Mn0.5Cd0.5S composites for enhanced photocatalytic hydrogen production under visible light

Journal Pre-proof In situ construction of NiSe/Mn0.5 Cd0.5 S composites for enhanced photocatalytic hydrogen production under visible light Xinwei Jiang, Haisheng Gong, Qiuwen Liu, Mingxia Song, Caijin Huang

PII:

S0926-3373(19)31185-3

DOI:

https://doi.org/10.1016/j.apcatb.2019.118439

Reference:

APCATB 118439

To appear in:

Applied Catalysis B: Environmental

Received Date:

14 May 2019

Revised Date:

20 September 2019

Accepted Date:

17 November 2019

Please cite this article as: Jiang X, Gong H, Liu Q, Song M, Huang C, In situ construction of NiSe/Mn0.5 Cd0.5 S composites for enhanced photocatalytic hydrogen production under visible light, Applied Catalysis B: Environmental (2019), doi: https://doi.org/10.1016/j.apcatb.2019.118439

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In situ construction of NiSe/Mn0.5Cd0.5S composites for enhanced photocatalytic hydrogen production under visible light Xinwei Jiang, a Haisheng Gong, a Qiuwen Liu, a Mingxia Songb,* [email protected] and Caijin Huanga,* [email protected]

State KeyLaboratory of Photocatalysis on Energy and Environment, College of Chemistry, Fuzhou

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a.

University, Fuzhou 350116, PR China. b.

Collaborative Innovation Center of Atmospheric Environment and Equipment Technology (CICAEET),

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School of Chemistry and Materials Science, Nanjing University of Information Science and Technology, Nanjing 210044, China. whom correspondence should be addressed

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*To

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Graphical abstract

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The well-matched structure between NiSe and Mn0.5Cd0.5S facilitates the charge transfer and prolongs the lifetime of photo-induced electrons, and thus improves photocatalytic activity.

Highlights Noble metal-free NiSe/Mn0.5Cd0.5S nanocomposites were prepared via an

in-situ hydrothermal synthesis. NiSe modified Mn0.5Cd0.5S exhibits enhanced photocatalytic activity (28.08 mmol/h/g) under visible light. The enhanced H2-evolution activity is due to the effective separation and

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prolonged lifetime of photo-induced carriers in the coupled heterojunction.

Abstract

MnxCd1-xS solid solution is an emerging semiconductor for photocatalytic water

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splitting with a tunable bandgap. However, it still leaves much room to improve the

photocatalytic activity. Herein, we modified Mn0.5Cd0.5S with noble metal-free NiSe

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by an in-situ hydrothermal synthesis. The as-obtained NiSe/Mn0.5Cd0.5S nanocomposites exhibited highly efficient photocatalytic H2 production under visible

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light and the highest hydrogen generation rate reaches 28.08 mmol/h/g, which is higher than that of Mn0.5Cd0.5S (13.71 mmol/h/g) and 1 wt% Pt/Mn0.5Cd0.5S (24.22 mmol/h/g). Through the detailed analyses of UV-vis DRS, photoluminescence (PL)

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spectra, time-resolved photoluminescence (TRPL) and photoelectrochemical tests, we found the well-matched structure between NiSe and Mn0.5Cd0.5S, which facilitates the

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charge transfer and prolongs the lifetime of photo-induced electrons, and thus improves photocatalytic activity.

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Keywords: Noble metal-free; NiSe/Mn0.5Cd0.5S; Photocatalytic hydrogen production.

1. Introduction Fossil fuels bring terrible environmental problems along with their combustion

and it is challengeable to solve the shortage of growing global energy by sustainable energy solutions in the 21st century [1,2]. Hydrogen is an ideal energy carrier and its large-scale renewable production is mainly from fossil fuels or by the high-energy consumption method [3]. Photocatalytic hydrogen production is an environment-friendly and cost-effective way to convert solar energy to chemical hydrogen energy [4-6]. Fujishima A and Honda K first reported TiO2 as a semiconductor electrode for photocatalytic hydrogen production since 1972 [7]. From then on, TiO2 becomes the most widely studied photocatalyst, but it has rather low

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photocatalytic activity due to its wide band gap (3.2 eV), which limits the absorption of solar energy [8,9]. To maximize the utilization of solar energy, visible-light-driven photocatalysts are the important research trend in photocatalysis. Recently, Many

semiconductors have been developed as photocatalysts, including metal oxides [10],

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metal sulfides [11], metal oxynitrides [12] and so on. Among them, CdS arouses great concern in recent years due to its suitable bandgap (2.4 eV) for visible light

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absorption and its conduction band position for the photocatalytic water splitting [13-15].

