Research Article pubs.acs.org/journal/ascecg
Rational Synthesis of Metal−Organic Framework-Derived Noble Metal-Free Nickel Phosphide Nanoparticles as a Highly Efficient Cocatalyst for Photocatalytic Hydrogen Evolution D. Praveen Kumar, Jiha Choi, Sangyeob Hong, D. Amaranatha Reddy, Seunghee Lee, and Tae Kyu Kim* Department of Chemistry and Chemical Institute for Functional Materials, Pusan National University, Busan 609-735, Republic of Korea S Supporting Information *
ABSTRACT: Facile preparation of metal−organic framework (MOF) derived earth-abundant nickel phosphide (Ni2P) by a simple, cost-effective procedure is described. Ni2P is recognized as a suitable replacement for expensive noble metal cocatalysts used for H2 production by water splitting. Ni2P nanoparticles were used to prepare a Ni2P/CdS composite with improved photocatalytic properties. Crystal structure and surface morphology studies showed that Ni-MOF spheres readily transform into Ni2P particles, and TEM images indicated the presence of Ni2P nanoparticles on CdS. The optical properties and charge carrier dynamics of the composite material exhibited better visible light absorption and improved suppression of charge carrier recombination. X-ray photoelectron spectra confirmed the presence of Ni2P on CdS. The synthesized materials were tested for photocatalytic hydrogen production with lactic acid as a scavenger under irradiation in a solar simulator. The rate of H2 production with Ni2P/CdS was 62 times greater than that with pure CdS. The superior activity of the composite material is attributed to the ability of Ni2P to separate the photoexcited charge carriers from CdS and provide good electrical conductivity. The optimized composite material also exhibited better photocatalytic activity than Pt cocatalyzed CdS. Based on the experimental results, a possible electron−hole transfer mechanism is proposed. KEYWORDS: Metal−organic frameworks, Ni2P, CdS, Photocatalysis, Water splitting
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INTRODUCTION Rapid increase in worldwide energy consumption and related environmental issues have triggered an urgent demand for renewable and clean energy sources.1 Water splitting is a promising approach for producing hydrogen fuel (H2), which is a safe, sustainable, and environmentally compatible source of energy.2 Various semiconductors have been applied to photocatalytic hydrogen production. In particular, NaTaO3 doped with La3 and Ga2O3 doped with Zn4 exhibit maximum water splitting rates when modified with NiO and Rh2yCryO3 cocatalysts, respectively. Solid solutions of ZnGeN2 and ZnO ((Zn1+xGe)(N2Ox)) and GaN and ZnO ((Ga1−xZnx)(N1−xOx)) are also used as photocatalytic materials for hydrogen evolution,5 but these materials are expensive. TiO2-based semiconductors are highly efficient at H2 production with the aid of Cu2O and CuO cocatalysts,6−8 but TiO2 is a wide band gap material that requires UV light for effective utilization. Among the known catalysts, CdS is one of the most promising materials for photocatalytic H2 production owing to its high activity under visible light, narrow band gap (Eg = 2.4 eV) and sufficiently negative band edge potential for the reduction of protons to H2.9−12 Nevertheless, the photocatalytic activity of pure CdS is low due to its fast electron−hole recombination and photocorrosion, which decrease the stability.13 Many © XXXX American Chemical Society
approaches, such as loading with an appropriate cocatalyst, have been explored to resolve these problems and to improve CdS activity toward H2 production. Cocatalysts can improve the transfer properties of excited charge carriers and thereby play an important role in promoting photocatalytic activity.14−16 Platinum and noble metals typically are used as cocatalysts for photocatalytic water splitting based on their exceptional activities. However, the high cost of these materials is not amenable to scale-up applications. Hence, the development of highly active, stable catalysts containing earth-abundant materials is an important objective. Since the initial development of oxide-based cocatalysts such as NiO17 and CuxO,18 many new sulfide-based materials including NiS,19,20 NiSx,21 CuS,22 and MoS223−26 have been investigated. However, these cocatalysts have the drawback of instability during the photocatalytic reaction. Recently, transition metal phosphides (TMPs), which are formed by alloying metals and phosphorus, have attracted attention for water splitting. These TMPs include FeP,27−29 CoP,30−33 Co2P,34 Ni2P,35−38,52 Cu3P,39 MoP,40 and WP.41 The advantages of transition metal phosphides include their Received: August 23, 2016
A
DOI: 10.1021/acssuschemeng.6b02032 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX
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Scheme 1. Material Preparation: Synthesis of Ni-BTC MOFs, Synthesis of Nickel Phosphides Derived from Ni−BTC Metal− Organic Framework, and Synthesis of Ni2P/CdS Composite
of pure CdS. This enhanced activity is attributed to the ability of MOF-derived Ni2P to separate the photoexcited charge carriers formed in CdS and indicates that the presence of Ni2P in composite as a cocatalyst promotes the effectiveness of Ni2P/CdS photocatalysts.
