Ordered mesoporous ZnGa2O4 for photocatalytic hydrogen evolution

Xiaomei Yang , Jinmiao Ma , Ru Guo , Xiaochen Fan , Ping Xue *, Xiaozhong Wang , Hui Sun , Qingfeng Yang and Xiaoyong Lai *
State Key Laboratory of High-Efficiency Utilization of Coal and Green Chemical Engineering, School of Chemistry and Chemical Engineering, Ningxia University, Yinchuan 750021, People's Republic of China. E-mail: xylai@nxu.edu.cn; ping@nxu.edu.cn; Fax: +86-0951-2062323; Tel: +86-0951-2061456

Received 16th April 2021 , Accepted 29th May 2021

First published on 2nd June 2021


Zinc gallate (ZnGa2O4) materials as a mixed oxide of d10 metal zinc and gallium have a wide range of applications because of their intriguing properties. Considerable effort has been devoted to nanostructuring ZnGa2O4 to regulate or enhance its properties and performance, although further investigation would be still necessary. Herein, for the first time, we demonstrated the nanocasting synthesis of ordered mesoporous ZnGa2O4 with highly crystalline frameworks by using mesoporous silica as the hard template, which exhibits excellent photocatalytic hydrogen (H2) evolution performance. The key for fabricating ordered mesoporous ZnGa2O4 was its improved stability over individual zinc oxide or gallium oxide in alkaline etching solution during the removal of the silica template, which allows for the maintenance of the ordered mesoporous structure in a finely etching silica process. The resultant ordered mesoporous ZnGa2O4 possesses an ultrathin framework of 3–5 nm and a large pore size of 11 nm as well as a relatively high surface area of ∼157 m2 g−1. An enhanced photocatalytic H2 evolution performance of 2.72 mmol h−1 g−1 could be achieved under ultraviolet (UV) light irradiation, which is almost 1.8 and 3.9 times as high as those of ZnGa2O4 nanoflowers (1.5 mmol h−1 g−1) and bulk ZnGa2O4 (0.7 mmol h−1 g−1).

1. Introduction

There is increasing interest in developing various advanced photocatalysis techniques since the pioneering work of Fujishima and Honda on photo-assisted water electrolysis at a TiO2 electrode,1 such as directly utilizing solar energy for splitting water2–8 or reducing carbon dioxide into fuels,9–13 and photodegrading organic pollutants.14–18 Photocatalyst materials play a key role in these systems, although the relative low efficiency of utilizing solar energy has critically limited the practical application. Therefore, numerous efforts have been devoted to designing and synthesizing various nanostructured photocatalysts with specific composition, high specific surface area and improved activities.19–23 To date, traditional transition metal oxide photocatalysts mainly composed of d0 metal ions, such as Ti4+,24–26 Zr4+,27–29 Nb5+,30–33 and Ta5+,34–38 have been frequently investigated. Among them, ordered mesoporous photocatalysts are of much interest.39 For example, Ying and co-workers synthesized ordered mesoporous titanium and niobium oxides with high specific surface area but low crystallinity and small pore size by a ligand-assisted templating method40,41 and Domen and co-workers modified this method and obtained ordered mesoporous tantalum oxides with well-crystalline frameworks and improved photocatalytic activities for overall water splitting under ultraviolet light irradiation.42,43 Zhao and co-workers also synthesized an ordered mesoporous black titanium oxide with highly visible-light-responsive activities by an evaporation-induced self-assembly (EISA) method combined with an ethylenediamine encircling process.44 Nevertheless, it remains necessary to further develop advanced photocatalysts with high activities.

Recently, p-block d10 metal oxides, such as ZnGa2O4, have attracted much attention and theoretical calculations reveal that these materials would have potential to be photocatalysts.45,46 Inoue and co-workers first demonstrated photocatalytic overall water splitting over ZnGa2O4 loaded with RuO2 cocatalysts, although bulk ZnGa2O4 obtained from the high temperature solid–solid reaction had a lower specific surface area of 2.3 m2 g−1 and exhibited a moderate H2 evolution performance of about 0.04 mmol h−1 g−1.47 Yan et al. reported a room-temperature route to mesoporous ZnGa2O4 with the high specific surface area of 110.4 m2 g−1 and improved photocatalytic activity for reducing CO2 to CH4.48 Wang and coworkers reported porous nanocrystalline ZnGa2O4 with a high specific surface area of 201 m2 g−1 and its high photocatalytic reactivity toward degradation of benzene pollutants, although its specific surface area dramatically reduced to 43 m2 g−1 during further crystallization at 200 °C, followed by the degradation of catalytic activity.49 ZnGa2O4 materials with an ordered mesoporous structure and a stable crystalline framework should be highly desirable, because their regular mesochannels could allow for improved mass-transfer and their frameworks at the nanoscale range could effectively increase the specific surface area for providing more active sites and also reduce the distance of photoinduced-charge migration from the bulk to the surface for facilitating photocatalytic action. To the best of our knowledge, however, there is no report on the successful synthesis of ordered mesoporous ZnGa2O4 materials and their application in photocatalysis.

