Solution - processed Bi 2 S 3 /BiVO 4 /TiO 2 ternary heterojunction photoanode with enhanced photoelectrochemical performance

: TiO 2 is an important component of photoelec - tric devices. How to broaden the light absorption of TiO 2 and accelerate the separation of photo - generated elec - trons and holes is the focus of the current research. Building heterojunction with narrow band gap semicon - ductor and TiO 2 is one of the important measures to improve its photoelectric performance. We prepared BiVO 4 /TiO 2 binary heterojunction by the simple hydrothermal method and analyzed the e ﬀ ect of BiVO 4 precursor solution concen - tration on the microstructure and photoelectric performance of the heterojunction. BiVO 4 /TiO 2 binary heterojunction can e ﬀ ectively improve the photoelectric performance of TiO 2 , and the transient current density reaches 85 μ A/cm 2 . To further boost the photocurrent of BiVO 4 /TiO 2 , Bi 2 S 3 was in situ grown on the heterojunction to form Bi 2 S 3 /BiVO 4 /TiO 2 ternary heterojunction. The results show that the band gap of Bi 2 S 3 /BiVO 4 /TiO 2 composites is signi ﬁ cantly narrowed compared with that of TiO 2 . The light absorption has been expanded to the visible range, and the photogenerated current density is also greatly boosted ( 0.514 mA/cm 2 ) . This Bi 2 S 3 /BiVO 4 /TiO 2 ternary heterojunction accelerates the separation of photo - carriers and improves the photo - electric performance of the device. The possible transport mechanism of photo - carriers in ternary heterojunction is analyzed. The current study provides an e ﬀ ective strategy for in situ construction of novel multicomponent heterojunction and provides a basis for the application of Bi 2 S 3 /BiVO 4 /TiO 2 in the optoelectronic ﬁ eld.


Introduction
Due to the process of the economic development, the negative effects of the highly developed industry are not anticipated enough, and the prevention is not in place, resulting in a global energy crisis and environmental pollution [1].Therefore, it is imperative to develop advanced technologies for environmental remediation, as well as energy storage and conversion [2].Solar energy is one of the most potential clean energy, which has attracted wide attention [3], due to its abundance, cleanliness, security, and sustainability [4].Solar cells are effective devices that can directly convert solar energy into electrical energy.Organic-inorganic hybrid perovskite thin film solar cells are the most potential photovoltaic devices.Photoanode is the core part of thin film solar cells.Photo-anode materials mainly include SnO 2 , ZnO, and TiO 2 .Because of the wide band gap, these common photo-anode materials have low absorption and utilization of visible light.How to make the photoelectric performance better of these photoanode materials is one of the research highlights.
Among these kinds of photo-anode materials, TiO 2 has the following advantages: excellent physical and chemical stability, nontoxic, nonpolluting, and low cost, which is still the main research object of photo-anode materials.However, TiO 2 has the defects of a wide band gap (about 3.2 eV) and electron-hole easy to recombination, which limit its optical and photoelectric properties.Therefore, TiO 2 needs to be modified [5], such as semiconductor recombination [6], doping [7], surface modification [8], and other methods, which can improve the performance of TiO 2 , broaden the optical absorption range, restrain the recombination of electrons and holes, improve the separation of photo-generated carriers, and greatly raise the photoelectric conversion efficiency of devices.Among them, combing narrow band gap semiconductor and TiO 2 to construct hetero-junction is the most widely used and effective method [9].For example, Bi 2 S 3 , CdS, Cu 2 S, and so on, these narrow band gap semiconductors can be combined with TiO 2 to build and form heterojunctions.In our previous researches [3,[10][11][12], we prepared Bi 2 S 3 nanomaterials and constructed TiO 2 /Bi 2 S 3 heterojunction with TiO 2 .The results showed that the TiO 2 /Bi 2 S 3 hetero-junction can effectively improve its photoelectric performance.