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However, low activity and photocorrosion limits the application of CdS in photocatalytic reaction [5,16]. One of the effective ways to overcome the obstacle is

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to integrate CdS with other metal sulfides to form solid solutions such as Zn1-xCdxS [17], (Zn0.95Cu0.05)1-xCdxS [18], (AgIn)xCd2(1-x)S2 [19]. For example, Li et al. [20] found Zn1-xCdxS solid solution has higher H2-production activity than CdS alone.

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Especially, MnxCd1-xS solid solution composed of CdS and MnS exhibits better photocatalytic activity and stability than pure CdS due to its bandgap tuned by

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adjusting x value in MnxCd1-xS and better tolerance to photocorrosion [21]. However, MnxCd1-xS exhibits fast combination of photo-induced electrons and holes in photocatalytic water splitting reaction. To overcome the drawbacks and enhance photocatalytic performance, several efforts have been made, such as doping metals (e.g., Fe, Cu, Ag, etc.) [22], semiconductor coupling [23] and co-catalyst modification [24]. For example, the use of Co3O4 as co-catalyst to modify Mn0.25Cd0.75S results in higher photocatalytic activity [23].

Ni-based co-catalysts have been widely explored in the field of photocatalysis to construct semiconductor heterojunctions in recent years [25-28]. Hong and co-workers [29] demonstrated that the highest hydrogen evolution rate of NiS/C3N4 photocatalyst was 250 times higher than that of native C3N4. CdS nanorods modified with Ni(OH)2 showed an H2-production rate of 5084 µmol/h/g [30]. Cd0.5Zn0.5S loaded with Ni2P improved the photocatalytic H2-evolution activity and stability [31]. As one of the Ni-based materials, nickel selenide is earth-abundant and low-cost [32-34]. NiSe, a typical p-type semiconductor, displays excellent electronic

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performance in electrocatalysis due to its good conductivity [35,36]. Also, it has been used as a co-photocatalyst in photocatalytic water splitting recently. NiSe/MnO2-CdS composites showed superb photocatalytic over-all water splitting activity and good photostability under visible light [37]. Our group [38] has synthesized NiSe/TiO2

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heterojunction significantly improving the photocatalytic H2 production compared with pure TiO2. This implies that NiSe is a good co-photocatalyst to construct

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heterojunction for efficient separation of photocarriers.

In this work, we modified Mn0.5Cd0.5S with NiSe nanoparticles using a

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solvothermal method. We adjusted x value in MnxCd1-xS and found the Mn0.5Cd0.5S sample has the best photocatalytic H2-production activity. The loading of NiSe over

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Mn0.5Cd0.5S showed enhanced photocatalytic hydrogen evolution under visible light. The samples were characterized by XRD, TEM, UV-vis DRS, XPS, PL, TRPL, EIS and so on. We have explored the reasons for enhanced photocatalytic hydrogen

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production. Finally, a possible photocatalytic mechanism of NiSe/Mn0.5Cd0.5S

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composite was also proposed based on the experimental results.

2. Experimental details 2.1. Materials

Manganese acetate tetrahydrate (Mn(CH3COO)2·4H2O), cadmium acetate dihydrate (Cd(CH3COO)2·2H2O), sodium sulfide (Na2S) and sodium sulfite (Na2SO3) were purchased from Aladdin Reagent Co., Ltd. Thioacetamide (CH3CSNH2), nickel sulfate hexahydrate (NiSO4·6H2O), H2PtCl6·6H2O, glycol (CH2OH)2 and absolute

ethanol (C2H5OH), were all obtained from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Sodium selenite (Na2SeO3) was supplied from Xiya Chemical Industry Co., Ltd. (Shandong, China). All the reagents were used without further purification.

