Porous WO 3 with Hierarchical Structure on FTO Substrates : Fabrication and Photoelectrochemical Water Oxidation Performance

WO3 is a type of oxide semiconductor material with bandgap of 2.5-2.8 eV and is considered a promising stuff that can be used in solar light-driven photocatalytic applications. In this work, WO3·0.33H2O films on fluorine doped tin oxide substrates were firstly deposited hydrothermally and then were annealed at 500°C to form WO3. The as-prepared WO3 films possess a hierarchical structure: short WO3 nanorods are assembled into large clusters in a radial form and the large clusters are distributed uniformly on the substrate to form the films. By employing photoelectrochemical etching technique, the WO3 nanorods were further treated to be nanopore-rich structure. When used as photoanodes, the porous WO3 films showed much enhanced photoelectrochemical water splitting performance in comparison with the non-etched WO3 films: under the illumination of simulated solar light and without using any oxygen evolution co-catalysts, the photocurrent increased from 0.06 mA/cm to 0.23 mA/cm at 1.23 V vs. reversible hydrogen electrode and the overpotential decreased from 1.0 V to 0.8 V vs. reversible hydrogen electrode. Such porous WO3 hierarchical nanostructures on conductive substrates based on simple hydrothermal deposition and photoelectrochemical etching are envisioned to provide valuable platforms for other solar-light-driven photocatalytic applications.


I
n recent years, WO 3 has attracted much attention as a photoanode material in photoelectrochemical (PEC) water splitting applications owing to its ~ 12% of solar spectrum absorption ability and the ideal band gap (E g = 2.5-2.8eV) [1][2][3].Compared to α-Fe 2 O 3 , another commonly used photoanode material, whose hole diffusion length is as short as ~ 2.5 nm, WO 3 possesses a moderate hole diffusion length of ~ 150 nm [4].At the same time, the inherent mobility of WO 3 is ~ 12 cm 2 /(V•s) , which is higher than the value of 1 cm 2 / (V•s) for rutile TiO 2 [5].These characteristics make WO 3 a very promising photoanode material for PEC water splitting applications.However, WO 3 also possesses drawbacks like sluggish kinetics of holes, slow charge transfer and rapid electron-hole recombination [6].To overcome such drawbacks, many substantial efforts have been made, in which increasing the specific surface of the WO 3 nanostructures via morphology control is considered to be one direct and effective way [7].For example, porous WO 3 flakes that were formed by treatment of the WO 3 flakes in ascorbic acid together with etching in poly(vinyl pyrrolidone) were reported to achieve much enhanced PEC performance in comparison with the non-etched WO 3 [8].
In this work, we report a feasible photoelectrochemical etching method to achieve porous WO 3 film from nanorod-like WO 3 film on fluorine doped tin oxide (FTO) substrates.Photoelectrochemical water oxidation experiments showed that after photoelectrochemical etching the WO 3 films acquired much enhanced performance.This method could be used to prepare other porous semiconductor nanostructures and thus acquire novel properties.

Sample preparation
WO 3 •0.33H 2 O films on FTO substrates were hydrothermally deposited using a similar procedure reported previously [9].Briefly, 5 mL of Na 2 WO 4 aqueous solution (0.04 M) was mixed with 5 mL of nitric acid (0.10 M) and the mixed solution was poured into a Teflon-lined stainless steel autoclave with a capacity of 15 mL.Clean FTO substrate (20×25 mm 2 ) was put into the autoclave, keeping the substrate against the wall of the Teflon liner and its conductive side facing down.The autoclave was heated at 110 °C for 3 h and then was cooled to room temperature.After washed with deionized water and ethanol, respectively, the film was dried in airhen was heat treated in air at 500 °C for 2 hours.After that, WO 3 film on FTO was obtained.
The photoelectrochemical etching was conducted in 0.1 M phosphate buffer solution (PBS) (pH=7.0)by using an electrochemical analyzer (CHI760D, Shanghai) with the typical three-electrode configuration at a potential of 1.8 V vs. reversible hydrogen electrode (RHE).The WO 3 film on FTO, a Pt plate and an Ag/AgCl electrode were used as the working, counter and the reference electrodes, respectively.A 500 W Xe lamp (AuLight, CEL-S500) coupled with an AM 1.5G filter (AuLight, PD-250 CEL-AM 1.5) was used to provide the simulated solar light with intensity of 100 mA/cm 2 , which was calibrated by using a solar power meter (Thorlabs, PM100USB) with a S302C detector (Thorlabs).The etching process was sustained for 2 h.

