Etching treatment of vertical WO₃ nanoplates as a photoanode for enhanced photoelectrochemical performance

Abstract

Vertically grown WO₃ nanoplates (WO₃NP) were successfully fabricated by a one-step hydrothermal process using citric acid as a structure directing agent. An innovative etching method was developed to obtain increased surface voids, active crystal facets and surface groups simultaneously, which led to a remarkably improved photocurrent density of ∼1.2 mA cm⁻² at 1.23 V vs. RHE, compared to 0.97 mA cm⁻² of pristine WO₃. Incident photon to current efficiency (IPCE) measurements also displayed a substantive increase of photoresponse in the intrinsic absorption range. Interestingly, a lower onset potential can be obtained after etching which is caused by the change of conduction and valence band positions. Moreover, the photoelectrocatalytic activity of WO₃ for degrading methylene blue (MB) was also evaluated. This effective design could provide a promising method to enhance the efficiency of photoelectrochemical performance based on WO₃ photoanodes.

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Introduction

Photoelectrochemical (PEC) water splitting using semiconductor thin films for solar hydrogen production has attracted considerable attention since 1972.^1,2 For the overall water splitting reaction involving two half reactions, the oxidation evolution reaction involving a four electron process is the slower one.³ Hence, development of efficient photoanodes for water oxidation is a critical subject. Among a variety of photoanode materials, WO₃ is regarded as one of the best candidates due to its suitable energy bandgap (2.5–2.8 eV) for optical absorption, high electron mobility (∼12 cm² V⁻¹ S⁻¹), long hole diffusion length (∼150 nm) and stability against photocorrosion in acidic solution.^4–8 Considerable efforts have been focused on improving the photoelectrochemical activity of WO₃, including doping of metal and non-metal elements, nanostructure engineering and crystal facet design.^8–15 It has been reported that vertically aligned WO₃ with a relatively small fraction of grain boundaries offers an effective channel for the directional transfer of electrons, which can efficiently prevent the recombination of photogenerated electrons and holes.^16–19 At the same time, two-dimensional nanostructures have drawn wide attention as they normally can provide large surface areas and low reflectance.^20–24 Hence, vertical plate-like WO₃ is a promising nanostructure for electrode films which have been well reported.^25–27 Yang et al. synthesized plate-like array WO₃ film using ammonium oxalate as a structure-directing agent without the assistance of a seed layer which showed excellent PEC performance.²⁵ Jiao et al. using sodium sulphate, ammonium sulphate and ammonium acetate selectively prepared three kinds of plate-like WO₃ films.²³ Su et al. reported that uniform WO₃ square nanoplate powders can be fabricated via the assistance of citric acid which changes the free energies of the different faces.²⁸ On the other hand, the performance of a semiconductor photocatalyst is known to vary, depending on its surface conditions such as surface atomic and electronic structures, and the adsorption and desorption of reactants, as well as surface abundance states.^29–31 Thus, the control of crystal facet growth is an important means to improve photocatalysts’ performance, which can lead to different energy band levels.^32–35 For instance, Xie et al. reported that a monoclinic WO₃ crystal with (002), (200), and (020) facets shows a much higher photocatalytic activity for O₂ evolution.³⁶ Zheng et al. fabricated WO₃ nanoplate films with a dominant crystal facet of (002) which gives a better photoelectrochemical performance.¹⁴

Wet chemical etching is considered a convenient and cost-effective method to increase surface area, providing more active sites for faster charge transfer.^37,38 In particular, it had been reported that the overpotential for oxygen and hydrogen evolution can be lowered after etching treatment, leading to a better solar-to-hydrogen production yield.^17,39,40 Thioacetamide (TAA) is usually used to provide a sulphur source and convert metal oxides into metal sulphides through an ion exchange process.^41,42 For instance, Wei et al. assembled hierarchical Bi₂S₃ by a multi-level bottom-up-then-up-down route.⁴¹ The obtained hollow-spherical nanostructures with networks of interweaving Bi₂S₃ nanowires are from the topotactic transformation of BiOCOOH nanosheets. Han et al. synthesised ZnO/ZnS/ZnIn₂S₄ core/shell type nanoarrays by a hydrothermal chemical conversion method.⁴³ In this case, TAA helps form a ZnS buffer layer on the surface of ZnO and simultaneously transforms nanorods into nanotubes.

