Rational design of Z-scheme system based on 3D hierarchical CdS

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Rational design of Z-scheme system based on 3D hierarchical CdS supported 0D Co9S8 nanoparticles for superior photocatalytic H2 generation Pengfei Tan, Yi Liu, Anquan Zhu, Weixuan Zeng, Hao Cui, and Jun Pan ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/acssuschemeng.8b01751 • Publication Date (Web): 02 Jul 2018 Downloaded from http://pubs.acs.org on July 2, 2018

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ACS Sustainable Chemistry & Engineering

Rational design of Z-scheme system based on 3D hierarchical CdS supported 0D Co9S8 nanoparticles for superior photocatalytic H2 generation Pengfei Tan a, Yi Liu a, Anquan Zhu a, Weixuan Zeng a, Hao Cui b *, Jun Pan a * a

State Key Laboratory for Powder Metallurgy, Central South University, Lushan

South Road 932, Changsha 410083, P. R. China b

Sino-Platinum Metals Co. Ltd., Kunming Institute of Precious Metals, People West

Road 121, Kunming 650106, P. R. China * Corresponding authors: *Jun Pan, E-mail: [email protected]; * Hao Cui, E-mail: [email protected].

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ABSTRACT Developing durable and efficient photocatalyst for H2 evolution is highly desirable to expedite current research on solar-chemical energy conversion. In this work, we rationally designed and synthesized a direct Z-scheme system based on three dimensional hierarchical CdS decorated with Co9S8 nanoparticles towards photocatalytic H2 evolution. The composition, microstructure and optical properties of the hybrids were thoroughly investigated. Photocatalytic performances revealed that the optimized CdS/Co9S8-15 composite exhibited the highest H2-evolution rate of 5.15 mmol h-1 g-1, which is approximately 6.8 and 257.5 times than that of CdS and Co9S8, respectively. In addition, this novel composite catalyst also displayed a long-term stability without apparent debasement in photocatalytic activity. Based on the analysis of UV–vis diffuse reflectance spectroscopy, photocurrent response, electrochemical impedance spectra and photoluminescence, the reinforced H2 evolution performance of the CdS/Co9S8 samples was attributed to a synergistic effect including boosted light absorption capacity, increased separation and transfer efficiency of photo-generated electron/hole pairs as well as much stronger reducibility of electrons in the conduction band of Co9S8. Finally, the photocatalytic mechanism for this composite was proposed and discussed in detail. KEYWORDS: Z-scheme, CdS/Co9S8, hierarchical, hydrogen evolution,


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photocatalytic INTRODUCTION Utilization of solar energy and semiconductor for clean H2 production through photocatalytic water splitting, which realizes the conversion of solar energy to chemical energy, has a huge potential to address the current energy and environmental crises.1-7 Unfortunately, most of developed single-component photocatalysts suffer from poor activity of H2 evolution because they can’t simultaneously fulfill the requirements: a wide light-absorption range and a high charge-separation efficiency as well as a strong redox ability, which are of vital importance to the performance of H2 production.8-12 In recent years, tremendous efforts have proved that constructing heterojunction or composite materials with favorable type-II or Z-scheme energy band structure and electron transfer route is an effective solution to these problems.13-20 Compared with traditional type-II mode, the direct Z-scheme system has attracted more and more extensive attention because it not only enormously extends the visible-light absorption region and accelerates the spatial separation efficiency of photo-generated charge carriers, but also effectively utilizes the higher reduction and oxidation potentials in the composite photocatalyst.15, 21-23 As a typical semiconductor material, CdS has been widely investigated to produce H2 because of its visible-light response and appropriate



