Controlled Fabrication of α-GaOOH and α-Ga2O3 Self-Assembly and

ARTICLE pubs.acs.org/JPCC

Controlled Fabrication of α-GaOOH and α-Ga2O3 Self-Assembly and Its Superior Photocatalytic Activity Manickavachagam Muruganandham,*,†,‡ Ramakrishnan Amutha,‡ Mahmoud S. M. Abdel Wahed,‡ Bashir Ahmmad,§ Yasushige Kuroda,§ Rominder P. S. Suri,† Jerry J. Wu,|| and Mika E. T. Sillanp€a€a‡ †

)

Water & Environmental Technology (WET) Center, Department of Civil and Environmental Engineering, Temple University, Philadelphia, Pennsylvania 19122, United States ‡ Laboratory of Green Chemistry, Faculty of Technology, Lappeenranta University of Technology, Patteristonkatu 1, FI-50100 Mikkeli, Finland § Department of Fundamental Material Science, Graduate School of Natural Science and Technology, Okayama University, 3-1-1 Tsushima, Okayama 700-8530, Japan Department of Environmental Engineering and Science, Feng Chia University, Taichung 407, Taiwan, ROC

bS Supporting Information ABSTRACT: In this article, we report the fabrication of gallium oxide (α-Ga2O3) microspheres (GOMs) by a self-assembly process. Gallium nitrate with oxalic acid in a hydrothermal process results in α-GaOOH, which was further converted into gallium oxide by calcinations at 450 °C for 3 h. We first report the formation of various morphological α-GaOOH by using the above-mentioned methodology. The influence of hydrothermal temperature and time on the crystal structure and its morphology was studied, and the results indicated that hydrothermal temperature played an important role in the final morphology of α-GaOOH. The flower-like α-GaOOH formed at 175 °C is converted into rodlike α-Ga2O3 after calcination at 450 °C, and the α-GaOOH microsphere and microrod formed at 200 and 225 °C retained their morphology during the calcination process, respectively. The synthesized α-GaOOH and α-Ga2O3 were characterized using X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), high-resolution transmission electron microscopy (HR-TEM), and nitrogen adsorption analysis. The XRD patterns indicated that well-crystallized α-GaOOH and α-Ga2O3 were formed in a hydrothermal and calcination process, respectively. The FE-SEM images indicated the formation of well-organized microspheres and microflowers, which were composed of nanoparticles and nanoplates, respectively. The photocatalytic degradation of Acid Orange 7 (AO7) dye and Cr(VI) reduction by using the synthesized GOM under UV light irradiation was investigated. The photocatalytic experiment showed superior photocatalytic activity of GOM having a higher efficiency than TiO2. We propose a plausible mechanism for the formation of various morphologies of α-GaOOH and α-Ga2O3.

1. INTRODUCTION The development of highly efficient active photocatalysts is one of the most important topics in photocatalysis research.1
3 It is well known that the size- and shape-controlled synthetic methodologies are of great interest in material chemistry. Thus, compounds with the same compositions, but different morphologies, are exhibiting remarkable differences in their properties.4 Gallium oxide hydroxide (GaOOH) is considered to be an important precursor for the direct preparation of wide-bandgap gallium oxide (Ga2O3), 4.2
4.9 eV.5 On the other hand, gallium oxide was also prepared by using gallium metal.6 Generally, GaOOH can be transformed into various phases (α, β, and γ) of Ga2O3 after the dehydration at suitable temperature.7
9 Therefore, the synthesis of GaOOH and its transformation into various Ga2O3 phases is an interesting research topic. Because the properties of Ga2O3 semiconductors are strongly dependent on the shape, size, and dimensionality of the particles, it would be r 2011 American Chemical Society

interesting to synthesize Ga2O3 with various morphologies for different applications.10 Wu et al. evaluated the photocatalytic activity of various morphologies of gallium oxides and found that the photocatalytic activity was strongly influenced by the morphology due to the difference in their surface properties.11 Similarly, Fu et al. investigated the photocatalytic activity of three polymorphs of gallium oxides (α, β, and γ forms) and concluded that the photocatalytic activity was strongly influenced by the crystal structure.12 These studies indicated that the α-G2O3 form was found to be more efficient than the other two forms of gallium oxides. Therefore, it is very important to synthesis various polymorphs of gallium oxides (α, β, and γ forms) with desired surface properties. Gallium oxide has been Received: June 7, 2011 Revised: November 19, 2011 Published: November 28, 2011 44

