Visible Light Harvesting and Spatial Charge Separation Over Creative

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Visible Light Harvesting and Spatial Charge Separation Over Creative Ni/CdS/Co Photocatalyst Hao Yang, Zhiliang Jin, Duanduan Liu, Kai Fan, and Guorong Wang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b01666 • Publication Date (Web): 24 Apr 2018 Downloaded from http://pubs.acs.org on April 24, 2018

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The Journal of Physical Chemistry

Visible Light Harvesting and Spatial Charge Separation Over Creative Ni/CdS/Co3O4 Photocatalyst

Hao Yanga, Zhiliang Jina*, Duanduan Liu, Kai Fan, Guorong Wang School of Chemistry and Chemical Engineering, North Minzu University, Yinchuan 750021, P.R.China

Abstract: In this case, the validity of the work on a stable nickel, cadmium sulfide, and spinel cobalt oxide composite proton-reduction photocatalyst are reported. Ni nanoparticles (acted as hydrogen collector) and Co3O4 nanoparticles (acted as oxygen collector) uniform assembles on the surface of CdS nanorods was prepared and the synergetic enhancement of light harvesting and charge management were simultaneously obtained and the efficient hydrogen evolution was revealed here as well. The Ni/CdS/Co3O4 composite photocatalyst shows a high H2 evolution yield of 207.63 µmol in 5h, which is greater 8.44 times than the pristine CdS (24.61µmol). Here, CdS nanorods with a rod-like structure provide a large number of attachment sites for Ni nanoparticles and the Co3O4 nanoparticles. The detail inner reason was comprehensively studied and understood by means of SEM, TEM, XRD, XPS, UV-vis DRS, especially, particular investigation of their photoelectrochemical properties with Photocurrent, Voltammetric Scanning, Fluorescence Spectra. etc. The high photocurrent response, the lower overpotential (-0.42 V vs SCE), the faster electron transfer rate (ket = 3.58 × 108 s−1) and the short fluorescence lifetime (2.74 ns) supported the efficient spatial charges transfer between CdS and Ni as well as Co3O4. The simultaneous loading

of bicocatalysts can significantly improve the photo-induced spatial charge separation of CdS because of Ni nanoparticles act as electron collectors to rapidly transfer electrons from CdS while Co3O4 acts as a holes collector to quickly transfer holes, and enhancing its photocatalytic performance as well.

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1. Introduction With the development of science and technology as well as the improvement of human living standards, the issue of energy depletion and environmental pollution has become a serious challenge to mankind in the 21st century. 1 It is the key to solve these problems finding an efficient, inexpensive and clean energy that can replace traditional fossil fuels. 2 Undoubtedly, hydrogen is the best energy due to its high heat value and pollution-free clean energy. However, the traditional method of production hydrogen seriously hampers its practical application due to its high cost.

3

Since the TiO2 photoelectrode was first reported by in 1972, the photocatalytic decomposition of water to produce hydrogen as a very viable approach has been extensively studied in the past few decades. 4 In general, it is regarded that photocatalysis contains three stages: light absorption process, photogenerated charge transfer process and surface reactivity. The light capture can be enhanced by selecting the semiconductor with appropriate bandgap or adding a sensitizer. Moreover, two important stages for determining the quantum efficiency of the photocatalytic process are charge transfer and surface reaction.

5-10

Selectively adding a suitable cocatalyst on the surface of

semiconductor is an effective method which could inhibit recombination of electrons and holes and increase the surface active sites. 11-13 Precious metal nanoparticles are universal cocatalysts for effective photocatalytic hydrogen evolution.

14-16

For example, TiO2 shows an excellently

photocatalytic performance after loading Pt nanoparticles.

17

Recently, many metal phosphides

have also been reported to significantly improve photocatalytic hydrogen evolution performance. 18-20

It is reported that Ni2P as cocatalyst supported CdS nanorods to exhibit efficient

photocatalytic hydrogen evolution activity.

21

In addition, it has been reported that it can also

suppress the recombination of photogenerated electron-hole pairs when bimetallic nanomaterials loaded on the surface of the semiconductor. 22, 23 Wang et al reported that metal Cd loaded on CdS can serve as an analogue of cocatalysts for enhancing the separation of electrons and holes.

24

Some metal oxide (such as IrO2, Cr2O3 and CoOx) has been widely studied as oxygen-evolution cocatalyst.

25, 26

For instance, Liu et al. reported that In2O3 acts as a cocatalyst to enhance the

charge separation of TiO2.

27

Moreover, the oxyhydroxide as a cocatalyst to promote

photocatalytic oxygen production has also been widely concerned. 28


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Although semiconductor surface-supported or oxidized cocatalysts can move electrons or holes rapidly, random flow of holes or electrons will increase recombination of photo-generated charge. It can solve the above problems by supporting cocatalyst with hydrogen evolution to induce rapid migration of photo-generated electrons, while loading the cocatalyst with oxygen evolution to induce rapid transfer of photogenerated holes. For example, Ta3N5 hollow sphere loading with Pt nanoparticles on the inner as electrons collector and loading with IrO2 on the outer as holes collector transfer electrons and holes, respectively, which decrease the recombination of photo-generated charge. 29 Qin et al. reported the porous TiO2 nanotubes with dual cocatalysts, Pt nanoclusters acting as electron collector deposited on the inner surface and CoOx as holes collector deposited outer surface, which exhibited effect of the spatially separated.

