A photoanode with plasmonic nanoparticles of earth abundant bismuth for photoelectrochemical reactions

Abstract

A wide range of technologies has been developed for producing hydrogen economically and in greener ways. Photoelectrochemical water splitting using photoelectrodes submerged in a bath electrolyte forms a major route of hydrogen evolution. The efficacy of water splitting is improved by sensitizing metal oxide photoelectrodes with narrow bandgap semiconductors that efficiently absorb sunlight and generate and transport charge carriers. Here we show that the efficiencies of photocurrent generation and photoelectrochemical hydrogen evolution by the binary TiO₂/Sb₂S₃ anode increase by an order of magnitude upon the incorporation of the earth-abundant plasmonic bismuth nanoparticles into it. The ternary electrode TiO₂/Bi nanoparticle/Sb₂S₃ illuminated with sunlight provides us with a photocurrent density as high as 4.21 mA cm⁻² at 1.23 V, which is fourfold greater than that of the binary electrode and tenfold greater than that of pristine TiO₂. By using bismuth nanoparticles, we estimate the incident photon to current conversion efficiency at 31% and solar power conversion efficiency at 3.85%. Here the overall impact of bismuth nanoparticles is attributed to increases in the open-circuit voltage (860 mV), which is by expediting the transfer of photogenerated electrons from Sb₂S₃ nanoparticles to the TiO₂ electrode, and short-circuit current (9.54 mA cm⁻²), which is by the plasmonic nearfield effect. By combining the cost-effective plasmonic bismuth nanoparticles with the narrow bandgap Sb₂S₃ on the TiO₂ electrode, we develop a stable, ternary photoanode and accomplish high-efficiency photocurrent generation and hydrogen evolution.

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Hydrogen is the most promising fuel for fulfilling the increasing global energy demand and cosseting the environmental costs of fossil fuels and greenhouse gases.^1–3 Photoelectrochemical (PEC) water splitting for hydrogen production is environmentally benign.^4–7 Even though TiO₂ is a promising and the most investigated n-type semiconductor photoelectrode for PEC water splitting, its efficiency to produce hydrogen is limited by the large bandgap (3.2 eV) or the absorption of UV radiation.^8–11 Therefore, TiO₂ is modified with narrow bandgap semiconductors such as CdS, BiVO₄ and Bi₂S₃, which is for extending the absorption edge to the visible and near infrared regions.^12–17 For example, an FTO/TiO₂/BiVO₄ heterojunction electrode provides higher PEC performance (4.11 mA cm⁻² at 1.23 V) over FTO/TiO₂ or FTO/BiVO₄.¹⁸ Similarly, heterojunction photoelectrodes composed of TiO₂ nanorods and Bi₂S₃ nanowires provide higher photocurrent density (2.4 mA cm⁻² at 0.7 V vs. RHE) than pristine TiO₂.¹⁹

Along with the natural abundance of Sb₂S₃ and the large optical absorption coefficient, the high stability and narrow bandgap (1.7–1.8 eV) make it a promising co-sensitizer of TiO₂.^20–25 Liu et al., used a WO₃/Sb₂S₃ heterojunction electrode and obtained four-fold enhancement of photocurrent density (1.79 mA cm⁻² at 0.8 V) over pristine WO₃.²⁶ Also, Song et al. reported a current density of 0.79 mA cm⁻² at an onset of −0.08 V for TiO₂/Sb₂S₃, while pristine TiO₂ shows poor performance under similar experimental conditions.²⁷ Nonetheless, such binary electrodes pose issues such as slow injection of electrons and high rates of carrier recombination. The efficiency of electron injection is improved by interfacing with noble metal nanoparticles (NPs) such as Ag and Au.^28–34 Here the metal NP not only lowers the rate of energy-wasting charge recombination in semiconductors but also enhances the photoreactivity due to the localized surface plasmon resonance (LSPR).^35–38 The collectively oscillating conduction band electrons in a plasmonic NP facilitate the absorption of a wide range of solar photons, which depends upon the metal as well as the size and shape of the NP. As an example, CdS/Ag/ZnO photoanodes show higher (3.13%) solar energy conversion efficiency than CdS/ZnO alone (1.35%).³⁹ Also, Li et al., fabricated CdS/Au/TiO₂ photoanodes and achieved photocurrent density as high as 4.07 mA cm⁻² at 0 V vs. the Ag/AgCl electrode.⁴⁰ Although bismuth (Bi) NPs expand the optical absorption window and suppress the rate of carrier recombination in conventional semiconductors,^41–46 the plasmonic properties of this earth-abundant NP⁴¹ are yet to be explored in PEC applications.

