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Self-Assembled Metastable γ-Ga2O3 Nanoflowers with Hexagonal Nanopetals for Solar-Blind Photodetection Yue Teng, Le Xin Song,* Anne Ponchel,* Zheng Kun Yang, and Juan Xia β-Ga2O3 is the most stable phase at room temperature among the five existing phases (α, β, γ, δ, and ⑀)[1,2] of Ga2O3. It is one of the widest bandgap semiconductor oxides (4.9 eV) and shows unique physical properties that make it interesting for high conductivity,[3] visible photoluminescence (PL),[4] photocatalytic activity,[5] adsorption capacity,[6] and optoelectronics.[7] The other metastable phases, like γ-Ga2O3, have been much less thoroughly investigated, partially due to the associated experimental difficulties in their synthesis and isolation. It was reported that γ-Ga2O3 materials can be obtained by sintering Ga(NO3)3[8] or GaOOH[9] at a certain temperature. However at a higher temperature, the metastable γ-Ga2O3 will be transformed into β-Ga2O3.[10] Thus, special attention is needed concerning the construction of pure γ-Ga2O3 materials, especially nanocrystals. Recently, Radovanovic and his colleagues demonstrated that γ-Ga2O3 nanocrystals show a size-tunable photoluminescence.[11] This allows us to think that the nanocrystals of γ-Ga2O3 with high ratios of surface to volume may lead to high performance in optoelectronics such as solar-blind photodetectors. Starting from this background, in this work we wish to report the synthesis of metastable γ-Ga2O3 nanoflowers with hexagonal nanopetals by the oxidation of metallic Ga in solutions. Our results indicate that a variation in the shape of the γ-Ga2O3 causes to a corresponding variation of photoresponse. This effect is of importance because it provides a possibility for improving the functionality or performance of photodetectors and related optoelectronic devices. A simple solvothermal method was used to realize the morphology-controlled synthesis of γ-Ga2O3 nanostructures. In a 50 mL flask, 0.1 g (1.4 mmol) of metallic Ga was mixed with 5.0 g (83 mmol) of urea using acetone as dispersant[12] at room temperature. Thereafter, the temperature in the flask was raised to 303 K and the system was stirred for 0.5 h. After Ga was fully
Y. Teng, Dr. L. X. Song, Z. K. Yang, J. Xia CAS Key Laboratory of Materials for Energy Conversion Department of Materials Science and Engineering University of Science and Technology of China Hefei 230026, PR China E-mail: [email protected] Y. Teng, Dr. L. X. Song, Z. K. Yang, J. Xia Department of Chemistry University of Science and Technology of China Jin Zhai Road 96, Hefei 230026, PR China Prof. A. Ponchel Université d’Artois Unité de Catalyse et Chimie du Solide UCCS UMR CNRS 8181, Faculté des Sciences Jean Perrin Rue Jean Souvraz, F-62300 Lens, France E-mail: [email protected]
DOI: 10.1002/adma.201402047
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melted, 10 mL concentrated hydrochloric acid (37%) and 40 mL acetone were added into the flask (i.e., the molar ratio of urea to HCl was 1:1.44). Then, the solution was completely transferred into a sealed stainless steel autoclave and heated to 473 K for 4 h. Finally, a white powder (γ-Ga2O3–1d, 0.067 g, 82%) was collected by filtration, washed several times with ethanol and distilled water, and dried in vacuum. The synthesis was repeated but with 12 mL concentrated hydrochloric acid, giving another white powder (γ-Ga2O3–2, 0.062 g, 76%). The X-ray diffraction (XRD, Figure 1a) patterns for the as-obtained samples show a face-centered cubic (fcc) structure of γ-Ga2O3 phase with a space group of Fd3m (JCPDS 20–0426).[13] The scanning electron microscopy (SEM) images of the γ-Ga2O3–1d are shown in Figure 1b and c. Clearly, the morphology of the particles (diameter, 1∼2 µm) seems to be a flower-like structure formed by the self-assembly of tens and hundreds of nanopetals (side length, 100–200 nm, height, 10–20 nm). The higher-magnification SEM images (Figure S1 in the Supporting Information, SI) show that the nanopetals exhibit a hexagonal feature. The transmission electron microscopy (TEM, Figure 1d) image not only verifies the existence of the self-assembly, but also suggests the presence of a hollow interior through a contrast between the edges and the center of the particle. Furthermore, the high-resolution TEM (HR-TEM, Figure 1e) image of a hexagonal petal tip indicates that the spacing between two adjacent lattice fringes is 0.29 nm, corresponding to the (220) plane of the fcc lattice. The selected area electron diffraction (SAED, Figure 1f) pattern shows six bright and sharp spots on a darker background with a six-fold symmetry, corresponding to the {220} reflections of the fcc γ-Ga2O3 crystal orientated in the [111] direction. This is an indication that the flat surface of the hexagonal petals is parallel to the (111) habit plane[14] (Figure S2 in the SI). In particular, there are the 1/3{422} reflections in the [111] electron diffraction pattern, which should be forbidden for a perfect fcc structure.[15] This phenomenon has been observed in plate-like crystals of metals.[16,17] To the best of our knowledge, it is the first example for Ga2O3 nanoparticles. The reason for this may be either: i) the existence of stacking faults lying parallel to the (111) surfaces and extending across the entire nanopetals,[16] ii) the presence of twin planes,[18] or, iii) a hexagonal-like monolayer on the nanopetal faces.[19] It should be noted that the HR-TEM image does not show a corresponding 3 × {422} lattice spacing (0.504 nm) of the fcc Ga2O3 crystal, which is different from those observed in metallic crystals.[16,20] The loss of superlattice spacings may be related to structural features such as large fringes and wider boundaries of the metal oxide nanocrystals compared with metal crystals. From the structural point of view, it is important to note that the oxidation process of Ga is accompanied by a structural
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COMMUNICATION Figure 1. a) XRD patterns of γ-Ga2O3–1d (A) and γ-Ga2O3–2 (B); b) FE-SEM image of γ-Ga2O3–1d; c) higher magnification image of a single nanoflower; d) TEM micrograph of a single nanoflower; e) HR-TEM image of a nanopetal; f) SAED pattern of a nanopetal; g) FE-SEM image of γ-Ga2O3–2; h) TEM micrograph of a single microsphere; i) HR-TEM image of a single microsphere; and, j) SAED pattern of a single microsphere.
Therefore, we speculate that the presence and hydrolysis of transformation from spherical (metallic Ga)[21] to flower-like urea is a key factor responsible for the generation, nucleation shape (the Ga2O3). An example of a time-dependent transforand growth of γ-Ga2O3. mation is illustrated in Figure 2. The result of growth experiments at different heating times of 1 h (γ-Ga2O3–1a), 2 h Several control experiments confirmed this hypothesis and (γ-Ga2O3–1b), and 3 h (γ-Ga2O3–1c) with other parameters the allowed us to evaluate processing dependent structural changes. same as in Figure 1b indicates that there is a clear trend from the nanoparticles (diameter, 80–100 nm) to the gradual emergence of the hexagonal nanopetals. The XRD patterns in Figure 2 confirm that all the samples are pure and iso-structural with the γ-Ga2O3–1d. The oxidation process of metallic Ga involving several steps is described by Equation 1–6. A schematic illustration explaining this oxidation process is shown in Figure 3. First of all, metallic Ga was oxidized into Ga3+ ions by the H+ ions of hydrochloric acid (Equation 1, Step I). Simultaneously, urea was reacted with the H+ ions and hydrolyzed to produce OH− ions, which is responsible for the high pH of the suspensions[22] (Equation 2 and 3). Then, the Ga3+ ions were hydrolyzed to form Ga2O3 in an alkaline solution through an equilibrium intermediate state[23] between Ga(OH)3 and GaOOH (Equation 4–6, Step II). Subsequently, the γ-Ga2O3 nanoparticles gradually grew to nanopetals (Figure 2 and Step III) and assembled into nanoflowers (Figure 1b, and Steps IV and V). Meanwhile, we considered that the release of the formed gases (Equation 1–3) may act as a gas-bubble template[24] for the creation of the hollow interior structural feature (Figure 1d). It should be reasonable to believe that Equation 3 and 4 could happen together and promote Figure 2. FE-SEM images and XRD patterns of the γ-Ga2O3 obtained using the same conditions each other in the solvothermal conditions. as Figure 1b but at different heating times of 1, 2, and 3 h.