30

Wang et al.

employ SiO2 as a template to synthesize C3N4 hollow spheres, which loading Pt particles inside as an electron collector while supporting Co3O4 on its outer surface as a holes collector. 31 Recently, Gong et al. reported that the construction of Pt@TiO2@In2O3@MnOx mesoporous hollow spheres reduce bulk and surface recombination by the thin heterojunctions and the spatially separated co-catalysts.

32

To a certain extent, there are also some drawbacks in loading the inner surface of

the hollow sphere with cocatalyst, although the hollow sphere structure enhances the separation of photo-generated electron and holes as described above. The closed hollow sphere structure hinders the contact of the inner space with the aqueous solution, which leads to the generation of H2 or O2 cannot be transferred out in time. A better way to solve this problem is that two different cocatalysts are loaded on the surface of the semiconductor, so that the generated H2 or O2 can directly transfer to the aqueous solution. In addition, most of the above methods use precious metal as a cocatalyst, increasing the cost of the catalyst. Therefore, it is very meaningful to develop non-precious metal cocatalyst for photocatalysis. CdS is a very suitable semiconductor photocatalyst due to its proper bandgap (2.4 eV), simple synthesis and low cost.

33-36

However, its photocatalytic activity is limited by the rapid

recombination of charge carriers and low photostability. Moreover, as a cocatalyst, non-noble Ni nanoparticles can enhance transfer of the photogenerated electron.

37, 38

For instance, Dong et al.

reported Ni nanoparticles loaded on g-C3N4 showed a high photocatalytic H2 production rate by a rapid light-assisted method.39 Xu et al. reported that surface modification of CdS nanoparticles with Ni nanocrystals by in situ photo-deposition, which showed an efficient photocatalytic activity



to split alcohols under visible light. catalyst has been reported.

41-43

40

Non-novel metal oxide Co3O4 as an oxygen evolution

Therefore, it is very promising that constructing an efficient,

low-cost and toilless composite potocatalyst with targeted space charge separation. In this work, CdS nanorods with uniform morphology were prepared by solvothermal method, then the Co3O4 particles were fixed on the surface of CdS nanorods by physical adsorption and the Ni nanoparticles were riveted to the surface of CdS nanorods by photoreduction (Scheme 1). This dual cocatalyst-modified composite catalyst shows high H2 evolution yield (207.63 µmol/10mg of H2 in 5h) with AQE (6.46% at 400 nm) and excellent electrochemical performance. The PL measurements show that the Ni/CdS/Co3O4 composite catalyst has an efficient separation of photogenerated charges and faster electron transfer rate constant (ket) (3.58 × 108s−1). Ni nanoparticles can act as electron collectors to rapidly transfer electrons accumulated on CdS while Co3O4 acts as holes collectors to quickly transfer holes, significantly enhancing the photocatalytic activity of the CdS under synergistic action.

Scheme 1．Synthesis process of Ni/CdS/Co3O4 composite photocatalyst.

2. Experimental 2.1 Preparation of photocatalysts 2.1.1 Preparation of CdS nanorods. All chemicals were purchased from Sinopharm chemical reagent Co., Ltd (China) and used without further purification. The CdS nanorods were prepared by means of our previous method. Specifically, 4.624 g of hydrated cadmium chloride (CdCl2•2.5H2O) and 4.624g of thiourea (CN2H4S) was added into 60


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mL of ethylenediamine (C2H8N2) and magnetically stirred 2.5h. Then, the mixed solution was transferred into a 100 mL polytetrafluoroethylene stainless autoclave and maintained 433K for 48 h. The CdS nanorods were collected by centrifuge and washed three times by N-hexane, ethanol and water, and vacuum dried at 333K for 10 h.

2.1.2 Preparation of Co3O4 nanoparticles. The 1 mmol (0.0125g) of cobaltous acetate tetrahydrate (C4H14CoO8) was dissolved in 50 mL of deionized water and stirred 15min to form settled solution. Then, the 0.8 mL of ammonium hydroxide (28%) was injected into above solution and stirred for 15 min. Subsequently, the above solution transferred into 100 mL polytetrafluoroethylene stainless autoclave and maintained 423K for 3 h. Co3O4 nanoparticles were collected by centrifuge and washed several times by ethanol and water, and dried at 333K for 10 h.

2.1.3 The synthesis of CdS/Co3O4 composite catalyst. Typically, the 0.1g of prepared Co3O4 nanoparticles were added to 100 mL of ethanol and dispersed by ultrasonic for 1h to form a uniform suspension. Then, 0.1g of prepared CdS nanorods were added into 5mL absolute ethyl alcohol and dispersed by ultrasonic for 1h to form a uniform suspension. Then, different volume of 1 mg/mL ethanol solution of Co3O4 (0.5, 1, 2 and 3mL) was added into above suspension of CdS, sonication was continued for 1 hour and then stirred for 3 hours. Subsequently, the mixed suspension was dried at 213K in a vacuum oven to remove ethanol to obtain a composite catalyst.