Characterization
X-ray diffractometer (XRD) patterns were recorded using a Bruker D8-Focus X-ray diffractometer (Cu K α1 radiation) with a voltage of 40 kV and a current of 40 mA and a scanning speed of 2°/min.The morphology of the samples was observed on a scanning electron microscope (SEM, FEI Quanta FEG 250).UV-Vis diffuse reflectance spectra were recorded using a Shimadzu UV-3101PC spectrometer.

Photoelectrochemical measurements
PEC water splitting measurements were carried out in the same three-electrode electrochemical cell and using the same electrochemical analyzer and Xe lamp mentioned above.The incident photon-to-electron conversion efficiency (IPCE) of the WO 3 photoanodes was measured using the same PEC measurement configuration just instead the simulated solar light with monochromatic light, which was obtained by using a monochromater (Zolix, Omni-λ 3005) coupled with the 500 W Xe lamp and calibrated with the same solar power meter coupled with an S120VC Si detector (Thorlabs).The reversible hydrogen potential can be converted from the potential measured versus Ag/AgCl reference electrode as follows: where E 0 Ag/AgCl is 0.1976 V at 25 °C.

RESULTS AND DISCUSSION
The crystal phases of the films on FTO glass substrates were confirmed by the X-ray diffraction (XRD).Figure 1a (blue pattern) shows the XRD pattern of the hydrothermally deposited film.As can be seen, except for the peaks from the FTO substrate, which are marked with circles, all the other peaks can be indexed to the orthorhombic tungsten oxide hydrate (WO 3 •0.33H 2 O, PCPDF No. 54-1012), and no peaks belong to other than these two materials were found, indicating that the deposited films are composed of pure WO 3 •0.33H 2 O.After annealing, as shown in Figure 1a (red and black patterns), both before and after photoelectrochemical etching, the dominant diffraction patterns are associated to the monoclinic WO 3 (JCPDS No. 83-0950, the bar-chart at the bottom) with very weak diffractions that can be indexed to the hexagonal WO 3 (JCPDS No. 33-1387, shown with black arrows).It suggests that the heat treatment can readily promote the WO 3 •0.33H 2 O to dehydrate to form WO 3 .In addition, in comparison with the diffraction lines of the referenced monoclinic WO 3 bar-chart, the prepared WO 3 samples all reveal much higher relative diffraction intensity for the (200) peak, indicating the existence of preferred orientation along [200] direction [10].Figure 1b-c show the scanning electron microscope (SEM) images of the WO 3 films before and after etching.It shows that the WO 3 film is composed of clusters with size of 3-5 μm, which are apart from each other and are distributed uniformly on the FTO substrate (Figure 1b).The SEM image with larger magnification shown in Figure 1c reveals that the cluster possesses hierarchal structure, which is composed of short nanorods grown in a radical mode.After etching, abundant pores are observed in the nanorods, shown in Figure 1d.The SEM images together with the XRD patters suggest that the photoelectrochemical etching does not change the crystal phase of the WO 3 , just leading to the pore-generation.In addition, Figure 1d also shows the existence of some blocks that do not possess the characteristic of porosity, which presumably is the small amount of the hexagonal WO 3 , indicating that they are not as easy to be etched as the monoclinic WO 3 , a subject that needs further study in the future.
The WO 3 films on FTO substrates were used as photoanodes and the PEC water splitting performance were investigated.Figure 2a shows the linear sweep voltammograms (LSV) of the WO 3 films.It shows that the dark current densities of the samples before and after etching are both small (<5 μA/cm 2 ).Under the illumination, the photocurrents of the both samples increased gradually.Compared to the sample before etching, the etched sample shows much enhanced PEC water splitting oxidation performance.For example, at potential of 1.23 V vs. RHE, the photocurrents of the samples before and after etching are 0.06 and 0.23 mA/cm 2 , respectively.At the same time, the etched sample showed an overpotential of 0.8 V vs. RHE, which is a little bit lower than that of the non-etched sample.The main reason for the photocurrent enhancement is believed to be attributed to the increased surface area of the etched sample due to the porous structure.The decrease of the overpotential is probably attributed to the numerous pores, too, which is considered to be beneficial for the hole transfer [11,12].Further investigations revealed that when the potential was lower than ~1.4 V vs. RHE the etching would not run obviously.Figure 2b shows the photocurrent variation of film during the etching process, which was conducted at a potential of 1.8 V vs. RHE and for 2 h.It shows that initially the photocurrent density increases fast and then slows down, indicating the equilibrium gradually reached.Although it is speculated that the etching itself will probably contribute the photocurrent, considering the fact that the etching would not happen when the applied potential is lower than 1.4 V vs. RHE, the enhancement in PEC water oxidation performance observed in Figure 2a is mainly resulted from the increase of the interface area of WO 3 /electrolyte.
To further understand the relationship between the photoactivity and the light absorption of the porous WO 3 , the incident photon to current conversion efficiency (IPCE) and UV-Vis absorption behaviour were measured.In comparison with the photocurrent density, IPCE is a better way to evaluate the photon absorption and conversion because it is independent from the light sources and filters used in different laboratories [13].IPCEs were calculated using this equation ( 2): where I (in A/cm 2 ) is the measured photocurrent density at a specific wavelength, λ (in nm) is the wavelength of incident light and J (in W/cm 2 ) is the measured irradiance at a specific  wavelength [14].Figure 3 shows the IPCE curve measured at 1.23 V vs. RHE and the UV-Vis absorption spectrum of the sample.The IPCE curve shows that the etched porous WO 3 film has response to the light with wavelength <470 nm.The corresponding UV-vis absorption spectrum also reveals an absorption onset of ~475 nm, which is consistent with the IPCE result.These results agree well with the band gap of WO 3 , i.e., ~ 2.6 eV [15].Additionally, from this IPCE curve it can be seen that the maximum photoresponse occurs in the UV region.For example, to the 320 nm light, an IPCE of ~12.5% is achieved.However, considering the fact that the distribution of the UV light in the solar light spectrum reaching to the earth is not dominant, the substantial response to the visible light shown by this porous WO 3 film contributes the main PEC water oxidation performance.

