Efficient photoelectrochemical water oxidation using a TiO₂ nanosphere-decorated BiVO₄ heterojunction photoanode

Abstract

Constructing heterojunctions by coupling dissimilar semiconductors is a promising approach to boost charge separation and charge transfer in photoelectrochemical (PEC) water splitting. In this work, we fabricated a highly efficient TiO₂/BiVO₄ heterojunction photoanode for PEC water oxidation via a simple hydrothermal method. The resulting heterojunction photoanodes show enhanced PEC performance compared to the bare BiVO₄ due to the simultaneous improvements in charge separation and charge transfer. Under simulated sunlight illumination (AM 1.5G, 100 mW cm⁻²), a high photocurrent of 3.3 mA cm⁻² was obtained at 1.23 V (vs. the reversible hydrogen electrode (RHE)) in a neutral solution, which exceeds those attained by the previously reported TiO₂/BiVO₄ heterojunctions. When a molecular Co–cubane catalyst was immobilized onto the electrode, the performance of the TiO₂/BiVO₄ heterojunction photoanode can be further improved, achieving a higher photocurrent density of 4.6 mA cm⁻² at 1.23 V, an almost three-fold enhancement over that of the bare BiVO₄. These results engender a promising route to designing an efficient photoelectrode for PEC water splitting.

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Introduction

With gradual, increasing consumption resulting in the depletion of traditional fossil fuels, new forms of energy are being investigated. Sunlight has attracted more interest as it is the most abundant renewable energy source.^1–3 Solar energy, a new type of renewable energy, has the advantages of being a clean and inexhaustible source of energy and is attracting research interest. Splitting water into hydrogen and oxygen using sunlight, via photoelectrochemical (PEC) devices, is a promising method of converting and storing solar energy as a chemical fuel.^2–5 To date, an enormous effort has been invested in this field, however, the development of a highly efficient PEC water splitting cell for solar-to-hydrogen fuel conversion remains a challenge.^6,7 And the PEC water splitting using semiconductors has received increasing attention, owing to the semiconductors have excellent light absorption characteristics, for example, bismuth vanadate (BiVO₄) and TiO₂. BiVO₄ has appropriate band-edge position (2.4 eV) and a wide range of absorption of sunlight.^8,9 However, the unmodified BiVO₄ has significant electron–hole recombination and poor water oxidation kinetics.¹⁰ Various strategies have been implemented to address these restrictions, such as nanostructured control,¹¹ elemental doping,^12,13 loading cocatalysts such as Co–Pi,^14–16 FeOOH¹⁷ and NiOOH,^10,18 construction of heterojunction,^19,20 and plasmonic enhancement.^21–23

In principle, constructing heterojunctions is the most effective and direct way to promote the efficiency of charge-separation in photoelectrodes. Many studies have investigated BiVO₄-based heterojunction photoanodes, such as WO₃/BiVO₄,^24–26 TiO₂/BiVO₄,^27–29 etc. These composites have been demonstrated to have higher PEC performance compared to BiVO₄. TiO₂ has been intensively studied as a candidate photoanode due to its favourable band-edge positions, high resistance to photocorrosion, physical and chemical stability, nontoxicity and low cost.^30,31 Recent studies have reported an improvement in PEC water oxidation using TiO₂/BiVO₄ heterojunctions.³² However, a high photocurrent density for this heterojunction electrode is scarce.

In this study, a facile hydrothermal method using modified nanoporous BiVO₄ photoanodes with TiO₂ nanospheres was proposed to develop TiO₂/BiVO₄ heterojunction photoanodes. The BiVO₄ layers in this photoanode absorb sunlight and the TiO₂ nanospheres facilitate the separation and transmission of the photogenerated charge in BiVO₄. The shift of TiO₂ and BiVO₄ interfacial Fermi levels result in the formation of an n–n junction at the interface,³³ promoting electron–hole separation. The proposed TiO₂/BiVO₄ heterojunction photoanodes had good PEC performance. A high photocurrent density of 3.3 mA cm⁻² was achieved at 1.23 V, compared to a reversible hydrogen electrode (RHE) in a neutral electrolyte, which is much higher than that of BiVO₄ and exceeds other TiO₂/BiVO₄ heterojunctions reported. To further optimise the kinetics of surface water oxidation of photoanodes, a molecular Co–cubane water oxidation catalyst (WOC) was immobilized on the electrode and an extremely high photocurrent density of 4.6 mA cm⁻² was achieved at 1.23 V vs. RHE.

