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CdS heterostructure for efficient photocatalytic hydrogen generation
Applied Surface Science 422 (2017) 962–969
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Full Length Article
Noble-metal-free NiO@Ni-ZnO/reduced graphene oxide/CdS heterostructure for efficient photocatalytic hydrogen generation Fayun Chen a,∗ , Laijun Zhang a , Xuewen Wang b,∗ , Rongbin Zhang b a b
College of Chemistry and Environmental Science, ShangRao Normal University, Shangrao 334001, PR China The Institute of Applied chemistry, The College of Chemistry, Nanchang University, 999# Xuefu Road, Nanchang 330031, PR China
a r t i c l e
i n f o
Article history: Received 16 March 2017 Received in revised form 23 May 2017 Accepted 24 May 2017 Available online 3 June 2017 Keywords: Ni ZnO/CdS Graphene Heterostructure Photocatalytic Hydrogen
a b s t r a c t Noble-metal-free semiconductor materials are widely used for photocatalytic hydrogen generation because of their low cost. ZnO-based heterostructures with synergistic effects exhibit an effective photocatalytic activity. In this work, NiO@Ni-ZnO/reduced graphene oxide (rGO)/CdS heterostructures are synthesized by a multi-step method. rGO nanosheets and CdS nanoparticles were introduced into the heterostructures via a redox reaction and light-assisted growth, respectively. A novel Ni-induced electrochemical growth method was developed to prepare ZnO rods from Zn powder. NiO@Ni-ZnO/rGO/CdS heterostructures with a wide visible-light absorption range exhibited highly photocatalytic hydrogen generation rates under UV–vis and visible light irradiation. The enhanced photocatalytic activity is attributed to the Ni nanoparticles that act as cocatalysts for capturing photoexcited electrons and the improved synergistic effect between ZnO and CdS due to the rGO nanosheets acting as photoexcited carrier transport channels. © 2017 Elsevier B.V. All rights reserved.
1. Introduction Photocatalytic hydrogen generation over semiconductors is a potential method of achieving solar-to-energy conversion [1–4]. During the photocatalytic hydrogen generation process, effective photo-reduction and −oxidation need to occur on different reaction sites to prevent the recombination of the photoexcited electrons and holes and counter reactions [5]. To restrict counter reactions and improve the transport of photoexcited carriers, a number of heterostructural materials with enhanced photocatalytic hydrogen activity have been developed [1,6–13]. The photocatalytic ability of heterostructures is related to the presence of suitable surface and interface states among the heterostructural phases. A high chargecarrier transport rate through the interfaces will induce an efficient synergistic effect. Thus, metals, graphene, graphene oxide (GO) and reduced graphene oxide (rGO) are widely used to fabricate carrier transport channels in photocatalytic materials to improve the interface/surface state [11,14–23]. Unlike graphene, rGO nanosheets involving a number of oxygen-containing functional groups and high surface areas are more suitable to form strong chemical bonding between the rGO and the heterostructural phases [14].
∗ Corresponding authors. E-mail addresses: [email protected] (F. Chen), [email protected] (X. Wang). http://dx.doi.org/10.1016/j.apsusc.2017.05.214 0169-4332/© 2017 Elsevier B.V. All rights reserved.
Cocatalysts are attached on the photocatalyst surface for capturing photoexcited electrons or holes and acting as photo-reductive or oxidative sites [24–28]. Noble metals and metal oxides are extensively used as cocatalysts for improving the photocatalytic hydrogen activity [3,29–31]. For example, TiO2 or CdS after loading of Pt nanoparticles exhibited significantly enhanced photocatalytic hydrogen generation rates [3,24,32,33]. However, noble metal nanoparticle loading will increase the cost of the photocatalytic materials. Non-noble metals can be loaded on the surface of photocatalysts via a reducing reaction, and can improve photocatalytic activity and reduce costs [1,25,26,34–40]. NiO or Ni is widely used in traditional catalytic reactions such as hydrogen reforming and CO oxidation [40,41]. In recent years, Ni-based nanoparticle cocatalysts are used on semiconductor photocatalysts and significantly enhance the photocatalytic activity [6,25,28,35,36,38,42]. Thus, it will be useful to develop Ni-based cocatalysts on more semiconductor photocatalysts. Previous reports [15,43] reported that ZnO-CdS heterostructures loaded with Pt nanoparticles as cocatalysts exhibited high photocatalytic hydrogen generation rates. However, the optimized content of Pt ranges from 1 to 3 wt%, which markedly increases the cost of the photocatalyst. In this study, NiO@Ni-ZnO/rGO/CdS heterostructures were designed and synthesized using a novel Ni-induced electrochemical and light-assisted growth process. The NiO@Ni-ZnO/rGO/CdS exhibit high and stable photocatalytic
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hydrogen generation rates under visible light irradiation due to NiO@Ni nanoparticles acting as cocatalyst and an enhanced synergistic effect between ZnO and CdS by the rGO nanosheets acting as a carrier transport channel.
at 100 ◦ C for 24 h. The reference ZnO/CdS was synthesized by the same method without addition of GO nanosheets.
2. Experimental
X-ray diffraction (XRD) patterns were recorded by a Rigaku diffractometer with Cu K␣ rradiation. Scanning electron microscopy (SEM) images and Energy Dispersive X-ray (EDX) spectroscopy profiles were acquired on an FEI Nova SEM 200. Transmission electron microscope (TEM) and High-resolution transmission TEM images were carried out on a Jeol-2100. The Brunauer–Emmett–Teller (BET) specific surface area and pore size distribution was obtained on ASAP–2010 M (Micrometritics). The ultraviolet (UV)-visible absorption spectra were performed on a UV–vis spectrophotometer (JASCO-V550). The contents of elements in the heterostructures were analyzed by an inductively coupled plasma-optical emission spectrometer (ICP-OES, Agilent-5100). X-ray photoelectron (XPS) spectra were analyzed by using a Thermo Escalab 250 system with a monochromatic Al K␣ (XPS) source. All of the binding energies were calibrated by the C 1 s peak (284.6 eV) produced by adventitious carbon. The electrochemical impedance spectra were measured by an electrochemical workstation (Princeton P-2273).
