Efficient and stable charge transfer channels for photocatalytic water splitting activity of CdS without sacrificial agents

Wei Chen a, Guo-Bo Huang *a, Hao Song b and Jian Zhang *bc
aSchool of Pharmaceutical and Materials Engineering, Taizhou University, Taizhou 318000, Zhejiang Province, PR China. E-mail: gbhuang973@163.com
bNew Energy Technology Engineering Lab of Jiangsu Province, School of Science, Nanjing University of Posts & Telecommunications, Nanjing 210023, PR China. E-mail: iamjzhang@njupt.edu.cn
cKey Laboratory for Organic Electronics and Information Displays, Institute of Advanced Materials, Jiangsu National Synergetic Innovation Center for Advanced Materials, Nanjing University of Posts & Telecommunications, Nanjing 210023, PR China

Received 23rd June 2020 , Accepted 11th August 2020

First published on 12th August 2020

Semiconductor-based durable photocatalysts for efficient water splitting have attracted much attention for the development of sustainable hydrogen energy production, but it is challenging for CdS to achieve the expectation of the absence of hole scavengers. Herein, it is reported that uniform CdS nanorods coated with ultrathin NiOOH were prepared as a photocatalyst for high-efficiency photocatalytic water splitting without using any hole sacrificial agents. For the novel surface dynamics features, electron accumulation on CdS was detected with in situ irradiated X-ray photoelectron spectroscopy and accelerated hole transfer was recorded as 49.6 ± 9.2 ps by femtosecond time-resolved transient absorption spectroscopy. The thickness dependence of the NiOOH wrapper with an ultimate continuous thickness of ∼4 nm not only achieved the record high value for photocatalytic hydrogen generation rate (118.6 μmol h−1 g−1) among CdS-based heterojunction photocatalysts without any scavengers, but also exhibited good photostability (over 25 h of cycling measurements). This work provides valuable guidelines for the design of next-generation, high performance CdS-based photocatalysts.

1. Introduction

Use of photocatalytic water splitting for hydrogen production has been considered as one of the most promising strategies for converting solar energy to hydrogen fuel.1,2 Although various earth-abundant semiconductor photocatalysts have been developed for water splitting in recent decades, the efficiency and durability of water splitting still needs to be substantially improved before its use in commercial applications.3 In the splitting process, the oxygen evolution half reaction is always identified as the rate limiting step due to the inherent slow kinetics of the four-electron transfer process. Therefore, except for the enhancement of light harvesting and charge separation, another significant strategy to enhance the photocatalytic efficiency is to accelerate surface charge transfer and injection to overcome the sluggish surface kinetics.4,5 Cadmium sulfide (CdS) has received great attention as a promising sustainability-inspired photocatalyst for hydrogen evolution from water in recent years, because of the intrinsic physical properties such as a favorable band gap for visible light absorption and appropriate band alignment for the relevant redox reactions.6,7 However, it is known that the holes accumulated on the valence band of sulfide triggers a photocorrosion reaction easily because of the low charge carrier separation and the efficiency of the injection of the holes.8–11 To solve these issues, numerous CdS-based composites are being constructed to accelerate charge separation and transfer by the formation of heterojunctions and a co-catalyst.12–15 For a heterojunction strategy, beyond traditional type II heterojunction,16 Z-scheme photocatalysts can increase the light absorption range and preserve photogenerated electron–holes with higher redox potentials.17,18 Following this mechanism, some oxygen generation half-reaction photocatalysts including CoS2,19 CuInS2,20 MnS,21 Fe2O3,22 WO3 (ref. 23) and BiVO4 (ref. 24) have been fabricated with CdS forming Z-scheme heterojunction photocatalysts. For a co-catalyst method, some hydrogen evolution reaction electrocatalysts, such as MoS2,25 MoOxSy,26 WS2 (ref. 27) and FeP,28 were usually coupled with CdS for the decreased hydrogen generation potential, which is beneficial for the transfer of photoexcited electrons. Regrettably, most of the previously reported photocatalysts still need scavengers (hole sacrificial agents) because of the high activity of hydrogen production. To avoid the consumption of the high-cost sacrificial agents and quantify the reduction yield accurately,29 it is imperative to explore an alternative method for efficient photocatalytic water splitting in the absence of sacrificial agents, and more importantly, uncover the hole transfer dynamics of CdS using ultrafast monitoring methods, but this is still to be done.

