Bi2Te3/Bi2Se3/Bi2S3 Cascade Heterostructure for Fast‐Response and High‐Photoresponsivity Photodetector and High‐Efficiency Water Splitting with a Small Bias Voltage

Abstract Large‐scale multi‐heterostructure and optimal band alignment are significantly challenging but vital for photoelectrochemical (PEC)‐type photodetector and water splitting. Herein, the centimeter‐scale bismuth chalcogenides‐based cascade heterostructure is successfully synthesized by a sequential vapor phase deposition method. The multi‐staggered band alignment of Bi2Te3/Bi2Se3/Bi2S3 is optimized and verified by X‐ray photoelectron spectroscopy. The PEC photodetectors based on these cascade heterostructures demonstrate the highest photoresponsivity (103 mA W−1 at −0.1 V and 3.5 mAW−1 at 0 V under 475 nm light excitation) among the previous reports based on two‐dimensional materials and related heterostructures. Furthermore, the photodetectors display a fast response (≈8 ms), a high detectivity (8.96 × 109 Jones), a high external quantum efficiency (26.17%), and a high incident photon‐to‐current efficiency (27.04%) at 475 nm. Due to the rapid charge transport and efficient light absorption, the Bi2Te3/Bi2Se3/Bi2S3 cascade heterostructure demonstrates a highly efficient hydrogen production rate (≈0.416 mmol cm−2 h−1 and ≈14.320 µmol cm−2 h−1 with or without sacrificial agent, respectively), which is far superior to those of pure bismuth chalcogenides and its type‐II heterostructures. The large‐scale cascade heterostructure offers an innovative method to improve the performance of optoelectronic devices in the future.


Introduction
The conversion of solar energy into electric and chemical energy by photoelectrochemical (PEC) devices has received widespread attention due to the promising prospects in solving energy-shortage and environmentpollution. [1] The key challenge in PEC devices is to develop photoelectrode materials with fast response and high photocurrent density under a small bias or without a bias voltage. [2] In the past few decades, n-type semiconductors such as TiO 2 and ZnO [3] have been widely investigated as photoanodes in PEC devices due to their strong redox capability. However, the low conductivity, limited carrier lifetime, rapid carrier recombination rate, and large bandgap in UV band only have prevented the energy conversion efficiency by these wide bandgap semiconductors. [4] In this regard, hunting for advanced materials such as visible-lightactive two-dimensional (2D) materials [5] is desirable for PEC devices due to their narrow bandgap, strong light-matter interaction, and high carrier mobility. As typical 2D materials, bismuth chalcogenides with a general structure Bi 2 X 3 (X = Te, Se, and S) are attractive and promising candidates for visible-light-driven PEC devices due to the narrow bandgap and environmental friendliness. Among them, Bi 2 Se 3 and Bi 2 Te 3 are the most studied topological insulators with the metallic Dirac surface states, [6] which are advantageous to the electron interface transmission. Furthermore, both Bi 2 Se 3 and Bi 2 Te 3 show a superior electrical conductivity and a small bandgap as well as a high surface mobility (10 4 cm 2 V −1 s −1 ). [7] In contrast, Bi 2 S 3 belongs to a typical n-type semiconductor with a relatively high photon-electron conversion efficiency (≈5%), a high absorption coefficient (10 4 -10 5 cm −1 ), and a narrow bandgap (≈1 eV). [8] Despite the strong and wide-band light absorption by bismuth chalcogenides, the rapid electron-hole pair recombination in them still slows down the interfacial kinetics and reduces the conversion efficiency in the photodetection and water splitting in PEC performance.

Results and Discussion
Chemical/physical vapor deposition (CVD/PVD) is a facile strategy to fabricate large-scale nanofilms. The uniform Bi 2 Se 3 film in a centimeter scale ( Figure S1a, Supporting Information) was synthesized by a PVD method (see Experimental Section for details). The characteristic Raman peaks in Figure 1a were found near 128.3 and 172.0 cm −1 , which match well the E g and A 1g vibration modes of Bi 2 Se 3 . [12] To further analyze the surface compositions and element states of the as-grown materials, the XPS spectra of Bi 4f and Se 3d signals in the Bi 2 Se 3 film were measured as shown in Figure 1b,c. There are two main peaks at 157.1 eV of Bi 4f 7/2 and 162.4 eV of Bi 4f 5/2 from Bi 2 Se 3 . As compared, two tiny peaks at 158.4 and 163.6 eV in Figure 1b are from Bi 2 O 3 , which may be due to the natural oxidation. [18] From the XPS spectrum of Se 3d core level, the Se 3d 5/2 (≈54.7 eV) and Se 3d 7/2 (≈53.8 eV) be-long to the Se 2− valence state. This analysis further confirms the successful formation of Bi 2 Se 3 films. Due to the similar physical and chemical properties of Bi 2 Se 3 and Bi 2 Te 3 , the same parameters were also used to grow Bi 2 Te 3 film by a PVD method with the morphology shown in Figure S1b (Supporting Information). The characteristic Raman peaks near 99.9 and 139.1 cm −1 in Figure 1d are the E 2 g and A 1g vibration modes of Bi 2 Te 3 . [19] The Bi 4f corelevel spectrum also exhibits four main peaks (162.4, 163.7, 157.1, and 158.4 eV) from Bi 3+ as shown in Figure 1e. The four peaks of Te 3d are 3d 5/2 (≈572.7 and 576.4 eV) and 3d 3/2 (≈583.2 and 586.8 eV) in Figure 1f. The synthesized Bi 2 Te 3 film with Bi and Te oxidation states may be due to the long-term air exposure in the measurement. [6] Compared with the PVD method, the Bi 2 S 3 film was synthesized by a CVD method to avoid the high decomposition under high temperature (see Experimental Section for details). The photograph and scanning electron microscopy (SEM) characterization of Bi 2 S 3 film suggests that the nanosheets are deposited and stacked a continuous and uniform film as shown in Figure S1c (Supporting Information). The Raman spectrum of Bi 2 S 3 in Figure 1g shows the A g vibration modes near 183.2 and 233.6 cm −1 , and the B 1g vibration mode near 259.2 cm −1 . [20] The XPS characterization in Figure 1h demonstrates that the peaks (162.4 and 157.1 eV) of Bi 4f are from Bi 3+ . The small peak at 159.8 eV is from Bi metal, which is caused by a bit precipitation of metal bismuth in the CVD reaction process. From the S 2p corelevel spectrum in Figure 1i, the peaks of S 2p 1/2 and 2p 3/2 energy levels are observed near 163.46 and 162.35 eV, respectively. Meanwhile, hexagonal phase Bi 2 Se 3 , hexagonal phase Bi 2 Te 3 , and orthorhombic phase Bi 2 S 3 have been confirmed by X-ray diffraction spectroscopy as shown in Figure S2 (Supporting Information).
