The formation and microstructural properties of uniform α-GaOOH particles and their calcination products

Journal of Alloys and Compounds 620 (2015) 217–227

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The formation and microstructural properties of uniform a-GaOOH particles and their calcination products Stjepko Krehula a,⇑, Mira Ristic´ a, Shiro Kubuki b, Yusuke Iida b, Martin Fabián c, Svetozar Music´ a - Boškovic´ Institute, P.O. Box 180, HR-10002 Zagreb, Croatia Division of Materials Chemistry, Ruder Department of Chemistry, Graduate School of Science and Engineering, Tokyo Metropolitan University, Minami-Osawa 1-1, Hachi-Oji, Tokyo 192-0397, Japan c Institute of Geotechnics, Slovak Academy of Sciences, 043 53 Košice, Slovakia a

b

a r t i c l e

i n f o

Article history: Received 22 July 2014 Received in revised form 15 September 2014 Accepted 16 September 2014 Available online 28 September 2014 Keywords: a-GaOOH a-Ga2O3 b-Ga2O3 FT-IR FE-SEM PL

a b s t r a c t Uniform a-GaOOH submicron particles of different shapes (spindles, rhombic rods, rhombic prisms, hierarchical particles) were synthesized by simple low-temperature (at 60 °C) or hydrothermal (at 160 °C) precipitation from the mixture of aqueous solutions of gallium(III) chloride and organic alkali tetramethylammonium hydroxide (TMAH) at various pH values (5, 7 or 9). The growth mechanism of these particles was discussed. Crystallographic directions and crystal planes in a-GaOOH particles were identified. Differences in crystallite size, thermal and infrared properties of a-GaOOH particles were observed and discussed. Uniform a-Ga2O3 and b-Ga2O3 particles of the same or similar shapes, containing holes due to dehydroxylation, were obtained by calcination of the corresponding a-GaOOH particles at 500 or 900 °C, respectively. Differences in crystallite size and infrared properties of obtained gallium oxide samples, as well as differences in photoluminescence properties of b-Ga2O3 samples were observed and discussed. Ó 2014 Elsevier B.V. All rights reserved.

1. Introduction

The main crystalline form of gallium oxyhydroxide (GaOOH) is

a-GaOOH (orthorhombic crystal system, space group Pbnm with Gallium(III) oxide (Ga2O3) is a wide bandgap semiconductor material which has been broadly investigated in recent years due to its suitable properties for a wide range of applications, for example as a catalyst, photocatalyst, luminescent material, gas sensor or antibacterial material. Five different crystalline forms (a, b, c, d and e form) of Ga2O3 are known [1–3], with a-Ga2O3 (rhombohedral crystal system, space group R-3c, a = 4.9825 Å, c = 13.433 Å [4]) (Fig. 1a) and b-Ga2O3 (monoclinic crystal system, space group C2/ m, a = 12.214 Å, b = 3.0371 Å, c = 5.781 Å, b = 103.83° [5]) (Fig. 1b) as the most important modifications. a-Ga2O3 has been mainly prepared by heating of gallium oxyhydroxide at 400–600 °C in air [6–14]. b-Ga2O3 is a stable gallium oxide form at high temperatures [1] and the most used one for various applications. It can be obtained by heating of other gallium oxides and (oxy)hydroxides [6,7,9–16], metallic gallium (with O2, H2O or other oxygen source) [17–19] or gallium arsenide [20] at high temperatures. The synthesis method and conditions have a strong influence on particle size, shape and crystallinity of gallium oxides, as well as their optical, thermal, electronic, electrical, catalytic and other properties. ⇑ Corresponding author. Tel.: +385 1 4561 094; fax: +385 1 4680 098. E-mail address: [email protected] (S. Krehula). http://dx.doi.org/10.1016/j.jallcom.2014.09.134 0925-8388/Ó 2014 Elsevier B.V. All rights reserved.

a = 4.5545 Å, b = 9.8007 Å, c = 2.9738 Å [7], or Pnma with a = 9.7907 Å, b = 2.9732 Å, c = 4.5171 Å [21]) (Fig. 1c) which is isostructural with diaspore (a-AlOOH) [22] and goethite (a-FeOOH) [23]. a-GaOOH, as a major precursor for the preparation of Ga2O3, can be synthesized at low temperature (<100 °C) [7,9,11,24–27] as well as by hydrothermal (>100 °C) [28–32], microwave [8,29] or sonochemical [33] treatment of diluted, partially neutralized or completely neutralized Ga(III) salt aqueous solutions at non-extreme pH values (usually in the range from pH 3 to pH 11). a-GaOOH nanorods can be also synthesized by hydrothermal treatment of an aqueous suspension of nanocrystalline Ga2O3 powders [34]. Spindle-like a-GaOOH particles were prepared by laser ablation of gallium metal in an aqueous solution [15]. a-GaOOH nanorods and nanowires were prepared by laser ablation of a gallium plate in aqueous solutions containing CTAB and PVP or PVA [35]. a-GaOOH nanoplates were prepared via a room-temperature ion exchange reaction between KGaO2 and CH3COOH [36]. Electrochemical deposition was used for the synthesis of a-GaOOH rhombic nanopillars with optical properties suitable for application as antireflection coating in solar cells [37]. It is well known that, in addition to the chemical composition and crystal structure, particle size and shape have a significant

