DOI: 10.1021/cg901420q
From β-Phase Particle to r-Phase Hexagonal-Platelet Superstructure over AgGaO2: Phase Transformation, Formation Mechanism of Morphology, and Photocatalytic Properties
2010, Vol. 10 2921–2927
Shuxin Ouyang,†,‡,§ Di Chen,§ Defa Wang,§ Zhaosheng Li,†,‡ Jinhua Ye,*,§,# and Zhigang Zou*,† † Ecomaterials and Renewable Energy Research Center (ERERC), Department of Physics, and National Laboratory of Solid State Microstructures, Nanjing University, 22 Hankou Road, Nanjing 210093, P. R. China, ‡Department of Materials Science and Engineering, Nanjing University, 22 Hankou Road, Nanjing 210093, P. R. China, §Photocatalytic Material Center (PCMC), National Institute for Materials Science (NIMS), 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan, and #International Center for Materials Nanoarchitectonics, National Institute for Materials Science (NIMS), 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan
Received November 14, 2009; Revised Manuscript Received March 23, 2010
ABSTRACT: We studied an orthorhombic-to-rhombohedral transformation of AgGaO2, which was performed in water at low temperature and accompanied by morphology variation, by using powder X-ray diffraction, UV-visible absorption spectroscopy, scanning electron microscopy (SEM), transmission electron microscopy, and photocatalytic 2-propanol oxidization. The experimental results revealed that the crystal growth of the β-phase (orthorhombic AgGaO2) to R-phase (rhombohedral AgGaO2) in water could be divided into phase-transformation, transition, and mature periods. The effects of water and temperature on the phase transformation and crystal-structure correction were clarified. It is found that the morphology of the R-phase sample is a superstructure, the shape of which is influenced by the phase-transformation temperature. The morphology variation of β-phase particle to R-phase hexagonal platelet is a solid-liquid-solid growth process via the time-line based SEM images. The formation mechanism of this superstructure may be attributed to the growing-space competition.
*To whom correspondence should be addressed. E-mail: jinhua.ye@ nims.go.jp (J.Y.);
[email protected] (Z.Z.).
not the phase-transformation process. The phase transformation depended on both temperature and water, which distinguishes this phase transformation from traditional ones and therefore motivates us to perform an in-depth study. The most appropriate technique for the study of traditional phase transformation, thermogravimetric-differential thermal analysis (TG-DTA), is inapplicable to studying such a phase transformation performed in water. In this study, powder X-ray diffraction (XRD), UV-visible absorption spectrum, and even photocatalytic properties were used to clarify the reaction course of β-AgGaO2 immersed in water. Most former investigations on photocatalysis were aimed at analyzing catalytic mechanisms20-23 or developing new materials for potential practical applications.24-30 However, the photocatalytic reaction was attempted to assist the study of phase transformation for the first time, because the chemical behavior for IPA photocatalytic oxidization could reflect the crystallization condition of samples. The results of experiments help to understand the effects of water and temperature in different reaction periods. Furthermore, the phase transformation of β-AgGaO2 to R-AgGaO2 is found to accompany morphology variation besides a change of crystal structure (as shown in Figure 1). Usual particles of β-phase finally become a superstructure with a unit of hexagonal platelet. Further investigations revealed that the shape of the unit of superstructure was influenced by the phase-transformation temperature. Although many multimetal-oxide superstructures with various units, such as nanoplatelets,31,32 nanorods,33-35 and nanotubes,36 have been prepared, reports on superstructure with hexagonal platelets are few. Time-line based scanning electron microscopy (SEM) images were employed for exploring the crystal growth process of β-phase particle to R-phase
r 2010 American Chemical Society
Published on Web 06/08/2010
