In-situ synthesis of strongly coupled Co2P-CdS nanohybrids: An

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In-situ synthesis of strongly coupled Co2P-CdS nanohybrids: An effective strategy to regulate photocatalytic hydrogen evolution activity Songsong Li, Lu Wang, Shuang Liu, Boran Xu, Nan Xiao, Yangqin Gao, Weiyu Song, Lei Ge, and Jian Liu ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/acssuschemeng.8b01190 • Publication Date (Web): 02 Jul 2018 Downloaded from http://pubs.acs.org on July 2, 2018

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ACS Sustainable Chemistry & Engineering

In-situ synthesis of strongly coupled Co2P-CdS nanohybrids: An effective strategy to regulate photocatalytic hydrogen evolution activity Songsong Li†,‡, Lu Wang,† Shuang Liu, ‡ Boran Xu, ‡ Nan Xiao, ‡ Yangqin Gao, ‡ Weiyu Song, † Lei Ge,*,†,‡, Jian Liu *,† †State

Key Laboratory of Heavy Oil Processing, College of Science, China University of Petroleum Beijing, No. 18 Fuxue Rd., Beijing 102249, People’s Republic of China.

‡ Department

of Materials Science and Engineering, College of Science, China University of Petroleum Beijing, No. 18 Fuxue Rd., Beijing 102249, People’s Republic of China.

Corresponding authors: Tel/Fax.: +86 1089739096. [email protected] (Jian Liu).

E-mail address: [email protected] (Lei.Ge),

ABSTRACT: Noble-metal-free photocatalyst Co2P-CdS was synthesized via a facile in-situ hydrothermal method for the first time to boost the photocatalytic H2 production performance. Such synthesis process allows the Co2P nanoparticles (NPs) to disperse on the surface of CdS sub-microspheres evenly and form unique intimate contact interfaces. The physical and photophysical properties of as-prepared Co2P-CdS composite samples were characterized by X-ray diffractometry (XRD), transmission electron microscope (TEM), UV-vis diffusion reflectance spectroscopy (DRS), X-ray photoelectron spectroscopy (XPS), photoluminescence (PL) and surface photovoltage spectroscopy (SPV). The results indicate that the photocatalytic H2 evolution activity of CdS sub-microspheres under visible light irradiation is significantly enhanced by introducing inexpensive Co2P as co-catalyst. The Co2P-CdS sample with loading ratio of 1.2 mol% Co2P gives the highest H2-production rate of 0.303 mmol·h-1, which was about 3 times higher than 0.5 wt% Pt loaded CdS sample. After further introduced K2HPO4 as sacrificial agent the H2-production rate value is reached 0.356 mmol·h-1 that is 41 times higher than pure CdS. The apparent quantum yield (AQY) of the Co2P decorated CdS sample is about 13.88% at 420 nm. These achieved results suggest that the synergistic effect between Co2P and CdS greatly enhances the photocatalytic activity of CdS. Moreover, a

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reasonable mechanism on the enhanced photocatalytic activity was proposed through density functional theory (DFT) calculation which was verified by surface photovoltage spectroscopy technique. KEYWORDS: Photocatalysis; Co2P-CdS; hydrogen evolution; CdS sub-microspheres. * Corresponding author at: State Key Laboratory of Heavy Oil Processing, China University of Petroleum Beijing, No. 18 Fuxue Road,

Beijing102249, PR China. Tel/Fax.: +86 1089739096.

E-mail address: [email protected] (Lei.Ge), [email protected] (Jian Liu).

INTRODUCTION Hydrogen (H2) is an environmentally friendly clean energy compared to fossil fuels which have caused serious environmental problems. In addition, semiconductor photocatalyst assisted solar-to-hydrogen conversion is one of the most attractive and sustainable solutions to produce hydrogen fuel from direct decomposition of water.1,2 Various efficient photocatalysts have been reported for water splitting, such as metal oxides, sulfides, phosphides and oxynitride semiconductors.3-6 However, majority of metal oxide semiconductors, such as TiO2,7,8 ZnO,9-10 CeO2 11-13 and WO3,14-15 usually have a wide band gap which limits their solar response to the UV-light range, therefore restricts their photocatalytic water splitting performance. In addition, g-C3N4 has received extensive attention nowadays due to its high specific surface area and non-toxicity characteristic and has achieved abundant achievements. However, the band gap has also limited its performance.16-20 Instead, most sulfides have received tremendous investigation because of the narrow band gaps they have, which is propitious to visible light photocatalytic activity. Among the various sulfide photocatalysts that including binary and ternary metal sulfides, cadmium sulfide (CdS) has a narrow band gap of 2.4 eV and exhibits prominent hydrogen production performance from photocatatlytic water splitting.21-22 However, the narrow band gap results in a much higher recombination rate of photo-excited electrons and holes, which is 2


