FULL P APER
Double-shelled GaN hollow spheres are prepared for photocatalytic reduction of CO
2through ammonia-gas nitridation of shape-analogous Ga
2O
3hollow spheres. Small amount oxygen in situ doping narrows the bandgap of the GaN hollow sphere which signifi cantly enhances the utilization of visible light. The large surface area of the hollow spheres provides more activity sites for the photoreduction reaction. The specifi c hollow structure also allows the trapping of the incident light for a longer time during photocatalysis process, which provides more opportunities for light absorption. Loading of Pt and RuO
2cocatalysts further enhances the separation of photogenerated elec- trons and holes to improve the CO
2reduction activity.
is of a direct bandgap with ≈3.4 eV, a very high inherent electron concentration above 10 19 cm −3 , and room-temperature mobilities of between 125 and 150 cm 2 V −1 sec −1 , [ 5 ] together with spontaneous polarization along with c -axis. [ 6 ] The associated built- in electric fi eld may facilitate to separate the generated electrons and holes to pre- vent combination. [ 7 ] GaN-based diodes are also highly effective blue-light-emitting devices. [ 8 ]
Considerable works have reported water splitting or other photocatalytic activities over GaN. [ 9 ] The valence and conduction bands of GaN well align with the water redox potentials, which is a prerequisite for effi - cient charge transfer between semiconductor and water-based electrolyte. [ 9f , 10 ] GaN also shows relatively good chemical sta- bility during photocatalysis process with more considerable resistance to corrosion in aqueous solutions under both acidic and basic solutions than other metal oxides, such as than ZnO and TiO 2. [ 9d , 11 ] For photocatalytic conversion of CO 2 , a series of experiments demonstrates that the energy of the conduction band of GaN is higher than that of most oxides, which makes it suitable for CO 2 reduction. [ 9h , 12 ]
It is known that the photocatalysis activity can in principle be improved through nanostructurization of photocatalysts, such as size decreasing for enhancing surface area, ultrathin architectures for shortening the transportation distance of the photogenerated charges from the interior to surface, [ 1e , 2d , 3d , 13 ] and controllable exposure of external crystal facets for a spatial charge separation for long duration. [ 14 ] Recently, hierarchical hollow microspheres have been drawing much attention due to their wonderful potential applications ranging from catalysis to electronics and energy. [ 15 ] Hollow structures prove good light absorbers in the fi eld of photocatalysis as the photons traped in the sealed space are scattered constantly to increase the absorbed quantity, consequently generate more excited elec- tron–hole pairs. [ 16 ]
In this work, we fabricated GaN double-shelled hollow spheres through ammonia-gas nitridation of shape-anal- ogous Ga 2 O 3 hollow spheres for photocatalytic reduction of CO 2. Different with general preparation of GaN using metalorganic vapor phase epitaxy growth, [ 17 ] or metal Ga as Ga source, [ 18 ] the present chemical method allows the residual oxygen in the Ga 2 O 3 to be in situ doping of the forming GaN hollow spheres, which narrows the bandgap of the GaN sphere, thus highly enhancing the utilization of visible light.