1. Introduction ABO2 compounds mainly belong to a rhombohedral system with the space group R3hm (delafossite structure) or an orthorhombic system with the space group Pna21 (cristobaliterelated structure).1,2 Since Kawazoe and co-workers prepared delafossite CuAlO2 film with both high optical transmissivity in the visible region and good p-type conductivity of 0.95 S/cm,3 delafossite ABO2 oxides, especially Cu- and Ag-based materials, have attracted intensive interest owing to the application as transparent conducting oxides.4-7 Recently, photocatalytic investigations have been involved in Cu- and Ag-based delafossite oxides, such as CuMO2 (M = Cr, La, Y),8-10 AgMO2 (M=Al, Ga, In),11-13 and AgLi1/3M2/3O2 (M=Ti, Sn).14 The studies of ABO2 oxides with cristobalite-related structure were first focused on their synthesis and crystal structures.2,15,16 Very recently, electronic structures and photocatalytic properties of Ag-based ABO2 oxides, such as AgAlO217 and Ag2ZnGeO4 [Ag(ZnGe)1/2O2],18 have attracted much attention. In 2006, Vanaja and co-workers reported that β-AgGaO2 (cristobalite-related structure) could be converted to R-AgGaO2 (delafossite structure) by hydrothermal treatment at 250 °C for four days. The obtained R-AgGaO2 sample was deposited on silicon and Al2O3 substrates by pulsed laser deposition and then the property of electrical conductivity was measured.19 In the same year, Maruyama et al. found that this phase transformation could take place in water at 70 °C under normal atmosphere, and studied the electronic structures and photocatalytic properties of R-AgGaO2 and β-AgGaO2.11 Both investigations were focused on the properties of material but
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Figure 1. Variations in crystal structure and morphology during the phase transformation from β-AgGaO2 to R-AgGaO2.
hexagonal-platelet superstructure. A suggested formation mechanism for such superstructure was proposed here. 2. Experimental Section Material Preparation. β-AgGaO2 was synthesized via a cationexchange method of treating NaGaO2 with molten AgNO3. The precursor, NaGaO2, was prepared using a solid-state reaction. A stoichiometric mixture of Ga2O3 (99.99%) and Na2CO3 (99.5%) was calcined at 900 °C for 12 h. Then, a cation-exchange reaction was carried out by heating the mixture of NaGaO2, AgNO3 (99.8%), and KNO3 (99.0%), with a molar ratio of 1.00: 1.03: 1.00, at 180-200 °C for 20 h. The product was washed repeatedly with distilled water to remove NaNO3, KNO3, and excess AgNO3. Finally, the β-AgGaO2 sample was dried in air at room temperature. R-AgGaO2 was prepared by a phase transformation from β-AgGaO2 in an aqueous environment. The phase-transformation reactions were performed at room temperature (22 °C), 28 °C, and 50 °C. Sample Characterization. Structural features and phase compositions of the samples were determined using powder XRD (RINT; Rigaku Corp., Japan) with Cu KR1 radiation. The diffuse reflectance spectra of the samples were recorded on a UV-visible spectrophotometer (UV-2500PC; Shimadzu Corp., Japan) with barium sulfate as the reference. Then the absorption spectra were obtained from the reflectance spectra by means of Kubelka-Munk transformations. The Brunauer-Emmett-Teller (BET) surface areas were measured via nitrogen physisorption (Gemini2360; Shimadzu Corp., Japan). A field emission scanning electron microscope (JSM 6500F; JEOL Corp., Japan) and a high-resolution transmission electron microscope (JEM 2010; JEOL Corp., Japan; operated at 200 kV) were employed for morphology observation. Activity Evaluation. A 300-W Xe arc lamp (7 A imported current, focused through a 45 mm 45 mm shutter window) was used as the light source of the photocatalytic reaction. The light beam was passed through a set of glass filters (HA30 þ U390 þ L42, 400 nm< λ < 530 nm) and a water filter (removing the infrared ray irradiation) before reaching the reactor. The reactor volume was 500 mL; it was equipped with a pyrex-glass lid as a window. Under such conditions, the incident light intensity was about 0.9 mW/cm2. The light intensities during the photocatalytic reaction were measured using a spectroradiometer (USR-40; Ushio Inc., Japan). The light intensity data were collected from 200 to 800 nm. The 400 mg of a sample was spread uniformly in an 8.5 cm2-plate that was located in the bottom of the reactor. Then the reactor was pretreated by artificial air [V(N2)/V(O2)=4:1] for 5 min to remove adsorbed gaseous impurities. The IPA was injected into the reactor to produce a concentration of 300-400 ppm. Before the irradiation, the reactor was
Figure 2. Thermal stabilities of R-AgGaO2 (a) and β-AgGaO2 (b). Metallic Ag (b) and Ga2O3 (1). kept in the dark for some time until ensuring an adsorption-desorption equilibrium of gaseous reactants on the sample. The concentrations of IPA and acetone were detected on a gas chromatograph (GC14B; Shimadzu Corp., Japan) with an FID detector (Details: Porapak Q and PEG1000 column; temperatures-injection port, 120 °C; column, 60 °C; detection, 200 °C. Maximum error is about 7%).