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successfully prepared by using thermal decomposition, thermal oxidation, carbothermal reduction, vapor
liquid
solid process, homogeneous precipitation using ammonia, and surface layer adsorption methods.13
18 However, the above-mentioned methodologies offer poor control over structural, morphological, and compositional properties and yield relatively low specific surface areas. Therefore, scientists are continuously exploring new methodologies to fabricate the desired morphology and surface properties for various applications. Fabrication of hollow materials with desired pore structures and complex morphologies and their potential applications in catalysis, separation, and controlled release have attracted much attention in terms of fundamental interest.1
3,19,20 Porous photocatalysts have been found to be highly efficient in the photocatalytic decomposition of pollutants.3,21 Hou et al. studied the photocatalytic degradation of benzene by using porous β-Ga2O3, and the results showed that the gallium oxide was found to be more efficient than TiO2.22 Therefore, the fabrication of such porous materials without using a template or catalyst in a simple methodology in large-scale production is highly desirable for catalytic applications. Thus, mesoporous photocatalysts offer a uniform and adjustable environment for encapsulating the target pollutants on the internal surface of the pores, which may increase the degradation rate substantially.23 Therefore, the fabrication of porous photocatalysts with a complex morphology and high photocatalytic activity is a highly challenging job for material scientists. Nevertheless, the use of templates and high fabrication costs may limit the applications of porous photocatalysts and may be overcome by developing a simple methodology.24 Moreover, the removal of templates needs additional energy and may inhibit the porous structures. Oxalic acid has been used as a coordination reagent (ligand) to prepare various nano/micromaterials.25,26 For example, Du et al. reported oxalic acid-assisted fabrication of Fe2O3 hollow urchins by hydrothermal treatment of a ferric nitrate
oxalic acid complex without using any other matrixes.27 Though various morphological Ga2O3 have been prepared, the self-assembled microsphere and microflower preparation is limited in the literature.13
18 In this research, we have successfully synthesized microflowers, microspheres, and microrod-shaped α-GaOOH by using gallium nitrate with oxalic acid in a hydrothermal process, and its thermal transformation into corresponding gallium oxide was investigated. The effect of various experimental parameters on the formation of GaOOH was investigated. The photocatalytic activity of α-Ga2O3 was evaluated by using Acid Orange 7 (AO7) and Cr(VI) reduction as model pollutants under UV light irradiation.

hydrothermal reaction. The Teflon cup was covered with stainless steel, and the autoclave setup was kept for the desired time in an oven. The precipitate formed was harvested by filtration and washed several times with deionized water and ethanol to remove possible impurities before being dried in an oven at 120 °C for 2 h. In the second step, the hydrothermally prepared GaOOH was decomposed at 450 °C for 3 h under oxygen atmospheric conditions. After the decomposition, the oven was allowed to cool to room temperature naturally, and the samples were then collected for further analysis. A 100 mL semibatch reactor made of glass with the dimensions of 9 cm in diameter and 15 cm in height was used to study the photocatalytic activity of the synthesized Ga2O3. All experiments have been carried out at 25 °C. In all cases, 75 mL of 10 mg/L AO7 dye or Cr(VI) containing 1 g/L of the photocatalysts was used. The resulting solution was then stirred continuously in the dark for 30 min to achieve the adsorption equilibrium on the photocatalysts. The photocatalytic run was then started under UV light irradiation (λ = 254 nm). A low-pressure mercury lamp (Pen-Ray), with an intensity of 5.5 mW/cm2, was used for the irradiation purpose. After the UV irradiation, the solution was extracted periodically from the reactor and immediately filtrated through a membrane filter in order to remove the photocatalyst. At least duplicated runs were carried out for each condition, averaging the results. The changes in the absorbance of AO7 dye and Cr(VI) concentrations were monitored by UV spectroscopy at 465 and 350 nm, respectively. The X-ray diffraction (XRD) patterns were recorded using an X0 Pert PRO PAN analytical diffractometer, with the scanned angle from 10° to 100°. High-resolution transmission electron microscope images were recorded using FE-TEM, a Philips CM-200 FEG-(S) TEM-Super Twin instrument. Samples for HR-TEM were prepared by ultrasonically dispersing the catalyst into ethanol and then placing a drop of this suspension onto a carboncoated copper grid and then drying it in the air. The working voltage of TEM was 80 kV. The morphology of the catalyst was examined using a Hitachi S-4100 scanning electron microscope (SEM). Prior to SEM measurements, the samples were mounted on a carbon platform that was then coated with platinum using a magnetron sputter for 10 min. The plate containing the sample was then placed in SEM for the analysis with the desired magnifications. The X-ray photoelectron spectra were collected on an ESCA-1000 X-ray photoelectron spectrometer (XPS), using Mg Kα X-ray as the excitation source. The UV
visible diffuse reflectance spectra were recorded using a spectrophotometer (JASCO-550). The thermogravimetric (TG) measurements were performed at temperatures from the room temperature to 973 K using an ULVAC TGD-9600 instrument. The sample was heated at a rate of 10 °C min
1 under air. The surface properties (surface area, pore size, and pore volume) of the gallium oxides were measured with an Autosorb-1-C surface analyzer (Quantachrome, the U.K.). All the samples were degassed at 120 °C for overnight before the measurements. All the surface properties measurements were duplicated, and the average values were reported. During the consecutive measurements, no obvious difference in surface area was noted and the difference was less than 2 m2/g in all the three samples.

