Hollow structured PtNix alloy as cocatalyst of CdS for hydrogen generation under visible light irradiation

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 2 8 1 0 4 e2 8 1 1 2

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Hollow structured PtNix alloy as cocatalyst of CdS for hydrogen generation under visible light irradiation Qianqian Ba a, Xinjia Jia a, Li Huang a, Xiying Li a,b, Li Gao a,b, Liqun Mao a,b,* a

Henan Engineering Research Center of Resource & Energy Recovery from Waste, Henan University, Kaifeng, 475004, PR China b Institute of Functional Polymer Composites, Henan University, Kaifeng, 475004, PR China

highlights

graphical abstract


The CdS photocatalysts with sheet structure was prepared.
PtNix hollow NPs acted as cocatalyst

was

applied

to

hydrogen

evolution.
PtNix

HNPs

modified

on

CdS

showed higher photocatalytic activity than Pt/CdS.

article info

abstract

Article history:

PtNix hollow nanoparticles (HNPs) as a cocatalyst was synthesized by a galvanic replace-

Received 23 May 2019

ment method at room temperature, and then was loaded on CdS. The hollow structure of

Received in revised form

PtNix alloy was confirmed by TEM and STEM. Photocatalytic hydrogen production reaction

29 August 2019

was performed for PtNix/CdS catalyst under 300 W Xe lamp (l  420 nm). It is noteworthy

Accepted 5 September 2019

that PtNi0.5/CdS shows the highest hydrogen evolution activity of 2.9 mmol/h with

Available online 25 September 2019

QE ¼ 51.24% at 420 nm, which is higher than that of situ-photodeposited Pt onto CdS (0.57 mmol/h). The enhancement of hydrogen evolution performance of PtNi0.5/CdS could

Keywords:

be attributed to the porous shell and hollow structure of PtNi0.5 NPs, as well as the strong

Hollow PtNix NPs

electronic coupling effect between Pt and Ni. Besides, the sheet structure of CdS in some

Photocatalyst

degree promoted the hydrogen production rate. Therefore, the PtNix HNPs could be a

CdS

promising cocatalyst of CdS for solar driven hydrogen evolution.

Hydrogen generation

© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

* Corresponding author. Henan Engineering Research Center of Resource & Energy Recovery from Waste, Henan University, Kaifeng, 475004, PR China. E-mail addresses: [email protected], [email protected] (L. Mao). https://doi.org/10.1016/j.ijhydene.2019.09.029 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

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Introduction Photocatalytic hydrogen generation from water splitting with solar energy is a potential solution for energy and environmental issues [1,2]. The development of the cocatalysts with high efficiency and low cost plays an essential role in this filed [3,4]. Recently, Pt-based materials have attracted increasing attention due to their outstanding catalytic activity. It is recognized that the catalytic reactions generally occur at the surface of catalysts, so increasing the surface area of catalysts is a good method to prompt catalytic performance. To date, some structures with large surface area have been already synthesized [5e14]. Among these structures, the Pt-based bimetal alloy with hollow structure has drawn much attention owing to its porous shell, excellent catalytic activity and low density [12e15]. The hollow structured NPs was usually synthesized by template method [16e18], however, the remove of template after reaction may destroy the structure of NPs. And now the most versatile and green method is galvanic replacement method which depends on the different standard reduction potential between metal ions [19]. Many hollow NPs have been reported since Sun et al. synthesized Au, Pt and Pd hollow structure with Ag as sacrificial template for the first time [20e23]. Many researchers focused on alloying large storage transition metals with Pt NPs, which can lower the catalysts costing and improve the catalytic preformance [24]. In numerous transition metals, Ni can be used as an ideal ligand to shape stable PteNi alloy, and the electronic coupling effect of Pt and Ni is beneficial to enhance its catalytic performance [25]. Hollow Pt-M (M ¼ Ni, Co, Ag, Pd, etc.) NPs have been reported for different applications, and it is often used for electrochemical catalysis. For example, Hu et al. [24] synthesized porous hollow PtNi nanocatalyst supported by graphene for methanol oxidation reaction (MOR), and the results showed that the porous nature of hollow NPs had better catalytic activity than that of solid counterpart, this mainly due to the presence of porous shell. Fu et al. [26] also synthesized three-dimensional porous PtNi hollow nanochains with galvanic replacement method, and the PtNi NPs with the Pt content of 77% exhibited superior activity towards oxygen reduction reaction (ORR), and its mass activity was 3 times higher than that of Pt/C. Zhang et al. [27] synthesized the hollow PtAg alloy using the above mentioned method. The hollow NPs showed better electrocatalytic performance in MOR compared with Pt/C. Gopinath et al. [28] demonstrated that PtAu alloy enhance the photocatalytic performance for hydrogen evolution reaction in Pt0.5Au/TiO2 system, which was mainly due to the synergistic effect between Pt, Au and porous TiO2 nanoparticles; Moreover, the interface between Pt and Au enhance the charge separation efficiency in turn minimize their recombination. These literature survey illustrated that porous hollow NPs had excellent catalytic performance. However, up to now, there are only a few papers about hollow NPs for photocatalytic hydrogen evolution [29], and applications of PteNi hollow NPs as cocatalyst of CdS for hydrogen production under visible light irradiation has been rarely reported.