Herein, we have demonstrated, for the first time, the nanocasting synthesis of ordered mesoporous ZnGa2O4 with highly crystalline frameworks by using mesoporous silica as the hard template (Scheme 1), which exhibits excellent photocatalytic hydrogen (H2) evolution performance. The resultant ordered mesoporous ZnGa2O4 possesses an ultrathin framework of 3–5 nm and a relatively high specific surface area of ∼157 m2 g−1 as well as a large pore size and a pore volume of 11 nm and 0.42 cm3 g−1, respectively. More importantly, the ordered mesoporous ZnGa2O4 materials can exhibit a high photocatalytic H2 generation rate of 2.72 mmol h−1 g−1, which is almost 1.8 and 3.9 times as high as those of ZnGa2O4 nanoflowers (1.5 mmol h−1 g−1) and bulk ZnGa2O4 (0.7 mmol h−1 g−1).

image file: d1qm00593f-s1.tif
Scheme 1 Illustration of ordered mesoporous ZnGa2O4 templated from mesoporous silica KIT-6.

2. Experimental section

2.1 Synthesis of ordered mesoporous ZnGa2O4 materials

Ordered mesoporous ZnGa2O4 was synthesized through the nanocasting method by using ordered mesoporous silica KIT-6, where KIT-6 was synthesized and kept at 40 °C according to the previous literature.50–53 0.54 g of gallium nitrate hydrate and 0.19 g of zinc nitrate hexahydrate were melted at 75 °C and then 2 g of KIT-6 was added into the above-mentioned liquid. The resultant KIT-6/precursor composite was ground three times at 75 °C every 30 min and then calcined at 850 °C for 6 h to thermally decompose zinc and gallium nitrates into ZnGa2O4. Finally, KIT-6 was etched at room temperature for 1 h by using a 2 M NaOH aqueous solution and the etching process was repeated two times. The resultant mesoporous ZnGa2O4 material was recovered by centrifugation, washed thoroughly with deionized water and ethanol, and dried at 70 °C for 12 h. The bulk ZnGa2O4 material was also synthesized through the similar process except without using KIT-6.

2.2 Synthesis of ZnGa2O4 nanoflowers

For comparison, ZnGa2O4 nanoflowers were synthesized by a similar process mentioned in the previous reports.54,55 0.42 g of gallium nitrate hydrate and 0.15 g of zinc nitrate hexahydrate were added into the solution of 10 mL of deionized water and 5 mL of ethylenediamine. The mixture was stirred for 40 min and then poured into a 25 mL stainless Teflon-lined autoclave. After being heated at 180 °C for 24 h in an oven, the autoclave was naturally cooled to room temperature. The product was collected by centrifugation, washed thoroughly with deionized water five times and with ethanol three times, and then dried at 60 °C for 12 h.

2.3 Characterizations

A powder X-ray diffraction (XRD) pattern was recorded with a Bruker AXS D8 diffractometer (Cu Kα), operating at 40 mA and 40 kV. The surface morphology of ZnGa2O4 samples was observed by a scanning electron microscope (SEM, ZEISS EVO18) with an acceleration voltage of 15 kV. Transmission electron microscopy (TEM) images were obtained using a Hitachi HT7700 electron microscope with an operating voltage of 120 kV. High-resolution TEM images, high-angle annular dark field scanning TEM (HAADF-STEM) images and elemental mapping images were recorded by an FEI Talos 200S electron microscope operating at 200 kV. N2 adsorption–desorption isotherms under liquid nitrogen (−196 °C) were obtained on the Autosorb-iQ-XR adsorption analyzer. Thermo Scientific ESCALAB 250Xi X-ray photoelectron spectroscopy (XPS) was used to observe the chemical environment of the samples. Diffuse reflectance spectra (DRS) were recorded on an ultraviolet-visible-near infrared spectrophotometer (JASCO V-770) equipped with an integrating sphere. Photocurrent response was obtained on an electrochemical analyzer (CHI 660E).