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urea/water because urea is insoluble in acetone, leading to a moderate decrease of the reaction rates. 2Ga + 6H+ → 2Ga 3+ + 3H2 ↑
(1)
CO(NH2 )2 + 2H+ + H2O → 2NH+4 + CO2 ↑
(2)
CO(NH2 )2 + 3H2 O → 2NH+4 + 2OH− + CO2 ↑
(3)
Ga 3+ + 3OH− → Ga(OH)3
(4)
Ga(OH)3 → GaOOH + H2O
(5)
Figure 3. Schematic illustration describing the formation of γ-Ga2O3–1d.
2GaOOH → Ga 2O3 + H2O (6) i) When the initial molar ratio of urea to HCl was decreased to 1:1.69, we obtained spherical γ-Ga2O3–2 with an average Moreover, our investigations have demonstrated that a temperature dependent phase transition from γ-phase to β-phase diameter of 1–1.5 µm as seen in Figure 1g and h. It should be stressed that the γ-Ga2O3–2 shows poor crystallization, with less occurs in the neighborhood of 973 K by sintering the γ-Ga2O3– defined diffraction lines. Its HR-TEM image (Figure 1i) depicts 1d in air. As seen in Figure 4, the phase structure of the similar lattice fringes as those of the γ-Ga2O3–1d, and its SAED γ-Ga2O3 could be maintained at lower temperatures such as 773 pattern in Figure 1j further implies a polycrystalline characterand 873 K. Subsequent heating resulted in a mixed-phase of istic. Diffraction rings with the diameters of 0.21 and 0.13 nm γ- and β-phase. Finally at 1073 K, an end-centered monoclinic correspond to the lattice planes of (533) and (400) of the fcc β-Ga2O3 crystal phase with a space group of C2/m (JCPDS 43–1012)[26] was obtained. We consider that such a phase tranγ-Ga2O3,[25] respectively. ii) When the molar ratio was further decreased to 1:2.17 or lower, no precipitation was observed. sition from the cubic structure to the more stable monoclinic Based on Equation (2), the molar ratio of urea to HCl should structure is likely to be ascribed to a strain field-induced distorbe 1:2 for the neutralization of the solution. Thus, an initial tion[27] between the crystal structures. As seen from Figure S7, ratio higher than 1:2 is needed to provide an alkaline condition the cubic (311) plane and the monoclinic (111) plane, as well as for the generation of Ga2O3 as described in Equation 4–6. iii) the cubic (440) plane and the monoclinic (403) plane, have very similar diffraction angles (and therefore similar d-spacings), As this ratio was increased to 1:0.72 or higher, no spheres or possibly implying an easy phase transition pathway between flowers may be seen, instead only irregular γ-Ga2O3 particles of the structures.[28] Also, we notice that the flower-like structure 0.5–2 µm in diameter were seen (Figure S3 in the SI). Taken together, these results indicate that the shape and size of the was maintained very well (Figure S8–S10) during the heating process, revealing that the phase transition is not accompanied γ-Ga2O3 crystals could be conveniently controlled by varying by a substantial change in symmetry. the ratio, thereby changing the pH of the solution. iv) When NH3·H2O was used to substitute for urea to adjust the pH, the The second objective of this research is directed to the understanding of the relationship between the structure and γ-Ga2O3 obtained using the same conditions as Figure 1b presents a bulk agglomerate structure (1–5 µm in diameter, Figure S4), emphasizing the importance of continuous contribution to pH regulation caused by slow hydrolysis of urea in creating well-ordered patterns and aligned morphologies of γ-Ga2O3. v) After the replacement of acetone with distilled water or butanone, although a large number of nanopetals were formed (Figure S5 and S6), they cannot assemble into perfectly spherical arrays but form instead a random plane network. This implies an effect of the polarity of solvents on the assembly of nanopetals. The mechanism of this effect is still poorly understood, but may be explained by a solvent-induced difference in the reaction rates (Equation 2 and 3). We consider that, unlike Figure 4. a) XRD patterns of the Ga O samples obtained using the same conditions as 2 3 water and butanone, the presence of acetone Figure 1b but at different sintering temperatures: 773, 873, 973, and 1073 K for 2 h. b) FE-SEM can moderately reduce reaction areas of image of the β-Ga2O3 obtained at 1073 K for 2 h.
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Current /nA
a 80 40 10 5 0 100 200 300 400 500 600 Time /s
b 80 Current /nA
properties of the as-obtained Ga2O3 crystals. Figure 5 shows the photoluminescence (PL) spectra of three groups of Ga2O3: γ-Ga2O3–1a, -1b, and -1d, γ-Ga2O3–2, and β-Ga2O3. Clearly, γ-Ga2O3–2 shows two broad emission features centered at 399.8 (3.10 eV) and 505.6 nm (2.45 eV), with the left stronger than the right. A dark purple spot under ultraviolet light was observed, corresponding to the wavelength of the stronger signal. However, in the cases of the γ-Ga2O3–1 series, the relative intensity of the two signals is reversed. This result is an indication of the influence of morphologies on luminescence properties.[29] In particular, two rare phenomena make this case of the series materials outstanding from the luminescence point of view. First, PL spectra of the crystallites shows a slight red shift (indicated by the red arrows in Figure 5) of the two bands from the γ-Ga2O3–1a (400.5 and 510.0 nm), 1b (409.7 and 512.8 nm), to 1d (415.2 and 518.4 nm). This could be associated with the larger particle size and more homogeneous particle distribution,[30] as shown in SEM photographs. Second, the luminescence intensity increases in the same order (see the red arrows), which may be due to the multiplication of defects during the growth of the crystals.[31] Also, we performed annealing experiments of the γ-Ga2O3–1a, b, and d at 873 K. Our results (Figure S11) indicate that there is no significant difference between the emission peaks of the materials before and after annealing, showing that the corresponding PL features are not related to residual chemicals. In addition, we found that the as-obtained β-Ga2O3 only shows one strong broad emission band centered at 531.9 nm (2.33 eV), which is similar to those reported by other researchers.[32] Furthermore, the photographs of the light spots in the figure clearly indicate the red shift of the band from the γ-Ga2O3–1a, -1b, to -1d, and from the γ-Ga2O3–2, γ-Ga2O3–1 series to the β-Ga2O3. The full-width-at-half-maximum of the band is about 133.9 nm, which is lower than those reported previously[33] and becomes smaller (70.3–108.4 nm) in the cases of the γ-Ga2O3 crystals. This implies a narrow size distribution of the latter. Brunauer–Emmett–Teller[34] (BET) and
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Figure 5. PL spectra of γ-Ga2O3–1a, -1b, and -1d, γ-Ga2O3–2, and β-Ga2O3 excited at 325 nm.
