In situ synthesis of g-C₃N₄/WO₃ heterojunction plates array films with enhanced photoelectrochemical performance

Abstract

g-C₃N₄/WO₃ heterojunction plate array films with enhanced photoelectrochemical (PEC) performance were successfully synthesized through a combination of hydrothermal and dipping-annealing methods. Urea aqueous solutions were prepared as the precursors to in situ synthesize g-C₃N₄ nanoparticles on the surface of WO₃ platelets. The PEC performances of the photoanodes were investigated by the photocurrent density and incident photon-to-current conversion efficiency (IPCE). As-prepared g-C₃N₄/WO₃ heterojunction films achieved a maximum photocurrent density of 2.10 mA cm⁻² at +2.0 V (vs. RHE), which was almost 3-fold higher than that of the pure WO₃ film (0.78 mA cm⁻²) under illumination. And the highest IPCE value increased from 25.1% to 53.1% after the g-C₃N₄ nanoparticles deposition. The enhanced PEC performance was attributed to the increased carriers density, better electron transport properties, longer electron lifetime and effective charge separation at the interface of heterojunction, which were confirmed by Mott–Schottky and electrochemical impedance spectroscopy (EIS). This study demonstrates that the low-dimensional morphological structure and in situ formation of heterojunction structure are expected to provide a promising photoanode for photoelectric catalysis.

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1. Introduction

To alleviate global energy and environment issues, efficient utilization of solar energy has been a key technology to achieve a prospective sustainable-energy society.¹ Since Fujishima and Honda used single crystal TiO₂ as a photocatalyst for the splitting of water under UV-light irradiation in 1972,² photocatalytic and photoelectrochemical (PEC) water splitting on semiconducting materials to convert solar energy into chemical energy has been studied extensively. However, the low solar-to-hydrogen (STH) efficiency for PEC water splitting is currently a bottle-neck that restricts its practical application of photocatalysis.³ For conventional TiO₂ photocatalyst, the narrow optical absorption range is a major problem because of its wide band gap (3.0 eV for rutile and 3.2 eV for anatase).⁴ As we known, a vast majority of the solar spectrum lies within the visible range,⁵ thus, great deals of semiconductors have been investigated, which can absorb more visible light. Among the numerous transition metal oxides, WO₃ has been considered to be a promising alternative for solar water splitting, because of its suitable band gap (∼2.65 eV) for visible light absorption,⁶ good electron transport properties,⁷ resilience to photocorrosion,⁸ good stability in acidic solution,⁹ moderate hole diffuse length (∼150 nm),¹⁰ and a positive enough valence band (VB) edge to provide a sufficient driving force for oxygen evolution.¹¹ Despite this, the efficiency of WO₃ for water oxidation is still low because of its fast photo-generated charge recombination. Some significant efforts have been made to improve the PEC performance, including metal^12–14 or nonmetal doping,^15–17 modification of WO₃ surface,¹⁸ the utilization of nanostructures and low-dimensional morphology.^19–22 It is known that there are increases of films' resistance and interfacial charge recombination in the nanoparticles films due to its numerous grain boundaries.²³ Comparatively, the low-dimensional nanostructured arrays can provide direct pathways for electron transport much faster than in nanoparticles films.^23–25 It has been suggested that the utilization of low-dimensional nanostructured arrays would be a promising approach to achieve better PEC performance.²⁶

Additionally, formation of hybrid heterojunction structures²⁷ has been becoming promising approaches to enhance the photocatalytic performance, which results in the efficient charge separation at the interface,²⁸ such as TiO₂/WO₃,^29,30 NiWO₄/WO₃ (ref. 31) and Fe₂O₃/WO₃.^32,33 Recently, graphitic carbon nitride (g-C₃N₄) has received wide attention since it has been regarded as the first metal-free photocatalyst under visible light illumination.^34,35 In general, g-C₃N₄ is prepared by pyrolysis of cyanamide,³⁶ melamine,³⁷ and more recently discovered thiourea and urea.^38,39 Many studies have confirmed that g-C₃N₄ is a good candidate for forming semiconductor heterojunctions with higher photocatalytic activity.^40–43 Due to its merits of low cost, reliable stability and suitable band gap of 2.7 eV, Zang et al. coupled the g-C₃N₄ with WO₃ nanoparticles, which showed high activity in methyl orange (MO) photodegradation.⁴⁴ Katsumata et al. prepared the g-C₃N₄/WO₃ composite with enhanced photocatalytic activity in H₂ production from triethanolamine aqueous solution,⁴⁵ because the photogenerated electrons of g-C₃N₄ have high reduction ability for hydrogen evolution. As g-C₃N₄ and WO₃ are both visible-light-driven photocatalysts, the g-C₃N₄/WO₃ heterojunction may be a promising candidate for efficient solar water splitting. However, both of the g-C₃N₄/WO₃ composites mentioned above were obtained by mechanical mixing.