Bismuth-based semiconductors have attracted research interest, mainly because of their stable chemical properties, suitable band gap, low cost, and easy preparation.BiVO 4 is a semiconductor material with properties such as good visible light driving activity, suitable band gap [13] (2.4 eV), low cost, splendid dispersion, nontoxic, and corrosion resistance [14,15].The construction of hetero-junction between BiVO 4 and TiO 2 can widen the absorption range to visible light and promote the dissociation of electron-hole pairs.Wang et al. [16] prepared snowflake-shaped BiVO 4 and TiO 2 microspheres, obtained TiO 2 /BiVO 4 Z-type hetero-junction through physical mixing, and used them for photocatalytic degradation of pollutants.Guo et al. [17] indicated that coupling BiVO 4 with TiO 2 by electrospinning enhances the photocatalytic degradation of rhodamine B dye. Khalil et al. [18] reported the role of exposed TiO 2 's (001) and (101) facets on the performance of BiVO 4 /TiO 2 photocatalytic fuel cells.At the same time, from the perspective of electronic structure, the hybridization of the 6 s orbital of Bi and the 2p orbital of O will cause the E VB to move up, which can accelerate the separation of electron and hole pairs in the process of electron transfer.On the basis of our previous research, we have learned that a single compound sensitizes TiO 2 , and its improved photoelectric performance is limited.
Co-sensitizers are promising materials that bring broader light absorption and an optimal cascade structure of energy level in quantum dots, leading to enhancing the charge transfer process and better photovoltaic performance than bare quantum dots [19].Among many photosensitive materials, BiVO 4 and Bi 2 S 3 are potential photon-absorbing materials, which have the tendency to capture the visible photons in the solar spectrum.The bandgap of BiVO 4 is 2.4 eV, and the bandgap of TiO 2 is 3.2 eV.Due to the energy band difference, TiO 2 and BiVO 4 can construct an efficient hetero-junction and promote carrier separation [17].BiVO 4 / TiO 2 photo-catalysts were prepared and reported [16][17][18].
Likewise, Bi 2 S 3 is an excellent light-absorbing material with the moderate band gap.For example, Han and Jia [20] prepared and used the 3D Bi 2 S 3 nanosheet-modified TiO 2 nanorod array.It showed good photo-electrochemical properties and high charge transfer efficiency.Wu et al. [21] effectively deposited Bi 2 S 3 nanoparticles on TiO 2 nanotube arrays by sequential chemical bath deposition.The results of electrochemical impedance spectroscopy and photoluminescence spectroscopy showed that the photo-generated electrons and holes of Bi 2 S 3 -TiO 2 composites were effectively separated under visible light excitation, and their photo-catalytic performance was greatly improved compared with a single TiO 2 nanotube.Annealing treatment on the Bi 2 S 3 /TiO 2 can effectively improve its photoelectric properties [12].
To further promote the photoelectric performance of TiO 2 , we first prepared TiO 2 /BiVO 4 binary heterojunction by the hydrothermal method, optimized the effect of the amount of BiVO 4 precursor reactant on the heterojunction, and optimized the process parameters.On the basis of this heterojunction, Bi 2 S 3 was in situ grown on BiVO 4 /TiO 2 binary heterojunction by the hydrothermal method, and then the Bi 2 S 3 /BiVO 4 /TiO 2 ternary heterojunction was formed.We optimized the hydrothermal growth time of Bi 2 S 3 .The experimental results show that the prepared Bi 2 S 3 /BiVO 4 / TiO 2 heterojunction exhibits excellent photoelectric characteristics, and its transient photoelectric current is greatly improved.Finally, the transport mechanism of photo-generated carriers in Bi 2 S 3 /BiVO 4 /TiO 2 heterojunction is analyzed.

Preparation of TiO 2
The TiO 2 materials were grown on the FTO substrate.The FTO was cleaned with acetone, absolute ethanol, and deionized water for 20 min by ultrasonic cleaning.A total of 15 mL of concentrated sulfuric acid and 15 mL of deionized water were mixed in a beaker.After uniform magnetic stirring, 0.5 mL of tetrabutyl titanate was added with a pipette gun, and magnetic stirring was continued for 40 min.The precursor solution was transferred to 50 mL polytetrafluoroethylene lining and placed into the reactor for the hydrothermal reaction, the reaction temperature is 150°C, and the reaction time is 12 h.After the reaction, the mixture is cooled to room temperature.The sample was taken out, washed alternately with deionized water and absolute ethanol, and dried at 60°C to obtain TiO 2 nanorod arrays.