abundance, low cost, and good electrical conductivity. Phosphorus produces a “weak ligand” effect upon forming metal−P bonds in metal phosphides and decreases the number of metal active sites through an ensemble effect.42−45 The structural arrangement of metal phosphides can facilitate the release of hydrogen from active sites. Even though several reports on metal phosphides for water splitting, in those reports they have prepared metal phosphides from yellow phosphorus, but this material is very toxic, and its usage is strictly restricted in some countries and existing synthetic methods suffer from complex operations, sophisticated apparatus, extreme conditions like calcinations processes under inert gas conditions. Thus, significant challenges remain in synthesizing Ni2P nanostructures in a simple and effective manner. Metal−organic frameworks (MOFs) are porous crystalline inorganic−organic hybrid materials that have attracted great interest in recent years.46 Their expanded application offers great promise, especially in the fabrication of metal phosphide nanostructures. Several MOF-derived materials have been developed for H2 production. Xu et al.47 reported that nanoporous FeP nanosheets can be synthesized by anionexchange reaction of inorganic−organic hybrids with phosphorus ions and used for electrocatalytic hydrogen production. Popczun et al. described the preparation and evaluation of CoP as an electrocatalyst for the hydrogen evolution reaction (HER) under strongly acidic conditions.33 Tian et al. used Cu3P nanowire arrays as a three-dimensional hydrogen-evolving cathode in acidic electrolytes.48 Tian et al.50 also reported Ni2P via Ni-BTC for electrocatalytic H2 production. Water splitting is generally achieved by photocatalysis or electrocatalysis but these two methods working function are totally different. In electrocatalysis, it is necessary to prepare electrodes containing synthesized catalysts and also needs to apply voltage but in the case of photocatalysis powder material suspension is enough. Powdered photocatalysts are beneficial for large-scale water splitting applications because of their simplicity. This method provides an opportunity to produce H2 in an energyefficient manner, whereby photoexcited electron−hole pairs are created by electromagnetic irradiation with energy greater than that of the band gap energy. The electrons photogenerated in this way reduce H+ to H2. Herein, we report the synthesis of Ni2P nanoparticles in a simple, cost-effective manner from a MOF and powdered photocatalyst. The system evolves hydrogen by water splitting under simulated solar radiation. MOF-derived Ni2P is extended to the preparation of a composite with CdS nanocrystals (Ni2P/CdS), which exhibits efficient H2 production. The rate of H2 production of the composite is 62 times greater than that
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EXPERIMENTAL SECTION
Synthesis of Ni-BTC MOFs. All chemicals were of analytical grade and were used without further purification. In a typical procedure, 1.274 g of nickel nitrate (Ni(NO3)2·6H2O) and 0.512 g of benzene1,3,5-tricarboxylic acid (C6H3(COOH)3, BTC) were dissolved in 70 mL of absolute methanol with stirring for 1 h. The alcoholic nickel nitrate solution was transferred into a 100 mL Teflon cup and heated in an autoclave at 150 °C for 24 h. The resulting green product was harvested by several rinse-precipitation cycles with methanol. The green Ni-BTC MOF powder was then dried under vacuum at 60 °C prior to characterization. Synthesis of Nickel Phosphides Derived from a Ni−BTC Metal−Organic Framework. In a typical preparation of Ni2P nanoparticles, 0.1 g of as-prepared Ni−BTC and 0.3 g of NaH2PO2 were mixed together, loaded into a covered ceramic crucible, and heated to 275 °C at a ramp rate of approximately 1 °C per min (2 h) in a muffle furnace. After cooling to room temperature, the product was washed several times with water and ethanol and dried under vacuum at 60 °C for 6 h. Synthesis of CdS Nanoparticles. Equimolar (0.1 M, 20 mL) solutions of Cd(OAc)2 and Na2S were prepared, and Na2S was slowly added dropwise to the Cd(OAc)2 solution with magnetic stirring. A yellow precipitate was obtained, washed many times with distilled water and ethanol, and dried at 80 °C for 12 h. Synthesis of Ni2P/CdS Composite. CdS catalysts with deposited Ni2P were prepared by the wet chemical method described in our previous paper.8 