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conduction band potential to reduce H+ to H2.24-25 Meanwhile, considerable research efforts have been undertaken to design and construct CdS-based Z-scheme photocatalytic systems to solve its innate defects such as narrow light-absorption range and the insufficient photo charge-carrier separation, thus improves its performance in hydrogen generation. For instance, the Z-scheme systems of CdS/BiVO4,26 CdS/CeO2,27 CdS/WO3,28 CdS/TiO2,29 CdS/Co9S8,30 and Co-C@Co9S831 have revealed highly efficient photocatalytic H2 production. However, in those systems, the structure of CdS is either zero dimensional (0D) nanoparticles or two dimensional (2D) nanosheet. Up to now, a direct Z-scheme photocatalytic system based on three dimensional (3D) hierarchical CdS structure for H2 evolution from water splitting has rarely been studied. As far as we know, the morphology of catalyst is crucial to the

photocatalytic

performance.32-34

Especially,

the

unique

3D

hierarchical structure has been demonstrated to possess more outstanding photocatalytic activity due to higher specific surface area, more active sites

and

boosted

light-absorption.35-39

For

example,

the

visible-light-driven H2 evolution rate of hierarchically porous CdS nanosheet is higher than that of pure CdS nanoparticles, due to the more valid

electron-hole

separation

and

enhanced

light

harvesting.40

Considering the feature of direct Z-scheme system and the peculiar properties of unique 3D hierarchical structure, it is therefore highly


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desirable to rationally design and synthesize novel and efficient direct Z-scheme system based on 3D hierarchical CdS so as to effectively improve the efficiency of water splitting. Recently, the noble-metal-free cobalt sulfide (CoxSy) has shown great promise in hydrogen evolution owing to its narrow band gap and a high flat-band potential as well as good stability.30, 41-44 Besides, we also note that 0D nanoparticle has the merits of short charge-migration distance and large surface area, while 3D structure can serve as a support to supply more

contact

areas

and

suppress

the

agglomeration

of

0D

nanoparticles.45-47. Inspired by these reports, we herein present a direct Z-scheme system of 0D Co9S8 nanoparticles anchored on 3D hierarchical CdS via facile two-step solvothermal process towards photocatalytic H2 generation. Our results demonstrated that the nanometer Co9S8 particles were successfully anchored on the surface of CdS through the induction of amino groups. The performance of the catalysts was evaluated by photocatalytic H2 evolution by using Na2S/Na2SO3 aqueous solution as sacrifice agent. The synthetized CdS/Co9S8 composites showed enhanced efficiency for H2 evolution in comparison to the individual component under visible light irradiation. Thereafter, the reason for the remarkable activity was investigated by conducting further experiments. Finally, on the basis of the experimental results, the photocatalytic mechanism for this composite was proposed and discussed in detail. This report creates



new opportunities to the rational design of direct hierarchical Z-scheme catalysts with special geometric structure for water splitting.

EXPERIMENTAL SECTION Preparation of Photocatalysts. The 3D hierarchical CdS was synthesized according to our previous works with some modifications.48 Typically, 2 mmol of cadmium acetate dihydrate (Cd(CH3COO)2·2H2O) and 10 mmol of thiourea (NH2CSNH2) were dissolved into the mixing solution containing 15 mL diethylenetriamine (DETA) and 45 mL ethyl alcohol (EtOH), then the solution was continuously stirred for about 30 min to form a transparent solution. Afterwards, the solution was transferred into a 100 mL Teflon-lined autoclave and heated at 120 °C for 20 h in an oven. After cooling to the room temperature naturally, the precipitate was collected by centrifugation, washing several times with ethanol and deionized water, and dried at 60 °C for 12 h. Co9S8 nanoparticles decorated 3D hierarchical CdS were prepared according to the following process: as illustrated in Scheme 1. Firstly, freshly made CdS nanoflower (100 mg) was added into 30 mL of water. The mixture was then sonicated for 10 min. Different amounts of Co(OAc)2 were then added at room temperature. After stirring for 2 h, the Na2S was added slowly to the above solution and makes up a Co/S mol ratio as 1:5 under vigorous stirring. Then the solution was continuously stirred for about 30 min, and subsequently was transferred into a 50 mL


