Core-shell structured ZnO@Cu-Zn–Al layered double hydroxides with enhanced photocatalytic efficiency for CO2 reduction

    Core-shell structured ZnO@Cu-Zn-Al layered double hydroxides with enhanced photocatalytic efficiency for CO 2 reduction Qiangsheng Guo, Qinghong Zhang, Hongzhi Wang, Zhifu Liu, Zhe Zhao PII: DOI: Reference:

S1566-7367(16)30019-X doi: 10.1016/j.catcom.2016.01.019 CATCOM 4564

To appear in:

Catalysis Communications

Received date: Revised date: Accepted date:

14 December 2015 14 January 2016 20 January 2016

Please cite this article as: Qiangsheng Guo, Qinghong Zhang, Hongzhi Wang, Zhifu Liu, Zhe Zhao, Core-shell structured ZnO@Cu-Zn-Al layered double hydroxides with enhanced photocatalytic efficiency for CO2 reduction, Catalysis Communications (2016), doi: 10.1016/j.catcom.2016.01.019

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Core-shell structured ZnO@Cu-Zn-Al layered double hydroxides with

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enhanced photocatalytic efficiency for CO2 reduction

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Qiangsheng Guoac, Qinghong Zhanga*, Hongzhi Wangb, Zhifu Liuc and Zhe Zhaoc*

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(aColleage of Materials and Engineering, Donghua University, Shanghai 201620, PR China. State key laboratory for modification of chemical fibers and polyer materials, Donghua

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University, Shanghai 201620, PR China. cSchool of Materials Science and Engineering,

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Shanghai Institute of Technology, Shanghai 201418, PR China.) Abstract

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Hierarchical ZnO@Cu-Zn-Al layered double hydroxides (LDHs) heterostructures was synthesized by a facial deposition-precipitation method, and it exhibited an enhanced

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photocatalytic efficiency for CO2 reduction. The physicochemical properties of as-prepared ZnO@LDHs were studied by SEM, XRD, BET, UV-vis and TEM technique. The improvement

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of photocatalytic efficiency is related two factors. On the one hand, LDHs species can increase the surface areas of catalysts effectively, which means that more adsorb active site for CO2 can be provided; On the other hand, the heterojunction was formed on the ZnO/LDHs interface hindering the recombination of excited charge carriers. Keywords: ZnO; layered double hydroxides; CO2 photoreduction; methane * Corresponding authors. E-mail addresses: [email protected];Fax:+86-021-67792855;Tel: +86-021-67792943, [email protected];Tel.＆Fax.:+86-21-60873079.

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1. Introduction

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As well known, the increase in CO2 concentration in the atmosphere is responsible for the

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problem of global warming [1]. Converting CO2 into organic fuel can attenuate the greenhouse

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effects and meet the increasing energy demands simultaneously. Therefore, many efforts have been made to develop an efficient photocatalyst for the conversion of CO2 [2]. Although various

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photocatalysts such as TiO2 [3], BiVO4 [4], Bi2WO6 [5] and Zn2GeO4 [6] have been investigated

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for the conversion of CO2 into small organic molecules (e.g. methane, C2+, methanol, ethanol, etc.), most of them show a low photocatalytic efficiency. Recently, layered double hydroxides

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(LDHs), a class of anion clays consisting of brucite-like host layers and interlayer anions [7],[8], has been received considerable attention because of the high sorption capacity for CO2 in the

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layered space [9],[10] and the tunable semiconductor properties (photocatalytic activity) via changing the metal cations [11]-[13]. For example, Kentaro et al. [7] carried out the

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photocatalytic conversion of CO2 in water with the presence of various LDHs. The results indicated that the basic site over the surface of photocatalyst was indispensable, which acted as the active site for CO2 adsorption. Despite of the high sorption capacity and the tunable semiconductor properties of LDHs, the relative low photocatalytic efficiency was still obtained for the immediate recombination of charges in the bulk and/or on the surface of semiconductor [14],[15]. Proper junctions formed in semiconductor-based photocatalysts can inhibit the recombination of photoinduced electrons and holes and lead to an enhanced efficiency in photocatalysis process [16]-[19]. Fabrication of junctions between LDHs and other different

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semiconductors has been studied in the field of the evolution of H2 and the degradation of organic

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pollutants in more recent years [20]-[22]. Li et al. [22] assembled Bi2MoO6/Zn-Al LDHs

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hierarchical heterostructures from Bi2MoO6 hierarchical hollow spheres and Zn-Al LDHs

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nanosheets by a low-temperature hydrothermal method. The results indicated that Bi2MoO6/Zn-Al LDHs heterostructure photocatalyst exhibited excellent stability and reusability,

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which mainly attributed to the efficient separation of photoinduced electrons and holes.

