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Article  |  Open Access  |  2 Apr 2024

NiFe Prussian blue analog cocatalyzed TiO2/In2S3 type-II heterojunction for solar water splitting

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Energy Mater 2024;4:400028.
10.20517/energymater.2023.101 |  © The Author(s) 2024.
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Abstract

Due to the excellent stability of titanium dioxide (TiO2), there is still value in improving its solar-to-hydrogen conversion efficiency through tremendous attempts. Metal sulfides with a narrow bandgap are good candidates to broaden the ultraviolet light absorption of TiO2 into the visible light region. However, sulfides suffer from the photocorrosion issue, leading to poor stability. Herein, a type-II heterojunction of TiO2/In2S3 is fabricated by a hydrothermal method, and a NiFe Prussian blue analog (NFP) overlayer is deposited on the surface of TiO2/In2S3 through a chemical bath deposition technique. Under AM1.5G illumination, a photocurrent density of 1.81 mA cm-2 can be obtained with NFP coated TiO2/In2S3 at 1.23 V vs. reversible hydrogen electrode, which is six folds of the photocurrent of TiO2. This photocurrent value can reach up to about 90% of its theoretical photocurrent. During a 12 h stability test, the TiO2/In2S3/NFP photoanode exhibits a high photocurrent retention of 95.17% after an initial transient decrease. The type-II heterojunction of TiO2/In2S3 can efficiently boost the charge separation because of the built-in electric field and enhance the visible-light absorption because of the narrow bandgap of In2S3. A NFP overlayer serves as the cocatalyst for water oxidation reaction due to its valence changes of nickel and iron elements. NFP cocatalyst can rapidly extract the photogenerated holes from In2S3 and then improve the charge separation/injection efficiencies. Thanks to chemical stability of NFP, its coating can also make In2S3 resistant to photocorrosion by physically separating the photoanode from the electrolyte. Therefore, there is a good synergistic effect between the TiO2/In2S3 heterojunction and NFP cocatalyst. This work provides some crucial insights for the interface engineering and material design in photoelectrochemical systems.

Keywords

Solar water splitting, heterojunction, cocatalyst, Prussian blue analog, indium sulfide

INTRODUCTION

Green hydrogen produced through solar water splitting is a promising and sustainable approach. There is no carbon footprint in the whole procedure. Photoelectrochemical (PEC) water splitting is one of the solar water splitting techniques, which has moderate solar-to-hydrogen efficiency and system complexity in comparison with photocatalysis and photovoltaic-electrolysis strategies[1,2]. The efficiency of PEC water splitting is still lower than the expected efficiency of 10%. A photoanode is a crucial limit component in one PEC device due to the sluggish kinetics of water oxidation reaction[3]. Among various n-type semiconductors for photoanodes, titanium dioxide (TiO2) is a good candidate, which possesses excellent PEC stability and unique electronic properties[4]. However, its PEC performance is limited by the fast charge recombination and the low light absorption[5]. Several strategies have been developed to improve the charge separation and suppress the charge recombination, such as doping[6-8], nanostructuring[9-11], cocatalyst modification[12-14], and heterojunction construction[15-17]. Constructing heterojunction is a widely accepted approach to enhance the charge separation with other semiconductors such as TiO2[15,18] and BiVO4[19]. However, it is hard to construct a type-II heterojunction with other semiconductors due to large bandgaps of TiO2.

The III-VI group chalcogenide semiconductors have recently received great attention due to their narrow bandgaps. Among the metal sulfides, indium sulfide (In2S3) has a bandgap of 2.0-2.3 eV, a high absorption coefficient, excellent conductivity, and low toxicity. Park et al. fabricated S, N-doped TiO2/In2S3 heterojunction and obtained a high photocurrent density of 2.73 mA cm-2 at 1.23 V vs. reversible hydrogen electrode (RHE), while S, N-doped TiO2 shows a photocurrent density of 1.93 mA cm-2[20]. However, the charge injection efficiency of TiO2/In2S3 was not improved compared to TiO2 photoanodes. This suggests the photogenerated holes would be accumulated on the surface of In2S3. Therefore, cocatalyst should be introduced to improve the charge injection efficiency from the photoanode to water molecules. Commonly used cocatalyst materials for this purpose include noble metals (such as Au and Pt)[21], phosphates[22], layered double hydroxides (LDH)[23], and Prussian blue analogs (PBAs)[24,25]. PBAs, a category of mixed-valence compounds, are characterized by a structure composed of two distinct or identical metal ions connected through cyanide bonds within a face-centered cubic (FCC) framework. They are considered chemically stable due to the strength of cyanide bridging, active in a wide pH range, and non-toxic. CoFe PBAs have been broadly used to modify BiVO4[26,27], Fe2O3[28], Fe2O3/Fe2TiO5[24], TiO2[29], and Sb:TiO2[30]. Further, a phosphate ions layer was inserted between Fe2O3 and CoFe PBA to drift the migration of photogenerated holes[31]. Hybrid CuFe-CoFe PBAs were also developed to modify BiVO4[32]. The cocatalytic effects of NiFe PBAs (NFP) have been investigated on the photoanodes of Zr-doped BiVO4[33] and ZnO/BiVO4[34]. All the reported results indicate that PBAs can improve the photocurrent and/or lower the onset potential of photoanodes. The probable mechanism is attributed to the formation of a favorable interface for efficient hole transfer and the catalytic activity of PBAs. Considering the photocorrosion of In2S3, the accumulated holes can break the In-S bond or attack S2- ions and then oxidize In2S3 rather than H2O[35]. Thereby, the In2S3 sulfide was decomposed into the soluble sulfate. Thanks to the excellent chemical stability of PBAs, it can be strongly expected that the PBAs coated TiO2/In2S3 heterojunction would possess a good PEC stability combined with an enhanced photocurrent.