Recently, the in situ formation of heterojunction has attracted much attention, which facilitates the interfacial contact of two components, optimizes the charge separation and transfer.⁴⁶ Fang et al. designed and in situ fabricated the BiOI/Bi₂S₃ heterojunction, which showed 3 times higher IPCE than pure BiOI photoanode, attributable to the better photogenerated charge carrier separation and transport efficiency.⁴⁷ Peng et al. used a solvothermal process and in situ growth method to fabricate the Bi₂WO₆/WO₃ heterojunctions with higher photocatalytic activities than pure WO₃ and Bi₂WO₆ for the degradation of rhodamine B (RhB). The heterojunctions could decompose 90% of RhB solution in 20 min under solar light irradiation, while Bi₂WO₆ and WO₃ just decomposed 20% to 30%.⁴⁸

In this work, we chose WO₃ plate-like arrays with direct pathways for electrons transport as the base material and urea solution as the precursor. A heterojunction film of g-C₃N₄/WO₃ was formed on the WO₃ surface in situ through a simple dipping-annealing process. The as-prepared g-C₃N₄/WO₃ heterojunction plates arrays films exhibited enhanced PEC performance. Through the various analysis techniques, a conclusion can be drawn that this g-C₃N₄/WO₃ film has advantages of increased carriers density, improved electrons' lifetime and effective separation of the photo-generated electron–hole pairs.

2. Experimental

2.1 Synthesis of WO₃ plate-like arrays films

The WO₃ plate-like arrays films were prepared according to our previous study.⁴⁹ In the typical hydrothermal synthesis process, as the tungsten source Na₂WO₄·2H₂O was reacted with HCl to form tungstic acid, with (NH₄)₂C₂O₄ as the structure-directing agent. The hydrothermal synthesis process was carried out at 120 °C for 12 h. Then the films were taken out and dried at 60 °C for 30 min.

2.2 Synthesis of g-C₃N₄/WO₃ heterojunction films

A dipping-annealing process was used to in situ synthesize the g-C₃N₄ nanoparticles on the surface of as-prepared WO₃ plate-like arrays films. In this method, different weight of urea (1, 5 and 10 g) dissolved in 50 ml of deionized water were prepared as the precursor. The corresponding g-C₃N₄ content is 1.14%, 2.13%, 3.05%, calculated from the data of EDS (at%, Fig. S1†). Then the WO₃ films were vertically dipped into above solution for 1 h. Finally the as-prepared films were calcined at 500 °C for 1 h under inert atmosphere with a heating rate of 5 °C min⁻¹.

2.3 Characterization

The crystalline phases of the samples were characterized by X-ray diffraction (XRD, D/Max2250, Rigaku Corporation, Japan) with Cu Kα (λ = 0.15406 nm) radiation. The microscopic morphologies were investigated by a scanning electron microscope (SEM, Nova NanoSEM 230) and high resolution transmission electron microscope (HRTEM, FEITECNAI G2 F20). The UV-vis spectra were obtained using a diffused reflectance ultraviolet and visible spectrophotometer (DR-UVS, Shimadzu 2450 spectrophotometer). Fourier-transform-infrared (FT-IR) measurements were performed by a spectrophotometer (NICOLET, 6700, USA). The surface electronic states of the heterojunction films were analyzed by X-ray photoelectron spectroscopy (XPS, K-Alpha 1063, Thermo Fisher Scientific).