2.1. Preparation of photocatalyst materials The NiO@Ni-ZnO/rGO/CdS heterostructures were prepared by a redox reaction, Ni-induced electrochemical and light-assisted growth processes. GO solution was synthesized by an improved Hummer method [44]. In the preparing process, 0.5 g of zinc powder was dispersed in 50 mL of deionized water under a magnetic stirring. Subsequently, 10 mL of GO aqueous solution (ca. 0.5 mg mL−1 GO) was added in the above solution with a constant magnetic stirring for 10 min. And the black precipitate of Zn/rGO was formed in the aqueous solution. A predetermined amount of nickel acetate aqueous solution was slowly dropped in the aqueous solution containing Zn/rGO under a constant stirring. The solution was maintained in an oven (60 ◦ C, 24 h) for the growth of NiZnO/rGO. The Ni-ZnO/rGO was washed three times with deionized water. An aqueous solution containing different amounts of CdCl2 was added in the above mixed solution with a constant magnetic stirring under light irradiation for 1 h. Afterward, 50 mL of aqueous solution containing Na2 S (the mole ratio of CdCl2 /Na2 S = 1:1.5) was dropwise added with a constant stirring under light irradiation for 1 h. The precipitate was washed with deionized water and then vacuum dried and at 80 ◦ C for 24 h. The powder was denoted as NiO@Ni-ZnO/rGO/CdS heterostructures. The reference ZnO/rGO/CdS was synthesized by a solution method. 26 mL of GO solution was added in a 100 mL of aqueous solution containing 0.02 mol zinc acetate. Then 50 mL of aqueous solution including 0.04 of mol NaOH was dropped in the solution. 0.006 mol of cadmium acetate was added in the solution under stirring for 10 min. Finally, 50 mL of aqueous solution including 0.006 mol of Na2 S was dropwise added under constant stirring for 30 min. The precipitate was washed with deionized water and dried
2.2. Characterization
2.3. Photocatalytic hydrogen generation measurement A gas-closed circulation photocatalytic hydrogen generation system with a vacuum was used to evaluate hydrogen generation rates. In the measurement process, 100 mg of photocatalyst powder was uniformly dispersed in 300 mL of aqueous solution including 7.2 g of Na2 S·9H2 O, 3.78 g of Na2 SO3 and 30 mL of triethanolamine. The amount of hydrogen generation was tested by a gas chromatograph (capillary column, thermal conductivity detector and argon gas serving as carrier gas). The irradiation light source used in the photocatalytic hydrogen generation measurement was a 300 W of Xe lamp (PLX-300). And a longpass filter with 420 nm was used to cut off UV light.
Fig. 1. Scheme of the fabrication process of NiO@Ni-ZnO/rGO/CdS heterostructure prepared by the Ni-induced electrochemical and light-assisted growth process.
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3. Results and discussion The NiO@Ni-ZnO/rGO/CdS heterostructures were synthesized by a redox reaction, Ni-induced electrochemical growth, and lightassisted loading CdS process (Fig. 1). Zn spheres with diameter of less than 10 m were used as the precursor to synthesize ZnO rods. The XPS spectra of GO nanosheets used for preparing Zn/rGO are shown in Fig. S1. The peaks at 284.6, 286.9, and 288.5 eV are corresponded to sp2 carbon, C O single bond and C O double bond, respectively. GO nanosheets with a number of oxygen-containing groups (Fig. S1), which process an oxidative ability and high surface area, can easily react with Zn spheres that possess a strong reductive ability in aqueous solution at room temperature (25 ◦ C) [43]. In the preparing process, GO was reduced to rGO and loaded on the surface of ZnO spheres (Fig. S2). Ni atoms replaced the Zn atoms and were well loaded on the surface of the Zn/rGO composite after adding an aqueous solution of nickel acetate. Compared to Zn (1.65), Ni with a high electronegativity (1.91), exhibits higher chemical stability and can form an original cell, where Ni|H2 O|H2 and Zn|ZnO server as the cathode and anode, respectively. Thus, the Zn/rGO composite was continuously converted into ZnO/rGO through a Niinduced electrochemical growth process. CdS nanoparticles were loaded on the surface of ZnO/rGO via a light-assisted growth process. All the above preparation processes were performed in an aqueous solution. Therefore, it is a very simple method to synthesize the NiO@Ni-ZnO/rGO/CdS heterostructures. XRD patterns of Ni-ZnO/rGO and NiO@Ni-ZnO/rGO/CdS synthesized by the Ni-induced electrochemical and light-assisted growth
Fig. 2. Left panel: XRD patterns of Zn/rGO, Ni-ZnO/rGO, NiO@Ni-ZnO/rGO/CdS heterostructure and CdS nanoparticles; Right panel: the local XRD patterns at 40–50◦ (2) of Ni-ZnO/rGO in the left panel.
process are shown in Fig. 2. No characteristic peaks of Zn powders were found in the XRD patterns of Ni-ZnO/rGO and all diffraction peaks were attributed to hexagonal ZnO (space group: P63mc), which indicates that Zn powders were completely converted into ZnO after 24 h of Ni-induced electrochemical growth. The characteristic peaks of rGO were not observed in the XRD patterns possibly because of the low rGO content (1 wt%), which is below the detection range of the XRD instrument. However, a peak centered at 44.5◦ (2) corresponded to metal Ni in the XRD patterns of Ni-ZnO/rGO,
Fig. 3. SEM images of (a) the precursor Zn powders, (b) Ni-ZnO/rGO, and (c) NiO@Ni-ZnO/rGO/CdS; (d) local magnified SEM image of NiO@Ni-ZnO/rGO/CdS.
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Fig. 4. (a) TEM and (b) local magnified TEM images of the NiO@Ni-ZnO/rGO/CdS heterostructure; high-resolution TEM image of (c) CdS and (d) Ni nanoparticles in the heterostructure.
which suggests that Ni nanoparticles are still present on the surface of ZnO after the electrochemical growth. After loading 30 at% CdS on Ni-ZnO/rGO by a light-assisted growth method, significantly broader diffraction peaks in the CdS phase were observed that correspond to nanosized CdS compared to the sharp peaks observed with the ZnO phase. However, no obvious characteristic diffraction peaks of Ni were found in the NiO@Ni-ZnO/rGO/CdS possibly owing to the oxidation of Ni after thermal treatment in an oven. SEM images of the precursor Zn powders are shown in Fig. 3a. The diameter of Zn micro-spheres is less than 10 m. A ZnO shell, formed by natural oxidation, covers the surface of the Zn spheres [15]. Ni-ZnO/rGO prepared from Zn spheres by the redox and Niinduced electrochemical processes are shown in Fig. 3b. The size of the ZnO hexagonal prisms mainly ranges from 100 to 800 nm, which is smaller than that of the ZnO rods prepared by a Ptinduced electrochemical process [45]. EDX elemental mapping images shown in Fig. S3 also confirmed that element Ni is present in the Ni-ZnO/rGO composite. In the Ni-induced electrochemical growth, the process may be discontinuous and unstable owing to the oxidation of Ni. Therefore, the growth of ZnO rods could be arrested during the electrochemical process. Fig. 3c and d show that the rGO nanosheets are effectively loaded onto the surfaces of the ZnO rods during the ZnO formation process. Under light irradiation, Cd2+ ions were easily adsorbed on the surfaces of the rGO nanosheets with negative charge centers formed by photoexcited electrons of ZnO and fully converted into CdS nanoparticles in the aqueous solution containing excessive S2− ions [2]. EDX
elemental mapping images of NiO@Ni-ZnO/rGO/CdS (Fig. S4) also suggest that Zn, O, Cd, S, and Ni elements are uniformly dispersed in the heterostructure. The N2 adsorption-desorption isotherms and pore size distribution of the heterostructure are shown in Fig. S5. The BET surface area and total pore volume are 60.1 m2 g−1 and 0.113 m3 g−1 , respectively. The averaged pore diameter is 7.5 nm. To explore the microstructures and the interface states, TEM images of the NiO@Ni-ZnO/rGO/CdS are obtained as shown in Fig. 4a and b. A number of nanoparticles dispersed on the surfaces of the rGO nanosheets corresponded to CdS, and rGO nanosheets were well loaded on the surface of the ZnO rods. The high-resolution TEM images (Fig. 4c) indicate that the crystalline CdS nanoparticles are formed by a light-assisted growth method in an aqueous solution. The averaged diameter of the CdS nanoparticles is approximately 8 nm. A nanoparticle with a crystal plane spacing of 0.21 nm, corresponding to the (111) crystal plane of Ni [46], was located on the surface of the ZnO rod as shown in Fig. 4d, which suggests that the metal Ni is still present on the surface of the ZnO rod after Ni-induced electrochemical growth of ZnO and light-assisted growth of CdS. However, the surfaces of Ni nanoparticles are oxidized into NiO owing to the instability of Ni nanoparticles. Thus, NiO@Ni nanoparticles replace the noble metal Pt and act as capturing sites for the photoexcited electrons, and then carry on the photo-reductive reaction in hydrogen generation. The UV–vis light absorption spectra of the composite photocatalysts are shown in Fig. 5. All the NiO@Ni-ZnO/rGO/CdS and Ni-ZnO/rGO show a wide visible light absorption owing to the
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Fig. 5. UV–vis light absorption spectra of CdS, Ni-ZnO/rGO, NiO@Ni-ZnO/rGO/CdS with different amounts of CdS nanoparticles (the mole ratios of ZnO/CdS varying from 1:0.1 to 1:0.8).