In this work, a type-II heterojunction of TiO2/In2S3 was first fabricated by a hydrothermal strategy. NFP catalyst was then grown on the surface of the TiO2/In2S3 heterojunction through a chemical bath technique. The ternary TiO2/In2S3/NFP photoanode shows a high photocurrent of 1.81 mA cm-2 at a potential of 1.23 V vs. RHE, which is six folds of the photocurrent of 0.30 mA cm-2 observed with the TiO2 photoanode. Based on the ultraviolet-visible (UV-vis) absorption spectroscopy of a TiO2/In2S3/NFP photoanode, the maximum achievable photocurrent is 2.03 mA cm-2. Therefore, there is a narrow gap between theoretical value and experimental value in this work. The charge injection efficiency and charge separation efficiency can reach up to 93.86% and 93.55% with TiO2/In2S3/NFP, respectively. After an initial transient decrease of the photocurrent, the TiO2/In2S3/NFP photoanode obtained a high photocurrent retention of 95.17% after a 12 h stability test. Thus, we fabricated a ternary TiO2/In2S3/NFP photoanode with a high photocurrent and good stability simultaneously. The detailed material characterizations, PEC performance measurements, and probable mechanism will be discussed below.

EXPERIMENTAL

Preparation of TiO2, TiO2/In2S3, and TiO2/In2S3 /NFP

All reagents were purchased from Aladdin and were used as received without further purification. Deionized water (DI, ~18.25 MΩ cm) was used during the experiments.

Preparation of TiO2: The fabrication of TiO2 nanorod arrays was accomplished using a reported hydrothermal method with some modifications[36]. Fluorine-doped tin dioxide conductive glass substrates (FTO, 2.5 cm × 1 cm, sheet resistance < 10 Ω sq-1) were ultrasonically cleaned in acetone, ethanol and DI sequentially for 30 min each. FTO glasses were dried with a N2 gas flow before use. To prepare the precursor, 20 mL of the concentrated HCl (38%) was dropped into 20 mL DI water. Then, 668 μL of titanium isopropoxide was infused into the above solution. The resulting solution was subsequently transferred into a 50 mL Teflon-lined stainless steel autoclave. FTO substrates were gently placed in the autoclave at an angle, ensuring that its conductive sides faced downward. Hydrothermal reactions were performed at 170 °C for 6 h. Following this, the samples underwent annealing in air at 350 °C for 1 h, with a heating rate of 5 °C min-1.

Preparation of TiO2/In2S3: To obtain the TiO2/In2S3 heterojunction, In2S3 was hydrothermally grown on the as-prepared TiO2. The indium nitrate and thioacetamide were dissolved into the DI water of 40 mL with a stoichiometric ratio of 1:3. The amount of indium nitrate was set at 0.05, 0.1, 0.2, and 0.3 millimoles. TiO2 was carefully placed in an autoclave with the film surface facing downward. Subsequently, the autoclave was subjected to a hydrothermal procedure at 180 °C for 12 h. After the autoclave had cooled to room temperature, the samples were taken out and washed with DI water. The samples were then transferred to a tube furnace, where they underwent annealing at 350 °C for 30 min under a nitrogen atmosphere. Based on the amount of indium nitrate used in the precursor, the TiO2/In2S3 samples are labeled as TiO2/In2S3-x.

Preparation of TiO2/In2S3/NFP: A layer of NiFe Prussian blue (NFP) was deposited on the surface of TiO2/In2S3 via a chemical bath method[24]. Firstly, 0.02 M Ni(NO3)2·6H2O and 0.02 M K3Fe(CN)6 were mixed under vigorous stirring. After that, TiO2/In2S3 was immersed in the above solution. The optimal temperature for NFP growth is 60 °C. The optimal reaction time is 3 h. After the growth of NFP, the samples were washed with DI water and then dried overnight in a 60 °C oven.

PEC performance measurements

PEC performance tests were performed using a three-electrode system with an electrochemical workstation (CHI 660E). TiO2/In2S3/NFP photoanodes were used as the working electrode; an Ag/AgCl electrode was used as the reference electrode, and a platinum wire was used as the counter electrode. The photoanodes were irradiated with the AM1.5G solar spectrum with an intensity of 100 mW cm-2 (CELHXF 300). Additionally, 1 M KOH aqueous solution (pH = 13.6) was employed as the electrolyte. Linear sweep voltammetry (LSV) measurements were performed at a scan rate of 20 mV s-1 with a potential range of 0-1.4 Vvs. RHE. The PEC impedance spectroscopy (PEIS) was recorded at a potential of 0.47 V vs. RHE with a perturbation of 10 mV under AM1.5 illumination. The frequency range is from 0.1 to 105 Hz. The obtained PEIS plots were fitted using Z-View software. Mott-Schottky (M-S) measurements were conducted in the dark at a fixed frequency of 2,000 Hz. Donor density (ND) and flat-band potential (Vfb) can be obtained from the M-S equation, denoted as:

$$ \begin{equation} \begin{aligned} \frac{1}{C^{2}}=\frac{2}{\left(\varepsilon \varepsilon_{\mathrm{r}} A^{2} e N_{\mathrm{D}}\right)}\left(V-V_{\mathrm{fb}}-\frac{K_{\mathrm{b}} T}{e}\right) \end{aligned} \end{equation} $$

In this study, the variables are defined as follows: A (electrode area), C (space charge capacitance), V (the applied potential), ε (dielectric constant with a value of 8.85 × 10-12 F m-1), εr (vacuum dielectric constant, εr = 100), e (elementary charge), Kb (Boltzmann constant with a value of 1.38 × 10-23 J K-1), and T (temperature).