In a typical preparation, 0.1 g each of CdS and Ni2P nanoparticles were suspended in 10 mL of ethanol in a 50 mL glass beaker. The mixture was ultrasonicated until the solvent evaporated. The powder was crushed in a mortar and dried at 60 °C for 6 h to obtain the final form of the photocatalyst. A schematic representation of the materials preparation is shown in Scheme 1. Characterization. Surface morphologies and elemental analyses were evaluated using an HITACHI S-4800 field emission scanning electron microscope (FESEM) equipped with an energy dispersive spectrometer (EDS, Inca 400, Oxford Instruments). The crystal structures of the samples were determined by X-ray diffraction (XRD) with a Bruker D8 Advanced X-ray diffractometer using Cu Kα radiation as the X-ray source. X-ray photoelectron spectroscopy (XPS) measurements were carried out to evaluate the chemical status and elemental composition of the samples with a monochromated Al Kα X-ray source (hν = 1486.6 eV) at an energy of 15 kV/150 W. Diffused reflectance spectra (DRS) were recorded with a UV−vis spectrometer (UV-1800 SHIMADZU, Japan). Photoluminescence (PL) spectra of the photocatalysts were collected at room temperature using a Hitachi F-7000 fluorescence spectrometer. An EG&G Princeton Applied Research PARSTAT 2263 instrument was used to collect the B
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Figure 1. (a) XRD analyses of Ni-MOF, Ni2P, CdS, and Ni2P/CdS composite. (b) UV−vis DRS and (c) PL spectra of CdS and Ni2P/CdS. impedance spectra at open circuit voltages from 100.0 kHz to 100.0 mHz (AC amplitude 10.0 mV). Metal content was determined by using inductively coupled plasma optical emission spectrometry (ICPOES) spectrometry. Photocatalytic Activity. Photocatalytic activity was tested in a Pyrex reactor equipped with a top loading port and sealed with a gastight rubber septum. The same port was used for outgassing and sampling. Typically, 1 mg of catalyst was dispersed in 15 mL of 20 vol % aqueous lactic acid solution. The gases present in the free space of the quartz reactor and dissolved in the reaction solution were removed by evacuating the reactor followed by purging with argon to obtain an inert atmosphere. Photocatalytic activity experiments were performed under simulated solar radiation. The H2 gas generated was collected and analyzed using an off-line gas chromatograph (GC). Control and blank (no catalyst) experiments were carried out under identical conditions. Three sets of experiments were carried out to check reproducibility. The recyclability study also was carried out 5 times to determine the stability of the photocatalyst. Each test was carried out as described above for 5 h under irradiation. After completion of each test, the gaseous products were evacuated, and the reactor was purged with argon.
Ni2P (2 wt %) was loaded onto CdS nanoparticles, no noticeable diffraction peaks belonging to Ni2P were observed. All reflections are evidence only of CdS and are indexed to a cubic phase, probably due to very strong diffraction peaks of CdS relative to the small amount of Ni2P dispersed on the CdS surface. However, EDS mapping clearly shows the presence of Ni and P in the Ni2P/CdS composite (Figure S1 in the Supporting Information (SI)). The transmission electron microscopy (TEM) analysis also provides evidence for the presence of Ni2P in the composite. The optical properties and charge carrier dynamics of the catalyst were analyzed by DRS and photoluminescence (PL) measurements. Figure 1b shows the UV−vis diffuse reflectance spectra of pure CdS and the Ni2P/CdS composite. The band edge of CdS occurs at ∼575 nm and exhibits a small shift toward longer wavelengths (650 nm) after loading with Ni2P. Tauc plots (plots of (αhν)2 vs (hν), where α, h, and ν denote the absorption coefficient, Plank constant, and frequency of light, respectively) were constructed for two representative samples of pure CdS and Ni2P/CdS composite to determine their energy gaps. From the intercepts of the straight lines in these plots, the band gaps of CdS and Ni2P/CdS are estimated to be 2.15 and 1.95 eV, respectively. The small decrease in band gap may arise after of Ni2P loading (Figure S2 in SI). According to Bao et al.49 the band gap of Pt deposited CdS decreases due to the increases in the particle size of CdS nanoparticles in Pt/ CdS. Based on this and the similarity of composites, we have also assumed that the reason for the decreases of band gap in Ni2P/CdS is due to CdS nanoparticles size increases while Ni2P loading process. PL analysis provides information about the electronic transition behavior of photocatalysts by recording emission spectra upon irradiating light of a particular wavelength. Spectra
