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Enhanced visible light activated hydrogen evolution activity over cadmium sulfide nanorods by the synergetic effect of a thin carbon layer and noble metal-free nickel phosphide cocatalyst
Journal of Colloid and Interface Science 525 (2018) 107–114
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Regular Article
Enhanced visible light activated hydrogen evolution activity over cadmium sulfide nanorods by the synergetic effect of a thin carbon layer and noble metal-free nickel phosphide cocatalyst Tengfei Wu, Peifang Wang ⇑, Yanhui Ao ⇑, Chao Wang Key Laboratory of Integrated Regulation and Resource Development on Shallow Lakes, Ministry of Education, College of Environment, Hohai University, No.1, Xikang road, Nanjing 210098, China
g r a p h i c a l a b s t r a c t Cheap and nontoxic carbon and nickel phosphide were used to modification the CdS nonorods. The synthesized CdS@C-Ni2P shows excellent hydrogen evolution activity and stability.
a r t i c l e
i n f o
Article history: Received 13 February 2018 Revised 14 April 2018 Accepted 17 April 2018 Available online 18 April 2018 Keywords: Water splitting Visible light Ni2P CdS Carbon layer
a b s t r a c t Photocatalytic water splitting is considered to be a promising strategy for addressing the global energy crisis through the expanded use of solar energy. Herein, cadmium sulfide (CdS) nanorods modified with a thin conductive carbon layer and a nickel phosphide co-catalyst, referred to as cadmium sulfide coated with a carbon layer and nickel phosphide (CdS@C/Ni2P, where @ indicates a core–shell structure), were synthesized and applied as a novel composite photocatalyst for water splitting. The optimized CdS@C/ Ni2P composite showed a high photocatalytic hydrogen generation rate of 32030 lmol h 1 g 1, which was approximately 19 times as high as that of pure CdS. We believed that the thin carbon layer acted as an electron acceptor to promote charge transfer and protect the CdS nanorods from photocorrosion. In addition, the surface loading of the nickel phosphide (Ni2P) cocatalyst was able to further draw photogenerated electrons from the cadmium sulfide coated with a carbon layer (CdS@C) heterojunction and provide active sites for hydrogen evolution. Thus, greatly enhanced hydrogen generation was achieved through a combination of carbon coating and surface cocatalyst loading. This development provides a new way to design composite photocatalysts with multiple junctions for efficient water splitting performance. Ó 2018 Published by Elsevier Inc.
⇑ Corresponding authors. E-mail addresses: [email protected] (P. Wang), [email protected] (Y. Ao). https://doi.org/10.1016/j.jcis.2018.04.068 0021-9797/Ó 2018 Published by Elsevier Inc.
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1. Introduction
2. Experimental
The depletion of fossil resources and the deepening crisis of environmental pollution from their consumption have motivated considerable effort to develop sustainable technologies [1–6]. Converting solar energy into hydrogen fuel is considered to be a promising way to generate clean and renewable energy [7– 14]. In particular, solar-driven photocatalytic water splitting has received considerable attention owing to the high product purity generated by this simple process [15–17]. To date, many different semiconductor based photocatalysts have been developed and studied for solar-driven hydrogen generation [18–20]. The design of inexpensive photocatalysts with good activity and stability is considered to be essential to the conversion process. Visible light driven cadmium sulfide is regarded as a promising material for photocatalytic hydrogen evolution among various photocatalysts because of its superior characteristics, such as a suitable band gap (2.4 eV) and appropriate potential conduction band [21–23]. Unfortunately, bare CdS is not yet ready for practical applications owing to the relatively low efficiency of electronhole separation and a tendency to undergo photocorrosion under irradiation [24,25]. A thin layer coated on the host photocatalyst can often facilitate the separation of light-induced electron-hole pairs. For example, Tang et al. showed that the intimate coaxial interfacial contact between a MoS2 shell and CdS core facilitated the transfer of photoexcited electrons to MoS2 [26]. As a promising material widely applied in the energy conversion field, carbon can also serve as a coating layer owing to its unique features, such as high conductivity, low-toxicity, and good chemical stability [27– 29]. Recently, it was reported that carbon coated cuprous oxide nanorods showed enhanced photocatalytic performance, attributed to the carbon layer facilitating charge transfer [30]. Considering the remarkable properties of carbonaceous materials and the limitations of CdS photocatalytic systems, coating CdS with a thin carbon layer might be a suitable strategy for constructing efficient photocatalytic systems. Active sites play an important role in the process of photocatalytic hydrogen production. Currently, noble-metal-based cocatalysts have been widely applied in photocatalytic systems to provide active sites for catalytic hydrogen evolution [31–33]. However, noble metals are rare and expensive. Therefore, there is a need to find noble-metal free co-catalysts with high efficiency. Of non-precious earth-abundant co-catalysts, transition metal phosphides have attracted considerable attention and have been confirmed to be suitable co-catalysts which can provide active sites to promote the hydrogen evolution of semiconductors [34,35]. For example, Xu et al. reported that Ni2P can act as a highly efficient co-catalyst when loaded on the surface of carbon nitride [36]. In order to combine all the advantages mentioned above and improve the photocatalytic performance of CdS, we designed and synthesized a highly efficient ternary photocatalyst system in which a carbon layer and Ni2P nanopaticles were used to modify CdS nanorods. The obtained hydrogen production yield reached 32030 lmol h 1 g 1, which was 19 times as high as that of pristine CdS. The carbon layer coated on the CdS nanorods featured high conductivity and good chemical stability, providing electron transport pathways and maintaining the stability of host photocatalyst. Furthermore, surface modification with Ni2P nanoparticles further improved the charge carrier separation and provided an abundance of active sites. Thus, excellent photocatalytic performance and stability were realized from the well-designed CdS@C/Ni2P semiconductor composites.
2.1. Materials All chemicals were purchased and used without further purification. Cadmium chloride (CdCl22.5H2O, Sinopharm Chemical Reagent Co., Ltd, AR, 99.0%), thiourea (NH2CSNH2, Sinopharm Chemical Reagent Co., Ltd, AR, 99.0%), ethylenediamine (C2H8N2, Sinopharm Chemical Reagent Co., Ltd, AR, 99.0%), ascorbic acid (C6H8O6, Sinopharm Chemical Reagent Co., Ltd, AR, 99.7%), nickel nitrate hexahydrate [Ni(NO3)6H2O, Sinopharm Chemical Reagent Co., Ltd, AR, 98.0%], sodium hydroxide (NaOH, Sinopharm Chemical Reagent Co., Ltd, AR, 96.0%), sodium hypophosphite (NaH2PO2, Aladdin Industrial Corporation, AR, 99.0%), sodium sulfide (Na2S, Aladdin Industrial Corporation, AR, 99.99%), sodium sulfide (Na2SO3, Sinopharm Chemical Reagent Co., Ltd, AR, 97.0%), sodium sulfate (Na2SO4, Sinopharm Chemical Reagent Co., Ltd, AR, 97.0%) were used as provided. 2.2. Preparation of CdS nanorods CdS nanorods were prepared by a reported method [37]. 9.26 g of CdCl22.5H2O and 9.26 g of thiourea in 120 mL ethylenediamine were added to a 200 mL Teflon-lined autoclave, which was maintained at 160 °C for 36 h. The yellow precipitates were collected and washed with absolute ethanol and deionized water several times. The obtained products were dried at 60 °C for 8 h in a vacuum oven. 2.3. Synthesis of CdS@C composites CdS@C heterostructure was prepared through a hydrothermal method. A 0.1-g portion of CdS nanorods were added into a mixture of 30 mL ultrapure water and 30 mL ethanol. After stirring for 1 h, 0.01 g of ascorbic acid was added into the solution. After stirring for another 1 h, the above solution was transferred to a 100 mL Teflon-lined stainless-steel autoclave and treated at 200 °C for 2 h. After cooling to room temperature, the sample was collected, washed several times and dried at 60 °C for 8 h. 2.4. Preparation of Ni2P nanoparticles Ni2P nanoparticles were synthesized according to a reported method [38]. First, an excess amount of NaOH solution (0.5 M) was added dropwise into 100 mL of an aqueous solution containing Ni(NO3)26H2O (200 mg) and sodium citrate (50 mg) under vigorous stirring. After stirring for 30 min, the precipitates were collected by centrifugation and dried at 80 °C overnight in vacuum oven. Then, the Ni(OH)2 and NaH2PO2 were mixed and ground in a mortar with a mass ratio of 1:5. The mixture was placed in a tube furnace and annealed at 300 °C for 1 h under an argon atmosphere (the ramp rate is 2 °C/min). The so-formed black solid was ground into powder, washed with deionized water and ethanol, and then dried in a vacuum oven overnight. 2.5. Preparation of CdS@C/Ni2P composites A certain amount of the prepared Ni2P composites was added into 30 mL of ethanol and it was exposed to ultrasonic for 30 min to obtain a suspension. Then, 0.2 g of the CdS@C composites was added into the Ni2P dispersion under stirring. After stirring for 1 h, the resulting suspension was transferred to a rotary evaporator to remove most of the ethanol before the products were dried in vacuum oven. We used the same method to obtain four samples