To evaluate the charge separation efficiency (ηsep) and charge injection efficiency (ηinj), a mixed electrolyte comprising 1 M KOH and 0.5 M Na2SO3 was employed. The calculation formulas for ηinj and ηsep are established as:

$$ \begin{equation} \begin{aligned} \eta_{\text {sep }}=J_{\mathrm{Na}_{2} \mathrm{SO}_{3}} / J_{\mathrm{abs}} \end{aligned} \end{equation} $$

$$ \begin{equation} \begin{aligned} \eta_{\text {inj }}=J_{\mathrm{H}_{2} \mathrm{O}} / J_{\mathrm{Na}_{2} \mathrm{SO}_{3}} \end{aligned} \end{equation} $$

The current density of $$J_{\mathrm{H}_{2} \mathrm{O}}$$ was measured in 1 M KOH, while the current density of $$J_{\mathrm{Na}_{2} \mathrm{SO}_{3}}$$ was measured in the presence of a sacrificial agent containing 0.5 M Na2SO3. Jabs represents the theoretical photocurrent density calculated based on light absorption, which is calculated based on the UV-vis absorption spectra of the photoanodes.

The transformation from the measured potential (EAg/AgCl) to the potential versus reversible hydrogen electrode (ERHE) is dictated by:

$$ \begin{equation} \begin{aligned} E_{\mathrm{RHE}}=E_{\mathrm{Ag} / \mathrm{AgCl}}+0.1971 \mathrm{~V}+0.0591~p H \end{aligned} \end{equation} $$

The electrochemical active surface area (ECSA) is typically quantified using the double-layer capacitance measured by cyclic voltammetry (CV). The magnitude of Cdl is calculated to assess the ECSA of the photoanodes. The formula for calculating ECSA is provided as:

$$ \begin{equation} \begin{aligned} C_{\mathrm{dl}}=\Delta J / \Delta E \end{aligned} \end{equation} $$

$$ \begin{equation} \begin{aligned} E C S A=C_{\mathrm{dl}} / C_{\mathrm{s}} \end{aligned} \end{equation} $$

Where Cdl represents the electrochemical double-layer capacitance, ΔJ is the recorded current density variation during the potential scan, and ΔE is the corresponding potential change. Cs signifies the specific capacitance, which is considered constant within the same electrolyte.

The transient decay time (τ) is calculated by:

$$ \begin{equation} \begin{aligned} D=\frac{I_{t}-I_{S}}{I_{i n}-I_{S}} \end{aligned} \end{equation} $$

where It represents the photocurrent at time t, Is denotes the photocurrent at steady state, and Iin signifies the photocurrent of the anodic spike. The time at which lnD = -1 is designated as the transient decay time (τ).

Materials characterizations

The crystal structure was investigated using an X-ray diffractometer (XRD, Rigaku Ultima IV, Japan) with a Cu Kα radiation source. Field-emission scanning electron microscopy (FESEM, thermo scientific Apreo 2C, Thermo Fisher) and energy-dispersive X-ray spectroscopy (EDS) were employed for morphological observations and compositional analysis of the prepared photoanodes. Transmission electron microscopy (TEM) was used to obtain atomic-resolution images (TEM, Talos F200S G2 instrument, USA). UV-vis absorption spectra of TiO2-based photoanodes were recorded in the wavelength range of 300-800 nm (Shimadzu UV-3600 Plus), while the UV-vis absorption spectra of In2S3 powder samples were measured on Lambda 1050 UV-Vis-NIR spectrophotometer. Surface elemental information of the samples was obtained using X-ray photoelectron spectroscopy (XPS, Escalab Xi+, Thermo Fisher). Ultraviolet photoelectron spectroscopy (UPS, Escalab Xi+, Thermo Fisher) and He I (21.22 eV) photon source were employed to obtain information about binding energies. The UPS spectra were analyzed to derive the valence bands (VB) and work functions of the photoanodes. Photoluminescence spectra (PL, Edinburgh FLS980) were obtained under ambient and unbiased conditions with an excitation wavelength of 425 nm.