addition of rGO nanosheets with a light absorption region greater than 1100 nm. However, the light absorption rate enhanced by the rGO sheets is not used for photocatalytic reactions because of its shielding effect. After the CdS nanoparticles are loaded on the NiZnO/rGO, a band of visible light ranging from 400 nm to 550 nm is observed in the NiO@Ni-ZnO/rGO/CdS. The photocatalytic hydrogen activity of the heterostructures is strongly affected by the mole ratio of ZnO and CdS. Therefore, NiO@Ni-ZnO/rGO/CdS with varying amounts of CdS was prepared in this work. The visible light absorbance ranging from 400 nm to 550 nm gradually increased with an increase in the loading content of CdS, whose light absorption edges are a little lesser than that of the CdS nanoparticles prepared by a solution method. The red shift of light absorption edges is due to the maturing of CdS nanoparticles in the NiO@NiZnO/rGO/CdS heterostructures [2]. To detect the chemical states of the heterostructural phases, the XPS spectra of NiO@Ni-ZnO/rGO/CdS and Ni-ZnO/rGO are shown in Fig. 6. In the synthesis process of ZnO/rGO, Ni nanoparticles were loaded onto the surfaces of the Zn spheres through a replacement process by the addition of an aqueous nickel acetate solution. The peaks of Ni 2p centered at 855. 8 and 887.3 eV suggest that Ni nanoparticles are present on the surface of the ZnO rods. Furthermore, the new peaks of Ni 2p centered at 862.0 and 879.7 eV indicated that Ni–O bonds were formed in the NiO@NiZnO/rGO/CdS owing to the oxidation of Ni. The NiO shells covered on the surface of Ni nanoparticles and formed NiO@Ni nanoparticles acting as cocatalysts on the surface of heterostructures in the photocatalytic reaction. The O 1s binding energies of 531.8, 531.5, and 530.4 eV in the heterostructures correspond to the O Zn bond in the ZnO rods, O Ni bond and the O C bond in the rGO nanosheets, respectively. The chemical bonds among ZnO, rGO, and CdS result in a significantly smoother charge-carrier transport process in the heterostructures. The ICP-OES result suggests that the Ni content in the NiO@Ni-ZnO/rGO/CdS prepared using 5% Ni is approximately 2.5%. The photocatalytic hydrogen generation rates of the heterostructures are greatly related to the sacrificial agents in an aqueous solution. Thus, the aqueous solution containing different sacrificial agents was used to estimate the photocatalytic activity under UV–vis light and visible light irradiation (Table 1). The NiO@Ni-ZnO/rGO/CdS exhibited low photocatalytic hydrogen generation rates of 225 and 114 mol h−1 under UV–vis and visible light irradiation in the aqueous solution containing 0.1 mol L−1 Na2 S and 0.1 mol L−1 Na2 SO3 , respectively. The hydrogen genera-
tion rates decreased to 175 and 73 mol h−1 in the aqueous solution containing 10 vol% C6 H15 NO3 . Unfortunately, no obvious hydrogen was detected in an aqueous solution containing CH3 OH (10 vol%). However, the NiO@Ni-ZnO/rGO/CdS heterostructures exhibited markedly improved photocatalytic hydrogen generation rates of 824 and 564 mol h−1 irradiated by UV–vis and visible light, respectively, in an aqueous solution containing 0.1 mol L−1 Na2 S, 0.1 mol L−1 Na2 SO3 , and 10 vol% C6 H15 NO3 . No hydrogen amount was detected in these solutions in the absence of photocatalysts under light irradiation. The above results suggest that composite sacrificial agents are suitable for photocatalytic hydrogen generation of the NiO@Ni-ZnO/rGO/CdS. The possible reason is that the sacrificial agents of Na2 S, Na2 SO3 and C6 H15 NO3 might be suitable to photo-oxidative reaction on the CdS and ZnO, respectively. The photocatalytic hydrogen generation rates over different photocatalysts are shown in Fig. 7a. The NiO@Ni-ZnO/rGO/CdS exhibited a hydrogen generation rate of 824 mol h−1 under UV–vis light irradiation, which is 588 times that of Ni-ZnO/rGO, 2.85 times that of the referenced ZnO/rGO/CdS and 7.5 times that of CdS nanoparticles. The low hydrogen generation rates of Ni-ZnO/rGO and CdS are attributed to the poor visible light absorption and surface combination of photoexcited electrons and holes, respectively. Moreover, the NiO@Ni-ZnO/rGO/CdS of visible light shows a hydrogen generation rate of 564 mol h−1 , which is 0.68 times that of UV–vis light. The high hydrogen generation rate of visible light suggests that the active photoexcited electrons used for the photocatalytic hydrogen generation reaction originate mainly from the CdS nanoparticles under light irradiation. The amount of CdS plays a key role in improving the photocatalytic activity of the heterostructures. The hydrogen generation rates of the heterostructures with different mole ratios of ZnO and CdS are shown in Fig. 7b. These heterostructures all exhibit higher hydrogen generation rates than those of the Ni-ZnO/rGO and CdS nanoparticles. The photocatalytic hydrogen generation rates of the NiO@Ni-ZnO/rGO/CdS markedly increased with increasing amounts of CdS nanoparticles. The NiZnO/rGO loading with 30 at% CdS nanoparticles exhibits the highest hydrogen generation rate and reaches 824 and 524 mol h−1 under UV–vis and visible light irradiation, respectively. The heterostructures exhibit higher light absorbance with increasing amounts of CdS. However, the corresponding hydrogen generation rates gradually decrease due to the decreased number of photocatalytic reaction sites on the surface of ZnO and maturing of the CdS nanoparticles. During the preparation process, NiO@Ni nanoparticles were introduced to form ZnO rods via an electrochemical method. The growth rate of the ZnO rods increased with increasing amount of Ni. The NiO@Ni nanoparticles also acted as cocatalysts for improving the photocatalytic activity on the surface of NiO@Ni-ZnO/rGO/CdS heterostructures. The loading amount of the cocatalyst will markedly affect the photocatalytic activity. Therefore, the NiO@Ni-ZnO/rGO/CdS heterostructures were prepared using different amounts of Ni and used for the photocatalytic hydrogen generation reactions (Fig. 7c). With the amount of Ni increase, the hydrogen generation rates gradually increase under light irradiation. When 5% Ni was used to prepare NiO@NiZnO/rGO/CdS, the heterostructure showed the highest hydrogen generation rate, which is higher than that of Pt-loaded ZnO rodsCdS in the previous work [43]. Meanwhile, the growth rate of ZnO from Zn powder rapidly increased with the addition of more nickel acetate. However, the photocatalytic activities of the NiO@NiZnO/rGO/CdS began to rapidly decrease. When 20% Ni was used to induce the growth of ZnO, the hydrogen generation rates of the heterostructure were only 198 and 135 mol h−1 under UV–vis and visible light irradiation, respectively. The low photocatalytic activities are attributed to the following reasons. i) Neither NiO nor Ni
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Fig. 6. High-resolution XPS spectra of C1s (a), Ni 2p (b), Zn 2p (c), O 1s (d), Cd 3d (e) and S 2p (f) in the NiO@Ni-ZnO/rGO/CdS and Ni-ZnO/rGO.