2.1.4 Synthesis of Ni/CdS/Co3O4 composite photocatalysts The Ni/CdS/Co3O4 composite photocatalyst was synthesized by photo-reduction method. Typically, 0.1 g of prepared CdS-Co3O4, 3mL of 0.1mol NiCl2 aqueous solution, 21ml of 0.1mol/L NaH2PO2 aqueous solution and 30mL of triethanolamine (TEOA) solution were added to 46mL of H2O. Subsequently, the above mixed solution was ultrasonically treated for 15 minutes to sufficiently disperse the components and degassed with N2 to remove the air. Photoreduction reaction was performed in a sealed quartz reaction flask sealed with a flat window on the side, it was irradiated by a 300W xenon lamp as a light source and controlling the distance is 20 cm between the reaction vessel and the light source. The reaction was held for 20 min and the product was collected by centrifugation. The final sample was washed three times with water and ethanol respectively. Then, the solid dried at 333 K for 4 hours in vacuum.



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The sample of CdS/Ni was obtained similar to the above preparation method except that CdS/Co3O4 was replaced with CdS.

2.2. Characterization of catalysts The morphology of the sample was characterized by a field-emission scanning electron microscope (SEM JSM-6701F.JEOL) and a transmission electron microscopy (TEM FEI Tecnai TF20). X-ray diffraction analysis (XRD, Rigaku RINT-2000) was used to determine the crystalline structure of the sample. X-ray photoelectron spectroscope (XPS, ESCALAB 250Xi) was used to identify the element composition and the chemical state of elements for samples. UV-2550 (Shimadzu) spectrometer was used to test the UV-vis diffuse reflectance spectra (using BaSO4 as the reference. Photoluminescence (PL) emission spectra and time-resolved PL measurements were obtained by a FLUOROMAX-4 spectrophotometer at room temperature (Horiba Scientific,). Photoelectrochemically measurements were manipulated using an electrochemical workstation (Versa stat 4, Advanced Measurement Technology, Inc. America).

2.3. Photocatalytic H2 evolution experiments Photocatalytic experiments were conducted in a quartz glass reactor ca. 62 cm3 and the opening of the reactor was sealed with a silicone rubber septum (PCX50A Discover, Perfect Light, Beijing, China). In a typical photocatalytic experiment, 10 mg of catalyst was suspended in 30 ml aqueous solution containing 0.35M Na2S·9H2O and 0.25M Na2SO3. Then, the system was degassed by bubbling N2 gas to remove oxygen. The photocatalytic process is carried out under 5-W light emitting diode lamp using magnetic stirring during the reaction to ensure the catalyst was suspended and dispersed in the solution. The amount of hydrogen evolution was detected by a gas chromatography with TCD detector (Tianmei GC7900, 5 A column, N2 as carrier). The apparent quantum efficiency (AQE) was also measured according to equation (1).

44, 45

Specifically, 20mg of Ni/CdS/Co3O4 sample was suspended in 100 ml aqueous solution containing 0.35M Na2S·9H2O and 0.25M Na2SO3. Then, the system was degassed by bubbling N2 gas to remove oxygen. Light source use a 300W xenon lamp and monochromatic wavelength radiation was controlled by a narrow-band filters with different wavelengths. Photon flux of the incident light was measured by a PL-MW200 photoradiometer (Perfect Light, Beijing, china). Calculated amount of H2 evolution in 30 minutes in different wavelengths after the photocatalyst was excited


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for 30min under a light (λ ≥ 420nm).

AQE =

×

× 100%

(1)

2.4. Photoelectrochemical measurements Preparation of working electrode: The Fluorine doped tin oxide (FTO) substrate was customized with a size of 1×2.5cm. Then, the prepared substrate was swashed by means of cleaning agent, acetone solution, isopropyl alcohol, ethanol and water under ultrasonic processing for 30 min. 5 mg of samples suspended in 200µL ethanol solution containing 20µL of 5% Nafion solution and dispersed by ultrasonication for 30 min. Subsequently, the suspension solution was placed into prepared substrate (controlling the coating area of about 1cm2). The working electrode is obtained after the painted electrode is dried in a vacuum oven at 333K for 3h. All PEC measurements were examined in a three-electrode system by an electrochemical workstation (VersaStat4-400, Advanced Measurement Technology, Inc.). The irradiation source was employed a 300W xenon lamp equipped with a filter (λ≥420nm) and 0.2M of Na2SO4 aqueous was used an electrolyte. A Pt plate electrode (1.5×2cm) is used as the counter electrode, a saturated calomel electrode (SCE) used as the reference electrode and the prepared electrode used as the working electrode.

