Xiaorong
Liang†
ac,
Jiale
Xie†
b,
Jinyun
Xiong
ac,
Liangping
Gong
ac and
Chang Ming
Li
*abc
aInstitute for Clean Energy & Advanced Materials, Faculty of Materials and Energy, Southwest University, Chongqing 400715, P. R. China. E-mail: ecmli@swu.edu.cn
bInstitute for Materials Science and Devices, Suzhou University of Science and Technology, Suzhou 215009, P. R. China
cChongqing Key Laboratory for Advanced Materials & Technologies of Clean Electrical Power Sources, Chongqing 400715, P. R. China
First published on 13th June 2018
BiVO4 is one of the most promising photoanode materials due to its relatively small band gap (∼2.4 eV) for efficient light absorption, and its suitable valence band edge for the oxygen evolution reaction. However, it achieves a photocurrent which is much lower than the theoretical value of 7.5 mA cm−2. Herein we prepare a multimetal oxide-based heterojunction W:BiVO4–FeCoW by a sol–gel approach with post-annealing. The heterojunction photoanode exhibits a photocurrent which is enhanced to ∼3.8 times that of a W:BiVO4 photoanode at 1.23 V vs. the reversible hydrogen electrode (RHE), and a significant negative shift of the flat-band potential of 280 mV. The experimental results demonstrate that the performance enhancement mechanism can be attributed to the significantly enhanced charge separation/transport and light absorption due to the multimetal oxide-modified heterojunction.
To further improve the performance of BiVO4 photoanodes, BiVO4 has been coupled with other single metal oxides to make heterojunctions, such as WO3/BiVO4,16,17 Bi2S3/BiVO4,18 TiO2/BiVO4,19 Co3O4/BiVO4,20 and ZnO/BiVO4.21 Fabrication of a heterojunction with multimetal oxides rather than a single metal oxide is considered to be more effective for the catalysis of OER by a photoanode.22–24 The OER property of a metal oxide is mainly related to the features of the metal valence states (d states) and oxygen valence states (p states), which determine the absorption energies of the metal oxides and the catalytic activity. The incorporation of some foreign metal elements could significantly influence the electronic structure due to the ligand effect and the strain effect.25,26 In addition, the heterojunction could improve the efficient separation of charge carriers, broaden the range of absorbed light, and suppress electron–hole recombination.27 Therefore, it is advisable to combine multimetal oxides with BiVO4 to form a heterojunction structure, such as ZnFe2O4/BiVO4 and FeCe-oxide/BiVO4, in order to promote PEC water splitting.28,29 It has been confirmed that the BiVO4 photoanodes with multimetal oxide coatings of either ZnFe2O4 or FeCe-oxide give greatly improved photocurrent density and stability. Furthermore, multimetal based compounds with good catalytic activity such as FeCoW oxy–hydroxide have been demonstrated in PEC water splitting and OER.30,31 Nevertheless, a heterojunction structure of W:BiVO4 modified by a ternary FeCoW multimetal oxide has not been investigated.
In this work, we synthesized a W:BiVO4–FeCoW photoanode by a sol–gel approach. The W:BiVO4–FeCoW heterojunction enhances the photocurrent density to 3.8 times that of W:BiVO4 and causes a significant negative shift of the flat-band potential of 280 mV in comparison to the pristine W:BiVO4 photoanode. The performance enhancement mechanism of this heterojunction toward water oxidation was subsequently explored. Besides, this work is very helpful in the study of the photocatalytic properties of a multiple-junction W:BiVO4–FeCoW photoanode, thus providing a new scientific insight into photoelectrochemistry.
The PEC performances were evaluated by measuring the current density–voltage (J–V) curves with a potentiostat (CHI660D, CH Instruments) at a scan rate of 20 mV s−1 with chopped light illumination. Stabilities were obtained by potentiostatic (I–t) measurements under intermittent illumination at a bias of 0.2 V vs. the saturated calomel electrode (SCE). Electrochemical impedance spectroscopy (EIS) was conducted in the frequency range of 0.1–105 Hz at an amplitude of 10 mV under AM 1.5 G illumination at open circuit potential using a CHI 660D workstation. Mott–Schottky plots were measured at the three frequencies of 1 kHz, 5 kHz, and 10 kHz in the dark. Intensity modulated photocurrent spectroscopy (IMPS) and impedance vs. potential measurements were carried out using a CIMPS photoelectrochemical workstation (Zahner) under 100 mW cm−2 visible light.