We report the ternary photoanode TiO₂/BiNP/Sb₂S₃ and its application in high efficiency PEC water splitting. The electrode is prepared by chemical bath deposition (CBD). We accomplish an order of magnitude higher efficiency of PEC hydrogen evolution by supplementing the conventional TiO₂ electrode with the narrow bandgap Sb₂S₃ and the plasmonic properties of BiNPs. The ternary electrode provides us with a photocurrent density as high as 4.21 mA cm⁻² at 1.23 V, the incident photon to current conversion efficiency (IPCE) at 31%, and solar power conversion efficiency at 3.85%. We hypothesize that BiNPs enhance the open circuit voltage by bending the band across the junctions, expediting the transfer of photogenerated electrons from Sb₂S₃ to TiO₂, and increase the short circuit current by the SPR effect.

The ternary photoanode TiO₂/BiNP/Sb₂S₃ was prepared by the doctor-blade technique and chemical bath deposition. The details of photoanode fabrication are provided in the Experimental section. The ternary electrode and BiNPs were characterized by FESEM, EDAX, HRTEM, Raman spectroscopy and XRD studies. Fig. 1a shows the surface morphology of the photoanode TiO₂/BiNP/Sb₂S₃, which aggregate randomly with no specific shape displaying high roughness. Further, the EDAX image confirms that the constituent elements in the ternary electrode are Ti, O, Bi, Sb and S (Fig. 1b) and it reveals that BiNPs are effectively incorporated on TiO₂ in the composite. The SEM and EDAX images of BiNPs, TiO₂/BiNP and TiO₂/Sb₂S₃ are shown in Fig. S1.† Further, the HRTEM image of the TiO₂/BiNP/Sb₂S₃ photoanode reveals the placement of BiNPs in between TiO₂ and Sb₂S₃ and the lattice fringes of individual components (Fig. 1c). Body centered tetragonal TiO₂ has a lattice spacing of 0.352 nm that corresponds to (101) (JCPDS-894921), while orthorhombic Sb₂S₃ has a lattice constant of 0.298 nm that is assigned to the (211) plane (JCPDS-421393). Plasmonic BiNPs have a lattice spacing of 0.316 nm which is assigned to the (012) plane (JCPDS-851331). The selected area electron diffraction (SAED) patterns (Fig. 1d) confirm the crystalline structures of BiNP, TiO₂ and Sb₂S₃. The TEM images of pure BiNPs show a spherical shape with an average particle size of around 5–10 nm (Fig. S2a†) and the lattice spacing of 0.32 nm relates to the (012) plane of Bi (JCPDS-851331) (Fig. S2b†). The XRD pattern of pure BiNPs is shown in the ESI (Fig. S2c†) and BiNPs possess a rhombohedral structure with the (012), (104), (110) and (202) planes corresponding to d-spacings of 3.25, 2.34, 2.24 and 1.83 Å respectively (JCPDS: 851331). No peaks of other bismuth compounds (such as Bi₂O₃) can be observed in the pattern which suggests that Bi³⁺ ions reduced to metallic Bi.^42,44,49 The XRD patterns of TiO₂, TiO₂/BiNP and Sb₂S₃ are shown in Fig. 1e, which demonstrate the body-centered tetragonal crystal lattice of TiO₂, rhombohedral structure of BiNPs and orthorhombic primitive lattice of Sb₂S₃. TiO₂ has a body-centered tetragonal crystal lattice with the (101), (103), (004), (200), (105), (211) and (204) planes corresponding to d-spacings of 3.51, 2.43, 2.36, 1.89, 1.69, 1.66, 1.47 and 1.36 Å respectively (JCPDS-894921). In TiO₂/BiNP, the presence of (012), (104) and (110) planes, confirms formation of TiO₂/BiNPs. Sb₂S₃ has an orthorhombic primitive lattice with the (130), (310), (230), (211), (221), (240), (501) and (351) planes corresponding to d-spacings of 3.54, 3.49, 3.16, 3.01, 2.74, 2.55, 1.93 and 1.73 Å respectively (JCPDS: 421393). Raman spectral analysis of BiNPs is shown in Fig. S2d,† where pure BiNPs show two characteristic vibrational bands at 68 and 91 cm⁻¹ corresponding to E_g and A_1g modes respectively. Furthermore, the Raman spectra of TiO₂, TiO₂/BiNP and TiO₂/BiNP/Sb₂S₃ (Fig. S3†) help us to clarify the structures of the electrode components. The anatase phase of TiO₂ was confirmed by the Raman bands at 144 and 635 cm⁻¹ that correspond to the E_g mode while 396 and 512 cm⁻¹ belong to B_1g and B_1g/A_1g modes respectively. In TiO₂/BiNP, the presence of peaks at 68 and 91 cm⁻¹, confirms that BiNPs are well incorporated with TiO₂ particles. Further, the introduction of Sb₂S₃ onto TiO₂/BiNP, shows peaks at 135, 188 and 431 cm⁻¹ that belong to the vibrational modes of SbS₃ units and Sb–Sb bonds in Sb₂S₃ respectively. The Raman spectra of a pure Sb₂S₃ sample are displayed in Fig. S4.†