Experimental section

Fabricate of the BiVO₄ photoanode

The BiVO₄ electrodes were prepared by using an electrodeposition process as reported.¹⁰ Briefly, the electrodeposited solution was prepared as follow: 2.91 g Bi(NO₃)₃·5H₂O (99.0%, AR) and 9.96 g KI (99.0%, AR) were dissolved in 150 mL deionized water respectively before its pH was adjusted to 1.7 by adding HNO₃, then this solution was mixed with 20 mL of absolute ethanol contained 1.49 g p-benzoquinone (99%) by stirring for a few minutes vigorously. A three-electrode cell was used for electrodeposition, with an FTO as the working electrode, an Ag/AgCl (3.5 M KCl) reference, and a platinum counter electrode. A CHI660 was used for electrodeposition studies. Deposition was carried out potentiostatically at −0.1 V vs. Ag/AgCl for 3 min at 30 °C (equivalent to passing a total charge of 0.45C cm⁻²). All prepared films were rinsed by deionised water and blow-dried with nitrogen. 10 mL of a dimethyl sulfoxide (DMSO, AR) solution containing 1.06 g vanadyl acetylacetonate (VO(acac)₂ 98%) was placed on the BiOI electrode. Then the BiOI electrodes were annealed at 450 °C (ramping rate = 2 °C min⁻¹) for 2 h. After annealing, the as deposited film was converted to crystalline BiVO₄ and amorphous V₂O₅, and pure BiVO₄ was obtained by dissolving the V₂O₅ in 1 M NaOH under stirring for several minutes.

Fabricate of the TiO₂/BiVO₄ photoanode

The TiO₂/BiVO₄ electrode was developed using a hydrothermal method. The TiO₂ nanospheres were formed using a hydrolysis reaction. 1 mL of TiCl₄ solution was added to 50 mL deionized water, and centrifuged for 5 min at 10 000 rpm until the suspended precipitate was spun down into a pellet. The supernatant was removed and precipitate resuspended in deionized water. The TiO₂ aqueous solution was subjected to ultrasonic treatment to promote the dispersion of TiO₂ nanospheres throughout the solution. A BiVO₄ electrode was immersed into the TiO₂ aqueous solution for 10 min (10 min-TiO₂/BiVO₄), 30 min (30 min-TiO₂/BiVO₄) and 60 min (60 min-TiO₂/BiVO₄) at 60 °C. Finally, the electrode was washed using deionized water and then calcined at 500 °C for 2 h in air (2 °C min⁻¹).

Development of a TiO₂ photoanode

A TiO₂ electrode was prepared using a spin-coating method. TiO₂ was deposited onto the FTO using spin coating at 1000 rpm for 20 s. The spin coating solution was the previously described solution in Section 2. After spin coating, the samples were annealed at 500 °C for 2 h with a temperature increase of 2 °C min⁻¹.

Synthesis of Co–cubane catalyst

The Co-catalyst was synthesised as below: Co(NO₃)₂·6H₂O (2.90 g, 10 mmol) and CH₃COONa·3H₂O (2.70 g, 20 mmol) added in 30 mL methanol and heated to refluxing temperature, then 4-cyanopyridine (10 mmol) was added to the reaction mixture. 30% hydrogen peroxide (5 mL) was slowly added into the reaction mixture, which was held at reflux for a further 4 h. The reaction mixture was cooled and concentrated in a rotary evaporator, and the aqueous layer was separated by adding CH₂Cl₂. The light pink aqueous layer was discarded, and the CH₂Cl₂ layer was dried using anhydrous Na₂SO₄. An olive-green compound precipitated out on addition of petroleum ether. Fig S1† is the structure of the Co–cubane. Fig S2† is the ¹H-NMR of the catalyst. ¹H-NMR (500 MHz, CD3OD): 8.65 (d, 8H), 7.62 (d, 8H), 2.2 (d, 12H).

Preparation of the Co–cubane/TiO₂/BiVO₄ photoanode

The catalyst solution is prepared by dissolving the Co–cubane catalyst into solution (CH₃OH/CH₂Cl₂/Nafion mixture (7 : 2.5 : 0.5, v/v)) at a concentration of 1 mM. A 10 μL of the cobalt catalyst was then cast onto the surface of the TiO₂/BiVO₄ electrode and dried at room temperature.