Barrett–Joyner–Halenda[35] (BJH) analyses (Figure S12–14) indicate that the γ-Ga2O3–1d (BET surface area, 31 m2·g−1; BJH pore size, 7 nm) and β-Ga2O3 (22 m2·g−1 and 3 nm) possess much larger specific surface areas and smaller pore sizes than the γ-Ga2O3–2 (12 m2·g−1 and 29 nm). This may be a reason why they exhibit a high PL intensity. These findings demonstrate the significant effects of crystal phases, morphologies, size and particle distribution of the Ga2O3 materials on their luminescence properties, prompting us to carefully look at their UV-vis optical absorption properties. The optical bandgaps of three representative Ga2O3 nanocrystals are depicted in Figure S15. It is evident that the absorption edge energies increase from the β-Ga2O3 (4.53 eV), γ-Ga2O3–1d (4.62 eV) to the γ-Ga2O3–2 (4.75 eV). This provides the first clear evidence that the optical bandgaps of Ga2O3 nanocrystals are related to their crystal phases. The bandgap energy difference of the same phase (γ-Ga2O3–1d and γ-Ga2O3–2) should be a reflection of the difference in the shape and size of the particles. It is our opinion that the PL luminescence does not originate from a near-bandgap transition in these materials but likely from the gallium-oxygen vacancy pairs created in the growth process of the crystals.[36,37] Finally two representative samples: γ-Ga2O3–1d and γ-Ga2O3–2 were applied to check the possibility of applications in solarblind photodetectors. The indium-tin oxide (ITO) electrode coated with a layer of Ga2O3 crystals was illuminated with a 254 nm ultraviolet lamp. Figure 6 shows the time-dependent photoresponse of the two materials. Several interesting and unexpected features have been noted in the current–time profiles when the bias voltage was set to 0.5 V. The dark currents of the γ-Ga2O3–1d and γ-Ga2O3–2 are only 0.30 and 4.4 nA, respectively. Achievement of such low dark currents at a bias of 0.5 V is extremely important to enhance signal-tonoise ratios. Upon ultraviolet illumination, the light current is increased to a constant higher value (instantaneously jump to 66 nA for the γ-Ga2O3–1d; intially instantaneously jump to
40 20 10 0
100 200 300 400 500 600 Time /s
Figure 6. Current-time curves of: a) γ-Ga2O3–1d, and, b) γ-Ga2O3–2 at 0.5 V.
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48 nA and then slowly increase to 54 nA for the γ-Ga2O3–2), leading to large light current–dark current ratios (220 for the γ-Ga2O3–1d and 12 for the γ-Ga2O3–2). From a comparison of the data, we see directly that although both the samples show high optoelectronic sensitivities, the γ-Ga2O3–1d has a much higher optoelectronic quality than the γ-Ga2O3–2. Additionally, the light and dark currents of the β-Ga2O3 are 60 and 1.4 nA, respectively, with a light/dark current ratio of 43 (Figure S16). These data indicate that the photoresponse of β-phase derived upon thermal annealing of γ-phase at 1073 K is much lower than that of the γ-Ga2O3–1d, but higher than that of the spherical γ-Ga2O3–2. These differences may be due to the difference in surface structures since the BET specific surface areas decrease in the order: γ-Ga2O3–1d > β-Ga2O3 > γ-Ga2O3–2. That is to say, the thin petal-like structure of γ-Ga2O3–1d and β-Ga2O3 can provide larger contact area and more opening sites for the photons to be absorbed in comparison with the spherical structure of γ-Ga2O3–2. To the best of our knowledge, this is the first example of the fabrication of solar-blind photodetectors using γ-Ga2O3. Also, we compared our results of the solar-blind photodetection performances of the γ-Ga2O3–1d, γ-Ga2O3–2 and β-Ga2O3 with those of reported nanomaterials (Table S1 in the SI), including ZnGa2O4 nanowires,[38]β-Ga2O3 nanowires,[39,40] and β-Ga2O3 nanobelts.[41] It is apparent that the γ-Ga2O3–1d nanoflowers exhibit a higher light/dark current ratio when compared to most of these nanomaterials. Some β-Ga2O3 nanostructures, such as β-Ga2O3 nanowires,[39] presented a higher light/dark current ratio than the γ-Ga2O3–1d nanoflowers, but the photoresponses of the nanomaterials were usually generated under a high bias voltage. Especially, the photoresponse time of the γ-Ga2O3–1d is shorter than 0.1 s, which is comparable to those of previous reports.[39,40] Short response time, low dark currents, and high light currents suggest that the γ-Ga2O3 nanomaterials will be important for the development of future solar-blind photodetector technologies. In summary, the present work developed a solvothermal synthetic approach for constructing Ga2O3 nanopetals, nanoflowers and microspheres. The phase formation of the Ga2O3 materials was found to be strongly dependent on the pH of the solution, so that their morphologies were easily regulated by changing experimental conditions such as solvents and the ratio of urea to HCl. The as-prepared Ga2O3 materials exhibited distinct PL properties associated with microstructures. Also, the γ-Ga2O3 nanoflowers show excellent solar-blind detection performance, e.g., a short response time and a large light current–dark current ratio. Taken together, these factors suggest that these Ga2O3 materials are promising candidates for solarblind photodetectors. Therefore, we believed that this study provides new and useful information on the controllable synthesis of Ga-based optoelectronic materials.
Experimental Section Preparation of the Ga2O3 Materials: In the preparation experiments, all the reagents were analytical grade and were used without further purification. γ-Ga2O3–1d was synthesized by a solvothermal process: In a 50 mL flask, 0.1 g (1.4 mmol) of metallic Ga was mixed with 5.0 g (83 mmol) of urea using acetone as dispersant, and the temperature
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of the flask was raised to 303 K, stirring for 30 min. After Ga was fully melted, 10 mL concentrated hydrochloric acid (37%) and 40 mL acetone was added into the flask. Then, the mixture was transferred into a sealed stainless steel autoclave and heated to 473 K for 4 h. Finally, a white powder (γ-Ga2O3–1d, 0.067 g, 82%) was collected by filtration, exhaustively washed with ethanol and distilled water and vacuum-dried. γ-Ga2O3–1a, 1b, and 1c were obtained using the same conditions but with the heating times: 1, 2, and 3 h, respectively. The synthesis is repeated but with 12 mL concentrated hydrochloric acid, giving another white powder (γ-Ga2O3–2, 0.062 g, 76%). Material Characterization: XRD measurements were conducted on a Philips X'Pert Pro X-ray diffractometer using a monochromatized Cu Kα radiation source (40 kV, 40 mA) with a wavelength of 0.1542 nm and analyzed in the range 20° ≤ 2θ ≤ 80°. FE-SEM images were performed using a Supra 40 operated at 5 kV. TEM, HR-TEM images, and SAED patterns were recorded with a JEF 2100F field-emission transmission electron microscope using an accelerating voltage of 200 kV. UV-Vis-NIR absorption spectra were evaluated employing a Shimadzu DUV-3700 spectrophotometer. Barium sulfate (BaSO4) powder was used to adjust baseline parameters. Nitrogen (N2) adsorption/desorption isotherms were obtained using Micromeritics ASAP-2000 at 77 K. PL analysis: PL measurements were performed on a Perkin Elmer Luminescence spectrometer L550B at room emperature (excited at 325 nm). Photoresponse Properties: Samples were coated on the ITO electrodes. The electrodes were immerged in saturated Na2SO4 solution. Current– time curves were acquired by a electrochemical analyzer system, CHI760 (Chenhua, Shanghai, China) in a three-compartment cell with a working electrode, a platinum plate counter-electrode, and a saturated calomel electrode reference-electrode under a bias voltage of 0.5 V using the excitation light of a UV transilluminator (XS-T5, 6W) as the light source (254 nm)
Supporting Information Supporting Information is available from the Wiley Online Library or from the author.