Fig. 2 EDS mappings of the W:BiVO4–FeCoW photoanode: (a0) SEM image, (a) Bi, (b) V, (c) Fe, (d) Co, (e) W. |
The crystal structures of the W:BiVO4 and W:BiVO4–FeCoW photoanodes were detected by XRD to confirm their compositions. The XRD patterns of the W:BiVO4 and W:BiVO4–FeCoW photoanodes are shown in Fig. 3a. The diffraction peaks from the FTO glass are marked with stars (*). The patterns of both W:BiVO4 and W:BiVO4–FeCoW photoanodes exhibit the peaks from the monoclinic structure of BiVO4 (PDF card no. 14-0688).34 This suggests that the W-doping does not change the crystalline structure of BiVO4. In the pattern of W:BiVO4, the peak marked with a cross (+) at 2θ = 28° corresponds to the (201) plane of Bi2O3 (PDF card no. 43-0452). This may be attributed to phase separation during the high temperature annealing. The XRD pattern of the W:BiVO4–FeCoW photoanode shows not only several characteristic peaks of W:BiVO4, but also three characteristic peaks (#) of the FeCoW multimetal oxide. However, these three peaks cannot be attributed to one of the known Fe-, Co- or W-based oxides. All these results suggest that the W:BiVO4–FeCoW photoanode exhibits a coexistence of both the W:BiVO4 and FeCoW multimetal oxide phases.
Fig. 3 (a) XRD patterns of W:BiVO4 and W:BiVO4–FeCoW films. (b) EDS pattern of W:BiVO4–FeCoW films. (c) UV-vis absorption spectra and Tauc plots of W:BiVO4, FeCoW and W:BiVO4–FeCoW films. |
The EDS pattern in Fig. 3b shows the existence of Fe, Co, W, Bi, V and O elements in the prepared W:BiVO4–FeCoW films. The optical absorption properties of the pristine W:BiVO4 and W:BiVO4–FeCoW photoanodes were investigated by UV-vis absorption spectra which are shown in Fig. 3c. The W:BiVO4–FeCoW photoanode has a wider wavelength absorption range than the pristine W:BiVO4 and FeCoW photoanodes. In contrast to the W:BiVO4 photoanode, the absorption edge of W:BiVO4–FeCoW shifts from 504 nm to 551 nm. In other words, a smaller band gap of ∼2.25 eV is achieved after FeCoW multimetal oxide modification and the band gap of the FeCoW photoanode is ∼2.30 eV. The optical band gap energy is calculated according to the equation,
λg = 1239.8/Eg | (1) |
The surface chemical states of the W:BiVO4–FeCoW photoanode were investigated by XPS and the results are shown in Fig. 4. According to the XPS spectra in Fig. 4a–f, elements including Bi, V, O, Fe, Co and W were detected. The Bi 4f peaks located at 164.0 and 158.7 eV are assigned to Bi 4f5/2 and Bi 4f7/2; the spin orbit splitting energy between the two peaks is 5.3 eV, confirming Bi3+ cations in W:BiVO4–FeCoW. The V 2p peaks located at 523.9 and 516.5 eV are assigned to V 2p1/2 and V 2p3/2; the spin orbit splitting energy between the two peaks is 7.4 eV, confirming V5+ cations in W:BiVO4–FeCoW.35–37 Moreover, the binding energy for O 1s at 529.5 eV may be attributed to the lattice oxygen in W:BiVO4–FeCoW, while binding energies between 530 eV and 532 eV are from the surface hydroxyl groups and/or oxygen deficient region.36,38
Fig. 4 XPS spectra of (a) Bi 4f, (b) V 2p, (c) O 1s, (d) Fe 2p, (e) Co 2p and (f) W 4f of the W:BiVO4–FeCoW photoanode. |
Fig. 4d shows the XPS spectrum of Fe 2p, which is characterized by two peak maxima located at 724.9 eV and 711.3 eV corresponding to Fe 2p1/2 and Fe 2p3/2 respectively, which are typical values of Fe3+. The other two marked peaks are attributed to two characteristic satellites of Fe3+.39 No peaks correspond to Fe (2p3/2 at 707 eV), Fe2+ (2p3/2 at 708 eV) or Fe4+ (2p3/2 at 713 eV),40 confirming the presence of the Fe(III) metal oxide in W:BiVO4–FeCoW. The XPS spectrum of Co 2p is analysed as displayed in Fig. 3e. The two peaks at binding energies of 781.0 eV and 796.3 eV, correspond to the Co 2p3/2 and Co 2p1/2 peaks, respectively. Besides, the peaks at 803.2 eV and 786.6 eV correspond to Co 2p1/2 and Co 2p3/2 satellite peaks. These are all consistent with the presence of Co2+, as reported before.41,42 Furthermore, this result indicates the presence of the Co(II) metal oxide in the FeCoW multimetal oxide. In Fig. 4f, the spin orbit splitting energy of W 4f between the W 4f7/2 and W 4f5/2 peaks is 2.1 eV. In addition, a broad peak at 41.5 eV corresponds to the tungsten oxide loss feature, confirming that V5+ is replaced by W6+ at VO43− tetrahedral sites; a similar result has been reported previously for W:BiVO4 photoanodes.36
Chopped linear sweep voltammetry (LSV) curves of W:BiVO4–FeCoW and W:BiVO4 photoanodes were produced by measuring the chopped polarization curve at a scan rate of 20 mV s−1 under standard AM 1.5 G irradiation in 1.0 M NaOH (Fig. 5a). They show that with an increase in applied potential, the photocurrent steeply increases and does not reach saturation. The photocurrent density and the onset potential of W:BiVO4–FeCoW are significantly improved in comparison to those of the pristine W:BiVO4 and FeCoW photoanodes. At 1.23 V (vs. RHE), the photocurrent density of W:BiVO4–FeCoW reaches 0.49 mA cm−2, while only 0.13 and 0.07 mA cm−2 are achieved for W:BiVO4 and FeCoW, respectively. Thus, the photocurrent density is increased by 280% due to the multimetal oxide coating. Similar improvement effects have also been observed on a hematite photoanode.43 It is noted that the PEC performance of FeCoW is the worst among those of the three photoanodes.