2.2. Synthesis of MnxCd1-xS (0 ≤ x ≤ 1) samples Mn0.5Cd0.5S samples were synthesized by a one-step hydrothermal method [24]. Firstly, 10x mmol of Mn(CH3COO)2·4H2O and 10(1-x) mmol of

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Cd(CH3COO)2·2H2O were dissolved into 40 mL deionized water. Then, 10 mmol of NH2CSNH2 was mixed into the solution with constant stirring for 30 minutes. The

solution was transferred to 100 mL stainless steel autoclave and maintained at 160 oC for 24 h then cooled to room temperature naturally. The product was washed with

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deionized water and ethanol several times each and dried in a vacuum oven 60 oC for

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12 h.

2.3. Synthesis of NiSe/MnxCd1-xS composites

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A certain amount of NiSO4·6H2O and Na2SeO3 (mole ratio = 1:1) were dissolved into a beaker containing 40 ml (CH2OH)2. Then, 0.1 g MnxCd1-xS was added with

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constant magnetic stirring. After 30 minutes, the mixture was put into the Teflon-lined autoclave of 100 mL capacity and the autoclave was sealed and maintained at 180 oC for 24 h then cooled to room temperature naturally. The resulting NiSe/MnxCd1-xS

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samples were washed with distilled water and ethanol several times each and dried at 60 oC for 12 h. For comparison, pure NiSe was synthesized with a similar method

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without adding MnxCd1-xS.

2.4. Characterization Powder X-ray diffraction (XRD) tests were measured on a Bruker D8 advance diffractometer with Cu Ka1 radiation at room temperature. The UV/Vis diffuse reflectance spectra (DRS) were investigated on a Varian Cary 500 Scan UV/Vis system. X-ray photoelectron spectroscopy (XPS) was carried out on a Thermo

Scientific 20 Escalab 250Xi spectrometer with Al (Kα) radiation. UPS spectra were tested with HeⅠexcitation (21.22 eV) in the ultrahigh chamber of the XPS instrument. Morphologies of samples were examined by Nova NanoSEM 230 Field Scanning Electron Microscopy (Czech Republic FEI Company). Transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) images were performed on Titan G2 60-300 with the image corrector at 300 kV. Photoluminescence (PL) spectra were conducted on an F-7000 Fluorescence Spectrophotometer. Time-resolved photoluminescence (TRPL) was measured on an Edinburgh FS1000 fluorescence spectrofluorometer, detected at 558 nm with

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excitation source (375 nm laser). Photoelectrochemical tests for the Mott–Schottky plots, the transient photocurrent and the electrochemical impedance spectroscopy (EIS) were carried out with a Bio-Logic SAS electrochemical workstation in a

three-electrode cell containing Na2SO4 (0.2 mol/L, pH 7) aqueous solution as the

electrolyte. The cyclic voltammograms (CV) were measured in the electrolyte of 0.5

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mol/L KCl aqueous solution containing 0.01 mol/L K3[Fe(CN)6]/K4[Fe(CN)6] (1:1). A Pt plate was used as counter electrode and an Ag/AgCl electrode as the reference

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electrode, the catalyst deposited on the fluorine-doped tin oxide (FTO) glasses with an area of 0.20 cm2 used as the working electrode. The photocurrent response

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experiment was equipped with a 300 W Xe lamp (PLS-SXE300) with a 420 nm cut-off filter as the light source. Mott–Schottky experiments were conducted with potential ranged from –0.6 to –0.4 V in the frequencies of 1.5, 1.0 and 0.5 KHz. The

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potentials were related to the RHE by the following calculations: E (vs. RHE) = E (vs. Ag/AgCl) + 0.197 + 0.059pH [33]. The electrochemical impedance spectroscopy (EIS) was conducted at an AC voltage magnitude of 10 mV in the frequency range of

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10-5 to 0.1 Hz. The apparent quantum efficiency (AQE) was also measured under the same photocatalytic reaction conditions with 20 mg photocatalysts and a 420 nm