RESULTS AND DISCUSSION

Properties of TiO2/In2S3/NFP

The morphology of TiO2-based photoanodes was characterized first. Figure 1A-C shows the top-view SEM images of TiO2, TiO2/In2S3, and TiO2/In2S3/NFP. Figure 1D-F displays the cross-sectional SEM images correspondingly. As shown in Figure 1A and D, we successfully synthesized ordered, vertically aligned TiO2 nanorod arrays on FTO substrates, with a diameter of approximately 0.15 μm and a length of 2.3 μm. In2S3 nanosheets assembled nanostructures can be clearly observed on the surface of TiO2 nanorods in Figure 1B and E. Cross-sectional EDS mappings of TiO2/In2S3 in Supplementary Figure 1A and B indicate that the coating of In2S3 is uniform in the whole TiO2 film. After the growth of NFP, the surface of TiO2/In2S3 is completely covered with NFP nanoparticles [Figure 1C and F]. As illustrated in Supplementary Figure 1C and D, Fe, Ni, C, and N elements are evenly distributed within the film thickness. EDS pattern of TiO2/In2S3/NFP is shown in Supplementary Figure 2. The mass percentages of TiO2, In2S3, and NFP calculated roughly from EDS pattern are ~70.13%, ~2.86%, and ~2.78%, respectively. Figure 1G and H presents TEM images of TiO2/In2S3 and TiO2/In2S3/NFP, respectively. In2S3 exhibits a nanosheet-like structure growing on the surface of TiO2 nanorods, while NFP nanoparticles appear to cover the TiO2/In2S3 heterojunction. Some sheet-like In2S3 nanostructures can also be observed in Figure 1H, suggesting that the growth of NFP did not significantly affect the structure of In2S3. In Figure 1I, a high-resolution TEM (HRTEM) image of TiO2/In2S3/NFP shows the formation of heterojunction among TiO2, In2S3, and NFP. Moreover, NFP nanoparticles exhibit a low crystallinity. Figure 1J and K presents an HRTEM image and selected area electron diffraction (SAED) on the TiO2 region. The interplanar spacings are measured as d(110) = 0.322 nm and d(101) = 0.237 nm, corresponding to the tetragonal rutile phase of TiO2. Figure 1L and M exhibits HRTEM and SAED on the region of In2S3. The lattice spacings of d(103) = 0.621 nm and d(109) = 0.33 nm illustrate that the synthesized In2S3 is β-In2S3 with a tetragonal phase. β-In2S3 has a defective spinel structure with the In3+ vacancies, which leads to the n-type characteristics of In2S3[35].

NiFe Prussian blue analog cocatalyzed TiO<sub>2</sub>/In<sub>2</sub>S<sub>3</sub> type-II heterojunction for solar water splitting

Figure 1. Morphology characterizations of TiO2-based photoanodes. (A-C) SEM images of TiO2, TiO2/In2S3, and TiO2/In2S3/NFP. (D-F) Cross-sectional SEM images of TiO2, TiO2/In2S3, and TiO2/In2S3/NFP. (G) TEM image of TiO2/In2S3. (H) TEM image of TiO2/In2S3/NFP. (I) HRTEM image of TiO2/In2S3/NFP. (J and K) HRTEM and SAED of TiO2 region. (L and M) HRTEM and SAED of In2S3 region.

XRD patterns shown in Figure 2A reveal the crystal structure of TiO2, TiO2/In2S3, and TiO2/In2S3/NFP. Some XRD peaks from the FTO substrate are marked with stars. The XRD peaks from the TiO2 film can be well attributed to the rutile phase of TiO2 (JCPDS No. 21-1276). The XRD peaks at 14.25, 27.43, 33.23, and 47.7° can be attributed to the (103), (109), (0012), and (2212) crystal planes of β-In2S3 (JCPDS No. 25-0390). Two XRD peaks at 17.3 and 24.57° correspond to the (200) and (220) crystal planes of NiFe(CN)6 (JCPDS No. 51-1897), respectively. Moreover, the XRD peaks of In2S3 can also be observed on the photoanode of TiO2/In2S3/NFP, suggesting the crystal structure of In2S3 is unaffected[37].

NiFe Prussian blue analog cocatalyzed TiO<sub>2</sub>/In<sub>2</sub>S<sub>3</sub> type-II heterojunction for solar water splitting

Figure 2. (A) XRD patterns of TiO2, TiO2/In2S3, and TiO2/In2S3/NFP. XPS spectra of (B) Ti 2p, (C) O 1s, and (D) S 2p of TiO2/In2S3. XPS spectra of (E) C 1s, (F) N 1s, (G) Ni 2p, and (H) Fe 2p of TiO2/In2S3/NFP. Inset of (F): the crystal structure of NiFe PBA. (I) Transmission spectra and UV-vis absorption spectra of TiO2 and TiO2/In2S3 with the different concentrations of indium nitrate in the precursor.