2.4 Photocatalytic H2 production evaluation

The photocatalytic reactions for H2 evolution were conducted on a commercial reaction system (CEL-PAEM-D8, Beijing China Education Au-light Co. Ltd, China) under UV-light irradiation.56 Typically, 50 mg of ZnGa2O4 samples were dispersed into a mixed aqueous solution containing 0.25 M Na2SO3 and 0.35 M Na2S (80 mL). The reaction system was kept at 6 °C. A 300 W Xe-lamp (CEL-HXUV300) accompanied with an optical filter of 200–400 nm was used as the UV-light source to vertically irradiate the reaction vessel to generate hydrogen (the light intensity was calibrated with a CEL-NP2000 optical power meter). The produced H2 was detected every hour by online gas chromatography (GC-7920, N2 as the carrier gas) using a thermal conductivity detector (TCD).

3. Results and discussion

It is usually hard to obtain ordered mesoporous amphoteric oxides such as individual zinc oxide or gallium oxide by directly templating mesoporous silica,57,58 which are easily dissolved in either the acid or alkaline solution during the etching silica template process, although the corresponding mixed oxide (ZnGa2O4) seems to possess enhanced stability in alkaline solution. Ordered mesoporous ZnGa2O4 could be synthesized by using zinc and gallium nitrates as mixed precursors and ordered mesoporous silica KIT-6 with a pore size of 5.6 nm (Fig. S1, ESI) as a hard template, where the etching silica template process should be carefully controlled and harsh etching would damage the mesostructure. Fig. 1 exhibits the low-angle XRD patterns of KIT-6 and mesoporous ZnGa2O4 before and after etching KIT-6. A sharp diffraction peak and two relatively weak but well-resolved diffraction peaks were observed in the XRD pattern of KIT-6 (Fig. 1a), which should be characteristic of a 3D cubic structure (Ia3d) and indexed as the (211), (220) and (332) reflection.50 After the formation of ZnGa2O4 frameworks within the channels of KIT-6, the diffraction contrast from mesopores and silica frameworks in the KIT-6/ZnGa2O4 composite correspondingly decreased, resulting in the reduction in intensity of diffraction peaks. A new diffraction peak appeared in the lower angle range, indexed as the (110) reflection of the space group I4132,59–61 suggesting the selective occupation of ZnGa2O4 frameworks within only one of two sets of double gyroid mesochannels in KIT-6 and forming a single uncoupled subframework. After etching KIT-6, the (110) diffraction peak exhibited more clearly in the XRD pattern of the resultant mesoporous ZnGa2O4, revealing the structural transformation from Ia3d to I4132 space group. The calculated lattice parameters from the respective d211 spacing are a0 = 20.9 nm for KIT-6, a0 = 20.3 nm for KIT-6/ZnGa2O4, and a0 = 19.2 nm for mesoporous ZnGa2O4, suggesting a somewhat structural shrinkage after high-temperature treatment and template removal. Wide-angle XRD patterns for all the ZnGa2O4 samples are shown in Fig. 2. All the diffraction peaks could be indexed to (111), (220), (311), (222), (400), (422), (511), (440), and (533) planes of ZnGa2O4 (JPCDS No. 71-0843), respectively, and no other crystalline impurities appeared. The XRD diffraction peaks for mesoporous ZnGa2O4 and ZnGa2O4 nanoflowers (Fig. S2, ESI) are significantly broader than those for bulk ZnGa2O4, which is in agreement with the reduction of crystallite size.54
image file: d1qm00593f-f1.tif
Fig. 1 Low-angle XRD patterns of (a) KIT-6 (b) KIT-6/ZnGa2O4 composites, (c) ordered mesoporous ZnGa2O4, respectively.

image file: d1qm00593f-f2.tif
Fig. 2 Wide-angle XRD patterns of (a) ordered mesoporous ZnGa2O4, (b) ZnGa2O4 nanoflowers, (c) bulk ZnGa2O4, respectively.

Typical SEM and TEM images for ordered mesoporous ZnGa2O4 products are shown in Fig. 3 and Fig. S3 (ESI). From their SEM and TEM images, we could clearly observe that the resultant mesoporous ZnGa2O4 products are mainly composed of some 500–1500 nm particles with periodically ordered mesostructures but irregular morphology. Predominantly uncoupled subframeworks with large pores of 11 nm can be observed in almost all the particles, which confirmed that ZnGa2O4 subframeworks only replicated one of two sets of enantiomeric mesochannels in KIT-6 templates, in line with the low-angle XRD data (Fig. 1). The high resolution (HR) TEM image (Fig. 3b) of the resultant mesoporous ZnGa2O4 shows the ultrathin subframeworks with a thickness of 3–5 nm, which is somewhat less than the pore size (5.6 nm) of KIT-6 templates, possibly due to the volume shrinkage of zinc and gallium nitrates during the thermal decomposition. The clear lattice fringes with a lattice spacing of 0.24 nm, which could be indexed to the (222) lattice plane of ZnGa2O4, also confirm the high crystallinity of the ZnGa2O4 subframeworks. The high angular dark field (HADF) scanning TEM (STEM) image and elemental maps demonstrate the homogeneous distribution of Zn, Ga and O elements within the ultrathin ZnGa2O4 frameworks (Fig. 3c–f).