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deleterious to the photocatalysis efficiency. Therefore, in order to boost the photocatalytic activity of CdS, it is critical to find an effective solution to suppress the high recombination rate of photo-excited charge carriers. Co-catalysts can not only serve as active sites for H2 evolution, but also contribute to accelerate the charge separation of photo-excited charge carriers.23 Benefit from their low Fermi energy levels, noble metals such as Pt,24 Au25 or alloys PtCo26 and AuPd27-29 were used as co-catalysts, which serve as electron sink to promote the photocatalytic H2 generation performance. However, most of these noble metals are expensive rare elements which could not be considered as satisfied co-catalyst candidates from economic perspective. Recently, metal phosphides, such as Ni2P, CoP, FeP and Cu3P, were studied as popular efficient co-catalyst because of their excellent electrocatalytic property as well as low production cost. Sun et. al30 reported a photocatalyst by integrating nickel phosphide co-catalyst with CdS nanorods, which exhibits enhanced photocatalytic durability and highly boosted photocatalytic hydrogen production rate. Chen et. al31 first found that cobalt phosphide (Co2P) can be adopted as a novel co-catalyst for CdS through a simple grinding strategy. Cheng et. al32 had reported a composite photocatalyst consisting of FeP nanoparticles and CdS nanocrystals for photocatayltic H 2 evolution in aqueous lactic acid solution under visible light irradiation. Yue et. al33 had demonstrated a novel hybrid photocatalyst by coupling p-Cu3P with n-TiO2, which shows an accelerated electron-hole pair separation and charge transfer speed with improved photocatalytic H2-evolution activity. Our group had also successfully decorated the CoP on Zn0.5Cd0.5S to achieve high H2 evolution activity via a two-step in-situ chemical deposition method.34 Zeng et. al incorporated 1D metal phosphides (Co2P) into 2D porous g-C3N4 nanosheets via a solution-phase method under ultrasonication leading to unprecedented opportunities on the highly efficient heterojunction photocatalysts for solar-to-H2 conversion.35 Lu and co-workers also accentuated that Co2P was a promising and highly efficient candidate as a cocatalyst for 3



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photocatalytic H2 evolution reaction.36 Nevertheless, according to our knowledge that there were no reports about the Co2P decorated CdS sub-microspheres with high photocatalytic H2 evolution performance through in-situ hydrothermal method. Herein, we have investigated the nano-hybrids of Co2P and CdS to achieve high photocatalytic activity of H2 evolution. After introducing the Co2P on CdS sub-microspheres through in-situ hydrothermal way, the photocatalytic activity of CdS sub-microspheres was significantly enhanced. The results illustrate that 1.2 mol% Co2P-CdS sample has the highest H2 production activity of 0.303 mmol·h-1, and this H2-production rate value is reached 0.356 mmol·h-1 after further introducing K2HPO4 as sacrificial agent, which is 41 times higher than pure CdS under the same conditions, and the corresponding apparent quantum efficiency is about 13.88 % at 420nm. The photocatalytic mechanism of Co2P-CdS composite samples were also investigated and explored with the help of DFT calculation and SPV technique.

EXPERIMENTAL Materials Cadmium acetate (Cd(Ac)2, Aladdin, 99.9%), thiourea (A.R., 99%), cobalt nitrate (Co(NO3)2·6H2O, Aladdin, 99.9%), lactic acid (A.R., 85.0-90%), and red phosphorus (P4, Xiya, 99.99%) were used as received without additional purification or treatment. Synthesis of the hybrid photocatalysts Synthesis of CdS sub-microspheres. In a typical process,37 0.368 g of Cd(Ac)2 and 0.42 g of thiourea were dissolved in 30 ml of de-ionized water and stirred for about 1 hour, after which the above suspension was transferred into a 50 ml autoclave and kept at 180 oC for 12 hours. And then the product was washed by centrifugation with distilled water and ethanol for several times. Finally, the final product was obtained via drying the 4


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washed solution under vacuum at 60 oC for 8 h. After that it was put into an autoclave and heated at 180 o

C for 18 h in order to keep the same condition with the composites of Co2P-CdS.