The composition and chemical states of the TiO2-based photoanodes were analyzed with XPS, as presented in Supplementary Figure 3 and Figure 2B-H. All elements of Ti, O, S, In, Ni, Fe, N, and C were successfully identified on the XPS survey spectra. In Figure 2B, Ti 2p1/2 and Ti 2p3/2 peaks can be observed at 458.6 and 464.3 eV on the TiO2 photoanode, respectively. No other peaks were observed at lower binding energy, indicating only the presence of Ti4+, not Ti3+[38]. For TiO2/In2S3, Ti 2p1/2 and Ti 2p3/2 peaks show a positive shift of 0.4 eV compared to the as-prepared TiO2. In Figure 2C, O 1s XPS spectra can be fitted with two peaks corresponding to Ti-O and oxygen vacancies (Ov)/S-O. The Ti-O peak also shows a positive shift of 0.35 eV after the growth of In2S3. It is important to mention that the S-O peak overlaps with the OV peak due to similar binding energies[20,39]. In Figure 2D, the S 2p XPS spectra of In2S3 powder exhibit two peaks at 161.61 and 162.82 eV, corresponding to the S 2p3/2 and S 2p1/2 peaks, respectively. After the formation of TiO2/In2S3 heterojunction, these two peaks display a negative shift of ~0.15 eV. The opposite shift direction of Ti 2p/O 1s and S 2p powerfully demonstrates the charge transfer from TiO2 to In2S3 at the heterojunction interface and the formation of a built-in electric field oriented from TiO2 to In2S3[16]. The C 1s spectrum of TiO2/In2S3/NFP in Figure 2E can be deconvoluted into three distinct peaks located at 284.8 (C-C), 285.8 (C≡N), and 288.6 eV (C=O). The N 1s spectrum of TiO2/In2S3/NFP depicted in Figure 2F confirms the presence of C≡N at 397.8 eV. The crystal structure of NFP is shown in the inset of Figure 2F. This demonstrates that the covering of NFP on the surface of TiO2/In2S3 heterojunction is successful. In Figure 2G, the peaks located at binding energies of 873.8 and 856.3 eV correspond to the Ni 2p1/2 and Ni 2p3/2 peaks, respectively. The Fe 2p peaks in Figure 2H confirm the presence of Fe3+ ions with characteristic binding energies of 708.46 and 721.4 eV[25]. Figure 2I depicts the transmission spectra and UV-vis absorption spectra of TiO2 and TiO2/In2S3 prepared with various amounts of indium nitrate. All samples exhibit pronounced UV absorbance, and with an increasing amount of indium nitrate, the light absorption range of TiO2/In2S3 progressively extends into the visible light region. The photographs of TiO2 and TiO2/In2S3 photoanodes in Supplementary Figure 4A can directly confirm the improved visible light absorption. This demonstrates that the In2S3 layer can effectively enhance the visible light absorption for more electron-hole pairs photogeneration.

PEC performance of TiO2/In2S3/NFP photoanode

In2S3 and NFP serve a light absorption layer and cocatalyst layer, respectively, which can greatly influence the PEC performance of the TiO2/In2S3/NFP photoanode. The amount of indium nitrate and the reaction time and temperature of NFP growth were optimized first in this work. For the following experiments, 0.05 mmol In(NO3)3, 3 h NFP growth time, and 60 °C NFP synthesis temperature were ascertained. The detailed optimization results are shown in Supplementary Figures 5-8.

PEC performances of the TiO2/In2S3/NFP photoanode are shown in Figure 3, compared with the control photoanodes of TiO2 and TiO2/In2S3. In Figure 3A, the photocurrent density of TiO2/In2S3/NFP is 1.81 mA cm-2, which is five times higher than that of TiO2 (0.30 mA cm-2). The TiO2/In2S3 heterojunction exhibits a photocurrent density of 1.3 mA cm-2. This clearly demonstrates the critical role of TiO2/In2S3 heterojunction fabrication. The applied bias photon-to-current efficiency (ABPE) was further calculated based on the light LSV curves, and the results are depicted in Figure 3B. TiO2/In2S3/NFP has the highest ABPE efficiency of 0.94% at 0.51 V vs. RHE, surpassing the as-prepared TiO2 by 5.8 times. As shown in Figure 3C, the normalized photovoltage decay curves show that the TiO2/In2S3/NFP photoanode attenuates the fastest under dark conditions. This suggests an improved separation of photogenerated electron-hole pairs[40]. Additionally, the photovoltage of TiO2/In2S3/NFP is around 130 mV higher than that of the as-prepared TiO2. The photovoltage was obtained by calculating the difference between OCPdark and OCPlight. The photovoltage value can reflect the degree of energy band bending at the semiconductor-electrolyte interface (SEI). A larger photovoltage indicates a more pronounced energy band bending and a stronger built-in electric field generated by the SEI[41], which can enhance the charge separation. Figure 3D illustrates the I-t curves under the chopped light, which can reflect the transient photocurrent response of photoanodes. Notably, no discernible transient peak can be observed, suggesting the effective separation of the photogenerated electrons and holes[42]. The origin of the photocurrent decay is believed to be due to the poor charge transport of TiO2/In2S3/NFP, which may be changed by doping TiO2. The inset of Figure 3D illustrates that the transient decay time (τ) becomes longer after the formation of TiO2/In2S3 heterojunction and the NFP modification. The increased τ of TiO2/In2S3/NFP indicates that the charge recombination has been suppressed efficiently[43]. Dark LSV curves of TiO2, TiO2/In2S3, and TiO2/In2S3/NFP electrodes are shown in Supplementary Figure 9. At the current density of 10 mA cm-2, the overpotential of TiO2/In2S3/NFP is significantly negative shift by 166 mV compared to TiO2/In2S3. This confirms the role of NFP overlayer as cocatalyst for water oxidation reaction.

NiFe Prussian blue analog cocatalyzed TiO<sub>2</sub>/In<sub>2</sub>S<sub>3</sub> type-II heterojunction for solar water splitting

Figure 3. (A) LSV curves, (B) ABPE plots, (C) normalized photovoltage decay and inset OCP, (D) transient photocurrent curves and inset transient decay time (τ) curves of TiO2, TiO2/In2S3, and TiO2/In2S3/NFP. (E) PEC stability curves of TiO2/In2S3 and TiO2/In2S3/NFP photoanodes at 1.23 V vs. RHE under AM1.5G illumination. Inset: SEM images of the tested TiO2/In2S3 and TiO2/In2S3/NFP photoanodes.