3. Results and Discussion 3.1. Phase Transformation. Key Factor for Phase Transformation. In Vanaja and coauthors’ work, the phase transformation of β-AgGaO2 converting to R-AgGaO2 was carried out using a hydrothermal reaction (in a Parr bomb at 250 °C).19 However, Maruyama et al. reported that this phase transformation could happen in water at 60-70 °C.11 It seems that the water and temperature, instead of high pressure, are necessary factors for the present reaction. Which is the key factor to drive the phase transformation has not been clarified. We assume that the water is a key factor because the R-AgGaO2 was successfully prepared at lower phase-transformation temperature (50 °C) in our first attempt (see in Figure 2). Two experiments were designed to prove our assumption. In the first experiment, the β-AgGaO2 and R-AgGaO2 (PTR-50 °C-3.5 h) were pretreated in a vacuum at room temperature, and then were heated at different high temperatures for 3 h. The R-AgGaO2 samples prepared from phase-transformation reactions are designated hereinafter as PTR-x°C-yh, where x and y represent heat-treatment
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Figure 3. Variation in the phase compositions with prolonged phasetransformation time.
temperature and time, respectively. As shown in Figure 2a, when β-AgGaO2 was heated at a temperature region between 550-640 °C with a step of 10 °C, β-AgGaO2 began to decompose at 610 °C and totally became Ag and Ga2O3 at 640 °C. In these experiments, no peak of R-AgGaO2 was observed in XRD patterns, indicating β-AgGaO2 could not convert to R-AgGaO2 in the absence of a water environment. If a phase transformation is not driven by a thermal effect, it will not reverse by heating. This is proved by the experiments of high-temperature treatments (600-700 °C) on the RAgGaO2 sample; as shown in Figure 2b, until R-AgGaO2 completely decomposed, no peak of β-AgGaO2 appeared in the XRD patterns. The decomposition temperature of R-AgGaO2 is 660 °C, which is a little lower than former report.1 This difference may be due to the different synthetic processes. In the second experiment, the β-AgGaO2 samples were immersed in water at different temperatures to study the effects of temperature on the phase-transformation rate. Figure 3 plots the change of ratios of R-phase to β-phase by prolonging the heating time. The ratios of R-phase to β-phase were measured by an external reference method (details related to the measurement are described in Supporting Information, SI-1). It is obvious that the phasetransformation rates accelerate with the heating temperature rising. The present reaction could be performed even at room temperature, indicating the temperature is not a key factor. The above-mentioned experiments demonstrate that the water is a key factor for the phase transformation of the β-phase to R-phase. The different solvents, such as n-hexane, C2H5OH, and mixture of C2H5OH and H2O (1:1, v%/v%), were adopted to study the role of water (as shown in Supporting Information, SI-2). When pure n-hexane or C2H5OH were used as a medium, no peak of R-phase was observed in the XRD patterns. However, if we supplied the mixture solvent of C2H5OH and H2O, the phase transformation could be executed, though the rate of phase transformation was slower than that in pure water. These results also reveal that the water is a key factor. We suggest that water is a necessary solvent for AgGaO2; the solubility of R-phase is lower than that of the β-phase, which drives the present phase transformation via a solid-liquid-solid process. Morphology observation from SEM shown in a later section will supply further evidence. Effect of Temperature on Photocatalytic Activity and Morphology. The R-phase samples prepared at different phase-
Figure 4. XRD patterns (a), UV-vis absorption spectra (b), and photocatalytic activities (c) of the PTR-22 °C-36 h, PTR-28 °C24 h, and PTR-50 °C-4.5 h samples.