Preparation of Co2P-CdS nanohybrids via in-situ hydrothermal method. The catalysts were synthesized through in-situ hydrothermal method. Firstly, 3 mmol CdS nanoparticles was dispersed in 35 ml deionized water, after which a certain amount of Co(NO3)2·6H2O was added according to designed molar ratio, and then the mixed solution was ultrasonicated and stirred for about half an hour. An excess of red phosphorous was added into the solution and stirred for another 1 hour, following which the mixed solution was transferred into an autoclave and kept at 180 oC for 18 h. After that, the final product was washed by deionized water and ethanol with centrifugation for three times respectively, and then dried in a vacuum oven at 60 oC for 8 h. By adjusting the volume of Co(NO3)2·6H2O and red phosphorous added into the solution, Co2P-CdS with different amounts of Co2P was obtained. Different nominal molar ratios of Co2P to CdS with R=0, 0.3, 0.5, 0.6, 0.8, 1, 1.2, and 1.4 mol% were prepared and characterized. The samples were named as H-Co2P-CdS. Co2P nanoparticles were also prepared with the same condition except no CdS was added. Preparation of Co2P-CdS composites via grinding method. The Co2P-CdS hybrid photocatalysts were synthesized by grinding a certain amount of CdS with Co2P NPs, and the samples were named as G-Co2P-CdS. Characterization The X-ray diffractometer (XRD) using Cu Kα as radiation was applied to investigate the crystallographic texture of all samples with 2θ ranges from 20° to 75°. Transmission electron microscopy (TEM) and high resolution transmission electron microscopy (HRTEM) were carried out by FEI Tecnai G2 F20 microscope. The morphology of the resulting samples was characterized by a field emission scanning electron microscopy (SEM, FEI Quanta 200F). The UV-vis diffuse reflection 5



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spectroscopy (DRS) of all samples was detected by Shimadzu UV-4100 spectrophotometer with BaSO4 used as reflectance standard. X-ray photoelectron spectroscopy (XPS, PHI 5300 ESCA system) was applied to analyze the element valence state of the samples; the signal of carbon at 284.8 eV was used as a reference to calibrate the binding energy. The PL spectrum was conducted on an F-7000 spectrophotometer with excitation wavelength of 400 nm. Photocatalytic activity The photocatalytic H2 evolution test was performed in Perfectlight Labsolar IIAG system equipped with a 300 ml quartz reactor, the temperature of the system was maintained at 4 oC by connecting to a low-temperature thermostat bath with circulating water. The employed visible light source is PLS-SXE 300UV Xe arc lamp combined with a UV-cutoff (≥400 nm) filter; the light source was positioned 15 cm away from the reactor to trigger the photocatalytic reaction. The irradiation light intensity was set to be 32 mW/cm2 confirmed by a radiometer. In a typical procedure, 50 mg of catalyst was dispersed in 100 ml of mixed aqueous solution which containing 10 ml lactic acid and 90 ml deionized water with a constant ultrasonic for about two minutes before the reactor installed to the system. In addition, 0.25 M and 0.35 M K2HPO4 were added in the above solution to further promote the performance of photocatalyst, respectively. What’s more, before irradiation, the system was evacuated for at least half an hour to remove the dissolved oxygen to ensure an anaerobic condition and magnetically stirred meanwhile. The gas produced during the reaction was detected by a gas chromatography (Beifen 3420A) and used high purity Argon as the carrier gas. The apparent quantum yield (AQY) was measured using 300W Xe-lamp (CEL-HXF300,CEAULIGHT,China) equipped with 420 nm (±5 nm) band pass cut filter. The apparent quantum yield (AQY) was calculated by the Eq. (1).

AQY (%) =

the number of reacted electrons  100 the number of incident photons 6


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=

the number of evolved H 2 molecules  2 100 the number of incident photons

(1)

Density functional theory (DFT) calculation All calculations were performed using DFT within the plane-wave pseudo-potential as implemented in the VASP code with a cutoff energy of 340 eV. The photogenerated electrons usually tend to delocalize in GGA-PBE calculations due to the self-interaction error. To overcome this issue, there are usually two methods: hybrid function and DFT methods. Here, because the hybrid function is quite time-consuming, we employ the DFT method as a major tool. The atomic positions were relaxed until the force on each atom was less than 0.05 eV/Å. Using the periodic slab model and self-consistent dipole correction, the averaging electrostatic potential in the planes perpendicular to the slab normal could be obtained. Bulk CdS forms a tetragonal lattice with space group P63mc and experimental lattice constants are: a = b = 4.136 Å, c = 3.713 Å. Bulk Co2P forms a tetragonal lattice with space group Pnma and experimental lattice constants are: a = 5.646 b = 3.513 Å, c = 6.608 Å. We built a periodic slab with four and four layers for CdS (002) and Co2P (112) facets, respectively. 2 × 2 and 1 × 1 surface unit cells were used, respectively. The vacuum gap thickness was set to be 12 Å. A Monkhorst−Pack grid of 9 × 9 × 1 k-points was used for all DFT calculations. SPV measurement The surface photovoltage (SPV) measurement was carried out on a surface photovoltage spectroscopy (PL-SPS/IPCE1000 Beijing Perfect Light Technology Co, Ltd). The measurement system is composed of a source of monochromatic light, a lock-in amplifier (SR830, Stanford research systems, Inc.), a light chopper (SR 540, Stanford research systems, Inc.), and a sample cell, respectively. The entire measurement was operated under ambient condition. 7



