# Photo-Oxidation of Water Using Nanocrystalline Tungsten Oxide under Visible Light

Authors: J. W. J. Hamilton; J. A. Byrne; P. S. M. Dunlop; N. M. D. Brown
Journal: International Journal of Photoenergy
 (2008)

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## Abstract

The photoelectrolysis of water to yield hydrogen and oxygen
using visible light has enormous potential for solar energy harvesting if suitable photoelectrode materials can be developed. Few of the materials with a band gap suitable for visible light activation have the necessary band-edge potentials or photochemical stability to be suitable candidates. Tungsten oxide (Ebg 2.8 eV) is a good candidate with absorption up to λ≈440 nm and known photochemical stability. Thin films of tungsten oxide were prepared using an electrolytic route from peroxo-tungsten precursors. The tungsten oxide thin films were characterised by FESEM, Auger electron spectroscopy, and photoelectrochemical methods. The magnitude of the photocurrent response of the films under solar simulated irradiation showed a dependence on precursor used in the film preparation, with a comparatively lower response for samples containing impurities. The photocurrent response spectrum of the tungsten oxide films was more favourable than that recorded for titanium dioxide (TiO2) thin films.  The WO3 photocurrent response was of equivalent magnitude but shifted into the visible region of the spectrum, as compared to that of the TiO2.


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## Body

## 1. Introduction

In 1969, Fujishima and Honda
reported the photoelectrolysis of water using single crystal rutile titanium
dioxide under UV irradiation [1]. Since then, the use of titanium dioxide photocatalysis
for environmental remediation of air and water has been intensively explored
and is well documented [2]. However, TiO2 is a wide band
gap semiconductor (anatase, Ebg = 3.2 eV) and absorbs light of wavelengths ≤387 nm.  Therefore, only a small proportion of the
solar energy spectrum (<5%) can be utilised. In order to facilitate further
advancement in the field of semiconductor photocatalysis using solar energy,
researchers are looking at other narrow band materials which absorb in the
visible region of the solar spectrum. Unfortunately, visible light absorbing
semiconductors are either unstable in contact with water or do not have the
necessary band edge potentials necessary for the desired electrochemistry. Tungsten
oxide has a reported band gap of 2.6 to 2.8 eV [3] with band edges located at Ecb − 0.3 V and EVB + 2.4 V at pH 7 [4],
making it a possible candidate material for the photoelectrolysis of water
using visible light. The preparation of WO3 is relatively well
explored and its use for the photoelectrolysis of water has also been reported
[5–14]. Additionally,
doped WO3 and multilayered photocatalysts incorporating WO3 have been investigated [14–16].However, the majority of
publications concerning preparation and characterisation of WO3 have
produced thin films by sol-gel synthesis. The use of sol-gel synthesis with
tungsten alkoxides has been reported to produce porous films with excessive
carbon impurities [17]. Zhitomirsky et al. previously reported that many metal
oxides, in which the metal of parent compound formed polyperoxoacids on
reaction with hydrogen peroxide, could be electrolytically deposited as oxide
thin films [18, 19]. Although they did not specifically report the electrolytic
deposition of tungsten oxide, other workers have used such a method to produce
electrochromic materials [20].In this work, peroxotungstic
acids were produced from a range of precursors. Previous literature [21] has
reported the production of peroxotungstates from otherwise highly stable
carbide under the action of H2O2. In this work tungsten
oxide, tungstic acid, and tungsten dodecaphosphate were used as precursors for the production of
polyperoxoacids of tungsten by digestion in H2O2, from
which thin films were prepared. Films were characterised by FESEM, AES, and
photoelectrochemical methods with comparison to sol-gel TiO2 films
as a reference standard.