2. EXPERIMENTAL SECTION Gallium nitrate, anhydrous oxalic acid, Acid Orange 7 dye, and potassium dichromate were purchased from Aldrich Chemical Co. Ltd. (Helsinki, Finland). All chemicals were of analytical grade and used as received without further purification. For all experimental work, Milli Q-Plus water (resistance = 18.2 M 3 Ω) was used. Hydrothermal experiments were performed by using water as a solvent. The synthesis of gallium oxide involves two steps. The first step was to prepare GaOOH by using gallium nitrate and oxalic acid in a hydrothermal process. About 50 mL of 0.0508 mol/L gallium nitrate and 50 mL of 0.155
0.666 mol/L oxalic acid were used for all experiments. Both the above-mentioned solutions were heated on the hot plate to around 90 °C and then mixed under constant magnetic stirring (250 rpm). After a while, the final solution was transferred into a 250 mL capacity Teflon cup for

3. RESULTS AND DISCUSSION Initially, hydrothermal syntheses were conducted by using various amounts of oxalic acid with the temperature ranging from 45

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175 to 225 °C for 3
20 h. The morphology and crystal phase of the synthesized GaOOH were analyzed by FE-SEM and XRD, respectively. Figure 1 shows the XRD pattern of GaOOH formed at 175, 200, and 225 °C for 10 h in a hydrothermal process. As shown in Figure 1, all the peaks can be indexed to a pure orthorhombic phase (space group: pbnm (62)) of α-GaOOH (JCPDS no. 06-0180) with cell constants a = 4.58 Å, b = 9.80 Å, and c = 2.97 Å. The XRD results clearly showed that, as the hydrothermal temperature increases from 175 to 225 °C, the intensities of the XRD peaks are varied, possibly due to differences in their size and morphology. The XRD pattern is quite

consistent with the earlier reports assigned for orthorhombic GaOOH.8,27,28 The sharp diffraction peaks demonstrated that the synthesized GaOOH is well-crystallized. The XRD results also indicated the absence of Ga(OH)3 phases. The XRD of α-GaOOH formed at 200 °C for 10 h did not show well-resolved peaks when compared with the other two XRD patterns and could be due to the small particle size of the microsphere formed at 200 °C. The morphologies of α-GaOOH formed in a hydrothermal process at various temperatures were analyzed. The FE-SEM results showed that the temperature played an important role in the final α-GaOOH morphology. The FE-SEM in Figure 2 showed that the morphologies of α-GaOOH formed at three different temperatures were quite different. Thus, at 175 °C for 10 h, an aggregate-free, uniform, hollow, flower-like morphology was noted and the sizes were in the range from 1 to 4 μm. The flower-like morphology was stacked by thin nanosheets, as evident from Figure 2A. It should be mentioned here that such a novel morphology has not been reported earlier, and we first introduced the new morphology in the GaOOH family. The FE-SEM picture also showed that the self-assembly of nanosheets created a hollow interior (inset in Figure 2A1). However, an increase in the hydrothermal temperature from 175 to 200 °C results in a hollow microsphere morphology and is quite different from the one formed at 175 °C. The microsphere was composed of small nanoparticles and created a porous surface structure, as shown in Figure 2B. Like microflowers, the microsphere also showed a hollow interior (see inset in Figure 2B1), and the sizes were ranging from 500 nm to 4 μm. We have also noted a small percentage of nanorods along with microspheres. These results indicated that the nanoparticles may grow like a rod as the hydrothermal reaction time increases. Interestingly, at 225 °C, we have noted that the high-aspect-ratio microrods and a few microspheres coexist. However, it is difficult to estimate the

Figure 1. XRD of α-GaOOH synthesized in a hydrothermal process: (A) 175, (B) 200, and (C) 225 °C for 10 h.

Figure 2. FE-SEM of α-GaOOH synthesized in a hydrothermal process at (A) 175 °C, (B) 200 °C, and (C) 225 °C for 10 h. 46

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Figure 3. FE-SEM of α-GaOOH synthesized in a hydrothermal process at 200 °C for (A) 3 h and (b) 6 h.

relative percentage of both morphologies. These results clearly indicated that the microsphere morphology is converted into microrods as the hydrothermal temperature increases. The XRD results of α-GaOOH formed at 225 °C showed a high intensity (110) peak that undoubtly confirmed the growth of the microrods at the abovementioned direction. The shape and size of the microrods were not uniform, and the length and width of the microrods were found to be 500 nm to 10 μm, and 400 nm to 2 μm, respectively. Literature results also indicated that the experimental conditions strongly influence the crystal growth. For example, Zhou et al., fabricated various morphological nano/microsized GaOOH by using gallium nitrate with NaOH at different hydrothermal temperatures and pHs.29 However, in our synthetic method, changing the decomposition temperature results in various morphological GaOOH, and therefore, the preparation method offers an environmentally benign, simple synthetic method for α-GaOOH fabrication. The influence of the hydrothermal reaction time on the morphology was investigated at 200 °C for 3, 6, and 10 h, and the results are presented in Figure 3. We have already presented the morphology of GaOOH formed at 10 h, as shown in Figure 2B. The FE-SEM results showed that, at the early stages (3 h), the GaOOH formed was possessing a flower-like morphology, as shown in Figure 3A. The microflowers formed at 200 °C for 3 h are composed of nanoparticles and possess an aggregate-free, uniform morphology. However, further increases in the hydrothermal reaction time from 3 to 6 h results in microspheres, also including a small percentage of microrods. It should be mentioned here that the nanoparticles on the microspheres (formed at 6 h) are loosely bound when compared with microspheres formed at 10 h, as shown in Figure 2B2. These results imply that the hydrothermal reaction time influenced the morphology at the early stages (3 h), which further had no effect on the microsphere morphology, except for the increase in the microrod percentage when the time was increased. We have also examined how the oxalic acid concentration is influencing the morphology of GaOOH in a hydrothermal process. While keeping gallium nitrate at 0.0508 mol/L, experiments were performed by changing the oxalic acid from 0.155 to 0.666 mol/L. The initially formed gallium oxalate complex was converted into α-GaOOH during the hydrothermal process, as evident from the XRD pattern presented in Figure 1. We have