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RESULTS AND DISCUSSION Characterization of Co2P-CdS hybrid samples The crystal structure of the as-prepared samples is characterized by XRD analysis. The XRD pattern of Co2P depicted in (Figure 1a) shows three main peaks with 2θ at 40.72°, 43.30°and 52.02°, which can be readily indexed to the (112), (211) and (020) crystal planes of hexagonal Co2P (JCPDS No. 65-2380), respectively. The broad diffraction peaks indicate that the Co2P synthesized via hydrothermal method is not very well crystalized. The XRD patterns of H-Co2P-CdS and G-Co2P-CdS samples are shown in (Figure 1b) and (Figure 1c). All samples can be well-indexed to the crystal structure of hexagonal CdS (JCPDS, 77-2306) with typical diffraction peaks at 2θ of 24.86°，26.45°, 28.22°, 43.78° and 51.86°, which are corresponding to the (100), (002), (101), (110), (103) and (112) crystal planes of CdS, respectively. However, there are no obvious diffraction peaks of Co2P in both H-Co2P-CdS and G-Co2P-CdS samples, which may indicate that the loading content of Co2P is too low to be detected. Nevertheless, the existence of Co2P in the composites can be clearly verified by TEM and XPS techniques which will be discussed later. In addition, the featured CdS diffraction peaks are retained after loading of Co2P without appearance of other unknown diffraction peaks, indicates that the crystal structure of CdS is not changed. Scanning electron microscopy (SEM) was applied to characterize the morphology and microstructure of the samples. As can be clearly seen in (Figure 2a), the CdS synthesized via hydrothermal method is in regular spherical shape with an average diameter of about 500 nm, and each CdS microsphere is composed by lots of even smaller particles. However, after the second hydrothermal process, the small particles on the surface of CdS microspheres have grew up and form microspheres with protruding surface as shown in the (Figure 2b) and (Figure 2c). The rougher microsphere surface is beneficial for providing more active sites for decoration of nanometer-sized Co2P.38 The size of the CdS 8


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microsphere after the second hydrothermal process was about 550 nm, which is slightly larger than the size of the pristine CdS. The schematic illustration of the synthesis procedure of CdS-Co2P cauliflower-structured sub-microsphere is depicted in (Figure 3). Nevertheless, the composition and crystal structure of the two samples were not changed according to the XRD results. TEM was further applied to investigate the detailed microstructural information of the composite samples. As shown in (Figure 4a), the morphology of the Co2P-CdS sample revealed by TEM is similar to that obtained from SEM as depicted in (Figure 2b) and (Figure 2c). A closer view of TEM shown in Figure 4b discloses that some nanoparticles are adhered on the surface of CdS. The HRTEM images for composites of H-Co2P-CdS and G-Co2P-CdS are shown in (Figure 4c) and (Figure 4d), respectively. From the HRTEM images, the lattice fringe of CdS phase with lattice spacing of d (002) =0.32 nm and d (112) =0.22 nm of Co2P can be clearly distinguished, which further confirms the results concluded from the XRD characterization. Furthermore, it can be clearly seen that there are overlaps between the lattice fringes of CdS and Co2P phase in (Figure 4c), which indicates a great bonding presents at the interface of the Co2P-CdS composites synthesized by in-situ hydrothermal method. On the contrary, no overlap in lattice fringes can be observed in (Figure 4d), and each of the lattice fringes of the two composites is regular and self-existent, all of these observations indicate that the interfaces formed in the Co2P-CdS compounds by grinding is not very strong. The strong bonding presents at the interface of H-Co2P-CdS sample is benefit to the charge transport of photo-induced charge carriers, which may enhance the photocatalytic performance of the composite photocatalyst as will be discussed latter. Meanwhile, scanning transmission electron microscopy (STEM) coupled with energy-dispersive X-ray spectroscopy (EDX) analysis of the H-Co2P-CdS composite sample reveals the presence of Cd, S, Co and P elements as shown in (Figure 4e), which is consistent with the HRTEM and XRD results.

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To confirm the surface chemical states of H-Co2P-CdS heterojunction sample, the X-ray photoelectron spectroscopy (XPS) was adopted. The C1s at 284.8 eV was used as a reference to correct the binding energies. In comparison with Co2P and CdS, H-Co2P-CdS is mainly consist of Cd，S, Co and P elements, which may verified that Co2P was successfully immobilized on the surface of CdS. The XPS spectra of Cd 3d in the sample of H-Co2P-CdS is shown in (Figure 5a) with binding energy of Cd 3d 5/2 and Cd 3d 3/2 at 404.7 and 411.5 eV, respectively, which shifted slightly toward the higher binding energy as compared to the Cd 2+ (404.6 eV and 411.4 eV) states in CdS. Furthermore, the binding energy of S 2p 3/2 (161.5 eV) and S 2p1/2 (162.7 eV) in (Figure 5b) are also shift slightly higher than pure CdS (161.27 eV and 162.45 eV). The shift of binding energy can be interpreted as the result of strong interaction formed between Co2P and CdS. Figure 5c presents the XPS spectrum with binding energy in the range of 770-820 eV, which are ascribed to the 2p