2.4 Photoelectrochemical measurements

The photoelectrochemical properties of the samples were characterized by a typical three-electrode electrochemical cell with an electrochemical analyzer (Zennium, Zahner, Germany). The as-prepared films, Pt sheet and Ag/AgCl electrode were used as the working electrode, the counter electrode and the reference electrode, respectively. Hereafter, the electrode potential is reported relative to the reversible hydrogen electrode (RHE), which is converted from the Ag/AgCl electrode using: E_RHE = E_Ag/AgCl + 0.6 V.⁵⁰ The photoelectrochemical measurements were carried out in 0.2 M Na₂SO₄ solution (pH = 7) under AM 1.5 G illuminations. An ultraviolet filter (λ > 400 nm) was used to remove UV irradiation. The voltage linear scanning speed was 20 mV s⁻¹. The Mott–Schottky plots were obtained at the AC frequency of 1 kHz, and the electrochemical impedance spectra (EIS) were measured at 1.4 V (vs. RHE) with the AC frequency of 100 mHz to 10 kHz. Meanwhile, the incident photo to current conversion efficiency (IPCE) was measured with the system consists of a xenon lamp light source (LSH-X150), a monochromator (Omni-λ300), a data collection system (DCS300PA) and a Si detector (QE-B3).

3. Results and discussion

3.1 Characterization of the synthesized photoanodes

The XRD patterns of WO₃, g-C₃N₄/WO₃ and g-C₃N₄ are shown in Fig. 1a. It was observed that WO₃ was indexed as monoclinic tungsten oxide (JCPDS data card no. 43-1035), while two broad diffraction peaks around 27.1 and 13.6° were observed for pure g-C₃N₄, corresponding to the (002) and (100) diffraction planes, respectively. The former is attributed to the long-range interplanar stacking of aromatic units, which corresponds to the interlayer distance of 0.327 nm; the latter with a much weaker intensity, is associated with interlayer stacking, which corresponds to a distance of 0.681 nm.³⁴ There is not any noticeable change in the phase by an introduction of g-C₃N₄, because the concentration of g-C₃N₄ was too low to be detected by XRD. No other peaks can be seen except for the FTO substrate in the composite heterojunction films.

Fig. 1 XRD patterns (a) and FT-IR spectra (b) of the g-C₃N₄/WO₃ heterojunction films.

Fourier-transform-infrared (FT-IR) spectra were employed to further confirm the composition information of g-C₃N₄/WO₃ composite, as indicated in Fig. 1b. The pure g-C₃N₄ is associated with the characteristic absorptions at wavenumbers of 1245, 1321, 1415, 1569 and 1631 cm⁻¹. The stretching vibration of C–N heterocyclics is reflected in the region of 1240–1590 cm⁻¹. The absorption peaks at 1321 and 1631 cm⁻¹ are attributed to both C–N and C=N stretching modes, respectively.⁵¹ When combined with WO₃, the characteristic absorptions of g-C₃N₄ for g-C₃N₄/WO₃ heterojunction are not noticeable, maybe due to its very low content. However, two new absorption peaks appear around 1112 and 2920 cm⁻¹, which demonstrates the existence of interactions between chemical bonds of pure g-C₃N₄ and pure WO₃.

The surface morphology and microstructures of the as-prepared samples were characterized by SEM. As shown in Fig. 2a, the pure WO₃ film reveals that numbers of plate-like arrays grow vertically on the FTO substrate. The average length and thickness of the WO₃ platelets were about 1.3 μm and 300 nm, respectively. The surface of the pure WO₃ platelets is smooth, while the surface of platelets with g-C₃N₄ nanoparticles is rough (Fig. 2b–d). The increased surface roughness, which attributed to the point corrosion of basic urea solution and the deposition of g-C₃N₄ nanoparticles, resulted in more active site in heterogeneous catalysis. The thicknesses of g-C₃N₄/WO₃ films are almost equal with the pure one (∼1.55 μm).

Fig. 2 SEM images of WO₃ films (a), WO₃/C₃N₄-1.14% (b), 2.13% (c), 3.05% (d).