Table 1 Photocatalytic hydrogen generation rates of the NiO@Ni-ZnO/rGO/CdS heterostructure (100 mg) in various sacrificial agents under UV–vis and visible light irradiation. Sacrificial Agent Light Source
10 vol% CH3 OH
0.1 M Na2 S + 0.1 M Na2 SO3
10 vol% C6 H15 NO3
0.1 M Na2 S + 0.1 M Na2 SO3 + 10 vol% C6 H15 NO3
0.1 M Na2 S + 0.1 M Na2 SO3 + 10 vol% C6 H15 NO3 (No catalysts)
UV–vis Vis
0 0
255 141
175 73
824 564
0 0
shows any photocatalytic activity in an aqueous solution. A number of inactive NiO@Ni nanoparticles cover the surface of the ZnO rods, which will markedly reduce the number of photocatalytic reaction sites. ii) NiO@Ni nanoparticles loaded on the surface act as cocatalyst to capture the photoexcited electrons and then enhance the photocatalytic activity. However, NiO@Ni nanoparticles began to grow and reduce the action of the cocatalyst after excessive addition of Ni. To explore the photocatalytic stability of the heterostructures, long-term photocatalytic hydrogen generation reactions over the (5%) NiO@Ni-ZnO/rGO/CdS heterostructure were performed in an aqueous solution, as shown in Fig. 7d. The total hydrogen generation amounts of 15 h reached 12.2 and 8.5 mmol under UV–visible and visible light ( ≥ 420 nm) irradiation, respectively. No obvious degradation of hydrogen generation rate was found in the three-cycle photocatalytic reactions, which suggests that the NiO@Ni-ZnO/rGO/CdS is stable in an aqueous solution containing S2− , SO3 2− , and C6 H15 NO3 . The amount of hydrogen generation of visible light from the NiO@Ni-ZnO/rGO/CdS was 2.9 mmol in the first cycle (5 h) can reach 71% that generated under UV–vis light irradiation. The high visible-light activity suggests that photoexcited electrons used for hydrogen reducing reactions originate mainly from the CdS nanoparticles. When Ni-induced electrochemical growth method was used to synthesize NiO@Ni-ZnO/rGO/CdS heterostructures and rGO nanosheets were introduced into the heterostructures, the improved photocatalytic hydrogen activity and stability likely occurs because of the following reasons:
1) NiO@Ni nanoparticles replace Pt nanoparticles and then act as cocatalysts on the surface of the NiO@Ni-ZnO/rGO/CdS. The photoexcited electrons are captured by NiO@Ni nanoparticles and are used for photo-reduction of hydrogen generation. Meanwhile, the NiO@Ni nanoparticles would also prevent recombination of the photoexcited electrons and holes owing to the photo-reductive and oxidative reactions occurring on different sites. Here, the NiO@Ni nanoparticles are well loaded on the surface of ZnO via a replacement reaction. The NiO covers on the surface of Ni and improves the stability of the heterostructures. 2) The synergistic effect between ZnO and CdS is the main reason for the improved photocatalytic hydrogen activity. The scheme of photocatalytic reaction process in the NiO@Ni-ZnO/rGO/CdS heterostructure is shown in Fig. 8. CdS nanoparticles with a wide light absorption range provide a number of photoexcited electrons for the photo-reductive reaction under light irradiation. In the heterostructures, the classic carrier transport method, photoexcited electrons from CdS transported to ZnO through rGO nanosheets and being used for photocatalytic hydrogen generation, plays the key role in improving the photocatalytic activity. The photo-oxidative reactions of sacrificial agents occur on the surfaces of CdS and ZnO. 3) rGO nanosheets with a high carrier transport rate are introduced into the ZnO/CdS heterostructure for constructing chargecarrier transport channels between ZnO and CdS (shown in Fig. 8). Chemical bonds formed between the semiconductor phases and rGO nanosheets ensure smooth photoexcited transport to improve the synergistic effect between ZnO and CdS.
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Fig. 7. (a) Photocatalytic hydrogen generation rates of Ni-ZnO/rGO, the reference ZnO/rGO/CdS, NiO@Ni-ZnO/rGO/CdS and CdS irradiated by UV–vis and visible ( ≥ 420 nm) light; Photocatalytic hydrogen generation rates of (b) NiO@Ni-ZnO/rGO/CdS with different amounts of CdS nanoparticles and (c) NiO@Ni-ZnO/rGO/CdS prepared using different amounts of Ni; (d) Time-circle photocatalytic hydrogen generation rates over the NiO@Ni-ZnO/rGO/CdS.
powder and were well loaded on the surface of the Zn spheres. Meantime, GO was also reduced to rGO. The rGO nanosheets with a high carrier transport rate are suitable to photoexcited electron transport in the heterostructures. The CdS nanoparticles were loaded on the rGO nanosheets via a light-assisted growth process. A strong chemical contact was formed in the heterostructures prepared by a redox replacement reaction and Ni-induced electrochemical growth methods, which protects the synergistic effect between ZnO and CdS and improves stability during a long-term photocatalytic hydrogen reaction.
4. Conclusion
Fig. 8. Scheme of photocatalytic reaction process in the NiO@Ni-ZnO/rGO/CdS heterostructure.
Electrochemical impedance spectra (shown in Fig. S6) exhibits that the resistance of the heterostructure decreases after the introduction of the rGO nanosheets compared to that of the ZnO/CdS. Thus, the photoexcited carrier transport will become smooth in the NiO@Ni-ZnO/rGO/CdS; this finding is confirmed by the high photocurrent intensity generated by the heterostructure under visible light irradiation (Fig.S7). 4) The synthesis method is a key factor for the improved photocatalytic hydrogen activities of the NiO@Ni-ZnO/rGO/CdS. In the redox reaction, the GO nanosheets were reduced by the Zn
Noble-metal-free NiO@Ni-ZnO/rGO/CdS heterostructures with a wide visible light absorption range are synthesized by redox replacement, Ni-induced electrochemical growth and lightassisted growth methods. The NiO@Ni-ZnO/rGO/CdS exhibits an improved photocatalytic hydrogen generation rate that reached 824 and 524 mol h−1 (100 mg) under UV–vis and visible light irradiation, respectively. Here, NiO@Ni nanoparticles loaded on the surface of ZnO act as cocatalysts for capturing the photoexcited electrons to improve the photocatalytic hydrogen activity. The synergistic effect between ZnO and CdS is markedly enhanced by rGO nanosheets, which forms a photoexcited carrier transport channel. The non-noble metal NiO@Ni, replacing Pt as a cocatalyst in the ZnO/rGO/CdS heterostructure, will substantially reduce the cost of photocatalysts.