3/2 and

2p

1/2

binding energies of the reduced Co

species in Co2P.39 The XPS peaks of Co 2p in H-Co2P-CdS sample was fitted by two main peaks at 781.4 (2p 3/2) and 797.1 eV coupled with two shake-up satellite peaks at 785.6 eV and 802.4 eV, which are corresponding to the oxidized Co species40, 41 originates from the cobalt phosphate formed on the surface of Co2P.41 The XPS peak at 133.4 eV shown in (Figure 5d) is assigned to the oxidized cobalt phosphorous as a result of air exposure.42 The XPS peaks at 129.2 eV and 130.4 eV indicate the representing of P-Co bonds in the H-Co2P-CdS composite sample. It can be clearly observed that the binding energies of Co 2p and P 2p have systematically shifted towards low binding energies compared with pure Co2P, which is attributed to change of surface electron density. It has been reported that the variation of binding energies related to the change of surface electron density, which results from electron transfer from CdS to Co2P. This process resulting in decreasing of electron density of CdS and increasing in Co2P, so the binding energies of Co 2p and P 2p decreased.43 All of these XPS characterization results lead to the conclusion that Co2P has been successfully synthesized and 10


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decorated on the surface of CdS microspheres . Photocatalytic H2 evolution activity The photocatalytic H2 production experiment was carried out under visible-light irradiation and 10 vol.% lactic acid was used as hole scavenger. Both of the H-Co2P-CdS and G- Co2P-CdS composite samples were characterized. As shown in (Figure 6a) and (Figure 6b), the photocatalytic H2 production rate of pure CdS is 0.0086 mmol·h-1. However, after loading the Co2P co-catalyst on the surface of CdS either via hydrothermal or grinding method, the photocatalytic H2 evolution performance of both Co2P-CdS composite samples is significantly enhanced. For both Co2P-CdS composite samples, the photocatalytic H2-production rate is proportional to the deposition ratio of Co2P at the initial stage. When the loading amount of Co2P reached 1.2 mol% in H-Co2P-CdS compsite sample, the photocatalyst displays the highest photocatalytic H2 evolution rate of 0.303 mmol·h-1 which is about 35 times and 3 times higher than pure CdS and Pt decorated CdS (Figure 6a), respectively. However, there is a noticeable decrease in photocatalytic H2 evolution rate further increasing the loading ratio of Co2P on the surface of CdS. This can be attributed to the excessive Co2P would cover the surface active sites and deteriorate the light absorption of CdS. As a contrast, the highest rate of photocatalytic H2 evolution is 0.131 mmol·h-1 achieved in 1.4 mol% G-Co2P-CdS composite sample, which is much lower than the highest value obtained in H-Co2P-CdS composite samples. In addition, pure Co2P shows no activity toward H2 evolution under visible-light irradiation, further confirm that the role of Co2P is co-catalyst rather than photocatalyst. The enhanced performance of H-Co2P-CdS composite sample with 1.2 mol% Co2P loading ratio may originate from several reasons. First, Co2P possesses strong electron capturing ability. The difference in work functions of Co2P and CdS results in a built-in field at the interface, which acts as driving force to promote the charge separation of the photo-excited electron-hole pairs. Furthermore, the intimate interfaces formed between CdS and Co2P can effectively boost the lifetime of 11



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charge carriers in the Co2P modified samples, and the rich interfacial contact area formed during the second hydrothermal process (Figure 4c) can provide an “express path” for the charge transfer of photo-generated carriers across the interface.11 According to the report by Kreuer,44 the H2 production rate can be dramatically enhanced after introduced K2HPO4 in the electrolyte solution, which is partly because the addition of HPO42- can lead to formation of hydrogen-chains of HPO42- to facilitate proton transport in the samples. In addition, the result has been further verified by Ye through probing the role of HPO42- in the process of H2 production.45 Therefore, we have also check the performance of 1.2 mol% H-Co2P-CdS composite sample in the presence of different amounts of K2HPO4. It can be clearly seen in Figure 7 that when 0.25 M K2HPO4 was added into the electrolyte solution with 10 vol% lactic acid, the sample showed the highest H2 production activity and reached a H2 production rate of 0.356 mmol·h-1. However, further increasing the concentration of K2HPO4 led to a slight decrease in photocatalytic H2 production activity, which may be caused by the change of pH of the solution.46 The photocatalytic H2 evolution in a time scale are summarized and presented in Figure 8 for H-Co2P-CdS composite samples with different Co2P contents. The apparent quantum yield (AQY) of H2 production for the 1.2 mol% H-Co2P-CdS composite sample was characterized by using a 300 W Xe lamp equipped with a 420 nm (± 5nm) band pass filter working as a monochromatic light source. A total duration of 5 hours was adopted for the test. During the test, the light intensity of the monochromatic light was measured with irradiatometer at each 1 hour. The AQY was calculated according to Equation 1 as shown in the exprimental section and the result is listed in table 1. As can be seen in (Figure 9) and table 1, the AQY of the sample is 13.88% in the first hour and keeps stable in the following 4 hours. Based on the above results and discussion, we can conjecture that the recombination ratios of 12