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RESULTS AND DISCUSSION The diffraction patterns of the synthesized materials are displayed in Figure 1a. The diffraction pattern of Ni-MOF (Ni-BTC) indicates its amorphous crystalline characteristic. The pattern from 2θ = 1 to 27°matches well with the previously reported XRD pattern of Ni-BTC.49,50 The distinct diffraction peaks of MOF-derived Ni2P nanoparticles (Ni2P) at 40.7°, 44.6°, 47.4°, and 54.2° can be perfectly indexed to the (111), (201), (210), and (300) planes of hexagonal Ni2P (JCPDS file no. 89-4864). The diffraction pattern clearly indicates that the Ni2P nanoparticles are crystalline and of high purity. CdS and Ni2P-loaded CdS (Ni2P/CdS) were indexed as the facecentered cubic phase of CdS (JCPDS no. 80-0019).51 When C
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Figure 2. (a) SEM image of as-synthesized Ni-BTC (MOF), (b) TEM image of CdS, (c) TEM image of Ni2P, (d) TEM image of Ni2P/CdS, (e) HRTEM of Ni2P/CdS, (f) linear scanning of EDS mapping of Ni2P/CdS, (g) TEM image of Pt/CdS, (h) HRTEM of Pt/CdS, and (i) linear scanning of EDS mapping of Pt/CdS.
of aggregated particles of Ni2P deposited on CdS. To further clarify this matter, we checked the composite material with high-resolution transmission electron microscopy (HRTEM), as shown in Figure 2e. This image confirms that Ni2P cocatalyst nanoparticles are deposited on the surface of CdS to form a Ni2P−CdS junction that exhibits lattice spacing of approximately 0.22 and 0.33 nm, which correspond to Ni2P and CdS, respectively. Figure 2f displays linear scanning of EDS mapping of Ni2P/CdS and it is clearly supports the presence of Cd, S, Ni, and P elements. Figure 2g displays the TEM image of Pt/ CdS, which confirms that the composite consists of aggregated particles of Pt deposited on CdS. To further clarify this matter, we checked the composite material with HRTEM, as shown in Figure 2h. This image confirms that Pt cocatalyst nanoparticles are deposited on the surface of CdS to form a Pt−CdS junction that exhibits lattice spacing of approximately 0.2 and 0.3 nm, which correspond to Pt and CdS, respectively. In addition, we have confirmed the presence of Cd, S and Pt elements by linear scanning of EDS mapping of Pt/CdS and showed in Figure 2i. Additional element mapping analyses imply the presence of Ni2P with fine dispersion in the composite materials (Figure S1 in the SI) and are in good agreement with previous reports.51 The XPS spectra of Ni2P/CdS provide clear evidence for the presence of Ni and P in the composite material (Figure 3).
were recorded by exciting samples at 410 nm. Figure 1c reveals that, the PL spectra of the CdS and composite Ni2P/CdS catalysts. From the PL data, it can be inferred that CdS shows distinct emission bands that probably are associated with near band-edge emission, while Ni2P/CdS exhibits behavior attributed to surface defects. According to the literature,56 photogenerated carriers are easily transferred to surface states, wherein electrons are not available for H2 evolution. After loading Ni2P onto CdS, the PL intensity of the emission bands becomes weaker, which suggests a fast photo induced electron transfer process in Ni2P/CdS. Moreover, the weaker intensity of the emission bands indicates that most surface states are passivated, making the electrons available for photocatalytic H2 evolution. We conducted morphological studies to characterize the detailed microstructure of the materials. Figure 2a contains a scanning electron microscopy (SEM) image of as-synthesized Ni-BTC (MOF), which confirms the presence of spherical shapes of uniform size. Figure 2b shows the TEM image of CdS nanoparticles, where it is evident that the lattice spacing of approximately 0.33 nm. Figure 2c shows the TEM image of Ni2P nanoparticles, where it is evident that the size of the spheres ranges from 5 to 10 nm. Figure 2d displays the TEM image of Ni2P/CdS, which confirms that the composite consists D
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Figure 3. XPS spectra of the Ni2P/CdS composite: (a) Cd 3d, (b) S 2p, (c) Ni 2p, and (d) P 2p.