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Teflon-lined autoclave and heated at 120 °C for 20 h in an oven. Finally, the obtained CdS/Co9S8 composites were collected and washed two times with water and absolute alcohol, then dried in vacuum at 40 °C for a whole night. According to this procedure, the theoretical weight ratios of Co9S8 in different CdS/Co9S8 composites were controlled to be 5%, 10%, 15% and 20%. And the corresponding samples were signed as CdS/Co9S8-5,

CdS/Co9S8-10,

CdS/Co9S8-15,

and

CdS/Co9S8-20,

respectively. (the actual weight ratios in the composites were listed in Table S1). The pure Co9S8 was also prepared for comparison following the same method without the addition of CdS.

Scheme 1. Scheme of the fabrication process of CdS/Co9S8 composites.

Characterization of the catalysts. The phase structures were analyzed by X-ray diffraction (D/max 2550, Rigaku Corporation). X-ray photoelectron spectroscopy (XPS) was performed on a Thermo ESCALAB250 X-ray photoelectron spectrometer. The inductively



coupled plasma (ICP) data was acquired on a Shimadzu ICPS-7500 equipment. The morphology of the catalysts was characterized using scanning electron microscope (Nova Nano SEM 230, FEI Electron Optics B.V) and field emission transmission electron microscopy (JEM-2100F, Japanese electronics Co., Ltd.). Elemental analyses were tested by using an elemental analyzer (VARIO EL111) attached to TEM. The Fourier transform infrared (FT-IR) spectra were recorded on a BRUKER TENSOR 27 spectrometer. The surface area was determined by the Brunauer-Emmett-Teller (BET) method on a BK132F surface area analyzer. UV-vis diffuse reflection spectra were carried out with a UV-vis spectrophotometer using BaSO4 as the standard reference (Evolution 220, Thermo Fisher Scientific). The photoluminescence (PL) was detected with 334 nm emission wavelength by FLS980 series of fluorescence spectrometers. Photocatalytic Activity. The H2 evolution experiments were carried out in a Pyrex glass cell which has a flat, round upside-window for external light incidence and is connected to a gas-closed gas circulation system. A commercial 300 W xenon lamp coupled with a UV cutoff filter (λ ≥ 420 nm) was used to simulate the visible-light source. In a typical reaction, 20 mg of photocatalyst was suspended in 100 mL of aqueous solution containing 0.35M Na2S and 0.25M Na2SO3 as sacrificial reagents, and then the suspension was stirred for 30 min. Before irradiation, the


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reaction system was pumped to vacuum. The temperature of the reactant solution was maintained at 6 °C by flowing cooling water during the whole reaction process. The gas chromatograph was employed once per hour to analyze the concentration of H2. The carrier gas was N2 (99.9%). Photoelectrochemical Measurements. The photoelectrochemical properties were investigated on a CHI 660E electrochemical workstation. In this typical three-electrode system, a platinum plate and a standard Ag/AgCl electrode were selected as counter electrode and reference electrode, respectively, and Na2SO4 aqueous solution was used as the electrolyte. The photocurrent responses were measured at 0.0 V during on-off cycling irradiated by using a xenon lamp with 420 nm cutoff filters. Electrochemical impedance spectroscopy (EIS) was conducted in the frequency range of 0.1 Hz and 100 KHz with an AC voltage magnitude of 5 mV. Moreover, Mott-Schottky plots with a frequency of 1000 Hz and amplitude of 10 mV were recorded to evaluate the flat band potential of the semiconductors.

