Dalton Transactions

(1)

Transactions

PAPER

Cite this:Dalton Trans., 2017,46, 10564

Received 10th December 2016, Accepted 6th January 2017 DOI: 10.1039/c6dt04668a rsc.li/dalton

Robust, double-shelled ZnGa

₂

O

₄

hollow spheres for photocatalytic reduction of CO

₂

to methane

Yuan Zhang,â,bPing Li,â,bLan-Qin Tang,â,b,cYong-Qiang Li,^bYong Zhou,*â,b,d Jun-Ming Liu*^band Zhi-Gang Zou^b,d

Robust, double-shelled ZnGa2O4hollow spheres were successfully fabricated by hydrothermally treating an aqueous solution containing Zn(II), Ga(III), and citric acid, followed by annealing at 600 °C, 700 °C, or 800 °C in air to remove the carbon species. The hollow structure is expected to trap incident photons to enhance the light absorbance. The sample annealed at 700 °C exhibited the optimized photocatalytic performance in the reduction of CO2in the presence of water vapor to methane. This property is ascribed to the improved crystallinity of the sample, which has fewer defect centers for the recombination of electron–hole pairs compared with that annealed at 600 °C. The reduced performance of the sample done at 800 °C relative to the one annealed at 700 °C is attributed to the formation of additional impurities besides ZnGa2O4, possibly due to partial Zn(II) evaporation at higher temperature leading to segregation of potential Ga-based oxides. RuO2and Pt were loaded onto the sample surface to greatly enhance the photocatalytic performance. The best photocatalytic performance was observed in the sample co-loaded with Pt and RuO2.

Introduction

Fossil fuels, such as petroleum and coal, are the primary energy source used by human beings. However, fossil fuels are non-renewable, and reserves are being exhausted much more rapidly than new ones are being made.¹ Thus, mimicking natural photosynthesis by the artificial photosynthesis of hydrocarbons has attracted considerable attention because of attractive prospects such as photocatalytic water splitting and the photocatalytic reduction of CO2into hydrocarbons in the presence of water.

Spinel ZnGa2O4 is an attractive transparent conducting oxide host.² Single-crystalline ZnGa2O4spinel phosphors give oﬀan intense ultraviolet emission.³ ZnGa2O4 is a wide band gap semiconductor, and its photoexcited electrons and holes are strong reductants and oxidants due to its high conduction band minimum (CBM) and low valence band maximum

(VBM).⁴ Its photoexcited electrons have an especially large mobility, which can enhance charge separation because of the large dispersion observed in the CB.⁵ In the area of photocatalytic research, its high electron mobility and high redox ability are beneficial for charges to quickly migrate from the interior to the exterior surface to participate in redox reactions.

So far, ZnGa2O4 has been reported in many photocatalytic areas: the photocatalytic splitting of water,^5–7 the photocatalytic reduction of CO2,^4,7–9 and the photocatalytic degra- dation of organic pollutants.¹⁰ Our group has reported a variety of ZnGa2O4 materials with diﬀerent morphologies for the photocatalytic reduction of CO2, including mesoporous ZnGa2O4 with a very large specific surface area,⁴ nanocubes with the (100) reaction active facets,⁷ and hierarchical nano- flowers comprised of nanosheets with quite thin thicknesses.⁸

Hollow structures have received increasing attention in many research areas because of their unique hollow characteristics, including in applications such as drug delivery, lithium- ion batteries, photocatalysis, gas sensors, and dye sensitized solar cells.¹¹For photocatalytic applications, hollow architec- tures can be utilized as light traps to allow for the multiscatter- ing of incident light for the enhancement of light absorption.¹² Thus, much attention has been paid to synthesize hollow structures of various photocatalytic materials. There are four main methods for synthesizing hollow structures:¹¹ (1) conventional hard templating synthesis, (2) sacrificial templating synthesis, (3) soft templating synthesis, and (4) template- free methods.

aKey Laboratory of Modern Acoustics, MOE, Institute of Acoustics, School of Physics, Nanjing University, Nanjing 210093, P. R. China

bNational Laboratory of Solid State Microstructures, Collaborative Innovation Center of Advanced Microstructures, and School of Physics, Nanjing University, 22 Hankou Road, Nanjing, Jiangsu 210093, P. R. China. E-mail: [email protected], [email protected]

cCollege of Chemistry and Chemical Engineering, Yancheng Institute of Technology, Yancheng 22401, P. R. China

dJiangsu Key Laboratory for Nano Technology, Ecomaterials and Renewable Energy Research Center (ERERC), Nanjing University, 22 Hankou Road, Nanjing, Jiangsu 210093, P. R. China

Published on 06 January 2017. Downloaded by NANJING UNIVERSITY on 15/11/2017 11:09:55.