It is known that charge carrier dynamics at the surface always strongly influence the catalytic characteristics of electrocatalysts and photocatalysts.30 Interface surroundings usually determine the surface electronic structure, which strongly influences the charge carrier dynamics characteristics.31 Selection of specific co-catalysts with the anomaly of highly-efficient hole injection, such as NiOOH,32 might accelerate the sluggish kinetics of oxygen generation over CdS photocatalysts by increasing the holes' migration and injection efficiency. The NiOOH nanocrystals have been reported as being effective oxygen evolution reaction electrocatalysts and oxygen generation co-catalysts due to their suitable oxygen generation potential and high hole injection efficiency.33,34 Ultrafast optical spectroscopy has been demonstrated as an effective strategy to estimate charge carrier kinetics by comparing charge carrier migration directly.35 For example, in black phosphorus/platinum heterostructure systems, the electron migration accelerated from 2.8 to 0.11 ps compared to that obtained with black phosphorus alone, observed by a femtosecond pump–probe microscopic optical system, which could explain the enhancement of photocatalytic efficiency adequately.36

To verify the scientific speculation based on the discussion previously, uniform CdS nanorods coated with an ultrathin NiOOH wrapper were prepared using an aerosol assisted chemical vapor deposition (AACVD) method followed by an atomic layer deposition (ALD) process. It is found that the photocatalytic performance of the hybrids possesses NiOOH shell thickness dependent features. The thickness dependence of the NiOOH wrapper with a continuous, ultimate thickness of ∼4 nm can not only achieve a record high value for photocatalytic hydrogen generation activity (118.6 μmol h−1 g−1) among CdS based heterojunction photocatalysts without any scavengers, but also exhibits good photostability (over 25 h cycling measurements) over the nanocomposite photocatalysts. The proposed core–shell nanohybrid photocatalysts with a novel charge transfer channel have great potential for visible light driven water splitting based on CdS nanocrystals.

2. Results and discussion

As illustrated in Fig. 1a, the vertically oriented 1D CdS nanorods grown on the fluorine-doped tin oxide (FTO) substrate were synthesized using an AACVD process according to a previous procedure.37 The NiOOH shells were subsequently coated on the surface of the CdS nanorods with different thicknesses by conveniently adjusting the deposition times of the ALD. The continuous ultimate thickness of the NiOOH shell was about 4 nm, and according to the deposition thickness, the samples were labeled as CdS (without NiOOH), CdS/2NiOOH (NiOOH with a 2 nm thickness), CdS/4NiOOH (NiOOH with a 4 nm thickness) and CdS/16NiOOH (NiOOH with a 16 nm thickness). The surface morphology of the as-deposited four electrodes can be recognized by the contrast between the top-view and cross-sectional view of field-emission scanning electron microscopy (FESEM) images as shown in Fig. 1b–e. In Fig. 1b, the original CdS nanorod electrode presents as mono-dispersed and self-supporting with diameters of ∼160 nm and lengths of over 2.0 μm. As far as is known, this was the maximum length-diameter ratio of 1D free-standing CdS until now, which has some unique configuration advantages for photocatalysis, such as for enhancement of the light absorption and scattering efficiency as well as to provide a fast electron transport pathway.38,39 After coating with a ∼2 nm thickness of NiOOH, it was found that the size and surface morphology of CdS/2NiOOH (Fig. 1c) remained good in comparison with the initial CdS at the current magnification. Increasing the deposition times continually, it was not difficult to observe that the diameters of the nanorods grew from ∼160 nm (CdS) to ∼170 nm (CdS/4NiOOH, Fig. 1d) and ∼200 nm (CdS/16NiOOH, Fig. 1e), which means that the NiOOH was deposited uniformly and closely on the surface of the CdS.
image file: d0ta06177h-f1.tif
Fig. 1 (a) Schematic showing the synthesis route of CdS/NiOOH core/shell nanorods. The SEM images of CdS (b), CdS/2NiOOH (c), CdS/4NiOOH (d), and CdS/16NiOOH (e). The insets top right in (b) to (e) show the corresponding cross sectional views of the SEM images.