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band-pass filter instead of the UV cut-off filter. Light intensity was measured by a PL-MW2000 optical power meter (Beijing PerfectLight Technology Ltd, China). The average intensity of irradiation and the irradiation area were measured to be 1.99 mW·cm-2 and 28.26 cm2, respectively. The AQE is calculated by the following formula： AQE (%) =

number of reacted electrons ×100 number of incident photons

=

number of evolved H2 molecules ×2 ×100 number of incident photons

2.5. Photocatalytic reaction Photocatalytic hydrogen reaction was conducted in a sealed gas circulation system of photocatalytic reaction with a 500 mL quartz photoreactor. Typically, 5 mg catalysts were dispersed in 50 mL of water containing Na2S (0.35 mol/L) and Na2SO3 (0.25 mol/L) as sacrificial reagents. Before reaction, the system was vacuumized for 15 minutes to remove air. Then, the photocatalytic H2 evolution reactions were

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performed under visible light (λ > 420 nm). The reaction temperature was maintained at 5 oC by the flowing of cooling water and the reactive suspension was stirred at 200 rpm by magnetic blender. The amount of produced H2 was analyzed by an online gas

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chromatograph (GC) equipped with a 5 Å molecular sieve column and sensitive

3. Results and discussion

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3.1. Catalyst structure analysis

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thermal conductive detector (TCD) with argon as the carrier gas.

Figure 1a depicts the phase structure of MnxCd1-xS samples by XRD analysis. The diffraction peaks located at 26.5, 30.8, 43.9 and 52.1° are ascribed to (111), (200),

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(220) and (311) lattice planes of cubic CdS (x = 0 for MnxCd1-xS), respectively. The diffraction peaks at 29.6, 34.3, 49.3 and 61.4° correspond to the (111), (200), (220)

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and (222) crystal planes of ɑ-MnS (x = 1 for MnxCd1-xS), respectively. As shown in Fig. 1a, the enlarged yellow area of the XRD patterns indicate the diffraction peaks of

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MnxCd1-xS gradually shift to higher angles with an increase of Mn content. The successive shifts of diffraction peaks imply that the MnxCd1-xS solid solution is formed rather than physical mixture of CdS and MnS. The incorporation of Mn2+ into the lattice of CdS decreases the fringe lattice distance of CdS since the radius of Mn2+ (0.46 Å) is lower than that of Cd2+ (0.97 Å) [3]. Figure S1 depicts that the diffraction peaks at 32.6, 44.3 and 49.6° match with (101), (102) and (110) lattice planes of hexagonal NiSe, respectively. Figure 1b presents that no obvious peak appears with

the increasing loading of NiSe on Mn0.5Cd0.5S due to the low contents of NiSe. The introduction of NiSe does not change the structure of Mn0.5Cd0.5S. Figure S2 displays the nitrogen adsorption–desorption isotherms and specific surface area of the Mn0.5Cd0.5S and 5 wt% NiSe/Mn0.5Cd0.5S composite. The N2 isotherms for Mn0.5Cd0.5S and 5 wt% NiSe/Mn0.5Cd0.5S are Type III and the hysteresis hoops are type H3 [39]. The BET specific surface area of Mn0.5Cd0.5S and 5 wt% NiSe/Mn0.5Cd0.5S are 19.0922 m2/g and 16.9039 m2/g, respectively.

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3.2. SEM and TEM analysis The morphologies of the samples were investigated by SEM and TEM technology. Figure S3a and S3b demonstrate the SEM images of Mn0.5Cd0.5S and 5 wt% NiSe/Mn0.5Cd0.5S. Figure 2 and S4 depict the TEM images of 5 wt%

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NiSe/Mn0.5Cd0.5S. The high-resolution TEM (HRTEM) (Fig. 2b) image demonstrates

that the lattice fringes of 0.336 (white mark) and 0.204 nm (yellow mark) are ascribed

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to the (111) plane of Mn0.5Cd0.5S and the (102) plane of NiSe, respectively. The tight attachment of NiSe on the interface of Mn0.5Cd0.5S with two different lattice fringes

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demonstrates the formation of heterostructure. Furthermore, analysis of EDX/HAADF mapping (Fig. 2c-h) presents that NiSe/Mn0.5Cd0.5S is composed of the elements S,

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Mn, Cd, Ni and Se, which are homogeneously distributed throughout the sample. The EDX spectrum demonstrated in Fig. S5 further proves the successful loading of NiSe on Mn0.5Cd0.5S. The co-existence of NiSe and Mn0.5Cd0.5S indicates the possibility for

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formation of intimately contacted heterojunction, which facilitates the transfer of

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photo-induced charges [40].