To further examine the stability of the W:BiVO4–FeCoW photoanode, I–t curves were measured at 1.23 V (vs. RHE) under illumination as shown in Fig. 5b. It can be observed that the photocurrent density of W:BiVO4–FeCoW decreases to 38% of its initial value after 2 h, which may be due to the corrosion of the photoanodes in the PEC reactions.44
The incident-photon-to-current-conversion efficiencies (IPCE) of the W:BiVO4–FeCoW and W:BiVO4 photoanodes at various bias potentials show a functional relationship between the photoelectric conversion efficiency and the incident light wavelength (Fig. 5c). With an increase of the voltage from 0.6 V (vs. RHE) to 1.6 V (vs. RHE), the corresponding IPCE increases at a given wavelength for a specific electrode, demonstrating the enhanced hole–electron separation of W:BiVO4–FeCoW for water oxidation. In addition, after applying a specific bias, the IPCE of the W:BiVO4–FeCoW photoanode is higher than that of the W:BiVO4 electrode at the same wavelength. The maximum IPCE of W:BiVO4–FeCoW occurs at ∼440 nm, while it occurs at ∼350 nm for W:BiVO4. This red shift is consistent with the UV-vis spectra (Fig. 3c), which show the reduced band gap of W:BiVO4–FeCoW; therefore a narrow band gap can effectively harvest solar energy, enlarge the range of the absorption wavelength and lead to the shifting of the maximum absorption peak to longer wavelength. The maximum IPCE value of W:BiVO4–FeCoW (10.71%) is 6.02 times higher than that of W:BiVO4 (1.78%) at 1.2 V (vs. RHE). Moreover, the visible light response range of the W:BiVO4–FeCoW photoelectrode is extended, which indicates that the FeCoW modification increases the visible light absorption obviously and leads to more efficient charge carriers.
Fig. 5d shows Mott–Schottky plots of W:BiVO4–FeCoW and W:BiVO4 photoanodes at 1 kHz, 5 kHz and 10 kHz, respectively. The slopes of these curves are all positive, indicating n-type semiconductor characteristics with electrons as the majority carriers. The slopes of these photoanodes become smaller with increasing frequency and the flat potential shows a frequency dependency due to the non-ideality of the electrode surface.45 W:BiVO4–FeCoW exhibits lower slopes than W:BiVO4, confirming its higher donor density in comparison to W:BiVO4. This result suggests that the W:BiVO4–FeCoW photoanode could improve charge transport by introducing huge numbers of electrons. At 5 kHz, the flat band potential of the W:BiVO4–FeCoW photoanode is ∼0.48 V vs. RHE, which is markedly lower than that of W:BiVO4 (∼0.76 V vs. RHE). The significant shift should be mainly due to the passivation of the surface states of W:BiVO4. It should be mentioned that the onset potential is not always equal to the flat band potential because the photocurrent in the vicinity of the flat band potential is suppressed by the recombination process.46
Charge recombination behaviors at the semiconductor–electrolyte interface (SEI) of the photoanodes were investigated by PEC impedance spectroscopy. The Nyquist and Bode plots of the W:BiVO4–FeCoW and W:BiVO4 photoanodes measured in 1.0 M NaOH under AM 1.5 G illumination at open circuit potential are given in Fig. 6a and b. Moreover, the equivalent circuits of W:BiVO4–FeCoW and W:BiVO4 used to fit the Nyquist plots are shown in the inset of Fig. 6a. Nyquist plots of both W:BiVO4–FeCoW and W:BiVO4 are composed of two semicircles, of which the semicircle at low frequency is related to the charge transfer process in the semiconductor depletion layer, and the one at high frequency is ascribed to the electron transfer in the Helmholtz layer. R1 (R′1) is the series resistance of the cell; R2 (R′2) and CPE1 (CPE′1) represent the semiconductor depletion layer resistance and the chemical capacitance, respectively. R3 represents the charge transfer resistance in the Helmholtz layer, while the CPE2 element corresponds to the recharged Helmholtz layer. According to the fitting results in Table 1, the W:BiVO4–FeCoW photoelectrode shows the lowest R2 of 300.8 Ω, suggesting its superior charge transfer process at the SEI, which can be attributed to the modification with FeCoW multimetal oxide. In contrast, the W:BiVO4 results in an R′2 value which is considerably higher (4612 Ω). This reveals that the improved performance is due to the more efficient charge separation and transport. The high value of CPE1 for W:BiVO4–FeCoW contrasts to the low value of CPE′1 for W:BiVO4 and also indicates better electrochemical activity after FeCoW multimetal oxide modification.