To observe the crystal structures and binding mode of g-C₃N₄ nanoparticles and WO₃ platelets, TEM image of g-C₃N₄/WO₃ heterojunction film is presented in Fig. 3. As can be seen in Fig. 3a, the base material WO₃ has plate-like structures. We can easily recognize the g-C₃N₄ nanoparticle from the HRTEM image (Fig. 3b), which was well attached to the WO₃ platelet. The lattice d-spacing of 3.749 Å, corresponding to the (020) plane was examined by Fourier transform for the WO₃ platelet, and lattice d-spacing of 3.274 Å (002) for the g-C₃N₄ nanoparticle.

Fig. 3 TEM image (a) and HRTEM image (b) of g-C₃N₄/WO₃ films.

The surface chemical composition of g-C₃N₄/WO₃ heterojunction film was detected by XPS. The overall XPS survey spectrum of the sample is shown in Fig. 4a, which shows the existence of all the elements of g-C₃N₄ and WO₃. It is observed that W 4f_7/2 and W 4f_5/2 peaks are located at 35.3 eV and 37.4 eV, respectively (Fig. 4b), which is consistent with the W⁶⁺.⁵² The O 1s main peak in Fig. 4c was dissociated into two peaks: W–O (530.1 eV) and surface hydroxide –OH (531.7 eV).³¹ The C 1s peaks (Fig. 4d) at 284.8 eV and 288.2 eV are ascribed to C–C and N=C–N₂ coordination.⁵³ The asymmetrical N 1s peaks (Fig. 4e) can be deconvoluted into three characteristic peaks: triazine rings (C–N–C, 399.4 eV), tertiary nitrogen (N–C₃, 400.1 eV) and amino functions (N–H, 401.6 eV).⁵⁴ Simultaneously, the potentials for top of the valence band (VB) for the samples were estimated (Fig. 4f). This result indicates that the potential for the top of VB for g-C₃N₄ was 0.8 eV more negative than that for WO₃. Based on this and optical absorption spectra, the energy level structure can be obtained.

Fig. 4 XPS spectra of the g-C₃N₄/WO₃ heterojunction films: (a) the survey scan; (b) W 4f; (c) O 1s; (d) C 1s; (e) N 1s; (f) VB-XPS spectra.

Fig. 5a depicts the UV-vis diffuse reflectance spectra of the pure WO₃ and g-C₃N₄/WO₃ films. For all samples, the optical absorption edge is estimated to be around 450 nm, which is consistent with the reported literature.⁴⁵ The Tauc plot of band gap is shown in Fig. 5b. The band gap energy (E_g) can be calculated by the Tauc formula:⁵⁵

(αhν)^1/2 = A(hν − E_g) (1)

where α, h, ν and A are the absorption coefficient, the Planck's constant, the light frequency and a constant, respectively. The pure WO₃ film has a band gap of about 2.65 eV, which is consistent with other reports.^56,57 After introducing g-C₃N₄ nanoparticles into WO₃ film, no obvious change was displayed of the band gap, because g-C₃N₄ and WO₃ have almost the same E_g.⁴⁴ This result implies that the g-C₃N₄/WO₃ composite photocatalysts are also responsible for the visible light region.

Fig. 5 UV-vis absorption spectra (a) and the band gap determination (b) of WO₃ and g-C₃N₄/WO₃ heterojunction films.

3.2 Photoelectrochemical study of as-prepared photoanodes

To investigate the PEC activity for water splitting, the g-C₃N₄/WO₃ and WO₃ films were used as photoanodes by linear sweep voltammetry (LSV) measurement under visible irradiation (Fig. 6). In Fig. 6a, the LSV was carried out with a potential range from 0.6 V to 2.0 V (vs. RHE) at a scan rate of 20 mV s⁻¹. It can be seen that the g-C₃N₄/WO₃ heterojunction films exhibited higher photocurrent density than the pure WO₃ film under illumination. At +2.0 V (vs. RHE), the photocurrent density of g-C₃N₄/WO₃ films with different content of g-C₃N₄ (1.14%, 2.13% and 3.05%) were 1.54, 2.10 and 1.48 mA cm⁻², while the untreated WO₃ film had a photocurrent of 0.78 mA cm⁻². The g-C₃N₄/WO₃ (2.13%) heterojunction film showed the best PEC performance, which exhibited an almost 3-fold higher photocurrent density than the pure WO₃ film. While the weight of urea increases to 10 g, the photocurrent density decreased. This probably because excess g-C₃N₄ may result in the electrons in CB of WO₃ combine with the holes in VB of g-C₃N₄ at the interface of the composite.⁴⁵

Fig. 6 Photocurrent densities (a) and IPCE (b) of the photoelectrodes.