Inspired by these above studies, herein we report a new vertical WO₃ nanoplate derived from a one-step hydrothermal method using citric acid as a structure-directing agent without the assistance of a seed layer under a mild temperature. Considering the ion exchange function of TAA, an etching treatment using TAA was developed. Instead of forming tungsten sulfide, a new type of WO₃NP was obtained with increased surface roughness which is beneficial for increasing the interfacial area. Meanwhile, the intensity of active crystal facets for photocatalysis belonging to monoclinic WO₃ prominently improved during this process due to the selective etching effect. The Mott–Schottky plots illustrate that the etching treatment decreases the band edge of WO₃. These multiple synergistic effects contribute to boosting the PEC performance of the WO₃NP.

Experimental

Synthesis

All chemicals were analytical grade without further purification. Sodium tungsten dehydrate (Na₂WO₄·2H₂O) and hydrochloric acid (HCl) were purchased from Ajax Finechem and Emsure, respectively. Citric acid and thioacetamide (TAA) were purchased from Sigma-Aldrich. Conductive fluorine-doped tin oxide (FTO, thickness around 2.3 mm, 15 Ω sq⁻¹) glasses were used for all working electrodes and purchased from a Chinese company. All water used in the experiments was Milli-Q water (18.2 MΩ).

WO₃ plate-like films were synthesized by a hydrothermal method using Na₂WO₄ as the precursor and citric acid as the structure-directing agent. In a typical synthesis, the precursor solution for hydrothermal treatment was prepared by dissolving 0.093 g of Na₂WO₄·2H₂O into 12 mL of Milli-Q water under constant stirring at room temperature. Then, 2 mL of 3 M HCl was added drop by drop with stirring for 10 min, followed by the addition of 0.196 g of citric acid and another 13 mL of deionized water into the above suspension. After continual stirring for 20 minutes, the as-prepared precursor was then transferred into a Teflon-lined stainless autoclave (50 mL volume). The FTO glass substrate was ultrasonically cleaned by acetone, ethanol and water followed by drying in a nitrogen stream and then immersed in the autoclave and leaned against the wall. Then, the autoclave was sealed and kept at 120 °C for 12 h. After cooling down to room temperature in the oven, the FTO substrate was taken out and rinsed with ethanol and Milli-Q water and then dried in a nitrogen stream. Finally, the as-prepared thin substrate was calcined in air at 500 °C for 2 h. An etching solution was prepared by dissolving 0.3 g of TAA into 40 mL of H₂O, and then a WO₃ plate-like film substrate was placed into the solution and kept at 90 °C for 10 h.

Characterization

X-ray diffraction (XRD) patterns were performed on an XRD spectrometer (Rigaku Miniflex with copper-Kα radiation) and the 2θ angle was between 20 and 80°. The morphologies and microstructures of the WO₃ plate-like films were investigated by a scanning electron microscope (SEM, JEOL, JSM-7001F) and a Transmission Electron Microscope (TEM, JEOL, 1010). Fourier-transform-infrared (FT-IR) measurements were obtained by a spectrophotometer (NICOLET, 6700, USA). Optical absorption was measured on a Shimadzu UV-2450 UV-Vis spectrophotometer in the range 300 to 800 nm.