3. Results and discussion. 3.1 Characterization structure of photocatalysts The crystal structures of CdS, CdS/Ni, CdS/Co3O4 and Ni/CdS/Co3O4 samples are tested by X-ray diffraction (XRD), as shown in Fig. 1. It can be clearly observed that pure CdS has six sharp peaks at 24.837 o, 26.534 o, 28.216 o, 43.737 o, 47.893 o and 51.889 o corresponding to the (100), (002), (101), (110), (103) and (112) plane, respectively, which is well agreement with the hexagonal phase CdS (PDF#77-2306). 46, 47 However, the diffraction peaks of the Ni nanoparticles and the Co3O4 particles are not observed in the composite catalyst because the content of Ni and Co3O4 was relatively small on the surface of the CdS.



Fig. 1. The X-ray diffraction (XRD) patterns of pure CdS, CdS/Ni, CdS/Co3O4 and Ni/CdS/Co3O4.

As shown in Fig. 2A, the scanning electron microscopy (SEM) images show that the pure CdS nanorods have a uniform size and smooth surface. The Co3O4 nanoparticles are decorated on the surface of CdS by the physical mixing method. Then, the composite catalyst is annealed at 573K for 2 hours to make the two components more tightly. As shown in Fig. 2B, the Cd/Co3O4 composite catalyst maintains the structure of CdS nanorods after annealing and the nanoparticles of Co3O4 present on the surface of CdS. As shown in Fig. 2C, Ni nanoparticles are well dispersed on the surface of CdS. Photodepositing Ni nanoparticles onto the surface of CdS has been reported in our previous work. 48 In TEOA solution, Ni2+ will be formed a [Ni(TEOA)2]2+ with TEOA and can be absorbed onto surface of CdS. Under the UV-visible light illumination, CdS is photo-excited to generate photogenerated electrons. The accumulated electrons, which are on the conduction band, transferred to the surface of CdS and combine with [Ni(TEOA)2]2+ to generate Ni nanoparticles. Fig. 2D shows the SEM of Ni/CdS/Co3O4 composite photocatalyst, it can be seen a large amount of particulates on the surface CdS nanorods. For the TEM pattern of Ni/CdS/Co3O4 composite catalyst (Fig. 2E), Co3O4 and Ni nanoparticles are successfully supported on the surface of the CdS nanorods can be observed. The energy-dispersive spectroscopy (EDS) spectrum (Fig. 2F) shows that the Ni/CdS/Co3O4 composite catalyst contains C, Cu, Cd, S, Ni, O and Co elements without other impurities. Among them, C and Cu elements were also detected because the carrier is used in the testing process for catalyst.


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Fig 2. The SEM patterns of A) pure CdS, B) CdS/Co3O4, C) CdS/Ni, D) Ni/CdS/Co3O4; E) The transmission electron microscopy (TEM) pattern of Ni/CdS/Co3O4; D) The EDS pattern of Ni/CdS/Co3O4.

The energy-dispersive spectroscopy (EDS) mapping spectrum clearly reflects the distribution of Ni and Co3O4 on the surface of CdS. As shown in Fig. 3, the signals of Cd, S, Ni, Co and O elements are clearly shown in Fig. 3B–F for the selected area in Fig. 3A. These results further confirm the presence of Ni and Co3O4 in the composite photocatalysts and the Ni and Co3O4 are uniform distribute to the surface of CdS nanorods.



Fig. 3. The EDS mapping pattern of Ni/CdS/Co3O4.

The surface chemical constitution and elements valence state of the composite catalyst Ni/CdS/Co3O4 are ascertained by the X-ray photoelectron spectroscopy (XPS) in Fig. 4. The XPS survey spectrum (Fig. 4A) depictes that all elements of S, Cd and Ni, Co and O are co-present in sample of Ni/CdS/Co3O4. In addition, C is also detected, which is because of the contamination introduced during the test. As shown in Fig. 4B, the high-resolution XPS spectrum of Cd 3d shows that the binding energy at 412.08eV and 405.34eV, which belongs to Cd 3d3/2 and Cd 3d5/2, respectively.

49

In Fig. 4C, the binding energy of S 2p at 162.82 eV and 161.67 eV, which can be

assigned to S 2p1/2 and S 2p3/2, respectively. 50, 51 In Fig.4D, O 1s XPS spectra can be divided into three peaks, the peaks at 533.21, 532.09 and 530.94eV are attributed to the absorbed O2, H2O and Co-O species, respectively.

52

The high-resolution XPS spectrum of Ni element (Fig.4E) showed

the binding energy at 871.71eV and the 853.09eV is obtained and corresponding to the nickel. In addition, the binding energy at 874.47eV and 855.85eV is assigned to Ni 2p1/2 and Ni 2p3/2,


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respectively, and the two satellite peaks at 861.2eV and 879.3eV are accompanied satellite.

40, 53

This is corresponding to NiO and come from testing process. The high-resolution XPS spectrum of Co element (Fig.4F) showed the peaks at 797.28eV and 781.64eV, which are ascribed to Co 2p1/2 and Co 2p3/2, respectively.

54

This result demonstrates that the Ni and Co3O4 nanoparticles

were successfully loaded onto the CdS surface and the Ni/CdS/Co3O4 photocatalyst was formed. In addition, XPS shows that the loading of Ni atom and Co atom are 0.63% and 0.5%, respectively, indicating that only a small amount of Ni and Co3O4 are decorated on the CdS nanorods.