View Article Online

View Journal | View Issue

(2)

In this work, we synthesized robust, double-shelled ZnGa2O4 hollow spheres. The unique hollow structure improved the absorbance of light, and the larger specific surface area provided more active reaction sites and absorbed more reactants. These characteristics make ZnGa2O4 hollow spheres a potential photocatalyst for the photocatalytic reduction of CO2 in the presence of water vapor. The present work provides a new method for fabricating multielement oxide hollow spheres, which can be applied to other fields besides photocatalysis.

Experimental

Synthesis and characterization

All reagents used here were analytically pure and commercially available and used without further purification. The reference sample (labeled as SSR) was obtained by the solid state reaction (SSR) of raw materials of ZnO and Ga2O3.^6c To obtain the precursor for ZnGa2O4 hollow spheres, 1 mmol of Zn(NO3)2·2H2O, 2 mmol of Ga(NO3)3and 8 mmol of citric acid (C6H8O7·H2O) were added to 40 ml of deionized water and stirred for 10 minutes to form a clear solution. The aqueous solution was then transferred to a 60 ml autoclave. The autoclave was sealed and kept at 200 °C for 20 hours for hydrothermal reaction. The products were washed with acetone and deionized water and centrifuged several times. The precipitates were dried in an oven at 60 °C in air. In addition, the ZnGa2O4

hollow spheres were obtained by calcinating the precipitates at 480 °C and further annealing at 600 °C, 700 °C, or 800 °C for 8 hours. These corresponding samples are labeled as S-600, S-700, and S-800, respectively.

The loading of the RuO2 cocatalyst on ZnGa2O4 hollow spheres was carried out by impregnating the sample with Ru3(CO)12in THF. The impregnated samples were stirred for 4 hours, dried at 60 °C and oxidized at 350 °C in air for 1 hour for conversion of the ruthenium complexes to ruthenium oxide. The loading of Pt was carried out by photochemical deposition with H2PtCl6·6H2O as the Pt source and CH3OH as the reductant under irradiation from a 300 W Xe arc lamp for 8 hours with stirring. The co-loading of RuO2 and Pt was carried out in the same manner as described above but with RuO2loading first. Then, the cocatalyst photocatalysts were fil- tered, washed thoroughly with deionized water and alcohol, and dried at 60 °C for 12 hours. For the co-loading of RuO2

and Pt on the ZnGa2O4 hollow spheres, the loading of RuO2

was performed first and then Pt was loaded according to the above methods.

The crystallographic phase of the samples was determined by using an X-ray diﬀractometer (XRD) (Rigaku Ultima III, Japan) using Cu Kαradiation (λ= 0.154178 nm). The XRD patterns were obtained over a scanning range of 10–80° at room temperature with a scan rate of 0.2° s⁻¹at 40 kV and 40 mA.

The morphology of the samples was observed by field emission scanning electron microscopy (FE-SEM, FEI NOVA NANOSEM 230). Transmission electron microscopy (TEM)

images, high-resolution transmission electron microscopy (HRTEM) images and selected area electron diﬀraction (SAED) patterns were obtained on a JEOL JEM-2100 microscope with a LaB6 filament and an accelerating voltage of 200 kV. The specific surface area of the samples was measured by nitrogen adsorption–desorption at 77 K on a surface area and porosity analyzer (Micromeritics TriStar, USA) and calculated by the BET method. The UV-vis diﬀuse-reflectance spectrum was recorded with a UV-vis spectrophotometer (Shimadzu UV-2550) at room temperature and transformed to the absorption spectrum according to the Kubelka–Munk relationship.

Photocatalytic tests

In the photocatalytic reduction of CO2, 0.1 g of the as-prepared ZnGa2O4 sample was uniformly dispersed on a circular glass reactor with an area of 4.2 cm². A UV-enhanced (200 to 350 nm) 300 W xenon arc lamp was used as the light source.