Acknowledgements This project was supported by the NSFC of China (no. 21071139). The authors thank the NSFC and the French Ministry of Foreign and European Affairs (MAEE) for their financial support (PHC Xu GUANGQI 2011, 26157TJ). Received: May 7, 2014 Revised: June 23, 2014 Published online: August 5, 2014
[1] a) T. Wang, P. V. Radovanovic, Chem. Commun. 2011, 47, 7161; b) T. Wang, P. V. Radovanovic, J. Phys. Chem. C 2011, 115, 18473. [2] a) A. Vimont, J. C. Lavalley, A. Sahibed-Dine, C. O. Arean, M. R. Delgado, M. Daturi, J. Phys. Chem. B 2005, 109, 9656; b) H. Y. Playford, A. C. Hannon, E. R. Barney, R. I. Walton, Chem. Eur. J. 2013, 19, 2803. [3] S. S. Farvid, N. Dave, P. V. Radovanovic, Chem. Mater. 2010, 22, 9. [4] a) L. Fu, Y. Q. Liu, P. Hu, K. Xiao, G. Yu, D. B. Zhu, Chem. Mater. 2003, 15, 4287; b) S. C. Vanithakumari, K. K. Nanda, Adv. Mater. 2009, 21, 3581. [5] a) X. Wang, Q. Xu, M. R. Li, S. Shen, X. L. Wang, Y. C. Wang, Z. C. Feng, J. Y. Shi, H. X. Han, C. Li, Angew. Chem. Int. Ed. 2012,
© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Mater. 2014, 26, 6238–6243
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[7]
[8] [9] [10] [11] [12]
[13]
[14]
[15]
[16] [17]
[18]
[19] [20] [21]
Adv. Mater. 2014, 26, 6238–6243
[22] X. W. Li, S. L. Xiong, J. F. Li, J. Bai, Y. T. Qian, J. Mater. Chem. 2012, 22, 14276. [23] N. M. Ghazali, M. R. Mahmood, K. Yasui, A. M. Hashim, Nanoscale Res. Lett. 2014, 9, 120. [24] a) C. Z. Wu, Y. Xie, L. Y. Lei, S. Q. Hu, C. Z. OuYang, Adv. Mater. 2006, 18, 1727; b) Z. C. Wu, M. Zhang, K. Yu, S. D. Zhang, Y. Xie, Chem. Eur. J. 2008, 14, 5346. [25] C. O. Arean, A. L. Bellan, M. P. Mentruit, M. R. Delgado, G. T. Palomino, Micropor. Mesopor. Mater. 2000, 40, 35. [26] S. F. Zhou, G. F. Feng, B. T. Wu, N. Jiang, S. Q. Xu, J. R. Qiu, J. Phys. Chem. C 2007, 111, 7335. [27] A. Molle, C. Wiemer, M. N. K. Bhuiyan, G. Tallarida, M. Fanciulli, G. Pavia, Appl. Phys. Lett. 2007, 90, 193511. [28] T. Yao, L. Liu, C. Xiao, X. D. Zhang, Q. H. Liu, S. Q. Wei, Y. Xie, Angew. Chem. Int. Ed. 2013, 52, 7554. [29] D. Hreniak, W. Strek, P. Gluchowski, M. Bettinelli, A. Speghini, Appl. Phys. B 2008, 91, 89. [30] C. Carrillo-Carrion, S. Cardenas, B. M. Simonet, M. Valcarcel, Chem. Commun. 2009, 35, 5214. [31] B. J. Jin, S. H. Bae, S. Y. Lee, S. Im, Mater. Sci. Eng. B 2000, 71, 301. [32] a) H. Q. Yang, R. Y. Shi, J. Yu, R. N. Liu, R. G. Zhang, H. Zhao, L. H. Zhang, H. R. Zheng, J. Phys. Chem. C 2009, 113, 21548; b) Y. D. Hou, L. Wu, X. C. Wang, Z. X. Ding, Z. H. Li, X. Z. Fu, J. Catal. 2007, 250, 12; c) Y. H. Zong, X. G. Meng, K. Fujita, K. Tanaka, Opt. Mater. Express. 2014, 4, 518. [33] R. Jangir, T. Ganguli, S. Porwal, P. Tiwari, S. K. Rai, I. Bhaumik, L. M. Kukreja, P. K. Gupta, S. K. Deb, J. Appl. Phys. 2013, 114, 074309. [34] S. Brunauer, P. H. Emmett, E. Teller, J. Am. Chem. Soc. 1938, 60, 309. [35] E. P. Barrett, L. G. Joyner, P. P. Halenda, J. Am. Chem. Soc. 1951,73, 373. [36] J. Geng, J. J. Zhu, H. Y. Chen, Cryst. Growth. Des. 2006, 6, 321. [37] G. G. Li, C. Peng, C. X. Li, P. A. P. Yang, Z. Y. Hou, Y. Fan, Z. Y. Cheng, J. Lin, Inorg. Chem. 2010, 49, 1449. [38] P. Feng, J. Y. Zhang, Q. Wan, T. H. Wang, J. Appl. Phys. 2007, 102, 074309. [39] P. Feng, J. Y. Zhang, Q. H. Li, T. H. Wang, Appl. Phys. Lett. 2006, 88, 153107. [40] P. Feng, X. Y. Xue, Y. G. Liu, Q. Wan, T. H. Wang, Appl. Phys. Lett. 2006, 89, 112114. [41] L. Li, E. Auer, M. Liao, X. Fang, T. Zhai, U. K. Gautam, A. Lugstein, Y. Koide, Y. Bando, D. Golberg, Nanoscale 2011, 3, 1120.
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[6]
51, 13089; b) Z. R. Tang, F. Li, Y. H. Zhang, X. Z. Fu, Y. J. Xu, J. Phys. Chem. C 2011, 115, 7880. a) H. Tsuneoka, K. Teramura, T. Shishido, T. Tanaka, J. Phys. Chem. C 2010, 114, 8892; b) M. R. Delgado, C. Morterra, G. Cerrato, G. Magnacca, C. O. Arean, Langmuir 2002, 18, 10255. a) Li. E. Auer, M. Y. Liao, X. S. Fang, T. Y. Zhai, U. K. Gautam, A. Lugstein, Y. Koide, Y. Bando, D. Golberg, Nanoscale 2011, 3, 1120; b) P. Feng, J. Y. Zhang, Q. H. Li, T. H. Wang, Appl. Phys. Lett. 2006, 88, 153107. D. Kisailus, J. H. Choi, J. C. Weaver, W. J. Yang, D. E. Morse, Adv. Mater. 2005, 17, 314. C. C. Huang, C. S. Yeh, New. J. Chem. 2010, 34, 103. M. R. Delgado, C. O. Arean, Z. Anorg. Allg. Chem. 2005, 631, 2115. T. Wang, S. S. Farvid, M. Abulikemu, P. V. Radovanovic, J. Am. Chem. Soc. 2010, 132, 9250. a) L. X. Song, Y. Teng, J. Chen, J. Phys. Chem. C 2012, 116, 22859; b) J. Chen, L. X. Song, J. Yang, J. Xia, Z. C. Shao, J. Mater. Chem. 2012, 22, 6251. a) V. N. Sigaev, N. V. Golubev, E. S. Ignat'eva, B. Champagnon, D. Vouagner, E. Nardou, R. Lorenzi, A. Paleari, Nanoscale 2013, 5, 299; b) S. F. Zhou, N. Jiang, K. Miura, S. Tanabe, M. Shimizu, M. Sakakura, Y. Shimotsuma, M. Nishi, J. R. Qiu, K. Hirao, J. Am. Chem. Soc. 2010, 132, 17945. a) M. Maillard, S. Giorgio, M. P. Pileni, Adv. Mater. 2002, 14, 1084; b) X. Y. Yu, Q. Q. Meng, T. Luo, Y. Jia, B. Sun, Q. X. Li, J. H. Liu, X. J. Huang, Sci. Rep. UK 2013, 3, 1. a) Q. Hua, W. X. Huang, J. Mater. Chem. 2008, 18, 4286; b) L. C. Liu, T. Wei, X. Guan, X. H. Zi, H. He, H. X. Dai, J. Phys. Chem. C 2009, 113, 8595. V. Germain, J. Li, D. Ingert, Z. L. Wang, M. P. Pileni, J. Phys. Chem. B 2003, 107, 8717. a) L. P. Jiang, S. Xu, J. M. Zhu, J. R. Zhang, J. J. Zhu, H. Y. Chen, Inorg. Chem. 2004, 43, 5877; b) C. Salzemann, J. Urban, I. Lisiecki, M. P. Pileni, Adv. Funct. Mater. 2005, 15, 1277. a) A. I. Kirkland, D. A. Jefferson, D. G. Duff, P. P. Edwards, I. Gameson, B. F. G. Johnson, D. J. Smith, Proc. R. Soc. London A 1993, 440, 589; b) C. Lofton, W. Sigmund, Adv. Funct. Mater. 2005, 15, 1197. R. Jin, Y. Cao, C. A. Mirkin, K. L. Kelly, G. C. Schatz, J. G. Zheng, Science 2001, 294, 1901. B. Rodriguez-Gonzalez, I. Pastoriza-Santos, L. M. Liz-Marzan, J. Phys. Chem. B 2006, 110, 11796. L. X. Song, J. Chen, L. H. Zhu, J. Xia, J. Yang, Inorg. Chem. 2011, 50, 7988.