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with different weight ratios of Ni2P. And the obtained samples are referred as CdS@C/Ni2P-1%, CdS@C/Ni2P-3%, CdS@C/Ni2P-5%, and CdS@C/Ni2P-8%. 2.6. Characterization The crystal structure measurements and phase identification of the prepared samples were performed with a Shimadazu XD-3A X-ray diffractometer with Cu Ka radiation. The microstructures were characterized with a transmission electron microscope (JEOL JEM-200CX, Japan). The chemical compositions of the synthesized sample were determined by X-ray photoelectron spectroscopy (XPS, PHI 5000 VersaProbe). To explore the optical properties of the as-prepared samples, a UV–vis spectrophotometer (Shimadzu UV-3600) with an integrating sphere was used. Photoluminescence (PL) spectra were measured with a fluorescence spectrophotometer (Hitachi F-700). 2.7. Electrochemical measurements Electrochemical measurements were performed on an electrochemical analyzer (CHI660D) in a standard three-electrode cell made of quartz, with a platinum plate as the counter electrode and standard Ag/AgCl electrode as the reference electrode. The working electrodes were made by the following procedure: catalyst power (10 mg) was dispersed into 0.2 mL of ethanol by ultrasonication for at least 30 min. This mixture was then drop-coated on the surface of a precleaned fluorine-doped tin oxide (FTO) substrate and dried in an oven at 150 °C for 2 h. The electrolyte was a 0.1 M Na2SO4 aqueous solution. The photocurrent response was measured at 0 V under irradiation from a 300 W Xe lamp. 2.8. Photocatalytic performance tests The photocatalytic hydrogen reduction experiments were performed in a 150-mL Pyrex reaction cell connected to a closed gas circulation and evacuation system. In a typical photocatalytic experiment, 5 mg of photocatalyst power suspended in a 50 mL solution containing 0.35 M sodium sulfite and 0.25 M sodium sulfide. This system was irradiated by a 300-W Xe lamp (CELHXF300, Beijing China Education Au-light Co., Ltd) equipped with a 400 nm cutoff filter. The gas evolved from water splitting was measured with an online gas chromatograph (GC-7900, TCD) with N2 as a carrier gas after 1 h of illumination. The reaction was continued for 5 h. Triplicate measurements were conducted for each sample, and the mean values are presented. 3. Results and discussion Typical X-ray Diffraction (XRD) patterns recorded from CdS@C/ Ni2P composites with different Ni2P contents along with CdS and CdS@C are shown in Fig. 1. The diffraction patterns of the pure CdS could be assigned to well-crystallized hexagonal wurtzite CdS (PDF# 65-3414) [39,40]. The main peaks centered at 24.8°, 26.5°, 28.2°, and 43.8° corresponded to the (1 0 0), (0 0 2), (1 0 1), and (1 1 0) planes, respectively. The CdS@C exhibited an almost identical XRD pattern to that of CdS, likely because of the amorphous nature of the carbon layer. In pure Ni2P, all the peaks could be indexed to the phase of Ni2P (PDF#65-3544) [38]. No obvious characteristic diffraction peaks for Ni2P were detected in the CdS@C/Ni2P composites owing to its low amount and weak diffraction intensity. The data clearly indicated that modification with the carbon layer and Ni2P had no effect on the crystalline phase of CdS. Microstructural analyses of the as-prepared samples were performed by transmission electron microscopy (TEM). A typical low
Fig. 1. Powder XRD pattern of the as-prepared CdS, CdS@C, Ni2P and CdS@C/Ni2P composites with different content of Ni2P.
magnification image shows that the pure CdS had a nanorod structure (Fig. 2a). As shown in Fig. 2b, the CdS@C composite almost maintained the same shape as that of the pure CdS except for the sharp edges smooth. High resolution TEM images of CdS@C (Fig. 2c) revealed that an amorphous layer was decorated on the surface of CdS, as marked by red lines. The result is apparent different with the pure CdS without carbon layer (inset of Fig. 2(a)). The carbon layer could act as an electron acceptor during the photocatalytic process. Fig. 2d shows that some Ni2P nanoparticles were randomly anchored on the surface of CdS@C. The introduction of the Ni2P co-catalyst would improve the hydrogen evolution rate by lowering the over potential and further promoting the separation of charge carriers. High-resolution X-ray photoelectron spectroscopy (XPS) was used to explore the surface chemical composition and valence states of the 5% CdS@C/Ni2P, as shown in Fig. 3a–e. Two sharp and symmetrical peaks located at 405.2 and 411.9 eV appeared in the Cd 3d XPS spectrum (Fig. 3a), which corresponded to signals from Cd 3d2/5 and Cd 3d3/2, respectively [41]. As depicted in Fig. 3b, the S 2p spectrum showed two peaks at 161.5 and 162.7 eV, which showed S was present in the S2 state [42]. In Fig. 3c, the strong peak at 284.8 eV in the C 1 s XPS spectrum originated from CAC bonds and revealed the formation of a carbon layer in the CdS@C/Ni2P composite. The other two small peaks located at 286.6 and 288.7 eV belonged to CAOAH and C@O, respectively, which originated from oxygen functional groups that were adsorbed on the carbon layer. Fig. 3d showed the spectrum of Ni 2p with three peaks located at 852.9, 856.1, and 873.8 eV, which were attributed to the signals of Ni (d+), Ni 2p3/2, and Ni 2p1/2 of Ni2+, respectively [43]. The peak appearing at 862.1 eV corresponded to the satellite of Ni 2p3/2 [36]. The highresolution spectrum of P 2p (Fig. 3e) was resolved into two peaks located at 129.5 and 133.3 eV, which can be assigned to the P 2p1/2 and P 2p3/2 in Ni2P [36,44]. The photocatalytic activity of the CdS@C/Ni2P composites with different Ni2P contents, together with those of CdS and CdS@C, were evaluated under Xe lamp irradiation (k 400 nm) (Fig. 4a). Without any modification, the CdS nanorods showed a relatively low hydrogen evolution rate (1668 lmol h 1 g 1). Because the carbon layer acted as a photo-generated electron acceptor, the carbon-coated CdS featured an enhanced hydrogen generation rate of 2826 lmol h 1 g 1, which was 1.7 times as high as that of pristine CdS nanorods. When the Ni2P was introduced to the CdS@C
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Fig. 2. (a) TEM images of pure CdS nanorods (inset: HRTEM images of CdS); (b) TEM images of CdS@C composite; HRTEM images of (c) CdS@C and (d) CdS@C/Ni2P.
composite, all the CdS@C/Ni2P composites showed enhanced photocatalytic activity, indicating that the Ni2P had a great effect on the rate of hydrogen production. The loading of 5% Ni2P helped the CdS@C to achieve the optimal photocatalytic hydrogen evolution rate of 32030 lmol h 1 g 1, which was 19 times as high as that of pure CdS. When the amounts of Ni2P were further increased, the CdS@C/Ni2P-8% composite featured a lower photocatalytic hydrogen evolution rate. There are two reasons contributed to the above phenomenon, one is the excess Ni2P nanoparticles shield the CdS nanorods from light and the other is the excess Ni2P could also become the recombination center. Table S1 lists the comparison of photocatalytic activity of hybrid CdS photocatalysts, indicating that CdS@C/Ni2P is efficient for photocatalytic hydrogen production. To verify the stability of the CdS@C/Ni2P-5%, repeated hydrogen production experiments were performed under the same conditions for four cycles, with each test conducted for 5 h. As shown in Fig. 4b, the photocatalytic hydrogen evolution rate of CdS@C/ Ni2P-5% showed almost no change in the repeated experiments, with a approximately 8% performance drop compared with that of first cycle. These results indicated that the synthesized CdS@C/ Ni2P heterojunction was robust and stable in the photocatalytic hydrogen production. It is generally acknowledged that the highly active solar-driven photocatalytic systems depend on a high light-harvesting capacity, efficient charge separation, timely transfer of charges to the surface for attendance of redox reactions, and an efficient co-catalyst with a low overpotential to enhance the kinetics of hydrogen generation [35,45,46]. Therefore, the optical properties, recombination process of photogenerated electrons and holes and photogenerated charge-carrier transfer were studied by analyzing the UV/Vis diffuse reflectance spectra (DRS), photoluminescence (PL) spectra and photocurrent responses, respectively. These results are helpful to understand the differences in the photocatalytic performance of bare CdS, CdS@C and the CdS@C/Ni2P composites. As shown in
Fig. 5(a), pure CdS nanorod showed a significant absorption edge at approximately 525 nm. The introduction of the carbon layer had little effect on the optical absorption likely because the carbon layer coated on the CdS was relatively thin. The band gap energy (Eg) values for CdS, CdS@C, CdS@C/Ni2P-5% were evaluated by the following formula [47]: (ahv)1/2 = hv Eg, where a, h, v, and Eg represent the absorption coefficient, Planck’s constant, the light frequency, and the band gap energy, respectively. As shown in Fig. 5(b), the Eg values of CdS, CdS@C, CdS@C/Ni2P-5% were 2.37, 2.36 and 2.33 eV, respectively. These results indicated that coating of the CdS nanorods by carbon and the Ni2P co-catalyst had no apparent effect on the band gap of CdS. However, the CdS@C/ Ni2P nanocomposites display significantly increased absorption ability over the entire wavelength range as Ni2P loading was increased. The effective absorption of sun-light was attributed to the high optical absorbance capacity of Ni2P nanoparticles. The PL emission spectra, shown in Fig. 6, were used to analyze the separation efficiency of the photoexcited charge carriers. The CdS@C nanocomposites showed a relatively low PL intensity, which might be attributed to the high conductivity of the carbon layer. After loading with the Ni2P nanoparticles, the PL intensity of the 5% CdS@C/Ni2P was further reduced, which indicated that the Ni2P co-catalyst could help to separate photogenerated carriers. In addition, it can be seen that the emission peak shifted to lower wavelength when the CdS was coated by a carbon layer. Then the peak shifted to more lower wavelength after the further coating of Ni2P nanoparticles. The difference wavelength in PL emission peaks could be mainly ascribed to the interaction between the CdS nanorods, carbon layer and Ni2P [48]. We confirmed that our CdS@C/Ni2P structure could limit the recombination of charge carriers, which is a key factor affecting photocatalytic hydrogen evolution in water splitting. Furthermore, the transient photocurrent responses of the pure CdS, CdS@C, and CdS@C/Ni2P-5% were used to explore the transfer and separation of charge carriers in the photocatalytic process
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Fig. 3. XPS spectra of (a) Cd 3d, (b) S 2p, (c) C 1s, (d) Ni 2p, and (e) P 2p, respectively.