Preparation of BiVO 4 /TiO 2 composite
BiVO 4 /TiO 2 composite films were synthesized by the simple hydrothermal method.First, 3 mmol (1.455 g) of Bi(NO 3 ) 3 •5H 2 O is weighed and dissolved in 25 mL of ethylene glycol under magnetic stirring, which is recorded as solution A. A total of 3 mmol (0.351 g) of NH 4 VO 3 is dissolved in 15 mL of hot deionized water (60°C) by stirring using a magnetic stirrer, and the magnetic stirring is continued for 30 min until the solution is uniform.Then, solution B is added to solution A under stirring.At this time, the solution appears bright orange.Stirring is continued for 15 min to obtain the precursor solution for preparing BiVO 4 .The prepared TiO 2 nanorod sample was placed into the polytetrafluoroethylene lining carefully in the conductive surface downward position at an angle of about 45°, and then the precursor solution prepared in advance is poured.The conductive glass about 1 cm long is placed without immersing in the solution.The hydrothermal reaction is conducted under the constant temperature of 120°C, and the reaction temperature is 10 h.The sample was cooled naturally to room temperature.After the reaction, the sample was rinsed with deionized water and dried to obtain BiVO 4 /TiO 2 sample.In the process of hydrothermal preparation of nanomaterials, the precursor concentration is an important variable.So, during the BiVO 4 growth, we set the reaction precursor concentration as 2, 3, and 4 mmol.For convenience, we marked the prepared samples BiVO 4 / TiO 2 as VT (2 mmol), VT (3 mmol), and VT (4 mmol).
2.4 Preparation of Bi 2 S 3 /BiVO 4 /TiO 2 composite Bi 2 S 3 /BiVO 4 /TiO 2 hetero-junction was also prepared by the hydrothermal method.A total of 1 mmol (0.076 g) of thiourea was weighed in an electronic balance and dissolved in 35 mL of deionized water through in situ conversion using thiourea as the sulfur source, and magnetic stirring was performed for 30 min.The conductive surface of the prepared BiVO 4 /TiO 2 composite film sample was tilted downward and placed into the lining, and then the sample was poured into the prepared sulfur source solution, leaving about 1 cm of conductive glass not immersed in the solution.The hydrothermal reaction was carried out at 170°C for 1.5, 3, 5, and 7 h, respectively.At the end of the reaction, the sample was washed and dried with deionized water and absolute ethanol to obtain Bi 2 S 3 / BiVO 4 /TiO 2 (BVT) heterostructure samples.The samples obtained are recorded as BVT (1.5 h), BVT (3 h), BVT (5 h), and BVT (7 h) according to the reaction time.Figure 1 shows the schematic of Bi 2 S 3 /BiVO 4 /TiO 2 ternary heterojunction preparation process.

Characterization
The phase composition was characterized by X-ray diffractometer (D8 Advanced X, Brooke, Germany), using   121), (040), ( 211), (150), and (161) planes (JCPDS No: 14-0688).The diffraction peak intensity of (121) and (040) planes increases with the BiVO 4 precursor concentration increase, indicating that more and more BiVO 4 materials were formed.These (121) and (040) crystal planes indicated the monoclinic phase BiVO 4 appear.Figure 3 shows the SEM images of TiO 2 nanorod array and BiVO 4 /TiO 2 composites (VT (2 mmol), VT (3 mmol), and VT (4 mmol)).Figure 3(a) shows the TiO 2 nanorod array SEM image.The TiO 2 nanorod array is very intuitive.These nanorods are evenly arranged and grown in the direction perpendicular to the conductive glass substrate, and no obvious defects can be seen.In addition, these nanorods are closely arranged with each other, presenting a rod-like array of quadrilateral structure, with good dispersion, and the top of the rod is relatively rough.The morphology of TiO 2 nanorod array prepared in this work is consistent with that reported by Zhu et al. [22] and Serikov et al. [23].The reason for the formation of this arrangement can be attributed to the fact that during the growth of TiO 2 crystal nucleus under acidic conditions, the (110) crystal surface was first adsorbed by Cl -, and the growth rate of this crystal surface decreased, making TiO 2 grow anisotropic along the [001] orientation.Figure 3(b)-(d) shows the SEM images of VT (2 mmol), VT (3 mmol), and VT (4 mmol), respectively.It is obvious that the peanut-like BiVO 4 is attached to the TiO 2 nanorod array.BiVO 4 particles are evenly distributed.When the concentration of the precursor of the reactant continues to increase, the morphology of BiVO 4 changes, showing a football shape with two ends.It shows that BiVO 4 /TiO 2 heterojunction was successfully prepared at the low-temperature hydrothermal method with 120°C, which is consistent with the analysis conclusion of the XRD diagram.