To optimize the heterostructure formation and confirm the band alignment, Bi 2 X 3 heterostructures (Bi 2 Te 3 /Bi 2 Se 3 , Bi 2 Se 3 /Bi 2 S 3 , and Bi 2 Te 3 /Bi 2 S 3 ) were first prepared by a twostep vapor phase deposition method. The uniform Bi 2 X 3 heterostructure films are observed from photographs and SEM images as shown in Figure S1d-f (Supporting Information) and the top layer films were successfully deposited onto the bottom layer films. From the Raman spectrum of Bi 2 Te 3 /Bi 2 Se 3 in Figure 2a, there are four vibration modes such as E g (126.9 cm −1 ) and A 1g (169.3 cm −1 ) from Bi 2 Se 3 and E 2 g (105.7 cm −1 ) and A 1g (146.7 cm −1 ) from Bi 2 Te 3 . Similarly, the A g (≈186.2 and 235.2 cm −1 ) and B 1g (≈263.7 cm −1 ) vibration modes from Bi 2 S 3 are also displayed in Figure 2b,c, which also confirms the formation of Bi 2 Se 3 /Bi 2 S 3 and Bi 2 Te 3 /Bi 2 S 3 , respectively. It is worth noting that the characteristic peaks have an obvious red-shift or blue-shift in these heterostructures. This is due to the interlayer coupling when the heterostructure interface is formed, which is the direct evidence of van der Waals heterostructures. [21] To determine the band offset parameters, high-resolution XPS is employed to evaluate the valence band offset (ΔE v ) at the Bi 2 X 3 interfaces. This offset is the energy difference of the Bi core levels between the heterostructure and the single component. In the Bi 2 Te 3 /Bi 2 Se 3 heterostructure, four main peaks of Bi 4f 5/2 and Bi 4f 7/2 are 164.6, 163.2, 159.2, and 157.9 eV, respectively, as shown in Figure 2d. Similarly, the Bi core levels of Bi 2 Se 3 /Bi 2 S 3 (163.6 eV of Bi 4f 5/2 and 158.3 eV of 4f 7/2 ) and Bi 2 Te 3 /Bi 2 S 3 (163.7 eV of Bi 4f 5/2 , 158.4 eV of Bi 4f 7/2 , and 161.1 eV of S 2p 3/2 ) heterostructures are also investigated as shown in Figure 2e,f. The corresponding Figure 1. a) Raman spectrum, and XPS spectra of b) Bi 4f core level and c) Se 3d core level of Bi 2 Se 3 film; d) Raman spectrum, and XPS spectra of e) Bi 4f core level and f) Te 3d core level of Bi 2 Te 3 film; g) Raman spectrum, and XPS spectra of h) Bi 4f core level and i) S 2p core level of Bi 2 S 3 film.
Te, Se, and S core levels are fitted and shown in Figure S3 (Supporting Information). These core levels in the heterostructures have a shift toward higher or lower binding energy compared with those of pure Bi 2 Te 3 , Bi 2 Se 3 , and Bi 2 S 3 , indicating the interfacial carrier redistribution when the heterostructure is formed.
The valence band spectra of Bi 2 S 3 , Bi 2 Se 3, and Bi 2 Te 3 were also measured by XPS to calculate the band arrangement structure at the interface. As shown in Figure 3a, the maximum valence bands (VBM) of the Bi 2 S 3 , Bi 2 Se 3 , and Bi 2 Te 3 films are 0.57, 0.30, and 0.03 eV, respectively. The valence band offset parameters (ΔE V ) of the Bi 2 Se 3 /Bi 2 S 3 , Bi 2 Te 3 /Bi 2 Se 3 , and Bi 2 Te 3 /Bi 2 S 3 films can be calculated as follows: [5,22] www.advancedsciencenews.com www.advancedscience.com Figure 2. a-c) Raman spectra and d-f) XPS spectra of Bi 4f core level in the Bi 2 Te 3 /Bi 2 Se 3 , Bi 2 Se 3 /Bi 2 S 3 , and Bi 2 Te 3 /Bi 2 S 3 , respectively.