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Full Length Article
Noble-metal-free NiO@Ni-ZnO/reduced graphene oxide/CdS heterostructure for efficient photocatalytic hydrogen generation Fayun Chen a,∗ , Laijun Zhang a , Xuewen Wang b,∗ , Rongbin Zhang b a b
College of Chemistry and Environmental Science, ShangRao Normal University, Shangrao 334001, PR China The Institute of Applied chemistry, The College of Chemistry, Nanchang University, 999# Xuefu Road, Nanchang 330031, PR China
a r t i c l e
i n f o
Article history: Received 16 March 2017 Received in revised form 23 May 2017 Accepted 24 May 2017 Available online 3 June 2017 Keywords: Ni ZnO/CdS Graphene Heterostructure Photocatalytic Hydrogen
a b s t r a c t Noble-metal-free semiconductor materials are widely used for photocatalytic hydrogen generation because of their low cost. ZnO-based heterostructures with synergistic effects exhibit an effective photocatalytic activity. In this work, NiO@Ni-ZnO/reduced graphene oxide (rGO)/CdS heterostructures are synthesized by a multi-step method. rGO nanosheets and CdS nanoparticles were introduced into the heterostructures via a redox reaction and light-assisted growth, respectively. A novel Ni-induced electrochemical growth method was developed to prepare ZnO rods from Zn powder. NiO@Ni-ZnO/rGO/CdS heterostructures with a wide visible-light absorption range exhibited highly photocatalytic hydrogen generation rates under UV–vis and visible light irradiation. The enhanced photocatalytic activity is attributed to the Ni nanoparticles that act as cocatalysts for capturing photoexcited electrons and the improved synergistic effect between ZnO and CdS due to the rGO nanosheets acting as photoexcited carrier transport channels. © 2017 Elsevier B.V. All rights reserved.
1. Introduction Photocatalytic hydrogen generation over semiconductors is a potential method of achieving solar-to-energy conversion [1–4]. During the photocatalytic hydrogen generation process, effective photo-reduction and −oxidation need to occur on different reaction sites to prevent the recombination of the photoexcited electrons and holes and counter reactions [5]. To restrict counter reactions and improve the transport of photoexcited carriers, a number of heterostructural materials with enhanced photocatalytic hydrogen activity have been developed [1,6–13]. The photocatalytic ability of heterostructures is related to the presence of suitable surface and interface states among the heterostructural phases. A high chargecarrier transport rate through the interfaces will induce an efficient synergistic effect. Thus, metals, graphene, graphene oxide (GO) and reduced graphene oxide (rGO) are widely used to fabricate carrier transport channels in photocatalytic materials to improve the interface/surface state [11,14–23]. Unlike graphene, rGO nanosheets involving a number of oxygen-containing functional groups and high surface areas are more suitable to form strong chemical bonding between the rGO and the heterostructural phases [14].
∗ Corresponding authors. E-mail addresses: [email protected] (F. Chen), [email protected] (X. Wang). http://dx.doi.org/10.1016/j.apsusc.2017.05.214 0169-4332/© 2017 Elsevier B.V. All rights reserved.
Cocatalysts are attached on the photocatalyst surface for capturing photoexcited electrons or holes and acting as photo-reductive or oxidative sites [24–28]. Noble metals and metal oxides are extensively used as cocatalysts for improving the photocatalytic hydrogen activity [3,29–31]. For example, TiO2 or CdS after loading of Pt nanoparticles exhibited significantly enhanced photocatalytic hydrogen generation rates [3,24,32,33]. However, noble metal nanoparticle loading will increase the cost of the photocatalytic materials. Non-noble metals can be loaded on the surface of photocatalysts via a reducing reaction, and can improve photocatalytic activity and reduce costs [1,25,26,34–40]. NiO or Ni is widely used in traditional catalytic reactions such as hydrogen reforming and CO oxidation [40,41]. In recent years, Ni-based nanoparticle cocatalysts are used on semiconductor photocatalysts and significantly enhance the photocatalytic activity [6,25,28,35,36,38,42]. Thus, it will be useful to develop Ni-based cocatalysts on more semiconductor photocatalysts. Previous reports [15,43] reported that ZnO-CdS heterostructures loaded with Pt nanoparticles as cocatalysts exhibited high photocatalytic hydrogen generation rates. However, the optimized content of Pt ranges from 1 to 3 wt%, which markedly increases the cost of the photocatalyst. In this study, NiO@Ni-ZnO/rGO/CdS heterostructures were designed and synthesized using a novel Ni-induced electrochemical and light-assisted growth process. The NiO@Ni-ZnO/rGO/CdS exhibit high and stable photocatalytic
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hydrogen generation rates under visible light irradiation due to NiO@Ni nanoparticles acting as cocatalyst and an enhanced synergistic effect between ZnO and CdS by the rGO nanosheets acting as a carrier transport channel.
at 100 ◦ C for 24 h. The reference ZnO/CdS was synthesized by the same method without addition of GO nanosheets.
2. Experimental
X-ray diffraction (XRD) patterns were recorded by a Rigaku diffractometer with Cu K␣ rradiation. Scanning electron microscopy (SEM) images and Energy Dispersive X-ray (EDX) spectroscopy profiles were acquired on an FEI Nova SEM 200. Transmission electron microscope (TEM) and High-resolution transmission TEM images were carried out on a Jeol-2100. The Brunauer–Emmett–Teller (BET) specific surface area and pore size distribution was obtained on ASAP–2010 M (Micrometritics). The ultraviolet (UV)-visible absorption spectra were performed on a UV–vis spectrophotometer (JASCO-V550). The contents of elements in the heterostructures were analyzed by an inductively coupled plasma-optical emission spectrometer (ICP-OES, Agilent-5100). X-ray photoelectron (XPS) spectra were analyzed by using a Thermo Escalab 250 system with a monochromatic Al K␣ (XPS) source. All of the binding energies were calibrated by the C 1 s peak (284.6 eV) produced by adventitious carbon. The electrochemical impedance spectra were measured by an electrochemical workstation (Princeton P-2273).
2.1. Preparation of photocatalyst materials The NiO@Ni-ZnO/rGO/CdS heterostructures were prepared by a redox reaction, Ni-induced electrochemical and light-assisted growth processes. GO solution was synthesized by an improved Hummer method [44]. In the preparing process, 0.5 g of zinc powder was dispersed in 50 mL of deionized water under a magnetic stirring. Subsequently, 10 mL of GO aqueous solution (ca. 0.5 mg mL−1 GO) was added in the above solution with a constant magnetic stirring for 10 min. And the black precipitate of Zn/rGO was formed in the aqueous solution. A predetermined amount of nickel acetate aqueous solution was slowly dropped in the aqueous solution containing Zn/rGO under a constant stirring. The solution was maintained in an oven (60 ◦ C, 24 h) for the growth of NiZnO/rGO. The Ni-ZnO/rGO was washed three times with deionized water. An aqueous solution containing different amounts of CdCl2 was added in the above mixed solution with a constant magnetic stirring under light irradiation for 1 h. Afterward, 50 mL of aqueous solution containing Na2 S (the mole ratio of CdCl2 /Na2 S = 1:1.5) was dropwise added with a constant stirring under light irradiation for 1 h. The precipitate was washed with deionized water and then vacuum dried and at 80 ◦ C for 24 h. The powder was denoted as NiO@Ni-ZnO/rGO/CdS heterostructures. The reference ZnO/rGO/CdS was synthesized by a solution method. 26 mL of GO solution was added in a 100 mL of aqueous solution containing 0.02 mol zinc acetate. Then 50 mL of aqueous solution including 0.04 of mol NaOH was dropped in the solution. 0.006 mol of cadmium acetate was added in the solution under stirring for 10 min. Finally, 50 mL of aqueous solution including 0.006 mol of Na2 S was dropwise added under constant stirring for 30 min. The precipitate was washed with deionized water and dried
2.2. Characterization
2.3. Photocatalytic hydrogen generation measurement A gas-closed circulation photocatalytic hydrogen generation system with a vacuum was used to evaluate hydrogen generation rates. In the measurement process, 100 mg of photocatalyst powder was uniformly dispersed in 300 mL of aqueous solution including 7.2 g of Na2 S·9H2 O, 3.78 g of Na2 SO3 and 30 mL of triethanolamine. The amount of hydrogen generation was tested by a gas chromatograph (capillary column, thermal conductivity detector and argon gas serving as carrier gas). The irradiation light source used in the photocatalytic hydrogen generation measurement was a 300 W of Xe lamp (PLX-300). And a longpass filter with 420 nm was used to cut off UV light.
