section 3.1). These phases probably act as insulating material and they block migration of the electrons from WO3 to FTO layer. The dependence of photocurrent and IPCE on annealing temperature has a similar trend for all three wavelengths. Annealing at 500 ◦ C results in the highest photocurrents and IPCE; therefore, this annealing temperature was used for all further experiments. This improvement could be explained by better adhesion of particles to the FTO glass support and by particle sintering which results in enhanced electron transfer.
Fig. 4. Polarization curve of 0.1 mg WO3 /cm2 layer annealed at three temperatures, 365 nm, 1 mW/cm2 and EE interface.
Fig. 5. IPCEs for 0.1 mg WO3 /cm2 particulate layers annealed at different temperature, EE interface.
IPCE =
(1)
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Fig. 6. Influence of sample irradiation direction to photocurrent for different amount of WO3 in the layer for a) 314 nm, b) 365 nm, c) 404 nm and d) AM1.5G.
Similar observation concerning the optimal annealing temperature at 500 ◦ C for WO3 on FTO glass was made by Kalanur et al. [12], and it was assigned to the change of crystal phase to monoclinic and to the change of morphology and the decrease in the surface area at higher temperature. Also Hong et al. [17], who prepared monoclinic WO3 particles by hydrothermal method, reported annealing temperature 450 ◦ C for the preparation of stable thick particulate layer from particle suspension by ‘doctor blade’ technique. Santato et al. [18] observed an increase in photocurrents with annealing temperature in the range 400–550 ◦ C and explained this by an increase in porosity, crystallinity and particle size but further increase of annealing temperature to 600 ◦ C has no positive effect. Unfortunately, photocurrents for annealing temperature 600 ◦ C were not reported. Thus our explanation of the negative influence of annealing temperature 600 ◦ C due to the reaction of WO3 particles with Sn and Na from the substrate is new and not yet published. Fig. 5 shows also the influence of light wavelength on IPCE for each annealing temperature. The energy of photons decreases with increasing wavelength from 314 to 404 nm and at the same time the number of photon increases. Together with increasing wavelength, the amount of absorbed light by WO3 film decreases [19, 20] which results in higher IPCE for shorter wavelengths. 3.2.2. Influence of WO3 amount, light source and direction of irradiation The evaluation based on measured photocurrents is more practical and we can guess the performance of photoelectrochemical water splitting. Therefore, in the next step we investigated the influence of WO3 amount, light source and side of irradiation of the layers. Fig. 6 shows the dependences of photocurrent on the mass of WO3 layer for different light sources and for two irradiation directions. The mass of the layer (mg WO3 /cm2 ) can be recalculated
to hypothetic layer thickness (assuming compact nonporous WO3 film). In this study the hypothetic layer thickness increases from 140 (for 0.1 mg WO3 /cm2 ) to 2800 nm (for 2 mg WO3 /cm2 ). The optimal mass of WO3 layer for which the photocurrent is maximal for sufficiently powerful light source is shown in Table 1. Maximal photocurrent at 314 nm was observed for layer containing 0.2 mg WO3 /cm2 (280 nm). With increasing layer mass photocurrent decreases. This can be explained by the value of absorption coefficient (˛) which can be recalculated to the light penetration depth (1/˛). Penetration depth of light with wavelength 314 nm in WO3 films was determined to 100 [19] or 70 nm [20]. For EE irradiation, it means that for layer thickness higher than 280 nm the part of the WO3 layer close to SE interface is not irradiated sufficiently and this results in the lower transport of electrons towards the FTO support, e.g. smaller photocurrent. Illumination from the SE interface by 314 nm light produces very low photocurrents. This is because of the low transmittance of FTO glass support which is equal to 0.081. When photocurrent is corrected on the light absorbed by FTO (divided by 0.081), we obtain hypothetical photocurrent around 61 A which is about 30% higher than for EE irradiation (≈40 A). The measured value of photocurrent for SE irradiation (often convenient geometry for application) can be improved by using a FTO quartz support, but at an incomparably higher cost. For illumination by 365 nm light, there is a shift of the optimal layer mass of WO3 to 0.5 mg/cm2 (700 nm). This shift is caused by smaller absorption coefficient/longer light penetration depth for 365 nm in comparison with 314 nm, e.g. 1/˛ of WO3 for 365 nm equal to 500 [19] and 250 nm [20], respectively. In the case of 404 nm, there is an increase of photocurrent with WO3 layer mass up to 0.5 mg/cm2 (700 nm) followed by plateau up to 2 mg WO3 /cm2 (2800 nm). This means that the optimal film thickness is between 700 and 2800 nm which is in agreement with