Figure 3a and b displays the Cd 3d and S 2p XPS spectra, respectively. The Cd 3d5/2 and Cd 3d3/2 binding energies occur at 405.04 and 411.76 eV, respectively, and the S 2p3/2 and S 2p1/2binding energies occur at 161.77 and 168.81 eV, respectively. Okotrub et al.54 synthesized CdS nanoparticles and found a similar pattern in the Cd 3d and S 2p XPS spectra. Figure 3c and d show the Ni 2p and P 2p XPS spectra, respectively. The peaks at 852.9 and 856.3 eV correspond to Ni(δ+) and Ni2+ of Ni2P, respectively. For P 2p XPS spectrum, the binding energy at 129.7 eV is attributed to the P(δ−) of Ni2P and 133.0 eV is ascribed to the surface nickel phosphate species. These results clearly indicate the presence of Ni2P in the synthesized composites. Xue et al.53 synthesized Ni2P by a microwave method and found binding energies by XPS that are in good agreement with our results. The actual composition of Ni for sample Ni2P/CdS should be characterized by ICP-OES test and confirmed by Ni wt % is 1.2, and these results are good agreement by earlier reports.56 To evaluate the photogenerated electron transfer in Ni2P/ CdS, the electrochemical impedance spectroscopy (EIS) measurements have been performed. Figure 4 shows Nyquist plots (Zim vs Zre) from EIS measurements of the CdS and Ni2P/CdS nanocomposites under simulated sunlight irradiation. The EIS spectra were recorded in buffer solution containing 4.0 mM [Fe(CN)6]3−/[Fe(CN)6]4− (1:1) and 0.1 M KCl using an alternating current voltage of 10 mV, with the frequency range of 0.1−100 kHz. Apparently, the Nyquist plot of the Ni2P/CdS shows a smaller arc diameter than that of the bare CdS nanostructures, indicating that the nanocomposite can efficiently transfer charge carriers and effect migration of photogenerated electron−hole pairs.63 In addition, the inset of Figure 4 also shows an equivalent circuit by fitting with experimental EIS data, where Rs is the solution resistance of the electrolyte solution, W is Warburg impedance, resulting from
Figure 4. Nyquist plots obtained for the CdS, Ni2P/CdS electrodes in 0.1 M KCl containing 4.0 mM [Fe(CN)6]3−/[Fe(CN)6]4− (1:1). (inset) Equivalent circuit for the corresponding data.
the diffusion of ions from the bulk electrolyte to the electrode interface, Rct is electron transfer resistance, Q is the constant phase element, RL is the film resistance, and CL is the interfacial double layer capacitance between an electrode and a solution, relating to the surface condition of the electrode.64 The Rct values were 7250 and 4883 kΩ for CdS and Ni2P/CdS, respectively. From the equivalent circuit, the obtained Rct value for Ni 2 P/CdS was much smaller than that of CdS nanostructures, indicating that Ni2P/CdS has excellent charge transfer capability. The above results all are in good accordance with the photocatalytic activities. The synthesized materials were tested for photocatalytic hydrogen production. The results are presented in Figure 5. Figure 5a shows the effect of the Ni2P loading on CdS. The amount of Ni2Pwas varied from 0 to 5.0 wt %. The composite E
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Figure 5. Photocatalytic activity of CdS and composites: (a) effect of Ni2P loading on CdS, (b) reproducibility test of 2% Ni2P/CdS, (c) comparison of photocatalysts, (d) recyclability test of 2% Ni2P/CdS. A 1 mg potion of catalyst was dispersed in 15 mL of 20 vol % aqueous lactic acid solution under simulated solar irradiation.