transformation temperatures (PTR-22 °C-36 h, PTR28 °C-24 h, PTR-50 °C-4.5 h) were characterized by XRD, UV-visible absorption spectra, and photocatalytic activity of 2-propanol (IPA) oxidization. Figure 4 shows these results. The XRD patterns and UV-visible absorption spectra of the PTR-22 °C-36 h and PTR-28 °C-24 h samples are similar but different from that of the PTR-50 °C-4.5 h sample. The photocatalytic IPA oxidization is a typical reaction to
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Figure 5. Effects of the phase-transformation temperature on the morphologies of the samples.
evaluate the catalytic property of a photocatalyst. Under light irradiation, the photocatalyst was excited to generate electrons and holes. One electron can combine with a molecular of O2 to form the 3 O2- radical, and then the 3 O2oxidizes IPA to become acetone. Therefore, the photooxidization of IPA to acetone is a one-photon process.37 As shown in Figure 4c, the rates of acetone evolution over the PTR22 °C-36 h, PTR-28 °C-24 h, and PTR-50 °C-4.5 h samples are 52.2, 64.9, and 154.7 ppm/h, respectively. The activity of PTR-50 °C-4.5 h samples is much higher than that of the other two samples, which is in good agreement with the results of XRD and UV-visible absorption spectra. We further investigated the morphologies of the three samples. Figure 5 shows the SEM images of samples. They are all superstructures; however, their units are coarse circular platelet, round-angle hexagonal platelet, and hexagonal platelet for the PTR-22 °C-36 h, PTR-28 °C-24 h, and PTR-50 °C-4.5 h samples, respectively. The higher phasetransformation temperature contributes to grow a more regular unit of superstructure. The difference of activities is not associated with the various morphologies of the samples, because the morphology of PTR-28 °C-24 h sample is close to that of PTR-50 °C-4.5 h sample, but their activities are quite different. The possibility that the BET surface area results in such a disparity in activity is also excluded (Supporting Information, SI-3). Although the temperature is not a key factor for the phase transformation, it affects the photocatalytic activities and morphologies of R-phase samples. The different activities related to crystal structures prompt us to perform further experiments to explore the original reason. Correction of Crystal-Structure Distortion. We have found that there are some differences (in the 2θ region of 34-42°) in the XRD patterns of the PTR-50 °C-3.5 h and PTR50 °C-4.5 h samples. To get further information, the PTR50 °C-4 h, PTR-50 °C-5 h, and PTR-50 °C-6 h samples were prepared. The XRD patterns of these five samples are shown in Figure 6. Enlarged profile of the 2θ region of 32-42° presents a transition from 3.5 to 4.5 h of heating time. After 4.5 h, the XRD patterns of the samples are similar. It is assumed that after the completion of the phase transformation, the crystal structure of R-AgGaO2 [(101) and (012) crystal planes] is distorted, but further heat treatment helps to correct this distortion. The results of the UV-visible absorption spectra and photocatalytic activities support the above assumption. As shown in Figure 7, although the absorption onsets of these samples are similar, it is obvious that the band edges red shift until the heating time reached 4.5 h. Figure 8 plots the change of photocatalytic activity and phase composition with prolonged heating time. This reaction in water is divided into three periods. In Period I which can be defined as the phase-transformation period, the photocatalytic activity is continuously enhanced with the R-phase
Figure 6. Effects of the heating-treatment time on the crystal structures of the samples when the water temperature is 50 °C.