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Table 1 Optimal film thickness and relation to light penetration depth. Irradiation wavelength (nm)
Optimal film mass (mg WO3 /cm2 )
Theoretical optimal film thickness (m)
0.2 0.5 0.5–2
0.28 0.70 0.7–2.8
Light penetration depth (m) 1/˛ [19]
314 365 404
the lowest reported absorption coefficient of WO3 film (in relation to 365 and 314 nm) that gives the light penetration depth equal to 1100 [19] and 500 nm [20], respectively. In the case of SE illumination, photocurrents increase for both 365 and 404 nm with WO3 layer mass. For layer mass 0.1–0.5 mg/cm2 , photocurrents are lower than those for EE illumination because of FTO glass transmittance (0.72 and 0.81 for 365 and 404 nm, respectively). When the photocurrent is corrected on the light absorbed by FTO (divided by 0.72), we obtain for 365 irradiation a hypothetical photocurrent around 55 A which is about 20% higher than for EE irradiation (≈45 A). But for layer mass 2 mg/cm2 , photocurrent for SE illumination is for both 365 and 404 nm higher than for EE illumination. This can be explained by the fact that the film thickness is significantly higher than the light penetration depth and the negative effect of FTO glass absorbance is eliminated by the positive effect of SE irradiation. This is more significant for 404 nm where the photocurrent for EE irradiation is 32 A/cm2 while that for SE irradiation corrected on light absorbed by FTO (divided by 0.81) is equal to ≈45 A/cm2 (about 40% higher). For irradiation by AM1.5G light source, the situation is similar to the irradiation wavelength 404 nm. Only values of photocurrents are nearly 10 times higher because of the significantly different light intensity and emission spectra.
3.2.3. Stability of photoelectrochemical performance Chopped light polarization curves of particulate layers (0.1 mg WO3 /cm2 ) annealed at different temperature were measured repeatedly (six times) to see the performance stability. Fig. 7 shows the decrease of the photocurrents (after six repetitions). Only the sample dried at 60◦ and the sample annealed at 300 ◦ C show very low stability of photocurrent which can be due to the loss of particles. The minimal decrease of photocurrent was observed for annealing temperatures 450 and 500 ◦ C where the loss of particles from layer is minimal due to particle sintering. Further decrease in stability for 600 ◦ C could be caused by the presence of undesirable phases produced by annealing (see Section 3.1).
0.1 0.5 1.1
1/˛ [20] 0.07 0.25 0.5
Fig. 8. Time evolution of H2 and measured photocurrents, 0.5 mg WO3 , 1 cm2 , AM1.5G, 1 sun.
3.3. Water splitting The time evolution of hydrogen formation on Pt counter electrode (while using prepared WO3 particle layer calcined at 500 ◦ C as working electrode for oxygen evolution) and corresponding photocurrents are shown in Fig. 8. After 2 h of irradiation the concentration of hydrogen in output gas (1 ml/min, Ar as carrier gas) is stabilized at 300 ppm (i.e. 0.3 l H2 /min). Interestingly, the current corresponding to the production of hydrogen is only 60% of the measured photocurrent. It means that around 40% of produced current is consumed at the cathode for another reduction reaction, e.g. oxygen reduction. Even the hydrogen concentration remains constant, photocurrent further decreases. Possible explanation for the current decrease can be in the loss of particles which can be higher than in the case of repeated short time photocurrent measurements (Fig. 7). The expected decrease in hydrogen concentration (due to photocurrent decrease) was not observed due to the decreased concentration of dissolved oxygen resulting in the increase in hydrogen current efficiency (from 60% at 2 h to 70% at 4 h).
4. Conclusions
Fig. 7. Relative decrease of the photocurrents after repeated use of particulate WO3 (0.1 mg WO3 /cm2 ), 365 nm, 1 mW/cm2 , EE interface.
The annealing of deposited particulate WO3 layers at temperatures 450–500 ◦ C results in better adhesion of particles to the FTO substrate and significant increase in photocurrent. The annealing at 600 ◦ C caused the formation of undesirable crystal phases (confirmed by XRD) and the significant decrease of photocurrent. For WO3 , mass of layer, corresponding to the maximal photocurrent, increases with the increasing wavelength of irradiation light in the range from 314 to 404 nm. The substrate/electrode side of illumination is advantageous for the films with higher mass of the photocatalyst (2 mg WO3 /cm2 ) and for the light source containing only visible or significant part of visible light.
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