( Figure S4, Supporting Information). The following relationship exists between the bandgap and the photon energy: Here, , h, , and A are the absorption coefficient, Planck's constant, optical frequency, and proportionality constant, respectively. Based on the calculation of Equation 4, the bandgaps of the Bi 2 S 3 , Bi 2 Se 3, and Bi 2 Te 3 films are 1.00, 0.82, and 0.77 eV, respectively. Therefore, the corresponding conduction band offset parameters (ΔE c ) of Bi 2 X 3 heterostructures can be calculated by the following Equations: According to the experimental data, the ΔE C1 value of Bi 2 Se 3 /Bi 2 S 3 is −0.04 eV, and the ΔE C2 value of Bi 2 Te 3 /Bi 2 Se 3 is -0.32 eV, and the ΔE C3 value of Bi 2 Te 3 /Bi 2 S 3 is −0.36 eV. Based on these results, the schematic diagram of the band arrangement of the Bi 2 Te 3 /Bi 2 Se 3 /Bi 2 S 3 cascade heterostructure is obtained as shown in Figure 3c. This band alignment of cascade heterostructure suggests the synchronization of the electron-hole movement as the photoexcited electrons can easily transfer from Bi 2 Te 3 to Bi 2 Se 3 and then from Bi 2 Se 3 to Bi 2 S 3 , while the photogenerated holes can easily transfer from Bi 2 S 3 to Bi 2 Se 3 and then from Bi 2 Se 3 to Bi 2 Te 3 . The Bi 2 Te 3 /Bi 2 Se 3 /Bi 2 S 3 cascade heterostructure was realized by a sequential deposition of Bi 2 S 3 , Bi 2 Se 3 , and Bi 2 Te 3 . The morphology by SEM ( Figure 3d) and photograph (the inset) suggests that the centimeter-scale Bi 2 Te 3 /Bi 2 Se 3 /Bi 2 S 3 film is formed uniformly. The Raman spectrum of the Bi 2 Te 3 /Bi 2 Se 3 /Bi 2 S 3 cascade heterostructure shows evident vibration modes from Bi 2 X 3 with the E 2 g (98.4 cm −1 ) and A 1g (140.7 cm −1 ) from Bi 2 Te 3 , the E g (121.4 cm −1 ) and A 1g (164.8 cm −1 ) from Bi 2 Se 3 , and the A g (184.2 and 236.5 cm −1 ) and B 1g (260.3 cm −1 ) from Bi 2 S 3 as shown in Figure 3e. This result is consistent with the XPS in Figure S5 (Supporting Information), which further confirms the successful formation of the Bi 2 Te 3 /Bi 2 Se 3 /Bi 2 S 3 cascade heterostructure.
Considering the well-matched staggered bandgap (Figure 3c), the cascade heterostructures are expected to improve the PEC performance as the built-in electric field at the multiple interfaces could promote the transmission of photoexcited electrons and holes. To validate the performance of these cascade heterostructures, we fabricated the PEC-type photodetectors and carried out the photodetection measurements. Herein, photocurrent density (I ph ) and photoresponsivity (R ph ) are often used to quantitatively evaluate photodetection performance: [23] I ph = (I light − I dark )∕S (8) where I light and I dark are the current responses under light illumination and dark states, respectively. In our experiment, the illumination area (S) and light power density (P ) are 0.7 cm 2 and 100 mWcm −2 , respectively. It is evident from Figure 4a that the photocurrent density of both pure Bi 2 X 3 and the heterostructures increases with the bias voltage. It is worth noting that the photocurrent density of the Bi 2 Te 3 /Bi 2 Se 3 /Bi 2 S 3 cascade heterostructure has a much higher value than those of pure Bi 2 X 3 and their related type-II heterostructures. To clearly evaluate the solar energy conversion efficiency, the applied bias photon-to-current efficiency (ABPE = I light ×(1.23−E RHE ) P × 100%) was calculated. [4a] As shown in Figure 4b, the cascade heterostructure displays the largest ABPE among Bi 2 X 3 and their related type-II heterostructures. The maximum ABPE reaches 1.58% at the 0.75 V versus RHE equal to −0.1 V ver-sus Ag/AgCl in our experiment, which is larger than those of BiVO 4 /CdS, [24] ZnO/CuS, [25] Fe 2 O 3 /NiFeOOH, [26] CdS/TiO 2 , [27] and Si/Au/TiO 2 . [28] The ABPE improvement mainly comes from the high light absorption, efficient photogenerated electron-hole separation, and fast carrier transport. As shown in Figure 4c from the I-V measurement under the chopped light illumination (100 mWcm −2 ), the PEC photodetector demonstrates a high photocurrent density without a bias or with a small bias. This suggests that this cascade heterostructure-based photodetector can be used as a highly sensitive self-powered photodetector as well as a low-bias photodetector with a low on-set potential. We further demonstrate the transient photocurrent response of both pure Bi 2 X 3 and heterostructures with a switching duration of 5 s as shown in Figure 4d,e under a low bias voltage (−0.1 V). Figure 4d demonstrates that the I ph and R ph values of Bi 2 S 3 reach 131.5 μA cm −2 and 1315 μA W −1 , which are higher than those of Bi 2 Se 3 (I ph ≈ 15.9 μA cm −2 , R ph ≈ 159 μA W −1 ) and Bi 2 Te 3 (I ph ≈ 12.3 μA cm −2 , R ph ≈ 123 μA W −1 ). This transient photocurrent response is determined by the carrier transportation and recombination rate of samples. Correspondingly, Figure 4e demonstrates the I ph values of Bi 2 Te 3 /Bi 2 Se 3 (301.2 μA cm −2 ), Bi 2 Se 3 /Bi 2 S 3 (1047.6 μA cm −2 ), and Bi 2 Te 3 /Bi 2 S 3 (1469.1 μA cm −2 ) are better than those of pure Bi 2 X 3 . It is evident from Figure 4e that the Bi 2 Te 3 /Bi 2 Se 3 /Bi 2 S 3 cascade heterostructure demonstrates the highest I ph and R ph values (I ph ≈ 2.2 mAcm −2 , R ph ≈ 22 mA W −1 ) among these pure bismuth chalcogenides and its type-II heterostructures. These values are 178 times larger than that of Bi 2 Te 3 , 138 times larger than that of Bi 2 Se 3 , and 17 times larger than that of Bi 2 S 3 , respectively. Compared with the Bi 2 X 3 heterostructure, the photoresponse of the cascade heterostructure is also greatly improved. The I ph and R ph values in the cascade heterostructure are 7.3, 2.1, and 1.5 times larger than those of the type-II Bi 2 Te 3 /Bi 2 Se 3 , Bi 2 Se 3 /Bi 2 S 3 , and Bi 2 Te 3 /Bi 2 S 3 heterostructures, respectively. The long-term It cyclic stability tests of Bi 2 Te 3 /Bi 2 Se 3 /Bi 2 S 3 heterostructure are measured in Figure S6a-c (Supporting Information). The photocurrent signal exhibits a reversible behavior with the switchable light on and off, indicating excellent reproducibility and stability. Further long-term photocurrent measurements show that the photocurrent density of the Bi 2 Te 3 /Bi 2 Se 3 /Bi 2 S 3 was maintained almost constant ≈2.24 mA cm −2 before and after one month under the illumination at −0.1 V as shown in Figure 4f. The results demonstrate that the cascade heterostructure-based photodetector also shows high stability. These improvements of the cascade heterostructure would be due to the rapid carrier transportation www.advancedsciencenews.com www.advancedscience.com and slow electron-hole pair recombination for the practical applications of PEC devices.
To further investigate the carrier dynamic process at the interface of heterogeneous structures, electrochemical impedance spectroscopy (EIS) was used as shown in Figure 4g. Generally, a smaller diameter in the EIS suggests a lower interfacial resistance, which can accelerate the charge transfer. In order to clearly understand the interface resistance, an equivalent circuit was constructed as shown in the inset in Figure 4g, where C PE and R s as well as R ct, represent the double layer capacitance, the electrolyte and charge transfer resistances, respectively. It is evident in Figure 4g that the semicircle diameter decreases in the order of Bi 2 Te 3 >Bi 2 Se 3 >Bi 2 S 3 >Bi 2 Te 3 /Bi 2 Se 3 >Bi 2 Se 3 /Bi 2 S 3 >Bi 2 Te 3 / Bi 2 S 3 >Bi 2 Te 3 /Bi 2 Se 3 /Bi 2 S 3 , which agrees well with LSV results. The results suggest that the cascade heterostructure shows a small interface resistance, due to the well-matched band structure between Bi 2 X 3 and ITO substrate. As such, the photoexcited electrons can transfer efficiently from Bi 2 Te 3 to ITO substrate and generate a high photoresponse.
Furthermore, Mott-Schottky curve is also used to analyze the performance of photodetectors. As shown in Figure 4h, it can be seen that the slope of the Mott-Schottky curves is positive, which indicates that Bi 2 X 3 and their heterostructures belong to n-type semiconductors. In addition, the flat band potential (V FB ) of the sample can be obtained by the tangent of the Mott-Schottky curve with the X-axis in Figure 4h. The V FB of Bi 2 Se 3 , Bi 2 Te 3 , and Bi 2 S 3 is calculated to be −0.53, −0.64, and −0.41 V (vs Ag/AgCl), respectively. Based on the V FB measurement of these n-type semiconductors, [29] the band energy position of Bi 2 Te 3 /Bi 2 Se 3 /Bi 2 S 3 in solution is also type-II alignment, which agrees with the results in Figure 3c. Because of the more negative CB position of Bi 2 Te 3 , the electrons in the CB of Bi 2 Te 3 will transfer to that of Bi 2 Se 3 and then to that of Bi 2 S 3 . [30] Furthermore, the inverse transfer direction of holes will greatly decrease the recombination of photogenerated carriers. These processes can also be found frequently in other cascade heterostructures such as ZnO/CdS/CdSe, ZnO/CdS/PbS, and MoS 2 /WS 2 /WSe 2 /Si. [14] Furthermore, the V FB of the Bi 2 Te 3 /Bi 2 Se 3 /Bi 2 S 3 cascade heterostructure is also calculated to be −0.86 V, which is larger than those of Bi 2 Te 3 /Bi 2 Se 3 (−0.70 V), Bi 2 Se 3 /Bi 2 S 3 (−0.73 V), and Bi 2 Te 3 /Bi 2 S 3 (−0.79 V) in Figure 4h. This suggests that the large band bending at the Bi 2 Te 3 /Bi 2 Se 3 /Bi 2 S 3 cascade interfaces, benefiting for the charge separation due to the significantly sharp heterostructure interface, which is also found in ZnIn 2 S 4 /TiO 2 and WO 3 /Bi 2 S 3 . [30] The free carrier density (N d ) can be obtained as follows: [31] where e = 1.6 × 10 −19 C is the electron charge, and 0 = 8.85×10 −14 Fcm −1 is the vacuum permittivity; r is the relative permittivity of Bi 2 X 3 (X = S, Te, Se). In addition, the OCP measurements in Figure 4i also verify the n-type semiconductors according to the low OCP under the light state. [33] Here, the photovoltage (V ph ) is defined as OCP dark −OCP light and the V ph is caused by the Fermi-level pinning