To reveal the PEC stability of TiO2/In2S3/NFP, a long-term stability test was performed nonstop for 12 h under AM1.5G illumination at 1.23 V vs. RHE, as depicted in Figure 3E. After 12 h, TiO2/In2S3/NFP exhibits an approximate 95% retention of photocurrent after an initial transient decrease, which is an 11% improvement over TiO2/In2S3 (84%). The high stability of TiO2/In2S3/NFP should be attributed to the robust NFP, which can also block the photocorrosion of In2S3 by physically separating the semiconductor from the electrolyte. The slight decrease of TiO2/In2S3/NFP should be attributed to the imperfect coating of NFP on the surface of TiO2/In2S3. It is worth noting that the photocurrent of TiO2/In2S3/NFP shows significant improvement from 1.81 to 3.1 mA cm-2 through the LSV measurements [Supplementary Figure 10]. As shown in Supplementary Figure 10C, ECSA was used as a key indicator for evaluating the catalytic performance of photoanodes[44]. After the stability test, TiO2/In2S3/NFP (5.06 × 10-2 mF cm-2) is 1.57 times the as-prepared photoanode (3.22 × 10-2 mF cm-2). This increase is consistent with the improvement of photocurrent, illustrating that the photocurrent enhancement is due to the enhanced ECSA of TiO2/In2S3/NFP[45]. This is consistent with the reported light-induced restorative effect of the Prussian blue material[33]. The inherent mechanism should be further investigated for answering this open question. The morphology and XPS characterizations of the tested TiO2/In2S3 and TiO2/In2S3/NFP photoanodes were performed, and the results are shown in Supplementary Figure 11 and the inset of Figure 3E. The NFP overlayer can greatly slow down the photocorrosion of In2S3. After the stability test, the Ni 2p peaks of TiO2/In2S3/NFP are similar to the peaks of Ni(OH)2[46]. This suggests that the as-prepared NFP cocatalyst is partially transformed into the active species of Ni(OH)2, which is consistent with the reported results[47]. As per the previous findings, the in-situ generated Ni(OH)2 from NFP can create NiOOH2-x that contains Ni4+ ions, which are highly active sites for water oxidation reactions. In addition, Supplementary Table 1 summarizes the comparison of the PEC performance of this work with the reported related photoanodes. The PEC performance of TiO2/In2S3 achieved in this study is comparable to or surpassing that of the reported photoanodes. This work further demonstrates the effectiveness of NFP as a cocatalyst for TiO2-based photoanodes. In the absence of any sacrificial agents, the photocurrent density of the TiO2/In2S3/NFP photoanode reaches 1.81 mA cm-2, which is six times that of the bare TiO2. In addition, we measured the photocurrent of TiO2, TiO2/In2S3, and TiO2/In2S3/NFP photoanodes under various light intensities, such as 40, 70, 100, 130, 160, 190, and 200 mW cm-2. There is a linear growth of the photocurrent with the light intensity in the measured range [Supplementary Figure 12]. Moreover, TiO2/In2S3 and TiO2/In2S3/NFP photoanodes show a higher slope of ~0.02 mA mW-1 than that of TiO2 (0.004 mA mW-1). This feature suggests that the TiO2/In2S3 heterojunction and the NFP cocatalyst overlayer can effectively facilitate the charge separation without limiting the hole transfer to the electrolyte[48].

The ECSA of TiO2, TiO2/In2S3, and TiO2/In2S3/NFP was further studied. CV curves [Supplementary Figure 13] were measured at scan rates of 5-100 mV s-1 in 1 M KOH, within a potential window of 0.625-0.725 V vs. RHE. The relationship between ΔJ and scan rate is illustrated in Figure 4A. The fitted dashed line provides the double-layer capacitance of TiO2-based photoanodes, determined by their slopes. After the coating of the NFP overlayer, the TiO2/In2S3/NFP photoanode presents a two-fold ECSA in comparison with TiO2. This increase is lower than the improvement of five times on the photocurrent. This implies a significantly synergistic effect among TiO2, In2S3, and NFP, not just the increase of ECSA. In order to investigate the charge transfer processes at the SEI interface, we conducted PEIS experiments under AM1.5G illumination at 0.47 V vs. RHE. The Nyquist plot was fitted using an equivalent circuit model, as shown in the inset of Figure 4B. It consists of a series resistance (Rs), a trapping resistance (Rtrap), a bulk phase constant phase element (CPEbulk), a charge transfer resistance (Rct,trap), and a trapping CPE (CPEtrap). The fitting parameters used are summarized in Table 1. Compared with TiO2/In2S3, the TiO2/In2S3/NFP photoanode shows a significant decrease in Rtrap (475.15 Ω) and Rct,trap (5711 Ω). This demonstrates that the NFP cocatalyst layer can boost the charge transfer kinetics at SEI. M-S plots were acquired at a fixed frequency of 2,000 Hz in the dark, as depicted in Figure 4C. All samples exhibit the positive slopes, illustrating their n-type conductivity. Evidently, TiO2/In2S3/NFP exhibits a more negative flatband potential, indicating the position of the Fermi level of the photoanode in relation to the potential of the reference electrode. The negatively shifted flatband potential is typically associated with a lowered Fermi level[49]. The donor density of TiO2/In2S3/NFP is 1.33 times of that of TiO2/In2S3, indicating the good conductivity of the NFP overlayer for charge transport. To further elucidate the kinetics of the TiO2-based photoanodes, we measured charge transfer efficiency (ηinj) and charge separation efficiency (ηsep), as shown in Figure 4D and E, respectively. At 1.23 V vs. RHE, TiO2/In2S3/NFP exhibits the highest ηinj of 93.86% and the highest ηsep of 93.55%. TiO2/In2S3 has a moderate ηinj and ηsep, corresponding to 84.58% and 80.67%, respectively. TiO2 only achieves a very low ηsep of 21.80%, indicating that the TiO2/In2S3 heterojunction plays a major role for the charge separation. The NFP cocatalyst overlayer can promote the charge injection at SEI and then facilitate the charge separation in the depletion region. Steady-state PL spectra of TiO2-based photoanodes measured with a light excitation of 420 nm are shown in Figure 4F. PL spectra reveal the radiative recombination of the photogenerated electrons and holes. The PL peaks located at 535.61 and 682.07 nm can be attributed to the excitonic PL process, in which the radiative transition from the sub-bands to the VB top occurs[50]. The sub-bands mainly result from the surface defects of the TiO2 photoanode. Therefore, the suppressed PL signal can be observed after the formation of TiO2/In2S3 heterojunction. The heterojunction can efficiently boost the process of the charge separation because of the built-in electric field. The modification of the NFP overlayer cannot further suppress the PL signal. This is because the PL measurements were conducted in air, not in the electrolyte. The charges transferred to the NFP overlayer cannot be migrated from NFP to the redox substance in the air. In other words, in the electrolyte, the NFP overlayer can migrate the charges from NFP to water molecules and promote the charge separation.

