Article pubs.acs.org/JPCC
Structure, Property, and Function of Gallium/Urea and Gallium/ Polyethylene Glycol Composites and Their Sintering Products: β- and γ‑Gallium Oxide Nanocrystals Le Xin Song,*,†,‡ Yue Teng,† and Jie Chen† †
Department of Chemistry, University of Science and Technology of China, Hefei 230026, P. R. China State Key Laboratory of Coordination Chemistry, Nanjing University, Nanjing 210093, P. R. China
‡
S Supporting Information *
ABSTRACT: The first part of the study is devoted to the comparison between the doping effect of urea (a small molecule) and polyethylene glycol (PEG, a long-chain polymer) on the physical property of metallic gallium (Ga). The physical properties of the Ga composited in the two materials, Ga/urea and Ga/PEG, were investigated by scanning electron microscopy, X-ray powder diffraction, X-ray photoelectron spectroscopy, differential scanning calorimetry, superconducting quantum interference device, and surface-enhanced Raman scattering spectra and compared with our previous results for the effect of macrocyclic hosts (e.g., cyclodextrins, calixarenes) on the physical modification of metallic Ga. Our data provide new direct evidence that the modification of physical properties of Ga is highly dependent on the nature of dopants used. For example, the addition of a small amount of urea causes a fundamental change in the crystallization behavior of Ga, and the presence of PEG results in the occurrence of a weak paramagnetism of Ga at high fields, both of which are completely different from the effect of other dopants. The other part of the study is devoted to demonstrating whether there is a significant difference in the oxidation process of metallic Ga and its composites. Our result gives a strong positive answer to the question. β- and γ-gallium oxide nanocrystals were obtained by sintering the Ga/urea composite at different temperatures and exhibited distinctive photoluminescence and photocatalysis properties. These results gave a strong impression that the introduction of different dopants leads metallic Ga to generate different features in microstructure, physical property, and especially chemical reactivity. We believe that the findings of this study have important implications for the development of inorganic materials.
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INTRODUCTION Recently, we found that the presence of a small amount of macrocyclic host molecules such as cyclodextrins (CDs) and calixarenes (CAs) resulted in a characteristic modification of metallic gallium (Ga), showing distinctive surface features, complex phase transitions, and unusual magnetic transformation.1,2 One of our major intentions was to investigate the interaction between organic hosts and metallic atoms. The encouraging experimental results of our research make us ask a question whether only the macrocyclic host molecules can produce significant effects on metallic Ga. This prompts us to design a feasibility study, aiming to determine whether such a characteristic modification of Ga is achievable in the presence of small molecules or long-chain polymers.3,4 Therefore, urea and polyethylene glycol 4000 (PEG) were used as dopants in this work. One of our present aims is to examine whether there is any evidence for our hypothesis that metallic Ga shows different responses to different dopants. Gallium oxide (Ga2O3) has five kinds of crystalline phases, that is, α-, β-, γ-, δ-, and ε-phases.5,6 The two polar phases, the β- and γ-phases, are particularly interesting in applications © 2012 American Chemical Society
because they have excellent pyroelectric, piezoelectric, and ferroelectric properties.7,8 In particular, the β-phase is a very important wide-band gap material (band gap, 4.9 eV) with potential applications in many fields such as optoelectronic and gas sensing devices.9−12 There are many reports on the preparation of β- and γ-Ga2O3 crystals,13,14 but most of them seem to lie on the use of Ga salts such as gallium nitrate, gallium chloride, gallium hydroxide, and gallium arsenide in the presence of solvents.15−17 Although metallic Ga can be oxidized very rapidly, its surface will be soon covered with gallium oxides in a few minutes in contact with the atmosphere, which prevents further oxidation of Ga even at 1073 K in air.18 Therefore, the other of our present aims is to investigate whether the presence of dopants influences the oxidation of Ga. In order to reach the two objectives of the study, we initially prepared two Ga materials, Ga/urea and Ga/PEG, using acetone as a dispersant. Subsequently, the materials are Received: June 27, 2012 Revised: September 10, 2012 Published: October 9, 2012 22859
dx.doi.org/10.1021/jp306318u | J. Phys. Chem. C 2012, 116, 22859−22866