To quantitatively assess the PEC activity at different wavelength, the incident-photon-to-current-efficiencies (IPCE) measurements of the photoelectrodes were performed at a bias voltage of 1.6 V (vs. RHE) in 0.2 M Na₂SO₄ solution, and the results are shown in Fig. 6b. IPCE can be calculated by IPCE = (1240I)/(λJ_light),⁵⁸ where I (mA cm⁻²), λ (nm) and J_light (mW cm⁻²) are the photocurrent density, the wavelength and power density of incident light, separately. All the photoelectrodes presented strong photo responses at 300–460 nm. Compared to the pure WO₃ film, the g-C₃N₄/WO₃ heterojunction film exhibited larger IPCE values. The formation of heterojunction resulted in enhanced efficiencies over the whole photo response region. The highest IPCE value of WO₃ film at ∼350 nm increased from 25.1% to 53.1% after depositing g-C₃N₄ nanoparticles, which is in accordance with the photocurrent results.

To elucidate the strong correlation between formation of heterojunction and the enhanced PEC performance, Mott–Schottky analysis under the visible light irradiation with a frequency of 1 kHz in a 0.2 M Na₂SO₄ aqueous electrolyte was carried out. The results were presented in Fig. 7. All the photoelectrodes exhibited an n-type characteristic with the positive slop, which reflected that electrons served as the majority carriers. The carrier density (N_d/cm⁻³) can be calculated based on the Mott–Schottky equation:⁵⁹

where ε and ε₀ are the dielectric constant of the semiconductor (50 for WO₃ (ref. 60)) and the vacuum permittivity (8.85 × 10⁻¹⁴ F cm⁻²), q the electron charge (1.60 × 10⁻¹⁹ C), and V the potential applied at the electrode, respectively. The electron density of WO₃ plate arrays film was calculated to be 8.9 × 10¹⁹ cm⁻³, which agrees with previous studies,^61,62 while the N_d value of g-C₃N₄/WO₃ (2.13%) film was 1.5 × 10²² cm⁻³. The formation of heterojunction thus led to an increase of 2 orders of magnitude in the carrier density. The bigger electron density in the g-C₃N₄/WO₃ film is believed to be one of the major factors contributing to the pronounced photocurrent density enhancement. The higher N_d value of g-C₃N₄/WO₃ film can be explained by effective charge separation at the interface of heterojunction, and much longer lifetime of photo-generated electrons.

Fig. 7 Mott–Schottky plots of (a) WO₃ and (b) g-C₃N₄/WO₃ films.

The flat band potential (V_fb) at semiconductor/electrolyte interface can also be estimated by the Mott–Schottky equation:⁶³

1/C² = (2/εε₀qN_d)[(V − V_fb) − kT/q] (3)

By extrapolating the X-intercepts of the liner region in Mott–Schottky plots, V_fb of pure WO₃ plate film was found to be ∼0.67 V vs. RHE, while that of g-C₃N₄/WO₃ heterojunction film shifted to the negative direction from 0.67 V to 0.52 V (1.14%), 0.43 V (2.13%) and 0.35 V (3.05%) vs. RHE. The typical n-type g-C₃N₄ has the values of V_fb around −1.13 V.⁶⁴ The negative shifts of the flat band were prominent, which indicated fewer barrier for oxygen evolution, larger accumulation of electrons and better separation of photo-generated carriers in the heterojunction film.⁶⁵