Photoelectrochemical measurements

The PEC performance of the BiVO₄ and TiO₂/BiVO₄ electrodes were tested in 0.1 M PBS solution and AM 1.5 G simulated sunlight illumination (100 mW cm⁻²). The active surface area of the BiVO₄ and TiO₂/BiVO₄ heterojunctions were 1 cm². A CHI660 was used for electrochemical PEC measurements which contained samples as the working electrode, an Ag/AgCl (3.5 M KCl) reference and a platinum counter electrode. EIS measurements was carried out at 0.6 V versus Ag/AgCl. Mott–Schottky measurements were performed under dark conditions, in the potential range −0.6 to +1.0 V versus Ag/AgCl with 500 Hz. All the potentials are presented against the reversible hydrogen electrode (RHE), and the conversion between the Ag/AgCl and RHE was performed using the equation:

E_RHE = E _Ag/AgCl + 0.059 pH + 0.197 V

Incident photon-to-current efficiency (IPCE) at each wavelength was determined using illumination from a 300 W Xe arc lamp and neutral density filters, to imitate the light from the sun. Monochromatic light was produced using an Oriel Cornerstone 130 monochromator with a 10 nm bandpass, and the output was measured using a photodiode detector. IPCE was measured at 0.6 V vs. RHE using the same three-electrode setup described above for the photocurrent measurements.

where: J is the photocurrent density, λ is the incident light wavelength and P_light is the measured irradiance.

The applied bias photon-to-current efficiency (ABPE) was calculated from a J–V curve, where: J is the photocurrent density, V_bias is the applied bias and P_in is the incident illumination power density (AM 1.5 G, 100 mW cm⁻²):

The η_tr was calculated using the equation:

η_tr = J_water/J_Na2SO3

where: J_water is the TiO₂/BiVO₄ electrode photocurrent density in 0.1 M PBS, J_Na2SO3 is the TiO₂/BiVO₄ electrode photocurrent density in 0.1 M PBS containing 0.1 M Na₂SO₃.

Results and discussion

A porous BiVO₄ was produced using the previously reported method.¹⁰ Fig. 1a shows the microstructure of the porous BiVO₄; the significant voids can be seen clearly. The porous structure of the BiVO₄ enables sites for the TiO₂ nanospheres to attach. TiO₂ nanospheres, with a range of diameter size 200 to 500 nm (illustrated by DLS in Fig S3†) were attached onto the BiVO₄ electrodes using a hydrothermal method. The SEMs of TiO₂/BiVO₄ electrodes with different hydrothermal times are shown in Fig. 1b–d, which show that the TiO₂ nanospheres were attached onto the BiVO₄ successfully and the quantity of TiO₂ nanospheres increased with immersed time. The elemental mapping results shown in Fig. 1e–h suggest the existence of O, Ti, Bi, and V elements, indicating a uniform distribution of TiO₂ nanospheres across the structure.

Fig. 1 SEMs of different electrodes. (a) BiVO₄, (b) 10 min-TiO₂/BiVO₄, (c) 30 min-TiO₂/BiVO₄, (d) 60 min-TiO₂/BiVO₄, and EDX-mapping of TiO₂/BiVO₄ are shown (e) Bi, (f) V, (g) O, (h) Ti.

The crystalline structures of the FTO, BiVO₄ and TiO₂/BiVO₄ samples were characterised using X-ray diffraction (XRD) patterns, as shown in Fig S4,† further confirming that TiO₂ attached to the porous BiVO₄ and that the crystalline structures of TiO₂ are anatase.

In order to gain more insight into TiO₂/BiVO₄ heterojunctions, X-ray photoelectron spectroscopy (XPS) characterisation was performed, which showed signals from Ti, O, Bi and V elements (Fig. S5†). The HR-XPS spectra of the TiO₂/BiVO₄ are shown in Fig. 2. Two binding energy peaks for a typical Bi element in BiVO₄ occurred at 158.9 and 164.2 eV, corresponding to the Bi 4f7/2 and Bi 4f5/2, suggesting that Bi is present as Bi³⁺ (Fig. 2a).³⁴ The binding energies of the vanadium(V) peaks were centred at 516.5 and 524.5 eV, corresponding to V 2p3/2 and V 2p1/2 orbits, respectively, characteristic of the presence of the V⁵⁺ oxidation state (Fig. 2b).³⁵ Two peaks for Ti 2p at 458.7 and 465.7 eV were assigned to Ti 2p3/2 and Ti 2p1/2, respectively, suggesting a Ti⁴⁺ oxidation state (Fig. 2c).³⁶ The peak at binding energy of 529.8 eV in Fig. 2d, indicates that oxygen was present as surface lattice oxygen and free oxygen.³⁷

Fig. 2 XPS spectra of the TiO₂/BiVO₄ electrode (a) Bi 4f, (b) V 2p, (c) Ti 2p, (d) O 1s.