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Self-Assembled Metastable γ-Ga2O3 Nanoflowers with Hexagonal Nanopetals for Solar-Blind Photodetection Yue Teng, Le Xin Song,* Anne Ponchel,* Zheng Kun Yang, and Juan Xia β-Ga2O3 is the most stable phase at room temperature among the five existing phases (α, β, γ, δ, and ⑀)[1,2] of Ga2O3. It is one of the widest bandgap semiconductor oxides (4.9 eV) and shows unique physical properties that make it interesting for high conductivity,[3] visible photoluminescence (PL),[4] photocatalytic activity,[5] adsorption capacity,[6] and optoelectronics.[7] The other metastable phases, like γ-Ga2O3, have been much less thoroughly investigated, partially due to the associated experimental difficulties in their synthesis and isolation. It was reported that γ-Ga2O3 materials can be obtained by sintering Ga(NO3)3[8] or GaOOH[9] at a certain temperature. However at a higher temperature, the metastable γ-Ga2O3 will be transformed into β-Ga2O3.[10] Thus, special attention is needed concerning the construction of pure γ-Ga2O3 materials, especially nanocrystals. Recently, Radovanovic and his colleagues demonstrated that γ-Ga2O3 nanocrystals show a size-tunable photoluminescence.[11] This allows us to think that the nanocrystals of γ-Ga2O3 with high ratios of surface to volume may lead to high performance in optoelectronics such as solar-blind photodetectors. Starting from this background, in this work we wish to report the synthesis of metastable γ-Ga2O3 nanoflowers with hexagonal nanopetals by the oxidation of metallic Ga in solutions. Our results indicate that a variation in the shape of the γ-Ga2O3 causes to a corresponding variation of photoresponse. This effect is of importance because it provides a possibility for improving the functionality or performance of photodetectors and related optoelectronic devices. A simple solvothermal method was used to realize the morphology-controlled synthesis of γ-Ga2O3 nanostructures. In a 50 mL flask, 0.1 g (1.4 mmol) of metallic Ga was mixed with 5.0 g (83 mmol) of urea using acetone as dispersant[12] at room temperature. Thereafter, the temperature in the flask was raised to 303 K and the system was stirred for 0.5 h. After Ga was fully
Y. Teng, Dr. L. X. Song, Z. K. Yang, J. Xia CAS Key Laboratory of Materials for Energy Conversion Department of Materials Science and Engineering University of Science and Technology of China Hefei 230026, PR China E-mail: [email protected] Y. Teng, Dr. L. X. Song, Z. K. Yang, J. Xia Department of Chemistry University of Science and Technology of China Jin Zhai Road 96, Hefei 230026, PR China Prof. A. Ponchel Université d’Artois Unité de Catalyse et Chimie du Solide UCCS UMR CNRS 8181, Faculté des Sciences Jean Perrin Rue Jean Souvraz, F-62300 Lens, France E-mail: [email protected]
DOI: 10.1002/adma.201402047
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melted, 10 mL concentrated hydrochloric acid (37%) and 40 mL acetone were added into the flask (i.e., the molar ratio of urea to HCl was 1:1.44). Then, the solution was completely transferred into a sealed stainless steel autoclave and heated to 473 K for 4 h. Finally, a white powder (γ-Ga2O3–1d, 0.067 g, 82%) was collected by filtration, washed several times with ethanol and distilled water, and dried in vacuum. The synthesis was repeated but with 12 mL concentrated hydrochloric acid, giving another white powder (γ-Ga2O3–2, 0.062 g, 76%). The X-ray diffraction (XRD, Figure 1a) patterns for the as-obtained samples show a face-centered cubic (fcc) structure of γ-Ga2O3 phase with a space group of Fd3m (JCPDS 20–0426).[13] The scanning electron microscopy (SEM) images of the γ-Ga2O3–1d are shown in Figure 1b and c. Clearly, the morphology of the particles (diameter, 1∼2 µm) seems to be a flower-like structure formed by the self-assembly of tens and hundreds of nanopetals (side length, 100–200 nm, height, 10–20 nm). The higher-magnification SEM images (Figure S1 in the Supporting Information, SI) show that the nanopetals exhibit a hexagonal feature. The transmission electron microscopy (TEM, Figure 1d) image not only verifies the existence of the self-assembly, but also suggests the presence of a hollow interior through a contrast between the edges and the center of the particle. Furthermore, the high-resolution TEM (HR-TEM, Figure 1e) image of a hexagonal petal tip indicates that the spacing between two adjacent lattice fringes is 0.29 nm, corresponding to the (220) plane of the fcc lattice. The selected area electron diffraction (SAED, Figure 1f) pattern shows six bright and sharp spots on a darker background with a six-fold symmetry, corresponding to the {220} reflections of the fcc γ-Ga2O3 crystal orientated in the [111] direction. This is an indication that the flat surface of the hexagonal petals is parallel to the (111) habit plane[14] (Figure S2 in the SI). In particular, there are the 1/3{422} reflections in the [111] electron diffraction pattern, which should be forbidden for a perfect fcc structure.[15] This phenomenon has been observed in plate-like crystals of metals.[16,17] To the best of our knowledge, it is the first example for Ga2O3 nanoparticles. The reason for this may be either: i) the existence of stacking faults lying parallel to the (111) surfaces and extending across the entire nanopetals,[16] ii) the presence of twin planes,[18] or, iii) a hexagonal-like monolayer on the nanopetal faces.[19] It should be noted that the HR-TEM image does not show a corresponding 3 × {422} lattice spacing (0.504 nm) of the fcc Ga2O3 crystal, which is different from those observed in metallic crystals.[16,20] The loss of superlattice spacings may be related to structural features such as large fringes and wider boundaries of the metal oxide nanocrystals compared with metal crystals. From the structural point of view, it is important to note that the oxidation process of Ga is accompanied by a structural
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Therefore, we speculate that the presence and hydrolysis of transformation from spherical (metallic Ga)[21] to flower-like urea is a key factor responsible for the generation, nucleation shape (the Ga2O3). An example of a time-dependent transforand growth of γ-Ga2O3. mation is illustrated in Figure 2. The result of growth experiments at different heating times of 1 h (γ-Ga2O3–1a), 2 h Several control experiments confirmed this hypothesis and (γ-Ga2O3–1b), and 3 h (γ-Ga2O3–1c) with other parameters the allowed us to evaluate processing dependent structural changes. same as in Figure 1b indicates that there is a clear trend from the nanoparticles (diameter, 80–100 nm) to the gradual emergence of the hexagonal nanopetals. The XRD patterns in Figure 2 confirm that all the samples are pure and iso-structural with the γ-Ga2O3–1d. The oxidation process of metallic Ga involving several steps is described by Equation 1–6. A schematic illustration explaining this oxidation process is shown in Figure 3. First of all, metallic Ga was oxidized into Ga3+ ions by the H+ ions of hydrochloric acid (Equation 1, Step I). Simultaneously, urea was reacted with the H+ ions and hydrolyzed to produce OH− ions, which is responsible for the high pH of the suspensions[22] (Equation 2 and 3). Then, the Ga3+ ions were hydrolyzed to form Ga2O3 in an alkaline solution through an equilibrium intermediate state[23] between Ga(OH)3 and GaOOH (Equation 4–6, Step II). Subsequently, the γ-Ga2O3 nanoparticles gradually grew to nanopetals (Figure 2 and Step III) and assembled into nanoflowers (Figure 1b, and Steps IV and V). Meanwhile, we considered that the release of the formed gases (Equation 1–3) may act as a gas-bubble template[24] for the creation of the hollow interior structural feature (Figure 1d). It should be reasonable to believe that Equation 3 and 4 could happen together and promote Figure 2. FE-SEM images and XRD patterns of the γ-Ga2O3 obtained using the same conditions each other in the solvothermal conditions. as Figure 1b but at different heating times of 1, 2, and 3 h.