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photo-induced charges have been greatly suppressed and the charge transfer of photo-excited electrons from the conduction band of CdS to Co2P has been accelerated. In order to verify the assumption we have performed photoluminescence (PL) spectra characterization for pure CdS and 1.2 mol% H-Co2P-CdS composite sample, and the result is shown in (Figure 10). The PL spectrum of pure CdS shows a strong emission in the range of 675 nm and 800 nm which indicates a severe recombination of charge carriers via inter-band states. However, the 1.2 mol% H-Co2P-CdS composite sample exhibits much lower PL emission in the same range, which can be attributed to the improved charges separation efficiency and suppressed recombination rate of photo-induced electron-hole pairs benefit from Co2P loading.41 The result will be further discussed with density functional theory (DFT) calculation later. The stability of the 1.2 mol% H-Co2P-CdS sample synthesized by hydrothermal method has been verified by photocatalytic cycling experiment shown in (Figure 11) and XRD test after photocatalytic reaction (figure 1(d)). Pure CdS is unstable during the visible-light irradiation because of the S2oxidation in CdS by photogenerated holes, which results in low H2 evolution cycling performance.42 However, the photocatalytic H2 evolution rate of 1.2 mol% H-Co2P-CdS keeps relative stable rate for about 16 hours (4 cycles), and from the analysis of the XRD after photocatalytic reaction, there is no significant change in both the position and intensity of the diffraction peaks, in addition, the XPS analysis (figure 6) of the after photocatalytic reaction sample has no obvious changes compared with before (figure 5). All these results indicate that it owns good photocatalytic H2 production stability. Photocatalytic mechanism investigation The application of Co2P as co-catalyst has significantly improved the photocatalytic H2 evolution performance of CdS. In order to explain the above experimental results theoretically, we carried out density functional theory (DFT) calculation using VASP code to derive the work function (Wm and Ws for metal and semiconductor, respectively) and the value of ECB and EVB of CdS. The interface model 13



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used for calculation is composed by CdS (002) and Co2P (112) as presented in (Figure 12a), the bonds formed between the atom of Co and S, P and Cd. As is well-known, band bending would be formed at the intimate contact interface between the metal and the semiconductor due to the different work functions they have.47 In our modeled system, the work function of the metallic Co2P (112) and CdS (002) surface is 4.26 eV and 6.33 eV, respectively. As depicted in (Figure 12b), when CdS and Co2P form intimate contact, the electrons will transfer from Co2P to CdS as the Fermi level of Co2P is higher than that of CdS, which leads to a downward bending of the energy bands of CdS until the Fermi level of each side is equal.48 When the equilibrium state is reached (unified Fermi levels), the separation and transfer of photo charges can be boosted as a result of the band bending formed at the interface, such that the photo-induced electron will be drifted to the surface of Co2P and the photo-induced holes will be accumulated on the surface of CdS. In this way, the recombination rate of photo-excited electron-hole pairs in CdS can be significantly depressed. As a result, the reduction reaction may occur on the surface of Co2P with plentiful electrons; meanwhile, the oxidation reaction may occur on the surface of CdS with holes. Based on the DFT calculation result, it can clearly show that proper band bending can be formed when loading Co2P on the surface of CdS, which leads to a dramatically enhanced performance of photocatalytic H2 production. The mechanism of enhanced photocatalytic H2 production in H-Co2P-CdS composite sample is proposed based on the above results: under visible-light irradiation, electrons in the valance band of CdS are excited to the conduction band to form photo-induced electrons. As a result of the proper band bending formed at the intimate interface, the photo-induced electrons are immediately transferred to Co2P once being excited, following which the transferred electron would participate in the reduction reaction to generate H2 from reduction of H+ as depicted in (Figure 13). Meanwhile, the photo-induced 14