RESULTS AND DISCUSSION Phase Structure. X-ray diffraction (XRD) was performed for analyzing the crystal phase of as-synthesized samples.49-50 Fig. 1 and Fig. S1 show the XRD patterns of pure Co9S8, CdS and CdS/Co9S8 composites. The peaks at 29.8°, 47.6° and 52.1° could be assigned to the (311), (511) and (440) of Co9S8 (JCPDS: 65-1765). For the specimen of



CdS, all the appeared diffraction peaks could be indexed to the hexagonal wurtzite CdS (JCPDS: 77-2306), and no impurity peaks were observed, confirming the high purity of the sample. As for CdS/Co9S8 composites, the characteristic peaks belonging to CdS could be obviously distinguished in all the composites. Besides, it should be note that the diffraction peaks corresponding to Co9S8 become manifest gradually with the increase of Co9S8 content. Especially for CdS/Co9S8-15 and CdS/Co9S8-20, it is apparent to find that the characteristic peaks in the XRD patterns are consistent with Co9S8 and CdS standard patterns, indicating that the catalysts are made up of CdS and Co9S8. The following results of XPS and TEM could also further assert this conclusion.

Fig. 1. XRD patterns of as-prepared (a) Co9S8, (b) CdS, and (c) CdS/Co9S8-15 samples.

Composition of the Catalyst. XPS was in-depth characterized to determine elemental composition and surface chemical states of the CdS/Co9S8-15 specimen. As shown in Fig. 2a, the XPS survey spectrum reveals the presence of Cd, S and Co elements in the hybrid. Besides, Fig.


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2b, 2c and 2d display the high-resolution XPS spectra of Cd 3d, S 2p and Co 2p for the CdS/Co9S8-15 sample, respectively. In the high-resolution XPS spectrum of Cd 3d (Fig. 2b), two peaks at binding energies of 405.2 and 411.9 eV, are corresponding to Cd 3d5/2 and Cd 3d3/2 species, respectively, indicating that the chemical states of Cd in the nanocomposite are +2.51 Moreover, The S 2p spectra core levels (Figure 2c) can be deconvoluted into two peaks located at 160.7 eV and 161.9 eV, which could be ascribed to S 2p3/2 and S 2p1/2.52-53 The binding energies of Cd and S are in good agreement with the reported values, thereby confirming the presence of CdS. The Co 2p XPS spectrum in Fig. 2d consists of two spinorbit doublets. The first doublet (with peaks at 778.5 eV and 780.8 eV) and the second doublet (with peaks at 796.1 eV) are assigned to the Co 2p3/2 and Co 2p1/2, respectively, indicating the coexistence of Co2+ and Co3+. The result is in good agreement with previous work.54-56 The remaining two peaks located at 784.8 eV and 800.3 eV are satellite peaks.57 The XPS analysis about the high-resolution spectra of Co 2p and S 2p together with the XRD results demonstrate the existence of Co9S8. In a word, the XPS results provide strong evidence that the synthesized nanocomposites comprise CdS and Co9S8 without any other impurities.



Fig. 2. XPS of CdS/Co9S8-15: survey scan (a) and high-resolution XPS spectra of (b) Cd 3d, (c) S 2p and (d) Co 2p.

Morphological Structure. Furthermore, the morphological structures of prepared catalysts were observed by scanning electron microscopy (SEM), transmission electron microscopy (TEM), and energy dispersive X-ray spectroscopy (EDS). It can be clearly observed that the pure CdS exhibited a 3D hierarchical flower-like superstructure with a diameter of ~500 nm (see Fig. S2). Fig. 3a shows the TEM image of pure CdS, from which, we can be aware of that the 3D hierarchical CdS was constructed by numerous thin and smooth interlaced nanosheets. Compared with that, TEM image of the as-prepared hybrid (Fig. 3b) evidently unfolds the nano-sized Co9S8 nanoparticles are tightly anchored on the surface of ACS Paragon Plus Environment