The phase structures of the four samples were examined using X-ray diffraction (XRD) analysis as shown in Fig. S1 (ESI) (the samples were scraped from the FTO glass). The diffraction peaks detected from the CdS sample were well indexed to the hexagonal phase CdS (JCPDS card no. 41-1049).40 However, only slight NiOOH signals could be detected from CdS/4NiOOH and CdS/16NiOOH due to the weak crystallites organized under limited deposition temperature (120 °C). The UV-vis absorption spectra (the samples were scraped from the FTO glass) were used to display the influence of the optical properties of the CdS electrode after merging with the NiOOH shell (Fig. S2, ESI). The bare CdS electrode showed an absorption edge at 528 nm, suggesting a band gap of ∼2.4 eV.41 After the introduction of NiOOH, the absorption range of the core/shell hybrids were increased from 528 to 800 nm due to the absorption in the visible region of NiOOH. However, the light absorption edge of the composites was maintained (only a slight red shift), indicating that the NiOOH cannot make an obvious change the band gap of the CdS.

Typical transmission electron microscopy (TEM) images of CdS, CdS/2NiOOH, CdS/4NiOOH and CdS/16NiOOH catalysts are shown in Fig. 2a–d. The bare CdS nanorods (Fig. 1a) were monocrystalline (the inset in Fig. 2a) with a smooth surface. After depositing a small amount of NiOOH (CdS/2NiOOH), a slight roughness can be seen in the TEM image in Fig. 2b. For the CdS/4NiOOH and CdS/16NiOOH samples, ∼4 nm and ∼16 nm shell thicknesses of the NiOOH (polycrystalline as seen from the inset of Fig. 2c and d) can be observed from the different electron penetrability between the core and shell (Fig. 1c and d), which matches the SEM images well. Fig. 2e and i show typical HRTEM images of CdS/4NiOOH and CdS/2NiOOH, in which the interplanar spacings of 0.20 and 0.363 nm agree well with NiOOH (210)42 and CdS (100),43 respectively. Meanwhile, the continuous ultimate thickness of the NiOOH shell was accurately measured as 3.9 nm. This ultrathin NiOOH coating can not only enhance visible light absorption, but also restrained the photocorrosion by forming an isolated layer between the catalysts and water, which was beneficial for photocatalytic water splitting.44 High-resolution EDS-lining images with scanning transmission electron microscope-high angle annular dark field (STEM-HADDF) of CdS/4NiOOH and CdS/2NiOOH are displayed in Fig. 2f, g, j and k and further reveal that NiOOH forms a shell of different thicknesses over the CdS core. Energy dispersive X-ray spectroscopy (EDS) also shows the gradient change in the proportion of the elements for the four samples (Fig. 2h and i, S3 and S4, ESI), which was consistent with expected results. In addition, EDS-mapping characterization (Fig. 2n and o) was compared using STEM-HADDF (Fig. 2m) to investigate the element composition and distribution over the four catalysts. The results showed that Cd elements were distributed in the core region of the nanorods only, whereas the Ni signals were located homogenously throughout the shell around the core (hybrid samples). Meanwhile, the precise atomic ratios of Cd[thin space (1/6-em)]:[thin space (1/6-em)]Ni of the hybrid samples were confirmed as: 40[thin space (1/6-em)]:[thin space (1/6-em)]1 for CdS/2NiOOH, 12[thin space (1/6-em)]:[thin space (1/6-em)]1 for CdS/4NiOOH and 2.6[thin space (1/6-em)]:[thin space (1/6-em)]1 for CdS/16NiOOH from the inductively coupled plasma-atomic emission spectrometry (ICP-AES) results (Fig. S5, ESI).

image file: d0ta06177h-f2.tif
Fig. 2 Low-resolution TEM images of CdS (a), CdS/2NiOOH (b), CdS/4NiOOH (c), and CdS/16NiOOH (d), the inserts in (a) to (d) are the corresponding SAED patterns. The HRTEM, STEM-HAADF with scanning direction, EDS-lining images and EDS spectrum of CdS/4NiOOH (e–h) and CdS/2NiOOH (i–l). The STEM-HAADF (m) and EDS-mapping images of Cd (n) and O (o).

X-ray photoelectron spectrometry (XPS) was used to identify chemical composition and elemental valence states of the samples' surfaces as shown in Fig. 3a and b. In Fig. 3a, the binding energy of Cd2+ in the Cd 3d5/2 and Cd 3d3/2 orbits were 405.1 and 411.8 eV, respectively, which were in agreement with previous reports of CdS.45 Merged with NiOOH, the Cd 3d peaks of CdS/2NiOOH shifted towards a lower binding energy by 0.15 eV, which was ascribed to the strong coupling interface induced charge transfer between the two components because of the tightly formed heterojunction, which prevented charge carrier recombination.46–48 A similar shift was also detected in S 2p XPS (0.12 eV negative shift, Fig. S6, ESI). For the Ni 2p spectrum of pure NiOOH film (Fig. 3b), the peaks located at 855.8 eV (2p3/2) and 873.2 eV (2p1/2) were attributed to the co-existence of Ni2+ and Ni3+.49 In addition, the satellite peaks of 2p3/2 and 2p1/2 spin orbits of the Ni element were found at about 862.2 and 879.6 eV, respectively. After coating of the NiOOH film on CdS, the 2p3/2 and 2p1/2 spin orbits of the Ni shift positively to 856.3 and 873.7 eV, respectively, which revealed that there was a reduction of electron density. The previous results of the absorption range of core/shell hybrid XPS analysis based on ground state measurements demonstrated that the NiOOH coating over the CdS to fabricate hybrid catalysts could effectively increase the charge carriers' migration.