Fig. 1 Characterization of the ternary photoanode and anode materials. (a) FESEM image, (b) EDAX image, (c) HR-TEM image and (d) SAED pattern of TiO₂/BiNP/Sb₂S₃, and (e) XRD patterns of TiO₂, TiO₂/BiNP and Sb₂S₃.

With the modification of the TiO₂ electrode using BiNPs and Sb₂S₃ sensitizers, the absorption band of the electrode extends from the UV to NIR region. Fig. 2 shows the absorption spectra of TiO₂, TiO₂/BiNP and TiO₂/BiNP/Sb₂S₃ electrodes, which reveal the strong absorption of light in the range of 400–800 nm by BiNPs. This corresponds to near UV to NIR absorption, which is confirmed by the absorption spectra of an aqueous solution of pure BiNPs (Fig. S5a and b†). The broad absorption in the UV-Vis-NIR region (Fig. 2) is due to the SPR of BiNPs.^49,50 The TiO₂ electrode selectively absorbs light with a wavelength smaller than 390 nm. The absorption onset of TiO₂ is expanded to the 400–750 nm region with the chemical bath deposition of the small bandgap Sb₂S₃ (Fig. S4b†). The bandgaps of TiO₂ and Sb₂S₃ employed in this work are estimated at 3.13 and 1.6 eV, respectively.

Fig. 2 Absorption spectra of the TiO₂ and TiO₂/BiNP electrodes. Inset: absorption spectrum of the TiO₂/BiNP/Sb₂S₃ electrode.

