Bi2S3 heterojunction nanostructures for photocatalytic overall water splitting

Journal Pre-proofs Z-scheme Bi2O2.33/Bi2S3 heterojunction nanostructures for photocatalytic overall water splitting Yinyi Ma, Xiao Jiang, Rongke Sun, Jianlong Yang, Xiaolin Jiang, Zhanqi Liu, Mingzheng Xie, Erqing Xie, Weihua Han PII: DOI: Reference:

S1385-8947(19)32430-1 https://doi.org/10.1016/j.cej.2019.123020 CEJ 123020

To appear in:

Chemical Engineering Journal

Received Date: Revised Date: Accepted Date:

30 July 2019 13 September 2019 30 September 2019

Please cite this article as: Y. Ma, X. Jiang, R. Sun, J. Yang, X. Jiang, Z. Liu, M. Xie, E. Xie, W. Han, Z-scheme Bi2O2.33/Bi2S3 heterojunction nanostructures for photocatalytic overall water splitting, Chemical Engineering Journal (2019), doi: https://doi.org/10.1016/j.cej.2019.123020

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Z-scheme Bi2O2.33/Bi2S3 heterojunction nanostructures for photocatalytic overall water splitting Yinyi Maa, Xiao Jianga, Rongke Suna, Jianlong Yangb, Xiaolin Jianga, Zhanqi Liua, Mingzheng Xieb, Erqing Xiea,c and Weihua Han*a,c

a

School of Physical Science and Technology, Lanzhou University, Lanzhou 730000, China.

b

College of Earth and Environmental Sciences, Lanzhou University, Lanzhou 730000, China.

c State

Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of

Science, Lanzhou, 730000, China.

*

Corresponding author.

E-mail: [email protected] (Weihua Han)

Abstract Photocatalysts with a wide response to solar spectrum especially to visible light are supposed to have a high energy conversion efficiency in solar water splitting. Heterogeneous binary composite photocatalysts exhibit outstanding performance in solar light absorption and photocarrier transfer/transport. In this work, a Z-scheme photocatalyst based on Bi2O2.33/Bi2S3 heterojunction is successfully designed and fabricated by assembling Bi2S3 nanoneedles on Bi2O2.33 nanosheets. Benefiting from the effective light absorption of Bi2S3 to solar light and the facilitated charge

transfer by the Z-scheme band structure of Bi2O2.33/Bi2S3, the composite photocatalyst exhibits a fascinating photocatalytic performance in water splitting. With the help of Pt as a co-catalyst and sacrificial agent, the production rate of H2 can reach 62.61 μmol·h-1（sample area 2 cm2）. The direct Z-scheme junction also exhibits the capacity of overall water splitting. Moreover, the stability of the heterojunction photocatalyst is optimized by depositing an ultra-thin amorphous TiO2 layer (about 3 nm) to avoid photocorrosion of Bi2S3. This work will provide a new insight into the design and development of efficient and long-life photocatalysts.

KEYWORDS: overall water splitting; photocatalyst; Z-scheme; bismuth compound.

1. Introduction With the demand for sustainable development of renewable energy, hydrogen fuel is considered as a promising candidate of future energy carriers.[1, 2] Solar water splitting which can covert solar energy into storable hydrogen fuel is believed to be an effective and sustainable strategy for solar energy harvesting. Considerable efforts have been devoted to improving the solar-to-hydrogen (STH) efficiency of water splitting with various strategies from materials design to device performance optimization.[3, 4] Generally, the STH efficiency is fatally determined by the band structure of photocatalysts. Their bandgap should be narrow to guarantee a certain amount of solar photons can be absorbed by the photocatalysts. And their absolute energy positions of conduction band (CB) and valence band (VB)edges should cover the water redox potentials to realize over all water splitting.[5, 6] However, few photocatalytic semiconductors can simultaneously meet these requirements. Some wide bandgap photocatalytic semiconductors, such as TaON,[7] TiO2,[8] CaTaO2,[9] have a appropriate band alignment, but their wide bandgap limits their light absorption in visible region which occupies most energy in the solar spectrum. Recent research shows that direct Z-scheme composite photocatalysts are promising material architecture to fulfill the band structure requirements.[10, 11] In a typical direct Z-scheme structure, two semiconductors were combined together and formed a heterojunction. Both the CB and VB of one semiconductor are higher than those of the other one. The photogenerated holes with lower oxidation ability in higher band structure semiconductor will combine with the photogenerated electrons with lower reduction ability in lower band structure semiconductor. The photogenerated electrons with higher reduction ability in higher band structure semiconductor and the photogenerated holes with higher oxidation ability in lower