Y. Zhang, Prof. Y. Zhou
Key Laboratory of Modern Acoustics MOE, Institute of Acoustics School of Physics
Nanjing University Nanjing 210093 , P. R. China E-mail: zhouyong1999@nju.edu.cn
Y. Zhang, Prof. Y. Zhou, Dr. L. Q. Tang, M. Wang, P. Li, W. G. Tu, Prof. J. M. Liu, Prof. Z. G. Zou National 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: liujm@nju.edu.cn
Y. Zhang, Prof. Y. Zhou, L. Q. Tang, M. Wang, P. Li, W. G. Tu, Prof. Z. G. Zou
Jiangsu Key Laboratory for Nano Technology
Ecomaterials and Renewable Energy Research Center (ERERC) Nanjing University
22 Hankou Road , Nanjing , Jiangsu 210093 , P. R. China Dr. L. Q. Tang
College of Chemistry and Chemical Engineering Yancheng Institute of Technology
Yancheng 22401 , P. R. China DOI: 10.1002/ppsc.201500235
Fabrication of Oxygen-Doped Double-Shelled GaN Hollow Spheres toward Effi cient Photoreduction of CO 2
Yuan Zhang , Yong Zhou , * Lanqin Tang , Meng Wang , Ping Li , Wenguang Tu , Junming Liu , * and Zhigang Zou
1. Introduction
Carbon dioxide (CO 2 ) is a kind of main greenhouse gas, and the annual yield of CO 2 is increasing because of combustion of fossil fuels. One of the best solutions to CO 2 problem is photocatalytic reduction of CO 2 to renewable hydrocarbons over photocatalysts. [ 1f,g ] Many effi cient photocatalysts were explored for fi xation of CO 2 , such as TiO 2 , [ 1 ] ZnGa 2 O 4 , [ 2 ] Zn 2 GeO 4 , [ 3 ] and metal-organic frameworks. [ 4 ] Wurtzite GaN (Gallium Nitride)
FULL P APER
2. Results and Discussion
The GaN hollow sphere was synthesized through nitridation of double-shelled hollow Ga 2 O 3 sphere in ammonia gas. The hollow sphere Ga 2 O 3 was fabricated through calcination of Ga species-containing carbon sphere in air, similar with our previous works. [ 19 ] In detail, the commercially available citric acid was utilized as carbon precursor of the carbon spheres.
The continuous hydrothermal process of mixed aqueous solu- tion of Ga(NO 3 ) 3 and citric acid induces the formation of Ga species-containing carbon sphere through thermal carboniza- tion process of the citric acid. Subsequent calcination in air allows progressively removal of the carbon, and simultaneously the internal Ga moiety grows into gallium oxides to form the hollow spheres. Figure 1 a shows the X-ray diffraction (XRD) pattern of the Ga 2 O 3 hollow sphere, which exhibits mainly two broadened peaks. The position of the two peaks can be indexed
to (311), (440) peaks of Ga 2 O 3 phase (JCPDS No.20-0426). The much broadening of the diffraction peaks demonstrates that the spherical shell is of low crystallinity, possibly originating from the very small grain as building blocks. The UV–vis absorp- tion spectrum of the Ga 2 O 3 hollow spheres shows a bandgap of 4.6 eV (Figure 1 b).
The fi eld emission scanning electron microscopy (FE-SEM) images clearly reveal that the formed Ga 2 O 3 exhibits regular microspheres with the general size ranging from 1.0−1.5 µm ( Figure 2 a). The hollow structure is implied with the quasi- transparent character of the sphere under electron beam irra- diation, as arrowheads marked in Figure 2 a. A breakage of the microsphere further shows the hollow feature of the Ga 2 O 3 sphere (Figure 2 b), which was also confi rmed with the ball-in- ball structure (Figure 2 c). The residual segment of the external shell of a seriously broken sphere was observed to cover on the internal sphere (Figure 2 d).
Figure 2. SEM images of the Ga 2 O 3 hollow spheres.
10 20 30 40 50 60 70 80
Intensity / a.u.
2θ/ degree
JCPDS#20-0426
(220) (311) (400) (422) (511) (440) (533)
(a)
200 250 300 350 400
3.5 4.0 4.5 5.0 5.5
(ahν)/ a.u.
hν / eV Eg = 4.60 eV
Absorbance / a.u.
Wavelength / nm (b)
Figure 1. a) XRD pattern and b) UV–vis absorption spectrum of the synthesized Ga 2 O 3 .
FULL P APER
Nitridation of the hollow Ga 2 O 3 in ammonia atmosphere results in the formation of the GaN spheres. The SEM images of GaN also display the similar hollow sphere structure ( Figure 3 ).
The transmission electron microscopy (TEM) image shows the double-shelled GaN hollow spheres, clearly indicative of the preservation of the hollow structure of the Ga 2 O 3 ( Figure 4 a).
The diffraction ring of the corresponding electron diffraction pattern demonstrates the polycrystalline property of the GaN (inset of Figure 4 b). The resolved lattice fringes separated by 0.19 nm is assigned to the (102) plane of the wurtzite phase of GaN (Figure 4 b). All the XRD peaks of the GaN can also be indexed to the wurtzite phase with lattice constants a = b = 0.