already presented the morphology of synthesized GaOOH by using 0.155 mol/L oxalic acid at 175 °C (Figure 2A) and 0.666 mol/L oxalic acid at 200 and 225 °C, as shown in Figure 2B,C, respectively. Thus, three additional experiments were carried out: one using 0.666 mol/L oxalic acid at 175 °C and another two using 0.155 and 0.666 mol/L oxalic acid at 200 °C for 10 h. The FE-SEM results are presented in Figure S1 (Supporting Information). The FE-SEM results presented in Figure 2 and Figure S1 (Supporting Information) showed that there is no appreciable difference in the GaOOH microflower and microsphere morphology as the oxalic acid is increased from 0.155 to 0.666 mol/L. However, it seems that the percentages of microrods were increased as the oxalic acid concentration increases at 200 °C. However, 0.666 mol/L oxalic acid at 175 °C also yields a microflower morphology with a small amount of a spherelike morphology, as shown in Figure S1 (Supporting Information). In conclusion, changing the oxalic acid concentration had no appreciable effect on the hydrothermal process. The conversion of α-GaOOH into α-Ga2O3 was facilitated under oxygen atmospheric conditions. Initially, we have examined the XRD and FE-SEM of the gallium oxide formed in the calcination process. Figure 4 depicts the XRD pattern of three gallium oxides formed at 450 °C for 3 h. The calcined product of α-GaOOH is purely Ga2O3, and no other materials are present. The formation of α-Ga2O3 is indicated in Figure 4a. The XRD patterns show that the synthesized α-Ga2O3 belongs to a hexagonal system with a = b = 4.979 Å, c = 13.425 Å (JCPDS 06-0503). However, the α-Ga2O3 formed from α-GaOOH at 200 °C showed broad XRD peaks, whereas two other gallium oxides formed at 175 and 225 °C showed sharp XRD peaks. This may be due to the difference in the particle sizes of these morphologies. Similar results were also noted in Figure 1 for the GaOOH formed at 200 °C, which was discussed earlier. We have noted similar observations in our earlier studies.4,24 On the other hand, we cannot completely rule out the possibility of the presence of the γ-phase as an impurity in Ga2O3 prepared at 200 °C. Thus, the speculation is based on the presence of higher-intensity XRD peaks at 36.1 and 64 (2θ). Both the α and the γ phases of Ga2O3 have XRD peaks at the above-mentioned position; however, the γ phase has higher intensities [100 and 70% at 36.22 and 64.23, respectively (JCPDS 20-0426)] than the α-phase Ga2O3. 47

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oxygen atmospheric conditions, and the results are shown in the Supporting Information in Figure S2. The TG indicated a gradual weight loss of 7.6% until 440 °C, which indicated the phase transformation (α-GaOOH to α-Ga2O3) during the calcination process. It indicates that the dehydroxylation process of GaOOH occurs, and α-Ga2O3 is formed via the reaction of GaOOH f 1 /2Ga2O3 + 1/2H2O, as shown in eq 3. The DTG results showed an endothermic peak at 432 °C, corresponding to the dehydration process shown in Figure S2 (Supporting Information). The thermal analysis result is consistent with earlier reports assigned for GaOOH transformation into Ga2O3.30 The formation of Ga2O3 has also been confirmed by EDX analysis, and the results are shown in Figure 4b. The EDX microanalysis demonstrates that the crystal consists of Ga and O elements, and the quantitative analysis confirmed that the Ga/O ratio nearly equals the stoichiometric ratio (2:3) of Ga2O3, as shown in Figure 4b, inset. The elemental mapping of both gallium and oxygen atoms in GOM is shown in Figure S3 (Supporting Information). These mappings showed a homogeneous distribution of both gallium and oxygen atoms in the microsphere. As we discussed earlier, the GaOOH formed at various hydrothermal temperatures possesses different morphologies, and therefore, it is interesting to investigate the stability of the GaOOH morphologies during the calcination process. The morphology of Ga2O3 formed at 450 °C for 3 h from the α-GaOOH, which were formed at three temperatures (175, 200, and 225 °C), is shown in Figure 5. The FE-SEM pictures in Figure 5B,C show that the morphology of α-Ga2O3 possesses the same microspheres and microrods as GaOOH morphologies that were formed at 200 and 225 °C, respectively. However, the GaOOH microflower morphology formed at 175 °C was changed into Ga2O3 microrods/plates, like the morphology in the calcination process. These results indicated that the GaOOH microflower is not stable during the dehydration process, and the reason for the unstable nature is not known. As we discussed earlier, the XRD results of the GaOOH prepared at three different temperatures possessed different peak intensities and were related to its morphological growth. Therefore, detailed research is necessary to understand the morphological changes and will be addressed in our future publications. Though various mechanisms have been proposed for the nano/microstructured gallium oxide growth, we have used a new synthetic methodology for the GaOOH preparation.8,30
32 Therefore, it is necessary to discuss the mechanism of various morphological GaOOH growths under different experimental conditions. Because we have not used any templates or surfactants for either the hydrothermal or the thermal decomposition process, the GaOOH and Ga2O3 could have been formed by a self-assembly process. As we discussed earlier, the hydrothermal decomposition of gallium oxalate results in GaOOH, as evident from the XRD results. Three different GaOOH morphologies were noted at three different hydrothermal temperatures, which could be due to the difference in the rate of gallium oxalate decomposition at a different hydrothermal temperature. As the gallium oxalate complex is completely decomposed into GaOOH, the initially formed nuclei growth could also be influenced by the hydrothermally generated pressure or temperature, as a result of different morphologies at different decomposition temperatures. Therefore, it is important to discuss the role of the hydrothermally generated pressure, temperature, and oxalic acid on the formation of different morphological GaOOH at different temperatures. Because the autoclave setup did not have a pressure monitoring