CONCLUSIONS
In summary, WO 3 films on FTO substrates were prepared firstly by hydrothermal deposition and then were heat treated at 500 °C for 2 h.The as-prepared WO 3 film possessed a hierarchical structure: the film was composed of large WO 3 clusters with size of 3-5 μm and the latter was composed of short nanorods grown in the radical mode.By employing a photoelectrochemical etching process, the WO 3 nanorods were etched to form abundant pores.When used as photoanodes to conduct PEC water splitting under AM 1.5 illumination at 100 mW/cm 2 , the porous WO 3 films revealed much enhanced water oxidation performance as compared to those that were not subject to photoelectrochemical etching.On the ground that WO 3 possesses several merits in the application of visible light-driven photocatalysis, such simple route to prepare hierarchical WO 3 nanostructures on FTO conductive substrates is very significant and it deserves further detailed study.At the same time, it is envisioned to provide valuable platforms for other solar-light-driven photocatalytic applications.

Figure 1 .
Figure 1.(a) XRD patterns of the sample films: the as-hydrothermally deposited (blue), after heat treated (black) and after heat treated and photoelectrochemically etched (red).The black arrows refer to the hexagonal WO 3 (JCPDS No.33-1387).SEM images of the samples before (b, c) and after (d) photoelectrochemical etching.

Figure 2 .
Figure 2. (a) LSV from the WO 3 films before and after etching under AM 1.5G illumination at 100 mW/cm 2 and in dark; (b) relationship of the etching photocurrent to the etching time conducted at a potential of 1.8 V vs. RHE.

Figure 3 .
Figure 3. IPCE and UV-Vis spectrum of the etched WO 3 film.