3189 nm, c = 0.5185 nm, α = β = 90°, γ = 120° (JCPDS No.
50-0792) ( Figure 5 a). Sharpness of the diffraction peaks indi- cates highly crystallinity. No other impure phases can be found in the pattern.
In sharp contrast to general GaN of wide bandgap ( E g = ≈3.4 eV), the present hollow GaN sphere displays light yellow color (Figure S1, Supporting Information), demon- strating the obvious visible-light absorption. The UV–vis absorption spectrum confi rms the absorption onset of ≈517 nm of the GaN hollow sphere, corresponding to the fi tted bandgap of ≈2.4 eV (Figure 5 b). X-ray photoelectron spectroscopy of the GaN hollow sphere displays the binding energy of Ga 2p 3/2 peak at 1118.3 eV, ( Figure 6 a). The detected O1s peak can be
resolved to two peaks centered at 530.8 and 531.8 eV, respec- tively (Figure 6 b). The former lower energy peak corresponds to the lattice oxygen and the latter to surface OH groups. [ 9c ] The source of the lattice oxygen and surface OH may originate from the residual oxygen atoms in the as-used Ga 2 O 3 and absorbed H 2 O, respectively. As the XRD pattern shows the only existence of GaN hexagonal wurtzite and no fi nding of gallium oxides, it indicates that the lattice oxygen should substitute for nitrogen atoms of the synthesized GaN, i.e., in situ doped with oxygen.
Similar natural existence of oxygen doping was also frequently observed in the synthesized Ta 3 N 5 through nitridation of Ta 2 O 5 , in which the optical bandgaps of O-doped Ta 3 N 5 are dependent on the oxygen concentration. [ 20 ] Therefore, the narrowing of the optical bandgap in O-doped GaN may be explained from impu- rity-related nitrogen defect. [ 9c ]
The CO 2 photoreduction mainly undergoes two courses, including oxidation and reduction processes. In the oxidation process, H 2 O is oxidized to O 2 (2H 2 O + 4h + → O 2 + 4H + ) with E °ox = 0.82 V versus NHE (Normal Hydrogen Electrode); in the reduction process, CO 2 is deoxidized to CH 4 (CO 2 + 8e − + 8H + → CH 4 + 2H 2 O) with E °red =−0.24 V versus NHE. [ 21 ] In order to explore the effect of the hollow structure on the photocatalytic activity, we also nitrided the commercial white Ga 2 O 3 powders at the same condition to obtain O-doped GaN powders with irregular morphology as comparison (We call it C-GaN and
Figure 4. TEM images of the double-shelled GaN hollow spheres.
Figure 3. SEM images of the double-shelled GaN hollow spheres.
FULL P APER
the corresponding FE-SEM was presented in Figure S2 in the Supporting Information). The UV–vis absorption spectrum of C-GaN is also conducted; the optical bandgap is estimated to be the same with GaN hollow spheres within experimental error (Figure 5 b ) . Figure 7 shows that the GaN hollow sphere renders yield of CH 4 at the fi rst hour about 0.079 µmol g −1 under UV–vis light illumination (Curve b), almost two times that of the C-GaN (≈0.039 µmol g −1) (Curve a). Reduction experiment of CO 2 preformed in the dark or absence of the photocatalyst shows no appearance of CH 4 , proving that the reduction reaction of CO 2 was driven by light under the photo- catalyst. Higher photocatalytic activity of the hollow structured GaN can be contributed to following two reasons: (1) the GaN hollow sphere possesses larger BET (Brunauer-Emmett-Teller) surface areas of 5.2 m 2 g −1 than the C-GaN of 3.7 m 2 g −1 . The large surface area of the hollow sphere provides more activity sites for the photoreduction reaction. For instance, the hollow sphere shows CO 2 adsorption quantity of 3.02 cm 3 g −1 , higher than C-GaN of 1.98 cm 3 g −1 . (2) The multiple-refl ection emer- gence inside the interior cavities of the hollow spheres could trap the incident light for a longer time during photocatalysis process, which provides more opportunities for light absorp- tion, consequently generates more electron–hole pairs for reduction of CO 2 . [ 22 ]
While cocatalysts are usually required to improve the water oxidation reaction of GaN and prevent degradation of the GaN surface during the reaction, [ 9a ] the generation rate of CH 4 over the present GaN hollow sphere could also be signifi cantly
enhanced by loading RuO 2 as electron sinkers (Curve c) and Pt effective for hole conduction (Curve d). Especially, co-loading Pt and RuO 2 allows the enhancement of more than an order of magnitude of the yield of CH 4 than pristine GaN hollow sphere (Curve e). To detect visible-light contribution to the CO 2 photoreduction, the light with the wavelength ( λ > 420 nm) was used for irradiation of the Pt and RuO 2 co-loaded GaN hollow sphere. The yield of CH 4 was detected to be 0.48 µmol g −1 h −1 (Curve f), which is almost about 61% of the yield under full wavelength of light. It demonstrates that the less substitution of nitrogen atoms with oxygen atoms can decrease the bandgap in wurtzite GaN, which offers a new idea for utilizing visible light energy.