band structure semiconductor are thus maintained.[12, 13] Direct Z-scheme optimized photocatalytic activity by combining narrow bandgap semiconductor which also addresses the light absorption and charge carrier separation issues simultaneously. For example, Chen et al. successfully prepared Cu2O/g-C3N4 composites based on type-II heterojunctions, which showed enhanced photocatalytic activity than single-component g-C3N4.[14] However, once a direct Z-scheme structure formed, their photocatalytic activity was significantly increased when combined with appropriate semiconductors, such as W18O49 and Fe2O3.[15, 16] The attractive performance was mainly attributed to the maintained strong redox photo-generated carriers in the direct Z-scheme structure. Although the aforementioned Z-scheme structure can solve the matching problem in band structure, a great number of charge trapping defects will be formed at the heterojunction interface due to the large crystal lattice mismatch and intrinsic interface defects. Considerable photogenerated carriers have been captured by the defects before reaching the photoelectrode/electrolyte interface without participating in the photoelectrochemical reactions.[17-20] Therefore, a high-quality heterojunction with fewer trap states may have a long lifetime of photogenerated carriers and thus high STH efficiency. Bismuth-based oxides have attracted wide attention in photocatalytic field due to their high dielectric permittivity, low toxicity, suitable band gap (2.8 eV) and a more positive valence band position.[21-24] Manipulating and modulating the stoichiometry of semiconductor nanomaterials has become a hot topic in recent years because it increases the density of free charges.[25] Recent reports show that nonstoichiometric Bi2O2.33 has better photocatalytic performance than stoichiometric Bi2O3.[26-28]

In this work, we designed a homologous Z-scheme heterojunction of Bi2O2.33/Bi2S3 by growing a thin layer of Bi2S3 nanoneedles on the Bi2O2.33 nanosheet surface with a wetchemistry method. The chemical composition in the junction will gradually change from one material to the other which eliminating the defect states from lattice mismatch and metal ion doping after thermal treatment. Associated with the high photocatalytic activity of nonstoichiometric Bi2O2.33 and the effective solar light absorption of Bi2S3, the Z-scheme photocatalyst are supposed to offer effective charge separation and transfer without sacrificing their redox capacity. As expected, the composite photocatalyst achieved in our experiment exhibits an excellent overall water splitting activity. The H2 evolution rate can reach 0.98 μmol·h-1, and the O2 evolution rate is about 0.5 μmol·h-1. Their stability was further improved by depositing an ultra-thin amorphous TiO2 layer (about 3 nm) to avoid Bi2S3 photocorrosion. ED BiOI

FTO

ED

SILAR CBD

Bi2O2.33/Bi2S3-TiO2

Bi2O2.33/Bi2S3

Bi2O2.33

Figure 1. Schematic synthesis process of the Bi2S3/Bi2O2.33-TiO2 composite.

2. Experimental The TiO2-protected Bi2S3/Bi2O2.33 composite photocatalyst was synthesized through the procedures diagramed in Figure 1. Briefly, the precursor BiOI nanosheets were firstly grown by electrodeposition on fluorine doped tin oxide (FTO) glass and annealing to convert into

Bi2O2.33. Then the Bi2O2.33/Bi2S3 heterojunction was formed by growing a layer of Bi2S3 nanoneedles on the Bi2O2.33 with successive ion layer adsorption reaction (SILAR) and chemical bath deposition (CBD). Finally, a protective layer of TiO2 was coated on the Bi2S3/Bi2O2.33 nanocomposite by electrodeposition to avoid photocorrosion. Some optical photographs have been taken in each step (Figure S1). It shows that BiOI is brown, Bi2O2.33 is white, and the color turns dark after Bi2S3 growth. There is no visible difference after coating TiO2 protective layer which may be due to its limited thickness. The details of each proce dure were provided as electronic supporting information (ESI).

3. Results and discussion 3.1 Material characterization (a)

(c)

(b) 300 nm

300 nm

Bi2O2.33

235 259 184

Bi2O2.33/Bi2S3

385 470

626

200 400 600 800 Raman Shift (cm-1)

Intensity (a.u.)

Intensity (a.u.)

(h)

1000

(f) (109) 0.27 nm

Bi2S3 (041) 0.23 nm

Experimental data Bi 4f Bi 4f7/2 S 2p Curve fitting Bi 4f5/2

S 2p3/2

S 2p1/2

Intensity (a.u.)

5 nm

20 nm

143 161 206

Bi2O2.33 (107) 0.305 nm

20

(i) Intensity (a.u.)

(e)

(d)

161 143

2 μm

2 μm

2 μm

(g)

300 nm

166 164 162 160 158 156 Binding energy (eV)

 Bi2O2.33















Bi2S3









Bi2O2.33/Bi2S3

Bi2O2.33 BiOI

30

40 50 2 (degree)

Experimental data Curve fitting 531.8 eV

60 O 1s

529.5 eV

537 534 531 528 525 Binding energy (eV)

Figure 2. Low and high magnification SEM images of a) BiOI, b) Bi2O2.33, c) Bi2O2.33/Bi2S3 heterostructures; d) TEM and e) HRTEM images of Bi2O2.33/Bi2S3 heterostructures; f) XRD patterns of BiOI, Bi2O2.33, Bi2O2.33/Bi2S3 heterostructures; g) Raman scattering spectra of bare Bi2O2.33 and Bi2O2.33/Bi2S3 composite; High-resolution spectrum of

h) Bi 4f and S 1s and i) O 1s.