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CdS without affecting the hierarchical structure. By magnification of the sample, the HRTEM image (Fig. 3c) shows that the size of Co9S8 particle is around 5 nm, and the lattice fringes of 0.336 nm and 0.176 nm could be assigned to the (002) and (440) crystal planes of hexagonal CdS and Co9S8, respectively. This confirms that an intimately contact occurred between 3D hierarchical CdS and Co9S8 nanoparticles in the CdS/Co9S8-15 nanocomposite and the close contact is favorable for the transfer and separation of photo-generated charges between these two components, which would greatly improve the activity of the samples. Moreover, it should be noted that the Co9S8 nanoparticles on the surface of CdS are much smaller and less aggregated than pure Co9S8 (~150 nm, Fig. S3). The desirable morphology can be ascribed to evenly distributed amino groups on the surfaces of CdS, which provides copious nucleation sites to couple Co precursors and induce corresponding dispersion and size control of Co9S8 nanoparticles on the surface of CdS under solvothermal conditions. The Fourier transform infrared (FT-IR) results confirm this speculation. As displayed in Fig. S4, similar with pure DETA, the characteristic vibration bands of -NH2 and -NH could also be observed, indicating that the amino groups are maintained during the formation of CdS.58 Nevertheless, the corresponding DETA vibration bands of the CdS/Co9S8-15 nanocomposite vanished, which should be the result of both Co9S8 nanoparticle coverage and the high temperature and



pressure reaction. The EDS spectrum (Fig. 3d) demonstrates the existence of Cd, S and Co elements (Cu is from the copper grid substrate), indicating the purity of the sample, which is consistent with the XRD results. Furthermore, STEM-EDS elemental mapping further supports the uniform distribution of Cd, S and Co elements in the CdS/Co9S8-15 nanocomposite (Fig. 3e).

Fig. 3. TEM (a) of pure CdS and TEM (b), HRTEM (c), EDX spectrum (d) as well as STEM-EDX elemental mapping (e) images of CdS/Co9S8-15 composite.

Nitrogen Adsorption−Desorption Analysis. Nitrogen adsorption– desorption isotherm measurements were performed to investigate porous ACS Paragon Plus Environment

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features of the as-prepared samples.22, 59-60 As shown in Fig. 4, all samples possess a type-IV isotherm, suggesting the existence of mesopores within the materials. Using these plots, the BET surface areas of Co9S8, CdS and CdS/Co9S8-15 are calculated to be ca. 49.967 m2/g, 98.871 m2/g and 108.085 m2/g, respectively (Table S2). Compared with pure Co9S8 and CdS, CdS/Co9S8-15 even shows a larger specific surface area, which could be ascribed to the fact that the relatively better dispersion of Co9S8 nanoparticles and the addition of 3D hierarchical CdS structures, which can be evidenced by SEM and TEM. Furthermore, the pore-size distributions of as-prepared samples were estimated using the BJH (Barrett-Joyner-Halenda) method. And the results are listed in the inset of Fig. 4 and Table S2. All the samples exhibit micropores and mesopores from 1 to 35 nm. As far as we know, the larger surface area is favorable to photocatalytic activity, which is mainly attributed to provide more active sites and efficient transport paths for reactants and products in photocatalytic reactions.61-62

Fig. 4. N2 adsorption-desorption isotherms and the corresponding pore size



distribution (inset) of the as-prepared samples.

Optical Properties and Energy Bands Alignments. The optical absorption properties of the pure CdS, Co9S8 and CdS/Co9S8 composites were analyzed through UV-Vis diffuse reflectance spectra (Fig. 5a and Fig. S5). As displayed in Fig. 5a, the absorption edge of pure CdS locates at about 520 nm, corresponding to the previous reports.48 As for pure Co9S8, it exhibits a very broad absorption in the range of 300-800 nm, that's to say it possesses a very strong light harvesting capacity from ultraviolet region to visible region. It is exciting to notice that the CdS/Co9S8-15 sample shows a stronger light absorption and extended visible light response in comparison with CdS. Simultaneously, the color of the samples changes (insets in Fig. 5a). These results indicate that the loading of Co9S8 could effectively facilitate the visible-light absorption of CdS, which means more photo-generated electrons participating in the H2 evolution reaction. In addition, the band gap energy of as-prepared samples are calculated by the equation αhv = A (hv-Eg)n/2, where α is the absorption coefficient, h is Planck constant, v is light frequency, A is a constant, Eg is the energy band gap, n is decided by the transition type of the semiconductor.63 The calculated Eg values of the pure CdS and Co9S8 are 2.47 and 1.25 eV, respectively (Fig. 5b). The Mott-Schottky (M-S) plots were conducted to determine the types of conductivity as well as the flat band potential for CdS and Co9S8. Seen from Fig. 5c, the positive slope of the M-S plots convincingly demonstrates that both CdS and ACS Paragon Plus Environment