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urea/water because urea is insoluble in acetone, leading to a moderate decrease of the reaction rates. 2Ga + 6H+ → 2Ga 3+ + 3H2 ↑
(1)
CO(NH2 )2 + 2H+ + H2O → 2NH+4 + CO2 ↑
(2)
CO(NH2 )2 + 3H2 O → 2NH+4 + 2OH− + CO2 ↑
(3)
Ga 3+ + 3OH− → Ga(OH)3
(4)
Ga(OH)3 → GaOOH + H2O
(5)
Figure 3. Schematic illustration describing the formation of γ-Ga2O3–1d.
2GaOOH → Ga 2O3 + H2O (6) i) When the initial molar ratio of urea to HCl was decreased to 1:1.69, we obtained spherical γ-Ga2O3–2 with an average Moreover, our investigations have demonstrated that a temperature dependent phase transition from γ-phase to β-phase diameter of 1–1.5 µm as seen in Figure 1g and h. It should be stressed that the γ-Ga2O3–2 shows poor crystallization, with less occurs in the neighborhood of 973 K by sintering the γ-Ga2O3– defined diffraction lines. Its HR-TEM image (Figure 1i) depicts 1d in air. As seen in Figure 4, the phase structure of the similar lattice fringes as those of the γ-Ga2O3–1d, and its SAED γ-Ga2O3 could be maintained at lower temperatures such as 773 pattern in Figure 1j further implies a polycrystalline characterand 873 K. Subsequent heating resulted in a mixed-phase of istic. Diffraction rings with the diameters of 0.21 and 0.13 nm γ- and β-phase. Finally at 1073 K, an end-centered monoclinic correspond to the lattice planes of (533) and (400) of the fcc β-Ga2O3 crystal phase with a space group of C2/m (JCPDS 43–1012)[26] was obtained. We consider that such a phase tranγ-Ga2O3,[25] respectively. ii) When the molar ratio was further decreased to 1:2.17 or lower, no precipitation was observed. sition from the cubic structure to the more stable monoclinic Based on Equation (2), the molar ratio of urea to HCl should structure is likely to be ascribed to a strain field-induced distorbe 1:2 for the neutralization of the solution. Thus, an initial tion[27] between the crystal structures. As seen from Figure S7, ratio higher than 1:2 is needed to provide an alkaline condition the cubic (311) plane and the monoclinic (111) plane, as well as for the generation of Ga2O3 as described in Equation 4–6. iii) the cubic (440) plane and the monoclinic (403) plane, have very similar diffraction angles (and therefore similar d-spacings), As this ratio was increased to 1:0.72 or higher, no spheres or possibly implying an easy phase transition pathway between flowers may be seen, instead only irregular γ-Ga2O3 particles of the structures.[28] Also, we notice that the flower-like structure 0.5–2 µm in diameter were seen (Figure S3 in the SI). Taken together, these results indicate that the shape and size of the was maintained very well (Figure S8–S10) during the heating process, revealing that the phase transition is not accompanied γ-Ga2O3 crystals could be conveniently controlled by varying by a substantial change in symmetry. the ratio, thereby changing the pH of the solution. iv) When NH3·H2O was used to substitute for urea to adjust the pH, the The second objective of this research is directed to the understanding of the relationship between the structure and γ-Ga2O3 obtained using the same conditions as Figure 1b presents a bulk agglomerate structure (1–5 µm in diameter, Figure S4), emphasizing the importance of continuous contribution to pH regulation caused by slow hydrolysis of urea in creating well-ordered patterns and aligned morphologies of γ-Ga2O3. v) After the replacement of acetone with distilled water or butanone, although a large number of nanopetals were formed (Figure S5 and S6), they cannot assemble into perfectly spherical arrays but form instead a random plane network. This implies an effect of the polarity of solvents on the assembly of nanopetals. The mechanism of this effect is still poorly understood, but may be explained by a solvent-induced difference in the reaction rates (Equation 2 and 3). We consider that, unlike Figure 4. a) XRD patterns of the Ga O samples obtained using the same conditions as 2 3 water and butanone, the presence of acetone Figure 1b but at different sintering temperatures: 773, 873, 973, and 1073 K for 2 h. b) FE-SEM can moderately reduce reaction areas of image of the β-Ga2O3 obtained at 1073 K for 2 h.
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Current /nA
a 80 40 10 5 0 100 200 300 400 500 600 Time /s
b 80 Current /nA
properties of the as-obtained Ga2O3 crystals. Figure 5 shows the photoluminescence (PL) spectra of three groups of Ga2O3: γ-Ga2O3–1a, -1b, and -1d, γ-Ga2O3–2, and β-Ga2O3. Clearly, γ-Ga2O3–2 shows two broad emission features centered at 399.8 (3.10 eV) and 505.6 nm (2.45 eV), with the left stronger than the right. A dark purple spot under ultraviolet light was observed, corresponding to the wavelength of the stronger signal. However, in the cases of the γ-Ga2O3–1 series, the relative intensity of the two signals is reversed. This result is an indication of the influence of morphologies on luminescence properties.[29] In particular, two rare phenomena make this case of the series materials outstanding from the luminescence point of view. First, PL spectra of the crystallites shows a slight red shift (indicated by the red arrows in Figure 5) of the two bands from the γ-Ga2O3–1a (400.5 and 510.0 nm), 1b (409.7 and 512.8 nm), to 1d (415.2 and 518.4 nm). This could be associated with the larger particle size and more homogeneous particle distribution,[30] as shown in SEM photographs. Second, the luminescence intensity increases in the same order (see the red arrows), which may be due to the multiplication of defects during the growth of the crystals.[31] Also, we performed annealing experiments of the γ-Ga2O3–1a, b, and d at 873 K. Our results (Figure S11) indicate that there is no significant difference between the emission peaks of the materials before and after annealing, showing that the corresponding PL features are not related to residual chemicals. In addition, we found that the as-obtained β-Ga2O3 only shows one strong broad emission band centered at 531.9 nm (2.33 eV), which is similar to those reported by other researchers.[32] Furthermore, the photographs of the light spots in the figure clearly indicate the red shift of the band from the γ-Ga2O3–1a, -1b, to -1d, and from the γ-Ga2O3–2, γ-Ga2O3–1 series to the β-Ga2O3. The full-width-at-half-maximum of the band is about 133.9 nm, which is lower than those reported previously[33] and becomes smaller (70.3–108.4 nm) in the cases of the γ-Ga2O3 crystals. This implies a narrow size distribution of the latter. Brunauer–Emmett–Teller[34] (BET) and
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Figure 5. PL spectra of γ-Ga2O3–1a, -1b, and -1d, γ-Ga2O3–2, and β-Ga2O3 excited at 325 nm.
Barrett–Joyner–Halenda[35] (BJH) analyses (Figure S12–14) indicate that the γ-Ga2O3–1d (BET surface area, 31 m2·g−1; BJH pore size, 7 nm) and β-Ga2O3 (22 m2·g−1 and 3 nm) possess much larger specific surface areas and smaller pore sizes than the γ-Ga2O3–2 (12 m2·g−1 and 29 nm). This may be a reason why they exhibit a high PL intensity. These findings demonstrate the significant effects of crystal phases, morphologies, size and particle distribution of the Ga2O3 materials on their luminescence properties, prompting us to carefully look at their UV-vis optical absorption properties. The optical bandgaps of three representative Ga2O3 nanocrystals are depicted in Figure S15. It is evident that the absorption edge energies increase from the β-Ga2O3 (4.53 eV), γ-Ga2O3–1d (4.62 eV) to the γ-Ga2O3–2 (4.75 eV). This provides the first clear evidence that the optical bandgaps of Ga2O3 nanocrystals are related to their crystal phases. The bandgap energy difference of the same phase (γ-Ga2O3–1d and γ-Ga2O3–2) should be a reflection of the difference in the shape and size of the particles. It is our opinion that the PL luminescence does not originate from a near-bandgap transition in these materials but likely from the gallium-oxygen vacancy pairs created in the growth process of the crystals.[36,37] Finally two representative samples: γ-Ga2O3–1d and γ-Ga2O3–2 were applied to check the possibility of applications in solarblind photodetectors. The indium-tin oxide (ITO) electrode coated with a layer of Ga2O3 crystals was illuminated with a 254 nm ultraviolet lamp. Figure 6 shows the time-dependent photoresponse of the two materials. Several interesting and unexpected features have been noted in the current–time profiles when the bias voltage was set to 0.5 V. The dark currents of the γ-Ga2O3–1d and γ-Ga2O3–2 are only 0.30 and 4.4 nA, respectively. Achievement of such low dark currents at a bias of 0.5 V is extremely important to enhance signal-tonoise ratios. Upon ultraviolet illumination, the light current is increased to a constant higher value (instantaneously jump to 66 nA for the γ-Ga2O3–1d; intially instantaneously jump to
40 20 10 0
100 200 300 400 500 600 Time /s
Figure 6. Current-time curves of: a) γ-Ga2O3–1d, and, b) γ-Ga2O3–2 at 0.5 V.