The volume of the reaction system was approximately 230 ml.

The reaction setup was vacuum-treated several times, and then high purity CO2gas was flown into the reaction setup to reach ambient pressure. Then, 0.4 ml of deionized water was injected into the reaction system as the reducer. The as- prepared photocatalysts were allowed to equilibrate in the CO2/H2O atmosphere for several hours to ensure that the adsorption of gas molecules was complete. A gas pump was used to accelerate gas diﬀusion, and circulating cooling water was used to guarantee the reaction carried out at room temperature. During irradiation, approximately 1 ml of gas was con- tinually taken from the reaction cell at given time intervals for subsequent CH4concentration analysis using a gas chromato- graph (GC-2014, Shimadzu Corp., Japan) equipped with an FID detector and a Plot Q capillary column (30 m × 0.53 mm × 20μm). Nitrogen gas was used as the carrier, and the temperature of the FID was approximately 200 °C.

Results and discussion

All X-ray diﬀraction peaks for the sample S-700 can be well indexed to the cubic spinel ZnGa2O4 phase based on JCPDS card 38-1240 (Fig. 1). No other impurity peaks could be found,

Fig. 1 XRD patterns of S-600, S-700, and S-800: (a) survey patterns and (b) magniﬁed patterns.

(3)

demonstrating that the synthetic ZnGa2O4is of a single phase.

Fig. 2 shows the FE-SEM images of S-700, which is comprised of sphere-like nanostructures with sizes ranging from 300 nm to 600 nm. The cracked spheres reveal their hollow nature (Fig. 2a and c), and there is clearly another sphere inside (Fig. 2c), demonstrating the ball-in-ball character of S-700. The TEM images further clearly demonstrate the hollow nature of the spheres from the dark edge and the pale intervals of the ZnGa2O4 microspheres (Fig. 3a and b). There is an obvious hollow sphere-like structure inside each sphere, also display- ing the ball-in-ball structure (Fig. 3c). These double-shelled hollow spheres may trap more incident photons to generate more electron–hole pairs. The high-resolution TEM image shows clear lattice fringes, the two spacings of which are 0.24 nm, corresponding to the (222) crystal plane, and 0.48 nm, corresponding to the (111) crystal plane, demonstrating that the hollow spheres are comprised of many small crystalline grains. The selected area electron diﬀraction rings further confirm the polycrystalline nature of the ZnGa2O4

hollow spheres.

The formation mechanism of the ball-in-ball ZnGa2O4

hollow spheres was tentatively explored. The Zn(II) and Ga(III) ions react with citric acid to form zinc citrate and gallium

citrate through ion exchange. During hydrothermal treatment, citrate may undergo polymerization and carbonization succes- sively to form carbon species embedding Zn(II) and Ga(III). The FE-SEM image reveals that the collected carbon species after hydrothermal treatment display spherical morphology.

Cracked spheres can also be found with the double-shell archi- tecture (Fig. 4). Therefore, the ball-in-ball structure of the ZnGa2O4hollow spheres was believed to inherit from the car- bonized precursor. Based on these observations, the present ZnGa2O4hollow spheres may follow a similar formation mechanism to that in our previous reports using gluconate salt as the carbon source to generate a family of multishelled binary metal oxide hollow microspheres.¹³ With the continuous hydrothermal process, zinc citrate and gallium citrate gradu- ally dehydrated into carbon spheres consisting of alternate car- bonaceous layers and embedding the released Zn(II) and Ga(III). The subsequent calcination process in air allows the progressive removal of the carbon species by the oxidation of carbon into CO2; simultaneously the embedded Zn and Ga species react and form crystalline ZnGa2O4 with a shell-layer structure, and the neighboring ZnGa2O4 shells condense together with the continuous annealing process, which enables the shells to become more and more densely packed to generate a double-shell hollow sphere. To our best knowl- edge, only a few ternary oxide multi-shell spheres have been reported.