## 2. Experimental

Tungsten oxide thin films
were produced via a modified electrolytic route. Peroxoprecursors were prepared
by dissolving 2.5 g of tungsten oxide, tungstic acid (Sigma-Aldrich Company Ltd., The Old Brickyard, New Road, Gillingham, Dorset
SP8 4XT), or tungsten dodecaphosphate
(BDH) in 15 cm3 of H2O2 (30%) and 10 cm3 H2O. The precursor slurries were heated at 70°C for 10
minutes to dissolve all the powder starting material, generating a viscous
yellow peroxotungsten precursor. During the powder digestion reactions, all
tungsten precursors exchange their ligands for peroxo ligands. The peroxotungsten species
produced by reaction of tungsten compounds with hydrogen peroxide are a complex
mixture. These include monotungstate species WO2(O2)22−, WO(OH)(O2)22−, bitungstate
species W2O3(O2)42−, and more complicated species
[22]. For this reaction scheme, only
the simplest species produced in the initial reaction is considered (see (1)).
During the extended heating, tungsten species undergo
condensation/polymerisation reactions, similar to those of a sol-gel precursor
(see (2)) [22]. These condensation reactions form complex chains and other
structures of repeating peroxotungstate units resulting in an increase in the
viscosity of the solution:(1)W−LX+3H2O2⟶WO2(O2)22−+6H++LXY−,(2)2WO2(O2)22−+2H+⟶W2O3(O2)22−+H2O. WO3 films were formed by electrochemical oxidation of the peroxotungstates on
tungsten foil and gas evolution were observed at the anode during
film formation. The presumed anodic half cell reaction is given in (3):
(3)2WO2(O2)22−⟶2WO3+3O2+4e−.
For AES analysis, films were
deposited on titanium metal foil to eliminate the tungsten signal of the
substrate from interfering. Anodic electrolytic deposition was carried out
under a constant current of 10 mA for 200 seconds on 1.5 cm-2 tungsten foil coupons.  The deposition
time was reduced to 60 seconds on titanium foil samples (1.5 cm2),
for elucidation of the growth mechanism. Constant current anodic deposition was
used as this gave more reproducible films in terms of microstructure. After
electrolytic treatment, the films were washed with distilled water and dried in
air. The tungsten foil samples were annealed at 500°C for 1 hour
with a temperature ramp of 5°C min-1.Titanium dioxide thin films
were prepared by a sol-gel route adapted from Murakami et al. [23]. Butan-1-ol
(10 cm3) was added to a round bottom flask, which was placed on a
balance and the balance rezeroed. Using a syringe, titanium butoxide was
removed from its sealed bottle via the rubber septum. With the tip of the
syringe immersed in the butanol, 0.377 g of butoxide was added to the flask. A
magnetic stirring bar was added and the flask was sealed with Parafilm™. The
round-bottomed flask was then transferred to a stirrer. Ammonium acetate
(0.0973 g) was dissolved in 0.2 cm3 of distilled water in a micro
beaker. The contents of the beaker were then washed into a clean, dry,
separating funnel with 15 cm3 of butan-1-ol and shaken to mix the
two solvents. The Parafilm™ was then removed from the neck of the
round-bottomed flask and the dropping funnel containing water, solvent, and
salt catalyst was attached at the quick fit junction. The contents of the
dropping funnel were added dropwise under constant
stirring. The reaction solution was finally aged for 1 hour before dip coating
(withdrawal rate 6 mm s-1).Tungsten oxide samples deposited
on titanium foil were imaged by field emission SEM and elemental analysis was
carried out by Auger electron spectroscopy (Perkin-ElmerΦ 660) with vacuum of 10-6 Torr with
electrons accelerated from a field
emission source at 4 kV accelerating
voltage to give a beam current of ∼2 μA.For electrochemical measurements,
electrical contacts were made by attaching copper wire with conductive silver
epoxy (Circuit Works, Chemtronics) to the back of the foil (cleaned to remove
oxide) and insulated using with negative photoresist (KPR resist, Casio
Chemicals) to give an exposed oxide area of 1 cm2.Electrochemical
characterisation of samples was performed under potentiostatic control (Autolab
PGSTAT30) in a one-compartment glass cell containing a quartz window with a
platinum basket counter electrode and a saturated calomel electrode (SCE)
reference electrode. All potentials are reported versus SCE. The supporting
electrolyte was 0.1 M LiClO4 and irradiation was provided by a 1 kW
Xe arc lamp (ss1000 Spears Robinson) with an AM1 filter to simulate solar
conditions. For monochromatic irradiation experiments, a monochromator (GM252,
set for 10 nm band pass) was positioned between the source (unfiltered) and the
photoelectrochemical cell. Linear sweep voltammograms were measured scanning
from −1.0 V to +1.5 V
at a scan rate 10 mV s-1. The current-time response was measured at
+1.0 V using chopped illumination (chopper Uniblitz vmm-t1, Vincent
Associates). The cell temperature was maintained at 25°C ± 2°C.