Fig. 4. X-ray photoelectron spectroscopy (XPS) patterns of Ni/CdS/Co3O4.



3.2 UV-vis diffuse reflectance and band structure The optical properties of the samples are studied by means of UV-Vis absorption spectra. As shown in Fig. 5A, the pure CdS nanorods showed an absorption edge at the wavelength of 520nm. After introduction of Co3O4, the light absorption edge of CdS/Co3O4 composite catalyst is almost no significant change. Compared with pure CdS, the absorption intensity of CdS/Ni, CdS/Co3O4 and Ni/CdS/Co3O4 were increase from 520nm to 680nm, which is attributing to the different colour. Increased absorption within these ranges does not enhance photoexcitation to generate electrons and holes and has no effect on the photocatalytic activity because these ranges are insufficient to excite the CdS semiconductor. The effect of optical properties on photocatalytic activity is further examined by the band gaps of samples. Fig. 5B shows that the estimated band gaps of CdS, CdS/Ni, CdS/Co3O4 and Ni/CdS/Co3O4 are 2.34, 2.31, 2.30 and 2.28eV, respectively. The narrowed bandgap of CdS/Ni, CdS/Co3O4 and Ni/CdS/Co3O4 samples is more conducive to the generation of photo-generated electrons and holes because the absorption edge of CdS is increase, so as to enhance the photocatalytic activity of H2 production. Furthermore, the variations in the band gap could probably cause a different degree of delocalization and mobility of photoexcited electron-hole pairs, which might then result in different photocatalytic efficiency. 55

Fig. 5. A) The UV-vis absorption spectres of pure CdS, CdS/Ni, CdS/Co3O4 and Ni/CdS/Co3O4; B) the band gaps of pure CdS, CdS/Ni, CdS/Co3O4 and Ni/CdS/Co3O4.

3. 3 Photocatalytic activity The photocatalytic H2 evolution activity of CdS, CdS-Ni, CdS-Co3O4 and Ni/CdS/Co3O4 is carried out under visible light irradiation in 0.35M Na2S and 0.25M Na2SO3 as holes sacrificial


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agent. As shown in Fig. 6, the pure CdS nanorods reveal a lowest H2 evolution amount is 24.61 µmol, which is due to the high recombination rate of photogenerated electron-hole. After adding Co3O4 nanoparticles, the composite catalyst exhibits the higher rate of H2 evolution compared with pure CdS nanoparticles. This is due to Co3O4 as holes collectors, which can quickly transmit the photogenerated holes, thus reducing the recombined rate of electron-hole. In particular, the CdS/Ni composite catalyst shows a higher amount of H2 evolution (106.99 µmol) after 5h illumination. It is attributed to the rapid transfer of photoelectric electrons when Ni nanoparticles are loaded onto CdS nanorods act as electronic capture agents and active sites. This results show that the single Co3O4 nanoparticles as holes capture agent and Ni nanoparticles as electron capture agent can inhibit the recombination of electron-hole, thus enhancing the activity of photocatalytic hydrogenation. We design two nanoparticles of Ni and Co3O4 simultaneously decorate the surface of CdS nanorods. The Ni/CdS/Co3O4 composite catalyst shows the highest hydrogen production of 207.63 µmol compared with the addition of a single component. This because Co3O4 as a hole-trapping agent can rapidly transport photogenerated holes while Ni nanoparticles as electron-trapping agents can also rapidly derive photogenerated electrons, thereby minimizing recombination of photogenerated electron-hole when their synergistic effect. The separation of photogenerated electron-hole enhances the photocatalytic activity. Overall, above results demonstrate that the composite catalyst with two kinds of cocatalysts simultaneously assembled has high efficiency of hydrogen production by photocatalytic water splitting. As shown in Fig. 6B, the hydrogen production of composite catalysts Ni/CdS/Co3O4 are 160.98, 178.38, 207.62, 342.02 and 240.40 µmol, respectively, for Co3O4 at 0, 0.5, 1, 2 and 3mL. Both co-catalysts act synergistically to improve the charge separation of CdS. There is a mutual balance between Ni and Co3O4 to transfer photo-generated electrons and holes. With the increase of the amount of Co3O4 added, this enhancement effect becomes greater. However, when excess Co3O4 is added, the balance is destroyed, so the hydrogen production activity of the composite catalyst begins to decrease.



Fig.6 A) Time dependent photocatalytic H2 evolution over CdS, CdS/Ni, CdS/Co3O4 and Ni/CdS/Co3O4. (10mg of catalyst added into 30mL solution with 0.35M Na2S and 0.25M Na2SO3 under the visible light irradiation.) B) The H2 evolution of composite catalysts Ni/CdS/Co3O4 for different loading of Co3O4.