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Herein, we prepared hollow PtNix NPs with different compositions by means of a galvanic replacement method, and next modified it onto CdS sheet. The activity of PtNix/CdS was tested for photocatalytic hydrogen evolution reaction using (NH4)2SO3 as sacrificial agent under visible light irradiation.

Experimental section Preparation of photocatalysts K2PtCl6$6H2O and (NH4)2SO3$5H2O were purchased from Afla Aesar Co. Ltd, NiCl2$6H2O was bought from Aladdin Industrial Co. Ltd, Cd(Ac)2$2H2O, CS(NH2)2 and Na2SO4 was bought from Tianjin Kermel Chemical Reagent Co. Ltd. HF was purchased from Luoyang Chemical Reagent Co. Ltd, NaBH4 was purchased by Shanghai Shanpu Chemical Reagent Co. Ltd, PVP (K ¼ 30) was bought from Sinopharm Chemical Reagent Co. Ltd, Ethanol was purchased from Anhui an’te food Co. Ltd. Nafion solution (5.0 wt %) was purchased from DuPont Co. Ltd. All materials were analytical reagents and were directly applied without any treatment. The ultrapure water was used throughout the experiment. CdS was synthesized by a hydrothermal method using HF as capping reagent [30]. PtNix hollow NPs (PtNix HNPs) were prepared via a galvanic replacement method. Typically, NiCl2$6H2O, PVP were dissolved into 50 mL ultrapure water. The mixed solution was ultrasound-treated for 15 min, and then purged with N2 for 15 min. Freshly prepared NaBH4 solution (20 mL, 0.5 mg/mL) was added into the solution dropby-drop under vigorously stirring, and then K2PtCl6 solution (20 mL, 6 mM) was added. After 2 h reaction, the products were separated by centrifuge and washed several times with ultrapure water and ethanol, respectively. In order to obtain PtNix NPs with different Ni/Pt molar ratio, adjusting the amount of NiCl2$6H2O while keeping other conditions unchanged. The as-prepared NPs were denoted as Pt, PtNi0.3, PtNi0.5, PtNi, Ni, respectively.

Characterization The crystal structure of samples were analyzed by X-ray diffraction (Bruker, Germany). The morphology of photocatalytic materials was examined by a Jeol JEM-2010 transmission electron microscope (TEM) coupled with selected area electron diffraction (SAED). The element distribution of product was performed on a Tecnai G2 F20S-TWIN (scanning) transmission electron microscopy (STEM). The surface electronic state was examined by an AXIS ULTRA X-ray photoelectron spectroscopy (XPS). The absorption properties were analyzed by Photoluminescence (PL) emission spectra and UVeVis diffuse reflection spectra (UVeVis DRS). PL were obtained on a FLS 980 fluorescence spectrophotometer. UVeVis DRS were collected on a UV-2600 UVeVis spectrophotometer with BaSO4 as the reference.

Electrochemical measurements The electrochemical measurements linear sweep voltammetry (LSV) curve were performed on a CH CHI660D

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electrochemical workstation with three-electrode system. A Pt wire and Ag/AgCl were used as the counter electrode and the reference electrode, respectively. The working electrode was obtained by coating photocatalysts on the glassy carbon electrode with an area of 0.71 cm2. The prepared details were as follows: 5 mg photocatalysts were dispersed into 1 mL 0.1 wt% nafion aqueous solution. Next, the slurry was injected into the glassy carbon electrode and was dried at 60  C for 2 h. The electrolyte used was 0.5 M Na2SO4 aqueous solution saturated with N2. Electrochemical impedance spectroscopy (EIS) was carried out at open-circuit potential with an AC voltage magnitude of 5 mV.