Figure 2a shows the morphology of the as-prepared BiOI nanosheets on FTO glass substrate. The nanosheets were vertically aligned on the FTO glass with a sized around 1 µm and a few nanometers in thickness. After annealing process, the morphology has not been maintained. It evolved from nanosheets into worm-like nanosheets and the nanostructure thickness was also increased to a few hundred nanometers when we increase the annealing temperature. The morphology dependence on the annealing temperature has been shown in Figure S2. In this procedure, the BiOI will be pyrolyzed into Bi2O3 and I2 which can be easily sublimed.[29] Bi2O3 will be further decomposed to form non-stoichiometric Bi2O2.33 when we further increased the annealing temperature as indicated by the XRD pattern in Figure S3. When the temperature reached 650 °C, a certain amount of nanostructures have disappeared due to the over-decomposition of Bi2O3. It shows that 600 °C is likely to be the most suitable annealing temperature to convert all the BiOI into Bi2O2.33 without over-sacrifice, which has been evidenced to have higher photocatalytic activity than that of stoichiometric Bi2O3.[28, 30] Figure 2b shows the resultant Bi2O2.33 nanostructures with an annealing temperature of 600 °C.All the BiOI nanosheets have completely converted into worm-like nanosheets which are also vertically aligned on the FTO glass substrate. Figure 2c shows typical SEM images of the Bi2O2.33/Bi2S3 heterostructures (CBD reaction time, 6 h). The Bi2S3 nanoneedles were evenly distributed and intertwined on the Bi2O2.33 nanosheets. The Bi2O2.33 nanosheets have turned back into nanosheets which may ascribe to the re-crystalline in the CBD process. The uniform and sparse distribution of the Bi2S3 nanoneedles was attributed to the successful Bi2S3 seeding on the Bi2O2.33 nanostructure. The seed distribution on the Bi2O2.33 nanostructure has been shown in Figure S4. The considerable Bi2S3 nanoparticles distributed on the nanostructure surface will serve as the crystal seeds or nucleus center for Bi2S3 nanoneedle growth. The

nanoneedle distribution was also influenced by the CBD reaction time which has been shown in Figure S5. In the first two hours, only nanotips can be observed on the nanosheets (Figure S5a). The nanotips grew up into nanoneedles with the CBD time increase and they interconnected together when the CBD time increased to 8 h. TEM images of the Bi2O2.33/Bi2S3 heterojunction structure are shown in Figure 2d and Figure S6. It is clear that a large number of Bi2S3 nanoneedles are uniformly interwoven on the entire Bi2O2.3 surface with the length of 100-200nm and diameter of 20-40nm. The HR-TEM image (Figure 2e) shows a clear lattice spacing is about 0.23±0.01 nm, which is corresponding to the (041) plane of Bi2S3. The lattice spacing of 0.27 and 0.305 nm is ascribed to the (109) and (107) plane of Bi2O2.33. X-ray diffraction (XRD) was carried out to track the material change in crystal after each procedure as shown in Figure 2f. The diffraction peaks of the XRD spectrum performed on BiOI nanosheets are well indexed to the tetragonal phase of BiOI (JCPDS: 73-2063). After annealing at 600 °C, the BiOI was turned into non-stoichiometric Bi2O2.33 evidenced by the well-indexed diffraction peaks to the Bi2O2.33 (JSPDS: 27-0051). The presentence of Bi2S3 diffraction peaks in the Bi2O2.33/Bi2S3 composite pattern proves the existence of Bi2S3 (JSPDS: 17-0320). The diffraction peaks of Bi2O2.33 phase are still retained and there are no other stray peaks observed, indicating the high purity of the two-phase heterogeneous structure. The composition and phase of heterojunction dependence on the CBD reaction times have been shown in Figure S7. Raman spectra are presented in Figure 2g and the strong peaks at 143, 161, 206, 386, 470 and 626 cm-1 are related to the tetragonal phase of β-Bi2O3.[31] After Bi2S3 loading, the Bi2O2.33 peaks became weak, only peaks at 143 and 161 cm-1 can be indistinctly observed. The peaks at 184 and 235 cm-1 correspond to the two Ag modes of Bi2S3, meanwhile the peaks at 259 cm-1 is correspond to its B1g modes.[32] The results indicated the simultaneous

