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Single-Crystalline β-Ga2O3 Hexagonal Nanodisks: Synthesis, Growth Mechanism, and Photocatalytic Activities Heqing Yang,*,† Ruyu Shi,†,‡ Jie Yu,†,‡ Ruini Liu,†,‡ Ruigang Zhang,†,‡ Hua Zhao,† Lihui Zhang,† and Hairong Zheng§ Key Laboratory of Macromolecular Science of Shaanxi ProVince, School of Chemistry and Materials Science, and School of Physics & Information Technology, Shaanxi Normal UniVersity, Xi’an 710062, China ReceiVed: June 21, 2009; ReVised Manuscript ReceiVed: NoVember 20, 2009
β-Ga2O3 hexagonal nanodisks were in situ grown on the surface of gallium grain and Si substrate via a reaction of metallic gallium with NH4Cl and H2O at 650-800 °C for 30 min. The as-synthesized nanodisks have perfect hexagonal shape with diagonals of 0.6-4.0 µm and thicknesses of 140-480 nm. Size of the hexagonal nanodisks increased with increasing heat treatment temperature. The formation of the Ga2O3 nanodisks results from the selective adsorption of Cl- ions on unsaturated Ga(I) and Ga(II) sites of β-Ga2O3 (100) surface. Photoluminescence and photocatalytic activity of the hexagonal Ga2O3 nanodisks were studied at room temperature. The results indicated that the hexagonal nanodisks display a stable blue-green emission band centered at 489 nm that originated from the recombination of an electron on an oxygen vacancy and a hole on a gallium-oxygen vacancy pair and high photocatalytic activity in the photodegradation of methyl orange. 1. Introduction Nanostructured materials have attracted considerable attention because of their unique electrical, optical, magnetic, and mechanical properties and enormous potential as fundamental building blocks for nanoscale electronic and photonic devices.1 The physical and chemical characteristics of the nanomaterials are strongly related to their size and shape. Therefore, synthesis of size- and shape-controlled nanostructures is important in controlling their physical and chemical properties. Monoclinic gallium oxide (β-Ga2O3), a wide-band-gap compound with a band gap of about 4.9 eV at 300 K, is an important functional material. It has a variety of applications including transparent conducting oxide,2 optical emitter for UV,3 photocatalysts,4 and high-temperature gas sensors5 due to its unique electrical, optical, and optoelectronic properties and chemical and thermal stability. Yu et al.6 first reported the growth of β-Ga2O3 nanowires on the inner wall of a quartz boat by evaporating Ga powder under Ar + H2 atmosphere. Subsequently, various physical and chemical routes such as chemical vapordeposition(CVD),7-14 microwaveplasmaCVD(MPCVD),15,16 metallorganic CVD (MOCVD),17,18 arc-discharge,19 and solution-phase growth20-25 have been employed to prepare β-Ga2O3 nanostructured materials with various geometrical morphologies. Up to now, nanowires,8 zigzag and helical nanostructures,9 nanobelts,15 nanorods,20,21,23 nanochains,11 nanotubes,16 nanowire arrays,18 ribbon-shaped tubes,14 nanopaintbrushes,16 and hollow microspheres24 of Ga2O3 and three-dimensional (3-D) Ga2O3/ In2O3 hierarchical heterostructures12 have been synthesized. The electrical,26 luminescent,3,8,18 optical waveguide,7 and gas sensing27 properties of Ga2O3 nanowires and nanobelts have been studied. However, photocatalytic ability of Ga2O3 nanomaterials * To whom correspondence should be addressed. E-mail: hqyang@ snnu.edu.cn. † School of Chemistry and Materials Science. ‡ Authors with equal contribution. § School of Physics & Information Technology.