under the on–off cycles (Fig. 7). The CdS@C/Ni2P-5% showed a remarkably higher photocurrent than that of the bare CdS and CdS@C, indicating that the combining of a carbon layer and Ni2P co-catalyst contributed to the efficient separation of electronhole pairs. Considering the above discussions, we are convinced that efficient separation of photocatalytic electron hole pairs could be achieved in the obtained CdS@C/Ni2P photocatalytic system. The increase in the number of electrons taking part in the redox reaction contributed to an increase of the photocatalytic hydrogen evolution rate. On the basis of the above results, we propose a possible reaction mechanism for photocatalytic hydrogen generation over the CdS@C/Ni2P system as presented in the Fig. 8. When the CdS@C/
Ni2P composite was exposed to visible light, the CdS nanorod were excited, that is, electrons generated in the valence band of CdS jumped to the conduction band, leaving holes in the valence band. The photogenerated electrons quickly transferred to the carbon layer due to its high conductivity and intimate contact with the CdS component. In addition, electrons could accumulate on the Ni2P nanoparticles, which acted as a co-catalyst which can reduce H2O to produce hydrogen. Meanwhile, most of the holes leaving were consumed by sacrificial regents. Alternatively, it is also possible that if the Ni2P nanoparticles were in direct contact with the uncoated CdS, photoelectrons in the conduction band of CdS might directly transfer to the Ni2P and then participate in hydrogen production.
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Fig. 4. (a) Comparison of photocatalytic hydrogen rates based on pure CdS, CdS@C and CdS@C/Ni2P with different contents of Ni2P; (b) recycling tests of CdS@C/Ni2P-5% composite.
Fig. 5. (a) The ultraviolet–visible diffuse reflectance spectra of CdS, CdS@C and CdS@C/Ni2P composites with different contents; (b) band gap determination of CdS, CdS@C and CdS@C/Ni2P-5%.
Fig. 6. Photoluminescence spectra of the as-prepared samples: CdS, CdS@C and CdS@C/Ni2P-5%.
Fig. 7. Transient photocurrent response of the pure CdS, CdS@C, and CdS@C/Ni2P5% under visible light irradiation.
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Fig. 8. Schematic illustration of charge separation and transfer over the CdS@C/Ni2P composites under visible light irradiation.
4. Conclusions
References
In summary, we successfully constructed a new noble metal free CdS@C/Ni2P photocatalyst for hydrogen production through water splitting. The ternary composites exhibited much higher photocatalytic performance for hydrogen generation than those of CdS. The photocatalytic hydrogen evolution rate of the optimized CdS@C/Ni2P composite was as high as 32030 lmol h 1 g 1. Based on the discussion in previous chapters, the improvement of the photocatalytic activity was also higher compared with other previously reported CdS-based composite photocatalysts. The high photocatalytic activity of the CdS@C/Ni2P composite was attributed to the following reasons. First, the intimate contact between CdS and the carbon layer assisted the charge separation and led to better photocatalytic performance. Second, the surface modification with the Ni2P co-catalyst can act as electron acceptor to further facilitate charge transfer and provide active sites for hydrogen evolution, leading to more effective use of electrons for photocatalytic hydrogen evolution. Finally, we believe that this work could offer a new strategy for developing composite photocatalysts with high performance through the combination of two modification strategies: carbon coating and cocatalyst deposition.
[1] A. Kudo, Y. Miseki, Heterogeneous photocatalyst materials for water splitting, Chem. Soc. Rev. 38 (2009) 253–278. [2] X. Zou, Y. Zhang, Noble metal-free hydrogen evolution catalysts for water splitting, Chem. Soc. Rev. 44 (2015) 5148–5180. [3] J. Liu, Y. Liu, N. Liu, Y. Han, X. Zhang, H. Huang, et al., Metal-free efficient photocatalyst for stable visible water splitting via a two-electron pathway, Science 347 (2015) 970–974. [4] J.P. Zou, L.C. Wang, J.M. Luo, Y.C. Nie, Q.J. Xing, X.B. Luo, et al., Synthesis and efficient visible light photocatalytic H2 evolution of a metal-free gC3N4/graphene quantum dots hybrid photocatalyst, Appl. Catal. B 193 (2016) 103–109. [5] S.J. Moniz, S.A. Shevlin, D.J. Martin, Z.X. Guo, J. Tang, Visible-light driven heterojunction photocatalysts for water splitting–a critical review, Energy Environ. Sci. 8 (2015) 731–759. [6] Y.P. Yuan, L.W. Ruan, J. Barber, S.C.J. Loo, C. Xue, Hetero-nanostructured suspended photocatalysts for solar-to-fuel conversion, Energy Environ. Sci. 7 (2014) 3934–3951. [7] H.G. Yu, X. Huang, P. Wang, J.G. Yu, Enhanced photoinduced-stability and photocatalytic activity of CdS by dual amorphous cocatalysts: synergistic effect of Ti(IV)-hole cocatalyst and Ni(II)-electron cocatalyst, J. Phys. Chem. C 120 (2016) 3722–3730. [8] G. Zhou, M.F. Wu, Q.J. Xing, F. Li, H. Liu, X.B. Luo, et al., Synthesis and characterizations of metal-free semiconductor/MOFs with good stability and high photocatalytic activity for H2 evolution: a novel Z-scheme heterostructured photocatalyst formed by covalent bonds, Appl. Catal. B 220 (2018) 607–614. [9] P. Wang, Y. Sheng, F.Z. Wang, H.G. Yu, Synergistic effect of electron-transfer mediator and interfacial catalytic active-site for the enhanced H2-evolution performance: A case study of CdS-Au photocatalyst, Appl. Catal. B 220 (2018) 561–569. [10] T. Hisatomi, J. Kubota, K. Domen, Recent advances in semiconductors for photocatalytic and photoelectrochemical water splitting, Chem. Soc. Rev. 43 (2014) 7520–7535. [11] S. Cao, J. Yu, Carbon-based H2-production photocatalytic materials, J. Photochem. Photobiol., C 27 (2016) 72–99. [12] J. Yuan, J. Wen, Y. Zhong, X. Li, Y. Fang, S. Zhang, et al., Enhanced photocatalytic H2 evolution over noble-metal-free NiS cocatalyst modified CdS nanorods/gC3N4 heterojunctions, J. Mater. Chem. A 3 (2015) 18244–18255. [13] W. Li, F. Chang, S. Dai, J. Yue, F. Hua, H. Hao, Fabrication of sulfur-doped gC3N4/Au/CdS Z-scheme photocatalyst to improve the photocatalytic performance under visible light, Appl. Catal. B 168–169 (2015) 465–471. [14] Z. Yan, X. Yu, A. Han, P. Xu, P. Du, Noble-metal-free Ni(OH)2-modified CdS/ reduced graphene oxide nanocomposite with enhanced photocatalytic activity for hydrogen production under visible light irradiation, J. Phys. Chem. C 118 (2014) 22896–22903. [15] J. Li, G. Zhan, Y. Yu, L. Zhang, Superior visible light hydrogen evolution of Janus bilayer junctions via atomic-level charge flow steering, Nat. Commun. 7 (2016) 11480.
Acknowledgements: We are grateful for grants from National Science Funds for Creative Research Groups of China (No. 51421006), Natural Science Foundation of China (51679063), the Key Program of National Natural Science Foundation of China (No. 91647206), the National Science Foundation of China for Excellent Young Scholars (No. 51422902), the National Key Plan for Research and Development of China (2016YFC0502203), Fundamental Research Funds (No. 2016B43814), and PAPD. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at https://doi.org/10.1016/j.jcis.2018.04.068.