existence of Bi2O2.33 and Bi2S3, and the heterojunction nanocomposite has been successfully achieved. Their surface electronic states and composition were analyzed with XPS. Figure 2h shows the XPS spectrum performed on the Bi2O2.33/Bi2S3 nanocomposite in the range of 156-167 eV. According to the requirements of charge balance, Bi3+ and Bi2+ ions must exist in Bi2O2.33 crystal. The two main peaks at 158.7 eV and 164.0 eV may be attributed to Bi2+, and the two shoulder peaks are attributed to Bi3+ in the non-stoichiometric Bi2O2.33.[33] While the peaks at 161.3 eV and 162.6 eV are attributed to S 2p3/2 and S 2p1/2 of Bi2S3.[32, 34] The two peaks at 529.5 eV and 531.8 eV (Figure 2i) are ascribed to lattice oxygen and hydroxyl groups of O1s, respectively. The survey spectrum has been shown in Figure S8. It shows that there are only C, Bi, O and S elements were detected. UV-visible absorption spectroscopy was used to investigate their light-harvesting ability (Figure 3a). The absorption band edges of bare Bi2O2.33 is about 440 nm, indicating poor absorption in the visible light region due to its large band gap. After being decorated with Bi2S3, the absorption edge extended to 800 nm which covers almost all the visible light region benefitted by the effective light absorption of narrow-bandgap Bi2S3. Their optical bandgaps were estimated according to Kubelka-Munk function (equation S1) by linearly extrapolating Tauc plot as shown in Figure 3b. The estimated optical bandgaps of Bi2O2.33 and Bi2O2.33/Bi2S3 are about 2.8 eV and 1.4 eV.

1.5

(b)

Bi2O2.33 Bi2O2.33/Bi2S3

1.2

15 Bi2O2.33

12

Bi2O2.33/Bi2S3

-1/2

]

Bi2S3

h2 [eV cm

0.9

Bi2S3

9

1/2

Absorbance (a.u.)

(a)

0.6 0.3

6 1.3 eV

3 0

800

700

600

500

400

1.4 eV

2.8 eV 3.0 2.7

2.4 2.1 h (eV)

1.8

1.5

Wavelenghth (nm)

Figure 3. a) UV-vis absorption spectra and b) the corresponding Tauc-plots.

2.2. Photoelectrochemical characterizations

12

IPCE (%)

Bi2S33 Seed-Dark Bi2O2.33/Bi2S3 Bi2O2.33/Bi2S3-Dark

4 0 -4

-0.2 -0.4 -0.6 Potential (V vs. Ag/AgCl)

-0.8

(d)

CPE1

500

-Z'' (ohm)

30

Bi2S33 Seed

8

400

Rs

300

R1

100 0 0

Rct

0.1M Na2S+0.1M Na2SO3

Bi2O2.33/Bi2S3

10

2.5

CPE2

100 200 300 400 500 600

Z' (ohm)

(e)

(c) Bi2O2.33

20

0

0.0

Bi2O2.33 Fitting Bi2O2.33/Bi2S3 Fitting

200

40

2.0

400

500 600 700 Wavelength (nm)

800

15

Current density (mA/cm2)

Bi2O2.33-Dark

2 1/Cs 1010(F-2cm4)

Current density (mA/cm2)

Bi2O2.33

16

12

Bi2O2.33 Bi2O2.33/Bi2S3

9 6 3 0 0

50

(f)

100 Time (s)

Bi2O2.33/Bi2S3 - 0.29 V Bi2O2.33- 0.13 V

CB

1.5 1.0

VB

0.5

Bi202.33

0.0 -0.6

-0.4

-0.2 0.0 0.2 0.4 Potential (V vs. NHE)

0.6

150

CB

--

-

(b)

(a)

+ + + +

VB Bi2S3

internal electric field

Figure 4. a) LSV curves of Bi2O2.33, Bi2S3 seed layer and Bi2O2.33/Bi2S3 composites; b) IPCE spectra of the samples; c) I-t curves of bare Bi2O2.33 (black), Bi2O2.33/Bi2S3 (red) at -0.2 V vs. Ag/AgCl; d) Nyquist and e) Mott-Schottky plots of the Bi2O2.33, Bi2O2.33/Bi2S3. The electrolyte is 0.2 M Na2SO4 and the pH value is 6.8; f) Energy band diagram between Bi2O2.33 and Bi2S3 with inverse band bending and internal electric field at interface.