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ZnO, a typical semiconductor material, has been investigated extensively for the photocatalytic reaction. The photocatalytic performance of ZnO photocatalyst is significantly influenced by its

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morphology due to surface defect and electronic structure [23],[24]. Furthermore, a ternary Cu-Zn-Al oxide catalyst has been employed for the synthesis of methanol from CO2

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hydrogenation, and it shows a high activity [25]-[27]. In this study, rod and belt ZnO catalysts enwrapped by Cu-Zn-Al LDHs were prepared and used for the photoreduction of CO2 to

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hydrocarbons. The catalysts were characterized with SEM, XRD, BET, TGA, UV-vis technologies and TEM. The relationship between the photocatalytic performance and the physicochemical properties, especially the adsorption of CO2 and the formation of heterojuction, were discussed. 2. Experimental For the sample preparation, characterization and photocatalytic test have been shown in the Supporting information. 3. Results and discussion

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3.1. Characterization of catalysts

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The SEM pictures of the as-prepared ZnO were presented in Fig. 1 (a) and (d). It can be seen

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that the ZnO materials exhibited typical rod-like and belt-like structure. The rod-like ZnO has

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regular shape with an average diameter of ~600 nm and a length of ~3.5 μm. The width of the belt-like ZnO varies from 50 nm to 200 nm, and the length of the belt is more than 10 μm. Fig. 1

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(b) and (e) show the morphology of ZnO enwrapped by Cu-Zn-Al layered double hydroxides

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(LDHs). Obviously, a coating of LDHs was covered on the surface of ZnO materials indicating that the ZnO@Cu-Zn-Al LDHs with a core-shell-like structure were obtained by the facile

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deposition-precipitation method in this study. Therefore, the rod-liked and belt-liked ZnO were marked R-ZnO and B-ZnO. Meanwhile, the rod-liked and belt-liked ZnO enwrapped by LDHs

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were noted R-ZnO@LDHs and B-ZnO@LDHs in the following description, respectively. To further investigate the morphology of ZnO@LDHs, the magnifications of Fig. 1 (b) and (e) were

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presented in Fig. 1 (c) and (f), respectively. It can be seen that the stacked LDHs nanosheets were intercross each other with a diameter ∼200 nm. Fig. 2 shows the XRD patterns of ZnO and ZnO@LDHs. Similar diffraction patterns are presented for the R-ZnO and B-ZnO, and the peaks at 31.7, 34.4, 36.2, 47.5, 56.5, 62.8 and 67.9 o correspond the (100), (002), (101), (102), (110), (103) and (112) planes of hexagonal ZnO (JCPDS file 89-1397), respectively. Moreover, no diffraction peaks of secondary phase were observed, which indicates that these two hexagonal ZnO are phase-pure. As for the ZnO@LDHs, the peaks appeared at 11.9, 23.7 and 39.4

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are assigned to the characteristic diffractions of

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Cu-Zn-Al LDHs with the interlayer anion of CO32- (JCPDS file 48-1024). Furthermore, with the

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coating of LDHs, except for a small decrease in the intensity of diffraction peaks, there is no

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change for the characteristic peaks of ZnO phase.