To evaluate the plasmonic effect of BiNPs on the PEC activity of the ternary electrode, we examined the photocurrent response by each photoanode (TiO₂ alone, TiO₂/Sb₂S₃ or TiO₂/BiNP/Sb₂S₃) by linearly sweeping the voltage (LSV plots) in a three-electrode system (Ag/AgCl as the reference electrode and Pt as the counter electrode), and the results are summarized in Fig. 3a. The pH of the electrolyte was set at 12.7 and the cells are illuminated with simulated sunlight (AM1.5). All the potentials reported against the reversible hydrogen electrode (RHE) are calculated using the Nernst equation. Upon chopped light illumination, the photocurrent density values for TiO₂, TiO₂/BiNP, TiO₂/Sb₂S₃ and TiO₂/BiNP/Sb₂S₃ photoanodes are 0.15, 0.61, 1.83 and 4.21 mA cm⁻², respectively, at 1.23 V. The highest photo-response in terms of current density is observed for TiO₂/BiNP/Sb₂S₃ at the lowest onset potential of 0.13 V. The enhanced photocurrent density of the TiO₂/BiNP/Sb₂S₃ electrode, which is promising for PEC water splitting, is attributed to the ability of Sb₂S₃ to absorb the visible-NIR light and the SPR of BiNPs. In addition, the efficiency of a photoelectrode is expressed in terms of solar to hydrogen conversion (STH). The STH conversion efficiencies for TiO₂, TiO₂/BiNP, TiO₂/Sb₂S₃ and TiO₂/BiNP/Sb₂S₃ are estimated at 0.03, 0.22, 0.51 and 1.56% at 0.6 V (Fig. 3b), which are calculated from short circuit current density and the intensity of incident light (ESI†). The TiO₂/BiNP/Sb₂S₃ photoanode provides us with 3- and 15-times higher STH conversion efficiency than TiO₂/Sb₂S₃ and TiO₂, respectively. Furthermore, estimation of the PEC water splitting performance of the TiO₂/Sb₂S₃/BiNP electrode was carried out by using LSV plots and the results are presented in Fig. S6a.† Upon chopped light illumination, the TiO₂/Sb₂S₃/BiNP electrode showed a maximum photocurrent density of 3.16 mA cm⁻² at 1.23 V. Further, the high STH conversion efficiency of the TiO₂/Sb₂S₃/BiNP electrode is 0.68% at 0.6 V (Fig. S6b†). Therefore, the TiO₂/Sb₂S₃/BiNP electrode provides lower photocurrent density and STH efficiency than the TiO₂/Bi/Sb₂S₃ electrode. These results suggested that BiNP-embedded TiO₂/Sb₂S₃ can be better understood based on the staggered energy alignment of the TiO₂/Bi/Sb₂S₃ compared to BiNPs grown on the surface of Sb₂S₃ (TiO₂/Sb₂S₃/BiNP) for improving PEC water splitting.

Fig. 3 PEC properties of photoanodes. (a) LSV plots, (b) solar to hydrogen conversion efficiency and (c) photocurrent stability plots of TiO₂, TiO₂/BiNP, TiO₂/Sb₂S₃ and TiO₂/BiNP/Sb₂S₃ electrodes. Here 0.1 M Na₂SO₃ and 0.1 M Na₂SO₄ are used as the electrolyte solution under simulated solar irradiation. (d) Temporal evolution of H₂ gas by TiO₂/Sb₂S₃ and TiO₂/BiNP/Sb₂S₃ electrodes.

The stability of the photocurrent response by TiO₂, TiO₂/BiNP, TiO₂/Sb₂S₃ and TiO₂/BiNP/Sb₂S₃ photoanodes is examined by chronoamperometric (I–t curve) studies. Also, the photocurrent responses of the electrodes are tested at 1.23 V versus RHE (0.6 versus Ag/AgCl) under simulated solar (AM1.5) irradiation, and the results are presented in Fig. 3c. The long-term stability of the ternary electrode is noteworthy; only little decay in performance is observed after 5100 s. H₂ evolution is quantified using gas chromatography and the amount of H₂ evolved is plotted as a function of time at 1.23 V vs. RHE. As shown in Fig. 3d, the TiO₂/BiNP/Sb₂S₃ photoanode provides maximum (3.2 mmol) H₂ evolution over 2 h, which is when compared to TiO₂/Sb₂S₃. The high photocurrent efficiency and H₂ evolution performances of TiO₂/BiNP/Sb₂S₃ are in good agreement.

The ability of photoelectrodes to convert the incident photons into electrons is examined by IPCE experiments in a two-electrode system, but without applying any external bias, as shown in Fig. 4a. The maximum IPCE for TiO₂ is 13% in the UV region (300–400 nm), which is negligibly small in the visible to NIR region (400–900 nm). We find an increase in the IPCE for the TiO₂ electrode supplemented with BiNPs, which is 15% in the UV region (300–400 nm) and additional contribution of 2–5% in the 400–800 nm region. This enhancement in IPCE performance by BiNPs is attributed to their SPR in the visible to NIR region. When compared to the 13% IPCE of TiO₂ alone, the TiO₂/Sb₂S₃ electrode shows ca. 20% IPCE in the 300–750 nm region, which is attributed to the small bandgap of Sb₂S₃. Thus, by combining the plasmonic effect of BiNPs with the small bandgap of Sb₂S₃ in TiO₂/BiNP/Sb₂S₃, we accomplish 31% IPCE for the ternary photoanode. Also, this result implies that BiNPs in the heterojunction suppress undesired recombination of photogenerated charge carriers in Sb₂S₃.