with a defined size and shape for oxidative decomposition of organic pollutants in water has not yet been examined. In contrary to one-dimensional (1-D) nanostructures, twodimensional (2-D) disk-shaped morphology is less reported, which has potential applications in light emitter, sensor, and catalyst. As reported, nanodisks of silver, cobalt, and Bi2Te3 were generally produced in solution environment through wet chemical reaction.28-30 However, synthesis of 2-D disklike structures by CVD is still considerably more difficult. In this regard, Dai et al.31 synthesized SnO diskettes by evaporating either SnO or SnO2 powders at elevated temperature. Xu et al.32 fabricated ZnO nanodisks with perfect hexagonal prism shape by evaporating a mixture of ZnO and graphite powders. However, the growth mechanism of the nanodisk is not clear yet. Dai et al.10 synthesized Ga2O3 nanosheets and nanoribbons by evaporating GaN at 1100 °C in the presence of oxygen. But, the as-synthesized 2-D Ga2O3 nanosheets always mix with 1-D nanobelts;10 namely, synthesis of simple 2-D Ga2O3 nanostructures has not been reported until now. In this contribution, we report on the synthesis of pute Ga2O3 nanodisks with perfect hexagonal prism shape by an in situ growth route. The hexagonal disks are new members in the family of Ga2O3 nanostructures, which were directly grown on the surface of gallium grains and Si substrates by the reaction of gallium grain with NH4Cl and H2O at 650-800 °C. Growth mechanism, photoluminescence of the hexagonal Ga2O3 nanodisks, and their photocatalytic activities in the degradation of methyl orange have been investigated and discussed. In addition, compared with all reported CVD methods for fabrication of Ga2O3 nanomaterials, this procedure does not require highly sophisticated equipment, rigorous vacuum conditions (40-300 torr),10,11 and very high temperature (1000-1200 °C).7-10 2. Experimental Section Synthesis of the hexagonal Ga2O3 nanodisks was carried out in a conventional high-temperature horizontal corundum tube
10.1021/jp905829w 2009 American Chemical Society Published on Web 12/10/2009
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Figure 1. Schematic diagram of experimental setup used for the synthesis of β-Ga2O3 hexagonal nanodisks.
furnace. The experimental setup is shown in Figure 1. A silicon foil was cleaned in absolute ethanol and acetone with ultrasonic irradiation for 10 min, respectively. After drying in air, the Si substrate was loaded into an alumina boat, and then a gallium grain (about 0.05-0.08 g) was placed on the substrate. The boat was placed at the center of a corundum tube that was inserted into the furnace. Another alumina boat containing 1.100 g of NH4Cl and 2 drops of deionized water was set upstream, and the distance between the two boats was 19 cm. Prior to heating, high-purity Ar (99.999%) was introduced into the alumina tube at a constant flowing rate of 2.5 L/h to purge the air inside. After 15 min, the furnace was heated to 300 °C at a rate of 6 °C/min and then to 650-800 °C at a rate of 8 °C/min and kept for 30 min under a constant Ar flow of 1.0 L/h. The temperature of the alumina boat containing NH4Cl and H2O is about 280-410 °C. After the system cooled to room temperature under Ar flow of 1.0 L/h, the gray samples were observed on the gallium grain and Si substrate. The as-synthesized samples were characterized and analyzed by scanning electron microscopy (SEM), transmission electron microscopy (TEM), and X-ray diffraction (XRD). SEM images were obtained using a FEI Quanta 200 scanning electron microscope at an accelerating voltage of 25 kV. An EAX energy-dispersive X-ray spectroscopy (EDS) facility attached to the SEM was employed to analyze chemical composition. TEM and electron diffraction images were obtained using a JEOL JEM-3010 transmission electron microscope at an accelerating voltage of 300 kV. Samples for HRTEM were prepared by dispersing a Ga2O3 powder on a carbon-coated copper grid. The XRD analysis was performed using a Rigaku DMX-2550/PC X-ray diffractometer with Cu KR radiation (λ ) 1.54 Å) at 40 kV and 40 mA. XPS measurements were performed by using a Kratos Axis ultra X-ray photoelectron spectrometer with an excitation source of AlKR ) 1486.7 eV.The photoluminescence (PL) spectra were measured at room temperature by using a He-Cd laser with 325 nm as the excitation source. The photocatalytic property of the Ga2O3 nanodisks was determined by measuring the decoloration of methyl orange solution. Ga grains covered with Ga2O3 nanodisks at 700 and 800 °C were added to 30 mL of 5.0 × 10-5 mol/L aqueous methyl orange (MO) solution, respectively. Subsequently, the mixed solution was transferred into a 50 mL quartz test tube and irradiated with a 300 W Hg lamp (365 nm) at a distance of about 8 cm (XPA-7 photochemical reactor, Xujiang Electromechanical Plant, Nanjing, China). At given irradiation time intervals, 5 mL samples were withdrawn from the test tube for analysis. The absorption spectra of these solutions were measured by using a 7U-1901 ultraviolet-visible spectrophotometer (Beijing Purkinje General Instrument Co. Ltd., Beijing, China).