To investigate the kinetics of the electrochemical reaction process at the electrode surface, the capacitance and resistance of the electrode materials which indicated the separation efficiency between the photo-generated electrons and holes,⁶⁶ electrochemical impedance spectroscopy (EIS) of the as-prepared electrodes were measured under the visible light illumination at an AC frequency varying from 10 kHz to 100 mHz with a 1.4 V (vs. RHE) bias. Fig. 8a shows the typical Nyquist curve of the g-C₃N₄/WO₃ heterojunction film and pure WO₃ film. The Nyquist plot collected for WO₃ and g-C₃N₄/WO₃ films, exhibits one semicircle, suggesting that the faradaic charge transfer is the limiting step for the electrochemical reaction process in the electrode surface.⁵⁹ The apparent difference can be observed that the EIS Nyquist curve of g-C₃N₄/WO₃ film has a smaller circular radius than that of the pure WO₃ film. According to the equivalent circuit which consists of solution resistance (R₁), charge transfer resistance (R₂) in parallel to the constant phase element (CPE₁),⁶⁷ the resistance R₁, R₂ were calculated to be 58.6 Ω, 236.8 Ω for the g-C₃N₄/WO₃ film (2.13%), and 43.6 Ω, 1028.1 Ω for the WO₃ film, respectively. This suggests that g-C₃N₄/WO₃ film owns lower charge transfer resistance than WO₃ film, corresponding to the better PEC performance. Meanwhile, Bode plots are transformed by the Z-view software as presented in Fig. 8b. The frequency peak (f_p) of g-C₃N₄/WO₃ film shifts from 67.7 Hz to 13.8 Hz compared with pure WO₃. From this, the lifetime of electrons for recombination with a time constant (τ) is calculated by the formula:⁶⁸

τ = 1/(2πf_p) (4)

and τ for WO₃ and g-C₃N₄/WO₃ are 2.4 ms and 11.5 ms, respectively. It means that the separation of photo-generated electron and hole has been improved, which may result in increased carriers density.

Fig. 8 EIS Nyquist plots (a) and Bode plots (b) of WO₃ and g-C₃N₄/WO₃ films.

On the basis of the energy level structure reported in previous literature,^44,45,69 the schematic diagram for the g-C₃N₄/WO₃ heterojunction film which illustrates the photo-generated carriers transport process is shown in Fig. 9. The CB and VB positions of WO₃ are about +0.74 and +3.4 V, respectively,⁷⁰ while those of g-C₃N₄ are about −1.13 and +1.57 V, respectively.⁷¹ Under the artificial solar light irradiation, both g-C₃N₄ and WO₃ would be excited and produce the photo-induced electrons and holes, simultaneously. In the g-C₃N₄/WO₃ heterojunction film, the photo-generated electrons of g-C₃N₄ can easily migrate to the CB of WO₃, and then transfer to the back-contacted FTO substrate through a direct pathway provided by the platelets. Similarly, the photo-generated holes of WO₃ will transfer to the VB of g-C₃N₄ and take part in the oxygen evolution reaction. Thus, the photo-generated electron–hole pairs will be separated at the interface of the heterojunction and the long-lived charge carriers will be induced, resulting in enhancing PEC performance.

Fig. 9 Depiction of the charge separation process in the g-C₃N₄/WO₃ films.

4. Conclusions

In summary, the g-C₃N₄ nanoparticles on the surface of two-dimensional (2D) WO₃ plates arrays were successfully in situ synthesized through a simple dipping-annealing method. The g-C₃N₄/WO₃ (2.13 at%) heterojunction film possessed the best PEC performance, and exhibited a 3-fold higher photocurrent density with respect to the pure WO₃ film under visible light irradiation. After introducing g-C₃N₄ nanoparticles, the g-C₃N₄/WO₃ heterojunction film owned larger carriers density, longer electrons' lifetime and more negative flat band potential. The enhanced PEC performance of the heterojunction film is mainly attributed to the effective separation of photo-generated electron–hole pairs at the interface and the improvement of electrons' lifetime. Besides that, the 2D ordered structures are favorable for highly efficient and directional transport of electrons. The in situ formation of heterojunction facilitates the interfacial contact of two components, and optimizes the charge separation. The study demonstrates that in situ formation of heterojunction structure and low-dimensional morphological structure are expected to provide a promising electrode for photoelectric catalysis.

Acknowledgements

This study was supported by the National Nature Science Foundation of China (51304253) and Hunan Provincial Innovation Foundation (CX2015B266).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra11464k

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