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[16] J. Chen, X.J. Wu, Y. Gong, Y. Zhu, Z. Yang, B. Li, et al., Edge epitaxy of twodimensional MoSe2 and MoS2 nanosheets on one-dimensional nanowires, J. Am. Chem. Soc. 139 (2017) 8653–8660. [17] D. Yue, X. Qian, M. Kan, M. Ren, Y. Zhu, L. Jiang, et al., Sulfurated [NiFe]-based layered double hydroxides nanoparticles as efficient co-catalysts for photocatalytic hydrogen evolution using CdTe/CdS quantum dots, Appl. Catal. B 209 (2017) 155–160. [18] X. Li, J. Yu, J. Low, Y. Fang, J. Xiao, X. Chen, Engineering heterogeneous semiconductors for solar water splitting, J. Mater. Chem. A 3 (2015) 2485– 2534. [19] K. Zhang, L. Guo, Metal sulphide semiconductors for photocatalytic hydrogen production, Catal. Sci. Technol. 3 (2013) 1672–1690. [20] X. Chen, S. Shen, L. Guo, S.S. Mao, Semiconductor-based photocatalytic hydrogen generation, Chem. Rev. 110 (2010) 6503–6570. [21] N.N. Chai, H.X. Wang, C.X. Hu, Q. Wang, H.L. Zhang, Well-controlled layer-bylayer assembly of carbon dot/CdS heterojunctions for efficient visible-lightdriven photocatalysis, J. Mater. Chem. A 3 (2015) 16613–16620. [22] W. Jiang, Y. Liu, R. Zong, Z. Li, W. Yao, Y. Zhu, Photocatalytic hydrogen generation on bifunctional ternary heterostructured In2S3/MoS2/CdS composite with high activity and stability under visible light irradiation, J. Mater. Chem. A 3 (2015) 18406–18412. [23] D.A. Reddy, H. Park, R. Ma, D.P. Kumar, M. Lim, T.K. Kim, Heterostructured WS2-MoS2 ultrathin nanosheets integrated on CdS nanorods for promoting charge separation and migration to improve solar-driven photocatalytic hydrogen evolution, Chemsuschem 10 (2017) 1563–1570. [24] J. Chen, X.J. Wu, L. Yin, B. Li, X. Hong, Z. Fan, et al., One-pot synthesis of CdS nanocrystals hybridized with single-layer transition-metal dichalcogenide nanosheets for efficient photocatalytic hydrogen evolution, Angew. Chem. Int. Ed. 54 (2015) 1210–1214. [25] Z. Wang, J. Peng, X. Feng, Z. Ding, Z. Li, Wide spectrum responsive CdS/NiTiO3/ CoS with superior photocatalytic performance for hydrogen evolution, Catal. Sci. Technol. 7 (2017) 2524–2530. [26] B. Han, S. Liu, N. Zhang, Y.J. Xu, Z.R. Tang, One-dimensional CdS@MoS2 coreshell nanowires for boosted photocatalytic hydrogen evolution under visible light, Appl. Catal. B 202 (2017) 298–304. [27] C. Yang, X. Zhang, J. Qin, X. Shen, R. Yu, M. Ma, et al., Porous carbon-doped TiO2 on TiC nanostructures for enhanced photocatalytic hydrogen production under visible light, J. Catal. 347 (2017) 36–44. [28] G. Fu, P. Zhou, M. Zhao, W. Zhu, S. Yan, T. Yu, et al., Carbon coating stabilized Ti (3+)-doped TiO2 for photocatalytic hydrogen generation under visible light irradiation, Dalton Trans. 44 (2015) 12812–12817. [29] S. Han, Y.C. Pu, L. Zheng, L. Hu, J.Z. Zhang, X. Fang, Uniform carbon-coated CdS core–shell nanostructures: synthesis, ultrafast charge carrier dynamics, and photoelectrochemical water splitting, J. Mater. Chem. A 4 (2016) 1078–1086. [30] L. Yu, G. Li, X. Zhang, X. Ba, G. Shi, Y. Li, et al., Enhanced activity and stability of carbon-decorated cuprous oxide mesoporous nanorods for CO2 reduction in artificial photosynthesis, ACS Catal. 6 (2016) 6444–6454. [31] L. Ma, K. Chen, F. Nan, J.H. Wang, D.J. Yang, L. Zhou, et al., Improved hydrogen production of Au–Pt–CdS hetero-nanostructures by efficient plasmon-induced multipathway electron transfer, Adv. Funct. Mater. 26 (2016) 6076–6083. [32] M. Luo, W. Yao, C. Huang, Q. Wu, Q. Xu, Shape effects of Pt nanoparticles on hydrogen production via Pt/CdS photocatalysts under visible light, J. Mater. Chem. A 3 (2015) 13884–13891.
[33] S. Naskar, F. Lübkemann, S. Hamid, A. Freytag, A. Wolf, J. Koch, et al., Synthesis of ternary and quaternary Au and Pt decorated CdSe/CdS heteronanoplatelets with controllable morphology, Adv. Funct. Mater. 27 (2017) 1604685. [34] Y. Xu, R. Xu, Nickel-based cocatalysts for photocatalytic hydrogen production, Appl. Surf. Sci. 351 (2015) 779–793. [35] J. Ran, J. Zhang, J. Yu, M. Jaroniec, S.Z. Qiao, Earth-abundant cocatalysts for semiconductor-based photocatalytic water splitting, Chem. Soc. Rev. 43 (2014) 7787–7812. [36] H. Zhao, S. Sun, P. Jiang, Z.J. Xu, Graphitic C3N4 modified by Ni2P cocatalyst: An efficient, robust and low cost photocatalyst for visible-light-driven H2 evolution from water, Chem. Eng. J. 315 (2017) 296–303. [37] J.S. Jang, U.A. Joshi, J.S. Lee, Solvothermal synthesis of CdS nanowires for photocatalytic hydrogen and electricity production, J. Phys. Chem. C 111 (2007) 13280–13287. [38] S. Cao, Y. Chen, C.J. Wang, X.J. Lv, W.F. Fu, Spectacular photocatalytic hydrogen evolution using metal-phosphide/CdS hybrid catalysts under sunlight irradiation, Chem. Commun. 51 (2015) 8708–8711. [39] X. Li, J. Zhu, H. Li, Comparative study on the mechanism in photocatalytic degradation of different-type organic dyes on SnS2 and CdS, Appl. Catal. B: Environ. s123–124 (2012) 174–181. [40] Y. Hu, X. Gao, L. Yu, Y. Wang, J. Ning, S. Xu, et al., Carbon-coated CdS petalous nanostructures with enhanced photostability and photocatalytic activity, Angew. Chem. 125 (2013) 5746–5749. [41] Y. Huo, X. Yang, J. Zhu, H. Li, Highly active and stable CdS–TiO2 visible photocatalyst prepared by in situ sulfurization under supercritical conditions, Appl. Catal. B 106 (2011) 69–75. [42] X.F. Ning, W.L. Zhen, Y.Q. Wu, G.X. Lu, Inhibition of CdS photocorrosion by Al2O3 shell for highly stable photocatalytic overall water splitting under visible light irradiation, Appl. Catal. B 226 (2018) 373–383. [43] Z. Yan, Z. Sun, X. Liu, H. Jia, P. Du, Cadmium sulfide/graphitic carbon nitride heterostructure nanowire loading with a nickel hydroxide cocatalyst for highly efficient photocatalytic hydrogen production in water under visible light, Nanoscale 8 (2016) 4748–4756. [44] W.Z. Gao, Y. Xu, Y. Chen, W.F. Fu, Highly efficient and selective photocatalytic reduction of nitroarenes using the Ni2P/CdS catalyst under visible-light irradiation, Chem. Commun. 51 (2015) 13217–13220. [45] Q. Xiang, F. Cheng, D. Lang, Hierarchical layered WS2/graphene-modified CdS nanorods for efficient photocatalytic hydrogen evolution, Chemsuschem 9 (2016) 996–1002. [46] Y.X. Pan, J.B. Peng, S. Xin, Y. You, Y.L. Men, F. Zhang, et al., Enhanced visiblelight-driven photocatalytic H2 evolution from water on noble-metal-free CdSnanoparticle-dispersed Mo2C@C nanospheres, Acs Sustain. Chem. Eng. 5 (2017) 5449–5456. [47] J.L. Yuan, J.Q. Wen, Q.Z. Gao, S.C. Chen, J.M. Li, X. Li, et al., Amorphous Co3O4 modified CdS nanorods with enhanced visible-light photocatalytic H2production activity, Dalton Trans. 44 (2015) 1680–1689. [48] W.L. Zhen, X.F. Ning, B.J. Yang, Y.Q. Wu, L. Zhen, G.X. Lu, The enhancement of CdS photocatalytic activity for water splitting via anti-photocorrosion by coating Ni2P shell and removing nascent formed oxygen with artificial gill, Appl. Catal. B 221 (2018) 243–257.