The PEC performance was investigated by linear sweep voltammetry (LSV) under threeelectrode configuration for sulfite oxidation process (0.1M Na2S + 0.1M Na2SO3). Sacrificial electron donors were used to study PEC performance without surface recombination. Figure 4a shows the LSV characteristics of Bi2O2.33 and Bi2O2.33/Bi2S3 electrodes under simulated sunlight with AM 1.5 G. The pristine Bi2O2.33 nanostructure film exhibits a photocurrent density of 3.0 mA/cm2 at 0 V vs. Ag/AgCl. It was improved to 5.9 mA/cm2 after the deposition of Bi2S3

seed layer and was further enhanced to 15.1 mA/cm2 after Bi2S3 nanoneedles growth with an optimized volume amount. The non-stoichiometric Bi2O2.33 has a better photoelectrochemical performance than Bi2O3 as evidenced in Figure S9. Modulating stoichiometric ratio can increase the free-charge density and result in improved light trapping and charge transfer efficiency.[25] Moreover, the Bi2S3 decoration amount is also an important factor which influences their final performance (Figure S10a). It indicates that when the CBD time is 6 hours, the photocurrent has the maximum value, which is about 5 times higher than that of Bi2O2.33. The non-stoichiometric Bi2O2.33 and the high-quality heterojunction formed with Bi2S3 are believed to contribute to the enhanced photocurrent density and photocatalytic activity as well by facilitating the separation and transport efficiency of photogenerated carriers. We noticed that all samples present a large dark current (Figure S10b)

which may due to the larger

specific surface area and more active areas for redox reactions as discussed in previous reports.[32] IPCE has been measured at a bias voltage of -0.2V vs. Ag/AgCl to quantitatively investigate the photoelectrochemical efficiency. The values were calculated by the equation S2 and the results are shown in Figure 4b. The pristine Bi2O2.33 has a weak photo-response and only in the wavelength range near 500 nm, with a maximum IPCE value of 12%. After decorated with Bi2S3, the Bi2O2.33/Bi2S3 composite showed enhanced photo-response in the entire visible light region and the IPCE can reach up to 27%. The IPCE results further confirmed the enhanced photon capture efficiency by Bi2S3 in PEC reactions. The weak photo-response of pristine Bi2O2.33 was also observed over 500 nm. This might be the result of the reaction between Bi2O2.33 and S2- ions, leading to the formation of Bi2S3 on the surface of Bi2O2.33,[35] which can be verified by the surface color change. After being measured in the sulfide solution, the color of the bare Bi2O2.33 film changed from pale yellow to black. In order to ensure the correctness of the analysis, LSV (Figure S11a) and IPCE (Figure S11b) were further tested in 0.2 M

Na2SO4 aqueous solution. Obviously, the nanocomposite still exhibited high photocurrent density and high IPCE. The light harvesting efficiency (LHE) of Bi2O2.33/Bi2S3 and pristine Bi2O2.33 are estimated by the equation S3. The LHE of Bi2O2.33/Bi2S3 is higher than that of pristine Bi2O2.33 in the entire absorption region (Figure S11c). If the performance enhancement is only introduced by the decorated Bi2S3, the composite should exhibit an increase in IPCE rather than absorbed photon-to-current efficiency (APCE).[36] However, the Bi2O2.33/Bi2S3 shows a significant enhancement in APCE (methods see the equation S4) in the visible region (Figure S12d), which means the separation ability of photogenerated electron-hole pairs increases. Figure 4c shows the transient photocurrent response of the Bi2O2.33 and Bi2O2.33/Bi2S3 composites under chopped illumination (measured at -0.2V vs. Ag/AgCl). Upon illumination, the photocurrent density of the sample increases rapidly. While the photocurrent density decreases instantaneously when the lamp is off (Joff). This rapid rise and fall in photocurrent density indicates that the carriers transport in the heterojunction material proceeds quickly. In addition, the Bi2O2.33/Bi2S3 composite shows a higher photocurrent density than that of Bi2O2.33, which is powerful evidence to verify the excellent photoelectric response and less charge recombination in the heterojunction structures. The Bi2O2.33/Bi2S3 composite displays a series of spikes upon turning the light on (Jon), indicating the excess holes accumulation at the photocatalyst/electrolyte interface. However, the accumulation of holes will self-oxidize the BiS bond, resulting in photocorrosion to the photocatalytic electrode as observed in the stability test. The stability of the Bi2O2.33/Bi2S3 composite electrode was tested at -0.2 V vs. Ag/AgCl under irradiation (Figure S12). The photocurrent density gradually decreased from 10.2 to 7.5 mA/cm2, implying severe photo-corrosion to the photocatalyst even in the presence of a sacrificial agent.