effect. In our experiment, the cascade heterostructure shows the highest V ph value (≈0.35 eV) than those of Bi 2 X 3 and its type-II heterostructures. This is because heterostructure formation could eliminate the Fermi-level pinning induced by the trap state, [34] resulting in a high V ph and a large band bending between the photoanode and electrolyte interface. The OCP measurement agrees well with the Mott-Schottky and EIS results as the sharp band bending in the cascade heterostructure can effectively suppress the surface carrier recombination and promote the surface charge transport ability. [35] To investigate the effect of light absorption range on the photodetection ability, the I-t measurement was conducted under the wavelengths ≥400 nm and ≥700 nm by adding band-pass optical filters. Similar to LSV results, the Bi 2 Te 3 /Bi 2 Se 3 /Bi 2 S 3 cascade heterostructure also shows the highest I ph (1.77 mA cm −2 and 373 μA cm −2 ) and R ph (29.5 and 18.65 mA W −1 ) than those of Bi 2 X 3 and its type-II heterostructures under the wavelengths ≥400 nm and ≥700 nm as shown in Figure 5a-d, respectively. The I ph and R ph values of Bi 2 X 3 and heterostructures are summarized in Table S1 (Supporting Information). In contrast, the I ph at the wavelength ≥400 nm is much larger than that of ≥700 nm due to the efficient light absorption in visible region. Additionally, the efficient charge separation and transfer in the cascade heterostructure also have a synergy effect to produce high photoresponse.
To deeply explore the visible-light-driven photodetector performance, the photoresponse of the Bi 2 Te 3 /Bi 2 Se 3 /Bi 2 S 3 cascade heterostructure at −0.1 V was investigated under different visible light wavelengths as shown in Figure 5e. The cascade heterostructure-photodetector keeps high I ph values in Table S2 (Supporting Information). Especially, the I ph reaches the maximum value ≈1243 μA cm −2 at the wavelength of 475 nm, which may be due to the high light-absorption at the special wavelength. [5] To estimate the sensitivity performance of the cascade heterostructure, the R ph as a function of the wavelength is displayed in Figure 5g. The R ph increases from 92 to 103 mAW −1 with the wavelength increased from 420 to 475 nm, and then it gradually decreases to 25.76 mA W −1 . The I ph and R ph values are much larger than those of previously reported PEC photodetectors as shown in Table 1. It is evident that the R ph value of the cascade heterostructure is approximately 10 3 times larger than those of 2D GeSe and SnS [1b,36] and ≈10 2 times larger than those of 2D SnS/SnSe 2 and Te@Se heterostructures. [37] This is mainly determined by the strong light absorption ability, well-matched band alignment, and specific atomic-level interfacial contact in the Bi 2 Te 3 /Bi 2 Se 3 /Bi 2 S 3 cascade heterostructure. The superior photoresponse performance of the cascade heterostructure suggests a great commercial potential in the visible-light-driven PEC photodetectors.
Further exploration of the photodetection behavior in the visible region without an applied voltage was also investigated as shown in Figure 5f and the relationship between the I ph (R ph ) and wavelengths is a similar to those in Figure 5e. The cascade heterostructure-based photodetector shows an excellent  www.advancedsciencenews.com www.advancedscience.com self-powered performance with the maximum I ph value of 42 μAcm −2 and R ph value of 3.5 mAW −1 at 475 nm, which are better than those of previously reported 2D materials-based PEC photodetector as shown in Table S3 (Supporting Information). On the one hand, the 2D Bi 2 X 3 have a strong light absorption efficiency beyond 10 4 cm −1 even in the infrared region due to the narrow bandgap, which can produce more photogenerated carriers compared with those of 2D nanosheets in Table S3 (Supporting Information). On the other hand, the Bi 2 Te 3 /Bi 2 Se 3 /Bi 2 S 3 cascade heterostructure generates strong built-in electric fields at the multiple interfaces and then promotes the photoexcited electron-hole transportation, resulting in a high self-powered capacity. This self-powered photodetector meets the practical application requirement in various harsh and complicated environments with low-energy consumption, and light weight. Apart from R ph , specific detectivity (D*) is another key parameter to estimate the photo-responsiveness of photodetectors. The It is evident that the cascade heterostructure also shows the strongest detectivity (8.96 × 10 9 and 1.99 × 10 9 Jones for −0.1 V and 0 V at 475 nm) than those of other wavelengths. Additionally, the D* values of the cascade heterostructure-based photodetectors are an order of magnitude larger than those of PEC-type photodetectors such as Bi, [44] Te, [45] BP, [46] SnS, [36a] and Te@Bi. [47] High crystallinity and efficient light absorption as well as fast interfacial charge transportation of cascade heterostructure lead to a low dark current and a high detectivity.