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Journal of Colloid and Interface Science journal homepage: www.elsevier.com/locate/jcis
Regular Article
Enhanced visible light activated hydrogen evolution activity over cadmium sulfide nanorods by the synergetic effect of a thin carbon layer and noble metal-free nickel phosphide cocatalyst Tengfei Wu, Peifang Wang ⇑, Yanhui Ao ⇑, Chao Wang Key Laboratory of Integrated Regulation and Resource Development on Shallow Lakes, Ministry of Education, College of Environment, Hohai University, No.1, Xikang road, Nanjing 210098, China
g r a p h i c a l a b s t r a c t Cheap and nontoxic carbon and nickel phosphide were used to modification the CdS nonorods. The synthesized CdS@C-Ni2P shows excellent hydrogen evolution activity and stability.
a r t i c l e
i n f o
Article history: Received 13 February 2018 Revised 14 April 2018 Accepted 17 April 2018 Available online 18 April 2018 Keywords: Water splitting Visible light Ni2P CdS Carbon layer
a b s t r a c t Photocatalytic water splitting is considered to be a promising strategy for addressing the global energy crisis through the expanded use of solar energy. Herein, cadmium sulfide (CdS) nanorods modified with a thin conductive carbon layer and a nickel phosphide co-catalyst, referred to as cadmium sulfide coated with a carbon layer and nickel phosphide (CdS@C/Ni2P, where @ indicates a core–shell structure), were synthesized and applied as a novel composite photocatalyst for water splitting. The optimized CdS@C/ Ni2P composite showed a high photocatalytic hydrogen generation rate of 32030 lmol h 1 g 1, which was approximately 19 times as high as that of pure CdS. We believed that the thin carbon layer acted as an electron acceptor to promote charge transfer and protect the CdS nanorods from photocorrosion. In addition, the surface loading of the nickel phosphide (Ni2P) cocatalyst was able to further draw photogenerated electrons from the cadmium sulfide coated with a carbon layer (CdS@C) heterojunction and provide active sites for hydrogen evolution. Thus, greatly enhanced hydrogen generation was achieved through a combination of carbon coating and surface cocatalyst loading. This development provides a new way to design composite photocatalysts with multiple junctions for efficient water splitting performance. Ó 2018 Published by Elsevier Inc.
⇑ Corresponding authors. E-mail addresses: [email protected] (P. Wang), [email protected] (Y. Ao). https://doi.org/10.1016/j.jcis.2018.04.068 0021-9797/Ó 2018 Published by Elsevier Inc.
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1. Introduction
2. Experimental
The depletion of fossil resources and the deepening crisis of environmental pollution from their consumption have motivated considerable effort to develop sustainable technologies [1–6]. Converting solar energy into hydrogen fuel is considered to be a promising way to generate clean and renewable energy [7– 14]. In particular, solar-driven photocatalytic water splitting has received considerable attention owing to the high product purity generated by this simple process [15–17]. To date, many different semiconductor based photocatalysts have been developed and studied for solar-driven hydrogen generation [18–20]. The design of inexpensive photocatalysts with good activity and stability is considered to be essential to the conversion process. Visible light driven cadmium sulfide is regarded as a promising material for photocatalytic hydrogen evolution among various photocatalysts because of its superior characteristics, such as a suitable band gap (2.4 eV) and appropriate potential conduction band [21–23]. Unfortunately, bare CdS is not yet ready for practical applications owing to the relatively low efficiency of electronhole separation and a tendency to undergo photocorrosion under irradiation [24,25]. A thin layer coated on the host photocatalyst can often facilitate the separation of light-induced electron-hole pairs. For example, Tang et al. showed that the intimate coaxial interfacial contact between a MoS2 shell and CdS core facilitated the transfer of photoexcited electrons to MoS2 [26]. As a promising material widely applied in the energy conversion field, carbon can also serve as a coating layer owing to its unique features, such as high conductivity, low-toxicity, and good chemical stability [27– 29]. Recently, it was reported that carbon coated cuprous oxide nanorods showed enhanced photocatalytic performance, attributed to the carbon layer facilitating charge transfer [30]. Considering the remarkable properties of carbonaceous materials and the limitations of CdS photocatalytic systems, coating CdS with a thin carbon layer might be a suitable strategy for constructing efficient photocatalytic systems. Active sites play an important role in the process of photocatalytic hydrogen production. Currently, noble-metal-based cocatalysts have been widely applied in photocatalytic systems to provide active sites for catalytic hydrogen evolution [31–33]. However, noble metals are rare and expensive. Therefore, there is a need to find noble-metal free co-catalysts with high efficiency. Of non-precious earth-abundant co-catalysts, transition metal phosphides have attracted considerable attention and have been confirmed to be suitable co-catalysts which can provide active sites to promote the hydrogen evolution of semiconductors [34,35]. For example, Xu et al. reported that Ni2P can act as a highly efficient co-catalyst when loaded on the surface of carbon nitride [36]. In order to combine all the advantages mentioned above and improve the photocatalytic performance of CdS, we designed and synthesized a highly efficient ternary photocatalyst system in which a carbon layer and Ni2P nanopaticles were used to modify CdS nanorods. The obtained hydrogen production yield reached 32030 lmol h 1 g 1, which was 19 times as high as that of pristine CdS. The carbon layer coated on the CdS nanorods featured high conductivity and good chemical stability, providing electron transport pathways and maintaining the stability of host photocatalyst. Furthermore, surface modification with Ni2P nanoparticles further improved the charge carrier separation and provided an abundance of active sites. Thus, excellent photocatalytic performance and stability were realized from the well-designed CdS@C/Ni2P semiconductor composites.
2.1. Materials All chemicals were purchased and used without further purification. Cadmium chloride (CdCl22.5H2O, Sinopharm Chemical Reagent Co., Ltd, AR, 99.0%), thiourea (NH2CSNH2, Sinopharm Chemical Reagent Co., Ltd, AR, 99.0%), ethylenediamine (C2H8N2, Sinopharm Chemical Reagent Co., Ltd, AR, 99.0%), ascorbic acid (C6H8O6, Sinopharm Chemical Reagent Co., Ltd, AR, 99.7%), nickel nitrate hexahydrate [Ni(NO3)6H2O, Sinopharm Chemical Reagent Co., Ltd, AR, 98.0%], sodium hydroxide (NaOH, Sinopharm Chemical Reagent Co., Ltd, AR, 96.0%), sodium hypophosphite (NaH2PO2, Aladdin Industrial Corporation, AR, 99.0%), sodium sulfide (Na2S, Aladdin Industrial Corporation, AR, 99.99%), sodium sulfide (Na2SO3, Sinopharm Chemical Reagent Co., Ltd, AR, 97.0%), sodium sulfate (Na2SO4, Sinopharm Chemical Reagent Co., Ltd, AR, 97.0%) were used as provided. 2.2. Preparation of CdS nanorods CdS nanorods were prepared by a reported method [37]. 9.26 g of CdCl22.5H2O and 9.26 g of thiourea in 120 mL ethylenediamine were added to a 200 mL Teflon-lined autoclave, which was maintained at 160 °C for 36 h. The yellow precipitates were collected and washed with absolute ethanol and deionized water several times. The obtained products were dried at 60 °C for 8 h in a vacuum oven. 2.3. Synthesis of CdS@C composites CdS@C heterostructure was prepared through a hydrothermal method. A 0.1-g portion of CdS nanorods were added into a mixture of 30 mL ultrapure water and 30 mL ethanol. After stirring for 1 h, 0.01 g of ascorbic acid was added into the solution. After stirring for another 1 h, the above solution was transferred to a 100 mL Teflon-lined stainless-steel autoclave and treated at 200 °C for 2 h. After cooling to room temperature, the sample was collected, washed several times and dried at 60 °C for 8 h. 2.4. Preparation of Ni2P nanoparticles Ni2P nanoparticles were synthesized according to a reported method [38]. First, an excess amount of NaOH solution (0.5 M) was added dropwise into 100 mL of an aqueous solution containing Ni(NO3)26H2O (200 mg) and sodium citrate (50 mg) under vigorous stirring. After stirring for 30 min, the precipitates were collected by centrifugation and dried at 80 °C overnight in vacuum oven. Then, the Ni(OH)2 and NaH2PO2 were mixed and ground in a mortar with a mass ratio of 1:5. The mixture was placed in a tube furnace and annealed at 300 °C for 1 h under an argon atmosphere (the ramp rate is 2 °C/min). The so-formed black solid was ground into powder, washed with deionized water and ethanol, and then dried in a vacuum oven overnight. 2.5. Preparation of CdS@C/Ni2P composites A certain amount of the prepared Ni2P composites was added into 30 mL of ethanol and it was exposed to ultrasonic for 30 min to obtain a suspension. Then, 0.2 g of the CdS@C composites was added into the Ni2P dispersion under stirring. After stirring for 1 h, the resulting suspension was transferred to a rotary evaporator to remove most of the ethanol before the products were dried in vacuum oven. We used the same method to obtain four samples