Figure 4. (a) XRD of α-Ga2O3 synthesized at 450 °C for 3 h by using α-GaOOH formed in a hydrothermal process at (A) 175, (B) 200, and (C) 225 °C for 10 h. (b) EDX analysis of α-Ga2O3 synthesized at 450 °C for 3 h by using α-GaOOH formed in a hydrothermal process at 200 °C for 10 h.

Moreover, microsphere growth at 110 directions could also be the reason for the appearance of high-intensity peaks at 36.1. The abovementioned speculation is in good agreement with the experimental results observed in Figure 1. Thus, at three different temperatures, the XRD peak intensities are varying, as shown in Figure 1, which may be due to the difference in their morphology (difference in crystal growth). Therefore, it is difficult to conclude the presence of a γ phase along with the α phase in Ga2O3 microspheres prepared at 450 °C. The phase of the gallium oxide formed from the α-GaOOH needs to be discussed. There is no consensus report available in the literature for the formation of various phases of Ga2O3 from GaOOH in the calcination process at different temperatures. For example, Huang et al. prepared β-Ga2O3 and γ-Ga2O3 from the calcinations of α-GaOOH at 750 and 500 °C, respectively.8 Yan et al. prepared α-Ga2O3 from α-GaOOH at 370 °C, and the phase transition (α-Ga2O3 to β-Ga2O3) occurred at 795 °C.9 However, the transformation of GaOOH to β-Ga2O3 was observed by Laubengayer et al. at temperatures higher than 300 °C.28 These discussions clearly indicated that further research is needed to understand the thermal conversion of GaOOH into various phases of Ga2O3, and such an investigation will be addressed in our future publications. To understand the thermal conversion of GaOOH to Ga2O3, the TG and DTG analysis of the former was studied under 48

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Figure 5. FE-SEM of α-Ga2O3 synthesized at 450 °C for 3 h by using α-GaOOH formed in a hydrothermal process at (A) 175, (B) 200, and (C) 225 °C for 10 h.

Scheme 1. Schematic Diagram of Various Morphological αGaOOH Formed in a Hydrothermal Process

facility, we were not able to measure the hydrothermally generated pressure at various temperatures. Moreover, controlled experiments showed that, in the absence of oxalic acid, the hydrolysis of gallium nitrate did not facilitate the microsphere morphology from 175 to 225 °C, as evident from the FE-SEM results presented in Figure S4 (Supporting Information). Similarly, when oxalic acid was replaced with sodium acetate in the hydrothermal process, it resulted in no microsphere morphology under the experimental conditions. The above-mentioned two experimental results confirmed the importance of oxalic acid on the formation of various morphological GaOOH. The formation of GaOOH in the hydrothermal decomposition of gallium oxalate and gallium nitrate hydrolysis (absence of oxalic acid) is shown in eqs 1 and 2. Tas et al. prepared GaOOH by the hydrolysis of gallium nitrate in the presence of urea and studied its transformation into Ga2O3.30 From eq 1, it is clearly understood that the gallium oxalate decomposition may generate CO and CO2 gases. 175 °C

Ga2 ðC2 O4 Þ3ðaqÞ þ H2 O f 2GaOOHðsÞ þ 2CO2 v þ 4CO v 175 °C

Ga3þ þ 2H2 O f GaOOH þ 3Hþ 450 °C

2GaOOH f Ga2 O3 þ H2 O

ð1Þ Moreover, at the same time, the formed CO and CO2 gases can act as a template for the microsphere formation, as shown in Scheme 1. A similar microsphere self-assembly was reported in hydrothermal as well as in thermal decomposition processes.5,33 The next question we should answer is why three different morphologies are formed at three different temperatures. As we discussed earlier, the microflowers and microspheres possess a hollow structure, which implies

ð2Þ ð3Þ

Because of its high surface energy, the initially formed nuclei may assemble into aggregates under the thermal energy. 49

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Figure 7. Nitrogen adsorption
desorption analysis of GOM prepared at 450 °C for 10 h, and inset shows corresponding pore size distribution.