Evaluation of photocatalytic activity Photocatalytic hydrogen evolution reactions were performed in a water-jacketed continuous stirred tank reactor (CSTR, F125 mm) at 283 ± 1 K. The details have been reported in our previous work [31,32]. In this work, we employed situ photodeposition method to synthesize Pt-s/CdS as a reference. In detail, 0.05 g CdS and 0.667 mL (1.5 mg/mL) K2PtCl6 were added into 50 mL 1.0 M (NH4)2SO3 aqueous solution, then the solution was illuminated under 300 W Xe lamp (l  420 nm) for 2 h before counting the hydrogen production amount. Generated hydrogen was monitored by gas chromatograph (GC9560, Huaai, China), and the volume of evolved hydrogen was measured every 30 min. The apparent quantum efficient (QE) [33] of the photocatalyst toward hydrogen generation is defined as: QE ¼ NH/NP*100%, Where NH is the moles of evolved hydrogen atom, NP is the moles of incident photons absorbed by the photocatalyst.

Results and discussion Synthesis and characterization of the hollow PtNix nanoparticles The hollow structured PtNix cocatalyst was synthesized via a galvanic replacement method using NiCl2 and K2PtCl6 as precursors, NaBH4 as a reductant, and PVP as a stabilizer. Firstly, Ni particles were formed by reducing Ni2þ with NaBH4, and the as-obtained Ni nanoparticles were absorbed on the PVP to protect the Ni nanoparticles from aggregation. Because the standard reduction potential of PtCl2 6 /Pt pair (0.74 V) is much higher than that of Ni2þ/Ni pair (0.25 V), once the PtCl6 2was added into the solution, the PtCl2 6 was quickly reduced to Pt by Ni2þ and next deposited on the surface of Ni particles. The continue dissolution of Ni core forms a chemical potential gradient between the core and exterior and pushes Ni nanoparticles out-migration, and the as-formed Pt and remaining Ni deposits on the surface of core, finally generates the hollow structure [34,35]. TEM was employed to observe the morphology and structure of as-prepared PtNi0.5 NPs. As shown in Fig. 1a, the particles exhibited uniform sphere with an average diameter of 30 nm. The bright interior and dark shell proved the hollow nature of PtNi0.5 NPs. It was obvious that the shell thickness of

PtNi0.5 HNPs was 2e4 nm, and it consisted of many small nanothrons, which may increase the amount of active sites, and thus improve the photocatalytic activity [36]. In addition, all the nanoparticles were well interconnected with each other which facilitated the fast mobility of charge carriers, and also the aggregation of particles were avoided due to the addition of PVP during the synthesis. The SAED image in Fig. 1b indicated that the PtNi0.5 NPs had evident polycrystalline and fcc structure. Furthermore, the diffraction ring pattern of PtNi0.5 indicated that the preferential growth of Pt was (111) plane direction. To investigate the elements distribution of PtNi0.5 NPs, S-TEM was used. Obviously, Pt and Ni element were distributed throughout the PtNi0.5 NPs (shown in the Fig. 1c). Moreover, the peak intensity of Pt and Ni element were higher in the edge than those of center, which further demonstrated the hollow structure of PtNi0.5 NPs. Fig. 1d depicted the XRD pattern of as-obtained PtNi0.5 HNPs. The main diffraction peaks located at 39.93 , 46.28 , 67.88 and 81.76 were corresponded to (111), (200), (220) and (311) crystalline planes of PteNi fcc structure, respectively. As compared with all the diffraction peaks, a peak at 39.93 shows high intensity than that of other which corresponds to the (111) plane of Pt and demonstrates that the preferential growth direction of Pt. These observations are well supported with that of SAED pattern of PtNi0.5 (Fig. 1b). No Ni or its oxides diffraction peaks were observed. As for (220) and (311) crystalline planes, there was obvious shift to higher angle compared to those of XRD pattern of pure Pt [JCPDS No.3652868], implying the formation of alloy of Pt and Ni [37]. In addition to the above factor, the peak broadening demonstrates that formation of nanocrystalline particles, which aids to enhance the active sites. Besides the hollow structure, another important factor affecting photocatalytic activity is the electronic coupling effect between Ni and Pt. Therefore, X-ray photoelectron spectroscopy (XPS) was employed to determine the surface chemical state of PtNi0.5 HNPs. It could be observed from Fig. 2a, that the 4f core-level spectrum consisted of two strong peaks for metallic platinum at 70.78 eV and 74.08 eV, demonstrated that Pt (0) predominated in the PtNi0.5 HNPs. With pure Pt as a reference, the BE of Pt element in PtNi0.5 appears to shift negatively, which demonstrates that the electronic stage of Pt changes when alloying with Ni. It could be beneficial to improve the photocatalytic hydrogen evolution activity [38]. The XPS spectra of Ni 2p was shown in Fig. 2b. Due to the oxidation of some portion of Ni on the surface, there were only peaks assigned to NiO. The electronic coupling effect between Pt and Ni is favorable to enhance the photocatalytic activity. These observations were consistent with the results of TEM, SAED and XRD. Considering the results obtained from XRD, TEM, STEM and XPS, it could confirm the formation of the hollow PtNi0.5 alloy. Fig. 3a and b showed the morphologies of PtNi0.5/CdS and the distribution of PtNi0.5 NPs over CdS, it could be seen that only few PtNi0.5 NPs (right circles) were dispersed over CdS, which may due to the low loading of PtNi0.5 NPs. HRTEM was used to investigate the contact between CdS and PtNi0.5 NPs, the images were shown in Fig. 3c. The lattice fringes with a