Fig. 4 IPCE and I/V/R characteristics of photoanodes. (a) Wavelength-dependent IPCE plots (b) Nyquist plots (symbol) with an equivalent circuit (dot lines) and (c) Mott–Schottky plots of pristine TiO₂, TiO₂/BiNP, TiO₂/Sb₂S₃ and TiO₂/BiNP/Sb₂S₃ photoanodes. (d) J–V data of TiO₂/Sb₂S₃ and TiO₂/BiNP/Sb₂S₃ photoanodes.

The kinetics of charge injection at the photoanode/electrolyte interfaces are studied by measuring the resistance across the electrode/electrolyte interface. Here impedance measurements were carried out under the light illumination of TiO₂, TiO₂/BiNP, TiO₂/Sb₂S₃ or TiO₂/BiNP/Sb₂S₃ electrodes immersed in a solution of 0.1 M Na₂SO₃ and 0.1 M Na₂SO₄ (1 : 1, v/v) and the plots were fitted into the equivalent circuits shown in the inset in Fig. 4b, where R_S, R_CT and CPE represent the electrolyte solution resistance, interfacial charge transfer resistance at electrode/electrolyte and constant phase element for photoelectrode/electrolyte, respectively. The fitted parameters are given in Table 2 (ESI†). All the electrodes have solution resistance (R_S) in the range of 4–9 Ω. The high R_CT value of 775 Ω for TiO₂ is due to low charge separation and the lowest R_CT value of 71 Ω is observed for the TiO₂/BiNP/Sb₂S₃ photoanode compared to other electrodes, indicating the facile transfer of charge carriers at the TiO₂/BiNP/Sb₂S₃ photoanode/electrolyte interface, which is consistent with the excellent PEC performance of the electrode. Also, these charge injection properties are consistent with the electron lifetime (τ) values estimated from Bode plots (Fig. S7†). The τ values are estimated using the relation τ = 1/(2πƒ_max) where ƒ_max is the frequency maxima. The τ values calculated for pristine TiO₂, TiO₂/BiNP, TiO₂/Sb₂S₃ and TiO₂/BiNP/Sb₂S₃ films are 1.13, 1.22, 1.46 and 1.65 ms, respectively. The large τ value corresponds to a high rate of carrier transport or low rate of undesired carrier recombination in the TiO₂/BiNP/Sb₂S₃ photoelectrode, which is attributed to the effect of BiNPs. In other studies, BiNPs increase the τ value of TiO₂/BiNP/Sb₂S₃, which is in good agreement with the higher photocurrent density. Furthermore, we construct the Mott–Schottky plots of electrodes (Fig. 4c), where the linear profiles with positive slopes confirm the n-type characteristics of the photoanodes.

To evaluate the role of plasmonic BiNPs on the power conversion efficiency of the ternary electrode, we constructed different photoelectrochemical solar cells and recorded the current–voltage (J–V) characteristics of the cells under 1 sun illumination (100 mW cm⁻²). Here the electrolyte used was polysulfide (0.1 M Na₂S and 0.1 M S) solution. The active area of the working electrode (TiO₂, TiO₂/Sb₂S₃, or TiO₂/BiNP/Sb₂S₃) was set at 0.15 cm², and the counter electrode used was C-fabric. The J–V responses of the cells are shown in Fig. 4d and the solar cell parameters are summarized in Table S1 (ESI†). The TiO₂/BiNP/Sb₂S₃ cell shows an open circuit voltage (V_OC) of ∼860 mV and a short circuit current density (J_SC) of 9.54 mA cm⁻², which are greater than those of the TiO₂/Sb₂S₃ electrode. The power conversion efficiency (PCE) of cells with TiO₂, TiO₂/Bi, TiO₂/Sb₂S₃ and TiO₂/BiNP/Sb₂S₃ electrodes is 0.019, 0.39, 2.99 and 3.85% respectively. The TiO₂/BiNP/Sb₂S₃ based cells achieved the highest PCE and V_OC was also the highest for this cell compared to the other electrodes. The high value of V_OC is attributed to the bending of bands and improvement of charge separation induced by BiNPs. We attribute the high J_SC value to the SPR effect of BiNPs, which increases the carrier density. These NPs not only transfer the photogenerated electrons from Sb₂S₃ to FTO via TiO₂ but also act as photosensitizers, which are obvious from the high J_SC and V_OC values of FTO/TiO₂/BiNP when compared with FTO/TiO₂. The J–V characteristics FTO-based electrodes are shown in the inset in Fig. 4d. Furthermore, the higher PCE (0.39%) of the FTO/TiO₂/BiNP electrode, which is when compared with FTO/TiO₂ (0.01%), suggests that BiNPs transfer photogenerated electrons to the external circuit. In addition, the TiO₂/Bi electrode shows high J_SC (0.56 mA cm⁻²) and V_OC (0.54 V) compared to the pure TiO₂ film which is due to different regions of the solar spectrum being absorbed and converted to current.