particles (inset in Figure 6A). We have also noted that the nanoparticles on the microsphere surface could grow like rods, as shown in Figure 6B. The microrod image showed a very sharp end and a smooth surface structure, as shown in Figure 6C. The lattice fringes image shown in Figure 6D confirmed that the particles are assembled by an oriented attachment, resulting in well-crystallized, defect-free gallium oxide. Figure 7 shows a typical nitrogen adsorption
desorption isotherm of gallium oxide. The isotherm displays the typical type IV curve, which is usually attributed to the predominance of the presence of mesoporosity. Though the isotherm results indicated the presence of mesoporosity, no clear evidence can be obtained from HR-TEM lattice fringes. Obviously, the FE-SEM and TEM pictures in Figures 2B and 6A show that the porous structure on the microsphere surface was formed between the particles. The BET surface area, pore size, and pore volume of the three synthesized Ga2O3 are presented in Table 1. As can be seen in Table 1, the microspheres possess a high surface area (61 m2/g) and a higher pore volume (0.193 cm3/g), followed by microplates/rods (46 m2/g) and microrods (10 m2/g). The pore size distribution was found to be from 4.7 to 12.3 nm in all three gallium oxides. The surface area of the Ga2O3 microspheres was found to be higher than that of nano/microsized Ga2O3 reported in the literature.9,11 Therefore, this synthetic methodology provides an opportunity to fabricate high-surface-area and porous surface structure gallium oxides. The narrow pore size distribution (inset of Figure 7) indicated a uniform pore size distribution on the microspheres' surface. Therefore, this synthetic methodology opens a door to a synthesis of precisely controlled surface structured α-Ga2O3 preparation. The diffuse reflectance spectra (DRS) of three α-Ga2O3 prepared at 450 °C for 3 h is shown in Figure 8. As expected, the synthesized gallium oxides showed absorption in the UV region. A small difference in the absorbance is due to the difference in their morphology and size. As we discussed earlier, we have noted microplates/rods, microspheres, and microrods α-Ga2O3, and the corresponding onset absorption wavelength and corresponding band gap were found to be 302 nm (4.1 eV), 292 nm (4.24 eV), and 304 nm (4. 07 eV), respectively. Earlier studies have also indicated that the surface properties of a photocatalyst strongly influenced the photocatalytic activity.34,35 Since the synthesized gallium oxide showed different surface properties and morphologies, it would be interesting to evaluate its photocatalytic activity. Fu et al. reported that the α-Ga2O3

Figure 6. HR-TEM images and lattice fringes of α-Ga2O3 synthesized at 450 °C for 3 h by using GaOOH formed in a hydrothermal process at 200 °C for 10 h.

that both morphologies could have been formed under similar mechanisms (CO and CO2 gas as template). However, due to the difference in the thermal energy and pressure at 175 and 200 °C, the self-assembly of initially formed GaOOH nanoparticles may proceed at a different growth rate, resulting in two different morphologies. The XRD pattern also supported the above speculated mechanism, which showed different intensities of the peaks, as shown in Figure 1. However, at 225 °C, the decomposition may proceed rapidly, leading to a fast growth of microrods at (100) directions, as evident from the highest-intensity peaks noted in the XRD pattern. The formation of Ga2O3 in the thermal decomposition process was already discussed, and the morphologies were not affected during the GaOOH-to-Ga2O3 conversion, except for the microflower morphology. However, the microflower morphology formed at 175 °C changed its morphology during the calcination process, and the reason for such changes is not understood clearly. We speculated that the unstable nature of the microflower morphology could be the reason for its morphology changes during the calcination process, as evident from the hydrothermal experimental results. Thus, the temperaturedependent GaOOH morphology indicated that the microrod morphology was formed at a higher temperature (225 °C), and these could have been formed via a microflower and microsphere intermediate morphology. To release the surface energy, the microflower changes into a microrod during the calcination process. These results indicated that the microrod is a highly stable/preferred morphology for the gallium oxide. However, we are not able to explain why the microsphere morphology is not changed during the calcination process, whereas the microflower morphology is. Thus, a multifold investigation is necessary and will be addressed in our future publications. The HR-TEM image of GOM is shown in Figure 6. The HR-TEM images in Figure 6A clearly show that the microsphere is composed of nanoparticles and that a porous surface structure is formed in between the 50

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Table 1. Band Gap and Surface Properties of α-Ga2O3 Formed at 450 °C for 3 h from GaOOH, Which Were Formed in a Hydrothermal Process at Various Temperatures hydrothermal temperature (used for GaOOH preparation) (°C)

calculated band gap (eV)

BET surface area (m2/g)

pore size (nm)

pore volume (cm3/g)

175 200

4.1 4.24

46 62

4.7 12.3

0.054 0.193

225

4.07

10

9.3

0.022

Figure 9. Degradation of AO7 dye in the presence GOM and TiO2 photocatalysts under different experimental conditions. [Photocatalysts] = 1 g/L, [AO7] = 10 mg/L, temperature = 25 °C. (A) Direct photolysis in the absence of photocatalysts. (B) α-Ga2O3 prepared at 450 °C for 3 h by using the α-GaOOH prepared at 175 °C for 10 h. (C) α-Ga2O3 prepared at 450 °C for 3 h by using the α-GaOOH prepared at 200 °C for 10 h. (D) α-Ga2O3 prepared at 450 °C for 3 h by using the α-GaOOH prepared at 225 °C for 10 h. (E) In the presence of TiO2 photocatalyst under UV light irradiation.