The PEC performance of the BiVO₄ and TiO₂/BiVO₄ was measured using a three electrode system which included the proposed electrode as the working electrode, an Ag/AgCl (3.5 M KCl) reference and a platinum counter electrode. Fig. 3a shows the linear sweep voltammetry (LSV) curves of the BiVO₄ and TiO₂/BiVO₄ electrodes. In the dark those electrodes show little photocurrent density. However, in the light, the differences in the TiO₂ on the BiVO₄ electrode resulted in a different photocurrent density, and TiO₂/BiVO₄ electrodes had a significantly enhanced photocurrent density compared to BiVO₄ and TiO₂ electrodes. The 30 min-TiO₂/BiVO₄ electrode had the highest photocurrent density, with a value of 3.3 mA cm⁻² at 1.23 V vs. RHE, which was 2.5 times that of BiVO₄. However, the 60 min-TiO₂/BiVO₄ electrode had lower photocurrent density compared to 30 min-TiO₂/BiVO₄, which may be a result of TiO₂ hindering charge transfer. A significant cathodic shift of onset potential was observed. For the 30 min-TiO₂/BiVO₄ electrode, the potential at 0.5 mA cm² was reduced greater than 500 mV, indicating an efficient charge separation or charge transfer originating from the formation of a heterojunction.

Fig. 3 (a) LSV curves of different TiO₂/BiVO₄ electrodes. (b) LSV of curves BiVO₄ and TiO₂/BiVO₄ in 0.1 M PBS containing 0.1 M Na₂SO₃. (c) The charge transfer efficiencies of different TiO₂ content on BiVO₄. (d) IPCEs of BiVO₄ and TiO₂/BiVO₄ electrodes.

To test the charge recombination behaviour of the TiO₂/BiVO₄ electrodes, 0.1 M Na₂SO₃ solution was added to the electrolyte as a hole scavenger for the PEC measurements. The LSV curves of TiO₂/BiVO₄ electrodes are shown in Fig. 3b. TiO₂/BiVO₄ electrodes show higher photocurrent density compared to the BiVO₄ electrode, demonstrating a heterojunction was formed and better charge separation occurred. The 30 min-TiO₂/BiVO₄ electrode had the highest photocurrent density, 5.4 mA cm⁻² at 1.23 V vs. RHE. The photocurrent can be described as: J = J_abs × η_sep × η_tr³⁸ the η_r values can be calculated as J_water/J_Na2SO3. As shown in Fig. 3c, all TiO₂/BiVO₄ electrodes had improved η_tr compared to the BiVO₄ electrode, over the range tested in this study. The 30 min-TiO₂/BiVO₄ electrode had the highest η_tr, 60% at 1.23 V vs. RHE, 3 times that of the BiVO₄ electrode. These results demonstrate that the construction of a TiO₂/BiVO₄ heterojunction could promote charge separation and charge transfer efficiencies. The incident-photon-to-current-conversion efficiency (IPCE) values are shown in Fig. 3d. The TiO₂/BiVO₄ was higher than BiVO₄ across the visible spectrum and at 520 nm the two electrodes reached 0, which is in agreement with the light absorbance of BiVO₄ and TiO₂/BiVO₄ electrodes (Fig S6†).

The photocurrent transient behaviours of the TiO₂/BiVO₄ electrodes were measured in 0.1 M PBS. A photocurrent spike was obtained using sudden illumination due to capacitive charging of the interface, and the spike decay due to the recombination of the electrons and holes that is associated with holes getting trapped at the electrode surface. A stabilised photocurrent was achieved when the charge generation and recombination rates were equal. Furthermore, the recombination rate could be reflected by longer transient decay times, which can be analysed on a logarithmic plot of parameter D, using the following equation:³⁹

D = (I_t − I_s)/(I_m − I_s)

where: I_t is the photocurrent density at time t, I_s is the stabilised photocurrent density and I_m is the photocurrent density spike. The transient decay time can be defined as the time at which ln D = −1. Fig. 4a shows the logarithmic plots of parameter D of the BiVO₄ and TiO₂/BiVO₄ electrodes. The transient decay time for TiO₂/BiVO₄ was 4.1 s, longer than that of BiVO₄ (1.5 s), suggesting a lower rate of electron and hole recombination in TiO₂/BiVO₄, which could be attributed to the TiO₂ nanospheres acting as hole resistors, resulting in increased efficiency of charge separation and prolonging the lifetime of the holes. As a result, abundant photogenerated charge carriers in the TiO₂/BiVO₄ electrode were effectively collected, leading to an improved PEC performance.

Fig. 4 (a) The logarithmic plots of the parameter D of the BiVO₄ and TiO₂/BiVO₄ electrodes. (b) Electrochemical impedance spectroscopy (EIS) of the TiO₂ under different soaking times.