The UV-vis absorbance spectrum demonstrates that the ZnGa2O4hollow spheres are an ultraviolet light-sensitive cata- lyst (Fig. 5). ZnGa2O4is a direct band gap semiconductor.³The band gap obtained from the UV-vis absorbance spectrumvia the direct gap formula is 4.6 eV, which is in accordance with the reported value.^3,8Although ZnGa2O4has a wide bandgap, it is a UV-light responding photocatalyst. However, a much negative energy edge of the conduction band endows the ZnGa2O4with a strong photocatalytic reduction ability, which is favourable for improving the photocatalytic performance.

Fig. 2 FE-SEM images of (a–c) S-700 and (d) SSR obtained by solid- state reaction.

Fig. 3 (a, b) TEM images of S-700 at diﬀerent magniﬁcations and (c) the corresponding high-resolution TEM.

Fig. 4 FE-SEM images of the collected product after hydrothermally treating an aqueous solution containing Zn(II), Ga(III), and citric acid.

(4)

Many reports have proven the capacity of ZnGa2O4 in the photocatalytic direct reduction of CO2 in the presence of H2O.^4,7–9 The mechanism involves photogenerated holes in the VB oxidize water to generate hydrogen ions by the reaction H2O → 1/2O2 + 2H⁺ + 2e⁻ (E^°_redox¼0:82 Vvs:NHE) and the photogenerated electrons in the CB reduce CO2to CH4by the reaction CO2 + 8e⁻ + 8H⁺ → CH4 + 2H2O

(E^°_redox¼ 0:24 Vvs:NHE).⁴ Fig. 6 shows the photocatalytic

activity of the ZnGa2O4hollow sphere synthesized at diﬀerent annealing temperatures. After irradiation for 8 hours, the sample S-700 exhibits the highest activity of 1.74 μmol g⁻¹. Nevertheless, the reference sample obtainedvia a solid state reaction (labelled as SSR) produced a trace amount of CH4. The reason for this originates from the large surface area of S-700 of 9.33 m²g⁻¹compared with that of the SSR sample of 0.22 m²g⁻¹. The very small specific surface area for SSR provides fewer active reaction sites and reduces gas adsorption.

The micrometre crystal size of the SSR sample (Fig. 2d) may also be responsible for the trace production of CH4. The large crystal size may lengthen the charge transport path from the interior to the outer surface. The reduction of CO2performed in the dark or in the absence of the photocatalyst shows no appearance of CH4, proving that the reduction of CO2 was driven by light and the photocatalyst.

It was found that the annealing temperature has a great eﬀect on the photocatalytic activity of the synthesized ZnGa₂O₄. The full width at half maximum of the XRD peaks of S-700 exhibits obviously narrower than that of S-600 (Fig. 1b).

This demonstrated the favored growth of the crystalline grains annealed at 700 °C, which reduced the grain boundaries and improved the crystallinity. Thus, the recombination of electrons and holes will reasonably decrease, which promotes the photocatalytic activity. The photocatalytic efficiency of S-700 is about three times higher than that of S-600. Nevertheless, the photocatalytic efficiency of S-800 is not better than that of S-700 although the crystallinity of S-800 is superior to that of S-700. It was noticed that there were several small impurity peaks in the XRD pattern of S-800. This may be possibly attributed to partial evaporation of Zn(II) at 800 °C, leading to segregation of potential Ga-based oxides. Such an impurity may act as recombination centers for electrons and holes to decrease the photocatalytic efficiency.

To further improve the efficiency of S-700, cocatalysts were loaded on the surface of the hollow spheres. The cocatalysts adopted were Pt and RuO₂, which were used as electron traps and hole traps, respectively.¹⁴ The cocatalyst is expected to effectively separate the photogenerated electron–hole pairs to improve the efficiency of CH₄generation (Fig. 7). As expected, the efficiency was significantly enhanced after loading either Pt or RuO₂. In particular, the efficiency was enhanced to more than seven times that of pristine S-700 after co-loading with Pt and RuO₂. The efficiency of the sample with RuO₂ and Pt coloaded was tested under Ar while other conditions were kept the same. It was found that no CH₄was detected. This result confirmed that the loading process has no effect on the CH₄ generation.

Conclusions

We successfully synthesized ternary oxide ZnGa2O4ball-in-ball hollow spheres for the photocatalytic reduction of CO2. The enhanced conversion eﬃciency can be ascribed to three main characteristics of spheres: (1) its hollow structure may facilitate Fig. 5 UV-vis absorption spectrum of S-700.