Photoanode | R 1/R1′ (Ω) | R 2/R2′ (Ω) | CPE1/CPE1′ (F) | R 3 (Ω) | CPE2 (F) |
---|---|---|---|---|---|
W:BiVO4–FeCoW | 25.71 | 300.8 | 9.38 × 10−6 | 3412 | 3.65 × 10−4 |
W:BiVO4 | 24.62 | 4612 | 5 × 10−6 | — | — |
Intensity-modulated photocurrent spectroscopy (IMPS) can efficiently minimize the photoinduced changes in band bending and was further used to identify the role of the FeCoW layer of the W:BiVO4–FeCoW photoanode. Two important parameters, namely the rate constants for charge transfer (ktr) and surface recombination (krec), were calculated using IMPS.47 An IMPS spectrum consists of two semicircles in the lower and upper quadrants, which correspond to the resistor–capacitor (RC) attenuation and the competition between charge transfer and recombination, respectively. The ratio of krec/ktr is positively proportional to the upper semicircle and a small value could indicate a charge transfer that is faster than the charge recombination.38 The W:BiVO4–FeCoW photoanode shows a smaller upper semicircle than the W:BiVO4 photoanode (Fig. 6c and d) at a specific voltage, suggesting that the charge recombination of W:BiVO4–FeCoW is much smaller than that of W:BiVO4. With an increase in the applied potential from 0.6 to 1.4 V vs. RHE, the upper semicircle of the W:BiVO4–FeCoW photoanode becomes progressively smaller. This is consistent with the theoretical relationship between water oxidation and oxidation potential, in which the reaction rate becomes faster with the increased applied potential. In contrast, this is not obvious in the upper semicircle of the W:BiVO4 photoanode with increasing applied potential, demonstrating that a higher level of charge recombination still occurs in the W:BiVO4 photoanode across a wide potential range.
The energy level diagram of the W:BiVO4–FeCoW photoanode is shown in Fig. 7. Both the FeCoW multimetal oxide and W:BiVO4 have a narrow band gap which favours the absorption of photons, consistent with the result from Fig. 3c. Under AM 1.5 G illumination, electrons are stimulated from the valence band to the conduction band in both the FeCoW multimetal oxide and the W:BiVO4 layers. More importantly, the heterojunction between the FeCoW multimetal oxide and W:BiVO4 can boost the charge separation and transport due to the triggered energy level structure. Moreover, the photocurrent of W:BiVO4–FeCoW is much larger than that of W:BiVO4 and FeCoW, mainly due to the charge transport and recombination rates (Fig. 5a). W:BiVO4–FeCoW can show improved charge transport due to the introduction of huge numbers of electrons and the electrochemical activity toward oxygen production. The lower charge transfer resistance of W:BiVO4–FeCoW can directly support this (Fig. 6a). Thus, the FeCoW multimetal oxide modification can not only enhance the charge separation and interfacial transfer, but also greatly suppress the charge recombination at the photoanode/electrolyte interface and remove the surface states of W:BiVO4 to favour the water oxidation reaction. In this stepwise band-edge energy structure, the FeCoW multimetal oxide acts as a charge transporter/sensitizer which can efficiently decrease the charge-transfer resistance, increase the electron density and consequently improve the PEC performance of the W:BiVO4–FeCoW photoanode.
Footnote |
† Contributed equally. |
This journal is © The Royal Society of Chemistry 2018 |