Fig. 1. Scheme of the fabrication process of NiO@Ni-ZnO/rGO/CdS heterostructure prepared by the Ni-induced electrochemical and light-assisted growth process.
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3. Results and discussion The NiO@Ni-ZnO/rGO/CdS heterostructures were synthesized by a redox reaction, Ni-induced electrochemical growth, and lightassisted loading CdS process (Fig. 1). Zn spheres with diameter of less than 10 m were used as the precursor to synthesize ZnO rods. The XPS spectra of GO nanosheets used for preparing Zn/rGO are shown in Fig. S1. The peaks at 284.6, 286.9, and 288.5 eV are corresponded to sp2 carbon, C O single bond and C O double bond, respectively. GO nanosheets with a number of oxygen-containing groups (Fig. S1), which process an oxidative ability and high surface area, can easily react with Zn spheres that possess a strong reductive ability in aqueous solution at room temperature (25 ◦ C) [43]. In the preparing process, GO was reduced to rGO and loaded on the surface of ZnO spheres (Fig. S2). Ni atoms replaced the Zn atoms and were well loaded on the surface of the Zn/rGO composite after adding an aqueous solution of nickel acetate. Compared to Zn (1.65), Ni with a high electronegativity (1.91), exhibits higher chemical stability and can form an original cell, where Ni|H2 O|H2 and Zn|ZnO server as the cathode and anode, respectively. Thus, the Zn/rGO composite was continuously converted into ZnO/rGO through a Niinduced electrochemical growth process. CdS nanoparticles were loaded on the surface of ZnO/rGO via a light-assisted growth process. All the above preparation processes were performed in an aqueous solution. Therefore, it is a very simple method to synthesize the NiO@Ni-ZnO/rGO/CdS heterostructures. XRD patterns of Ni-ZnO/rGO and NiO@Ni-ZnO/rGO/CdS synthesized by the Ni-induced electrochemical and light-assisted growth
Fig. 2. Left panel: XRD patterns of Zn/rGO, Ni-ZnO/rGO, NiO@Ni-ZnO/rGO/CdS heterostructure and CdS nanoparticles; Right panel: the local XRD patterns at 40–50◦ (2) of Ni-ZnO/rGO in the left panel.
process are shown in Fig. 2. No characteristic peaks of Zn powders were found in the XRD patterns of Ni-ZnO/rGO and all diffraction peaks were attributed to hexagonal ZnO (space group: P63mc), which indicates that Zn powders were completely converted into ZnO after 24 h of Ni-induced electrochemical growth. The characteristic peaks of rGO were not observed in the XRD patterns possibly because of the low rGO content (1 wt%), which is below the detection range of the XRD instrument. However, a peak centered at 44.5◦ (2) corresponded to metal Ni in the XRD patterns of Ni-ZnO/rGO,
Fig. 3. SEM images of (a) the precursor Zn powders, (b) Ni-ZnO/rGO, and (c) NiO@Ni-ZnO/rGO/CdS; (d) local magnified SEM image of NiO@Ni-ZnO/rGO/CdS.
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Fig. 4. (a) TEM and (b) local magnified TEM images of the NiO@Ni-ZnO/rGO/CdS heterostructure; high-resolution TEM image of (c) CdS and (d) Ni nanoparticles in the heterostructure.
which suggests that Ni nanoparticles are still present on the surface of ZnO after the electrochemical growth. After loading 30 at% CdS on Ni-ZnO/rGO by a light-assisted growth method, significantly broader diffraction peaks in the CdS phase were observed that correspond to nanosized CdS compared to the sharp peaks observed with the ZnO phase. However, no obvious characteristic diffraction peaks of Ni were found in the NiO@Ni-ZnO/rGO/CdS possibly owing to the oxidation of Ni after thermal treatment in an oven. SEM images of the precursor Zn powders are shown in Fig. 3a. The diameter of Zn micro-spheres is less than 10 m. A ZnO shell, formed by natural oxidation, covers the surface of the Zn spheres [15]. Ni-ZnO/rGO prepared from Zn spheres by the redox and Niinduced electrochemical processes are shown in Fig. 3b. The size of the ZnO hexagonal prisms mainly ranges from 100 to 800 nm, which is smaller than that of the ZnO rods prepared by a Ptinduced electrochemical process [45]. EDX elemental mapping images shown in Fig. S3 also confirmed that element Ni is present in the Ni-ZnO/rGO composite. In the Ni-induced electrochemical growth, the process may be discontinuous and unstable owing to the oxidation of Ni. Therefore, the growth of ZnO rods could be arrested during the electrochemical process. Fig. 3c and d show that the rGO nanosheets are effectively loaded onto the surfaces of the ZnO rods during the ZnO formation process. Under light irradiation, Cd2+ ions were easily adsorbed on the surfaces of the rGO nanosheets with negative charge centers formed by photoexcited electrons of ZnO and fully converted into CdS nanoparticles in the aqueous solution containing excessive S2− ions [2]. EDX
elemental mapping images of NiO@Ni-ZnO/rGO/CdS (Fig. S4) also suggest that Zn, O, Cd, S, and Ni elements are uniformly dispersed in the heterostructure. The N2 adsorption-desorption isotherms and pore size distribution of the heterostructure are shown in Fig. S5. The BET surface area and total pore volume are 60.1 m2 g−1 and 0.113 m3 g−1 , respectively. The averaged pore diameter is 7.5 nm. To explore the microstructures and the interface states, TEM images of the NiO@Ni-ZnO/rGO/CdS are obtained as shown in Fig. 4a and b. A number of nanoparticles dispersed on the surfaces of the rGO nanosheets corresponded to CdS, and rGO nanosheets were well loaded on the surface of the ZnO rods. The high-resolution TEM images (Fig. 4c) indicate that the crystalline CdS nanoparticles are formed by a light-assisted growth method in an aqueous solution. The averaged diameter of the CdS nanoparticles is approximately 8 nm. A nanoparticle with a crystal plane spacing of 0.21 nm, corresponding to the (111) crystal plane of Ni [46], was located on the surface of the ZnO rod as shown in Fig. 4d, which suggests that the metal Ni is still present on the surface of the ZnO rod after Ni-induced electrochemical growth of ZnO and light-assisted growth of CdS. However, the surfaces of Ni nanoparticles are oxidized into NiO owing to the instability of Ni nanoparticles. Thus, NiO@Ni nanoparticles replace the noble metal Pt and act as capturing sites for the photoexcited electrons, and then carry on the photo-reductive reaction in hydrogen generation. The UV–vis light absorption spectra of the composite photocatalysts are shown in Fig. 5. All the NiO@Ni-ZnO/rGO/CdS and Ni-ZnO/rGO show a wide visible light absorption owing to the
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Fig. 5. UV–vis light absorption spectra of CdS, Ni-ZnO/rGO, NiO@Ni-ZnO/rGO/CdS with different amounts of CdS nanoparticles (the mole ratios of ZnO/CdS varying from 1:0.1 to 1:0.8).