The carrier transport and recombination dynamics of the composite were further investigated by EIS under light illumination. Figure 4d shows the Nyquist plots of the Bi2O2.33 and Bi2O2.33/Bi2S3 electrode. Each curve in the Nyquist plots contains two semicircles. The curve was fitted with a series connection of an RC (resistor and capacitor) circuit model. A smaller arc radius was observed for the Bi2O2.33/Bi2S3 electrode compared with that of pristine Bi2O2.33 electrode. The smaller radius in the Nyquist plot means the small corresponding charge transfer resistance. The fitted values of each component are shown in Table S1. The results show that the composite structure has a fast interfacial charge transfer and effective carrier separation ability. PL spectroscopy was used to further confirm the enhanced carrier separation efficiency (Figure S13). Generally, the higher PL intensity indicates a greater probability of charge recombination. The inclusion of Bi2S3 in the hybrids decreases emission intensity, which suggests the less carrier recombination in the Bi2S3/Bi2O2.33 in comparison with pristine Bi2O2.33. To reveal the mechanism underlying the enhanced performance, Mott-Schottky (M-S) plots (Figure 4e) was measured in a dark environment to evaluate the band structure of the composite. Flat band potential (Vfb) and charge carrier density (Nd) of the electrode can be determined by M-S relationships. Nd can be obtained from the slope of the M-S plots using equation S5 and the results have been listed in Table S2. The Bi2O2.33/Bi2S3 composite electrode gives rise to a much smaller slope than that of the pristine Bi2O2.33 electrode, representing a higher carrier concentration. In addition, the positive slope indicates that the Bi2O2.33/Bi2S3 electrode is an Ntype semiconductor.[37] On the other hand, in a specific electrolyte, Vfb reflects the position of Fermi energy level, which is very close to the minimum value of the CB of N-type semiconductor.[38] Usually, the Vfb can be extrapolated from the X-axis intercept of the linear region in the M-S plot. The estimated values of the Bi2O2.33 and Bi2O2.33/Bi2S3 electrodes are

about -0.19 V and -0.29 V (vs. NHE at pH 7). Generally, when two semiconductors contacts together, free electrons will migrate from one semiconductor to the other due to the Fermi level difference until they align at the same level.[39] Since the Fermi level of Bi2S3 is much higher than that of Bi2O2.33. Once Bi2S3 and Bi2O2.33 contact together, the different in Fermi energy levels will generate a space charge region and an internal electric field from Bi2S3 (+) to Bi2O2.33 (-) which drive the energy band edges up or down at the interface (Figure 4f).[40, 41] The downward bent band allows electrons to flow out while suppressing the outflow of holes. Conversely, holes can move along the upward band while electrons are forbided.[15] Therefore, under photoexcitation, the reverse band bending is expected to drive direct recombination between the electrons in the Bi2O2.33 CB and the holes of Bi2S3 VB, thereby realizing a direct Z-sheme transfer process of photogenerated carriers. 2.3 Photocatalytic water splitting (c)

(a) 250

Bi2O2.33/Bi2S3-2h

150

Bi2O2.33/Bi2S3-4h

E(V) vs NHE PH=7

200

Bi2O2.33/Bi2S3-6h Bi2O2.33/Bi2S3-8h

100 50 0 0

1

(1wt %) Pt/

2 3 Time (h)

VB

1

Bi202.33 2.8 eV

2

Bi2O2.33/Bi2S3

Produced H2 Produced O2

(1wt %) Pt/

Produced H2

Bi2O2.33

Produced O2

2

Time (h)

3

e- e- e-

Bi2S3 1.3 eV

CB VB

Bi202.33 2.8 eV

h+ h+

VB h+ h+ h+

(d) 40

e-

H+/H2

Bi2S3 1.3 eV h+ h+

H2O/O2 OH-/•OH

VB h+ h+ h+

Type-Ⅱ

(e)

Direct Z-Scheme 40

Bi2O2.33/Bi2S3

425 nm

0 1

eCB

4

2

0

CB

3

6

4

e- e- e-

0

Intensity (a.u.)

Gas production (μmol)

(b)

e-

eCB

-1

4

30 20

40 min 30 min 20 min 10 min

10 0 375 400 425 450 475 500 525 550

Wavelength (nm)

Intensity (a.u.)

Bi2O2.33

H2 generation μmol)

300

Bi2O2.33

30 20

Bi2S3

TAOH

10 0 10

15

20 25 30 35 Irradiation (min)

40

Figure 5. a) Photocatalytic hydrogen evolution under full arc irradiation and b) Photocatalytic overall water splitting by Bi2O2.33 and Bi2O2.33/Bi2S3; c) Schematic illustration of traditional type-II heterojunction and direct Z-scheme charge transfer mechanism; d) PL spectra change of TA solution in presence of Bi2O2.33/Bi2S3 measured after different irradiation times; e) Comparison of PL spectra changes in the presence of different samples (excitation at 315 nm).