3. Conclusion
GaN hollow spheres were prepared for photocatalytic reduc- tion of CO 2 through ammonia-gas nitridation of Ga 2 O 3 hollow spheres. A small amount of oxygen doping effectively enhances the absorption of incident light. The large surface area of the hollow spheres provides more activity sites for the photoreduction reaction. The specifi c hollow structures also allow the trapping of the incident light for a longer time during photocatalysis process, which provides more opportunities for light absorption. Loading of Pt and RuO 2 cocatalysts further enhances the separation of photogenerated electron and hole to improve the CO 2 reduction reaction.
1124 1120 1116 1112
Intensity / a.u.
Binding energy / eV
Ga2p3/2 1118.3
(a)
536 532 528 524
Intensity / a.u.
Binding energy / eV O1s
(b)
531.8
530.8
Figure 6. XPS of GaN (when using Al Kα as light source, N1s peak is overlapped with the Auger peak of Ga element.).
10 20 30 40 50 60 70 80 90
In ten s ity / a.u.
2 θ / degree
JCPDS#50-0792
(100) (002)(101) (102) (110) (103) (112)(200) (201)(004) (202) (104)
(a)
300 400 500 600 700 800
1.5 2.0 2.5 3.0 3.5
(ahν)2 / a.u.
hν / eV Eg=2.4 eV
(b)
A b s o rb an ce / a .u .
Wavelength / nm
Figure 5. a) XRD pattern and b) UV–vis absorption spectrum of the GaN hollow spheres (black) and C-GaN (red).
FULL P APER
4. Experimental Section
Sample Fabrication : To fabricate GaN hollow spheres, 1 mmol Ga(NO 3 ) 3 and 3 mmol citric acid (C 6 H 8 O 7⋅H 2 O) were mixed and stirred in 40 mL H 2 O to form a clear solution as precursor. The precursor was hydrothermally treated in a 60 mL Tefl on-lined autoclave at 200 °C with a heating rate of 5 °C min −1 for 20 h. The reactor was cooled to room temperature naturally. The resultant product was ultrasonically treated and washed with acetone and deionized water mutually several times, and then dried at 60 °C. The dry sample was calcined at 500 °C for 3 h to form Ga 2 O 3 hollow spheres. [ 19 ] The Ga 2 O 3 hollow spheres were calcined at 900 °C in a quartz tube under NH 3 atmosphere at the rate of 100 sccm at a heating rate of 3 °C min −1 for 90 min. The NH 3 continuously fl owed until the sample was cooled to room temperature naturally. Finally, the yellow GaN hollow spheres were obtained. As comparison to GaN hollow spheres, commercial Ga 2 O 3 was purchased and also nitrided to polycrystalline GaN powders. The loading of RuO 2 cocatalyst on GaN hollow spheres was carried out by impregnating sample with Ru 3 (CO) 12 in THF (Tetrahydrofuran), and the loading of Pt was carried out with photochemical deposition with H 2 PtCl 6⋅6H 2 O as Pt source. The co-loading of RuO 2 and Pt was carried out as described above with RuO 2 loading fi rst. [ 3a ]
Characterization : Thermogravimetric analysis was carried out (Pyris 1DSC, PerkinElmer USA) at a heating rate of 20 °C min −1 from 25 to 750 °C in air. The crystal structure of the powder sample was investigated by an X-ray diffractometer (Rigaku Ultima III, Japan) using Cu Kα radiation ( λ = 0.154178 nm) at 40 kV and 40 mA. The XRD pattern was obtained over the scanning range of 5°–90° at room temperature with a scan rate of 0.2° S −1 . The morphology of the powders was examined by FE-SEM (FEI NOVA NANOSEM 230). TEM images and high-resolution TEM images were obtained on a JEOL JEM-2100 microscope with a LaB 6 fi lament and an accelerating voltage of 200 kV. The surface states of sample were analyzed with X-ray photoelectron spectroscopy (XPS) (PHI 5000 VersaProbe), using Al Kα as light source. The XPS spectrum was calibrated with respect to the binding energy of the adventitious C1s peak at 284.8 eV. The specifi c surface