image file: d1qm00593f-f3.tif
Fig. 3 (a and b) TEM images, (c) high angular dark field (HADF) scanning TEM (STEM) images of ordered mesoporous ZnGa2O4 and the corresponding elemental maps for (d) Zn, (e) Ga and (f) O, respectively.

N2 adsorption–desorption isotherms for all the ZnGa2O4 samples are shown in Fig. 4. The resultant mesoporous ZnGa2O4 products show typical type-IV isotherms with a hysteresis loop, indicating a characteristic of mesoporous structures including capillary condensation.50 The corresponding specific surface area and total pore volume are about 157 m2 g−1 and 0.42 cm3 g−1, respectively, which are significantly higher than those for ZnGa2O4 nanoflowers (94 m2 g−1 and 0.33 cm3 g−1) and bulk ZnGa2O4 (13 m2 g−1 and 0.13 cm3 g−1). It should be reasonable to consider that the framework thickness of the mesoporous ZnGa2O4 was effectively limited to 3–5 nm by the silica framework of KIT-6 (the theoretically estimated specific surface areas for straight 3 and 5 nm thick ZnGa2O4 frameworks with smooth surfaces are calculated to be about 216 and 130 m2 g−1, respectively). Pore size distribution (Fig. 5) of mesoporous ZnGa2O4 shows that its pore size is centered at 11 nm, in accordance with the TEM image. These results combined with XRD and TEM data well demonstrated a relatively successful replication from KIT-6 to ordered mesoporous ZnGa2O4 with ultrathin crystalline frameworks and large pore size. Both high specific surface area and large mesopores of the resultant ordered mesoporous ZnGa2O4 should be especially beneficial for wide application owing to more active sites and the enhanced mass-transfer, in heterogeneous catalysis and chemical sensing.

image file: d1qm00593f-f4.tif
Fig. 4 N2 adsorption–desorption isotherms of (a) ordered mesoporous ZnGa2O4, (b) ZnGa2O4 nanoflowers, (c) bulk ZnGa2O4, respectively.

image file: d1qm00593f-f5.tif
Fig. 5 Pore size distribution of ordered mesoporous ZnGa2O4.

The diffuse reflectance absorption spectrum of mesoporous ZnGa2O4 is shown in Fig. 6. A typical semiconductor absorption with an edge of 277 nm is observed for mesoporous ZnGa2O4 as compared to 283 nm for ZnGa2O4 nanoflowers and bulk ZnGa2O4, whose corresponding band gaps (Eg) are 4.47 and 4.39 eV, respectively. The slight blue shift in the band gap may be related to the smaller framework thickness of mesoporous ZnGa2O4. The valence band of mesoporous ZnGa2O4 could be estimated to be 3.49 V (versus normal hydrogen electrode, NHE) from its valence band XPS spectra (Fig. S4, ESI), whereas its conduction band corresponds to −0.98 V (versus NHE). Such an ultrathin framework structure is also favorable for the interfacial separation and transferring of photoinduced-electrons and holes, which could be indeed confirmed by the enhanced photocurrent response of the mesoporous ZnGa2O4-based electrode in 0.5 M Na2SO4 aqueous solution (Fig. S5, ESI). Therefore, improved performance could be expected in heterogeneous photocatalysis.

image file: d1qm00593f-f6.tif
Fig. 6 The UV/Vis diffuse reflectance absorption spectrum of (a) ordered mesoporous ZnGa2O4, (b) ZnGa2O4 nanoflowers, (c) bulk ZnGa2O4, respectively.