Figure 2. SEM images of the samples grown on the surface of gallium grain (a-c) and silicon foil (e-f) at 650 (a and d), 700 (b and e), and 800 °C (c and f) for 30 min in an Ar gas atmosphere.
3. Results and Discussion Morphologies of the samples grown on the surface of gallium grain and silicon foil at 650, 700, and 800 °C for 30 min are shown in Figure 2. Figure 2a-c shows the SEM images of the samples grown on the surface of gallium grain at 650, 700, and 800 °C, respectively. It is clearly seen that a small quantity of nanodisks covered the gallium grain after heating at 650 °C. The nanodisks have hexagonal shape with typical diagonals of 1.8-2.2 µm and a thickness of about 170 nm. Size, thickness, and yield of the hexagonal nanodisks increased with increasing heat treatment temperature. When the temperature was increased to 700 and 800 °C, the typical diagonal of the hexagonal nanodisks is 2.2-2.6 and 3.0-3.7 µm, respectively, and the thickness is about 220 and 460 nm, respectively. In addition, some nanodisks intercross with each other to form complicated networks on the gallium grains after heating at 700 and 800 °C (Figure 2b-c). Figure 2d-f shows the SEM images of the samples grown on Si substrates at 650, 700, and 800 °C, respectively. The SEM observations indicate that the samples obtained at 650 °C consist of a quantity of hexagonal nanodisks. The nanodisks have rough sides and diagonals varying from 0.6 to 0.7 µm and a thickness of about 110 nm. When the reaction temperature was increased to 700 °C, the hexagonal nanodisks have typical diagonals of 1.7-2.0 µm and a thickness of about 160 nm. Moreover, aggregated nanodisks were observed (Figure 2e). As the temperature was increased to 800 °C, diagonals of the hexagonal nanodisks are 3.3-4.0 µm, the thickness is about 200 nm (Figure 2f), and a few nanodisks gather together (Figure 2f). The diagonal and thickness of samples obtained at different reaction temperature are summarized in Table 1, which indicates that the final sizes of samples depend on the reaction temperature. Figure 3a-b shows the EDS patterns of the samples grown on the gallium grain and Si substrate at 800 °C for 30 min, respectively. The EDS
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TABLE 1: Diagonal and Thickness of Samples Obtained at Different Reaction Temperatures
sample on Ga grain sample on Si substrate
reaction temperature (°C)
diagonal (µm)
thickness (nm)
650 700 800 650 700 800
1.8-2.2 2.2-2.6 3.0-3.7 0.6-0.7 1.7-2.0 3.3-4.0
170 220 460 110 160 200
patterns reveal that the hexagonal nanodisks on the gallium grain and Si substrate consist of Ga and O elements. Samples grown on the gallium grain at 650, 700, and 800 °C and the silicon substrate at 800 °C for 30 min were charactered by XRD, and the results are shown in Figure 4. Peaks at 2θ ) 30.04, 31.54, 32.94, 35.14, 38.24, 45.49, 48.54, 57.5, 60.8, 64.5, 70.0, 72.12, and 77.82° are observed in XRD patterns of the samples grown on the gallium grain (Figure 4a-c). According to JCPDS card no. 76-0573, the samples are Ga2O3 with a monoclinic structure, and peaks are assigned to (4j01), (002), (1j11), (111), (3j11), (3j13), (112), (510), (020), (512), (022), (204), and (603) diffraction lines, respectively. Diffraction lines of Ga2O3 were observed together with the (111) diffraction line of Si (JCPDS no. 65-1060) for the samples grown on the Si substrate. Further structural details of the samples grown on gallium grain at 800 °C for 30 min were studied by TEM observations. Figure 5a shows a typical TEM image of an isolated hexagonal nanodisk. The nanodisk has perfect hexagonal shape with a uniform side length of about 1.1 µm and diagonal of about 2.5 µm. The corresponding selected-area electron diffraction (SAED) pattern (Figure 5b) can be identified as the [101] zone axis