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Fig. 1 e (a) TEM image, (b) SAED pattern, (c) S-TEM image (green-Pt, red-Ni) and (d) XRD spectra of PtNi0.5 NPs. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

(a)

(b)

Pt 4f

4f 4f

Ni 2p Ni 2p

Intensity / (a.u.)

Intensity / (a.u.)

Ni 2 2p

82

80

78

76

74

72

70

68

66

Binding energy / eV

64

880

870

860

850

840

Binding energy / eV

Fig. 2 e XPS spectra of (a) Pt 4f and (b) Ni 2p.

spacing of 0.356 nm correspond to (100) crystal plane of hexagonal CdS, and these of 0.221 nm and 0.225 nm are ascribed to (111) plane of Pt [30,39e42]. The strong electronic interaction between PtNi0.5 NPs and CdS facilitates fast electron transport at the interface of PtNi0.5 and CdS for better photocatalytic performance [29].

Fig. 4 displayed the TEM images of PtNix NPs with different compositions. As observed, the morphologies and size of as-obtained PtNix NPs relied on the molar ratio of Ni/ Pt in the precursors. Pt NPs (Fig. 4a) displayed in the form of dendritic particle with an average diameter of 4 nm. Ni samples (Fig. 4b) presented the fibrous structure, and it

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Fig. 3 e (a) & (b) TEM image and (c) HRTEM of PtNi0.5/CdS.

Fig. 4 e TEM images of (a) Pt-g, (b) Ni, (c) Pt Ni0.3, (d) PtNi0.5 and (d) PtNi.

might form nanofilm. Reducing the Ni/Pt molar ratio to 0.3 from 0.5, the hollow structure nanospheres (Fig. 4c) could be obtained, whereas thinner shell and partial crush also existed due to the small ratio of Ni/Pt [43]. At Ni/Pt molar ratio of 0.5:1, there were whole hollow nanospheres (Fig. 4d), suggesting this molar ratio of Ni/Pt is optimum to synthesize hollow structure. Further increasing the Ni/Pt molar ratio, such as the PtNi NPs in Fig. 4e, the nanoparticles depicted non-uniform morphology and size. Besides some hollow nanoparticles were observed, some solid NiePt alloy also formed. This phenomenon might be caused by deficient PtCl26 to oxidize the Ni particles [44]. These results explained that the galvanical displacement method could be employed to synthesize hollow nanoparticles with different compositions.

Hydrogen evolution properties Fig. 5 showed the effect of the amount of PtNi0.5 NPs on the hydrogen generation rate of a CdS photocatalyst under visible light (l  420 nm) with (NH4)2SO3 as sacrificial agent. CdS with hexagonal phase was prepared using a hydrothermal method. It possessed snow-flake and sheet structure with six petals, and the average petal length is 5 mm [30]. The unique structure could shorten the electron delivery distance from the bulk to surface, and thus to some extent prompt the photocatalytic performance [45,46]. The photocatalytic activity of pure CdS under the experimental conditions was negligible (not shown). After PtNi0.5 HNPs loading, the hydrogen generation rate of CdS was significantly enhanced. The rate gradually increased, and reached the highest value (2.9 mmol/h) with