The Fermi level of BiNPs, and conduction (CB) and valence band (VB) positions of TiO₂ and Sb₂S₃ are obtained from cyclic voltammetry (Fig. S8†). The procedure for calculations of band positions in the energy level diagram is as shown in the ESI.† The mechanism of PEC performance of the ternary photoanode TiO₂/BiNP/Sb₂S₃ is schematically presented in Fig. 5. Here the simulated solar illumination of the electrode leads to the generation of electron/hole pairs in the conduction (CB)/valence bands (VB) of Sb₂S₃. The photogenerated holes in the VB of Sb₂S₃ react with the sacrificial reagent (SO₃²⁻) and hydroxyl ion to produce SO₄²⁻ and oxygen. On the other hand, BiNPs assist the transport of photogenerated electrons through the external circuit to the counter electrode (Pt) where protons are reduced to liberate hydrogen. In this hypothetical mechanism, the primary role of BiNPs is to expedite the transfer of electrons from the conduction band of Sb₂S₃ to that of TiO₂ and finally to the back contact FTO.

Fig. 5 A scheme of a PEC cell constructed using the ternary photoanode TiO₂/BiNP/Sb₂S₃, showing the band alignment, charge transport and redox reactions under the simulated solar irradiation.

In summary, by combining the low bandgap Sb₂S₃ with the plasmonic BiNPs in the ternary TiO₂/BiNP/Sb₂S₃ photoanode, we find that the efficiencies of photocurrent generation and photoelectrochemical hydrogen evolution increase by an order of magnitude when compared with the pristine TiO₂ electrode. This ternary photoanode shows photocurrent density as high as 4.21 mA cm⁻² at 1.23 V with a solar to hydrogen conversion efficiency of 1.56% at 0.6 V. BiNPs deliver the SPR effect to Sb₂S₃ and increase the rate of electron transport from Sb₂S₃ to TiO₂, and as a result, the maximum of IPCE becomes 31% and the solar power conversion efficiency becomes 3.85%. By expediting the transfer of photogenerated electrons from Sb₂S₃ to the TiO₂ electrode, BiNPs increase the open circuit voltage up to 860 mV. Also, with the plasmonic nearfield effect, the short circuit current reaches up to 9.54 mA cm⁻². Overall, by combining the earth-abundant and cost-effective plasmonic BiNPs and the narrow bandgap Sb₂S₃ on the TiO₂ electrode, we develop a stable, ternary photoanode and accomplish high efficiency photocurrent generation and hydrogen evolution.

Experimental

Materials and samples

Antimony chloride (SbCl₃), sodium thiosulfate (Na₂S₂O₃), bismuth nitrate pentahydrate [Bi(NO₃)₃·5H₂O], dimethyl sulfoxide (DMSO), sodium sulfate (Na₂SO₄), elemental sulfur, methanol and acetone were obtained from Aldrich. Titanium tetrachloride (TiCl₄), sodium borohydride (NaBH₄), sodium sulfide (Na₂S), sodium sulfite (Na₂SO₃) and tannic acid were obtained from Merck. TiO₂ paste (18NR-T) used for electrode preparation was obtained from Dyesol. Deionized water (resistivity ∼ 18.2 Ω cm) was obtained from a Millipore Direct-Q3 UV system. Fluorine doped tin oxide (FTO) glass (sheet resistance ∼ 25 Ω cm⁻²) was obtained from Pilkington.