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48 nA and then slowly increase to 54 nA for the γ-Ga2O3–2), leading to large light current–dark current ratios (220 for the γ-Ga2O3–1d and 12 for the γ-Ga2O3–2). From a comparison of the data, we see directly that although both the samples show high optoelectronic sensitivities, the γ-Ga2O3–1d has a much higher optoelectronic quality than the γ-Ga2O3–2. Additionally, the light and dark currents of the β-Ga2O3 are 60 and 1.4 nA, respectively, with a light/dark current ratio of 43 (Figure S16). These data indicate that the photoresponse of β-phase derived upon thermal annealing of γ-phase at 1073 K is much lower than that of the γ-Ga2O3–1d, but higher than that of the spherical γ-Ga2O3–2. These differences may be due to the difference in surface structures since the BET specific surface areas decrease in the order: γ-Ga2O3–1d > β-Ga2O3 > γ-Ga2O3–2. That is to say, the thin petal-like structure of γ-Ga2O3–1d and β-Ga2O3 can provide larger contact area and more opening sites for the photons to be absorbed in comparison with the spherical structure of γ-Ga2O3–2. To the best of our knowledge, this is the first example of the fabrication of solar-blind photodetectors using γ-Ga2O3. Also, we compared our results of the solar-blind photodetection performances of the γ-Ga2O3–1d, γ-Ga2O3–2 and β-Ga2O3 with those of reported nanomaterials (Table S1 in the SI), including ZnGa2O4 nanowires,[38]β-Ga2O3 nanowires,[39,40] and β-Ga2O3 nanobelts.[41] It is apparent that the γ-Ga2O3–1d nanoflowers exhibit a higher light/dark current ratio when compared to most of these nanomaterials. Some β-Ga2O3 nanostructures, such as β-Ga2O3 nanowires,[39] presented a higher light/dark current ratio than the γ-Ga2O3–1d nanoflowers, but the photoresponses of the nanomaterials were usually generated under a high bias voltage. Especially, the photoresponse time of the γ-Ga2O3–1d is shorter than 0.1 s, which is comparable to those of previous reports.[39,40] Short response time, low dark currents, and high light currents suggest that the γ-Ga2O3 nanomaterials will be important for the development of future solar-blind photodetector technologies. In summary, the present work developed a solvothermal synthetic approach for constructing Ga2O3 nanopetals, nanoflowers and microspheres. The phase formation of the Ga2O3 materials was found to be strongly dependent on the pH of the solution, so that their morphologies were easily regulated by changing experimental conditions such as solvents and the ratio of urea to HCl. The as-prepared Ga2O3 materials exhibited distinct PL properties associated with microstructures. Also, the γ-Ga2O3 nanoflowers show excellent solar-blind detection performance, e.g., a short response time and a large light current–dark current ratio. Taken together, these factors suggest that these Ga2O3 materials are promising candidates for solarblind photodetectors. Therefore, we believed that this study provides new and useful information on the controllable synthesis of Ga-based optoelectronic materials.
Experimental Section Preparation of the Ga2O3 Materials: In the preparation experiments, all the reagents were analytical grade and were used without further purification. γ-Ga2O3–1d was synthesized by a solvothermal process: In a 50 mL flask, 0.1 g (1.4 mmol) of metallic Ga was mixed with 5.0 g (83 mmol) of urea using acetone as dispersant, and the temperature
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of the flask was raised to 303 K, stirring for 30 min. After Ga was fully melted, 10 mL concentrated hydrochloric acid (37%) and 40 mL acetone was added into the flask. Then, the mixture was transferred into a sealed stainless steel autoclave and heated to 473 K for 4 h. Finally, a white powder (γ-Ga2O3–1d, 0.067 g, 82%) was collected by filtration, exhaustively washed with ethanol and distilled water and vacuum-dried. γ-Ga2O3–1a, 1b, and 1c were obtained using the same conditions but with the heating times: 1, 2, and 3 h, respectively. The synthesis is repeated but with 12 mL concentrated hydrochloric acid, giving another white powder (γ-Ga2O3–2, 0.062 g, 76%). Material Characterization: XRD measurements were conducted on a Philips X'Pert Pro X-ray diffractometer using a monochromatized Cu Kα radiation source (40 kV, 40 mA) with a wavelength of 0.1542 nm and analyzed in the range 20° ≤ 2θ ≤ 80°. FE-SEM images were performed using a Supra 40 operated at 5 kV. TEM, HR-TEM images, and SAED patterns were recorded with a JEF 2100F field-emission transmission electron microscope using an accelerating voltage of 200 kV. UV-Vis-NIR absorption spectra were evaluated employing a Shimadzu DUV-3700 spectrophotometer. Barium sulfate (BaSO4) powder was used to adjust baseline parameters. Nitrogen (N2) adsorption/desorption isotherms were obtained using Micromeritics ASAP-2000 at 77 K. PL analysis: PL measurements were performed on a Perkin Elmer Luminescence spectrometer L550B at room emperature (excited at 325 nm). Photoresponse Properties: Samples were coated on the ITO electrodes. The electrodes were immerged in saturated Na2SO4 solution. Current– time curves were acquired by a electrochemical analyzer system, CHI760 (Chenhua, Shanghai, China) in a three-compartment cell with a working electrode, a platinum plate counter-electrode, and a saturated calomel electrode reference-electrode under a bias voltage of 0.5 V using the excitation light of a UV transilluminator (XS-T5, 6W) as the light source (254 nm)
Supporting Information Supporting Information is available from the Wiley Online Library or from the author.