## 3. Results and Discussion

To examine the electrolytic growth
of tungsten oxide, samples were prepared on titanium foil to ensure that the WO3 thin film grown by the electrolytic process was not simply due to
anodisation of the tungsten foil itself. FESEM analysis showed that the films
were nonuniform with more WO3 deposition around the edges of the
electrode. The uneven deposition observed is not uncommon for electrodeposited films and is due to edge effects.The surface of the sample showed grain-like
structures (Figure1). Features were selected for elemental analysis (Figure 1,
points 1–3) using AES and
the corresponding spectra are given in Figure 2.Figure 1
SEM showing WO3 film and locations selected for AES analysis.Figure 2
AES spectra from points selected in Figure1.The surface showed
significant amounts of tungsten and oxygen in addition to the titanium of the
underlying support electrode. The concentration of elements in the selected
points is given in Table1.Table 1
Elemental
by AES analysis of points in SEM (Figure1).
%COTiWPoint 133.2432.5119.1315.12Point 225.2642.4328.104.20Point 325.8341.8724.058.26This analysis showed the WO3 to have grown in islands with the highest concentration of tungsten measured in
the grain-like features (point 1 in Figure 1 and Table 1). This is also shown
in elemental colour mapped SEM images produced using AES (Figure 3). In the
colour mapped images, the lighter colours indicate higher concentrations of the
element and darker colours representing lower concentrations, and black
indicating the absence of the element.Figure 3
Colour coded SEM images showing the elemental distribution on the surface.Imaging of samples produced from
other precursors (tungsten oxide, tungsten dodecaphosphate) showed a similar
growth with islands of material building up preferentially at the edges. The samples
produced from the tungsten dodecaphosphate precursor contained a small quantity
of phosphorus (see Figure4).Figure 4
AES spectrum for the WO3 film prepared from the dodecaphosphate precursor showing a small signal due to phosphorous contamination.The photoelectrochemical
characterisation of the films using LSV showed a typicaln-type response with
no anodic current observed in the dark and photocurrent a potentials more
positive than 0.0 V (Figure 5). The films produced from the tungstic acid
precursor gave the highest relative photocurrent.Figure 5
Comparison of photocurrent response of WO3 film electrodes prepared from different precursors, that is, tungsten dodecophosphate (green line), tungsten oxide (blue line), tungstic acid (red line) and with dark current response (dark blue).Comparison of
photocurrent densities produced from WO3 thin films and TiO2 at
+1.0 V are given in Table 2. The WO3 films had a higher photocurrent
density than that of the TiO2 thin films prepared via a sol-gel
route under solar simulated irradiation.Table 2
A comparison of photocurrent densities produced
under solar simulated conditions at +1.0V.
MaterialTungsten oxideTitanium dioxide via sol-gel routePrecursorTungsten oxideTungstic acidTungsten dodeca phosphatePhotocurrent (mA cm-1)0.6740.9740.1340.266Figure6 shows the current-time response of the tungstic
acid derived WO3 film under monochromatic irradiation showing a
small photocurrent response in the visible (λ > 420 nm—not normalised to monochromator output).Figure 6
Photocurrent response of WO3 under monochromatic irradiation (bandwidth 10 nm).Using light
intensity measurements the photocurrent produced under monochromatic
irradiation was used to calculate
incident photon to current conversion efficiency.(4)%IPCE=(JI0F)×100, where J is the photocurrent density, I0 is the incident light
intensity (band pass 10 nm), and F is
Faraday’s constant. A comparison of the photon conversion efficiency between WO3 and TiO2 thin films is given in Figure 7 which shows both materials
exhibited ∼7% efficiency at peak wavelengths 355 nm
and 380 nm, respectively.Figure 7
Comparison of the %IPCE of TiO2 (pink line) and WO3 (blue line) as a function of wavelength.The response spectrum of WO3 extended to longer wavelengths into the visible region of the spectrum.


## 4. Conclusions

Surface characterisation of WO3 films prepared from peroxo-polytungstate
precursors confirmed the growth of WO3. Photoelectrochemical
measurements under simulated solar irradiation showed that the WO3 films behaved as n-type semiconductors producing anodic current only under
irradiation. Comparison of samples produced from different precursors showed
that the films prepared using the tungstic acid precursor gave the highest
relative photocurrent. The lower photocurrent from the films prepared from
tungsten dodecaphosphate could be attributed to phosphorus impurities in the
film. The WO3 films gave a higher photocurrent response under
simulated solar irradiation compared to sol-gel derived TiO2 films.
Photocurrent response spectra measured under monochromatic irradiation showed
similar peak efficiency (∼7%) at 355 nm for TiO2 and 380 nm for WO3.
However, the photocurrent spectral response for the WO3 films was
red-shifted into the visible region of the spectrum. Further work is needed to
determine the O2 and H2 yield under prolonged, simulated
solar irradiation.



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*Source: 185479-2008-03-27.xml*