image file: d0ta06177h-f3.tif
Fig. 3 High-resolution XPS for Cd 3d of CdS and CdS/2NiOOH (a), Ni 2p of CdS/2NiOOH and CdS/4NiOOH (b), Cd 3d (c) and Ni 2p (d) of CdS/4NiOOH in the dark and under a blue laser irradiation (488 nm, 100 mW).

To investigate the transfer and accumulation behavior of the photogenerated charge carriers between the CdS core and NiOOH shell, in situ irradiated XPS50 was implemented under a blue light illumination (488 nm, 100 mW). In particular the focus was on the CdS/4NiOOH catalyst because it showed optimal photocatalytic performance for water splitting. In the dark, the Cd 3d spectrum from the CdS/4NiOOH catalyst displayed two sharp peaks at about 404.9 and 411.6 eV which were attributed to the 3d5/2 and 3d3/2, respectively (Fig. 3c). Upon light irradiation, the Cd 3d binding energy exhibited a slight negative shift (∼0.2 eV), indicating an accumulation of excess electrons for the CdS. Similarly, two strong signals located at 856.7 and 874.3 eV corresponding to the binding energies of Ni 3d3/2 and Ni 3d1/2, respectively, accompanied by two satellite peaks (862.4 eV for Ni 2p3/2 and 881.0 eV for Ni 2p1/2) were obtained from the CdS/4NiOOH sample without light (Fig. 3d). The peaks shifted about 0.1 eV towards the higher binding energy under light irradiation, which suggested there was a decrease in the electron density.51 Such shifts confirmed that the photoexcited electrons accumulated on the CdS upon light illumination over the CdS/NiOOH photocatalyst. Quantitative evidence for the charge carrier directed migration on the hybrid catalysts is discussed next.

The photocurrent density-potential curves of CdS, CdS/2NiOOH, CdS/4NiOOH and CdS/16NiOOH were collected using linear sweep voltammetry under AM 1.5 G illumination in 0.5 M Na2SO4 electrolyte without hole scavengers (Fig. 4a). It can be seen that the pure CdS curve maintains a steady growth in the photocurrent current with the onset potential of ∼0.38 V and reaches to ∼0.3 mA cm−2 at the threshold potential of water splitting (1.23 eV vs. RHE), and this was similar to other reports on pure CdS photoelectrochemical (PEC) characteristics without sacrificial agents.52 When coated with an NiOOH shell, all three of the heterojunction electrodes displayed significantly improved photocurrent densities with a slight reduction of onset potential (∼0.037 V vs. RHE), which was attributed to the effective photoexcited electron–hole separation over the hybrid photocatalysts according to the qualitative analysis of charge carriers discussed previously (in situ irradiated XPS). In particular, the photocurrent density from the CdS/2NiOOH photoanode was drastically increased to 6.62 mA cm−2 at 1.23 V (vs. RHE), which was 22.6 times higher than that of the bare CdS electrode. Fig. 4b presents the photoresponse curves of the as-prepared photoanodes with the light on/off cycles under a potential of 1.23 V vs. RHE. All the photoanodes showed quick photoresponses through the fast change between photocurrent and dark current. Interestingly, the photocurrent density of the CdS/2NiOOH decreased gradually and was outperformed by that of CdS/4NiOOH after 25 min, suggesting a co-existence of high-activity and instability when compared with the results for the other two hybrid electrodes. Notably, a photocurrent spike at the initial time of irradiation was found in the CdS/2NiOOH curve, demonstrating that the recombination process occurred,53 whereas a negligible photocurrent spike was recorded for the CdS/4NiOOH sample, indicating more efficient electron–hole pair separation between CdS and NiOOH. The photocurrent density of the CdS/4NiOOH photoanode, from the photocurrent stability tests (Fig. S7, ESI) showed only ∼5% decay after continuous illumination over 12 h, which was comparable to or higher than those figures reported in recent work without scavengers,52,54 however, quite large degradation could be observed over a pure CdS photoanode in 4 h, again revealing the good photostability of CdS/4NiOOH. The incident-photon-to-current conversion efficiency (IPCE) spectra were measured on CdS and CdS/4NiOOH to further evaluate the PEC performance for water splitting under the same light source irradiation (Fig. S8, ESI). As expected, the IPCE value of CdS/4NiOOH electrode was recorded as 38.2% at 400 nm, which was almost eight times that of the bare CdS electrode (4.8%) at the same wavelength. These enormous differences in IPCE prove the vital role that NiOOH plays, which could build an efficient charge transfer channel between CdS and the electrolyte.