Figure 7. Effects of the heating-treatment time on the UV-vis absorption spectra of the samples when the water temperature is 50 °C.
percentage increasing because the photocatalytic performance of the R-phase is higher than that of the β-phase as the former reports.11,12 The 4.5 h is set to be a demarcation since there is a jump of activity. During 3.5-4.5 h, defined as Period II, the average activity is 116.5 ppm/h. However, in Period III (beginning at 4.5 h), this value reaches 144.7 ppm/h, which is an enhancement of 24% in contrast to Period II. The characteristic of Period II is the transition which means the correction of crystal-structure distortion and the absorption band edges have a red shift. After these advantageous changes, the properties of the sample have reached a maturity. This contributes to the higher photocatalytic performance. For the PTR-50 °C-4.5 h sample, the apparent photonic efficiency (APE) of IPA converting to acetone was measured (Supporting Information, SI-4; the evaluation method refers to our former study12). The achieved APE was 4% under the condition of λ = 425 ( 12 nm. We also studied the effects of water and temperature on the correction of
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Figure 8. Influence of the heating-treatment time on the photocatalytic activities of the samples when the water temperature is 50 °C.
crystal-structure distortion. According to Figures 2b and 6, it is found that the structural correction could not be driven in the absence of a water environment even if high temperature was used. Furthermore, a β-phase sample was kept in water at room temperature for a long time. The structural correction was not observed (Supporting Information, SI-5). These results indicate both water and temperature are necessary factors for the correction of crystal-structure distortion. 3.2. Growth Mechanism of Hexagonal-Platelet Superstructure. The former reports on multimetal oxides with special morphologies mainly concentrated on Bi-based oxides.38-41 For Ag-based oxide, only AgIn(WO4)2 with a morphology of nanotube was developed.42 Here, it is found that a superstructure with hexagonal platelet of R-AgGaO2 could form after phase transformation from β-AgGaO2. Figure 9 shows morphology change of β-phase transforming to R-phase when the water temperature is 50 °C. The β-phase sample (Figure 9a) is a usual particle. After heating treatment in water for 1.75 h, as shown in the yellow frame of Figure 9, grain boundary and grain dissolve to provide a stock for growing R-phase hexagonal platelet (as denoted by yellow arrows); some part has formed the superstructure (as displayed in red frame). The nascent R-phase sample presents a shape of round-angle hexagonal platelet (Figure 9c). Figure 9d depicts that many parts of the particle become hexagonal-platelet superstructures when heated for 2.5 h. The whole sample seems to form a hexagonal-platelet superstructure after 3 h, but there is still 12% β-phase in the sample (Figure 9e). This phenomenon is reasonably explained by the fact that the phase transformation is solid-liquid-contact and therefore an outside-in reaction. Finally, after the β-phase totally converts to the R-phase, the morphology of the sample is a hexagonal-platelet superstructure (Figure 9f). These timeline based SEM images reveal that the morphology change of the β-phase particle to R-phase hexagonal platelet is based on the solid-liquid-solid growth mechanism similar to former reports in the literature.43,44 As shown in Figure 10, to investigate the unit of superstructure, a single hexagonal platelet of the PTR-50 °C-4.5 h sample was separated by ultrasonic treatment. The selected area electron diffraction (SAED) pattern from this hexagonal platelet reveals its single crystal nature. High-resolution transmission electron microscopy (HRTEM) image displays the resolved lattice fringes of 0.297 and 0.296 nm, which corresponds to the (006) and (060) planes of the rhombohedral AgGaO2 structure, respectively.