The Journal of Physical Chemistry C
Article
for 5 h. Finally, the solutions were collected for measurements of UV−vis spectroscopy with a wavelength range from 190 to 900 nm to determine the content of dyes in the solutions. Instruments and Methods. X-ray powder diffraction (XRD) patterns of samples were obtained with the Philips X'Pert Pro X-ray diffractometer with monochromatized Cu Kα irradiation (λ = 1.542 Å) at 40 kV and 40 mA and analyzed in the range of 10° ≤ 2θ ≤ 90°. A JEOL JSM-6700F fieldemission scanning electron microscope (FE-SEM) operated at 5 kV was used to examine the morphology of the Ga samples. X-ray photoelectron spectra (XPS) were recorded on an ESCALAB250 spectrometer, with Al excitation radiation (1486.6 eV) under ultrahigh vacuum conditions (2.00 × 10−9 Torr). To compensate for the surface charging effect, all the values of binding energy were referenced to the C1s neutral carbon peak at 284.6 eV, with an energy resolution of 0.16 eV. Magnetic measurements were carried out with a fully automated magnetometer (MPMS-7XL from Quantum Design) with an ultrasensitive superconducting quantum interference device (SQUID) with utilization of a vibrating sample magnetometer. Photoluminescence (PL) measurements were performed on a Perkin-Elmer Luminescence spectrometer L550B at room temperature (excited at 325 nm, filter at 380 nm). Absorption spectra were recorded on a Shimadzu UV 2401-(PC) spectrometer at room temperature over the wavelength range from 190 to 800 nm, using quartz cells with a 1 cm optical path. Surface-enhanced Raman scattering (SERS) detection of the Ga samples was acquired by a LABRAM-HR Confocal Laser Micro Raman spectrometer operated with a 514.5 nm laser excitation in the range 100−2000 cm−1 and a laser power of 2.5 mW at the sample for an exposure time of 10 s at room temperature, with a resolution of 0.6 cm−1. R6G and CV were used as probe molecules. The Ga samples were dispersed on a silica substrate (1 cm × 1 cm) after suspended in alcohol. Then, the R6G and CV reagents were dissolved into pure water (10−3 M), and 20 μL of the probe solution was dropped onto the silica substrate.
characterized by a series of techniques. Our results support the hypothesis that different dopants may cause different changes in physical properties of Ga. Then, the materials were sintered at different temperatures. β- and γ-Ga2O3 nanocrystals were obtained by sintering the Ga/urea material at relatively low temperatures. This provides evidence that the existence of dopants can effectively promote the oxidation of Ga. Finally, we further evaluated the photoluminescence (PL) and photocatalytic properties of the β- and γ-Ga2O3 nanocrystals. In short, the work not only strengthens our earlier argument that physical properties of Ga can be modified by the addition of a small amount of dopants but also provides new insights into the role of dopants in improving the oxidation reaction of Ga. We believe that the findings can broaden the scope of supramolecular chemistry19 and may contribute to the design and preparation of metallic oxide nanoparticles.
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EXPERIMENTAL SECTION Materials. Urea and PEG were purchased from Shanghai Chemical Reagent Company. Metallic Ga and anhydrous ethanol were obtained from Sinopharm Chemical Reagent Company. Acetone was supplied by Yangzhou Hubao Chemical Reagent Company and dried with anhydrous sodium sulfate before use. Crystal violet (CV), Congo red, methyl orange, methylene blue, and rhodamine B were from Aladdin Chemistry Co. Ltd. Rhodamine 6G (R6G) was purchased from Sigma-Aldrich. All other chemicals were of general-purpose reagent grade unless otherwise stated. Preparation of Solid Ga Materials. Two Ga materials were prepared according to the following procedure. Metallic Ga (500 mg, 7.17 mmol) was added to 10 mL of acetone in the presence of PEG (50 mg, 0.013 mmol) in a round-bottom flask, followed by vigorous stirring at 320 K for 5 h. After acetone was evaporated in air at room temperature, the Ga material containing PEG was obtained and marked as Ga/PEG. Ga/ urea was obtained in the same way with a 10:1 mass ratio of Ga (500 mg, 7.17 mmol) to urea (50 mg, 0.83 mmol). The control sample of metallic Ga was obtained with the treatment of acetone under the same conditions. Sintering Experiments. A Ga/urea composite with 1:100 mass ratio of Ga (0.055 g, 0.79 mmol) to urea (5.545 g, 92.3 mmol) was prepared with the same method as mentioned above. The composite (5.6 g) was sintered at different temperatures673, 873, and 1073 Kand different atmospheresin a muffle furnace in air and in a tube furnace (Nabertherm, M7/11, with a program controller) in nitrogen. Then, the sintering products were cooled down to room temperature in a desiccator over P2O5 and weighed. Finally, white Ga2O3 powders formed. The amounts of the Ga2O3 obtained are 74 mg at 673 K in air, 73 mg at 773 K in air, 75 mg at 873 K in air, 74 mg 873 K in nitrogen, 77 mg at 1073 K in air, and 76 mg at 1073 K in nitrogen. However, sintering the Ga/PEG composite with a 1:100 mass ratio of Ga to PEG until 873 K did not lead to the formation of Ga2O3 in air. Even at 1073 K, the yield of the Ga2O3 obtained in air is only 11.2% (6 mg). Preparation of Photocatalysis Samples. Initially, the dye solutions (25 mL, 6.0 mg·L−1) of Congo red, methyl orange, methylene blue, and rhodamine B were prepared in distilled water. Subsequently, Ga2O3 (5.0 mg, 0.072 mmol) was added to the dye solutions and magnetically stirred in the dark for 30 min to reach equilibrated adsorption. Then, the solutions were placed in three-necked flasks and irradiated under visible light
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RESULTS AND DISCUSSION Surface Features and Stacking Structures of the Ga/ Urea and Ga/PEG Materials. The FE-SEM image of the pure Ga material shows that it has a uniform spherical structure, with typical diameters ranging from 2 to 7 μm.1 However, the Ga/ urea and Ga/PEG composites exhibit a nanoporous honeycomb pattern with an average cavity diameter of 20−50 nm and a spongy structure, respectively. Interestingly, in both cases, no microspheres were observed. These figures, taken together with our previous findings,1,2 allow us to give a complete description of the importance of dopants and their influence on the size and shape of Ga materials. That is to say, the surface features of Ga composites are directly dependent on the addition of a small amount of dopants. Figure 2 displays the XRD patterns of the metallic Ga and its two composites: Ga/urea and Ga/PEG. All three Ga materials can be indexed to an orthorhombic α-Ga phase (JCPDS 892735), but each of them has a distinctive characteristic close packed plane (CCPP). For example, the CCPPs of Ga in the metallic Ga, Ga/urea, and Ga/PEG are 131 (45.2°), 131 (45.2°), and 221 (51.6°), respectively. This result indicates that the introduction of a small amount of dopants has caused a change in stacking properties of the Ga atoms in the materials. From this figure, we observed that the crystal growth of the 22860
dx.doi.org/10.1021/jp306318u | J. Phys. Chem. C 2012, 116, 22859−22866
The Journal of Physical Chemistry C
Article
Figure 3. DSC curves of metallic Ga, Ga/urea, and Ga/PEG. Figure 1. FE-SEM images of Ga/urea (a) and Ga/PEG (b).
258.4, 239.0, and 246.2 K corresponding to β-, γ-, and εphases.22−24 Especially, the α-phase of Ga has shifted to higher temperatures at 304.7 in the Ga/PEG and 306.2 in the Ga/ urea. The endothermic enthalpies of the phase transitions are a reflection of contents of different phases of Ga in the materials. We notice that the content of α-Ga in metallic Ga (105.9 J·g−1) decreases slightly in the Ga/PEG (95.0 J·g−1) but markedly in the Ga/urea (26.5 J·g−1). Instead, the content of β-Ga in the Ga/urea (20.7 J·g−1), which is comparable to that of α-Ga, is significantly higher than in the Ga/PEG (3.3 J·g−1). It is important to mention that the ε-Ga phase occurs in Ga/urea but does not exist in metallic Ga and Ga/PEG. This may be because urea molecules, like β-CD25−27 and CA-6,2,28 can act as a host due to the formation of an ordered channel with a diameter of over 0.7 nm to allow the penetration of Ga atoms (diameter 0.36 nm).2,20,29 The present results further support the fact that different dopants may produce different effects on the phase transition property of metallic Ga.1,2 The content increase in the metastable phases and the content decrease in the stable phase of Ga should be an important property, because a metastable phase always has a higher free energy than its stable counterpart, leading to increased activity.30,31 Although limited data are available describing the relation between the phase transition behavior of Ga and the nature of dopants, our data suggest that the phase transition process of Ga can be controlled by the introduction of a small amount of dopants. Magnetic Properties of the Ga/Urea and Ga/PEG Materials. The magnetization (M, emu·g−1) curves of metallic Ga and its two composites at 4.5 K are shown in Figure 4. All three Ga materials exhibit superconductor properties at this temperature,32 but their magnetic hysteresis loops have completely different parameters. First, the metallic Ga exhibits very low remanence (Mr, 5.4 × 10−5 emu·g−1) and relatively high coercivity (Hc, 121 Oe). On the contrary, the Ga/urea and Ga/PEG present relatively high Mr (>5.0 × 10−3 emu·g−1) and very low Hc (