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Co9S8 are the typical n-type characteristic.64 Based on the M-S equation, the flat band potentials (Ufb) for CdS and Co9S8 are -0.58 and -1.07 V vs. NHE, respectively. The results are in good agreement with previous reports.30-31, 64 Generally, the flat band potential is approximately equal to the conduction band potential (ECB) for n-type semiconductor.65 Thus the CB of CdS and Co9S8 are roughly reckon up to be -0.58 and -1.07 V. Meanwhile, the values of valance band potential (EVB) are 1.89 and 0.18 V vs. NHE calculated by equation of EVB = ECB + Eg. Detailed calculation results and the band structure of the composite are plotted in Fig. 5d.

Fig. 5. (a) UV–Vis DRS of CdS, Co9S8, and CdS/Co9S8-15 samples, inset is the digital



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photographs of the samples (A: CdS, B: Co9S8, C: CdS/Co9S8-15). (b) Plots of (αhν)2 versus hν for CdS and Co9S8 samples. (c) Mott–Schottky plots of CdS and Co9S8 (inset). (d) Schematic potential energy diagram of CdS and Co9S8.

Photocatalytic Properties. Photocatalytic H2 generation activities of the as-synthesized samples were investigated under visible light irradiation by virtue of Na2S/Na2SO3 as sacrificial reagents. Fig. 6a summarizes the H2 production amount over the first four hours of pure CdS, Co9S8 and different ratios of CdS/Co9S8 composites. Obviously, the amount of hydrogen steadily increases with time. Meanwhile, the corresponding H2 evolution rates of the samples were also displayed in Fig. 6b. It could be clearly seen that Co9S8 nanoparticles exhibits negligible

photocatalytic

activity

for

H2

production

with

the

H2-production rate of 0.02 mmol h-1 g-1 due to nanoparticles aggregation and easy recombination of photo-induced charges. Bare CdS also displays a rather low H2 evolution efficiency (0.76 mmol h-1 g-1). However, excitingly, all the composites show remarkably enhanced rates of H2 evolution after loading Co9S8 nanoparticles onto the surface of hierarchical CdS. Especially, when the content of Co9S8 is 15 wt%, the CdS/Co9S8-15 demonstrates the optimal rate of up to 5.15 mmol h-1 g-1, approximately 6.8 and 257.5 times than that of pure CdS and Co9S8. Moreover, it also should be noted that an appropriate amount of Co9S8 is critical to the enhancement of activity. For instance, a reduced hydrogen evolution activity is observed when further increasing the Co9S8 content to 20 wt% in the composite. This phenomenon arose from the reason that ACS Paragon Plus Environment

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the amount affects its dispersion, contact with sacrificial agent/water molecules and even the light absorption. In addition, the photo-stability of the optimal sample was also evaluated by cycling hydrogen evolution experiments in the presence of a prolonged visible light irradiation of 16 h. Fig. 7 displays the data of cycling experiments of H2 production over CdS/Co9S8-15 catalyst under irradiation of visible light. Clearly, no noticeable decrease of activity in hydrogen production during the four successive recycles is observed, indicating that the as-prepared composite possesses good stability for hydrogen evolution.