Figure 3. EDS patterns of the samples grown on gallium grain (a) and Si substrate (b) at 800 °C for 30 min in an Ar gas atmosphere.
Figure 4. XRD patterns of the samples obtained on Ga grain at 650 (a), 700 (b), and 800 °C (c) and silicon substrate at 800 °C (d) for 30 min in an Ar gas atmosphere.
Figure 5. TEM image and SAED pattern of the samples grown on gallium grain at 800 °C for 30 min in an Ar gas atmosphere.
projection of the monoclinic Ga2O3 reciprocal lattice. A highresolution TEM (HRTEM) image is displayed in Figure 5c. As shown in the HRTEM image, the fringe spacing of the observed lattice planes is about 0.26 and 0.28 nm, respectively, which agrees with the (111j) and (2j02) lattice planes of the monoclinic Ga2O3, respectively. The SAED and HRTEM results indicate that the nanodisk grew mainly along the six symmetric directions perpendicular to (111j) (1j1j1), (1j11), (11j1j), (2j02), and (202j) facets, and the nanodisks are enclosed by ((100) top and bottom surfaces.10 To illuminate the role of NH4Cl in the formation of Ga2O3 hexagonal nanodisks, 1 drop of 1.0 mol/L HCl solution was added to a gallium grain (about 0.040 g) on a cleaned silicon foil lain on an alumina boat and then dried naturally in air to form a gallium grain coated with GaCl3 · xH2O. The boat containing GaCl3 · xH2O-coated gallium grain was placed at center of the corundum tube that was inserted into the furnace. The furnace was heated to 700 °C under a constant Ar flow of 1.0 L/h and kept for 30 min. The samples grown on the gallium grain were characterized by SEM and XRD, and the results are shown in Figures 6a and b. The SEM observations (Figure 6a) indicated that products synthesized by heating GaCl3 · xH2Ocoated gallium grain are also composed of a large quantity of perfect hexagonal nanodisks. The side lengths and thicknesses of the hexagonal nanodisks are about 5.5 µm and 500 nm, respectively. The XRD measurement (Figure 6b) showed that the hexagonal nanodisk is Ga2O3 with the monoclinic and unknown structures (JCPDS card no. 76-0573 and no. 06-0509). In addition, equal weight (NH4)2SO4 was used instead of NH4Cl. The samples obtained via the same experimental procedure were characterized by SEM. The results reveal that the as-obtained samples are composed of a small quantity of microspheres instead of hexagonal nanodisks (Figure 6c). Therefore, it is reasonable to conclude that HCl produced by decomposition of NH4Cl plays an important role in the formation of Ga2O3 hexagonal nanodisks. To further illuminate the role of Cl- in the formation of Ga2O3 hexagonal nanodisks, the Ga2O3 nanodisks grown on the Ga grains at 700 °C for 30 min were characterized by XPS spectra, and the results are shown in Figure 7. The binding energies obtained in the XPS analysis were corrected for specimen charging through referencing the C1s to 284.6 eV. The binding energy of Ga 2p3/2 and 2p1/2 is
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Figure 7. XPS spectra of products grown on Ga grains at 700 °C in an Ar gas atmosphere.