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16

16

14

14 12

10 8 6

1% 2% 3% 4%

4 2 0 0

1

2

3

4

H2Yield / mmol

12

rH mmol/h

H2 Yield / mmol

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 2 8 1 0 4 e2 8 1 1 2

10 8 6 4 2 0

5

0

5

10

Time / h Fig. 5 e The effect of the amount of PtNi0.5 HNPs on the hydrogen generation rate of a CdS photocatalyst under visible light (l ≥ 420 nm).

c

d

mmol/h

r

H2 Yield / mmol

10

d

b e a

b

8 6

e

4

a

2 0 0

1

2

25

c

14 12

20

Fig. 7 e Time course of photocatalytic hydrogen production over PtNi0.5/CdS (PtNi0.5 dosage: 2.0 wt%) ptotocatalyst. Reaction conditions: 0.05 g catalyst, 300 W Xe lamp (l ≥ 420 nm), 1 M (NH4)2SO3 (50 mL).

3

4

5

Time / h (a) Pt-s/CdS, (b) PtNi0.3/CdS, (c) PtNi0.5/CdS, (d) PtNi/CdS, (e) Ni/CdS

Fig. 6 e Hydrogen generation rate of PtNix/CdS photocatalyst under visible light (l ≥ 420 nm).

the loading of 2.0 wt%. When the loading was 1.0 wt%, the hydrogen generation rate (1.95 mmol/h) was less than that of 2.0 wt% due to the insufficient active sites. When PtNi0.5 NPs loading was higher than 2.0 wt%, the hydrogen generation rate reduced, which because the PtNi0.5 cocatalyst blocked the adsorption of light photo of CdS. Fig. 6 showed the effect of PtNix NPs (2.0 wt%) with different compositions on the CdS for hydrogen evolution activity. Photocatalytic activity of Pt-s/CdS and Ni/CdS were also evaluated and compared with these of the results obtained from other photocatalyst. Pt-s/CdS and Ni/CdS showed low hydrogen production rate about 0.57 and 1.18 mmol/h, respectively. Hydrogen production rate was significantly increased from 1.77 to 2.9 mmol/h at Ni/Pt molar ratio of 0.3 and 0.5 respectively. Decreasing of activity observed when Ni/ Pt molar ratio increased to 1.0. It could be seen that the rate of hydrogen generation was increased with the increase of Ni content in PtNix NPs, and reached to the maximum of

2.9 mmol/h (Ni/Pt ¼ 0.5, QE ¼ 51.24%). Decreasing of hydrogen generation when loading of cocatalyst lower than Ni/Pt molar ratio 0.5, which indicated that low number of interface between PteNi NPs and CdS. As compared with the previously reported value [47e49], the PtNix/CdS photocatalysts possessed higher activity and better application value. These results revealed that Ni/Pt ratio is a crucial factor to enhance the photocatalytic H2 generation activity. Fig. 7 shows the photocatalytic stability of PtNi0.5/CdS for H2 production under visible light irradiation for long run. The same photocatalyst was used for five repeating cycles, in prior to conduct next cycle, the catalyst was thoroughly washed with ultrapure water and dried, and added into 50 ml 1 M (NH4)2SO3 aqueous solution. Then the reaction solution was purged with N2 for 30 min. It was observed that the hydrogen production rate for first cycle reached maximum of 15.04 mmol. After 5 h of the first cycle, the H2 production amount was slightly decreased, it might due to the

CdS Pt/CdS PtNi0.5/CdS

1.2

1.0

Absorbance

16

15

Time / h

0.8

0.6

0.4

0.2

0.0 200

300

400

500

600

700

800

Wavelength / nm Fig. 8 e UVeVis DRS of CdS, Pt-s/CdS and PtNi0.5/CdS.

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CdS Pt/CdS PtNi0.5/CdS

1400 1200

Counts

1000 800 600 400 200 0

8.0k

10.0k

12.0k

14.0k

16.0k

Time / ns Fig. 9 e Fluorescence lifetime of CdS, Pt-s/CdS and PtNi0.5/ CdS photocatalyst.