Fig. 6 (a) The amounts of hydrogen generated of the samples in four hours under visible light irradiation, (b) the apparent hydrogen generation rate over the corresponding samples.



Fig. 7. The recycling test of photocatalytic H2 evolution for CdS/Co9S8-15 under visible light irradiation

The Charge Separation and Transfer Performances. To verify the essential factors for the superior activity of such CdS/Co9S8 species, the photocurrent responses together with the electrochemical impedance spectroscopy (EIS) were used to probe the charge separation and migration efficiency of as-prepared samples.14, 66 As shown in Fig. 8a, when CdS, Co9S8, and CdS/Co9S8-15 electrodes were used as working electrodes, periodic on/off photocurrent responses are observed upon visible light irradiation. The CdS/Co9S8-15 sample exhibits highest photocurrent density by nearly 2.5 and 7.5 times than pure CdS and Co9S8, indicating uppermost separation efficiency of the photo-excited charge carriers. Notably, no noticeable decrease in current density is observed during several successive recycles, which is in consistent with the results of the cycling hydrogen generation experiments. In addition, to evaluate the excellent charge mobility of CdS/Co9S8-15 under visible light, EIS spectra of the above electrodes were also tested, and the corresponding results are displayed in Fig. 8b. Generally, in the EIS Nyquist plot, the ACS Paragon Plus Environment

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smaller semicircle arc indicated faster interfacial charge transport.67 It is easy to find that the arc radius of the CdS/Co9S8-15 composite was the smallest, indicating the hybrid possesses the fastest interfacial electron transfer compared to that of the CdS and pure Co9S8. The results of photocurrent measurement and EIS data definitely demonstrate that the introduction of Co9S8 could accelerate the separation and interfacial transport efficiency of the photo-generated charge carriers, thus enhanced the photocatalytic performance. It is well acknowledged that the lower PL intensity indicates the faster separation of the charge carriers, resulting in higher photocatalytic activity.68 The separation ability of photo-induced charge carriers over the bare CdS and optimized CdS/Co9S8-15 nanocomposites were further understood by photoluminescence (PL) spectra. As presented in Fig. 9, the PL intensity of CdS/Co9S8-15 nanocomposite is remarkably lower than CdS, revealing that the recombination rate of photo-induced electron-hole pairs was suppressed by loading Co9S8, which is crucial to boost the H2 generation activity of photocatalyst. The PL result is consistent with the former performance for H2 evolution and our above-mentioned discussion about photocurrent and EIS data.



Fig. 8. (a) Transient photocurrent responses and (b) EIS Nyquist plots for Co9S8, CdS and CdS/Co9S8 composites.

Fig. 9. PL spectra of CdS and CdS/Co9S8-15 under an excitation wavelength of 334 nm.

Discussion of Mechanism for Enhanced Photocatalytic Activities. Based on the present staggered energy band arrangement between CdS and Co9S8 (Fig. 5d), two different electron transfer modes, referred as conventional type-II (Fig. 10a) or direct Z-scheme (Fig. 10b), may happen in the as-synthesized hybrids for photocatalytic water splitting. In the traditional type-II transfer mechanism, the photo-generated electrons in the CB of Co9S8 would migrate to the CB of CdS with the reason that