Preparation of photoanodes

The TiO₂/BiNP/Sb₂S₃ ternary photoanode was prepared by the doctor-blade technique. The photoanode was fabricated as follows: TiO₂ paste was coated onto the pre-cleaned FTO substrates by the doctor-blade method, which was followed by annealing the TiO₂ film at 500 °C for 30 min to provide a thin layer of TiO₂ on FTO. Further, the TiO₂ film was immersed in a hot (70 °C) solution (40 mM in isopropanol) of TiCl₄ for 30 min. The film was rinsed with DI water and annealed at 500 °C for 30 min to provide the uniform TiO₂ electrode. Further, BiNPs were grown onto the TiO₂ electrode. In brief, Bi(NO₃)₃·5H₂O was dissolved in DMSO and a solution containing a mixture of NaBH₄ and tannic acid was slowly added, which brought about the reduction of Bi³⁺ to Bi⁰ and the obtained BiNPs were dispersed in water for further studies. Finally, the TiO₂ film was vertically introduced into the bismuth precursor solution and sonicated for 15 min to form the TiO₂/BiNP film, which was brown in color. Further, Sb₂S₃ was grown onto the TiO₂/BiNP film by the chemical bath deposition process to form the ternary electrode TiO₂/BiNP/Sb₂S₃.^47,48 Briefly, 400 mg of SbCl₃ was dissolved in 2.5 mL of acetone, which was followed by the addition of 72.5 mL of deionized water to yield a turbid solution that subsequently converted into a white precipitate. Subsequently, 3.95 g of Na₂S₂O₃ dissolved in 25 mL of water was added to the SbCl₃ solution under stirring. The TiO₂/BiNP film was vertically dipped in the Sb₂S₃ solution at 25 °C for 60 min, which was followed by rinsing the electrode with deionized water and drying on a hot plate at 60 °C. Finally, the film was annealed at 300 °C for 15 min under an argon atmosphere to yield the dark-orange colored TiO₂/BiNP/Sb₂S₃ photoanode. Preparation of the electrode is illustrated in the ESI.†

Characterization techniques

The surface morphologies of the samples were analyzed using a field emission scanning electron microscope (FESEM-Zeiss supra 40). High-resolution transmission electron microscope (HR-TEM) images were recorded using a TECNAI G-2 FEI instrument operating at 300 kV. The powder X-ray diffraction (XRD) patterns of samples were measured on a PANalytical, Xpert PRO instrument. The Raman spectra of photoanode materials were recorded by using a Bruker Senterra dispersive Raman microscope spectrometer with an excitation wavelength of 532 nm. The absorption spectra of the synthesized electrodes were recorded on a UV-Vis spectrophotometer (Shimadzu UV-3600). PEC measurements were performed in an aqueous electrolyte containing a 1 : 1 mixture of 0.1 M Na₂SO₃ and 0.1 M Na₂SO₄, using a three electrode system with Pt and Ag/AgCl as the counter and reference electrodes, respectively. The current versus potential (I–V) characteristics of photoanodes were recorded using a LOT-Oriel-Autolab with a 150 W xenon arc lamp with 100 mW cm⁻² intensity. Chronoamperometric (I–t) plots, Mott–Schottky plots, cyclic voltammetry (CV) and electrochemical impedance spectra (EIS) were measured on an Autolab PGSTAT 302N equipped with NOVA 1.9 software. The irradiance intensity was confirmed by calibrating with a 2 cm × 2 cm Si Reference Cell and re-affirmed with a radiant power meter from Newport (842-PE). The IPCE was measured using a quantum efficiency measurement system (Oriel IQE-200), where 250 W quartz tungsten and halogen lamps were used as the light sources, the monochromator path length was 1/8M and the spot size was 1 mm × 2.5 mm rectangular at focus. This instrument gave the IPCE values as a function of wavelength directly.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

PS thanks CSIR-HRDG, New Delhi for providing fellowship. CS, SSKR and VB acknowledge the financial support under the SPARC program of the Science and Engineering Research Board of India. VB acknowledges financial support under the MEXT JSPS Grant-in-Aid for Scientific Research B (19H02550). HM and VB acknowledge financial support under the MEXT JSPS Special Advancement Research Grant (18H05205).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/d0na00641f

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