Figure 6. SEM image (a) and XRD pattern (b) of the products obtained by heating GaCl3 · xH2O-coated gallium grain at 800 °C in an Ar gas atmosphere. (c) SEM image of the samples grown on Ga grain by using equal weight (NH4)2SO4 instead of NH4Cl.
identified at 1117.8 and 1144.8 eV, respectively (Figure 7a), and the peak at about 530.6 eV corresponds to O1S, which is derived from Ga2O333 (Figures 7b). The peaks at about 199.3 and 200.8 eV can be assigned to Cl 2p3/2 and 2p1/2, respectively (Figure 7c), which might come from Cl- ions absorbed on the surface of the Ga2O3 nanodisks. The XPS spectra suggest that the formation of the Ga2O3 nanodisks may originate from the selective adsorption of Cl- ions. In order to understand the growth process of the β-Ga2O3 nanodisks, the samples collected after heating at 700 and 800 °C without keeping were characterized with SEM, and the results are shown in Figure 8. The SEM observations indicate that the samples grown on the surface of Ga grain are also hexagonal nanodisks. The side lengths of the hexagonal nanodisks are 1.1-2.6 and 1.5-3.9 µm, respectively. The samples grown on the Si substrate at 700 °C for 0 min are mainly
Figure 8. SEM images of the samples grown on gallium grain (a-c) and silicon substrate (b-d) at 700 (a and b) and 800 °C (c and d) for 0 min in an Ar gas atmosphere.
nanodisks with rough sides. The perfect hexagonal nanodisks grown on the Si substrate at 800 °C for 0 min have diagonals in the range of 4.5-5.0 µm and a thickness of about 300 nm. Figure 9 shows the EDS patterns of the samples grown on the
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Figure 9. EDS patterns of the samples grown on gallium grain (a) and silicon substrate (b) at 700 °C for 0 min in an Ar gas atmosphere.
surface of gallium grain and Si substrate at 700 °C for 0 min, respectively. The EDS patterns reveal that the hexagonal nanodisks on the gallium grain and Si substrate consist of Ga and O elements. These results reveal that the growth of hexagonal nanodisks starts during the heating process. Until now there are two well-accepted mechanisms for the growth of 1D nanostructure, the vapor-liquid-solid (VLS) and the vapor-solid (VS) mechanisms. In the present study, no catalyst was employed, and no liquid droplets were found at the ends of the hexagonal nanodisks. Moreover, the geometrical morphologies of the samples obtained on the gallium grain and silicon substrate are the same. Therefore, we consider that the growth of the hexagonal nanodisks is not controlled by the VLS process but is governed by the VS process. During heating under the flow of N2, the metallic Ga grain was first melted to form liquid Ga, the water was vaporized to form H2O vapor, and then the NH4Cl was decomposed into NH3 and HCl. The liquid Ga reacted with H2O vapor to form Ga2O3, the Ga2O3 reacted with Ga to form Ga2O vapor, and the Ga2O vapor was further oxidized by H2O into Ga2O3. In addition, the HCl may have reacted with the liquid Ga or Ga2O3 to form GaCl3 vapor, the GaCl3 vapor reacted with H2O to form the Ga2O3, and the Ga vapor also may have reacted with H2O to form Ga2O3. The chemical reactions to form Ga2O3 can be formulated as follows:
NH4Cl(s) f NH3(g) + HCl(g)
(1)
Ga(l) + H2O(g) f Ga2O3(s) + H2(g)
(2)
Ga2O3(s) + Ga(l) f Ga2O(g)
(3)
Ga2O(g) + H2O(g) f Ga2O3(s) + H2(g)
(4)
HCl(g) + Ga(l) f GaCl3(g) + H2(g)
(5)
HCl(g) + Ga2O3(s) f GaCl3(g) + H2O(g)
(6)
GaCl3(g) + H2O(g) f Ga2O3(s) + HCl(g)
(7)
Ga(g) + H2O(g) f Ga2O3(s) + H2(g)
(8)