Recycling experiments were carried out to evaluate the stability of the Ni2P/CdS photocatalyst. Figure 5d shows the rate of H2 production tested for five cycles with 5 h irradiation in a solar simulator with the optimized catalyst (2 wt % Ni2P/ CdS). An almost identical amount of H2 generation was recorded in all the five experiments, but minor reductions occurred in the second and third cycles. This behavior is attributed to the oxidation of lactic acid, which leads to a decrease in lactic acid concentration and formation of oxidized products that may adsorb on the surface of the photocatalyst and block the penetration of light to the active sites. After again we added 3 mL of lactic acid and performed the photoexperiment for two more cycles, the rate of H2 production retains as like first cycle. A plausible mechanism for efficient H2 production under solar irradiation using lactic acid as a hole scavenger and Ni2P/ CdS as a photocatalyst is depicted in Figure 6. Excitation of semiconducting CdS in Ni2P/CdS by visible light generates electron−hole pairs at the conduction (CB) and valence bands (VB), respectively. These electrons are scavenged by the Ni2P nanoparticles on the CdS surface followed by reaction with protons to produce molecular hydrogen. The proposed reaction mechanism is in a good agreement with an earlier report.36,52,57−62 Sacrificial agents (water, lactic acid, or its intermediates) are consumed by photogenerated holes in the VB of CdS, which results in the generation of protons and intermediates. We have verified the pH of reaction solutions
catalyst shows a very high rate of H2 generation compared to that achieved with CdS. The presence of Ni2P in the catalyst triggers a greater rate of H2 production, which increases with increasing Ni2P loading to an optimum level at 2.0 wt % and then decreases. Under optimum conditions, the rate of H2 production is 33,480 μmol h−1 g−1, which is 62 times greater than the rate for pure CdS nanoparticles (534 μmol h−1 g−1). This high increase in photocatalytic activity is attributed to the fine dispersion and good electrical conductivity of the Ni2P nanoparticles. At lower Ni2P loadings (0.5 and 1.0 wt %), less H2 production is observed due to the smaller number of catalytically active sites. Above the optimum level (>2.0 wt % Ni2P), the lower rate of H2 formation is attributed to coverage of the CdS surface by excess Ni2P, which obstructs the incident light and prevents the generation of electrons from CdS. The Pt loading on CdS for H2 production was optimized and 2 wt % of Pt shows the best performance (Figure S3 in the SI). To confirm the reproducibility of H2 production with the optimized catalyst (2 wt % Ni2P/CdS), the experiment was repeated three times and almost identical results were obtained (Figure 5b). The optimized results were also compared with those for pure Ni2P, pure CdS and Pt/CdS cocatalyst as shown in Figure 5c. The rate of H2 production for 2 wt % Ni2P/CdS was 112, 62, and 2.6 times greater than that for pure Ni2P (297 μmol h−1 g−1), pure CdS (534 μmol h−1 g−1), and Pt/CdS (12,420 μmol h−1 g−1), respectively. F
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ACS Sustainable Chemistry & Engineering Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by National Research Foundation of Korea (NRF) grants funded by the Korean government (MEST and MSIP; 2014R1A4A1001690). This research was also supported in part by the Global Research Laboratory Program [Grant No. 2009-00439] and the Max Planck POSTECH/KOREA Research Initiative Program [Grant No. 2011-0031558] through the MEST’s NRF funding.
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Figure 6. Proposed reaction mechanism of Ni2P/CdS.
before and after the photo experiments and observed variation in pH from 2.06 to 2.13. We assumed that the changes in pH of reactions solutions are due to the formation of oxidized products. According to Zhang et al.55 the pH of the reaction solutions is different before and after reaction, originating from the formation of oxidized intermediates from lactic acid to pyruvic acid after photoreaction. The presence of Ni2P minimizes the recombination of photogenerated charge carriers and enhances the rate of H2 production.
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CONCLUSIONS A nanoparticles nickel phosphide (Ni2P) cocatalyst has been synthesized via a simple MOF procedure using cheap and abundant elements. X-ray photoelectron spectra and TEM images confirm the interaction of Ni2P with the surface of CdS nanoparticles. Optical properties indicate that the CdS band edge in Ni2P/CdS is shifted to longer wavelength. PL spectra reveal a decrease in the rate of charge carrier recombination rate in Ni2P/CdS. Ni2P acts as an effective cocatalyst on the CdS surface for photocatalytic H2 production by water splitting under simulated solar irradiation. The rate of hydrogen production is 62 times greater than that of pure CdS and 2.6 greater than that of expensive Pt cocatalyzed CdS, which illustrates a great improvement in the photocatalytic activity by Ni2P. Catalytic experiments demonstrate the reproducibility and recyclability of the Ni2P/CdS catalyst over at least three cycles without any significant loss in activity.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.6b02032. Elemental analyses, FESEM mapping with EDAX patterns, and Tauc plots for band gap calculations (PDF)
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REFERENCES
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DOI: 10.1021/acssuschemeng.6b02032 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX
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DOI: 10.1021/acssuschemeng.6b02032 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX