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influence on different properties of solid polycrystalline materials. The uniformity of particle size and shape is also an important factor that provides high-quality properties for different sophisticated applications of these materials. Generally, the invention of new routes in the synthesis of uniform colloid particles has been one of the major goals over the last few decades [38–42]. Several works have been published concerning the synthesis of uniform gallium oxyhydroxide and oxide particles. Hamada et al. synthesized uniform spherical a-GaOOH particles (100 nm in diameter) by heating (at 98 °C) a diluted aqueous solution of Ga3+ ions in the presence of sulphate ions [26]. Zhang et al. synthesized uniform a-GaOOH rods and spindles from GaCl3–H2O–diethyleneglycolNaOH solutions by hydrothermal treatment at 180 °C and the initial pH of 6 or 8, respectively [28]. Qian et al. synthesized monodispersed single crystalline a-GaOOH spindles by precipitation from aqueous GaCl3-NH3 solutions at pH 10 or 11 [11]. Li et al. synthesized uniform Dy-doped a-GaOOH submicron spindles and ellipsoids as well as hierarchical microspheres by precipitation from Ga3+–Dy3+ aqueous solutions having different initial pH values (pH 4, 6, 7 or 9) [43]. Uniform a-GaOOH nanospindles were produced by the room temperature aging of a gallium(III) chloride aqueous solution in the presence of His–Ser dipeptide as an additive [44]. Since a limited number of works has been published on the synthesis and properties of uniform gallium oxyhydroxide and oxide particles of different shapes, we have systematically investigated the influence of experimental conditions on the formation and properties of these particles. In this sense, we report a new simple route for the synthesis of uniform a-GaOOH submicron particles of different shapes by precipitation from the suspension obtained by mixing aqueous solutions of gallium(III) chloride and strong organic alkali tetramethylammonium hydroxide (TMAH). A similar method has been confirmed as successful in the synthesis of uniform goethite (a-FeOOH) submicron particles [45–47]. In many inorganic syntheses TMAH is a source of hydroxyl ions and also plays the role of a structural template. It is generally known that the type of alkali utilized in the synthesis of metal oxides may influence their nano/microstructure and phase composition. Uniform a-Ga2O3 and b-Ga2O3 particles of different shapes containing holes were prepared by calcination of the corresponding a-GaOOH particles at 500 or 900 °C, respectively. Prepared samples were analyzed by several instrumental techniques. Differences in properties of gallium oxide samples were observed and discussed.

2. Experimental 2.1. Sample preparation Gallium(III) chloride (GaCl3), ultra dry, 99.999% (metals basis), and a tetramethylammonium hydroxide (TMAH, (CH3)4NOH) aqueous solution (25% w/w, electronic grade 99.9999%), both supplied by Alfa AesarÒ , were used. Milli-Q water prepared in own laboratory was used in all experiments. Predetermined volumes of a GaCl3 aqueous solution and water were mixed, then TMAH was added gradually with strong mixing in order to reach pH values 5, 7 or 9, respectively. The aqueous suspensions thus formed were vigorously shaken for 5 min, then heated at 60 °C in a plastic bottle (until complete precipitation was obtained), or 160 °C (for 2 h) using the Parr general-purpose bomb (model 4744) comprising a Teflon vessel and a cup. The exact experimental conditions for a-GaOOH samples preparation are given in Table 1. After the required heating time the precipitates were cooled to room temperature and subsequently washed with Milli-Q water to remove the ‘‘neutral electrolyte’’. The ultraspeed Sorvall RC2-B centrifuge was used. Thus obtained a-GaOOH precipitates were dried at 60 °C overnight. a-Ga2O3 and b-Ga2O3 samples were obtained by calcination of a-GaOOH precursor samples in a furnace for 2 h at 500 or 900 °C, respectively. The temperature in the furnace was raised to the corresponding value at a rate of 5 °C/ min. Calcined samples were cooled down to room temperature naturally within the furnace. All samples were characterized by X-ray powder diffraction (XRD), thermal field emission scanning electron microscopy (FE-SEM) and Fourier transform infrared (FT-IR) spectroscopy. a-GaOOH samples were also analyzed by thermogravimetric analysis (TGA), differential thermogravimetric analysis (DTG) and differential thermal analysis (DTA). b-Ga2O3 samples were additionally analyzed by photoluminescence (PL) spectroscopy.

2.2. Instrumentation An APD 2000 X-ray powder diffractometer (Cu Ka radiation, graphite monochromator, NaI-Tl detector) supplied by ItalStructures (G.N.R. s.r.l., Novara, Italy) was used. The full width at half maximum (FWHM) values of the diffraction lines were obtained by fitting a pseudo-Voigt function to experimental data using the WinDust32 program (ItalStructures). Crystallite size was estimated from FWHM values (after correction for instrumental broadening) of the XRD line (1 1 0) for aGaOOH, line (1 1 0) for a-Ga2O3 and line (1 1 1) for b-Ga2O3, using the Scherrer equation [48]. A JEOL thermal field emission scanning electron microscope (FE-SEM, model JSM-7000F) was used for observation of the morphology of samples. The specimens were not coated with an electrically conductive surface layer. Fourier transform infrared (FT-IR) spectra were recorded at RT using a Perkin– Elmer spectrometer (model 2000). The FT-IR spectrometer was connected to a PC with the installed IRDM (IR Data Manager) program to process the recorded spectra. The specimens were pressed into small discs using a spectroscopically pure KBr matrix. Thermogravimetric analysis (TGA), differential thermogravimetric analysis (DTG) and differential thermal analysis (DTA) were carried out by Rigaku Thermo Plus TG8120 between RT and 1000 °C at the heating rate of 10 °C min1 under nitrogen gas flow of 100 mL min1 and a-Al2O3 standard. PL spectra were recorded using PC1 (ISS, USA) spectrofluorimeter using 300 W Xe lamp. Samples were measured under 90° scattering geometry with an excitation wavelength of 254 nm.

3. Results and discussion 3.1. Uniform GaOOH particles 3.1.1. X-ray powder diffraction The presence of orthorhombic gallium oxyhydroxide a-GaOOH (JCPDS PDF card No. 54-0910, space group Pbnm) as a single phase in all prepared samples (Table 1) was confirmed by analyzing X-ray powder diffraction patterns (Fig. 2). Differences in the relative intensities of certain diffraction lines in XRD patterns indicate the preferred orientation of a-GaOOH particles in some samples. The diffraction pattern of sample A-160 shows lower relative intensities of diffraction lines with the non-zero Miller index l (for example line 111) in comparison with lines with the index l equal to zero (for example line 110) which indicates elongation of a-GaOOH particles in the direction of crystallographic c-axis. Similar XRD patterns were recorded on a-GaOOH rod-shape particles [34,35]. This difference in intensity is less manifest in the patterns of samples A-60, N-60 and N-160. The smallest difference in relative intensities is observed in the XRD patterns of samples B-60 and B-160, which indicates the smallest preferred orientation of a-GaOOH particles in these samples, i.e., the least elongated particles in the direction of crystallographic c-axis. Crystallite sizes in a-GaOOH samples are given in Table 2. Among the samples formed at 60 °C the largest crystallites were formed at neutral pH (sample N-60) where the precipitation was fastest (Table 1). The growth of a-GaOOH crystallites, as well as that of a-GaOOH particles consisting of n crystallites can be related to the state of gallium in aqueous solutions. Depending on the pH in an aqueous medium, gallium(III) forms Ga(OH)2+, GaðOHÞþ 2, 2 GaðOHÞ3 , GaðOHÞ [49]. Calculations based on the 4 or Ga2 ðOHÞ8 hydrolysis constants indicate that at pH 1 gallium(III) exists primarily as Ga3+, at pH 3 primarily as GaðOHÞ2þ , at pH 4 primarily as GaðOHÞþ 2 , and at pH 7 practically only in the form of hydrated GaðOHÞ3 . Since gallium is an amphoteric element, Ga(OH)3 is solu2 ble in alkaline solutions forming gallates, GaðOHÞ 4 and Ga2 ðOHÞ8 . The equilibrium of these complexes at a given pH plays an important role in the formation of a-GaOOH microstructure, as well as the adsorption of gallium(III) on metal oxide surfaces [50]. Sato and Nakamura [25] reported that amorphous GaðOHÞ3 , precipitated by adding various alkalis to GaCl3 aqueous solutions, crystallized to a-GaOOH. The dissolution/recrystallization kinetics in dependence on pH is an additional factor influencing the overall crystallization process. In the present work the pH values were

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Fig. 1. Crystal structures of (a) a-Ga2O3 [4], (b) b-Ga2O3 [5] and (c) a-GaOOH [7,21] phases represented by the cation polyhedra. The space group, crystallographic axes and unit cell are designated.

Table 1 Conditions for the preparation of a-GaOOH samples. Tetramethylammonium hydroxide (TMAH) was used as an alkali for the adjustment of the initial pH value. Phase composition of solid samples was determined by X-ray powder diffraction. Sample

A-60 N-60 B-60 A-160 N-160 B-160

[GaCl3] (mol dm3)

0.10 0.10 0.10 0.10 0.10 0.10

pH Initial

Final

5 7 9 5 7 9

2.5 5.0 10 2.5 4.0 10.5

measured at room temperature; however, at temperature raised up to 160 °C the pH values change and this should be also taken into account when discussing the crystallite (particle) growth. Moreover, in basic pH medium the (CH3)4N+ cation may display

Temperature (°C)

Aging time

Phase composition

60 60 60 160 160 160

7 days 1 day 2 days 2h 2h 2h

a-GaOOH a-GaOOH a-GaOOH a-GaOOH a-GaOOH a-GaOOH

template role in the formation of the corresponding microstructure. Crystallite size in a-GaOOH samples hydrothermally formed at 160 °C was smaller in the sample formed under neutral pH conditions (sample N-160) in comparison with samples formed under

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Fig. 2. XRD powder patterns of prepared a-GaOOH samples. Positions and intensities of the diffraction lines of a-GaOOH as given in the Powder Diffraction File (PDF) of the International Centre for Diffraction Data (ICDD) are shown at the bottom.

Table 2 Crystallite size of prepared a-GaOOH, a-Ga2O3 and b-Ga2O3 samples estimated from the FWHM values of XRD (1 1 0) lines for a-GaOOH and a-Ga2O3 and of the (1 1 1) line for b-Ga2O3 using the Scherrer equation. Sample

Crystallite size (nm)

a-FeOOH A-60 N-60 B-60 A-160 N-160 B-160

(1 1 0)

a-Ga2O3 (1 1 0)

b-Ga2O3 (1 1 1)

39 52 27 69 45 72

40 67 85 75 72 75

57 50 54 31 33 29

acidic or basic conditions (samples A-160 and B-160) (Table 2). A similar result was obtained by Zhao et al. [31] on a-GaOOH samples obtained by hydrothermal treatment (at 180 °C) of Ga(NO3)3– NaOH aqueous solutions at different pH values. 3.1.2. Scanning electron microscopy The morphology of a-GaOOH particles was analyzed by FE-SEM (higher magnification image in Fig. 3 and larger area image in Fig. SD1 in Supplementary data). The sample formed at 60 °C and initial pH 5 (sample A-60) consists of spindle-shaped particles of an average length of about 800 nm and average width of about 300 nm (Fig. 3a). Particles of this shape are common for a-GaOOH formed in an acidic medium at low temperatures [7,25,43]. The rough surface of these particles indicates an aggregation of initially

formed a-GaOOH nanoparticles. A similar particle shape has been observed for b-FeOOH samples formed under similar conditions (acidic medium, presence of chloride ions) [51–53]. Zhao et al. [31] observed that a rise in temperature to 100 °C affected the transformation of spindle-shaped particles to a-GaOOH rods. a-GaOOH particles formed at 60 °C and initial pH 7 (sample N-60) were mostly in the form of somewhat elongated rhombic prisms of an average length of about 600 nm and a rhombic (diamond-like) face (shown by arrows) of a side length of about 200 nm (Fig. 3c and Fig. SD1c in Supplementary data). Crystal planes and edges are clearly visible. Uniform hierarchical a-GaOOH particles of an average length of about 500 nm and width of about 300 nm were formed at 60 °C and initial pH 9 (sample B-60, Fig. 3e). These particles consist of few rhombic prisms grown together. a-GaOOH sample formed at 160 °C and initial pH 5 (sample A-160) consists of particles in the form of rhombic rods elongated in the c-axis direction of an average length of about 1.5 lm and width of about 300 nm (Fig. 3b). This observation is in line with the preferred orientation of a-GaOOH particles noticed from the reduced intensity of the (1 1 1) diffraction line relative to the (1 1 0) line in the XRD pattern of the same sample (Fig. 2). Elongation of particles in the c-axis direction is caused by the a-GaOOH crystal structure (isomorphous to diaspore, a-AlOOH, and goethite, a-FeOOH) with double chains of the edge-sharing GaO3(OH)3 octahedra separated by two empty octahedral sites running in the c-axis direction (Fig. 1c). The growth of a-GaOOH crystals appears to proceed most readily by the addition of growth units to the sites at the (0 0 1) plane (at the end of the double chains), similarly to the formation of isostructural a-FeOOH rods [45,46]. Generally, the direction of the crystal growth of metal oxides and hydroxides in a solution strongly depends on the medium pH. The formation of rod-shaped a-GaOOH particles elongated in the [0 0 1] direction is favoured in the acidic conditions [7,28,31]. Particles formed at 160 °C and initial pH 7 (sample N-160) were in the form of rhombic prisms of an average length of about 600 nm and the edge of a rhombic face (0 0 1) of about 200 nm (Fig. 3d), similar to the particles obtained at 60 °C and the same pH. A basal rhombic face (0 0 1) is indicated by an arrow in the Fig. 3d. Uniform hierarchical a-GaOOH particles were formed at 160 °C and initial pH 9 (sample B-160), with an average length of about 500 nm and width of 300 nm (Fig. 3f). These particles were similar in shape and size to the particles formed at 60 °C and same pH (Fig. 3e), with only a slight difference in the surface morphology (particle edges were more rounded in sample B-160). The shape of a-GaOOH crystals in samples A-160, N-160 and B-60 is clarified in Fig. 4. The basal face (0 0 1) of the rods in sample A-160 and prisms in sample N-160 is a rhombus with edge length of about 200 nm and two characteristic angles, a of about 50° and b of about 130°. The (0 0 1) basal face of this shape has been observed in a-GaOOH rods prepared in other studies [16,35,37], and in b-Ga2O3 rods obtained by heating of a-GaOOH rods [16,28]. The basal face of hierarchical a-GaOOH particles in sample B-60 consists of few overlapping rhombi with the same characteristic a and b angles. A similar (0 0 1) basal face has been already observed in a-GaOOH nanoplates [36] and in a-GaOOH nanopillars [37]. In the prismatic crystal form {h k 0}, with crystal faces parallel to  the c-axis, the angle ahk0 between crystal faces (h k 0) and (hk0)  are given and the angle b between crystal faces (h k 0) and (hk0) hk0

by equations (explained in Fig. SD2 in Supplementary data for the case of the crystal form {1 1 0}):

  a=h b=k

ahk0 ¼ 2arctan

  b=k bhk0 ¼ 2arctan a=h

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Fig. 3. FE-SEM images of a-GaOOH samples (a) A-60, (b) A-160, (c) N-60, (d) N-160, (e) B-60, and (f) B-160. Arrows show the presence of rhombic faces.

where a and b are unit cell parameters of a-GaOOH (a = 4.5545 Å, b = 9.8007 Å). Excellent agreement with measured angles of about 50° and 130° was reached for the rhombic prism crystal form {1 1 0} where a110 = 49.85° and b110 = 130.15°. This conclusion is in line with an earlier assignation of crystal faces in a-GaOOH prismatic nanorods by Huang and Yeh [35]. 3.1.3. Infrared spectroscopy The FT-IR spectra of prepared a-GaOOH samples in the characteristic wavenumber range (1200–250 cm1) are shown at Fig. 5. Strong bands at about 1020 cm1 and 950 cm1 are assigned to Ga–O–H bending vibrations [31,54]. Analogous to the case of goethite (a-FeOOH) [55,56], an oxyhydroxide of the same structure, these two bands can be designated as a dOH band (with the transition moment lying in the a–b plane) and a cOH band (with the transition moment parallel to the crystallographic c-axis). Relative intensities and positions of these two bands vary in dependence on a-GaOOH particle shape. The intensity of the cOH band is lower in the spectra of samples A-60, A-160, N-60, N-160 containing a-GaOOH particles elongated in the c-axis direction (Fig. 3), probably due to the preferred orientation with respect to the infrared beam. A similar reduced intensity of the cOH band in the IR spectrum was observed in the case of a-FeOOH particles elongated

in the crystallographic c-axis direction [57–62]. Furthermore, the positions of these two peaks in the IR spectra of synthesized aGaOOH samples were shifted from 946 to 962 cm1 and from 1009 to 1035 cm1, respectively. The IR spectra with a higher wavenumber (higher energy) of dOH and cOH bands correspond to a-GaOOH samples with stronger Ga–O–H  O–Ga hydrogen bonds, which are located in the structure of a-GaOOH (Fig. 1c) between the double chains of the GaO3(OH)3 octahedra. The IR-absorption bands at wavenumbers lower than 800 cm1 are attributed to Ga–O stretching or lattice vibrations [31]. Positions and intensities of these bands depend on a-GaOOH particle shape. The absorption maximum at 654 cm1 in the FT-IR spectrum of a-GaOOH rods elongated in the direction of crystallographic c-axis (sample A-160) was shifted to lower wavenumbers (633 cm1) in the spectrum of shorter a-GaOOH particles (sample B-160). In addition to that, the intensity of this absorption band increased significantly, as well as the intensity of the shoulder band at about 520 cm1. In the spectra of the same samples the absorption maxima at 472 and 392 cm1 (sample A-160) were shifted to higher wavenumbers (496 and 415 cm1, respectively). The influence of particle shape on the position of lattice vibration bands was also observed in the FT-IR spectra of a-FeOOH samples [45,46].

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Fig. 4. Selected details from the FE-SEM images of a-GaOOH samples A-160, N-160 and B-60 with designated characteristic angles (a and b) in the basal rhombic faces (left) and the illustration of particle shapes in these samples (right).

Fig. 5. Characteristic parts of the FT-IR spectra of prepared a-GaOOH samples.

3.1.4. Thermal analysis a-GaOOH samples were also analyzed by thermal methods (TGA, DTG, DTA) and the obtained curves are shown in Fig. 6. Peak positions in DTA and DTG curves and TGA weight loss are summarized in Table 3. The TGA curves of different a-GaOOH samples (Fig. 6a) show strong differences in weight loss as well as in temperature of weight loss (Table 3). Generally, weight loss was higher

in a-GaOOH samples formed at lower temperatures (60 °C), indicating a higher water content in these samples compared with hydrothermally (160 °C) prepared samples. If samples prepared at the same temperature are compared, the highest weight loss (the highest water content) was in the sample formed in neutral conditions and the lowest weight loss (the lowest water content) was in the sample formed in basic conditions. The higher weight loss in samples formed at lower temperatures and in neutral conditions was probably affected by the higher content of amorphous gallium hydroxide in these samples or by the higher content of adsorbed water on the surface of a-GaOOH crystallites. Gallium hydroxide is more stable at lower temperatures and in neutral conditions, therefore its transformation to a-GaOOH in these conditions is slower in comparison with higher temperature and acidic or basic conditions. DTG curves (Fig. 6b) show a strong influence of the preparation conditions on the temperature of maximum weight loss (Table 3). This temperature is lowest for the sample prepared at low temperature in an acidic medium (A-60) and highest for the sample prepared hydrothermally in a basic medium (B-160) which has the highest crystallinity (Table 2) and lowest weight loss (Table 3). The shoulder at about 250 °C in the DTG curve of samples N-60, B-60 and N-160 can be attributed to the presence of a certain quantity of amorphous gallium hydroxide in these samples. The DTA curves of a-GaOOH samples (Fig 6c) also show significant differences in temperature of a-GaOOH dehydroxylation, a-Ga2O3 recrystallization and a-Ga2O3 to b-Ga2O3 transformation (Table 3). Generally, by comparing DTA results with crystallite size of a-GaOOH samples obtained by XRD (Table 2) it can be inferred that an increase in crystallite size involves a rise in temperature of a-GaOOH dehydroxylation, a-Ga2O3 recrystallization and a-Ga2O3 to b-Ga2O3 transformation. 3.2. Uniform a-Ga2O3 and b-Ga2O3 particles

3.2.1. X-ray powder diffraction The X-ray powder diffraction patterns of a-Ga2O3 samples obtained by calcination of a-GaOOH samples at 500 °C are shown

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Fig. 7. XRD powder patterns of a-Ga2O3 samples obtained by calcination of aGaOOH samples at 500 °C. Positions and intensities of the a-Ga2O3 diffraction lines as given in the Powder Diffraction File (PDF) of the International Centre for Diffraction Data (ICDD) are shown at the bottom.

Fig. 6. Thermal analysis of prepared a-GaOOH samples: (a) thermogravimetric analysis (TGA), (b) differential thermogravimetric analysis (DTG), and (c) differential thermal analysis (DTA).

in Fig. 7. All diffraction lines in these patterns correspond to the rhombohedral phase a-Ga2O3 (JCPDS PDF card No. 06-0503, Fig. 1a). Significant differences in the intensities of particular diffraction lines (for example the strongest 104 and 111 lines) are caused by differences in a-Ga2O3 particle shapes, i.e., by the preferential orientation of elongated particles in respect to the X-ray beam during the XRD pattern recording. The X-ray powder diffraction patterns of b-Ga2O3 samples obtained by calcination of a-GaOOH samples at 900 °C are shown in Fig. 8. All diffraction lines in these patterns correspond to the monoclinic phase b-Ga2O3 (JCPDS PDF card No. 41-1103, Fig. 1b). Some differences in the relative intensities of diffraction lines due to different particle shapes and their preferential orientation are visible, like in the case of a-Ga2O3 patterns. Crystallite sizes in a-Ga2O3 and b-Ga2O3 samples are given in Table 2. By comparison with data for a-GaOOH it can be noticed

Table 3 Peak positions in DTA and DTG curves and TGA weight loss measured on different a-GaOOH samples. Sample

A-60 N-60 B-60 A-160 N-160 B-160

Dehydroxylation of a-GaOOH (°C) (DTA)

(DTG)

300 378 383 391 387 403

302 255, 363 245, 375 388 257, 382 401

Recrystallization of a-Ga2O3 (DTA) (°C)

a-Ga2O3 ? b-Ga2O3 (DTA) (°C)

Weight loss (TGA) (%)

525 550 530 505, 580 555 500, 580

640 650, 745 650, 745 743 741 750

12.5 14.4 12.0 9.7 11.0 8.2

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Fig. 8. XRD powder patterns of b-Ga2O3 samples obtained by calcination of aGaOOH samples at 900 °C. Positions and intensities of the b-Ga2O3 diffraction lines as given in the Powder Diffraction File (PDF) of the International Centre for Diffraction Data (ICDD) are shown at the bottom.

that crystallite size in a-Ga2O3 samples is generally larger than in corresponding a-GaOOH samples. However, further calcination and transformation to b-Ga2O3 induced a reduction (except in the case of sample A-60) in crystallite size. A reduction in crystallite size during the a-Ga2O3 to b-Ga2O3 transformation was greater in the case of samples obtained by calcination of hydrothermally prepared a-GaOOH.