To clarify the growth of BiVO 4 on TiO 2 , we tested and analyzed the cross section of the BiVO 4 /TiO 2 sample.Figure 4(a) shows the cross-sectional SEM image of BiVO 4 /TiO 2 .It can be seen that the TiO 2 nanorods are aligned neatly and grown vertically on the FTO substrate.On the top of the TiO 2 nanorods, there are small projections composed of scales.From the top view of SEM images (Figure 3(b-d)), we have For this BiVO 4 /TiO 2 binary heterojunction, it was characterized by TEM. Figure 5(a) shows the TEM image of the BiVO 4 /TiO 2 composite film.We can see that there are mulberry-like substances on the TiO 2 nanorods.This mulberry-like substance is BiVO 4 .Figure 5 Transient photocurrent response and electrochemical impedance are important parameters to evaluate the photo-electrochemical properties of photo-anode materials.Figure 6 shows the transient photocurrent diagrams and electrochemical impedance diagrams of BiVO 4 /TiO 2 composites and TiO 2 nanorod array.The generation and disappearance of photocurrent are analyzed and characterized by controlling the light source switch.When the light is irradiated on the heterojunction, the photocurrent will be generated instantaneously.When the light is turned off, the photocurrent returns to zero, which indicates that the electron-hole pairs generated by the light excitation are separated.The VT (3 mmol) composite shows the maximum photocurrent density (0.0825 mA/cm 2 ), followed by VT (2 mmol) and then VT (4 mmol).The photocurrent response of the sample directly reflects the generation and transfer of photogenerated carriers [24].Compared with pure TiO 2 nanorod array samples, the photocurrent density of BiVO 4 /TiO 2    Solution-processed Bi 2 S 3 /BiVO 4 /TiO 2 ternary heterojunction photoanode  7 36.1,62.9, and 69.8°match with the (101), (002), and (112) crystal planes of TiO 2 , which are in better agreement with the standard PDF#21-1276, and thus, the prepared TiO 2 is known to be rutile.In the XRD pattern of the binary composite VT (see the VT curve in Figure 7(a)), there is an obvious diffraction peak at 18.9°, which matches with the standard PDF#14-0688 of BiVO 4 , corresponding to the (011) crystal plane of BiVO 4 , and after the growth of Bi 2 S 3 , the diffraction peak of BiVO 4 at this point is not obvious, probably due to adhering of Bi 2 S 3 , which affects BiVO 4 diffraction peak intensity.Moreover, the diffraction peaks at 28.8 and 54.6°are corresponding to the (121) and (013) crystal planes of BiVO 4 , respectively.Figure 7(b) is a partially enlarged view of Figure 7(a).The diffraction peaks of Bi 2 S 3 match well with the standard PDF#17-0320, and the diffraction peak positions at 25.2, 33.9, and 45.7°match with the (310), (311), and (440) crystal planes of Bi 2 S 3 in BVT (1.5 h), the diffraction peak at 25.2°is almost absent, presumably due to the short hydrothermal time and the low amount of Bi 2 S 3 generated by conversion.In addition to this, some characteristic peaks of conductive glass SnO 2 were observed.In this XRD pattern, the diffraction peaks corresponding to Bi 2 S 3 , BiVO 4 , and TiO 2 can be found for the samples obtained at different hydrothermal times, indicating that the ternary photo-anode composites were successfully prepared.