Photoelectrochemical measurements

The photoelectrochemical measurements were performed in a standard three-electrode photoelectrochemical cell with a quartz window and tested on a CHI660 electrochemical workstation (CH Instruments). A Pt and an Ag/AgCl electrode were used as the counter and reference electrode respectively. An aqueous solution containing 0.5 M H₂SO₄ was used as the electrolyte. A Xenon lamp (150 W, Newport) was used to simulate sunlight and the photocurrent was measured under solar AM 1.5G illumination (100 mW cm⁻²). IPCE measurements were carried out using the Xenon lamp and a monochromator with a bandwidth of 20 nm. The illumination area was 0.785 cm². The degradation experiments were performed in a standard three-electrode reactor with 100 mL MB (5 mg L⁻¹) aqueous solution. The solution was stirred in darkness to achieve an adsorption–desorption equilibrium in the first 30 min. Then the photoelectrocatalytic degradation of MB was carried out with a potential voltage of 1.0 V under UV-vis light. The dye concentration was measured every 20 min by a Shimadzu UV-1800 spectrophotometer at a wavelength of 664 nm. All PEC experiments were purged with a nitrogen stream for 30 min before the electrochemical tests at room temperature.

Results and discussion

Morphology and characterization

After the hydrothermal process and then calcining in air, the FTO glass is covered with a uniform and thin layer which presents a light yellow colour. Fig. 1 shows SEM and TEM images of the WO₃NP films before and after the etching treatment. A high density and uniform vertical alignment of WO₃ plates covers the FTO substrate. The WO₃NP exhibit an edge length in the range 200 nm to 500 nm and a thickness of 50–100 nm. The SEM cross-section image of WO₃NP films (insert in Fig. 1a) indicates that the thickness of the structured thin film is approximately 600–800 nm. There is no obvious difference between the pristine WO₃NP and the etched one in the top-view images at low magnification (Fig. 1a and b), while a rougher surface can be seen in Fig. 1c and d. The TEM images show that the etched film has a porous morphology with holes of various sizes (Fig. 1f). Compared with the relatively uniform and unwounded surface of the pristine one in Fig. 1e, it is suggesting that TAA is effective in etching the WO₃NP to generate holes on the surface. The different void sizes may be attributed to the selective etching of TAA caused by the differences in crystal structure and surface energy on the WO₃ surface.^17,44,45 More images after the etching treatment are shown in Fig. S1.†

Fig. 1 SEM (a and c) and TEM (e) images of pristine WO₃NP; SEM (b and d) and TEM (f) images of the etched WO₃NP; inset in (a): SEM image of the cross section of the films.

The XRD patterns of the WO₃ platelet can be well indexed as the typical crystalline phase of monoclinic WO₃ (JCPDS no. 43-1035) with 23.1° (002), 23.7° (020) and 24.4° (200) planes as shown in Fig. 2. The etched WO₃NP show no observable shifts of the main characteristic diffraction peaks compared to the pristine one, indicating that the main crystal structure is preserved after the etching process. Meanwhile, it is noteworthy that the intensity of the characteristic (020) and (200) peaks, which have been reported as the active crystal planes for photocatalysis, became obviously strengthened. Particularly, the intensity of the (200) peak is almost three-fold higher compared with that of the pristine one, suggesting that TAA could be effective for selectively exposing the (200) planes.³⁶

Fig. 2 XRD patterns of the WO₃NP films: as-prepared (a) and after etching (b).

The energy-dispersive X-ray spectroscopy (EDX, ESI Fig. S2†) of the etched WO₃NP film demonstrates that W, O, N, S, and Sn elements are contained in the product. The Sn element has come from the FTO glass and small amounts of N and S elements have come from the hydrolysis of TAA. The FT-IR spectra of the samples were also employed to confirm the composition information, as shown in Fig. 3. The absorption band of pure WO₃NP in the range of 600–1000 cm⁻¹ belongs to the characteristic stretching vibration of W–O–W groups.⁴⁶ After the etching process, the absorption peaks at wavenumbers of 1184, 1443, 1634 and 3401 cm⁻¹ became noticeable. The absorption peaks at 3401 and 1443 cm⁻¹ are attributed to the asymmetric and symmetric stretching vibration modes of N–H. The absorption at 1634 cm⁻¹ originates from the stretching deformation vibration of molecular water. The residual C–S band at 1184 cm⁻¹ may be caused by an interaction between WO₃ and TAA.^47,48

Fig. 3 FT-IR spectra of the WO₃NP films: as-prepared (a) and after etching (b).