To quantitatively evaluate the efficiency of the cascade heterostructure based photodetectors, the external quantum efficiency (EQE) and incident photon-to-current efficiency (IPCE) are calculated based on the incident photon as a function of the wavelength as follows: Here, q is 1.6 × 10 −19 C; h is 6.63 × 10 −34 J s; c is 3 × 10 8 m s −1 ; is the incident wavelength. The calculated EQE and IPCE are summarized in Figure 5i and Table S4 (Supporting Information). It is clear that the cascade heterostructure photodetector displays a broad and high EQE and IPCE values in the visible region. Significantly, the cascade heterostructure exhibits the highest EQE (26.17% and 0.88% for −0.1 and 0 V) and IPCE (27.04% and 0.91% for −0.1 V and 0 V) at the wavelength of 475 nm in consistent with the wavelength-dependent photoresponse results in Figure 5g. These results further suggest that the cascade heterostructure can effectively accelerate the carrier separation and transportation under a small bias voltage with a large junction area.
To evaluate the sensitivity of the photodetector, the response time (t res , response from 10% to 90%) and recovery time (t rec , recombination from 90% to 10%) are measured under the single wavelength as shown in Figure 6a. Interestingly, the t res and t rec are on a millisecond scale, which is almost not influenced by the incident wavelength. Figure 6b,c show the typical t res (8 ms) and t rec (6.9 ms) of the Bi 2 Te 3 /Bi 2 Se 3 /Bi 2 S 3 cascade heterostructure at 475 nm. Even without a voltage bias voltage, the cascade heterostructure still keeps the fast response of t res (8 ms) and t rec (8 ms) at different incident wavelengths as shown in Figure 6d-f. The t res value is comparable to the t rec value, suggesting that few defects and trap centers are involved in carrier separation and recombination. The rapid photoresponse characteristics in the cascade heterostructure are far superior to those of 2D materials-based photodetectors. For instance, the response time in the cascade heterostructure is an order of magnitude faster than previously reported Bi 2 S 3 -based photodetector (t res ≈100 ms and t rec ≈100 ms). [48] On the one hand, the rapid response could result from the internal electric field of the multi-staggered band alignment that induces a fast charge transfer and separation efficiently. On the other hand, the rapid response could result in a strong redox reaction, which unambiguously promotes the PEC process, leading to a high photocurrent density.
To further evaluate the light sensitive properties of the photodetector, the incident light power-dependent photoresponse is investigated with the power intensity from 50 to 100 mW cm −2 at −0.1 V as shown in Figure 6g. The extracted I ph from Figure 6g increases from 1130 to 2390 μA cm −2 at −0.1 V. Similarly, without a bias voltage, the I ph also increases proportionally from 104 to 178 μA cm −2 as shown in Figure S7 (Supporting Information). Furthermore, the I ph increase almost linearly with the increment of power intensity as shown in Figure 6h, which can be fitted by I ph ∝P 0.98 for both with and without a bias voltage. Based on the photocurrent generation principle, I ph would be linearly dependent on the incident power density under ideal conditions for the photoconductive detector. [49] As such, the linear power dependence suggests the high crystallinity of the Bi 2 Te 3 /Bi 2 Se 3 /Bi 2 S 3 cascade heterostructure with relatively low defects and traps. [50] Apart from the I ph values, the detailed R ph values are also calculated and summarized into Table S5 (Supporting Information). The extracted R ph values keep 22.8 ± 0.32 and 1.91 ± 0.15 mAW −1 at −0.1 and 0 V as shown in Figure 6h, suggesting a high PEC detection stability even under a weak visible light. It is worth pointing out that the R ph values in the cascade heterostructure are ≈10 2 -10 3 times larger than those of GeSe and Te nanosheets based self-powered photodetector. [1b,45] The large-area junction interface and few trap states are responsible for this improvement. The self-powered characteristics are well explained based on the energy-band structure in Figure 6i. The built-in electric field at the semiconductor interface can ensure that photodetector works well even without a bias voltage. The charge transfer in the heterstrucrure is mainly determined by both Fermi level and the energy difference (ΔE cv ) between CB of semiconductor I and VB of semiconductor II. [51] According to the band arrangement of the Bi 2 Te 3 /Bi 2 Se 3 /Bi 2 S 3 heterostructure, a smaller work function (higher Fermi level) of semiconductor Bi 2 S 3 than those of Bi 2 Se 3 and Bi 2 Te 3 , which is characterized by ultraviolet photoelectron spectroscopy measurements as shown in Figure S8 (Supporting Information). Furthermore, the energy difference (ΔE cv ≈ 0.73 eV) between CB of Bi 2 S 3 and VB of Bi 2 Se 3 is far larger than that of ΔE c (0.07 eV). Similarly, the ΔE cv (0.5 eV) between CB of Bi 2 Se 3 and VB of Bi 2 Te 3 is larger than that of ΔE c (0.32 eV) as shown in Figure 3b. The built-in electric field at the Bi 2 Te 3 /Bi 2 Se 3 /Bi 2 