Hydrogen production activity of the composite nanomaterial was evaluated by splitting water under simulated sunlight illuminate. The practical performance has been recorded and presented in Figure 5a by using Na2S/Na2SO3 as sacrificial agents and in-situ photo-deposited Pt (1wt %) as a co-catalyst (Figure S14). Among the sample series, Bi2O2.33/Bi2S3 composite with a CBD time of 6 h exhibits the highest H2 evolution rate, which is about 62.61 μmol·h-1. Whereas, the pristine Bi2O2.33 photocatalyst showed no detectable hydrogen evolution activity in the same situation. The cycling experiment of water splitting as shown in Figure S15. In addition, we extended the application of Bi2O2.33/Bi2S3 composite to overall water splitting without hole-scavenging sacrificial agent. The detected generation rates are 0.98 μmol·h-1 for hydrogen and 0.5 μmol·h-1 for oxygen when illustrated by a 500W Xenon lamp (Figure 5b). The molar ratio of the evolution H2 and O2 is about 1.95 which is pretty close to the theoretical ratio of 2.0. This might be introduced by the inevitable leaking of the measuring system to H2. Compared with recent reports on representative Z-scheme photocatalytic overall water splitting systems, Bi2O2.33/Bi2S3 has obviously superior photocatalytic activity (Table S3). Based on the band alignment between Bi2O2.33 and Bi2S3, there are two possible charge transfer paths in the heterojunction as illustrated in Figure 5c. .However, it is well known that the CB position of the Bi2O2.33 is lower than proton reduction potential.[9] Photoelectrons cannot undergo thermodynamic reaction with protons to produce hydrogen due to the overpotential. However, hydrogen was detected in this process which indicates that the photoelectrons generated in Bi2O2.33 tend to combine with the photoholes generated in Bi2S3, and the formed junction must be a direct Z-scheme structure. To futher confirm this inference, the concentration of hydroxyl radicals (•OH) in the electrolyte was roughly estimated with PL by using terephthalic acid (TA) as probe molecules.[42] The PL spectrum (Figure 5d) shows a

maximum intensity at 425 nm (excitation at 315 nm).The emergence of the PL signal indicates that •OH has been produced and the VB position of Bi2O2.33 is more positive than E (•OH/OH−) which is enough to oxidize OH− into •OH. For Bi2O2.33 and Bi2O2.33/ Bi2S3, the increase in PL peak intensity at 425 nm maintains a similar increase, indicating that the potential for oxidizing OH− to •OH is only exhibited by Bi2O2.33/ Bi2S3 (Figure 5e).[43] However, that of Bi2S3 almost kept the same in the same situation which implies that there is no •OH presence due to the higher VB position of Bi2S3 than OH− oxidation potential. If the photo-generated holes in the VB of Bi2O2.33 flow to the VB of Bi2S3, all the photo-generated holes are in the VB of Bi2S3 and their oxidation potential are insufficient for water splitting. There would be no TAOH detected in the electrolyte. However, we verified the existence of TAOH with the increased PL intensity with the reaction time which is solid evidence of the Z-scheme structure. The attractive photocatalytic activity of the composite photocatalyst was mainly attributed to the following three factors. One is the successful formation of a Z-scheme structure between Bi2O2.33 and Bi2S3 which facilitates the charge separation and transfer dynamics without sacrificing the redox capacity of photogenerated carriers. The other one is that the heterojunction is homologous which gradually changes from one material to the other without dramatic change in lattice and chemical composition. Theoretically, less defect traps will exist in the junction can significantly decrease the recombination of photogenerated carriers and thus promise a high charge separate rate. Furthermore, we believe the non-stoichiometric Bi2O2.33 also plays an important role in enhancing the photocatalytic activity by its high conductivity and light capturing. 2.4 Lifespan extension with TiO2 protective layer

Photocorrosion is a severe issue that hinders the practical applications of sulfide photocatalysts. An ultra-thin amorphous TiO2 layer was decorated on the surface of the Bi2O2.33/Bi2S3 composite photocatalyst to keep them from photocorrosion. After depositing TiO2 protective layer, there is no observable morphological changes and has little effect on the crystal structure (Figure S16). This may be due to the amorphous crystalline states and limited decoration amount. The TEM image (Figure 6a) and the HR-TEM image (Figure 6b) show that the Bi2S3 needle is completely covered with a TiO2 layer which has a thickness of about 3 nm. The corresponding fast Fourier transform (FFT) pattern shows that the TiO2 layer is amorphous, whereas the Bi2S3 is highly crystalline. The existence of amorphous TiO2 was further confirmed by XPS (Figure S17) and EDX (Figure S18) spectroscopy. (a)

(b)

FFT Bi2S3 (221) 0.28 nm

2 nm

12 9

Bi2S3 (e)

(d) Pristine Bi2O2.33/Bi2S3 Bi2O2.33/Bi2S3/TiO2-1 Bi2O2.33/Bi2S3/TiO2-2 Bi2O2.33/Bi2S3/TiO2-3

Current density (mA/cm2)

Current density (mA/cm2)

(c) 15

FFT

TiO2

Bi2O2.33

10

6 3 0 -3 -0.8 -0.6 -0.4 -0.2 Potential (V vs. Ag/AgCl)