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Fig. S2 shows the TGA curves of the as-prepared ZnO and ZnO@LDHs. The weight loss is less than 2 wt% for these two ZnO material, which is ascribed to the evaporation of adsorbed

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water. There are three weight loss stages for Cu-Zn-Al LDHs. The first step (<150 oC)

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corresponds to the evaporation of adsorbed and interlayer intercalated water. In the range of 150∼350 oC, the weight loss is related to two overlapping events of the dehydroxylation and the

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decomposition of the interlayer CO32- [28],[29]. As the temperature is higher than 400 oC, the layered LDHs structure began to collapse. The TGA curves of ZnO@LDHs are similar to that of

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LDHs, suggesting that the LDHs are coated successfully on the surface of ZnO materials. The amount of LDHs coated on ZnO was evaluated by calculating weight loss of TGA, and the data

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were summarized in Table S1. It can be seen that the amount of LDHs for R-ZnO@LDHs and B-ZnO@LDHs is approximately 17 wt% and 27 wt%, respectively. The N2-adsorption/desorption isotherms were carried out to study the surface area and porosity of ZnO, Cu-Zn-Al LDHs, and ZnO@LDHs materials. As shown in Fig. 3, LDHs and ZnO@LDHs samples present the type IV isotherms with a H3 hysteresis loops (P/P0 > 0.8), indicating the presence of a porous structure, which is ascribed to the formation of slit-shaped pores. The specific surface area and pore volumes determined from the Brunauer-Emmett-Teller (BET) isotherms are summarized in Table S1. The surface areas of 21.8 and 26.8 m2g-1 were

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obtained for R-ZnO@LDHs and B-ZnO@LDHs, respectively. Obviously, the values of the

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surface areas are significantly higher than that of ZnO. Moreover, the pore volumes of the

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materials increase greatly after the LDHs coating. The increase in the surface area and pore

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volume of ZnO@LDHs is attributed to the existence of hierarchical LDHs, which has been illustrated in Fig. 1 (c) and (f).

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Fig. S3 shows the UV-vis diffuse reflectance spectra of the samples. Only one absorption

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region is observed for ZnO material, and the absorption peak appears at UV light region (λ<360 nm). The absorbance of the B-ZnO is slight higher than that of R-ZnO. In the case of LDHs, there

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are two absorption regions. One is in the UV light region with a peak appearing at 240 nm, and the other locates in the visible light region at the wavelength range above 600 nm. The

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characteristic absorption bands of both ZnO and LDHs are presented in the profile of ZnO@LDHs, indicating that no change occurs for the energy gap of ZnO and LDHs. Comparing

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to the uncoated ZnO, the intensity in the absorbance band enhances markedly, and this means more photons can be obtained for the photocatalytic reaction. Fig. 4 shows the TEM images of ZnO@Cu-Zn-Al LDHs. From Fig. 4(a) and (c), it can be seen that the rod/belt ZnO act as the core, the LDHs layer is coated on the surface of ZnO, and the light difference between the rod/belt core and the shell, which is consistent with the results of SEM. As shown in the high-resolution TEM images, the periodicity of the fringes of the core is 0.282 and 0.521 nm, corresponding to the (100) and (001) planes of hexagonal ZnO [30],[31]. The nanoparticles of the LDHs shell show a fringe spacing of 0.375 nm and 0.228 nm, which is

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close to the reported values of the (006) and (015) plane of rhombohedra LDHs, respectively.[32]

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3.2. Catalytic activity

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heterojunction nanostructure of ZnO and LDHs phases.

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It reveals that the interface between ZnO and LDHs can be clearly resolved, reflecting the formed

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Fig. S5 shows the compared tests of photo catalytic CO2 reaction at room temperature and 200 o

C. The tests results indicate the enhancing temperature greatly increased the photocatalytic

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activity. Consequently, all photo reactions were carried out at 200 oC. Meanwhile, the blank tests

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reflect that as-synthesized ZnO were active for CO2 photoreduction. As shown in Fig. 5(1), the visible light photocatalytic activities of ZnO, LDHs, and ZnO@LDHs were evaluated by the photoreduction of CO2 to hydrocarbons under Xenon lamp irradiation. After 5 hours irradiation,

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CH4 cumulative yields for rod and belt ZnO at 200 oC are 10.6 and 12.8 μmolg-1, respectively.