To characterise the kinetics of the charge transfer process of the TiO₂/BiVO₄ and BiVO₄ electrodes, electrochemical impedance spectroscopy (EIS) tests were performed at 1.23 V vs. RHE in 0.1 M PBS under simulated solar light illumination. Fig. 4b shows Nyquist at frequencies from 1 Hz to 100 kHz. The BiVO4 electrode had the greatest resistance to charge transfer and the 30 min-TiO₂/BiVO₄ had the lowest charge transfer resistance. However, the 60 min-TiO₂/BiVO₄ electrode had a lower charge transfer resistance, which may be a result of the TiO₂ impeding the transfer of surface charge, further indicating the optimum amount of TiO₂ was achieved in the 30 min-TiO₂/BiVO₄ electrode. The above results suggest that the TiO₂ facilitates the BiVO₄ electrode charge separation and transfer.

In order to further investigate the PEC performance, a molecular Co–cubane water oxidation catalyst was immobilised onto the TiO₂/BiVO₄ electrode. As the LSV curves show in Fig. 5a, the Co–cubane/TiO₂/BiVO₄ photoanode exhibits a high photocurrent density of 4.6 mA cm⁻² at 1.23 V vs. RHE, 3.1 times higher than that of BiVO₄, indicating that the Co–cubane was acting as an efficient OER catalyst, further enhancing the charge transfer. Furthermore, after 400 s photoelectrolysis, the photocurrent density of Co–cubane/TiO₂/BiVO₄ could maintain more than 2.6 mA cm⁻², which is 2.6 times than that of the bare BiVO₄ (Fig S7†). The half-cell photoconversion efficiencies of BiVO₄ and Co–cubane/TiO₂/BiVO₄ electrodes were calculated using the LSV results, (Fig. 5b). The maximum value of Co–cubane/TiO₂/BiVO₄ achieved 1.48% at 0.72 V, approximately 6 fold compared to BiVO₄. The XPS spectra of the Co–cubane/TiO₂/BiVO₄ electrode was shown in Fig S8,† which indicated the element of Co was exited.

Fig. 5 (a) LSV curves of BiVO₄ and Co–cubane/TiO₂/BiVO₄ electrodes. (b) ABPEs of BiVO₄ and Co–cubane/TiO₂/BiVO₄ electrodes. (c) The charge transfer mechanism of TiO₂/BiVO₄ electrode.

To explore the charge transfer mechanism between the BiVO₄ and TiO₂, Mott–Schottky plots of the BiVO₄ and TiO₂ were performed under dark conditions.⁴⁰ In Fig S9b,† the linear part of Mott–Schottky plots of TiO₂/BiVO₄ rather than BiVO₄ show the gentlest slope and the more negative flatband potential. The gentle slope means the modifying with TiO₂ can obviously increase the density of the carriers.⁴¹ From Fig S9a and c,† the CB position of BiVO₄ and TiO₂ were 0.12 eV and 0.2 eV. And from Fig S10,† the band gap energies of BiVO₄ and TiO₂ were 2.42 eV and 3.09 eV. Therefore, the VB position of BiVO₄ and TiO₂ were 2.54 eV and 3.29 eV, calculated using E_CB = E_VB − E_g. Compared the BiVO₄, the TiO₂ approach to the intrinsic semiconductor, So we suppose the charge transfer mechanism shown the Fig. 5c, When these two types of semiconductor materials are closely joined together, the heterojunction structure was formed. At this moment, the two semiconductors have a uniform Fermi level and the system is in equilibrium. An electron transfer from TiO₂ to BiVO₄ possibly occurred, providing evidence for the efficient charge transfer between BiVO₄ and TiO₂.

Conclusions

In summary, a simple hydrothermal method was proposed to produce a TiO₂/BiVO₄ heterojunction electrode. The proposed TiO₂/BiVO₄ electrode provided a high performance for the oxidation of PEC water. The TiO₂/BiVO₄ heterojunction results indicate good charge separation and transfer efficiency. When a molecular Co–cubane catalyst was attached onto the TiO₂/BiVO₄ electrode, a further improvement in the PEC performance was achieved, with a photocurrent of photocurrent density of 4.6 mA cm⁻² at 1.23 V, more than 3 times for BiVO₄ alone. This study provides a simple and effective approach in the production of heterojunction photoelectrodes for increasing the performance of PEC, which could be used to design efficient photocatalytic systems in future studies.

Conflicts of interest

The authors declare no conflict of interest.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (21401092, 21671089, 21773100), the Shenyang Natural Science Foundation of China (F16-103-4-00), Scientific Research Found of Liaoning Province (LT2017010, 20170540409, 20180510011).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c8ra09072f

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