Figure 8 displays the SEM images of TiO 2 , BiVO 4 / TiO 2 , and Bi 2 S 3 /BiVO 4 /TiO 2 composite films.This rodlike TiO 2 nanoarray is consistent with those reported by Zhu et al. [22] and Serikov et al [23].The reason for the formation of this arrangement can be attributed to the preemptive adsorption of Cl − on the (110) crystal plane in the growth of TiO 2 nuclei under acidic conditions, which reduces the growth rate of this crystal plane and makes TiO 2 grow anisotropically along the [001] orientation [26].Figure 8(b) shows the SEM image of BiVO 4 /TiO 2 binary heterojunction.The peanut-like BiVO 4 grains appeared on the basis of the original TiO 2 .Figure 8(c)-(f) shows the SEM images of Bi 2 S 3 /BiVO 4 /TiO 2 heterojunctions prepared by growing Bi 2 S 3 using different hydrothermal times of 1.5, 3, 5, and 7 h, respectively.In Figure 8(c)-(f), when the hydrothermal time is 1.5 h, the in situ transformation has grown long strips of Bi 2 S 3 , stacked together in a burr-like manner, but with little content.With the gradual increase of hydrothermal time, the content of Bi 2 S 3 increased with the increase of hydrothermal time, and the content of long strips of Bi 2 S 3 reached the highest at the hydrothermal time of 5 h, the grains were clear and evenly attached to the surface of BiVO 4 , and when the hydrothermal time was 7 h, the morphology of some Bi 2 S 3 changed from long strips to short rods and flakes, which were closely piled up and covered up the BiVO 4 , which was difficult to observe.The tight structure between TiO 2 , BiVO 4 , and Bi 2 S 3 further confirmed the successful preparation of Bi 2 S 3 /BiVO 4 /TiO 2 heterojunctions using different hydrothermal times, which was consistent with the analytical conclusion of XRD plots.For the in situ growth of Bi 2 S 3 , cysteine also can be used as the sulfur source [27].The shape of Bi 2 S 3 obtained by using cysteine as the sulfur source is significantly different from that obtained by using thiourea, indicating that the sulfur source has a great impact on the formation of Bi 2 S 3 , which will also be our plan for further research.
We have tested and analyzed the cross-sectional morphology of the Bi 2 S 3 /BiVO 4 /TiO 2 ternary heterojunction, as shown in Figure 9(a).TiO 2 nanorods grown on FTO substrates and some BiVO 4 and Bi 2 S 3 nanomaterials can be clearly seen.The morphology of BiVO 4 is consistent with the results observed in top view SEM.Bi 2 S 3 exhibits a rod-like structure, which is consistent with the SEM results shown in Figure 8(f).A line scan test was conducted on the yellow line marked portion in Figure 9 to analyze the elemental composition of the substance.In Figure 9(b), it can be found that the distribution of Si, Sn, O, Na, and Mg elements is mainly in the substrate.These elements mainly come from glass and FTO films.The Ti element is mainly distributed in the middle of the online scanning, which corresponds to the hydrothermal generation of TiO 2 nanorods.Bi, S, and V mainly focus on the top portion of the line scanning test, which correspond to BiVO 4 and Bi 2 S 3 nanomaterials.The cross-sectional SEM and line scanning test results of the Bi 2 S 3 /BiVO 4 /TiO 2 ternary heterojunction are consistent with the XRD results.
To investigate the surface chemical compositions and the chemical states of each element, XPS measurement was recorded [28,29].XPS testing was performed on both BiVO 4 /TiO 2 and Bi 2 S 3 /BiVO 4 /TiO 2 samples, as shown in Figure 10    composite.In Figure 10(d), the peak of Ti 2p 3/2 is located at 458.5 eV.The energy differences between Ti 2p 1/2 and Ti 2p 3/2 are 7.3 eV (for BiVO 4 /TiO 2 ) and 6.7 eV (for Bi 2 S 3 / BiVO 4 /TiO 2 ).This indicates that the formation of Bi 2 S 3 has a certain impact on the peak position of the Ti element.In Figure 10(e), the O1s peak splits into a main peak at 529.1 eV and a small shoulder at 530.7 eV, which are corresponding to the lattice oxygen and surface adsorbed oxygen, respectively.However, in Figure 10(f), it is found that the peak position of O1s simultaneously moves slightly toward the high energy portion.For the Bi 4f high-resolution XPS spectra, the two peaks of Bi 4f at 163.7 eV and 158.4 eV/158.5 eV binding energy, corresponding to Bi 4f 5/2 and Bi 4f 7/2 , respectively, can be proved to be Bi 3 + in Bi 2 S 3 .
Comparing Figure 10(g) and (h), it is found that a weak peak appears in Figure 10(h).The peak of S2p at 161.0 eV binding energies corresponds to S 2p 1/2 , which indicates that the S element is S 2− in Bi 2 S 3 .In Figure 10(i), the binding energy peaks at 523.3 and 515.9 eV correspond to V 2p 1/2 and V 2p 3/2 and are attributed to V 5+ in BiVO 4 .In Figure 10(j), the binding energy peaks are located at 524.2 and 516.8 eV, respectively.In the sample Bi 2 S 3 /BiVO 4 /TiO 2 , the peak position of V also shifted slightly toward the high energy direction.From the XPS pattern analysis and combined with the XRD pattern, the sample contains TiO 2 , BiVO 4 , and Bi 2 S 3 , indicating that the Bi 2 S 3 /BiVO 4 /TiO 2 ternary compliant photoanode material was successfully prepared, which is matched with the expected results.