Figure 8. Diffuse reflectance spectra of α-Ga2O3 prepared at 450 °C for 3 h by using the α-GaOOH prepared at different hydrothermal temperatures: (A) 175, (B) 200, and (C) 225 °C for 10 h.

photocatalyst is more efficient than β and γ forms of Ga2O3.12 As we discussed earlier, all the synthesized Ga2O3 possesses an α form, which is expected to yield a higher photocatalytic activity. We have examined the photocatalytic activity of the synthesized αGa2O3 photocatalysts under UV light irradiation (λ = 254 nm). The photocatalytic activity was evaluated by using Acid Orange 7 (AO7) dye degradation and Cr(VI) reduction as model pollutants, and the results were compared with TiO2. Figures 9 and 10 depict AO7 dye and Cr(VI) normalized absorbance (A/A0) against irradiation time, respectively. Here, A and A0 denote absorbance after irradiation time and at initial time, respectively. Initially, various controlled experiments have been performed in the absence of photocatalysts and light sources. The results clearly showed that AO7 (10 mg/L) dye was susceptible to direct photolysis under UV light irradiation and about 86.7% degradation was noted after 60 min. These results indicated that UV light energy is enough to degrade AO7 dye. In the absence of light irradiation, a small percentage of dye removal was observed, which was due to the adsorption of dye molecules on the αGa2O3 microspheres' surface. However, in the Cr(VI) reduction process, UV direct photolysis had no effect and about 15% removal was noted in the absence of light irradiation, which was due to the adsorption on the photocatalytic surfaces. The difference in the direct photolysis effect of AO7 dye and Cr(VI) is mainly due to the difference in their decomposition mechanism. Thus, photogenerated electrons are involved in the Cr(VI) reduction process in the photocatalyzed process, whereas in the photolysis (absence of photocatalysts), such electron generation is not possible. Therefore, no Cr(VI) reduction is observed under UV light irradiation. The photocatalytic activity of the α-Ga2O3 microplates, microspheres (GOM), and microrods showed 62, 91.2, and 53% of AO7 degradation in 15 min,

Figure 10. Photocatalytic reduction of Cr(VI) in the presence of GOM and TiO2 photocatalysts under different experimental conditions. [Photocatalysts] = 1 g/L, [Cr(VI)] = 10 mg/L, temperature = 25 °C. (A) In the presence of GOM photocatalysts and absence of UV light. (B) In the presence of GOM photocatalysts and UV light. (C) In the presence of GOM photocatalysts, UV light, and 14 mg/L of oxalic acid. (D) In the presence of GOM photocatalysts, UV light, and 28 mg/L of oxalic acid. (E) In the presence of TiO2 photocatalysts, UV light, and 28 mg/L of oxalic acid.

respectively. These results confirmed that the synthesized microsphere is highly efficient, which may be due to its high surface area, as indicated in Table 1. Similar results were also reported in the literature.24 We have compared the efficiency of GOM 51

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photocatalysts with Degussa TiO2. TiO2 photocatalysts yield about 64.6% of AO7 dye degradation in 15 min, and however, at the same time, GOM microsphere photocatalysts showed 91.2% removal. These results imply that the GOM is 1.4 times more efficient than TiO2. Similar results were also noted by Hua et al. for the decomposition of benzene under UV light irradiation.22 We have also compared the efficiency of both GOM and TiO2 for the Cr(VI) reduction under UV light irradiation. Figure 10 shows Cr(VI) reduction under various experimental conditions. The photocatalytic reduction of Cr(VI) to Cr(III) is mainly facilitated by photoexcited electrons, and therefore, better electron
hole separation can be achieved by using oxalic acid as a hole (h+) scavenger in the reduction process.36,37 Therefore, photocatalytic Cr(VI) reduction was studied in the presence, as well as in the absence, of oxalic acid as a hole scavenger.30 About 66.8, 75.4, and 82.4% of Cr(VI) reduction was noted in the absence and presence of 14 and 28 mg/L of oxalic acid at 60 min, respectively. As expected, the presence of a hole scavenger substantially enhances the reduction process, due to better electron
hole separation.36,37 However, the optimum amount of hole scavenger on the reduction process was not studied. We have compared the photocatalytic Cr(VI) reduction efficiency of GOM with TiO2 in the presence of 28 mg/L of oxalic acid as a hole scavenger. Thus, TiO2 showed 63.9% of Cr(VI) reduction, whereas at the same time, 82.4% reduction was noted in α-Ga2O3 photocatalysts, as shown in Figure 10. These results showed that the synthesized GOM photocatalyst is more efficient than TiO2. Similar results were also reported in the literature.38 We have also examined the reusability of GOM photocatalysts in three consecutive cycles by using AO7 as a model pollutant. The entire catalytic reusability test has been investigated under identical reaction conditions. The results indicated (not presented in the figure) that the GOM photocatalyst retains its photocatalytic activity in three consecutive cycles. These results imply that the GOM photocatalyst can be used in the long term in environmental remediation processes. In conclusion, the synthesized α-Ga2O3 is a potential candidate for photocatalyzed environmental remediation processes.

gallium oxide is a potential candidate for environmental remediation processes.