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with different weight ratios of Ni2P. And the obtained samples are referred as CdS@C/Ni2P-1%, CdS@C/Ni2P-3%, CdS@C/Ni2P-5%, and CdS@C/Ni2P-8%. 2.6. Characterization The crystal structure measurements and phase identification of the prepared samples were performed with a Shimadazu XD-3A X-ray diffractometer with Cu Ka radiation. The microstructures were characterized with a transmission electron microscope (JEOL JEM-200CX, Japan). The chemical compositions of the synthesized sample were determined by X-ray photoelectron spectroscopy (XPS, PHI 5000 VersaProbe). To explore the optical properties of the as-prepared samples, a UV–vis spectrophotometer (Shimadzu UV-3600) with an integrating sphere was used. Photoluminescence (PL) spectra were measured with a fluorescence spectrophotometer (Hitachi F-700). 2.7. Electrochemical measurements Electrochemical measurements were performed on an electrochemical analyzer (CHI660D) in a standard three-electrode cell made of quartz, with a platinum plate as the counter electrode and standard Ag/AgCl electrode as the reference electrode. The working electrodes were made by the following procedure: catalyst power (10 mg) was dispersed into 0.2 mL of ethanol by ultrasonication for at least 30 min. This mixture was then drop-coated on the surface of a precleaned fluorine-doped tin oxide (FTO) substrate and dried in an oven at 150 °C for 2 h. The electrolyte was a 0.1 M Na2SO4 aqueous solution. The photocurrent response was measured at 0 V under irradiation from a 300 W Xe lamp. 2.8. Photocatalytic performance tests The photocatalytic hydrogen reduction experiments were performed in a 150-mL Pyrex reaction cell connected to a closed gas circulation and evacuation system. In a typical photocatalytic experiment, 5 mg of photocatalyst power suspended in a 50 mL solution containing 0.35 M sodium sulfite and 0.25 M sodium sulfide. This system was irradiated by a 300-W Xe lamp (CELHXF300, Beijing China Education Au-light Co., Ltd) equipped with a 400 nm cutoff filter. The gas evolved from water splitting was measured with an online gas chromatograph (GC-7900, TCD) with N2 as a carrier gas after 1 h of illumination. The reaction was continued for 5 h. Triplicate measurements were conducted for each sample, and the mean values are presented. 3. Results and discussion Typical X-ray Diffraction (XRD) patterns recorded from CdS@C/ Ni2P composites with different Ni2P contents along with CdS and CdS@C are shown in Fig. 1. The diffraction patterns of the pure CdS could be assigned to well-crystallized hexagonal wurtzite CdS (PDF# 65-3414) [39,40]. The main peaks centered at 24.8°, 26.5°, 28.2°, and 43.8° corresponded to the (1 0 0), (0 0 2), (1 0 1), and (1 1 0) planes, respectively. The CdS@C exhibited an almost identical XRD pattern to that of CdS, likely because of the amorphous nature of the carbon layer. In pure Ni2P, all the peaks could be indexed to the phase of Ni2P (PDF#65-3544) [38]. No obvious characteristic diffraction peaks for Ni2P were detected in the CdS@C/Ni2P composites owing to its low amount and weak diffraction intensity. The data clearly indicated that modification with the carbon layer and Ni2P had no effect on the crystalline phase of CdS. Microstructural analyses of the as-prepared samples were performed by transmission electron microscopy (TEM). A typical low
Fig. 1. Powder XRD pattern of the as-prepared CdS, CdS@C, Ni2P and CdS@C/Ni2P composites with different content of Ni2P.
magnification image shows that the pure CdS had a nanorod structure (Fig. 2a). As shown in Fig. 2b, the CdS@C composite almost maintained the same shape as that of the pure CdS except for the sharp edges smooth. High resolution TEM images of CdS@C (Fig. 2c) revealed that an amorphous layer was decorated on the surface of CdS, as marked by red lines. The result is apparent different with the pure CdS without carbon layer (inset of Fig. 2(a)). The carbon layer could act as an electron acceptor during the photocatalytic process. Fig. 2d shows that some Ni2P nanoparticles were randomly anchored on the surface of CdS@C. The introduction of the Ni2P co-catalyst would improve the hydrogen evolution rate by lowering the over potential and further promoting the separation of charge carriers. High-resolution X-ray photoelectron spectroscopy (XPS) was used to explore the surface chemical composition and valence states of the 5% CdS@C/Ni2P, as shown in Fig. 3a–e. Two sharp and symmetrical peaks located at 405.2 and 411.9 eV appeared in the Cd 3d XPS spectrum (Fig. 3a), which corresponded to signals from Cd 3d2/5 and Cd 3d3/2, respectively [41]. As depicted in Fig. 3b, the S 2p spectrum showed two peaks at 161.5 and 162.7 eV, which showed S was present in the S2 state [42]. In Fig. 3c, the strong peak at 284.8 eV in the C 1 s XPS spectrum originated from CAC bonds and revealed the formation of a carbon layer in the CdS@C/Ni2P composite. The other two small peaks located at 286.6 and 288.7 eV belonged to CAOAH and C@O, respectively, which originated from oxygen functional groups that were adsorbed on the carbon layer. Fig. 3d showed the spectrum of Ni 2p with three peaks located at 852.9, 856.1, and 873.8 eV, which were attributed to the signals of Ni (d+), Ni 2p3/2, and Ni 2p1/2 of Ni2+, respectively [43]. The peak appearing at 862.1 eV corresponded to the satellite of Ni 2p3/2 [36]. The highresolution spectrum of P 2p (Fig. 3e) was resolved into two peaks located at 129.5 and 133.3 eV, which can be assigned to the P 2p1/2 and P 2p3/2 in Ni2P [36,44]. The photocatalytic activity of the CdS@C/Ni2P composites with different Ni2P contents, together with those of CdS and CdS@C, were evaluated under Xe lamp irradiation (k 400 nm) (Fig. 4a). Without any modification, the CdS nanorods showed a relatively low hydrogen evolution rate (1668 lmol h 1 g 1). Because the carbon layer acted as a photo-generated electron acceptor, the carbon-coated CdS featured an enhanced hydrogen generation rate of 2826 lmol h 1 g 1, which was 1.7 times as high as that of pristine CdS nanorods. When the Ni2P was introduced to the CdS@C
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Fig. 2. (a) TEM images of pure CdS nanorods (inset: HRTEM images of CdS); (b) TEM images of CdS@C composite; HRTEM images of (c) CdS@C and (d) CdS@C/Ni2P.
composite, all the CdS@C/Ni2P composites showed enhanced photocatalytic activity, indicating that the Ni2P had a great effect on the rate of hydrogen production. The loading of 5% Ni2P helped the CdS@C to achieve the optimal photocatalytic hydrogen evolution rate of 32030 lmol h 1 g 1, which was 19 times as high as that of pure CdS. When the amounts of Ni2P were further increased, the CdS@C/Ni2P-8% composite featured a lower photocatalytic hydrogen evolution rate. There are two reasons contributed to the above phenomenon, one is the excess Ni2P nanoparticles shield the CdS nanorods from light and the other is the excess Ni2P could also become the recombination center. Table S1 lists the comparison of photocatalytic activity of hybrid CdS photocatalysts, indicating that CdS@C/Ni2P is efficient for photocatalytic hydrogen production. To verify the stability of the CdS@C/Ni2P-5%, repeated hydrogen production experiments were performed under the same conditions for four cycles, with each test conducted for 5 h. As shown in Fig. 4b, the photocatalytic hydrogen evolution rate of CdS@C/ Ni2P-5% showed almost no change in the repeated experiments, with a approximately 8% performance drop compared with that of first cycle. These results indicated that the synthesized CdS@C/ Ni2P heterojunction was robust and stable in the photocatalytic hydrogen production. It is generally acknowledged that the highly active solar-driven photocatalytic systems depend on a high light-harvesting capacity, efficient charge separation, timely transfer of charges to the surface for attendance of redox reactions, and an efficient co-catalyst with a low overpotential to enhance the kinetics of hydrogen generation [35,45,46]. Therefore, the optical properties, recombination process of photogenerated electrons and holes and photogenerated charge-carrier transfer were studied by analyzing the UV/Vis diffuse reflectance spectra (DRS), photoluminescence (PL) spectra and photocurrent responses, respectively. These results are helpful to understand the differences in the photocatalytic performance of bare CdS, CdS@C and the CdS@C/Ni2P composites. As shown in
Fig. 5(a), pure CdS nanorod showed a significant absorption edge at approximately 525 nm. The introduction of the carbon layer had little effect on the optical absorption likely because the carbon layer coated on the CdS was relatively thin. The band gap energy (Eg) values for CdS, CdS@C, CdS@C/Ni2P-5% were evaluated by the following formula [47]: (ahv)1/2 = hv Eg, where a, h, v, and Eg represent the absorption coefficient, Planck’s constant, the light frequency, and the band gap energy, respectively. As shown in Fig. 5(b), the Eg values of CdS, CdS@C, CdS@C/Ni2P-5% were 2.37, 2.36 and 2.33 eV, respectively. These results indicated that coating of the CdS nanorods by carbon and the Ni2P co-catalyst had no apparent effect on the band gap of CdS. However, the CdS@C/ Ni2P nanocomposites display significantly increased absorption ability over the entire wavelength range as Ni2P loading was increased. The effective absorption of sun-light was attributed to the high optical absorbance capacity of Ni2P nanoparticles. The PL emission spectra, shown in Fig. 6, were used to analyze the separation efficiency of the photoexcited charge carriers. The CdS@C nanocomposites showed a relatively low PL intensity, which might be attributed to the high conductivity of the carbon layer. After loading with the Ni2P nanoparticles, the PL intensity of the 5% CdS@C/Ni2P was further reduced, which indicated that the Ni2P co-catalyst could help to separate photogenerated carriers. In addition, it can be seen that the emission peak shifted to lower wavelength when the CdS was coated by a carbon layer. Then the peak shifted to more lower wavelength after the further coating of Ni2P nanoparticles. The difference wavelength in PL emission peaks could be mainly ascribed to the interaction between the CdS nanorods, carbon layer and Ni2P [48]. We confirmed that our CdS@C/Ni2P structure could limit the recombination of charge carriers, which is a key factor affecting photocatalytic hydrogen evolution in water splitting. Furthermore, the transient photocurrent responses of the pure CdS, CdS@C, and CdS@C/Ni2P-5% were used to explore the transfer and separation of charge carriers in the photocatalytic process
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Fig. 3. XPS spectra of (a) Cd 3d, (b) S 2p, (c) C 1s, (d) Ni 2p, and (e) P 2p, respectively.