The absorption response properties of pure TiO 2 , binary photocatalyst VT, and ternary photocatalyst BVT (5 h) were carried out.In Figure 11 wavelengths less than 400 nm, and after modification with BiVO 4 , the light absorption edge expands to 600 nm, indicating a red shift, along with a certain increase in light absorption intensity.After in situ conversion to generate ternary composite photocatalysts, the light absorption intensity increases significantly, and the light absorption limit is further broadened.It is mainly due to the adhesion of BiVO 4 and Bi 2 S 3 , which narrows the band gap of the composite.These results indicate that the comodification of BiVO 4 and Bi 2 S 3 significantly improves light absorption intensity and extends the light absorption range.In conclusion, it can be guessed that the ternary composite photocatalyst has the strongest response to visible light absorption, which is significantly stronger than TiO 2 nanorods and binary materials.According to the Tauc plot fit in Figure 11(b) and the band gap equation: , so the band gaps of TiO 2 , binary heterojunction VT, and ternary heterojunction BVT (5 h) are 3.01, 2.36, and 1.98 eV, respectively, and the BVT (5 h) with narrowed band gap shows the strongest absorption of visible light.
To investigate the photogenerated carrier transport mechanism, electrochemical tests were performed.The photocurrent response of the sample in the electrolyte solution can directly reflect the photogenerated carrier production and transfer [24].Figure 12 shows the transient photocurrent plots, electrochemical impedance plots, and Mott-Schottky plots of Bi 2 S 3 /BiVO 4 /TiO 2 with different hydrothermal times.According to Figure 12(a), the Bi 2 S 3 /BiVO 4 /TiO 2 composites prepared with different hydrothermal times are all very sensitive to light irradiation.When light is irradiated, the electrons inside the composite material are jumped by light excitation to produce photo-generated electron-hole, so there is a large instantaneous photocurrent, which corresponds to the sharp part in the curve, and with the increase of light time, a part of the broad-generated electron-hole pairs inside the material will be compounded, and the compounded carriers and separated carriers reach some kind of balance and tend to the steady state.When the light source is removed, the current decreases instantaneously.The photocurrent of the ternary heterojunction is significantly larger than TiO 2 and BiVO 4 /TiO 2 composites when illuminated, and the photocurrent density increases gradually with the increase of hydrothermal time, reaching a maximum value of 0.514 mA/cm 2 at 5 h.However, the photocurrent density decreased when the hydrothermal time reached 7 h, probably due to the accumulation of too much Bi 2 S 3 , which increases the interfacial resistance, while a new composite may be formed centers [30], an increase in electron-hole pair complexation, and a decrease in the separation rate.The EIS profiles of different photo-anode materials are shown in Figure 12(b), and the curvature of the curves are basically presented, and the radius of curvature is TiO 2 > VT > BVT (7 h) > BVT (1.5 h) > BVT (3 h) > BVT (5 h) in a descending order, and the smaller radius of curvature indicates that electrons encounter less obstruction in the transmission process, and the corresponding AC impedance is smaller.Therefore, BVT (5 h) has the smallest interfacial resistance and the smallest electron loss during charge transfer compared with other samples, which is more favorable for interfacial electron transfer and suppresses carrier complexion at the same time.The slopes of Mott-Schottky (M-S) curves of different samples are positive (Figure 12 samples are all n-type semiconductors.The flat-band potential E fb can be calculated based on the following formula: where C denotes the capacitance at the electrolyte interface, e and q are the charge (1.6 × 10 −19 C), ε is relative permit- tivity, ε 0 is the vacuum permittivity (8.85 × 10 −14 F cm −1 ), N D is the charge carrier concentration, E is the applied potential, E fb is the flat-band potential, k B is the Boltzmann con- stant (1.381 × 10 −23 J K −1 ), and T is the temperature [31,32].E fb was calculated from the intercept of the 1/C 2 curve versus the x-axis [33], and the calculation values are shown in Table 1.The flat-band potentials of different samples are BVT (7 h) > BVT (3 h) > BVT (1.5 h) > BVT (5 h) in a descending order, and the flat-band potential of the BVT (5 h) sample is negative, indicating that it has the highest carrier concentration and the lowest charge recombination rate with the best photo-generated electron transport performance.The stability of current in photoanode materials under illumination is an issue that requires attention.We conducted a light stability test on the sample (BVT (5 h)) with the best photocurrent.photocurrent presents a downward trend.After 600 s of light irradiation, the photocurrent basically reaches a stable value.