The evaluations of photocatalytic water-splitting H2 production for all the ZnGa2O4 samples were conducted in an 80 mL aqueous solution containing 50 mg of photocatalyst and sacrificial reagents of Na2SO3 and Na2S under the UV light irradiation of a 300 W Xe lamp. As shown in Fig. 7a, the ordered mesoporous ZnGa2O4 exhibits a H2 evolution performance of 13.6 mmol g−1 under UV light irradiation for 5 h and the average H2 evolution rate is calculated to be about 2.72 mmol h−1 g−1, which is 1.8 times higher than that for ZnGa2O4 nanoflowers (1.5 mmol h−1 g−1), although the specific surface area (157 m2 g−1) of the former is only 1.67 times higher than that (94 m2 g−1) of the latter. That could be because of the ultrathin frameworks of 3–5 nm in the resultant ordered mesoporous ZnGa2O4 that not only allow for high specific surface area (providing more active sites) but also shorten the diffusion path of photo-generated charges from the interior to the surface (reducing the recombination of the photo-generated electron and hole),48 thus resulting in higher photocurrent density and more photo-generated electrons for effective H2 evolution. The apparent quantum efficiency for the ordered mesoporous ZnGa2O4 has been carefully investigated, which is ca. 0.48% at 254 nm, 0.92% at 275 nm, 1.3% at 280 nm, and 1.5% at 295 nm, respectively. The average apparent quantum efficiency in the range of 200–400 nm for the ordered mesoporous ZnGa2O4 is ca. 0.23% as compared to 0.13% for ZnGa2O4 nanoflowers. After three reaction runs (Fig. 8), the mesoporous ZnGa2O4 sample retained 90% of the initial photocatalytic activity under UV-light irradiation and reached a stable H2 evolution rate of 2.44 mmol h−1 g−1, superior to those previously reported in the literature (Table S1, ESI).47,55,62–65 A long-time H2 evolution rate test for 24 h also demonstrated the good stability in photocatalytic activity (Fig. S6, ESI) and its structural stability was also confirmed (Fig. S7, ESI). However, the ordered mesoporous ZnGa2O4 with ultrathin frameworks and high specific surface area seems to be still utilized well below its potential as compared to bulk ZnGa2O4. Nevertheless, if this photocatalysis system was performed in the absence of cocatalysts, the surface recombination of the photo-generated electron and hole may be non-negligible. XPS analysis (Fig. S8 and S9, ESI) shows that the Ga/Zn atomic ratio on the surface of the mesoporous ZnGa2O4 sample before and after photocatalytic water splitting H2 evolution is about 1.60 and 2.58 respectively, suggesting that there may be partly gallium and zinc dissolution and some derived surface defects, which would play a role of the recombination centre for the photo-generated electron and hole.66–68 Therefore, there should be still a great potential to further enhance photocatalytic H2 production performance of the ordered large-pore mesoporous ZnGa2O4 with ultrathin frameworks if combining with suitable cocatalysts and suppressing the surface recombination.

image file: d1qm00593f-f7.tif
Fig. 7 Time course of photocatalytic water-splitting H2 evolution on (a) ordered mesoporous ZnGa2O4, (b) ZnGa2O4 nanoflowers, (c) bulk ZnGa2O4, respectively.

image file: d1qm00593f-f8.tif
Fig. 8 Stability test of ordered mesoporous ZnGa2O4 during photocatalytic water splitting H2 evolution (a) 1st run, (b) 2nd run, and (c) 3rd run.

4. Conclusions

In summary, we successfully synthesized an ordered large-pore mesoporous ZnGa2O4 photocatalyst through a nanocasting method by using ordered mesoporous silica as the hard template, which is constructed with ultrathin crystalline frameworks of 3–5 nm and possesses a high specific surface area of 157 m2 g−1. The resultant ordered mesoporous ZnGa2O4 with both high specific surface area and ultrathin crystalline frameworks exhibited excellent photocatalytic H2 evolution and was significantly superior to bulk ZnGa2O4 because of more active sites derived from higher surface area and more photo-generated electrons for H2 evolution resulting from the shortened migration path of photo-generated charges from the interior to the surface. More investigations on the bulk composition adjustment and surface functionalization of such ordered mesoporous ZnGa2O4 with both high specific surface area and ultrathin crystalline frameworks may be worthy for further improving its photocatalytic H2 evolution.

Conflicts of interest

There are no conflicts to declare.


This work was partly supported by the National Natural Science Foundation of China (No. 52062043), the National First-Rate Discipline Construction Project of Ningxia (NXYLXK2017A04) and the Talent-introducing Special Foundation for Key Research Program of Ningxia (2018BEB04032). X. Lai thanks the Ningxia Fostering Program for Innovative Leading Talents in Science and Technology (KJT2017003) and the Program for Youth Excellent Scholars of State Key Laboratory of High-Efficiency Utilization of Coal and Green Chemical Engineering and the Natural Science Foundation of Ningxia University (ZR1719).


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Electronic supplementary information (ESI) available: N2 adsorption–desorption isotherms, SEM image, DRS spectra, XPS spectra, photocurrent response and comparison of photocatalytic H2 evolution rate. See DOI: 10.1039/d1qm00593f
These authors contributed equally to this work.

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