under the on–off cycles (Fig. 7). The CdS@C/Ni2P-5% showed a remarkably higher photocurrent than that of the bare CdS and CdS@C, indicating that the combining of a carbon layer and Ni2P co-catalyst contributed to the efficient separation of electronhole pairs. Considering the above discussions, we are convinced that efficient separation of photocatalytic electron hole pairs could be achieved in the obtained CdS@C/Ni2P photocatalytic system. The increase in the number of electrons taking part in the redox reaction contributed to an increase of the photocatalytic hydrogen evolution rate. On the basis of the above results, we propose a possible reaction mechanism for photocatalytic hydrogen generation over the CdS@C/Ni2P system as presented in the Fig. 8. When the CdS@C/
Ni2P composite was exposed to visible light, the CdS nanorod were excited, that is, electrons generated in the valence band of CdS jumped to the conduction band, leaving holes in the valence band. The photogenerated electrons quickly transferred to the carbon layer due to its high conductivity and intimate contact with the CdS component. In addition, electrons could accumulate on the Ni2P nanoparticles, which acted as a co-catalyst which can reduce H2O to produce hydrogen. Meanwhile, most of the holes leaving were consumed by sacrificial regents. Alternatively, it is also possible that if the Ni2P nanoparticles were in direct contact with the uncoated CdS, photoelectrons in the conduction band of CdS might directly transfer to the Ni2P and then participate in hydrogen production.
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Fig. 4. (a) Comparison of photocatalytic hydrogen rates based on pure CdS, CdS@C and CdS@C/Ni2P with different contents of Ni2P; (b) recycling tests of CdS@C/Ni2P-5% composite.
Fig. 5. (a) The ultraviolet–visible diffuse reflectance spectra of CdS, CdS@C and CdS@C/Ni2P composites with different contents; (b) band gap determination of CdS, CdS@C and CdS@C/Ni2P-5%.
Fig. 6. Photoluminescence spectra of the as-prepared samples: CdS, CdS@C and CdS@C/Ni2P-5%.
Fig. 7. Transient photocurrent response of the pure CdS, CdS@C, and CdS@C/Ni2P5% under visible light irradiation.
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Fig. 8. Schematic illustration of charge separation and transfer over the CdS@C/Ni2P composites under visible light irradiation.
4. Conclusions
References
In summary, we successfully constructed a new noble metal free CdS@C/Ni2P photocatalyst for hydrogen production through water splitting. The ternary composites exhibited much higher photocatalytic performance for hydrogen generation than those of CdS. The photocatalytic hydrogen evolution rate of the optimized CdS@C/Ni2P composite was as high as 32030 lmol h 1 g 1. Based on the discussion in previous chapters, the improvement of the photocatalytic activity was also higher compared with other previously reported CdS-based composite photocatalysts. The high photocatalytic activity of the CdS@C/Ni2P composite was attributed to the following reasons. First, the intimate contact between CdS and the carbon layer assisted the charge separation and led to better photocatalytic performance. Second, the surface modification with the Ni2P co-catalyst can act as electron acceptor to further facilitate charge transfer and provide active sites for hydrogen evolution, leading to more effective use of electrons for photocatalytic hydrogen evolution. Finally, we believe that this work could offer a new strategy for developing composite photocatalysts with high performance through the combination of two modification strategies: carbon coating and cocatalyst deposition.
[1] A. Kudo, Y. Miseki, Heterogeneous photocatalyst materials for water splitting, Chem. Soc. Rev. 38 (2009) 253–278. [2] X. Zou, Y. Zhang, Noble metal-free hydrogen evolution catalysts for water splitting, Chem. Soc. Rev. 44 (2015) 5148–5180. [3] J. Liu, Y. Liu, N. Liu, Y. Han, X. Zhang, H. Huang, et al., Metal-free efficient photocatalyst for stable visible water splitting via a two-electron pathway, Science 347 (2015) 970–974. [4] J.P. Zou, L.C. Wang, J.M. Luo, Y.C. Nie, Q.J. Xing, X.B. Luo, et al., Synthesis and efficient visible light photocatalytic H2 evolution of a metal-free gC3N4/graphene quantum dots hybrid photocatalyst, Appl. Catal. B 193 (2016) 103–109. [5] S.J. Moniz, S.A. Shevlin, D.J. Martin, Z.X. Guo, J. Tang, Visible-light driven heterojunction photocatalysts for water splitting–a critical review, Energy Environ. Sci. 8 (2015) 731–759. [6] Y.P. Yuan, L.W. Ruan, J. Barber, S.C.J. Loo, C. Xue, Hetero-nanostructured suspended photocatalysts for solar-to-fuel conversion, Energy Environ. Sci. 7 (2014) 3934–3951. [7] H.G. Yu, X. Huang, P. Wang, J.G. Yu, Enhanced photoinduced-stability and photocatalytic activity of CdS by dual amorphous cocatalysts: synergistic effect of Ti(IV)-hole cocatalyst and Ni(II)-electron cocatalyst, J. Phys. Chem. C 120 (2016) 3722–3730. [8] G. Zhou, M.F. Wu, Q.J. Xing, F. Li, H. Liu, X.B. Luo, et al., Synthesis and characterizations of metal-free semiconductor/MOFs with good stability and high photocatalytic activity for H2 evolution: a novel Z-scheme heterostructured photocatalyst formed by covalent bonds, Appl. Catal. B 220 (2018) 607–614. [9] P. Wang, Y. Sheng, F.Z. Wang, H.G. Yu, Synergistic effect of electron-transfer mediator and interfacial catalytic active-site for the enhanced H2-evolution performance: A case study of CdS-Au photocatalyst, Appl. Catal. B 220 (2018) 561–569. [10] T. Hisatomi, J. Kubota, K. Domen, Recent advances in semiconductors for photocatalytic and photoelectrochemical water splitting, Chem. Soc. Rev. 43 (2014) 7520–7535. [11] S. Cao, J. Yu, Carbon-based H2-production photocatalytic materials, J. Photochem. Photobiol., C 27 (2016) 72–99. [12] J. Yuan, J. Wen, Y. Zhong, X. Li, Y. Fang, S. Zhang, et al., Enhanced photocatalytic H2 evolution over noble-metal-free NiS cocatalyst modified CdS nanorods/gC3N4 heterojunctions, J. Mater. Chem. A 3 (2015) 18244–18255. [13] W. Li, F. Chang, S. Dai, J. Yue, F. Hua, H. Hao, Fabrication of sulfur-doped gC3N4/Au/CdS Z-scheme photocatalyst to improve the photocatalytic performance under visible light, Appl. Catal. B 168–169 (2015) 465–471. [14] Z. Yan, X. Yu, A. Han, P. Xu, P. Du, Noble-metal-free Ni(OH)2-modified CdS/ reduced graphene oxide nanocomposite with enhanced photocatalytic activity for hydrogen production under visible light irradiation, J. Phys. Chem. C 118 (2014) 22896–22903. [15] J. Li, G. Zhan, Y. Yu, L. Zhang, Superior visible light hydrogen evolution of Janus bilayer junctions via atomic-level charge flow steering, Nat. Commun. 7 (2016) 11480.
Acknowledgements: We are grateful for grants from National Science Funds for Creative Research Groups of China (No. 51421006), Natural Science Foundation of China (51679063), the Key Program of National Natural Science Foundation of China (No. 91647206), the National Science Foundation of China for Excellent Young Scholars (No. 51422902), the National Key Plan for Research and Development of China (2016YFC0502203), Fundamental Research Funds (No. 2016B43814), and PAPD. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at https://doi.org/10.1016/j.jcis.2018.04.068.