3.2.2. Scanning electron microscopy The FE-SEM images of a-Ga2O3 and b-Ga2O3 samples obtained by calcination (at 500 or 900 °C, respectively) of a-GaOOH samples prepared at 60 °C are shown in Fig. 9 and in Figs. SD3 and SD4 in Supplementary data. The shape of initial a-GaOOH particles is preserved in a-Ga2O3 samples obtained at 500 °C, but calcination at 900 °C caused a partial change of particle shape due to the collapse of structural holes formed in particles by dehydroxylation. A similar retention of the morphology of a-GaOOH particles after heating to 250 or 500 °C and its collapse after heating to 750 or 1000 °C was observed in the case of a-GaOOH spindle-like (zeppelin-like) particles [7]. A better stability of particle structure and shape during heating was observed in the case of a-Ga2O3 and b-Ga2O3 samples obtained from a-GaOOH samples synthesized at 160 °C (Fig. 10 and Figs. SD5 and SD6 in Supplementary data). Tiny holes of up to several tens of nanometers in diameter are well visible at the surfaces of both a-Ga2O3 and b-Ga2O3 particles. However, particle shape in these gallium oxides was the same as the initial aGaOOH particles shape. Lower water content in hydrothermally prepared a-GaOOH samples is the possible reason for a better stability of particle shape during high temperature calcination due to a lower content of structural holes in thus formed porous gallium oxide particles. As a-GaOOH and a-Ga2O3 are both based on the hexagonal close packing (h c p) of oxygen anions, the stacking sequence of layers of close packed oxygen anions ABABAB is preserved during dehydroxylation. The direction of stacking of these layers is [1 0 0] in a-GaOOH (Fig. 1c) and [0 0 1] in a-Ga2O3 (Fig. 1a). The direction of the elongation of a-GaOOH rods, [0 0 1] direction, transforms to [0 1 0] direction in a-Ga2O3 rods obtained by heating to 500 °C. In b-Ga2O3 oxygen anions form a slightly disordered face centered cubic (fcc) lattice with gallium cations occupying both octahedral and tetrahedral interstices (Fig. 1b). The transformation of a-Ga2O3 to b-Ga2O3 requires greater structural changes than the transformation a-GaOOH to a-Ga2O3 and this is probably one of the reasons why some a-Ga2O3 particle shapes collapsed during the transformation to b-Ga2O3 particles. 3.2.3. Infrared spectroscopy The FT-IR spectra of a-Ga2O3 samples obtained by calcination at 500 °C are shown in Fig. 11. Different features in the spectra are well visible and these differences can be attributed to the different shapes of particles in a-Ga2O3 samples. The infrared spectra of other corundum-type oxides isostructural with a-Ga2O3 (a-Al2O3,

Fig. 9. FE-SEM images of a-Ga2O3 and b-Ga2O3 samples obtained by calcination at 500 or 900 °C, respectively, of a-GaOOH samples synthesized at 60 °C.

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Fig. 10. FE-SEM images of a-Ga2O3 and b-Ga2O3 samples obtained by calcination at 500 or 900 °C, respectively, of a-GaOOH samples synthesized at 160 °C.

a-Fe2O3, a-Cr2O3) of different particle shapes have been studied and interpreted using the phonon theory based on the scattering of electromagnetic radiation by small particles [63]. Particle shape was shown as an important factor which influences the position of infrared absorption bands. A gradual shift of IR bands from 346, 439 and 485 cm1 in the spectrum of significantly elongated a-Ga2O3 particles (sample A-160) to 353, 435 and 493 cm1 in the spectrum of slightly elongated a-Ga2O3 particles (sample N-160) and, finally, to 365, 416 and 530 cm1 in the spectrum of almost isodimensional a-Ga2O3 particles (sample B-160) is present. In addition to that, the distance between IR bands at 524 and 485 cm1 in the spectrum of sample A-160 is reduced in the spectrum of sample N-160 and these bands eventually overlap in the spectrum of sample B-160. Similar tendencies are present in the IR spectra of a-Ga2O3 samples A-60, N-60 and B-60 (Fig. 11, left). The FT-IR spectra of b-Ga2O3 samples obtained by heating to 900 °C are shown in Fig. 12. Features of the IR spectra were changed depending on particle shape, similar to the IR spectra of a-Ga2O3 samples. A gradual shift of IR bands from 313, 456 and 667 cm1 in the spectrum of significantly elongated b-Ga2O3 particles (sample A-160) to 319, 465 and 669 cm1 in the spectrum of slightly elongated b-Ga2O3 particles (sample N-160) and, finally, to 325, 496 and 687 cm1 in the spectrum of almost isodimensional b-Ga2O3 particles (sample B-160) is present. Similar tendencies are present in the IR spectra of b-Ga2O3 samples A-60, N-60 and B-60 (Fig. 12, left). Fig. 11. Characteristic parts of the FT-IR spectra of prepared a-Ga2O3 samples.