’ ASSOCIATED CONTENT

bS

Supporting Information. The FE-SEM of GaOOH prepared under different experimental conditions, thermal analysis, and elemental mapping of Ga2O3 are presented. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected], mmuruganandham@ yahoo.com. Phone: +1 215 204 7802. Fax: +1 215 204 0622.

’ ACKNOWLEDGMENT EU Transfer of Knowledge Marie Curie grants MKTD-CT2006-042637 and partial support from the Water and Environmental Technology (WET) Center, Temple University, are acknowledged for financial support of the study. ’ REFERENCES (1) Chen, X.; Shen, S.; Guo, L.; Mao, S. S. Chem. Rev. 2010, 110, 6503–6570. (2) Maeda, K.; Domen, K. J. Phys. Chem. C 2007, 111, 7851–7861. (3) Muruganandham, M.; Kusumoto, Y. J. Phys. Chem. C 2009, 113, 16144–16150. (4) Muruganandham, M.; Kusumoto, Y.; Okamoto, C.; Amutha, M.; Abdulla-Al-Mamun, Md.; Bashir, A. J. Phys. Chem. C 2009, 113, 19506–19517. (5) Binet, L.; Gourier, D. J. Phys. Chem. Solids 1998, 59, 1241–1249. (6) Sharma, S.; Sunkara, M. K. J. Am. Chem. Soc. 2002, 123, 12288–12293. (7) Qian, H.-S.; Gunawan, P.; Zhang, Y.-X.; Lin, G.-F.; Zheng, J.-W.; Xu, R. Cryst. Growth Des. 2008, 8, 1282–1287. (8) Huang, C.-C.; Yeh, C.-S. New. J. Chem. 2010, 34, 103–107. (9) Yan, S.; Wan, L.; Li, Z.; Zhoua, Y.; Zou, Z. Chem. Commun. 2010, 46, 6388–6390. (10) Xiang, X.; Cao, C. B.; Zhu, H. S. J. Cryst. Growth 2005, 279, 122–128. (11) Hou, Y.; Zhang, J.; Ding, Z.; Wu, L. Powder Technol. 2010, 203, 440–446. (12) Hou, Y.; Wu, L.; Wang, X.; Ding, Z.; Li, Z.; Fu, X. J. Catal. 2005, 250, 12–18. (13) Michorczyk, P.; Ogonowski, J. Appl. Catal., A 2003, 251, 425–433. (14) Dai, Z. R.; Pan, Z. W.; Wang, Z. L. J. Phys. Chem. B 2002, 106, 902–904. (15) Gundiah, G.; Govindaraj, A.; Rao, C. N. R. Chem. Phys. Lett. 2002, 351, 189–194. (16) Chun, H. J.; Choi, Y. S.; Bae, S. Y.; Seo, H. W.; Hong, S. J.; Park, J.; Yang, H. J. Phys. Chem. B 2003, 10, 9042–9046. (17) Yada, M.; Ohya, M.; Machida, M.; Kijima, T. Langmuir 2000, 16, 4752–4755. (18) Sun, X.; Li, Y. Angew. Chem., Int. Ed. 2004, 43, 3827–3831. (19) Botterhuis, N. E.; Sun, Q. Y.; Magusin, P.; van Sante, A.; Sommerdijk, N. Chem.—Eur. J. 2006, 12, 1448–1456. (20) Wang, H. N.; Wang, Y. H.; Zhou, X. F.; Zhou, L.; Tang, J. W.; Lei, J.; Yu, C. Z. Adv. Funct. Mater. 2007, 17, 613–617. (21) Amutha, R.; Akilandeswari, S.; Ahmmad, B.; Muruganandham, M.; Sillanp€a€a, M. J. Nanosci. Nanotechnol. 2010, 10, 8438–8447. (22) Hou, Y.; Wang, X.; Ding, Z.; Fu, X. Environ. Sci. Technol. 2006, 40, 5799–5803.

4. CONCLUSIONS We have successfully fabricated microflowers, microspheres, and microrods morphological α-GaOOH without using any templates in a hydrothermal process. The hydrothermal experimental conditions strongly influenced the morphology of GaOOH. The influence of oxalic acid concentration, hydrothermal temperature, and time on the morphology was investigated. The oxalic acid played a major role in the formation of various microstructured GaOOH. The XRD pattern indicated the formation of wellcrystallized α-GaOOH. The α-Ga2O3 was prepared by calcinations of α-GaOOH at 450 °C without changing its microsphere and microrod morphology. However, the calcination process induced the α-GaOOH microflower morphology into an α-Ga2O3 microplate-like morphology. The FE-SEM images showed that the microflowers and microspheres possess hollow structures and are self-assembled by nanoplates and nanoparticles, respectively. The nitrogen adsorption analysis indicated the presence of a macroporous surface structure. The photocatalytic degradation experiments showed that the synthesized α-Ga2O3 microsphere is more efficient than microplates and microrods. The microsphere is 1.4 times more efficient than TiO2 in AO7 dye degradation and Cr(VI) reduction. These results indicated that the synthesized 52

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