image file: d0ta06177h-f4.tif
Fig. 4 Linear sweep voltammogram curves of CdS, CdS/2NiOOH, CdS/4NiOOH and CdS/16NiOOH (a), and photoresponse measurements (b) for various samples at an applied voltage of 1.23 V vs. RHE. The amount of hydrogen evolution of the obtained samples in pure water without any sacrificial agents (c), and monochromatic light quantum efficiency (QE) of CdS/4NiOOH (d). The room temperature PL spectra (e), and EPR (f) of the four samples. All the PEC measurements were performed in 0.5 M Na2SO4 electrolyte under AM 1.5 G sunlight illumination (100 mW cm−2).

The photocatalytic activity of the four as-synthesized samples (the samples were scraped from the FTO glass) was determined using photocatalytic testing for the hydrogen evolution reaction under visible light irradiation (λ ≥ 420 nm) in deionized water without any hole scavengers. From Fig. 4c, the bare CdS nanorods generate a negligible amount of hydrogen after a continuous 5 h experiment (8.6 μmol) without sacrificial agents. It was surprising that the CdS/4NiOOH sample not only showed the highest hydrogen evolution rate of 115.6 μmol h−1 (over 67 times higher than that of pure CdS), but also displayed a stable photocatalytic activity compared with CdS/2NiOOH. As a comparison, the stability of CdS (Fig. S9, ESI) and CdS/4NiOOH (Fig. S10, ESI) were then evaluated using long-term hydrogen evolution measurements over 25 h. The hydrogen generation rates over CdS/4NiOOH remained good after five cycles, demonstrating the high photostability of the CdS/4NiOOH nanorods. In addition, the XRD (Fig. S11, ESI), TEM (Fig. S12, ESI), XPS (Fig. S13, ESI) and ICP-AES (Fig. S14, ESI) results of the CdS and CdS/4NiOOH catalysts after the photostability tests (25 h) also verified the high structural stability of the CdS/4NiOOH catalyst. For the oxidation half-reaction, the concentration of H2O2 gradually increased over 20 h (Fig. S15, ESI), which indicated that H2O could be oxidized during the water splitting process. It is worth noting that such a performance obtained from the CdS/4NiOOH electrode also outperformed most of the documented CdS-based hybrid photocatalysts,55–57 as indicated in Table S1 (ESI).

The apparent QE of the hydrogen generation from the CdS/4NiOOH hybrids achieved the maximum value of 3.67% at 420 nm, which decreased gradually in the direction of longer wavelengths: 2.98%, 1.73%, 0.33%, and 0.04% at 450, 500, 550, and 625 nm, respectively, as shown in Fig. 4d. The photoluminescence (PL) spectrum is an important strategy to describe the trapping, migration, and recombination of photoexcited charges in photocatalysis.58 As shown in Fig. 4e, under 405 nm excitation, steady-state PL quenching indicated the substantially prohibited recombination of photoexcited charges59 in CdS/2NiOOH and CdS/4NiOOH, which were beneficial for photocatalytic activity. The ALD was considered to be an accurate film production technology at the atomic level.60 However, the surface characterization of the original CdS could be affected after forming the new interface by precise merging of the NiOOH shell by the ALD process. From the results of the electron paramagnetic resonance (EPR) spectra (Fig. 4f), the clear resonance signals can be assigned to S defects (vacancies) with the g-factor value located at 2.003, and indeed, the distinct increase of S vacancies from CdS/2NiOOH and CdS/4NiOOH could improve the photocatalytic activity effectively, which was verified by the results of the water splitting experiments.61 Then, electrochemical impedance spectroscopy (EIS) was conducted to explore the charge transfer resistance between the photoanode and electrolyte over the four electrodes (Fig. S16, ESI). From Fig. S15 (ESI), it can be seen that the resistance of the CdS/2NiOOH electrode was smaller than that of the CdS, CdS/4NiOOH and CdS/16NiOOH, which could be ascribed to the accelerated electron–hole separation and faster collection of charge carriers.