The Ga2O3 formed by reactions 4, 7, and 8 directly deposited on the surface of Ga grain and Si substrate to form Ga2O3 crystalline nuclei by the V-S process. The nuclei further grew into hexagonal nanodisks. The monoclinic β-form of Ga2O3 is the most stable polymorph, which belongs to the C2/m (or C2h3) space group with lattice parameters a ) 12.23 ( 0.02 Å, b ) 3.04 ( 0.01 Å, c ) 5.80 ( 0.01 Å, and β ) 103.7 ( 0.3°.34 There are four Ga2O3 in the unit cell. The lattice is composed of two types of coordination for Ga3+ ions in this structure, namely, tetrahedral and octahedral, hereafter referred to as Ga(I) and Ga(II), and three types of oxygen ions, referred to as O(I), O(II), and O(III).31 According to Bermudez35 and Gonzalez,36
Figure 10. Lateral view of the (100) plane after Cl- adsorption on unsaturated Ga(I) and Ga(II) sites of β-Ga2O3 (100) facet.
β-Ga2O3 (100) surface is a polar surface, and the polar (100) surface termination consists of a layer of unsaturated Ga(I) and Ga(II) sites. It is well-known that the growth of certain surfaces can be impeded by using additives that preferentially adsorb to specific crystal faces. In the CVD system, in addition to Ga2O3, there are HCl, NH3, and H2 vapors. Cl- in HCl have electron pairs, may serve as ligands to Ga, and adsorb selectively on unsaturated Ga(I) and Ga(II) sites of β-Ga2O3 (100) surface during the nucleation and growth of the Ga2O3 nanodisks. The most likely structure is shown in Figure 10. The presence of Cl- ions impedes the growth of the (100) surface of Ga2O3 nuclei, and thus Ga2O3 nanodisks are obtained. Moreover, the supersaturation ratio of Ga2O3 vapor formed by reactions 4, 7, and 8 is high in the CVD system because that volatilization of GaCl3, Ga2O, and Ga is very easy. Although the difference of surface energies can lead to their different growth rates, the high supersaturation ratio makes the effect of the surface energy difference on the growth very small.37 As a result, the growth rates of (111j), (1j1j1), (1j11), (11j1j), (2j02), and (202j) facets of the disklike crystalline nuclei become quite close, and thus the Ga2O3 nanodisks with perfect hexagonal prism are obtained. When equal weight (NH4)2SO4 was used instead of NH4Cl, there are not HCl and Cl- ions in the CVD system, and so the products are not the Ga2O3 hexagonal disks. Nanodisks with rough sides were obtained on Si substrate at 650 °C for 30 min and 700 °C for 0 min, which may result from the lack of gaseous Ga2O3 formed via reactions 4, 7, and 8. The concentration of Ga2O3 vapor is lesser at low temperature and the initial growth stage and does not feed the growth of hexagonal disks completely, and thus nanodisks with rough sides are obtained. Ga2O3 vapor pressure increases with increasing the reaction temperature and time, and thus the nanodisks with rough side grew into perfect hexagonal nanodisks. PL spectra of the β-Ga2O3 hexagonal nanodisks grown on the gallium grain at 800 °C were measured at room temperature with 325 nm He-Cd laser excitation, and the results are shown in Figure 11. A broad strong emission peak centered at 489 nm is observed in the blue-green range, which is similar to the PL spectra of nanowires,3,7 nanobelts,13 and microbelts38 of Ga2O3 reported previously.The full width at half-maximum of the bluegreen peak is about 139 nm. The PL property of the β-Ga2O3 nanostructures with different morphologies3,7,8,18 has been extensively studied in the last decades. It was found that the PL behaviors of the β-Ga2O3 nanostructures have a connection with their growth conditions. According to the previous
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Figure 11. Room-temperature PL spectrum of the β-Ga2O3 nanodisks grown on the gallium grain at 800 °C for 30 min in an Ar gas atmosphere.