addition of rGO nanosheets with a light absorption region greater than 1100 nm. However, the light absorption rate enhanced by the rGO sheets is not used for photocatalytic reactions because of its shielding effect. After the CdS nanoparticles are loaded on the NiZnO/rGO, a band of visible light ranging from 400 nm to 550 nm is observed in the NiO@Ni-ZnO/rGO/CdS. The photocatalytic hydrogen activity of the heterostructures is strongly affected by the mole ratio of ZnO and CdS. Therefore, NiO@Ni-ZnO/rGO/CdS with varying amounts of CdS was prepared in this work. The visible light absorbance ranging from 400 nm to 550 nm gradually increased with an increase in the loading content of CdS, whose light absorption edges are a little lesser than that of the CdS nanoparticles prepared by a solution method. The red shift of light absorption edges is due to the maturing of CdS nanoparticles in the NiO@NiZnO/rGO/CdS heterostructures [2]. To detect the chemical states of the heterostructural phases, the XPS spectra of NiO@Ni-ZnO/rGO/CdS and Ni-ZnO/rGO are shown in Fig. 6. In the synthesis process of ZnO/rGO, Ni nanoparticles were loaded onto the surfaces of the Zn spheres through a replacement process by the addition of an aqueous nickel acetate solution. The peaks of Ni 2p centered at 855. 8 and 887.3 eV suggest that Ni nanoparticles are present on the surface of the ZnO rods. Furthermore, the new peaks of Ni 2p centered at 862.0 and 879.7 eV indicated that Ni–O bonds were formed in the NiO@NiZnO/rGO/CdS owing to the oxidation of Ni. The NiO shells covered on the surface of Ni nanoparticles and formed NiO@Ni nanoparticles acting as cocatalysts on the surface of heterostructures in the photocatalytic reaction. The O 1s binding energies of 531.8, 531.5, and 530.4 eV in the heterostructures correspond to the O Zn bond in the ZnO rods, O Ni bond and the O C bond in the rGO nanosheets, respectively. The chemical bonds among ZnO, rGO, and CdS result in a significantly smoother charge-carrier transport process in the heterostructures. The ICP-OES result suggests that the Ni content in the NiO@Ni-ZnO/rGO/CdS prepared using 5% Ni is approximately 2.5%. The photocatalytic hydrogen generation rates of the heterostructures are greatly related to the sacrificial agents in an aqueous solution. Thus, the aqueous solution containing different sacrificial agents was used to estimate the photocatalytic activity under UV–vis light and visible light irradiation (Table 1). The NiO@Ni-ZnO/rGO/CdS exhibited low photocatalytic hydrogen generation rates of 225 and 114 mol h−1 under UV–vis and visible light irradiation in the aqueous solution containing 0.1 mol L−1 Na2 S and 0.1 mol L−1 Na2 SO3 , respectively. The hydrogen genera-
tion rates decreased to 175 and 73 mol h−1 in the aqueous solution containing 10 vol% C6 H15 NO3 . Unfortunately, no obvious hydrogen was detected in an aqueous solution containing CH3 OH (10 vol%). However, the NiO@Ni-ZnO/rGO/CdS heterostructures exhibited markedly improved photocatalytic hydrogen generation rates of 824 and 564 mol h−1 irradiated by UV–vis and visible light, respectively, in an aqueous solution containing 0.1 mol L−1 Na2 S, 0.1 mol L−1 Na2 SO3 , and 10 vol% C6 H15 NO3 . No hydrogen amount was detected in these solutions in the absence of photocatalysts under light irradiation. The above results suggest that composite sacrificial agents are suitable for photocatalytic hydrogen generation of the NiO@Ni-ZnO/rGO/CdS. The possible reason is that the sacrificial agents of Na2 S, Na2 SO3 and C6 H15 NO3 might be suitable to photo-oxidative reaction on the CdS and ZnO, respectively. The photocatalytic hydrogen generation rates over different photocatalysts are shown in Fig. 7a. The NiO@Ni-ZnO/rGO/CdS exhibited a hydrogen generation rate of 824 mol h−1 under UV–vis light irradiation, which is 588 times that of Ni-ZnO/rGO, 2.85 times that of the referenced ZnO/rGO/CdS and 7.5 times that of CdS nanoparticles. The low hydrogen generation rates of Ni-ZnO/rGO and CdS are attributed to the poor visible light absorption and surface combination of photoexcited electrons and holes, respectively. Moreover, the NiO@Ni-ZnO/rGO/CdS of visible light shows a hydrogen generation rate of 564 mol h−1 , which is 0.68 times that of UV–vis light. The high hydrogen generation rate of visible light suggests that the active photoexcited electrons used for the photocatalytic hydrogen generation reaction originate mainly from the CdS nanoparticles under light irradiation. The amount of CdS plays a key role in improving the photocatalytic activity of the heterostructures. The hydrogen generation rates of the heterostructures with different mole ratios of ZnO and CdS are shown in Fig. 7b. These heterostructures all exhibit higher hydrogen generation rates than those of the Ni-ZnO/rGO and CdS nanoparticles. The photocatalytic hydrogen generation rates of the NiO@Ni-ZnO/rGO/CdS markedly increased with increasing amounts of CdS nanoparticles. The NiZnO/rGO loading with 30 at% CdS nanoparticles exhibits the highest hydrogen generation rate and reaches 824 and 524 mol h−1 under UV–vis and visible light irradiation, respectively. The heterostructures exhibit higher light absorbance with increasing amounts of CdS. However, the corresponding hydrogen generation rates gradually decrease due to the decreased number of photocatalytic reaction sites on the surface of ZnO and maturing of the CdS nanoparticles. During the preparation process, NiO@Ni nanoparticles were introduced to form ZnO rods via an electrochemical method. The growth rate of the ZnO rods increased with increasing amount of Ni. The NiO@Ni nanoparticles also acted as cocatalysts for improving the photocatalytic activity on the surface of NiO@Ni-ZnO/rGO/CdS heterostructures. The loading amount of the cocatalyst will markedly affect the photocatalytic activity. Therefore, the NiO@Ni-ZnO/rGO/CdS heterostructures were prepared using different amounts of Ni and used for the photocatalytic hydrogen generation reactions (Fig. 7c). With the amount of Ni increase, the hydrogen generation rates gradually increase under light irradiation. When 5% Ni was used to prepare NiO@NiZnO/rGO/CdS, the heterostructure showed the highest hydrogen generation rate, which is higher than that of Pt-loaded ZnO rodsCdS in the previous work [43]. Meanwhile, the growth rate of ZnO from Zn powder rapidly increased with the addition of more nickel acetate. However, the photocatalytic activities of the NiO@NiZnO/rGO/CdS began to rapidly decrease. When 20% Ni was used to induce the growth of ZnO, the hydrogen generation rates of the heterostructure were only 198 and 135 mol h−1 under UV–vis and visible light irradiation, respectively. The low photocatalytic activities are attributed to the following reasons. i) Neither NiO nor Ni
F. Chen et al. / Applied Surface Science 422 (2017) 962–969
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Fig. 6. High-resolution XPS spectra of C1s (a), Ni 2p (b), Zn 2p (c), O 1s (d), Cd 3d (e) and S 2p (f) in the NiO@Ni-ZnO/rGO/CdS and Ni-ZnO/rGO.