NiFe Prussian blue analog cocatalyzed TiO<sub>2</sub>/In<sub>2</sub>S<sub>3</sub> type-II heterojunction for solar water splitting

Figure 4. (A) Determination of capacitance (Cdl) values through ΔJvs. scan rate plots for TiO2, TiO2/In2S3, and TiO2/In2S3/NFP. (B) Nyquist plots of TiO2-based photoanodes measured at 0.47 V vs. RHE under AM1.5G illumination. Inset shows the equivalent circuit for alternating current (AC) impedance fitting. (C) M-S plots of TiO2-based photoanodes measured at a fixed frequency of 2,000 Hz in the dark. (D) Surface charge injection efficiency (ɳinj) of TiO2-based photoanodes. (E) Bulk charge separation efficiency (ɳsep) of TiO2-based photoanodes. (F) PL spectra of TiO2, TiO2/In2S3, and TiO2/In2S3/NFP.

Table 1

Electrical element parameters of TiO2-based photoanodes extracted from PEIS after the equivalent circuit model fitting

PhotoanodeRs (Ω)Rtrap (Ω)CPEbulk (F)Rct,trap (Ω)CPEtrap (F)
TiO219.48863.744.37E−715,4722.61E−5
TiO2/In2S318.79857.524.23E−713,5151.13E−5
TiO2/In2S3/NFP8.48475.151.61E−75,7111.41E−5

Probable mechanism of PEC improvement

To determine the energy band positions of TiO2 and In2S3, Tauc plots were generated using UV-vis absorption spectra [Figure 5A]. UPS spectra of TiO2 and In2S3 were further measured to determine the work functions and the edge of the VB. During the UV-vis absorption spectra and UPS measurements of In2S3, the In2S3 powder, which was collected from the hydrothermal residues, was used. The photograph of the In2S3 powder is shown in Supplementary Figure 4B. The bandgaps (Eg) of TiO2 and In2S3 can be derived from the Tauc plots, corresponding to 3.01 and 1.92 eV, respectively. The two bandgap values are consistent with the reported results[20]. The work functions of 4.39 eV for TiO2 and 4.08 eV for In2S3 can be obtained from the UPS spectra in Figure 5B and C, respectively. Furthermore, the energies from the VB edge to the Fermi level are 2.78 and 1.61 eV for TiO2 and In2S3, respectively. These experimental data offer detailed insights into the energy band structures of TiO2 and In2S3, benefitting our understanding of the electron structure and PEC processes of the TiO2/In2S3 photoanode.

NiFe Prussian blue analog cocatalyzed TiO<sub>2</sub>/In<sub>2</sub>S<sub>3</sub> type-II heterojunction for solar water splitting

Figure 5. (A) Tauc plots of TiO2 and In2S3. (B) UPS spectrum of TiO2. (C) UPS spectrum of In2S3. (D) Schematic diagrams of the energy band structures for TiO2 and In2S3 before and after their contact. (E) Mechanism diagram of TiO2/In2S3/NFP photoanode for solar water splitting.