The stability of the Ni/CdS/Co3O4 composite for photocatalytic H2 evolution is carried out, as shown in Fig. 7A. The stability experiment is run for 25 hours over 5 cycles, replace the sacrificial solution of the system every 5 hours and replace the air in the system with N2. It can be seen that the catalyst basically retains the initial hydrogen production after 5 cycles, indicating that the catalyst is very stable. This is mainly due to the Co3O4 particles as holes collectors to quickly transfer photogenerated holes to avoid the photocorrosion of CdS. In addition, Ni nanoparticles are able to rapidly transfer photogenerated electrons so that the electrons accumulated on the CdS rapidly react with the protons. In addition, TEM images before and after the stability testing were also provided. As shown in Fig. 7B and Fig. 7C, the CdS almost maintained the original rod structure before and after the test, although some slight changes occurred. Stability test shows that the catalyst has good stability and practical significance.

Fig.7. A)The photocatalytic hydrogen production stability of Ni/CdS/Co3O4; B) The TEM images before the


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stability testing; C) The TEM images after the stability testing.

The composition of the solution in the photocatalytic system is an important factor for affecting the activity of the catalyst. The addition amount of Na2S and Na2SO3 in the system is also studied as shown in Fig.8. It can be seen the Ni/CdS/Co3O4 catalyst showed the highest H2 evolution activity when the system containing 0.35 M Na2S and 0.25M Na2SO3 (the molar ratio is 1: 1.4). The hydrogen yield of the Ni/CdS/Co3O4 catalyst is only 154.54 µmol when the molar ratio of Na2S and Na2SO3 is 1: 0.8. It decreases to 144.36 µmol when the molar ratio of Na2S and Na2SO3 increased to 1: 1.8. This result demonstrated that low or high molar ratio of Na2S and Na2SO3 aqueous solution are not conducive to the photocatalytic hydrogen production for Ni/CdS/Co3O4 catalyst．

Fig. 8. A) The effect of sacrificial agent composition on hydrogen generation activity of Ni/CdS/Co3O4; B) The AQE of Ni/CdS/Co3O4 under different wavelength from 400 to 550nm.

In order to investigate the wavelength dependence of photocatalytic H2 evolution, the AQE of Ni/CdS/Co3O4 have also examined under a wide range of visible light irradiation from 400 to 550nm. As shown in Fig. 8B, the AQE of the composite catalyst gradually increases as the monochromatic wavelength increases and the highest AQE of 14.7% is obtained at 550nm because to its higher potential of photon.

56

At the monochromatic wavelength of 550nm, the composite

catalyst cannot produce hydrogen, so its AQE is zero.

3.4 Optical and photoelectrochemical properties The PL spectrum is an effective way to reflect the trapping, migration and recombination of



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photogenerated charges in the photocatalysis. The low PL intensity corresponds to an efficient separation of photo-generated electron holes and excellent photocatalytic activity.

57, 58

As shown

in Fig.9a, the PL spectrum of CdS shows a strong emission band at 520 nm under the excitation wavelength of 380 nm, which is assigned to the excitonic band edge emission of CdS.

59

After

loading of Ni and Co3O4 nanoparticles cocatalysts, the PL emission of CdS exhibites a serious quenched phenomenon, which verified the recombination of photogenerated electron-hole pairs is significantly reduced. In particular, this quenching efficiency is most pronounced when both of cocatalysts are added simultaneously, which is consistent with photocatalytic H2 evolution. This shows that Ni and Co3O4 particles simultaneously loaded on the CdS surface to produce a synergistic effect. Ni acts as an electron trapping agent to rapidly transfer photogenerated electrons while Co3O4 acts as a hole-trapping agent to rapidly transfer photogenerated holes, thus significantly enhancing the photogenerated charge separation.

Fig.9. The PL spectra of pure CdS, CdS/Ni, CdS/Co3O4 and Ni/CdS/Co3O4.

Table 1. Kinetic analysis of emission decay for CdS, CdS/Co3O4, CdS/Ni, and the Ni/CdS/Co3O4 samples Parameters

A1

τ1

A2

τ2

A3

τ3

ket

χ2

Samples

s-1

(%)

(ns)

(%)

(ns)

(%)

(ns)

(ns)

CdS

26.62

1.51

37.73

5.47

35.65

62.50

3.98

1.81

CdS/Co3O4

22.05

1.10

41.77

4.78

36.18

56.32

3.39

1.73

2.47×108

CdS/Ni

24.43

1.11

41.15

4.67

34.42

54.51

3.17

1.75

2.38×108

Ni/CdS/Co3O4

26.58

0.98

40.09

4.56

33.33

56.12

2.74

1.82

3.58×108


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In order to further explore the effect of adding Ni nanoparticles and Co3O4 nanoparticles for the charge carrier of CdS nanorods, time-resolved PL measurements are exhibited. As a function of time, the TRPL are fit using a tri-exponential decay model using the following Equation (1). 58

! = "$+,,,. #$ exp − /*$ !

(1)

Where I is the normalised emission intensity; τi is respectively decay lifetime of luminescence; Bi is the corresponding weight factors.