Fig. 6 CH₄yieldversustime over SSR, S-600, S-700, and S-800.

Fig. 7 CH₄yieldversustime over S-700 loaded with (a) 1 wt% RuO₂, (b) 1 wt% Pt, and (c) 1 wt% RuO2and 1 wt% Pt.

(5)

the absorbance of light, (2) the high specific surface area can absorb more CO2 and H2O and provide more active reaction sites, and (3) the high crystallinity prevents the recombination of electrons and holes. The cocatalysts can further promote the separation of electrons and holes to enhance the eﬃciency considerably.

Acknowledgements

This work was supported by 973 Programs (no. 2014CB239302 and 2013CB632404), the NSF of China (no. 21473091, 21473082, 51272101, 51202005, and 21603183), the NSF of Jiangsu Province (no. BK2012015 and BK20130425), and the Jiangsu Postdoctoral Science Foundation (1601062B).

References

1 S. Das and W. M. A. Wan Daud, Renewable Sustainable Energy Rev., 2014,39, 765–805.

2 H. Kawazoe and K. Ueda, J. Am. Ceram. Soc., 1999, 82, 3330–3336.

3 L. Zou, X. Xiang, M. Wei, F. Li and D. G. Evans, Inorg.

Chem., 2008,47, 1361–1369.

4 S. C. Yan, S. X. Ouyang, J. Gao, M. Yang, J. Y. Feng, X. X. Fan, L. J. Wan, Z. S. Li, J. H. Ye, Y. Zhou and Z. G. Zou,Angew. Chem., Int. Ed., 2010,122, 6544–6548.

5 K. Ikarashi, J. Sato, H. Kobayashi, N. Saito, H. Nishiyama and Y. Inoue,J. Phys. Chem. B, 2002,106, 9048–9053.

6 (a) X. X. Xu, A. K. Azad and J. T. S. Irvine, Catal. Today, 2013,199, 22–26; (b) N. Kumagai, L. Ni and H. Irie,Chem.

Commun., 2011, 47, 1884–1886; (c) C. M. Zeng, T. Hua, N. J. Hou, S. Y. Liu, W. L. Gao, R. H. Cong and T. Yang, Mater. Res. Bull., 2015,61, 481–485.

7 S. C. Yan, J. J. Wang, H. L. Gao, N. Y. Wang, H. Yu, Z. S. Li, Y. Zhou and Z. G. Zou, Adv. Funct. Mater., 2013,23, 758– 763.

8 Q. Liu, D. Wu, Y. Zhou, H. B. Su, R. Wang, C. F. Zhang, S. C. Yan, M. Xiao and Z. G. Zou, ACS Appl. Mater.

Interfaces, 2014,6, 2356–2361.

9 Z. Wang, K. Teramura, S. Hosokawaab and T. Tanaka, J. Mater. Chem. A, 2015,3, 11313–11319.

10 (a) B. J. Ma, K. Y. Lin, W. G. Su and W. Y. Liu, Appl.

Surf. Sci., 2014, 317, 682–687; (b) W. W. Zhang, J. Y. Zhang, X. Lan, Z. Y. Chen and T. M. Wang, Catal.

Commun., 2010, 11, 1104–1108; (c) X. Chen, H. Xue, Z. H. Li, L. Wu, X. X. Wang and X. Z. Fu, J. Phys. Chem.

C, 2008, 112, 20393–20397; (d) Z. S. Li, B. L. Li, Z. H. Liu, D. H. Li, C. Y. Ge and Y. P. Fang, RSC Adv., 2014,4, 48590–48595.

11 X. W. Lou, L. A. Archer and Z. C. Yang,Adv. Mater., 2008, 20, 3987–4019.

12 W. G. Tu, Y. Zhou, Q. Liu, Z. P. Tian, J. Gao, X. Y. Chen, H. T. Zhang, J. G. Liu and Z. G. Zou, Adv. Funct. Mater., 2012,22, 1215–1221.

13 Z. P. Tian, Y. Zhou, Z. D. Li, Q. Liu and Z. G. Zou,J. Mater.

Chem. A, 2013,1, 3575–3579.

14 S. C. Feng, X. Y. Chen, Y. Zhou, W. G. Tu, P. Li, H. J. Li and Z. G. Zou,Nanoscale, 2014,6, 1896–1900.