0.0

8 6 4 2 0

20 40 60 80 100 120 140 160

Bi2O2.33/Bi2S3/TiO2

Light on

10

Bi2O2.33/Bi2S3-TiO2

Current density (mA/cm2)

10 nm

TiO2

Bi2O2.33/Bi2S3

8 6 4

Light off Light off

2 0

0

300 600 900 1200 1500 1800

Time (s)

Time (s)

Figure 6. TEM and HRTEM images of Bi2O2.33/Bi2S3-TiO2 (a, b); c) LSV curves of Bi2O2.33/Bi2S3-TiO2 with different

charge

densities

pristine:

Bi2O2.33/Bi2S3,

Bi2O2.33/Bi2S3/TiO2-1

(0.1

C/cm2 passed),

Bi2O2.33/Bi2S3/TiO2-2 (0.2 C/cm2 passed) and Bi2O2.33/Bi2S3/TiO2-3 (0.3 C/cm2 passed); d) I-t curves Bi2O2.33/Bi2S3-TiO2 at -0.2 V vs. Ag/AgCl; e) Stability test of the Bi2O2.33/Bi2S3 for 300 s and Bi2O2.33/Bi2S3TiO2 for 1800 s measured at -0.2 V vs. Ag/AgCl.

The thickness of TiO2 protective layer can be rationally controlled with the charge passing amount during deposition. The goal is to obtain a layer of TiO2 that completely covers the surface of the original sample to prevent direct contact between Bi2O2.33/Bi2S3 composite and electrolyte. The thickness should be sufficiently thin to avoid affecting the transport of photogenerated carriers. The PEC performance influenced by the TiO2 film thickness is shown in Figure 6c. We found that the optimum thickness was achieved when the charge amount was 0.1 C/cm2. To investigate the efficacy of the TiO2 protective layer, a transient photocurrent density test was performed to evaluate the photoresponse as shown in Figure 6d. The photocurrent density rapidly increased to a steady-state once light irradiated. Compared with the Bi2O2.33/Bi2S3 composite without TiO2 protective layer, there were no obvious photocurrent peaks observed (Figure 4c), indicating that the loaded TiO2 can effectively prevent carrier accumulation. This also indicates that the TiO2 protective layer has passivated some surface traps and thus suppresses recombination of electron-hole pairs, which is consistent with previous studies on protective layers.[44, 45] A long-term stability test under illumination is shown in Figure 6e with a bias potential of -0.2 V vs. Ag/AgCl. The photocurrent density of Bi2O2.33/Bi2S3 composite electrode without TiO2 protective layer continually decreased till the light off. The oxidation of the S-metal bonds in Bi2S3 by the photo-generated holes which have a strong oxidizing ability may lead to performance degradation. The performance degradation of sulfide photocatalysts caused by photocorrosion greatly limits their practical application although they have intrinsic advantage in solar light absorption and water oxidation. We addressed this issue by depositing a layer of amorphous TiO2 as protective media to block the direct contact between semiconductor photocatalysts and chemical electrolyte. The results show that the TiO2 protective layer can effectively decrease the photo-corrosion to sulfide

photocatalyst and thus extend their lifespan. There is no significant performance degradation in 1800 s during light continually illustrating. 4. Conclusion In conclusion, we designed a homologous heterojunction photocatalyst based on nanostructured Bi2O2.33 and Bi2S3 which has a Z-scheme band alignment structure for potential application in overall water splitting. A layer of amorphous TiO2 was used to prevent the photocorrosion to Bi2S3 and extend the lifespan. Our results show the composite photocatalyst has excellent photocatalytic activity in overall water splitting which the H2 evolution rate of 0.98 μmol·h-1 and 0.5 μmol·h-1 for O2 evolution under simulated solar light irradiation. The photocatalytic performance was effectively maintained by the TiO2 protective layer and there is no significant decline in 1800 s. The enhanced photocatalytic activity of Bi2O2.33/Bi2S3 composite was attributed to the favorable separation and transport efficiency in a Z-scheme structure, the homogeneous gradually changed heterojunction with fewer defect traps and the high photocatalytic activity of non-stoichiometric Bi2O2.33. This work provides a novel strategy to achieve high performance photocatalyst by rational designing the band alignment and material composition.

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Graphical Abstract

H2 e- e-

E(V) vs NHE PH=7

-1

CB

e- e- e-

0

CB

1

O2

Bi2S3 1.3 ev

VB

Bi202.33 2.8 ev

2 3

H+

h+ h+

H+/H2

H2O/O2

VB h+ h+ h+

H2O

Highlights 

Achievement of the high interface quality between Bi2O2.33 and Bi2S3.



The unique Z-scheme features effectively promote the separation and transfer of photogenerated electron-hole pairs.



The composite photocatalyst exhibits overall water splitting activity without any sacrificial agents.