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the CB potential of Co9S8 was more negative than that of CdS, which inevitably leads to the drop in electron reduction potential.13, 65 In this case, the electrons accumulated on the CB of CdS will experience difficulty in reducing H+ into H2, especially in the absence of Pt as a co-catalyst.69 These may not well interpret the dramatically enhancement of photocatalytic H2 production (Fig. 6). According to recent reports.30-31 the enhanced photocatalytic H2-evolution activity could be properly explained following the direct Z-scheme charge transfer mechanism for the CdS/Co9S8 system. When both CdS and Co9S8 were excited by visible light irradiation, owing to the short electron-migration distance and their close band positions, the electrons in the CB of CdS could quickly transfer to the VB of Co9S8 and combine rapidly with holes there at the formed ohmic contact in the solid-solid interface.8, 65 The electrons in the CB of Co9S8 could reduce H+ into H2, while the holes in the VB of CdS could be consumed by the sacrificial agent (Na2S/Na2SO3 aqueous solution) under visible light irradiation. This Z-scheme mechanism not only effectively accelerated the separation and extended the lifetime of photo-generated charge carriers but also improved the redox ability of the photocatalyst because of the enhanced redox potential, thus promoting the H2 evolution ability of the system. To validate the aforementioned Z-scheme mechanism, we performed the photo-deposition of Pt nanoparticles to confirm the Z-scheme charge transfer pathway. Generally,



Pt4+ in H2PtCl6 can be reduced to Pt0 in the form of Pt nanoparticles by the photo-induced electrons. Therefore, as-obtained Pt nanoparticles tend to accumulate around electron-rich sites.14, 22, 37 As shown in the HRTEM image in Fig. S6, the Pt nanoparticles gathered on/around Co9S8 nanoparticles rather than randomly dispersed on the CdS substrate, implying that photo-induced electrons in the CB of Co9S8 remain to reside on the original site as predicted by Z-scheme route instead of transferring to the CB of CdS through the conventional type-II mechanism. The results undoubtedly demonstrated that the construction of Z-scheme system, rather than the type-II system, is the reason for a superior charge separation efficiency in the composite, thus significantly improves H2 generation performance.

Fig. 10. Schematic diagram of mechanism for the photocatalytic H2 evolution under visible-light irradiation over CdS/Co9S8 composites: (a) Traditional type-II mode and (b) Z-scheme mode.


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On the basis of the aforementioned results and discussions, it is concluded that the improved photocatalytic H2-evolution activity of the CdS/Co9S8 composites could be ascribed to a synergistic effect including enhanced light absorption capacity, more efficient separation of photo-generated electron/hole pairs as well as much stronger reducibility of electrons caused by Z-scheme system. CONCLUSION In summary, we rationally designed and synthesized a direct Z-scheme mode photocatalyst via facile solvothermal method, among which 3D hierarchical CdS was used to support the 0D Co9S8 nanoparticles for the first time. The coupling of hierarchical CdS with Co9S8 can trigger significantly improved photocatalytic H2 evolution. The 3D hierarchical CdS acting as supporting material not only can take advantage of its own distinguished properties such as high specific surface area and strong optical absorption, but also induce the dispersion and size control of Co9S8 nanoparticles thus to inhibit particle agglomeration. The introduction of Co9S8 can increase the light absorption (narrow bandgap), making more optical energy to participate in H2O reduction. Moreover, the incorporation with special 3D hierarchical CdS superstructure leads to the formation of direct Z-scheme structure, which does not only facilitate the electron–hole separation and transfer but also improve the reduction ability of the hybrid system. The improved light harvesting ability,



increased separation and transfer of charge carriers and stronger reducibility of electrons could be responsible for the improved photocatalytic H2 evolution. This work can provide new insight to the design and development of Z-scheme photocatalysts for water splitting.

ASSOCIATED CONTENT Supporting Information XRD, SEM, TEM, ICP values, surface area, pore size, pore volume parameters, DRS and FT-IR spectra of the prepared samples, including 6 figures and 2 tables.

AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] *E-mail: [email protected]. ORCID Jun Pan: 0000-0002-5454-5554 Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS This work is financial supported by the National Science Foundation of China (11674398), the Science Fund for Distinguished Young Scholars of Hunan Province (2015JJ1016), the Fundamental Research Funds for the Central Universities of Central South University (2017zzts099).


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Synopsis. A photocatalytic Z-scheme system based on 3D hierarchical CdS decorated with Co9S8 nanoparticles was rationally designed to generate sustainable hydrogen.


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Rational design of Z-scheme system based on 3D hierarchical CdS

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