report,3,38 the luminescence of β-Ga2O3 nanostructures is mainly attributed to oxygen vacancies (VO· ), gallium vacancies, and gallium-oxygen vacancy pairs ((VO, VGa)′) generated because of partially incomplete oxidation and crystallization during the growth of the nanostructures. These defects would induce new energy levels in the band gap. The blue light emission originates from oxygen vacancies (VO· ) and gallium-oxygen vacancy pairs ((VO, VGa)′). The (VO, VGa)′ and the VO· may act as the acceptors and donors, respectively. After excitation of the acceptor, a hole on the acceptor and an electron on a donor are created according to the following formal equation:
(VO, VGa) + VO· + hν f (VO, VGa)× + V×O An electron in donor level (V×O) may be captured by a hole on an acceptor level ((VO, VGa)×) to form a trapped exciton. The trapped exciton recombines radiatively to produce the blue emission.36 In our experiment, the gallium grains were oxidized in a mixed gas atmosphere of N2, HCl, NH3, and H2O at 650-800 °C, a relatively oxygen deficient environment, which might result in the generation of (VO· ) and (VO, VGa)′ in the as-synthesized hexagonal nanodisks. As a result, the PL feature of the hexagonal Ga2O3 nanodisks is similar to those reported previously.3,8,13,18,39 To evaluate the photocatalytic activity of the hexagonal Ga2O3 nanodisks, absorption spectra of the MO, MO solutions containing a Ga grain, and Ga2O3 nanodisk-loaded Ga grains obtained at 700 and 800 °C after UV light irradiation for 5 h were measured, and the results are shown in Figure 12a. The MO shows a maximum absorption band at 465 nm (curve I in Figure 12a). The absorption intensity of the peak was reduced in the presence of Ga grain and Ga grains covered with Ga2O3 nanodisks (curves II-IV in Figure 12a). The reduction of the absorption peak is quickest in the presence of Ga grains loaded with Ga2O3 nanodisks obtained at 700 °C. Time-dependent absorption spectra of the MO solution containing samples obtained at 700 °C during the irradiation are illustrated in Figure 12b. It can be seen that the maximum absorbance at 465 nm decreased with irradiation time. The decoloration of solution implies the destruction of the dye chromogen. Since no new absorption peak was observed, the MO has been decomposed. It is obvious that the hexagonal Ga2O3 nanodisks were effective photocatalysts for the direct degradation of methyl orange. The fitting of absorbance maximum plot versus time indicates an exponential decay as shown in Figure 12c. The normalized concentration of the solution equals the normalized maximum absorbance, so we use A0/A to take the place of C0/C. The photodegradation of MO catalyzed by the hexagonal Ga2O3 nanodisks fits pseudo-first-order reaction well, i.e., -dc/dt ) Kt, or ln(C0 /C) ) Kt, where C0 and C are the initial and actual
Figure 12. (a) Absorption spectra of MO solution (I) and MO solutions containing Ga grain (II) and Ga grains loaded with Ga2O3 nanodisks obtained at 800 (III) and 700 °C (IV) after ultraviolet irradiation for 5 h. (b) Absorption spectra of MO solution containing Ga grains loaded with Ga2O3 nanodisks obtained at 700 °C after UV irradiation for different times. (c) Fitting of absorbance maximum plot vs time.