Compared with the indirect or qualitative evidence discussed previously (steady-state PL, EPR, EIS and in situ irradiated XPS), pump–probe techniques in the various spectral regions were well established for revealing the photogenerated charge carrier, charge transfer dynamics in condensed phases, such as femtosecond time-resolved transient absorption (fs–TA) spectroscopy.62Fig. 5a presents the fs–TA spectral profile under different delay times and probe wavelengths over CdS, CdS/2NiOOH, CdS/4NiOOH and CdS/16NiOOH. For the bare CdS spectra, the dominant feature was two reversed transient bleach signals centered at about 500 nm and 470 nm, which were well aligned with the 1S exciton bleach band63 and the Stark-effect-induced absorption,64 respectively. Coated with different contents of NiOOH, the hybrid catalysts show similar spectral features as that of pure CdS (Fig. 5a). However, the hybrid system has more complex transient spectra with long tails extending to 600 nm.65 The charge carriers transfer behavior was distinctly changed in the presence of NiOOH according to the similar 1S exciton bleach under the longer delay times, indicating the enhancement of hole migration from CdS to NiOOH in the three hybrids.66 The corresponding kinetics were described subsequently at the transient absorption at 468 nm (∼2.65 eV) and the maximum bleach wavelengths at 500 nm (∼2.48 eV) in Fig. 5b. From Fig. 5b, it was seen that NiOOH shell with an appropriate thickness (CdS/2NiOOH) had an effect on the decay rate of hybrids in the first ∼50 ps, producing an even faster decay afterwards, which acted successfully as an efficient charge transfer channel. The exciton bleach recovery rates reflected the hole transfer rate of the four samples directly, which were recorded as CdS (1978 ± 415 ps) < CdS/16NiOOH (862 ± 109 ps) < CdS/2NiOOH (49.6 ± 9.2 ps) < CdS/4NiOOH (52.9 ± 9.5 ps). Overall, all the previous results verified that the well-constructed ultrathin NiOOH shell not only acted as a fast charge transfer pathway, but also served as a durable protective layer on CdS nanorods for high-efficiency photocatalytic water splitting without using any hole scavengers (Fig. 5c and S17, ESI).

image file: d0ta06177h-f5.tif
Fig. 5 Femtosecond time-resolved transient absorption spectra of CdS, CdS/2NiOOH, CdS/4NiOOH and CdS/16NiOOH following excitation at selected delay times following excitation by a 400 nm pump (a) and extracted normalized kinetics at ∼470 nm and ∼504 nm (b). Schematic illustration of the enhanced mechanism for photocatalytic performance over the as-prepared photocatalysts (c).

3. Conclusion

In summary, 1D CdS/NiOOH core/shell nanostructures with tunable shell thickness were fabricated, characterized, and applied as photocatalysts for visible light water splitting without any sacrificial agents. The optimal performance over the CdS/4NiOOH hybrid catalyst originates from the coating of an ultrathin NiOOH wrapper with the ultimate continuous thickness of ∼4 nm, which offers a medium for fast spatial charge separation and hole migration and additionally a protector to restrain photocorrosion by isolating CdS from the electrolyte. Moreover, for the first time, in situ irradiated XPS and femtosecond time-resolved transient absorption spectroscopy revealed the mechanism of the improved photocatalytic performance for the heterojunction samples. Photogenerated electron accumulation on the conduction band of CdS as well as almost two-fold enhancement of the hole transfer rate of CdS obtained from the previous operando spectroscopic techniques exhibited good charge separation through accelerated hole injection. This approach provides a valuable reference for employing a precisely constructed shell as an effective catalytic, protective interface to stabilize and improve the photocatalytic water splitting over the CdS photocatalyst.

Conflicts of interest

The authors declare no competing financial interests.


We acknowledge the National Natural Science Foundation of China (51602161), the China Postdoctoral Science Foundation (2019M650120), the National Synergetic Innovation Center for Advanced Materials (SICAM), the Zhejiang Provincial Natural Science Foundation (LQ19E020001) and the Key Research and Development Projects of Zhejiang Province (2020C04004).

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Electronic supplementary information (ESI) available. See DOI: 10.1039/d0ta06177h

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