3.3. XPS analysis XPS was conducted to study the surface elemental composition and valence states

of the sample. The survey spectrum of 5 wt% NiSe/Mn0.5Cd0.5S composite is depicted in Fig. 3a. In the Mn 2p XPS spectrum (Fig. 3b), the binding energies at 652.1 and 641.1 eV correspond to Mn 2p1/2 and Mn 2p3/2 of Mn2+, respectively [3]. Figure 3c presents two peaks at 411.5 and 404.7 eV, which are ascribed to Cd 3d5/2 and Cd 3d3/2

binding energies of Cd2+, respectively [41]. The XPS spectrum of S is displayed in Fig. 3d, S 2p peaks at 162.3 and 161.1 eV are in agreement with S 2p1/2 and S 2p3/2 of S2-, respectively [24]. As displayed in Fig. 3e, the peak at 54.0 eV is attributed to Se2[42]. The peaks located at 872.9 and 855.4 eV presented in Fig. 3f can be assigned to Ni2+ ions [43]. From XPS results, we further confirm that NiSe/Mn0.5Cd0.5S composites have been successfully synthesized. In addition, compared with pure Mn0.5Cd0.5S, the binding energy of Cd 3d state for 5 wt% NiSe/Mn0.5Cd0.5S and 10 wt% NiSe/Mn0.5Cd0.5S shifts to higher energy region (Fig. S6a), whereas, Se 3d peaks of 5

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wt% NiSe/Mn0.5Cd0.5S and 10 wt% NiSe/Mn0.5Cd0.5S shift to lower binding energy in comparison with Se 3d peak of NiSe (Fig. S6b). This change of binding energy can be induced by the variation of electron density on the interface of semiconductors, which can be achieved by transfer of electrons between different semiconductors [44]. The

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XPS results reveal a strong interfacial interaction between Mn0.5Cd0.5S and NiSe

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[45,46].

3.4. UV-vis DRS analysis

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The optical properties of Mn0.5Cd0.5S and NiSe/Mn0.5Cd0.5S composites were investigated by UV-vis diffuse reflectance spectra displayed in Fig. 4a. The spectrum

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of pure Mn0.5Cd0.5S indicates the edge absorption at around 600 nm and the edge absorption shifts to higher wavelengths with the increase of NiSe contents. This may be due to the strong inherent absorption of NiSe in the visible light region (Fig. 4c).

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This indicates that the efficient utilization of solar energy can be obtained. The bandgap of Mn0.5Cd0.5S and NiSe can be estimated by the Kubelka–Munk function in

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the following way:

α (1– 𝑅)2 = s 2R Where F(R) is the Kubelka–Munk function, R the reflectance, s the scattering factor F(R) =

and F(R) is proportional to extinction coefficient (α). Based on the Kubelka–Munk function, there is modification of multiplying the F(R) function by hυ to the power of n alone with an electronic transition as follows: (F(R)•hυ)n. n = 2 is for direct allowed

transitions and n = 1/2 is for indirect allowed transitions. As revealed in Fig. 4b and 4d, from a plot of (F(R)•hυ)2 vs. hυ, the bandgap energy of Mn0.5Cd0.5S and NiSe can be calculated by extrapolating the slope to F(R) → 0, which is 2.33 and 2.03 eV, respectively [47,48].

3.5. Photocatalytic activity and stability The photocatalytic H2-production activities of all the samples were investigated in Na2S (0.35 mol/L) and Na2SO3 (0.25 mol/L) aqueous solution under visible light for

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three hours. The rate of hydrogen production against the weight of catalyst is

presented in Fig. S7 for assessing appropriate amount of catalyst in photocatalytic

reaction. Figure 5a displays that pure CdS (x = 0) and MnS (x = 1) shows very low

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H2-production activity rate of 2.53 and 1.67 mmol/h/g, respectively. This is related to

poor separation ability of photogenerated charge carriers over CdS and wide bandgap

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of MnS limiting light response [49,50]. As for MnxCd1-xS (0 ≤ x ≤ 1) solid solution, with the increase of x value (0 ≤ x ≤ 1) in MnxCd1-xS, photocatalytic activity increases