The schematic diagram of the energy band structure before and after the contact between TiO2 and In2S3[Figure 5D] illustrates that, initially, there is a disparity in the Fermi levels between TiO2 and In2S3, with In2S3 exhibiting a higher Fermi level. When In2S3 was grown on the surface of TiO2, electrons began to migrate from In2S3 to TiO2, gradually aligning the Fermi levels of both materials. Finally, a classical type-II heterojunction was formed with TiO2 and In2S3. An internal built-in electric field was generated at the interface of TiO2/In2S3 heterojunction. The electric field direction is from TiO2 to In2S3. The energy band structure of TiO2/In2S3 type-II heterojunction facilitates the separation of the photogenerated charges due to the electric field force. Specifically, it is energy-favorable that the photogenerated electrons are transported from the conduction band (CB) of In2S3 to the CB of TiO2. For the photogenerated holes, the large VB offset at the interface of TiO2/In2S3 can provide tremendous driving force and push the holes to the side of In2S3. The separated holes can be migrated to the NFP cocatalyst overlayer. The NFP catalyst captured holes rapidly, preventing the recombination of electrons and holes at the TiO2/In2S3 interface and thus enhancing the utilization of photogenerated charge carriers. More importantly, the NFP catalyst can promote the hole participation in the water oxidation reaction at SEI[16]. As shown in Supplementary Figure 14, the NFP cocatalyst can remarkably improve the electrochemical activity and provide the active species through the state variation of nickel and iron elements. Therefore, thanks to the type-II heterojunction of TiO2/In2S3 and the efficient NFP cocatalyst, a higher photocurrent density can be achieved with TiO2/In2S3/NFP photoanodes. The theoretical photocurrent density (2.03 mA cm-2) of TiO2/In2S3/NFP can be estimated from its UV-vis absorption spectrum shown in Supplementary Figure 15. The experimental photocurrent of 1.81 mA cm-2 is ~90% of the above theoretical value. In brief, as shown in Figure 5E, TiO2 and In2S3 can absorb the UV and visible light, respectively, and form the type-II heterojunction. The type-II heterojunction can promote the charge separation and suppress the charge recombination due to the built-in electric field at the interface of TiO2/In2S3. A NFP layer, as cocatlyst and protector, can enhance the water oxidation reactions through the transition of Ni2+/Ni3+ and Fe2+/Fe3+ and protect the In2S3 layer from the photocorrosion, respectively.

CONCLUSIONS

In conclusion, we fabricated a ternary TiO2/In2S3/NFP photoanode by the facile hydrothermal and chemical bath methods. This photoanode achieved a photocurrent density of 1.81 mA cm-2 at the potential of 1.23 V vs. RHE, reaching around 90% of the theoretical maximum value. Its charge injection efficiency and charge separation efficiency are near 95%. Compared with a pristine TiO2 photoanode, the kinetics of the TiO2/In2S3/NFP photoanode was significantly improved, such as a low charge transfer resistance, and a suppressed PL signal. The TiO2/In2S3 type-II heterojunction significantly suppresses the charge recombination at the surface of TiO2 and facilitates the separation of electron-hole pairs under the assistance of the built-in electric field. The NFP cocatalyst overlayer substantially decreases the charge transfer resistance, and promotes the catalytic activity for water oxidation reactions, thereby further improving the PEC performance of TiO2/In2S3. Moreover, the NFP can protect the In2S3 layer from the photocorrosion in the electrolyte. Therefore, the synergistic effect of the TiO2/In2S3 heterojunction and NFP cocatalyst determines the finally good PEC performance of the TiO2/In2S3/NFP photoanode. These findings provide some critical insights for the interface engineering and material design in PEC systems.

DECLARATIONS

Authors’ contributions

Conceptualization, methodology, investigation, data curation, formal analysis, writing-original draft: Zhang M

Conceptualization, methodology, investigation, formal analysis, funding acquisition, supervision, writing-review and editing: Yang P, Xie J

Methodology, investigation, formal analysis: Tao W, Pang X, Su Y, Peng P, Zheng L, Li R, Wang S, Huang J, Zou L

Availability of data and materials

The data are made available upon request to authors.

Financial support and sponsorship

This work was supported by the Sichuan Science and Technology Program (2022NSFSC1272), the Scientific Research Starting Project of SWPU (2021QHZ018), the Chengdu Education Bureau Program (QYGG004, QYGG010), the School of New Energy and Materials Program (2022SCNYTZHCL013), and the Production-Education Integration Demonstration Project of Sichuan Province “Photovoltaic Industry Production-Education Integration Comprehensive Demonstration Base of Sichuan Province (Sichuan Financial Education [2022] No.106)”.

Conflicts of interest

All authors declared that there are no conflicts of interest.

Ethical approval and consent to participate

Not applicable.

Consent for publication

Not applicable.

Copyright

© The Author(s) 2024.

Supplementary Materials

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Zhang M, Yang P, Tao W, Pang X, Su Y, Peng P, Zheng L, Li R, Wang S, Huang J, Zou L, Xie J. NiFe Prussian blue analog cocatalyzed TiO2/In2S3 type-II heterojunction for solar water splitting. Energy Mater 2024;4:400028. http://dx.doi.org/10.20517/energymater.2023.101

AMA Style

Zhang M, Yang P, Tao W, Pang X, Su Y, Peng P, Zheng L, Li R, Wang S, Huang J, Zou L, Xie J. NiFe Prussian blue analog cocatalyzed TiO2/In2S3 type-II heterojunction for solar water splitting. Energy Materials. 2024; 4(3): 400028. http://dx.doi.org/10.20517/energymater.2023.101

Chicago/Turabian Style

Zhang, Ming, Pingping Yang, Wenyan Tao, Xiangui Pang, Youyi Su, Pai Peng, Lin Zheng, Runhan Li, Shuxiang Wang, Jing Huang, Li Zou, Jiale Xie. 2024. "NiFe Prussian blue analog cocatalyzed TiO2/In2S3 type-II heterojunction for solar water splitting" Energy Materials. 4, no.3: 400028. http://dx.doi.org/10.20517/energymater.2023.101

ACS Style

Zhang, M.; Yang P.; Tao W.; Pang X.; Su Y.; Peng P.; Zheng L.; Li R.; Wang S.; Huang J.; Zou L.; Xie J. NiFe Prussian blue analog cocatalyzed TiO2/In2S3 type-II heterojunction for solar water splitting. Energy Mater. 2024, 4, 400028. http://dx.doi.org/10.20517/energymater.2023.101

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