S 3 heterostructure interface favors the type-II charge transfer process. Furthermore, the enhanced photocurrent in the heterostructure is solid evidence to verify the type-II instead of direct Z-scheme heterostructure in Figure S9 (Supporting Information), which is consistent with the ultraviolet photoelectron spectroscopy measurement results. The photoresponse of Bi 2 S 3 /Bi 2 Se 3 and Bi 2 Se 3 /Bi 2 S 3 (Bi 2 Te 3 /Bi 2 Se 3 and Bi 2 Se 3 /Bi 2 Te 3 ) heterostructures was also measured as shown in Figure S10a,b (Supporting Information). Compared with Bi 2 S 3 /Bi 2 Se 3 /ITO heterostructure, much more photogenerated electrons of Bi 2 Se 3 /Bi 2 S 3 /ITO heterostructure are collected at ITO substrate and then generate a higher photocurrent as shown in Figure S11 (Supporting Information). This is in consistent with our experimental results as shown in Figure S10a (Supporting Information). Furthermore, the photocurrent of Bi 2 S 3 is larger than that of Bi 2 S 3 /Bi 2 Se 3 /ITO heterostructure. This is because the photogenerated electrons of Bi 2 Se 3 quickly transfer toward Bi 2 S 3 and then participate in water reduc-tion reaction as shown in Figure S11d (Supporting Information). The results further confirm that the Bi 2 Se 3 /Bi 2 S 3 belongs to type-II heterostructure instead of Z-scheme heterostructure as shown in Figure S11 (Supporting Information). Similarly, the photocurrent measurement in Figure S10b (Supporting Information) also demonstrates that the Bi 2 Te 3 /Bi 2 Se 3 heterostructure also belongs to type-II heterostructure. Under the light illumination, the photoexcited electron-hole pairs are separated by the internal electric field. In details, the photoinduced electrons transmit from valence band (VB) to conduction band (CB) of the Bi 2 X 3 semiconductors and the electrons in the CB of Bi 2 Te 3 would go into the CB of the Bi 2 Se 3 and then flow into Bi 2 S 3 as shown in Figure 6i.
Different from other photodetectors, the PEC photodetectors can work in electrolytes and the electrolytes as ion channels to complete the current loop. Under the light illumination, this PEC-type photodetector not only can collect the photogenerated electrons to produce electric signal, but also can produce H 2 due to the photogenerated electrons undergoing the redox reactions. A vacuum gas circulation system combined with a gas chromatograph was employed to detect the amount of H 2 production (see Experimental Section for details). Under the bias voltage of −0.1 V, the H 2 productions are 0.23, 0.09, and 0.34 mmol cm −2 for Bi 2 Se 3 , Bi 2 Te 3, and Bi 2 S 3 within 2.5 h as shown in Figure 7a. The results suggest that the Bi 2 X 3 films show strong activity under a small bias voltage as shown in Table S6 (Supporting Information). Especially, compared with previously reported Bi 2 S 3 nanowires, [52] our chemical vapor deposited Bi 2 S 3 film shows the almost two-fold increase in the PEC water splitting performance as shown in Table S6 (Supporting Information). This is mainly due to the reason that the atomic-level interfacial contact between the centimeter-scale Bi 2 X 3 film and ITO substrate can accelerate the charge transport to counter electrode for reduction reaction.
To further improve the water splitting performance, the Bi 2 X 3 heterostructures are investigated on the PEC hydrogen production under the same measurement conditions in Figure 7b. The H 2 productions of Bi 2 Te 3 /Bi 2 Se 3 , Bi 2 Se 3 /Bi 2 S 3 , and Bi 2 Te 3 /Bi 2 S 3 films are 0.43, 0.51, and 0.83 mmol cm −2 within 2.5 h which are 4.78, 5.67, and 9.22 times larger than that of Bi 2 Te 3 under the same conditions. Interesting, the H 2 www.advancedsciencenews.com www.advancedscience.com production rate of Bi 2 Te 3 /Bi 2 S 3 is also superior to those of TiO 2 /Bi 2 Se 3 , Bi 2 Te 3 /V 0.04 -Sb 2 Te 3 , Bi 2 S 3 /BiVO 4 , and Bi 2 S 3 @TiO 2 heterostructures [10][11]53] as shown in Table S6 (Supporting Information). This improvement in the Bi 2 X 3 heterostructures is mainly attributed to several merits. I) Two-step vapor deposited van der Waals heterostructures can effectively avoid the alloy and defect-center formation. II) The similar crystal structure among 2D Bi 2 X 3 materials is beneficial for forming a good-contact and a large-area heterostructure interface. III) The conducting surface state facilitates the charge transfer at the large-area interface. IV) The formed type-II heterostructure can effectively separate electron-hole pairs and participate the redox reaction. As a representative Bi 2 X 3 heterostructure, the charge transfer process of Bi 2 Se 3 /Bi 2 S 3 can be described as following. Under the visible light illumination, the photogenerated electrons transmit from VB to CB of Bi 2 Se 3 and Bi 2 S 3 semiconductors and the electrons in the Bi 2 Se 3 would go into the CB of the Bi 2 S 3 . Much more accumulated electrons in the Bi 2 S 3 would transit to the counter electrode and generate a higher H 2 production rate than those of pure Bi 2 X 3 . Similarly, the interfacial charge transfer also improves the H 2 production rates in the Bi 2 Te 3 /Bi 2 Se 3 and Bi 2 Te 3 /Bi 2 S 3 heterostructures.