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dramatically then decreases [4,24,41]. Particularly, Mn0.5Cd0.5S exhibits the highest H2-production rate of 13.71 mmol/h/g. Pure NiSe shows very low photocatalytic

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activity (0.44 mmol/h/g) depicted in Fig. 5b. It is noted that the introduction of NiSe leads to significant improvement in the photocatalytic H2-evolution performance. With the increase of amounts of NiSe loading on Mn0.5Cd0.5S, the photocatalytic H2

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generation of the composite increases at first, then declines with further loading of

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NiSe. The possible reason is that appropriate amounts of NiSe as a co-catalyst can lower the recombination rate of photo-excited electrons and holes, resulting in enhanced photocatalytic H2-production activity. However, excessive loading of NiSe may shield the incident light and cover the active sites of Mn0.5Cd0.5S surface leading to a decrease of hydrogen production [3]. In particular, 5 wt% NiSe/Mn0.5Cd0.5S shows the highest hydrogen production rate of 28.08 mmol/h/g, which is twice than pure Mn0.5Cd0.5S. We also compared the photocatalytic hydrogen evolution of 1 wt%

Pt/Mn0.5Cd0.5S with pure Mn0.5Cd0.5S and 5 wt% NiSe/Mn0.5Cd0.5S (Fig. S8). We loaded Pt on Mn0.5Cd0.5S with a typical photodeposition method using H2PtCl6·6H2O as the precursor. It is clear that 1 wt% Pt/Mn0.5Cd0.5S significantly improved the photocatalytic activity (24.22 mmol/h/g) compared with Mn0.5Cd0.5S. However, the photocatalytic H2 production of 1 wt% Pt/Mn0.5Cd0.5S is still lower than 5 wt% NiSe/Mn0.5Cd0.5S, which confirms that NiSe is a high efficient co-photocatalyst for promoting photocatalytic hydrogen evolution. Moreover, we prepared a physical

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mixture of Mn0.5Cd0.5S and NiSe for comparison. It is obvious that the photocatalytic hydrogen production of the physical mixture of Mn0.5Cd0.5S and NiSe (15.57

mmol/h/g) is slightly higher than pure Mn0.5Cd0.5S (13.71 mmol/h/g), but lower than

that of 5 wt% NiSe/Mn0.5Cd0.5S (28.08 mmol/h/g). The photocatalytic H2-production

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activity of 5 wt% NiSe/ɑ-MnS (1.92 mmol/h/g) and 5 wt% NiSe/CdS (11.46

mmol/h/g) are also lower than that of 5 wt% NiSe/Mn0.5Cd0.5S (Fig. S8). Furthermore,

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the AQE of 5 wt% NiSe/Mn0.5Cd0.5S was calculated to be 3.3 % at 420 nm, which was

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3 times higher than that of pure Mn0.5Cd0.5S (1.1 %). In addition, the action spectra of hydrogen production for 5 wt% NiSe/Mn0.5Cd0.5S were achieved under light irradiation with different wavelengths (Fig. S9). The photocatalytic performance of 5

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wt% NiSe/Mn0.5Cd0.5S matches well with the optical absorption, suggesting that the photocatalytic reaction is mainly triggered by the captured visible photons [51].

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The stability of 5 wt% NiSe/Mn0.5Cd0.5S composite was investigated by a five-time cycling test. Figure 6a exhibits no noticeable change occurs in the

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hydrogen-evolution rate during five-time recycles, indicating the good photostability and reusability of NiSe/Mn0.5Cd0.5S nanocomposites. As revealed in Fig. 6b, there is no obvious change in XRD patterns comparing 5 wt% NiSe/Mn0.5Cd0.5S with 5 wt% NiSe/Mn0.5Cd0.5S after 9 h of photocatalytic H2-production reaction, which further proves high stability in the photocatalytic process. Moreover, no obvious change in morphology of 5 wt% NiSe/Mn0.5Cd0.5S was observed after 3 h of reaction (Fig. S3c and S3d).