Photoelectric conversion mechanism
Whether the heterojunction can effectively separate carriers depends on its energy band position.The E CB edge of the semiconductor at the point of zero charge can be calculated by the empirical equations: where E VB is the valence band-edge potential, E CB is the conduction band-edge potential, X is the electronegativity of the semiconductor, expressed as the geometric mean of the absolute electronegativity of the constituent atoms, E e is the energy of free electrons on the hydrogen scale (about 4.5 eV), and E g is the band gap of the semiconductor.Taking the absolute electronegativity values for Bi, V, S, Ti, and O as 4.69, 3.6, 6.22, 3.45, and 7.54, respectively, the X can be obtained, which are 5.56 eV (Bi 2 S 3 ), 6.16 eV (BiVO 4 ), and 5.81 eV (TiO 2 ).On the basis of the aforementioned equations, we can obtain E CB and E VB values of different semiconductors (Table 2).
Based on the calculated values, E CB and E VB positions of BiVO 4 are about 0.46 and 2.86 eV (relative to normal hydrogen electrode).Generally speaking, we deem BiVO 4 as an intrinsic semiconductor, and its Fermi level (E F ) would be in the middle of E CB and E VB .So, the BiVO 4 's E F position is approximately around 1.6 eV [18,34,35].In addition, the TiO 2 's E F position is about −0.1 eV.When TiO 2 and BiVO 4 contact to form a heterojunction, their Fermi levels reach a unified state.After thermodynamical equilibrium, BiVO 4 's E F reached to be the same as that of TiO 2 's E F (−0.1 eV).So the E CB and E VB of BiVO 4 will shift more negative potential.The E CB position of BiVO 4 will change from 0.46 to −1.24 eV, and the E VB position will change from 2.86 to 1.16 eV.After thermodynamic equilibrium, photogenerated electrons are transferred from BiVO 4 to TiO 2 E CB due to energy band difference.Figure 13 displays the energy band diagram of TiO 2 and BiVO 4 before and after the heterojunction formation.After the formation of heterojunction between TiO 2 and BiVO 4 , due to the difference in energy band, the electrons in the E CB of BiVO 4 will transfer to the E CB of TiO 2 to realize the separation of photogenerated carriers.
Under the simulated sunlight, both Bi 2 S 3 and BiVO 4 will produce free electrons and holes.Based on the traditional II-type heterojunction mechanism, the photo-generated electrons in Bi 2 S 3 transfer from its E CB to the BiVO 4 's E CB .The E CB position of Bi 2 S 3 is higher than that of BiVO 4 .Likewise, generated holes in BiVO 4 will  transfer to E VB of Bi 2 S 3 due to its E VB d position, which is more positive than that of Bi 2 S 3 .Eventually, the photogenerated electron-hole pairs are completely separated.