Considering the interfacial carrier transportation, the Bi 2 Te 3 /Bi 2 Se 3 /Bi 2 S 3 heterostructure with a multi-staggered bandgap and multiple interfaces was designed and the H 2 production can reach 1.04 mmol cm −2 , which is 4.5, 11.6, and 3.06 times larger than those of Bi 2 Se 3 , Bi 2 Te 3, and Bi 2 S 3 , respectively. Similarly, the H 2 production performance of Bi 2 Te 3 /Bi 2 Se 3 /Bi 2 S 3 is also far larger than those of Bi 2 X 3 -based heterostructures as shown in Table S6 (Supporting Information). Especially for a low bias voltage, the H 2 production in the cascade heterostructure is 17.5 times larger than the Bi 2 S 3 -BiOBr/TiO 2 heterostructures. [54] The highest H 2 production rate in the cascade heterostructure is realized among the 2D Bi 2 X 3 materials and related heterostructures, which mainly come from the efficient charge transport and proper band alignment as shown in Figure 7c. The electron-hole pairs are separated in the cascade heterostructure based photoanode under the simulated sunlight illumination. The well-matched band energy edge at the Bi 2 Te 3 /Bi 2 Se 3 /Bi 2 S 3 cascade heterostructure interfaces provides an efficient carrier highway to deliver electrons to counterpart electrode and reduce the recombination of photogenerated carriers. Under a small applied voltage, the carrier transport rate was also greatly improved due to the external electric field. Meanwhile, the holes participate in the oxidation reaction with the sacrificial agent (Equations 14-17) and the corresponding schematic diagram of the photocatalytic mechanism is also displayed in Figure 7c. The S 2− 2 production can be efficiently inhibited by the mixing SO 2− 3 ions and then produce S 2 O 2− 3 ions. In the chemical reaction process, the holes are consumed and further slow the carrier recombination, resulting in a high PEC performance.
In order to deeply understand the relationship between PEC and photocatalytic H 2 production, the H 2 amount of the Bi 2 Te 3 /Bi 2 Se 3 /Bi 2 S 3 cascade heterostructure at the bias voltage of −0.1 and 0 V under 100 mW cm −2 was measured in Figure 7d. The result shows that the H 2 production can reach 0.12 mmol cm −2 within 2.5 h even under 0 V. Significantly, H 2 production can greatly be improved to 1.04 mmol cm −2 under a small external bias voltage of −0.1 V due to the synergistic effect. However, hydrogen evolution takes place at the high-cost consumption of electron sacrificial agents in the PEC processing. In our experiment, the PEC and photocatalytic H 2 production of the Bi 2 Te 3 /Bi 2 Se 3 /Bi 2 S 3 cascade heterostructure in pure water was further tested as shown in Figure 7e. The results show that the Bi 2 Te 3 /Bi 2 Se 3 /Bi 2 S 3 cascade heterostructure can produce 13.81 μmol cm −2 of photocatalytic H 2 production and 28.64 μmol cm −2 of PEC hydrogen production within 2 h, respectively. This is mainly from water reduction reaction, which can be described as 2H 2 O + 2e -= 2OH -+H 2 . This result shows that the Bi 2 Te 3 /Bi 2 Se 3 /Bi 2 S 3 cascade heterostructure can realize high-activity photocatalytic H 2 production and PECH 2 production even without using sacrificial agent.

Conclusion
The Bi 2 Te 3 /Bi 2 Se 3 /Bi 2 S 3 cascade heterostructure with a wellmatched band alignment was optimized and prepared by a vapor phase deposition method. The cascade heterostructure-based PEC photodetector exhibits a fast response at a millisecond level, a high photoresponsivity in 10 2 mAW −1 scale, and a high detectivity beyond 10 9 Jones under a small bias of −0.1 V. Furthermore, the Bi 2 Te 3 /Bi 2 Se 3 /Bi 2 S 3 photodetector also demonstrates a superior self-powered capability, displaying a broad photoresponse in the visible region. The excellent photodetection performance in the Bi 2 Te 3 /Bi 2 Se 3 /Bi 2 S 3 cascade heterostructure is mainly attributed to the efficient charge transfer at the multiple interfaces, which is characterized by EIS, OCP, and Mott-Schottky measurements. Benefiting from their proper band position, efficient light harvesting ability, and high charge transport efficiency, the cascade heterostructure also shows a superior photocatalytic activity and the H 2 generation rate can reach 0.416 mmol cm −2 h −1 and 14.320 μmol cm −2 h −1 with or without the sacrificial agent, respectively. The incorporation of merits of cascade heterostructure is very promising for PEC photodetector and water splitting applications.
Synthesis of Bi 2 X 3 Materials: In the experiment, 2D uniform Bi 2 Se 3 films were deposited onto ITO substrate by using a PVD method. Before the growth process, Ar gas flow (200 sccm) was filled the furnace to drive away air and the furnace was pumped down to 100 pa. During the growth process, 5 mg Bi 2 Se 3 powder was heated to 500°C within 20 minutes and kept for 5 min and the vapor was carried onto the ITO substrate by 50 sccm Ar gas. The Bi 2 Se 3 nanosheet deposits onto the ITO substrate at the distance of 10 cm from the source, where the temperature is 350°C.