3.2.4. Photoluminescence spectroscopy The photoluminescence spectra of selected b-Ga2O3 samples are shown in Fig. 13. A strong and broad blue luminescence band centered at about 450 nm (2.76 eV) and a weak UV luminescence shoulder at about 368 nm (3.37 eV) were observed in the PL spectrum of b-Ga2O3 sample N-160. These bands are also visible in the spectra of samples A-160 and B-160, but their intensity is very weak. b-Ga2O3 samples A-60, N-60 and B-60 showed no significant bands in the PL spectra. It has been suggested that the origin of blue luminescence in b-Ga2O3 could be the recombination of electrons from donors formed by oxygen vacancies and holes from acceptors formed by gallium vacancies [36,64]. Weak UV luminescence in b-Ga2O3 has been explained to originate from the recombination of self-trapped excitons [64]. Blue luminescence of low intensity in b-Ga2O3 samples A-160 and B-160 as

well as its absence in samples A-60, N-60 and B-60 can be explained by good crystallinity of these samples, with no significant quantity of structural defects and vacancies due to their formation at high temperature (900 °C) in the presence of oxygen. The highest intensity of blue luminescence in the PL spectrum of b-Ga2O3 sample N-160 can be explained by the highest number of defects in the structure of this sample, which could be related to the highest number of holes observed by FE-SEM in the prismatic particles of this sample in comparison with other b-Ga2O3 samples (Fig. 10). This can be related to the highest water content in precursor a-GaOOH particles prepared in neutral conditions (sample N-160) in comparison with other a-GaOOH samples hydrothermally formed at 160 °C.

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Fig. 12. Characteristic parts of the FT-IR spectra of prepared b-Ga2O3 samples.

 Thermal analysis showed significant differences in thermal weight loss and thermal transformation temperatures of different a-GaOOH samples. A higher weight loss in samples formed at lower temperature and in samples formed in neutral conditions indicates a higher water content, which can be explained by the higher content of amorphous gallium hydroxide and/or adsorbed water in these samples. Generally, the temperatures of thermal transition were higher for a-GaOOH samples of higher crystallinity.  Uniform a-Ga2O3 and b-Ga2O3 particles of different shapes, containing nano/microstructural holes, were obtained by heating the corresponding a-GaOOH particles to 500 or 900 °C, respectively.  The a-GaOOH particles of better crystallinity, obtained by hydrothermal treatment at 160 °C, showed a better conservation of shape during high-temperature phase transformation to b-Ga2O3 at 900 °C in comparison with the a-GaOOH particles obtained at 60 °C.  a-GaOOH, a-Ga2O3 and b-Ga2O3 samples of different crystallinity, particle size and shape, prepared under different conditions, showed differences in the intensities of lines in XRD powder patterns and differences in the band position in infrared spectra.  b-Ga2O3 samples showed differences in photoluminescence properties. A strong and broad blue luminescence band, which indicates the presence of structural defects, was observed only in the case of b-Ga2O3 sample N-160 having the highest content of holes formed by dehydroxylation of a-GaOOH.  The simple method described in this work could be used for the synthesis of well defined gallium oxides for advanced applications (catalysts, photocatalysts, gas sensors or luminescent materials).

Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jallcom.2014. 09.134. References

Fig. 13. Photoluminescence spectra of selected b-Ga2O3 samples.

4. Conclusions  Uniform a-GaOOH particles of different shapes (spindles, rhombic rods, rhombic prisms, hierarchical particles) were synthesized by simple low temperature (at 60 °C) or hydrothermal (at 160 °C) precipitation from GaCl3-TMAH aqueous solutions with pH adjusted to 5, 7 or 9.  a-GaOOH particles elongated in the direction of c-axis (spindles, rhombic rods) were obtained in an acidic medium, rhombic prisms in neutral and hierarchical particles in a weakly basic medium.  a-GaOOH crystallite size was strongly dependent on the synthesis pH and temperature. A possible template role of (CH3)4N+ at alkaline pH values influenced the microstructure of a-GaOOH particles.  Analysis of angles between crystal faces led to conclusion that the predominant crystal faces in a-GaOOH particles are (1 1 0) and (0 0 1).

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