3.6. Charge transfer and separation and the possible photocatalytic mechanism. The radiative recombination process of photo-induced carriers in photocatalyst was characterized by photoluminescence (PL) spectra, time-resolved PL spectra (TRPL) and photoelectrochemical tests. Figure 7a depicts PL spectra of Mn0.5Cd0.5S and NiSe/Mn0.5Cd0.5S with an excitation wavelength at 360 nm and an emission wavelength at 558 nm, respectively. The PL peak intensity of NiSe/Mn0.5Cd0.5S is strongly quenched for comparison with Mn0.5Cd0.5S. Moreover, for the

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NiSe/Mn0.5Cd0.5S samples, the intensity of emission decreases with the increase of mass ratio of NiSe. The TRPL spectra (Fig. 7b) also indicate that the lifetime of 5 wt% NiSe/Mn0.5Cd0.5S composite (15.21 ns) is much higher than pure Mn0.5Cd0.5S (9.83 ns), suggesting effective charge transfer from Mn0.5Cd0.5S to NiSe. The interface

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charge separation efficiency can be further investigated by the photoelectrochemical

technique. Figure 7c depicts the periodic on/off photocurrent response of Mn0.5Cd0.5S

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and 5 wt% NiSe/Mn0.5Cd0.5S nanocomposite. The photocurrent density for 5 wt% NiSe/Mn0.5Cd0.5S is much higher than native Mn0.5Cd0.5S, suggesting less

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recombination and faster photo-induced charges migration on photocatalyst. Electrochemical Impedance Spectroscopy (EIS) was an efficient way to study electron

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transfer efficiency and interface reaction ability. Figure 7d demonstrates the Nyquist plots fitted to the equivalent Randle circuit and the fitted values of Rct were 246535 and 100548 Ω for Mn0.5Cd0.5S and 5 wt% NiSe/Mn0.5Cd0.5S, respectively. 5 wt%

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NiSe/Mn0.5Cd0.5S exhibits a smaller arc radius than Mn0.5Cd0.5S, indicating that the construction of NiSe/Mn0.5Cd0.5S heterojunction leads to a lower charge transfer

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resistance and thus higher separation and transfer rate of photo-induced carriers [52], which helps to enhance photocatalytic activity. The cyclic voltammograms (CVs) for Mn0.5Cd0.5S and 5 wt% NiSe/Mn0.5Cd0.5S are shown in Fig. S10. It can be seen that the current density over the electrode of 5 wt% NiSe/Mn0.5Cd0.5S is larger than pure Mn0.5Cd0.5S, suggesting that more efficient separation and migration of photo-induced electron–hole pairs over the 5 wt% NiSe/Mn0.5Cd0.5S than the Mn0.5Cd0.5S [53]. The conduction band (CB) potential of Mn0.5Cd0.5S (–0.47 V vs. RHE) was determined by

Mott–Schottky plots in Fig. 8a, which suggests a typical n-type semiconductor for Mn0.5Cd0.5S. The valence band (VB) potential of Mn0.5Cd0.5S was measured 1.86 V according to the equation: EVB = Eg + ECB. The electronic band position of NiSe was determined by UPS in Fig. 8b. The Fermi level (EF) and work function were measured –4.88 and 4.88 eV (vs. vacuum), respectively, by subtracting the width of the He I UPS spectrum from the excitation energy (21.22 eV). The valence band maximum (EVB) is located at –6.4 eV (vs. vacuum) and the value of relative valence band maximum (EVB) is 1.96 V depending on the relationship between the absolute vacuum

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energy (Eabs) and the normal electrode potential (E◦) following: Eabs = – 𝐸◦ – 4.44

[54]. Therefore, the CB potential of NiSe was calculated to be –0.07 V in relation to

the value of Eg determined in Fig. 4d. Based on the band structure of Mn0.5Cd0.5S and

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NiSe, a possible mechanism for increased photocatalytic H2 production could be

proposed (Fig. 9). When Mn0.5Cd0.5S is coupled with NiSe with a well-matched band

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structure, the CB position of Mn0.5Cd0.5S (–0.47 V) is more negative than that of NiSe (–0.07 V) and the VB position of NiSe (1.96 V) is more positive than that of

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Mn0.5Cd0.5S (1.86 V). The photogenerated electrons will transfer from the CB level of Mn0.5Cd0.5S to that of NiSe with rich electrons accumulated on the CB position of NiSe to generate hydrogen under visible light [55] Also, the photo-induced holes will

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migrate from the VB position of NiSe to Mn0.5Cd0.5S to oxidize the S2- and SO32sacrificial reagents to S2O32-, which reduces the wasteful energy and enhances

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photocatalytic hydrogen evolution. Thus, such a well-matched band structure drives photo-induced electrons and holes to migrate in opposite directions, resulting in the

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effective separation of photo-induced charges for enhanced photocatalytic hydrogen production.