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holes generated in the valance band of CdS are tend to participate in the oxidation reaction to oxidize lactic acid. In addition, using K2HPO4 as sacrificial agent is an effective way to further boost the photocatalytic H2 production rate. In the presence of K2HPO4, HPO42- acts as a mediator which participates directly in the photocatalytic reaction, such that H+ from HPO42- instead of H2O is combined with the photogenerated electrons to produce H2. The remaining PO43- will capture the H+ from water to HPO42- again which completes the proton-reduction cycle.49 As a result, the H2 production process with the assistance of HPO42- is more effective than direct produce H2 from water decomposition. In order to further verify the validity of the proposed mechanism, surface photovoltage (SPV) technique is employed to investigate the photo-generated charge transfer properties of the H-Co2P-CdS composite samples. The SPV spectra of H-Co2P-CdS composite samples with different loading ratio of Co2P are shown in (Figure 14). As is well-known, a stronger photoelectric signal corresponds to a higher charge separation efficiency.50 The SPV spectra revelas that all of the composite samples have a strong response at the wavelength region from 300 nm to 525 nm, which means the photo-excited charge carriers generated by light absorption in this range are effectively separated. However, the photovoltage response of pure CdS is much weaker than the 1.2 mol% H-Co2P-CdS composite sample, which implies the charge transfer and charge separation efficiency in H-Co2P-CdS composite sample is much stronger than pure CdS. Nevertheless, the SPV signal will decrease when excessive Co2P is introduced on the surface of CdS, which is caused by the light attenuation effect of Co 2P to prevent enough light reaching the surface of CdS. The results of SPV spectra and the above test illustrate that the band alignment at the interface formed between Co2P and CdS plays an important role in charge carrier separation. With decoration of Co2P on the surface of CdS, the transfer of electrons from CdS to Co2P is promoted, and consequently results in enhanced photocatalytic activity.

CONCLUSIONS 15



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In summary, we have successfully synthesized a series of noble-metal-free Co2P-CdS composite photocatalysts through both in-situ hydrothermal method and grinding method. With detailed microstructural characterization of the Co2P-CdS composite samples synthesized by the two methods, it is revealed that intimate interfaces are formed for the samples prepared by in-situ hydrothermal method, which is demonstrated to be beneficial for photocatalytic H2 production. A highest photocatalytic H2 evolution rate of 0.356 mmol ·h-1 with a good photocatalytic stability is obtained in the sample of 1.2 mol% H-Co2P-CdS, which is 41 times higher than pure CdS. Moreover, DFT calculation is performed to explain the mechanism of enhanced photocatalytic H2 production in H-Co2P-CdS composite sample. Finally, the technique of surface photovoltage (SPV) provides solid support for the proposed mechanism derived from DFT calculation. Our results have proved that Co2P is a promising low-cost non-noble metal co-catalyst for CdS in the photocatalytic H2-production process. AUTHOR INFORMATION Corresponding Authors * E-mail:

[email protected] (L. G.).

* E-mail:

[email protected] (J. L.).

Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS This work was financially supported by the National Science Foundation of China (Grant No. 51572295, 21273285 and 21003157), Beijing Nova Program (Grant No. 2008B76), and Science Foundation of China University of Petroleum, Beijing (Grant No. KYJJ2012-06-20 and 2462016YXBS05).

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ZnIn2S4 sub-microspheres with enhanced visible-light photocatalytic hydrogen production activity. Dalton T. 2017, 46, 10620-10629, DOI: 10.1039/c7dt00819h. Figure Captions: Figure 1. XRD patterns of as-prepared (a) Co2P, (b) H-Co2P-CdS and (c) G-Co2P-CdS, (d) H-Co2P-CdS after photocatalytic reaction samples. Figure 2. SEM images of (a) CdS without second hydrothermal, (b) CdS with second hydrothermal process, (c) 1.2 mol% H-Co2P-CdS samples. Figure 3. Synthesis procedure of CdS-Co2P cauliflower-structure sub-microspheres. Figure 4. TEM images of (a) Co2P-CdS sub-microsphere (b) Petal of Co2P-CdS sub-microsphere (c) HRTEM of H-Co2P-CdS (d) HRTEM of G-Co2P-CdS (e) EDX of H-Co2P-CdS. Figure 5. XPS spectra of (a) Cd 3d (b) S 2p; (c) Co 2p; (d) P 2p for CdS, Co2P and H-Co2P-CdS. Figure 6. XPS spectra of (a) Cd 3d (b) S 2p; (c) Co 2p; (d) P 2p for H-Co2P-CdS after photocatalytic reaction. Figure 7. Rate of H2 evolution over Co2P-CdS composite samples (a) H-Co2P-CdS (b) G-Co2P-CdS with different Co2P contents. Figure 8. Rate of H2 evolution over 1.2 mol% H-Co2P-CdS with different K2HPO4 contents. Figure 9. Photocatalytic H2 evolution over H-Co2P-CdS composite samples with different Co2P contents under visible light. Figure 10. Image of H2 evolution and apparent quantum yield (AQY) with 420 nm (± 5 nm) band pass filter (1.2 mol% H-Co2P-CdS). Table. 1. Apparent quantum yield (AQY) and light intensity over 1.2 mol% H-Co2P-CdS at different time. Figure 11. Photoluminescence (PL) spectra of pure CdS and 1.2 mol% H-Co2P-CdS samples. 24


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Figure 12. Cycling runs for the photocatalytic H2 evolution in the presence of the 1.2 mol%

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H-Co2P-CdS composite sample under visible light irradiation.

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Figure 13. Schematic of photocatalysis by H-Co2P-CdS composites. (a) The illustration of combination

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between metal Co2P (Co: blue, P: purple) and semiconductor CdS (S: yellow, Cd: dull yellow). (b)

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Calculated work function of metal Co2P and CdS.