Acknowledgements This project was supported by the NSFC of China (no. 21071139). The authors thank the NSFC and the French Ministry of Foreign and European Affairs (MAEE) for their financial support (PHC Xu GUANGQI 2011, 26157TJ). Received: May 7, 2014 Revised: June 23, 2014 Published online: August 5, 2014
[1] a) T. Wang, P. V. Radovanovic, Chem. Commun. 2011, 47, 7161; b) T. Wang, P. V. Radovanovic, J. Phys. Chem. C 2011, 115, 18473. [2] a) A. Vimont, J. C. Lavalley, A. Sahibed-Dine, C. O. Arean, M. R. Delgado, M. Daturi, J. Phys. Chem. B 2005, 109, 9656; b) H. Y. Playford, A. C. Hannon, E. R. Barney, R. I. Walton, Chem. Eur. J. 2013, 19, 2803. [3] S. S. Farvid, N. Dave, P. V. Radovanovic, Chem. Mater. 2010, 22, 9. [4] a) L. Fu, Y. Q. Liu, P. Hu, K. Xiao, G. Yu, D. B. Zhu, Chem. Mater. 2003, 15, 4287; b) S. C. Vanithakumari, K. K. Nanda, Adv. Mater. 2009, 21, 3581. [5] a) X. Wang, Q. Xu, M. R. Li, S. Shen, X. L. Wang, Y. C. Wang, Z. C. Feng, J. Y. Shi, H. X. Han, C. Li, Angew. Chem. Int. Ed. 2012,
© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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[7]
[8] [9] [10] [11] [12]
[13]
[14]
[15]
[16] [17]
[18]
[19] [20] [21]
Adv. Mater. 2014, 26, 6238–6243
[22] X. W. Li, S. L. Xiong, J. F. Li, J. Bai, Y. T. Qian, J. Mater. Chem. 2012, 22, 14276. [23] N. M. Ghazali, M. R. Mahmood, K. Yasui, A. M. Hashim, Nanoscale Res. Lett. 2014, 9, 120. [24] a) C. Z. Wu, Y. Xie, L. Y. Lei, S. Q. Hu, C. Z. OuYang, Adv. Mater. 2006, 18, 1727; b) Z. C. Wu, M. Zhang, K. Yu, S. D. Zhang, Y. Xie, Chem. Eur. J. 2008, 14, 5346. [25] C. O. Arean, A. L. Bellan, M. P. Mentruit, M. R. Delgado, G. T. Palomino, Micropor. Mesopor. Mater. 2000, 40, 35. [26] S. F. Zhou, G. F. Feng, B. T. Wu, N. Jiang, S. Q. Xu, J. R. Qiu, J. Phys. Chem. C 2007, 111, 7335. [27] A. Molle, C. Wiemer, M. N. K. Bhuiyan, G. Tallarida, M. Fanciulli, G. Pavia, Appl. Phys. Lett. 2007, 90, 193511. [28] T. Yao, L. Liu, C. Xiao, X. D. Zhang, Q. H. Liu, S. Q. Wei, Y. Xie, Angew. Chem. Int. Ed. 2013, 52, 7554. [29] D. Hreniak, W. Strek, P. Gluchowski, M. Bettinelli, A. Speghini, Appl. Phys. B 2008, 91, 89. [30] C. Carrillo-Carrion, S. Cardenas, B. M. Simonet, M. Valcarcel, Chem. Commun. 2009, 35, 5214. [31] B. J. Jin, S. H. Bae, S. Y. Lee, S. Im, Mater. Sci. Eng. B 2000, 71, 301. [32] a) H. Q. Yang, R. Y. Shi, J. Yu, R. N. Liu, R. G. Zhang, H. Zhao, L. H. Zhang, H. R. Zheng, J. Phys. Chem. C 2009, 113, 21548; b) Y. D. Hou, L. Wu, X. C. Wang, Z. X. Ding, Z. H. Li, X. Z. Fu, J. Catal. 2007, 250, 12; c) Y. H. Zong, X. G. Meng, K. Fujita, K. Tanaka, Opt. Mater. Express. 2014, 4, 518. [33] R. Jangir, T. Ganguli, S. Porwal, P. Tiwari, S. K. Rai, I. Bhaumik, L. M. Kukreja, P. K. Gupta, S. K. Deb, J. Appl. Phys. 2013, 114, 074309. [34] S. Brunauer, P. H. Emmett, E. Teller, J. Am. Chem. Soc. 1938, 60, 309. [35] E. P. Barrett, L. G. Joyner, P. P. Halenda, J. Am. Chem. Soc. 1951,73, 373. [36] J. Geng, J. J. Zhu, H. Y. Chen, Cryst. Growth. Des. 2006, 6, 321. [37] G. G. Li, C. Peng, C. X. Li, P. A. P. Yang, Z. Y. Hou, Y. Fan, Z. Y. Cheng, J. Lin, Inorg. Chem. 2010, 49, 1449. [38] P. Feng, J. Y. Zhang, Q. Wan, T. H. Wang, J. Appl. Phys. 2007, 102, 074309. [39] P. Feng, J. Y. Zhang, Q. H. Li, T. H. Wang, Appl. Phys. Lett. 2006, 88, 153107. [40] P. Feng, X. Y. Xue, Y. G. Liu, Q. Wan, T. H. Wang, Appl. Phys. Lett. 2006, 89, 112114. [41] L. Li, E. Auer, M. Liao, X. Fang, T. Zhai, U. K. Gautam, A. Lugstein, Y. Koide, Y. Bando, D. Golberg, Nanoscale 2011, 3, 1120.
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[6]
51, 13089; b) Z. R. Tang, F. Li, Y. H. Zhang, X. Z. Fu, Y. J. Xu, J. Phys. Chem. C 2011, 115, 7880. a) H. Tsuneoka, K. Teramura, T. Shishido, T. Tanaka, J. Phys. Chem. C 2010, 114, 8892; b) M. R. Delgado, C. Morterra, G. Cerrato, G. Magnacca, C. O. Arean, Langmuir 2002, 18, 10255. a) Li. E. Auer, M. Y. Liao, X. S. Fang, T. Y. Zhai, U. K. Gautam, A. Lugstein, Y. Koide, Y. Bando, D. Golberg, Nanoscale 2011, 3, 1120; b) P. Feng, J. Y. Zhang, Q. H. Li, T. H. Wang, Appl. Phys. Lett. 2006, 88, 153107. D. Kisailus, J. H. Choi, J. C. Weaver, W. J. Yang, D. E. Morse, Adv. Mater. 2005, 17, 314. C. C. Huang, C. S. Yeh, New. J. Chem. 2010, 34, 103. M. R. Delgado, C. O. Arean, Z. Anorg. Allg. Chem. 2005, 631, 2115. T. Wang, S. S. Farvid, M. Abulikemu, P. V. Radovanovic, J. Am. Chem. Soc. 2010, 132, 9250. a) L. X. Song, Y. Teng, J. Chen, J. Phys. Chem. C 2012, 116, 22859; b) J. Chen, L. X. Song, J. Yang, J. Xia, Z. C. Shao, J. Mater. Chem. 2012, 22, 6251. a) V. N. Sigaev, N. V. Golubev, E. S. Ignat'eva, B. Champagnon, D. Vouagner, E. Nardou, R. Lorenzi, A. Paleari, Nanoscale 2013, 5, 299; b) S. F. Zhou, N. Jiang, K. Miura, S. Tanabe, M. Shimizu, M. Sakakura, Y. Shimotsuma, M. Nishi, J. R. Qiu, K. Hirao, J. Am. Chem. Soc. 2010, 132, 17945. a) M. Maillard, S. Giorgio, M. P. Pileni, Adv. Mater. 2002, 14, 1084; b) X. Y. Yu, Q. Q. Meng, T. Luo, Y. Jia, B. Sun, Q. X. Li, J. H. Liu, X. J. Huang, Sci. Rep. UK 2013, 3, 1. a) Q. Hua, W. X. Huang, J. Mater. Chem. 2008, 18, 4286; b) L. C. Liu, T. Wei, X. Guan, X. H. Zi, H. He, H. X. Dai, J. Phys. Chem. C 2009, 113, 8595. V. Germain, J. Li, D. Ingert, Z. L. Wang, M. P. Pileni, J. Phys. Chem. B 2003, 107, 8717. a) L. P. Jiang, S. Xu, J. M. Zhu, J. R. Zhang, J. J. Zhu, H. Y. Chen, Inorg. Chem. 2004, 43, 5877; b) C. Salzemann, J. Urban, I. Lisiecki, M. P. Pileni, Adv. Funct. Mater. 2005, 15, 1277. a) A. I. Kirkland, D. A. Jefferson, D. G. Duff, P. P. Edwards, I. Gameson, B. F. G. Johnson, D. J. Smith, Proc. R. Soc. London A 1993, 440, 589; b) C. Lofton, W. Sigmund, Adv. Funct. Mater. 2005, 15, 1197. R. Jin, Y. Cao, C. A. Mirkin, K. L. Kelly, G. C. Schatz, J. G. Zheng, Science 2001, 294, 1901. B. Rodriguez-Gonzalez, I. Pastoriza-Santos, L. M. Liz-Marzan, J. Phys. Chem. B 2006, 110, 11796. L. X. Song, J. Chen, L. H. Zhu, J. Xia, J. Yang, Inorg. Chem. 2011, 50, 7988.
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