Table 1 e Results of double-exponential fitting to the fluorescence of CdS, Pt-s/CdS and PtNi0.5/CdS. Sample

t1 (ns)

a1 (%)

t2 (ns)

a2 (%)

tave (ns)

CdS Pt-s/CdS PtNi0.5/CdS

1026.34 1002.23 1052.3

63.59 59.30 57.38

7428.98 7726.52 7969.31

36.41 40.70 42.62

6184.41 6657.68 6925.26

photocorrosion of CdS. After first cycle, the photocatalytic hydrogen generation activity stabilized at about 13.52 mmol, which was 89.89% of original value (15.04 mmol). Besides, no significant change in the activity was observed for long run. These results indicated that PtNi0.5/CdS photocatalyst possessed good stability and could be regarded as a promising photocatalyst for practical hydrogen production.

Absorption and electrochemical properties The UVeVis DRS of CdS, Pt-s/CdS and PtNi0.5/CdS photocatalysts were characterized to understand the role of PtNi0.5 alloy and the visible-light harvesting ability. As shown in Fig. 8, the band gap of all the samples are 2.3 eV based on the

absorption edge of about 540 nm, suggesting PtNi0.5 NPs only modified on the surface of CdS instead of doping in its lattice [50]. Additionally, the PtNi0.5/CdS had obvious enhanced absorption within visible-light region due to the light-absorption of PtNi0.5 NPs. It is believed that the lifetime of a semiconductor photocatalyst is related to the lifetime of photo-generated carriers. The lifetime of CdS, Pt/CdS and PtNi0.5/CdS photocatalyst were displayed in Fig. 9. It could be observed that all of the lifetime curves were fitted well by a double exponential function, and the curve of PtNi0.5/CdS exhibited a right shift along with time compared with alone CdS, indicating that modifying PtNi0.5 NPs onto CdS prolonged the lifetime of photocatalyst. Moreover, the average lifetime value could be calculated via a function (tave¼ (a1t21 þ a2t22)/(a1t1 þ a2t2) [51]. From the Table 1, we could find that the PtNi0.5/CdS showed longer lifetime than that of bare CdS. These results indicated that PtNi0.5 NPs loaded on the surface of CdS effectively facilitated the separation of photo-induced electrons and holes, thus improved the photocatalytic hydrogen evolution activity [52]. To investigate the effect of PtNi0.5 HNPs on enhancing the photocatalytic hydrogen generation, the overpotential of hydrogen production of CdS and PtNi0.5/CdS were performed by a liner sweep voltammetry (LSV) method. From Fig. 10a, it could be clearly seen that the current densities of PtNi0.5/CdS were distinctly prompted compared with bare CdS, indicated that PtNi0.5 NPs could effectively improve the photocatalytic hydrogen evolution activity [53]. Furthermore, the Tafel plots (the insert figure in Fig. 10a) of PtNi0.5/CdS photocatalyst (ca. 0.48 V) exhibit lower overpotential than that of pure CdS (ca. 0.60 V), suggested that hydrogen evolution reaction is easier to occur over PtNi0.5/CdS than bare CdS. In addition, electrochemical impedance spectroscopy (EIS) was conducted to evaluate the charge-carrier transfer ability of CdS and PtNi0.5/ CdS. As shown in Fig. 10b, PtNi0.5/CdS exhibited smaller semicircular diameter than that of bare CdS, implied that PtNi0.5/CdS possessed a faster charge-carrier transfer rate, which was beneficial to enhance the hydrogen production rate [54e56]. Overall, the PtNix NPs modified on the surface of CdS enhances the photocatalysts visible-light harvesting ability, reduces the over-potential of hydrogen evolution and facilitates the separation of photo-generated charge carriers, and thus improves the hydrogen evolution performance.

Fig. 10 e (a) LSV curves and (b) EIS spectra of electrodes that consisted of CdS and PtNi0.5/CdS.

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 2 8 1 0 4 e2 8 1 1 2

Conclusion [13]

In summary, hollow PtNix nanoparticles were prepared by a galvanic displacement method, and were modified on the surface of CdS sheet for photocatalytic hydrogen production reaction. The PtNi0.5/CdS photocatalyst exhibited highest rate of H2 generation (2.9 mmol/h), and was higher than that of situ-photodeposition of Pt/CdS (0.57 mmol/h). We suggest that the hollow nature of PtNi0.5 alloy, and the electronic coupling effect between Pt and Ni, and the sheet morphology of CdS are beneficial to enhance the H2 generation activity. This work reduces the usage of Pt and improves the hydrogen production activity, which would be of directive significance for the design of cocatalysts.

[14]

[15]

[16]

[17]

Acknowledgments This work was financially supported by National Natural Science Foundation of China (No.51602091).

[18]

[19]

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