concentration of MO, respectively, and K is the apparent rate constant of the degradation. In our experiment, K is found to be 0.145/min. The photocatalytic properties in the degradation of the MO of the Ga2O3 nanodisks suggest that the samples should have potential application in water treatment. In addition, the Ga2O3 nanodisks grew directly on the Ga grains, and thus callback of the catalyst is very easy in the practical application. It is generally accepted that the catalytic process is mainly related to the adsorption and desorption of molecules on the surface of the catalyst. The high specific surface area can provide more reactive adsorption/desorption sites for photocatalytic reactions. The SEM images shown in Figure 2 and Table 1 indicated that the size of Ga2O3 nanodisks grown on the Ga grains increases with an increase in the heating temperature. The surface area increases with reducing the size of the Ga2O3 nanodisks. Thus, Ga2O3 nanodisk-loaded Ga grain obtained at 700 °C has larger surface area, and its photocatalytic ability is stronger than that of the sample obtained at 800 °C. 4. Conclusion We demonstrate in situ growth of β-Ga2O3 hexagonal nanodisks on the surface of gallium grain and Si substrate by
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the reaction of gallium grain with NH4Cl and H2O at 650-800 °C. A selective adsorption mechanism is proposed to account for the growth of the hexagonal nanodisks. The PL spectrum manifests a broad and strong blue emission band centered at 489 nm originated from the recombination of an electron at an oxygen vacancy and a hole at a gallium-oxygen vacancy pair. The as-prepared β-Ga2O3 hexagonal nanodisks showed an excellent photocatalytic ability to degrade MO, which is expected to be useful in sewage water treatment. Moreover, this simple and mild approach to fabricate 2-D Ga2O3 nanostructures can be easily scaled up and potentially extended to the synthesis of other compound nanostructures. Acknowledgment. We acknowledge financial support from the National Natural Science Foundation of China (Grant No. 20573072) and Specialized Research Fund for the Doctoral Program of Higher Education (Grant No. 20060718010). We thank Miss Ying Li and Mr. Dichun Chen from Xi’an University of Technology for their assistance in XPS and TEM analysis. References and Notes (1) Tian, B. Z.; Kempa, T. J.; Lieber, C. M. Chem. Soc. ReV. 2009, 38, 16. (2) Edwards, D. D.; Mason, T. O.; Goutenoire, F.; Poeppelmeier, K. R. Appl. Phys. Lett. 1997, 70, 1706. (3) Wu, X. C.; Song, W. H.; Huang, W. D.; Pu, M. H.; Zhao, B.; Sun, Y. P.; Du, J. Chem. Phys. Lett. 2000, 328, 5. (4) Yuliati, L.; Hattori, T.; Itoh, H.; Yoshida, H. J. Catal. 2008, 257, 396. (5) Trinchi, A.; Kaciulis, S.; Pandolfi, L.; Ghantasala, M. K.; Li, Y. X.; Wlodarski, W.; Viticoli, S.; Comini, E.; Sberveglieri, G. Sens. Actuators, B 2004, 103, 129. (6) Zhang, H. Z.; Kong, Y. C.; Wang, Y. Z.; Du, X.; Bai, Z. G.; Wang, J. J.; Yu, D. P.; Ding, Y.; Hang, Q. L.; Feng, S. Q. Solid State Commun. 1999, 109, 677. (7) Wang, F.; Han, Z. H.; Tong, L. M. Physica E 2005, 30, 150. (8) Chun, H. J.; Choi, Y. S.; Bae, S. Y.; Seo, H. W.; Hong, S. J.; Park, J. H.; Yang, H. J. Phys. Chem. B 2003, 107, 9042. (9) Zhan, J. H.; Bando, Y.; Hu, J. Q.; Xu, F. F.; Golberg, D. Small 2005, 1, 883. (10) Dai, Z. R.; Pan, Z. W.; Wang, Z. L. J. Phys. Chem. B 2002, 106, 902. (11) Dai, L.; You, L. P.; Duan, X. F.; Lian, W. C.; Qin, G. G. J. Cryst. Growth 2004, 267, 538.
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