Table 1 Photocatalytic hydrogen generation rates of the NiO@Ni-ZnO/rGO/CdS heterostructure (100 mg) in various sacrificial agents under UV–vis and visible light irradiation. Sacrificial Agent Light Source
10 vol% CH3 OH
0.1 M Na2 S + 0.1 M Na2 SO3
10 vol% C6 H15 NO3
0.1 M Na2 S + 0.1 M Na2 SO3 + 10 vol% C6 H15 NO3
0.1 M Na2 S + 0.1 M Na2 SO3 + 10 vol% C6 H15 NO3 (No catalysts)
UV–vis Vis
0 0
255 141
175 73
824 564
0 0
shows any photocatalytic activity in an aqueous solution. A number of inactive NiO@Ni nanoparticles cover the surface of the ZnO rods, which will markedly reduce the number of photocatalytic reaction sites. ii) NiO@Ni nanoparticles loaded on the surface act as cocatalyst to capture the photoexcited electrons and then enhance the photocatalytic activity. However, NiO@Ni nanoparticles began to grow and reduce the action of the cocatalyst after excessive addition of Ni. To explore the photocatalytic stability of the heterostructures, long-term photocatalytic hydrogen generation reactions over the (5%) NiO@Ni-ZnO/rGO/CdS heterostructure were performed in an aqueous solution, as shown in Fig. 7d. The total hydrogen generation amounts of 15 h reached 12.2 and 8.5 mmol under UV–visible and visible light ( ≥ 420 nm) irradiation, respectively. No obvious degradation of hydrogen generation rate was found in the three-cycle photocatalytic reactions, which suggests that the NiO@Ni-ZnO/rGO/CdS is stable in an aqueous solution containing S2− , SO3 2− , and C6 H15 NO3 . The amount of hydrogen generation of visible light from the NiO@Ni-ZnO/rGO/CdS was 2.9 mmol in the first cycle (5 h) can reach 71% that generated under UV–vis light irradiation. The high visible-light activity suggests that photoexcited electrons used for hydrogen reducing reactions originate mainly from the CdS nanoparticles. When Ni-induced electrochemical growth method was used to synthesize NiO@Ni-ZnO/rGO/CdS heterostructures and rGO nanosheets were introduced into the heterostructures, the improved photocatalytic hydrogen activity and stability likely occurs because of the following reasons:
1) NiO@Ni nanoparticles replace Pt nanoparticles and then act as cocatalysts on the surface of the NiO@Ni-ZnO/rGO/CdS. The photoexcited electrons are captured by NiO@Ni nanoparticles and are used for photo-reduction of hydrogen generation. Meanwhile, the NiO@Ni nanoparticles would also prevent recombination of the photoexcited electrons and holes owing to the photo-reductive and oxidative reactions occurring on different sites. Here, the NiO@Ni nanoparticles are well loaded on the surface of ZnO via a replacement reaction. The NiO covers on the surface of Ni and improves the stability of the heterostructures. 2) The synergistic effect between ZnO and CdS is the main reason for the improved photocatalytic hydrogen activity. The scheme of photocatalytic reaction process in the NiO@Ni-ZnO/rGO/CdS heterostructure is shown in Fig. 8. CdS nanoparticles with a wide light absorption range provide a number of photoexcited electrons for the photo-reductive reaction under light irradiation. In the heterostructures, the classic carrier transport method, photoexcited electrons from CdS transported to ZnO through rGO nanosheets and being used for photocatalytic hydrogen generation, plays the key role in improving the photocatalytic activity. The photo-oxidative reactions of sacrificial agents occur on the surfaces of CdS and ZnO. 3) rGO nanosheets with a high carrier transport rate are introduced into the ZnO/CdS heterostructure for constructing chargecarrier transport channels between ZnO and CdS (shown in Fig. 8). Chemical bonds formed between the semiconductor phases and rGO nanosheets ensure smooth photoexcited transport to improve the synergistic effect between ZnO and CdS.
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Fig. 7. (a) Photocatalytic hydrogen generation rates of Ni-ZnO/rGO, the reference ZnO/rGO/CdS, NiO@Ni-ZnO/rGO/CdS and CdS irradiated by UV–vis and visible ( ≥ 420 nm) light; Photocatalytic hydrogen generation rates of (b) NiO@Ni-ZnO/rGO/CdS with different amounts of CdS nanoparticles and (c) NiO@Ni-ZnO/rGO/CdS prepared using different amounts of Ni; (d) Time-circle photocatalytic hydrogen generation rates over the NiO@Ni-ZnO/rGO/CdS.
powder and were well loaded on the surface of the Zn spheres. Meantime, GO was also reduced to rGO. The rGO nanosheets with a high carrier transport rate are suitable to photoexcited electron transport in the heterostructures. The CdS nanoparticles were loaded on the rGO nanosheets via a light-assisted growth process. A strong chemical contact was formed in the heterostructures prepared by a redox replacement reaction and Ni-induced electrochemical growth methods, which protects the synergistic effect between ZnO and CdS and improves stability during a long-term photocatalytic hydrogen reaction.
4. Conclusion
Fig. 8. Scheme of photocatalytic reaction process in the NiO@Ni-ZnO/rGO/CdS heterostructure.
Electrochemical impedance spectra (shown in Fig. S6) exhibits that the resistance of the heterostructure decreases after the introduction of the rGO nanosheets compared to that of the ZnO/CdS. Thus, the photoexcited carrier transport will become smooth in the NiO@Ni-ZnO/rGO/CdS; this finding is confirmed by the high photocurrent intensity generated by the heterostructure under visible light irradiation (Fig.S7). 4) The synthesis method is a key factor for the improved photocatalytic hydrogen activities of the NiO@Ni-ZnO/rGO/CdS. In the redox reaction, the GO nanosheets were reduced by the Zn
Noble-metal-free NiO@Ni-ZnO/rGO/CdS heterostructures with a wide visible light absorption range are synthesized by redox replacement, Ni-induced electrochemical growth and lightassisted growth methods. The NiO@Ni-ZnO/rGO/CdS exhibits an improved photocatalytic hydrogen generation rate that reached 824 and 524 mol h−1 (100 mg) under UV–vis and visible light irradiation, respectively. Here, NiO@Ni nanoparticles loaded on the surface of ZnO act as cocatalysts for capturing the photoexcited electrons to improve the photocatalytic hydrogen activity. The synergistic effect between ZnO and CdS is markedly enhanced by rGO nanosheets, which forms a photoexcited carrier transport channel. The non-noble metal NiO@Ni, replacing Pt as a cocatalyst in the ZnO/rGO/CdS heterostructure, will substantially reduce the cost of photocatalysts.
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