It is also reported that Z-type heterojunction may also be formed between Bi 2 S 3 and BiVO 4 [36].When Bi 2 S 3 and BiVO 4 absorbing the sunlight, there are numbers of free electrons and holes in the E CB and E VB , respectively.At this time, the electrons in the E CB of BiVO 4 will transmigrate to the E VB of Bi 2 S 3 , accelerating the separation of electrons and holes in BiVO 4 and Bi 2 S 3 .Z-scheme heterojunction of BiVO 4 /Bi 2 S 3 allows the electron transferring from BiVO 4 to Bi 2 S 3 and accumulated the electron and hole in BiVO 4 and Bi 2 S 3 , respectively.In the aforementioned discussion, the possible carrier transmission paths between BiVO 4 /TiO 2 and BiVO 4 / Bi 2 S 3 were analyzed.Bi 2 S 3 was in situ grown on BiVO 4 by the hydrothermal method.The ternary heterojunction Bi 2 S 3 /BiVO 4 /TiO 2 was formed between TiO 2 , BiVO 4 , and TiO 2 .According to the electrochemical test, the Bi 2 S 3 / BiVO 4 /TiO 2 heterojunction showed excellent photoelectric response, which is mainly due to the rapid separation between electrons and holes in the heterojunction.Figure 14 shows the two possible transmission processes of the electrons and holes in Bi 2 S 3 /BiVO 4 /TiO 2 heterojunction, one is the traditional II-type heterojunction carrier transmission and the other is Z-scheme carrier transmission.In the conventional II-type heterojunction, it produces photo-generated carriers and transfers rapidly.The electron-hole pairs will be produced under the sun irradiation.The electrons in the E CB of Bi 2 S 3 will transfer to the E CB of BiVO 4 and TiO 2 .At the same time, the holes in the E VB of BiVO 4 and TiO 2 will shift to the E CB of Bi 2 S 3 , finally achieving the rapid separation of electron-hole pairs.There is another possible carrier transmission mode.Double Z-type heterojunction was formed in Bi 2 S 3 /BiVO 4 /TiO 2 composite.When the visible light illuminate the Bi 2 S 3 /BiVO 4 /TiO 2 composite, the electron-hole pairs will be generated in each part.The electrons in the E CB of TiO 2 and BiVO 4 will directly jump into the E VB of Bi 2 S 3 , which will lead to the accumulation of electrons in the E CB of Bi 2 S 3 and also lead to the accumulation of the holes in the E VB of BiVO 4 and TiO 2 .It is beneficial to improving the photocurrent response of the Bi 2 S 3 /BiVO 4 /TiO 2 hetero-junction.Compared with the traditional II-type heterojunction, this double-Z heterojunction has higher transmission and separation efficiency of photogenerated carriers, which is more conducive to improving the photoelectric performance of the device.

Conclusions
In our work, the Bi 2 S 3 /BiVO 4 /TiO 2 composites were created utilizing a simple hydrothermal method.Bi 2 S 3 /BiVO 4 /TiO 2 composite has stronger visible light absorption capacity than that of pure TiO 2 and BiVO 4 /TiO 2 , mainly due to the formation of ternary heterojunction.With the increase of the concentration of BiVO 4 precursor solution, the morphology of BiVO 4 changes from peanut to olive.Photo-electrochemical performance tests confirmed that BiVO 4 /TiO 2 has better electron-hole pair separation and transport efficiency.Using thiourea as the sulfur source, BiVO 4 is partially transformed into Bi 2 S 3 on the BiVO 4 /TiO 2 heterojunction and finally forms the Bi 2 S 3 /BiVO 4 /TiO 2 heterojunction.The results showed that the Bi 2 S 3 /BiVO 4 /TiO 2 has much higher photoelectric performance than pure TiO 2 and BiVO 4 / TiO 2 .This extraordinary improvement in photoelectric performance is mainly due to the formation of heterojunction and the reduction of TiO 2 optical band gap due to BiVO 4 and Bi 2 S 3 .The simple method presented in this work could be used to fabricate composite heterojunction with excellent photoelectric performance in different fields, such as photo-anode, photo-catalyst, and photodetector.
(b) shows the electron diffraction of the TiO 2 nanorods.Comparing the d values and angle measured in Figure 5(b) with the XRD standard card (JCPDS 21-1276, I41/and space group, a = 3.785 nm), we found that the measured d values were consistent with the lattice plane spacing of (1-11) TiO 2 and (-211) TiO 2 , and then the phase of TiO 2 was confirmed.By using the same method, BiVO 4 was also calibrated and confirmed.The results of TEM are consistent with the results of SEM, XED, and line scan.

3. 2 Figure 7 Figure 6 :
Figure 7 shows the XRD patterns of Bi 2 S 3 /BiVO 4 /TiO 2 heterojunctions prepared by the hydrothermal method with different hydrothermal time.The diffraction peak positions at
(a).The full spectrum shows five elements, Bi, O, Ti, V, and S for Bi 2 S 3 /BiVO 4 /TiO 2 .All elemental binding energies are calibrated with C 1s binding energy.Comparing the full spectra of BiVO 4 /TiO 2 and Bi 2 S 3 /BiVO 4 / TiO 2 samples, it can be found that there is a peak of S 2s at 225 eV in the spectral line of Bi 2 S 3 /BiVO 4 /TiO 2 (Figure 10(b)).

Figure 13 :
Figure 13: The diagram of the energy band before and after TiO 2 and BiVO 4 formation heterojunction.

Table 1 :
Flat band potential E fb of samples with different hydrothermal times