area of the samples was measured by nitrogen adsorption–desorption at 77 K on surface area and porosity analyzer (Micromeritics TriStar, USA) and calculated by the BET method. The CO 2 adsorption quantity of the samples was also measured at 273 K. The UV–vis diffuse-refl ectance 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 Reduction of CO 2 : In the photocatalytic reduction of CO 2 , 0.1 g of the as-prepared GaN sample was uniformly dispersed on the circular glass reactor with an area of 4.2 cm 2 . A 300 W Xenon arc lamp was used as the light source. The volume of reaction system was about 230 mL. The reaction setup was vacuum-treated several times, and then the high purity CO 2 gas was followed into the reaction setup for reaching ambient pressure. 0.4 mL of deionized water was injected into the reaction system as reducer. The as-prepared photocatalysts were allowed to equilibrate in the CO 2 /H 2 O 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 that the reaction was carried out at room temperature.
During the irradiation, about 1 mL of gas was continually taken from the reaction cell at given time interval for subsequent CH 4 concentration analysis by using a gas chromatograph (GC-2014, Shimadzu Corp., Japan) equipped with a FID (Flame Ionization Detector) detector and Plot Q capillary column (30 m × 0.53 mm × 20 µm). Nitrogen gas was used as the carrier, and the temperature of FID was about 150 °C.
Supporting Information
Supporting Information is available from the Wiley Online Library or from the author.
Acknowledgements
This work was supported by the 973 Programs (Grant Nos.
2014CB239302 and 2013CB632404), National Natural Science
0 1 2 3 4 5
0.00 0.05 0.10 0.15 0.20
CH4 / μmol g-1
Time / hour (a) (b)
1 3 5
0 2 4
0 1 2 3 4
CH4 / μmol g-1
Time / hour
(d) (c) (e)
1 3 5
0 2 4
0.0 0.5 1.0 1.5 2.0 2.5
CH4 / μmol g-1
Time / hour (f)
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9
CH4 / μmol g-1 h-1
Sample mark
a b c d e f
(g)
Figure 7. CH 4 yield versus time over a) C-GaN, b) GaN hollow spheres, c) 1 wt% RuO 2 -loaded GaN hollow spheres, d) 1 wt% Pt-loaded GaN hollow spheres, e) 1 wt% RuO 2 , and 1wt% Pt-coloaded GaN hollow spheres under UV–vis light irradiation, and f) GaN hollow spheres loaded with RuO 2 and Pt under visible light irradiation, and g) time average yield of every sample.
FULL P APER
Foundation of China (Grant Nos. 21473091, 51272101, and 51202005), and National Science Foundation of Jiangsu Province (Grant Nos.
BK2012015, BK20130425, and BK20130053), Fundamental Research Funds for the Central Universities (020414380001), and the National Natural Science Foundation of China (Grant No. 51431006).
Received: November 19, 2015 Revised: March 6, 2016 Published online: March 31, 2016
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