As shown in Fig. 9B, the time-resolved photoluminescence (TRPL) spectra of CdS-Ni, CdS-Co3O4 and Ni/CdS/Co3O4 exhibit a faster PL decay compared with CdS nanorods, revealing the efficient charges separation efficiency. The fitting parameters are given in Table 1. The pure CdS reveal that two fast components of the emission decay respective lifetimes are 1.51 and 5.47 ns, respectively, and one slower component with a lifetime is 62.50 ns. These lifetimes are all reduced after loading of cocatalysts. To better evaluate the influence of cocatalyst Ni and Co3O4 nanoparticles on CdS emission decay, we calculate the average lifetimes according to Equation (2). 60, 61

< * >= ∑$+,,,. #$ *$ / ∑$+,,,. #$ *$

(2)

Where is the average lifetime, τi is respectively decay time of the individual components; Bi is the corresponding weight factors.

234 = 5

, 6,7898:∗>?@A8:

−5

, 6,7898:>?@A8:∗

(3)

Where ket is the upper limit of electron transfer rate constant, tF, donor*–acceptor and tF, donor–acceptor* are donor of energy and acceptor of energy, respectively.

Compared with the pure CdS, the average lifetime of CdS loaded with Ni, Co3O4 and both of them are decreased from 3.98ns to 3.39, 3.17 and 2.74 ns, respectively. The upper limit of electron transfer rate constant (ket) is calculated according to Equation (3) and result shows in Table 1. Based on the fastest decay component, the estimated value of ket is 2.47 × 108s−1, 2.38 × 108 s−1 and 3.58 × 108s−1 for CdS/Co3O4, CdS/Ni, and Ni/CdS/Co3O4, respectively. These results indicate that loading of Ni or Co3O4 on the surface of CdS provide a path for photogenerated charge transfer, which competes with the excited state deactivation of CdS, thus reducing PL lifetime and increasing the separation of photogenerated charge, thereby enhancing photocatalytic activity. 61 In order to gain a deeper insight into the electron transfer process for catalysts, photoelectrochemical properties are performed in 0.2 M Na2SO4 electrolyte. As shown in Fig.10A, the Transient photocurrent-time curves of CdS, CdS/Co3O4, CdS/Ni and Ni/CdS/Co3O4 are



obtained by an on-off cycles of intermittent irradiation under visible light irradiation. It can be observed that pure CdS shows the lowest photocurrent density, indicating the photogenerated charge of CdS has the slowest transfer rate and serious recombination rate of photogenerated charges. In contrast, the photo-current density of the CdS-Ni and CdS-Co3O4 electrodes all increase after adding Ni nanoparticles and Co3O4 nanoparticles, respectively. This indicates that the addition of Ni nanoparticles and Co3O4 nanoparticles facilitates the transfer of photogenerated charges and suppresses the recombination of photogenerated charges. In particular, when Ni nanoparticles and Co3O4 particles are simultaneously loaded onto the surface of CdS nanorods, the Ni/CdS/Co3O4 sample shows the highest photocurrent density. This may be attributed to the Ni nanoparticles can rapidly transfer the photogenerated electron accumulated on the CdS while Co3O4 as holes trap rapidly transfers the photogenerated holes on the CdS resulting in the highest photocurrent density of Ni/CdS/Co3O4. The photocurrent response of all the samples shows a trend with an order CdS < CdS/Co3O4 < CdS/Ni < Ni/CdS/Co3O4, which is consistent with the hydrogen production and PL results. Electrochemical impedance spectroscopy (EIS) is an effective method to reflect the charge transfer speed in the catalyst electrode. Fig. 10B shows the EIS Nyquist plots of CdS, CdS/Co3O4, CdS-Ni and Ni/CdS/Co3O4. As well known, the diameter of the semicircle in the Nyquist plot corresponds to the impedance of the electrode, and the larger the radius, the larger the impedance. It shows the semicircle diameter presents a trend in order CdS > CdS-Co3O4 > CdS-Ni > Ni/CdS/Co3O4, which indicates that the impedance of the CdS electrode is the largest. The CdS/Ni and CdS/Co3O4 electrode is smaller than that of the CdS electrode, and the Ni/CdS/Co3O4 electrode is the smallest. This result shows that the charge transfer rate is the fastest in the sample Ni/CdS/Co3O4 compared with other samples. The photoelectrochemical results are in good agreement with the above testing results, which indirectly shows that the addition of Ni nanoparticles and Co3O4 nanoparticles can rapidly transfer the photogenerated charges and inhibit the recombination of photogenerated charges.


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Fig.10. A) Transient photocurrent response curve; B) The electrochemical impedance spectroscopy (EIS) pattern.

C) Linear voltammetry scanning curve; D) Open-circuit voltage response curves of CdS, CdS/Ni, CdS/Co3O4 and Ni/CdS/Co3O4 electrodes in 0.2 mol/L Na2SO4 aqueous solution. The scan rate was 0.1 mV s−1.

The electrocatalytic H2 evolution activities of different electrodes are also examined by the linear sweep voltammetry (LSV) method. The photocatalytic activity of H2 evolution is dependent on the overpotential of samples.