Figure 9. Morphology change of β-phase transforming to R-phase when the water temperature is 50 °C.
Figure 10. SEM and TEM images of PTR-50 °C-4.5 h sample.
In Vanaja and co-workers’ study, when the technique of pulsed laser deposition was adopted to grow R-AgGaO2 thin film, they observed the nucleation and growth of the film in the form of nanorods. From the HRTEM image, their lattice spacing of 0.223 nm coincides with the crystal plane (104).19,45 We suggest the possible reason for the formation of hexagonal platelets including two aspects. First, these {006} planes are with the lowest energy (the relatively closepacked plane) and therefore could preferentially grow. There is no effect of template induction or ion-absorption restriction. Thus, the formation of platelet-like structure is prior to other shapes. Second, as we know, the rhombohedral symmetry is similar to the hexagonal symmetry. The hexad-axis characteristic influences the growing extension.
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Crystal Growth & Design, Vol. 10, No. 7, 2010 Scheme 1. Suggested Growth Mechanism of the Hexagonal-Platelet Superstructure
The suggested formation mechanism of hexagonal-platelet superstructure is illustrated in Scheme 1. Scheme 1A is a particle of the β-phase sample. After heating in water, a nascent hexagonal platelet (denoted as Unit I) grows on the particle (Scheme 1B) according to the solid-liquid-solid growth mechanism. It can freely extend because of adequate growing space. Scheme 1C shows that a small superstructure forms on the surface of the particle. Enlarged image reveals the details of the small superstructure. The second-formed hexagonal platelet (denoted as Unit II) is confined by the Unit I, and thus it grows around Unit I. Unit III also rounds Unit I and Unit II to extend, and so forth. The similar phenomena appear in other parts of this particle (Scheme 1D). Finally, the growing-space competition induced to form the superstructure that is aggregated by the single crystal platelets (Scheme 1E). 4. Conclusions Phase Transformation (1) This phase transformation is irreversible. (2) In this phase transformation, the water but not the temperature is the essential factor. However, the higher temperature accelerates the transformation course. (3) In phase transformation at 50 °C, the β-phase converts to mature R-phase through a transition period. After the phase transformation, the crystal structure of R-AgGaO2 is distorted. The structural distortion of R-phase is corrected during the transition period. (4) Both water and temperature are necessary to correct crystal-structure distortion. Crystal Growth (1) When phase-transformation temperature is varied from room temperature to 50 °C, the morphologies evolves from coarse circular-platelet superstructure to hexagonal-platelet superstructure. Therefore, the higher temperature is beneficial to grow a regular unit of superstructure. (2) The morphology change of the β-phase particle to R-phase hexagonal platelet is based on the solidliquid-solid growth mechanism. (3) The phase transformation is an outside-in reaction. Because the various R-phase hexagonal platelets compete for the growing space among each other, they finally form the superstructure. Every hexagonalplatelet unit is a single crystal. Photocatalytic Properties. The mature R-phase shows higher activity than the R-phase with structural distortion.
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Acknowledgment. Financial support from the National Natural Science Foundation of China (Nos. 50732004 and 20773064), China-Japan cooperation project of science and technology (2009DFA61090), and the National Basic Research Program of China (973 Program, 2007CB613305) is gratefully acknowledged. J.Y. would like to thank the financial support from the World Premier International Research Center Initiative on Materials Nanoarchitectonics, MEXT. S.O. is grateful to Dr. Guangcheng Xi for helpful discussion in the growth mechanism of hexagonal platelet. Supporting Information Available: Standard curve of the phase composition with external reference method standardizing, effects of the solvents on the phase transformation, phase compositions and BET surface areas of the samples, apparent photonic efficiency of the sample PTR-50 °C-4.5 h, and XRD patterns of the samples treated at room temperature. This information is available free of charge via the Internet at http://pubs.acs.org/.
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