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Figure 14. Schematic illustration of charge transfer and H2 evolution mechanism involving HPO42− for

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H-Co2P-CdS photocatalyst under visible light irradiation.

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Figure 15. SPV spectra of pure CdS and H-Co2P-CdS composites with different molar ratios of Co2P.

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Figure 2. SEM images of (a) CdS without second hydrothermal, (b) CdS with second hydrothermal process, (c) 1.2 mol% Co2P-CdS samples.

Figure 3. Synthesis procedure of CdS-Co2P cauliflower-structure sub-microspheres.

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Figure 4. TEM images of (a) Co2P-CdS sub-micosphere (b) Petal of Co2P-CdS sub-micosphere (c) HRTEM of H-Co2P-CdS (d) HRTEM of G-Co2P-CdS (e) EDX of H-Co2P-CdS.

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Figure 5. XPS spectra of (a) Cd 3d (b) S 2p; (c) Co 2p; (d) P 2p for CdS, Co2P and H-Co2P-CdS.

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Figure 6. XPS spectra of (a) Cd 3d (b) S 2p; (c) Co 2p; (d) P 2p for H-Co2P-CdS after photocatalytic reaction.

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1

(a) 0.303

0.30 0.25

0.231

0.20

0.173 0.141

0.15

0.122 0.104

0.10

0.086

0.05

H2

production rate (mmolh-1)

0.35

0.0086

0.023

0.00

0%

0.3% 0.5% 0.6% 0.8% 1.0% 1.2% 0.12% 1.4% Loading amount (mol %) Pt

2

0.14

H2 production rate (mmolh-1)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(b) 0.131

0.12

0.111

0.10

0.09

0.08

0.057

0.06

0.045 0.045 0.04

0.023 0.025 0.02

0.0086 0.00

0 % 0.3 % 0.5 % 0.6 % 0.8 % 1.0 % 1.2 % 1.4 % 1.6 %

3

Loading amount (mol%)

4 5

Figure 7. Rate of H2 evolution over Co2P-CdS composite samples (a) H-Co2P-CdS (b) G-Co2P-CdS with different Co2P contents.

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0.40

0.356

0.35

H2 production rate (mmolh-1)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 1 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0.30

0.34

0.303

0.25 0.20 0.15 0.10 0.05 0.00

1.2%

0.25 M

0.35 M

Samples Figure 8. Rate of H2 evolution over 1.2 mol% H-Co2P-CdS with different K2HPO4 contents.

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Figure 9. Photocatalytic H2 evolution over H-Co2P-CdS composite samples with different Co2P contents under visible light.

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0.12

14

0.10

12

0.08

10 8

0.06 6

AQY %

1 2 3 1 4 2 5 3 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

H2 evolution (mmol)


0.04 4 0.02

2

0.00

0 1

2

3

4

5

Irradition time (h) Figure 10. Image of H2 evolution and apparent quantum yield (AQY) with 420 nm (±5 nm) band pass filter (1.2 mol% H-Co2P-CdS).

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Table. 1 Apparent quantum yield (AQY) and light intensity over 1.2 mol% H-Co2P-CdS at different time.

Irradiation Time (h)

1

2

3

4

5

Light intensity (mW/cm2)

4.7

4.2

4.7

4.8

4.7

AQY（%）

13.88

11.89

12.80

11.73

12.80

CdS

Intensity (a.u.)

1 2 3 1 4 2 5 3 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1.2 mmol%Co2P-CdS

550

600

650

700

750

800

Wavelength (nm) Figure 11. Photoluminescence (PL) spectra of pure CdS and 1.2 mol% H-Co2P-CdS samples.

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1.6 1.4 1.2

H2 evolution (mmol)

1 2 3 1 4 2 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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1.0 0.8 0.6 0.4 0.2 0.0 0

2

4

6

8

10

12

14

16

Irradiation time (h) Figure 12. Cycling runs for the photocatalytic H2 evolution in the presence of the 1.2 mol% H-Co2P-CdS composite sample under visible light irradiation.

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(a)

Cd

S

Co

P

(b)

Figure 13. Schematic of photocatalysis by Co2P-CdS composites. (a) The illustration of combination between metal Co2P (Co: blue, P: purple) and semiconductor CdS (S: yellow, Cd: dull yellow). (b) Calculated work function and band bending of metal Co2P and CdS.

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Figure 14. Schematic illustration of charge transfer and H2 evolution mechanism involving HPO42− H- Co2P-CdS photocatalyst under visible light irradiation.

Figure 15. SPV spectra of pure CdS and H-Co2P-CdS composites with different molar ratios of Co2P. 38


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This catalyst not only has good stability but can splitting water for producing hydrogen sustainably.


In-situ synthesis of strongly coupled Co2P-CdS nanohybrids: An

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