62

As shown in Fig. 10C, the overpotential of CdS, CdS/Co3O4,

CdS/Ni and Ni/CdS/Co3O4 are -0.54, -0.48, -0.45 and -0.42V, respectively. Compared with CdS electrodes, the low overpotential is obtained after loading of the Ni and Co3O4 nanoparticles and Ni/CdS/Co3O4 electrodes shows the lowest overpotential. This is further proves the Ni and Co3O4 nanoparticles act as an important roles in efficient H2 evolution and Ni/CdS/Co3O4 is an excellent photocatalyst for H2 evolution, which results are consistent with the photocurrent experiment and photocatalytic performance. To investigate the promotion of photocurrent CdS electrode after the addition of cocatalyst, open circuit photovoltage (OCPV) measurements are examined. The measurements are carried out by recording the open circuit photovoltage when turning on and turning off the light. Open circuit voltage is defined as the difference in the Fermi levels between the photoanode and the counter electrodes. 63 As shown in Fig. 10D, at the beginning of illumination, the photovoltage response of



electrodes increases sharply and gradually became flat with the increase of illumination time. Subsequently, after turning off the light, the voltage of the electrodes returns to the original level again. The redox equilibrations determine the open circuit photovoltage of electrodes under dark conditions. When the turning on the light, the electrodes of semiconductor is excited to generate charges and the accumulating electrons in the electrode are transferred by workstation. Therefore, the photovoltage has been a dramatic increase. Once the electron accumulation reaches a maximum a steady state, the increase of the photovoltage has become sloe, which because the competition with charge recombination.

64

Therefore, the highest open circuit voltage response

proves that the addition of cocatalyst Ni and Co3O4 nanoparticles enhances the separation of photogenerated charges for CdS electrode and enhances its photocatalytic performance.

3.5 The mechanism for H2 evolution Based on the above results, we have speculated possible mechanism of photocatalytic hydrogen production by water spitting under visible light irradiation. As depedicted in scheme 2, under visible light irradiation，the photogenerated electrons of CdS migrated to the conduction band while the photogenerated hole is remained in the valence band. Then, electrons and holes will then randomly move toward the CdS surface. In this process, a portion of the electrons and holes will recombine, which is an important factor that restricts the activity of the catalyst. In this work, Ni nanoparticles are loaded on the surface of CdS acting as electrons traps to rapidly transfer electrons, simultaneously Co3O4 acting as holes traps to rapidly transfer holes. Thus, the separation of photogenerated electrons and holes will be greatly enhanced by directing and rapid transfer for electrons and holes. Subsequently, the electrons transferred to the Ni nanoparticles will combine with protons in water to form H2 because Ni nanoparticles have very low Fermi levels. Accordingly, the holes transferred to the Co3O4 nanoparticles will be consumed by the SO32- and S2- to generate SO42- and S2O32-. In addition, the Ni nanoparticles and Co3O4 nanoparticles cause the enriched electron and holes on the CdS surface to separately participate in the reaction, thereby inhibiting the recombination of electron-hole pairs on the CdS surface.


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Scheme 2 The proposed photocatalytic mechanism for hydrogen evolution by water splitting under visible light irradiation.

4. Conclusions We successfully design and prepare the Ni/CdS/Co3O4 composite photocatalyst with spatial charge separation to produce hydrogen by photocatalytic water spitting. Specifically, CdS nanorods with a rod-like structure are provide a large number of attachment sites for Ni nanoparticles and the Co3O4 nanoparticles. The Ni nanoparticles are successfully loaded onto the surface of the CdS nanorods by a simple photo-reduction method and the Co3O4 nanoparticles are integrated by a physically mixed method. The results of SEM, TEM and XRD indicate that the Ni nanoparticles and Co3O4 nanoparticles are uniform assemble on the surface of CdS. The kinetics of photocatalytic H2 evolution show that the composite catalyst Ni/CdS/Co3O4 has excellent photocatalytic hydrogen production activity, which is about 8.44 times higher than that of the original CdS nanorods. This can be attributed to the rapid transfer of electrons by the Ni nanoparticles acting as electrons collectors while Co3O4 acts as holes collectors to quickly transfer holes. The PL results confirm the electron transfer rate constant (ket) is 3.58 × 108s−1 for composite catalyst Ni/CdS/Co3O4, which is faster than that of CdS-Ni (2.38 × 108s−1) and CdS-Co3O4 (2.47 × 108s−1). The fluorescence lifetime of Ni/CdS/Co3O4 (2.74ns) is shorter compared with CdS (3.98ns), CdS-Ni (3.17ns) and CdS-Co3O4 (3.39ns), which demonstrate the efficient separation of photogenerated charges. In addition, the electrochemical measurements reveal that the Ni/CdS/Co3O4 has high photocurrent response and low overpotential (-0.42 V vs SCE). These



results demonstrate that simultaneously loading Ni nanoparticles acting as hydrogen collector and Co3O4 nanoparticles as oxygen collector on the surface of CdS are effective strategies for improving its photocatalytic hydrogen production performance.

Acknowledgements This work was financially supported by the Chinese National Natural Science Foundation ( 41663012 and 21263001).

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Visible Light Harvesting and Spatial Charge Separation Over Creative

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