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[16] J. Chen, X.J. Wu, Y. Gong, Y. Zhu, Z. Yang, B. Li, et al., Edge epitaxy of twodimensional MoSe2 and MoS2 nanosheets on one-dimensional nanowires, J. Am. Chem. Soc. 139 (2017) 8653–8660. [17] D. Yue, X. Qian, M. Kan, M. Ren, Y. Zhu, L. Jiang, et al., Sulfurated [NiFe]-based layered double hydroxides nanoparticles as efficient co-catalysts for photocatalytic hydrogen evolution using CdTe/CdS quantum dots, Appl. Catal. B 209 (2017) 155–160. [18] X. Li, J. Yu, J. Low, Y. Fang, J. Xiao, X. Chen, Engineering heterogeneous semiconductors for solar water splitting, J. Mater. Chem. A 3 (2015) 2485– 2534. [19] K. Zhang, L. Guo, Metal sulphide semiconductors for photocatalytic hydrogen production, Catal. Sci. Technol. 3 (2013) 1672–1690. [20] X. Chen, S. Shen, L. Guo, S.S. Mao, Semiconductor-based photocatalytic hydrogen generation, Chem. Rev. 110 (2010) 6503–6570. [21] N.N. Chai, H.X. Wang, C.X. Hu, Q. Wang, H.L. Zhang, Well-controlled layer-bylayer assembly of carbon dot/CdS heterojunctions for efficient visible-lightdriven photocatalysis, J. Mater. Chem. A 3 (2015) 16613–16620. [22] W. Jiang, Y. Liu, R. Zong, Z. Li, W. Yao, Y. Zhu, Photocatalytic hydrogen generation on bifunctional ternary heterostructured In2S3/MoS2/CdS composite with high activity and stability under visible light irradiation, J. Mater. Chem. A 3 (2015) 18406–18412. [23] D.A. Reddy, H. Park, R. Ma, D.P. Kumar, M. Lim, T.K. Kim, Heterostructured WS2-MoS2 ultrathin nanosheets integrated on CdS nanorods for promoting charge separation and migration to improve solar-driven photocatalytic hydrogen evolution, Chemsuschem 10 (2017) 1563–1570. [24] J. Chen, X.J. Wu, L. Yin, B. Li, X. Hong, Z. Fan, et al., One-pot synthesis of CdS nanocrystals hybridized with single-layer transition-metal dichalcogenide nanosheets for efficient photocatalytic hydrogen evolution, Angew. Chem. Int. Ed. 54 (2015) 1210–1214. [25] Z. Wang, J. Peng, X. Feng, Z. Ding, Z. Li, Wide spectrum responsive CdS/NiTiO3/ CoS with superior photocatalytic performance for hydrogen evolution, Catal. Sci. Technol. 7 (2017) 2524–2530. [26] B. Han, S. Liu, N. Zhang, Y.J. Xu, Z.R. Tang, One-dimensional CdS@MoS2 coreshell nanowires for boosted photocatalytic hydrogen evolution under visible light, Appl. Catal. B 202 (2017) 298–304. [27] C. Yang, X. Zhang, J. Qin, X. Shen, R. Yu, M. Ma, et al., Porous carbon-doped TiO2 on TiC nanostructures for enhanced photocatalytic hydrogen production under visible light, J. Catal. 347 (2017) 36–44. [28] G. Fu, P. Zhou, M. Zhao, W. Zhu, S. Yan, T. Yu, et al., Carbon coating stabilized Ti (3+)-doped TiO2 for photocatalytic hydrogen generation under visible light irradiation, Dalton Trans. 44 (2015) 12812–12817. [29] S. Han, Y.C. Pu, L. Zheng, L. Hu, J.Z. Zhang, X. Fang, Uniform carbon-coated CdS core–shell nanostructures: synthesis, ultrafast charge carrier dynamics, and photoelectrochemical water splitting, J. Mater. Chem. A 4 (2016) 1078–1086. [30] L. Yu, G. Li, X. Zhang, X. Ba, G. Shi, Y. Li, et al., Enhanced activity and stability of carbon-decorated cuprous oxide mesoporous nanorods for CO2 reduction in artificial photosynthesis, ACS Catal. 6 (2016) 6444–6454. [31] L. Ma, K. Chen, F. Nan, J.H. Wang, D.J. Yang, L. Zhou, et al., Improved hydrogen production of Au–Pt–CdS hetero-nanostructures by efficient plasmon-induced multipathway electron transfer, Adv. Funct. Mater. 26 (2016) 6076–6083. [32] M. Luo, W. Yao, C. Huang, Q. Wu, Q. Xu, Shape effects of Pt nanoparticles on hydrogen production via Pt/CdS photocatalysts under visible light, J. Mater. Chem. A 3 (2015) 13884–13891.
[33] S. Naskar, F. Lübkemann, S. Hamid, A. Freytag, A. Wolf, J. Koch, et al., Synthesis of ternary and quaternary Au and Pt decorated CdSe/CdS heteronanoplatelets with controllable morphology, Adv. Funct. Mater. 27 (2017) 1604685. [34] Y. Xu, R. Xu, Nickel-based cocatalysts for photocatalytic hydrogen production, Appl. Surf. Sci. 351 (2015) 779–793. [35] J. Ran, J. Zhang, J. Yu, M. Jaroniec, S.Z. Qiao, Earth-abundant cocatalysts for semiconductor-based photocatalytic water splitting, Chem. Soc. Rev. 43 (2014) 7787–7812. [36] H. Zhao, S. Sun, P. Jiang, Z.J. Xu, Graphitic C3N4 modified by Ni2P cocatalyst: An efficient, robust and low cost photocatalyst for visible-light-driven H2 evolution from water, Chem. Eng. J. 315 (2017) 296–303. [37] J.S. Jang, U.A. Joshi, J.S. Lee, Solvothermal synthesis of CdS nanowires for photocatalytic hydrogen and electricity production, J. Phys. Chem. C 111 (2007) 13280–13287. [38] S. Cao, Y. Chen, C.J. Wang, X.J. Lv, W.F. Fu, Spectacular photocatalytic hydrogen evolution using metal-phosphide/CdS hybrid catalysts under sunlight irradiation, Chem. Commun. 51 (2015) 8708–8711. [39] X. Li, J. Zhu, H. Li, Comparative study on the mechanism in photocatalytic degradation of different-type organic dyes on SnS2 and CdS, Appl. Catal. B: Environ. s123–124 (2012) 174–181. [40] Y. Hu, X. Gao, L. Yu, Y. Wang, J. Ning, S. Xu, et al., Carbon-coated CdS petalous nanostructures with enhanced photostability and photocatalytic activity, Angew. Chem. 125 (2013) 5746–5749. [41] Y. Huo, X. Yang, J. Zhu, H. Li, Highly active and stable CdS–TiO2 visible photocatalyst prepared by in situ sulfurization under supercritical conditions, Appl. Catal. B 106 (2011) 69–75. [42] X.F. Ning, W.L. Zhen, Y.Q. Wu, G.X. Lu, Inhibition of CdS photocorrosion by Al2O3 shell for highly stable photocatalytic overall water splitting under visible light irradiation, Appl. Catal. B 226 (2018) 373–383. [43] Z. Yan, Z. Sun, X. Liu, H. Jia, P. Du, Cadmium sulfide/graphitic carbon nitride heterostructure nanowire loading with a nickel hydroxide cocatalyst for highly efficient photocatalytic hydrogen production in water under visible light, Nanoscale 8 (2016) 4748–4756. [44] W.Z. Gao, Y. Xu, Y. Chen, W.F. Fu, Highly efficient and selective photocatalytic reduction of nitroarenes using the Ni2P/CdS catalyst under visible-light irradiation, Chem. Commun. 51 (2015) 13217–13220. [45] Q. Xiang, F. Cheng, D. Lang, Hierarchical layered WS2/graphene-modified CdS nanorods for efficient photocatalytic hydrogen evolution, Chemsuschem 9 (2016) 996–1002. [46] Y.X. Pan, J.B. Peng, S. Xin, Y. You, Y.L. Men, F. Zhang, et al., Enhanced visiblelight-driven photocatalytic H2 evolution from water on noble-metal-free CdSnanoparticle-dispersed Mo2C@C nanospheres, Acs Sustain. Chem. Eng. 5 (2017) 5449–5456. [47] J.L. Yuan, J.Q. Wen, Q.Z. Gao, S.C. Chen, J.M. Li, X. Li, et al., Amorphous Co3O4 modified CdS nanorods with enhanced visible-light photocatalytic H2production activity, Dalton Trans. 44 (2015) 1680–1689. [48] W.L. Zhen, X.F. Ning, B.J. Yang, Y.Q. Wu, L. Zhen, G.X. Lu, The enhancement of CdS photocatalytic activity for water splitting via anti-photocorrosion by coating Ni2P shell and removing nascent formed oxygen with artificial gill, Appl. Catal. B 221 (2018) 243–257.