Fig. 4 illustrates the UV-visible optical absorption spectra of the pristine WO₃NP and etched WO₃NP films. The optical absorption edge of the pristine one is estimated to be around 456 nm, while that of the etched WO₃NP shows a slight red shift to 468 nm and a little enhancement in the visible light region. The band gap energy (E_g) can be further extrapolated using the Tauc formula:

αhv = A(hv − E_g)ⁿ (1)

where h is Planck’s constant, ν is the frequency of light, A is a constant, and n is equal to 2 for an allowed indirect transition or 1/2 for an allowed direct transition.⁴⁹ For WO₃, the constant is 2. The calculated E_g value of pure WO₃NP film is about 2.72 eV.⁵⁰ A slight decrease of the band gap (∼0.1 eV) can be observed for the etched WO₃NP, which should be beneficial to harvesting more visible light. This decrease is attributed to residual groups such as N–H and C–S groups on the surface of WO₃NP which can be identified by the FT-IR spectra.

Fig. 4 UV-vis absorption of pristine WO₃NP and the etched WO₃ film.

Photoelectrochemical study

Photocurrent density curves for pristine WO₃NP and the etched WO₃NP film were employed in a potential range from 0.2 to 1.8 V vs. Ag/AgCl (0.4 to 2.0 V vs. RHE) at a scan rate of 5 mV s⁻¹ and are presented in Fig. 5a. The current densities of the films were negligible when the light was chopped. The highest photocurrent density of the pristine WO₃NP is 1.3 mA cm⁻² (Fig. 5a, black curve), comparable to previous reports.²³ The etched one (red curve) exhibits the highest photocurrent density of ∼1.44 mA cm⁻², indicating the etching effects on the photoelectrochemical activity of WO₃. Encouragingly, a lower onset potential of the photocurrent was obtained upon the etching process, which is important to increase the photocurrent density and to achieve the realization of p/n PEC water splitting devices.^40,51,52 Fig. 5b shows the transient photocurrent responses with chopping light at 1.23 V (vs. RHE). The photocurrent decreases to almost zero rapidly as soon as the light turns off, and returns to a certain value when the light is on again. Particularly, the photocurrent density of the etched WO₃NP is 1.2 mA cm⁻², which is almost 20% higher than that of the pristine one (0.97 mA cm⁻²). During the process of etching, the voids on the surface appear and make more crystal lattices and active crystal planes become exposed. The exposed photocatalytic active crystal planes contribute to the improvement of the photocurrent density. To investigate the strong correlation between etching and the enhancement of the PEC performance, Mott–Schottky analysis was conducted with a frequency of 1 kHz in a 0.5 M H₂SO₄ aqueous electrolyte. Both samples exhibited positive slopes, demonstrating that the semiconductors were n-type (Fig. 5c), which means that electrons served as the majority of carriers in the materials.⁵³ In addition, the flat band potential (V_FB) and the donor density (N_D) can be calculated from the intercept and slope respectively from the plots. By extrapolating the X-intercepts of the linear region, V_FB of WO₃NP and the etched one are found to be 1.16 and 1.02 V (vs. RHE), respectively. Such a negative shift of the flat band potential indicates a higher carrier concentration, fewer barriers for oxygen evolution, and better separation of the photo-generated carriers after the etching process which allows more efficient charge transfer from the WO₃NP film to the FTO current collector.^47,54 The conduction band can be estimated since it has been reported that the bottom of the conduction band in many n-type semiconductors is more negative by 0.1–0.3 V than the flat band potential.^55,56 Based on the result of V_FB and the bandgap values of the semiconductors, the conduction and valence band (CB and VB) positions of WO₃ can be roughly estimated to be 0.86 and 3.58 V vs. NHE, whereas the CB and VB after the etching process can be calculated to be 0.73 and 3.35 V vs. NHE respectively. The negative shift of the conduction band after etching verifies the lower onset potential of the photocurrent. The photoelectrochemical properties of photoanodes can also be evaluated by the light energy to chemical energy efficiency as follows:

ε(%) = J_p{(E⁰_r − |E_a|)}/I₀ × 100 (2)

where J_p is the photocurrent density acquired from the photocurrent density curves. E⁰_r and E_a are the standard reversible potential and applied potential of the working electrode, respectively. I₀ is the power density of the incident light.⁵⁷ As shown in Fig. 5d, the maximum photoconversion efficiency of the etched WO₃NP film (0.383%) is greater than that of the pristine WO₃NP (0.216%). The PEC stability of the photoelectrode can be seen in Fig. S3.† After 60 min testing, the performance can still be maintained at 72.3% (from 1.20 to 0.86 mA cm⁻²). The result is better than that reported by Li et al. and Yagi et al.^58,59

Fig. 5 (a) Photocurrent density curves for pristine WO₃NP and the etched WO₃NP film; (b) photocurrent–time plots with chopping light at 1.23 V (vs. RHE); (c) Mott–Schottky plots of the as-prepared samples; (d) photoconversion efficiency of the as-prepared samples.

To evaluate the external quantum efficiency of the samples at different wavelength regions, the incident photo to current conversion efficiency (IPCE) measured at 1.23 V vs. RHE in 0.5 M H₂SO₄ solution is displayed in Fig. 6. IPCE can be calculated by the equation below:

IPCE = 1240I/(λJ) (3)

where I is the measured photocurrent density (mA cm⁻²), J is the irradiance at a specific wavelength (mW cm⁻²), and λ is the wavelength of incident light (nm).²¹ The pristine WO₃NP have a photoresponse in the near-UV region with a lower IPCE value of ∼35%, whereas the etched one has a higher IPCE profile (∼60%), indicating that the effect of etching contributes to more active PEC reaction sites.

Fig. 6 IPCE spectra of the two types of WO₃NP films measured at 1.23 V vs. RHE.

The photoelectrocatalytic properties of the two WO₃NP films are also investigated using MB as the target pollutant (Fig. 7). In Fig. 7a, it can be observed that nearly 70% of MB was removed by the etched WO₃NP film within 150 min, which shows a 10% improvement compared with that of the bare WO₃NP film. In order to quantitatively measure the degradation process, the kinetics was evaluated using the pseudo-first-order kinetics equation (ln(C₀/C) = kt).⁶⁰ C₀ is the concentration of MB after absorption in the dark and C is the concentration at every interval time. t is the illumination time. k reflects the reaction rate constant derived from the slope of the equation. The reaction rate constant is shown in Fig. 7b; the photoelectrocatalytic rate constant for the etched WO₃NP films is 0.9 × 10⁻² min⁻¹, while the bare one is only 0.7 × 10⁻² min⁻¹.

Fig. 7 (a) The photoelectrocatalytic degradation of MB under UV-vis light illumination at an applied bias of 1.0 V; (b) the reaction kinetics curves.

Conclusions

In summary, we synthesized a new type of vertical plate-like WO₃ array by a simple hydrothermal method and etching treatment. This etching process modified the surface of the nanoplates with increased surface voids, as well as an enhanced intensity of photocatalytically active crystal planes. Subsequently it led to a lower onset potential caused by the change of band positions. The photocurrent density and photoelectrocatalytic degradation performance in the WO₃NP films were also enhanced. Hence, the chemical etching treatment using TAA provides an efficient approach to design highly efficient photoanodes.

Acknowledgements

The financial support from Zhejiang scientific and technological projects (no. 2009R50002-20) is gratefully acknowledged. Zhefei Zhao thanks the ARC Centre of Excellence for Functional Nanomaterials.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra11750c

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