4. Conclusion

In conclusion, we present an in-situ synthesis of NiSe/Mn0.5Cd0.5S nanocomposites by using a facile hydrothermal treatment. The as-prepared 5 wt% NiSe/Mn0.5Cd0.5S sample exhibits the highest photocatalytic H2-production rate of

28.08 mmol/h/g under visible light, which is twice than pure Mn0.5Cd0.5S and also higher than 1 wt% Pt/Mn0.5Cd0.5S. The improved photocatalytic activity attributes to the construction of NiSe/Mn0.5Cd0.5S nanocomposites which lowers charge transfer resistance and facilitates the separation and transfer of photo-induced carriers, thus prolongs the electron lifetime. In this work, we constructed a noble metal-free NiSe/Mn0.5Cd0.5S heterostructure photocatalyst with high photocatalytic activity, which is beneficial for practical photocatalytic application.

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Conflicts of interest There are no conflicts of interest to declare.

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Acknowledgments

The authors are grateful for the financial support of the National Natural Science

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Foundation of China (Nos. U1662112, 21273038, 21543002 and 11305091) and the Open Project Program of the State Key Laboratory of Photocatalysis on Energy and Environment (Grant Nos. SKLPEE-KF201703, SKLPEE-KF201807), Fuzhou

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University.

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Figures

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Figure 1. (a) XRD patterns of as-prepared CdS, MnS and MnxCd1-xS (0 ≤ x ≤ 1) and (b) Mn0.5Cd0.5S and NiSe/Mn0.5Cd0.5S composites with different weight loading ratios of NiSe.

ro of -p re lP na ur Jo Figure 2. (a) TEM and (b) HRTEM image of 5 wt% NiSe/ Mn0.5Cd0.5S; (c) HADDF-STEM image of 5 wt% NiSe/Mn0.5Cd0.5S; Elemental mapping images of (d) S, (e) Mn, (f) Cd, (g) Ni and (h) Se.

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Figure 3. XPS spectra of (a) survey, (b) Mn 2p, (c) Cd 3d, (d) S 2p, (e) Se 3d, and (f) Ni 2p for 5 wt% NiSe/Mn0.5Cd0.5S composite.

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Figure 4. UV–vis DRS of (a) Pure Mn0.5Cd0.5S and NiSe/Mn0.5Cd0.5S nanocomposites with different contents of NiSe; Bandgap of (b) Mn0.5Cd0.5S; (c) UV–vis DRS of NiSe and (d) Bandgap of NiSe derived from Tauc plots.

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Figure 5. H2 production of (a) MnxCd1-xS (0 ≤ x ≤ 1) and (b) NiSe/Mn0.5Cd0.5S composites loading with different weight of NiSe.

Figure 6. (a) Cycling test of H2 production for 5 wt% NiSe/Mn0.5Cd0.5S and (b) XRD patterns of 5 wt% NiSe/Mn0.5Cd0.5S before and after 9 h of reaction.

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Figure 7. (a) PL spectrum of pure Mn0.5Cd0.5S and NiSe/Mn0.5Cd0.5S composites; (b) TRPL spectra; (c) Transient photocurrent responses and (d) Electrochemical impedance spectroscopy (EIS) Nyquist plots of Mn0.5Cd0.5S and 5 wt% NiSe/Mn0.5Cd0.5S composite. In the equivalent Randle circuit, RS is the ohmic resistance electrolyte solution, CPE is the constant phase element for the electrode/electrolyte interface, and Rct is charge transfer resistance across the electrode/electrolyte interface [56].

Figure 8. (a) Mott–Schottky plots of Mn0.5Cd0.5S with frequency of 1.5, 1.0 and 0.5 KHz and (b) UPS spectrum of NiSe.

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Figure 9. The band structure and charge transfer and separation of NiSe/Mn0.5Cd0.5S under visible light.