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Under the same condition, the CH4 cumulative yield over LDHs reaches 16.8 μmolg-1, and this value is higher than that of rod and belt ZnO. The increased CH4 yield can be ascribed to 2D layered space of LDHs. Both a higher surface area and pore volume, as a result of 2D the layered space, promote the accessibility of CO2 molecular to surface sites. Furthermore, a much higher the CH4 cumulative yield is obtained over the ZnO@LDHs. The activity of core-shell hierarchical structures is about 3.1 fold and 3.4 fold higher than that of LDHs samples, respectively. Considering the comparative surface area and pore volume, the improved photocatalytic activity of ZnO@LDHs is relative with the electronic coupling between the LDHs nanosheets and ZnO in such semiconductor heterostructure. The perfect interfacial contact at the

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ZnO/LDHs interface allows fast and efficient transfer of the photogenerated electrons or holes

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from the ZnO core to the LDHs shell, which depresses the recombination of charge carrier and

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enhances the reduction CO2 adsorbed species in Cu-Zn-Al LDHs. As illustrated in Fig. 5(2), the

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LDHs accept photogenerated electrons transferred from ZnO because it has a more positive conduction band(ECB) (+0.05 eV) than ZnO (-0.33 eV), while photogenerated hole of LDHs will

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migrate to the valance band(EVB) of ZnO due to the more negative EVB of ZnO(see ESI). The

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charge carrier transfer between ZnO core to the LDHs shell effectively depresses the recombination and is favour to CO2 reduce. In conclusion, the results clearly indicate that the

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photocatalytic performance of ZnO@LDHs hierarchical structures can be greatly enhanced via introducing LDHs nanosheets. As shown in Fig. 5(2), the stacked LDHs increase the surface

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areas, which enhanced the absorption capability of CO2 molecules. Meanwhile, the heterojunction formed at ZnO/LDHs interface improves the separation of electrons and holes, and

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prevents charge-carrier recombination in photocatalysts. 4. Conclusions

ZnO enwrapped by Cu-Zn-Al LDHs with hierarchical structures was synthesized by a deposition-precipitation method. A core-shell-like structure was observed for as-prepared ZnO@LDHs material, and a heterojunction was formed at the ZnO/LDHs interface. The resulting ZnO@LDHs catalyst exhibits significantly an enhanced activity in photoreduction of CO2 to hydrocarbons. The improvement of photocatalytic activity can be ascribed to the increase in the

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amount of adsorbed CO2 and the formation of hierarchical structure, which depresses the

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recombination of charge carrier.

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Acknowledgements

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This research is supported by the Youth Science and Technology Excellence Sail Plan of Shanghai (No. 14YF1410800) and the National Natural Science Foundation of China (No.

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21502116, 21302127).

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Figure captions

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Fig. 1. SEM images of bare ZnOand ZnO@Cu-Zn-Al LDHs composites

(a)R-ZnO; (b) R-ZnO@LDHs; (c) magnification of Fig.(b); (d) B-ZnO; (e) B-ZnO@LDHs; (f)

Fig. 2. XRD patterns of ZnO and ZnO@LDHs

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magnification of Fig.(e).

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(a)R-ZnO; (b) R-ZnO@ LDHs; (c) B-ZnO; (d) B-ZnO@LDHs.

Fig. 3. N2-sorption isotherms of Cu-Zn-Al LDHs, ZnO and ZnO@LDHs composites

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(a)B-ZnO; (b) R-ZnO; (c) B-ZnO@LDHs; (d) R-ZnO@LDHs; (e) Cu-Zn-Al LDHs

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Fig. 4. TEM and HRTEM images of ZnO@Cu-Zn-Al LDHs (a)TEM of rod-like ZnO@Cu-Zn-Al LDHs; (b) HRTEM of rod-like ZnO@Cu-Zn-Al LDHs; (c) TEM of belt-like ZnO@Cu-Zn-Al LDHs; (d) HRTEM of belt-like ZnO@Cu-Zn-Al LDHs

Fig. 5. CH4 evolution(1) and schematic illustration(2) of the photocatalytic conversion of CO2 over ZnO, LDHs, and ZnO@LDHs catalysts under visible light.

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Figure 5 60

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Graphical Abstract

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Hierarchical ZnO@Cu-Zn-Al layered double hydroxides heterostructures exhibited an enhanced photocatalytic efficiency for CO2 reduction.

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The

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structured

ZnO@Cu-Zn-Al

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by

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The special structures of ZnO@LDHs increase the surface areas and CO2 adsorbed capacity

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of catalysts effectively.

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The perfect interfacial contact at the ZnO@LDHs interface promotes the separation of

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electron-hole pairs.

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fabricated

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deposition-precipitation method. 

was

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Highlights

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