DOI: 10.1021/cg1004984
Understanding Hydrothermal Titanate Nanoribbon Formation
2010, Vol. 10 3618–3625
Kunlanan Kiatkittipong,† Changhui Ye,‡ Jason Scott,† and Rose Amal*,† †
ARC Centre of Excellence for Functional Nanomaterials, School of Chemical Engineering, The University of New South Wales, Sydney, NSW 2052, Australia, and ‡Key Laboratory of Materials Physics, Institute of Solid State Physics, Chinese Academy of Sciences, Hefei 230031, China Received April 15, 2010; Revised Manuscript Received June 1, 2010
ABSTRACT: Titanate nanoribbons were prepared by hydrothermal treatment of neat TiO2 P25 with 10 M NaOH at 200 °C. Hydrothermal aging times were varied to assist with understanding the transformation process while the impact of postsynthesis acid washing on nanoribbon formation kinetics was assessed. Nanoribbon evolution was observed to occur via the formation of intermediate amorphous titanate particles. The final nanoribbon product was found to comprise a layered titanate framework with sodium cations interspersed between the sheets. Acid washing served to exchange the Naþ with Hþ to give H2Ti3O7. Moreover, acid washing accelerated nanoribbon formation, providing a well-defined (i.e., no amorphous particles) nanoribbon sample after 15 h compared to 40 h when acid washing was not used. The difference in formation rates was attributed to interlayer formation of the intermediates in the hydrogen bond array (i.e., Ti-O-H) during H2Ti3O7 being faster than intermediate formation in the sodium bond array (i.e., Ti-O-Na). Acid washing of the nanoribbon samples after extended hydrothermal synthesis times (>24 h) led to fragmenting of the structure.
1. Introduction In the past decade, synthesizing metal oxides with controlled architectures has been the subject of increasing interest due to the beneficial properties the structures can provide.1,2 In particular, titanium oxide nanostructures with high physical and chemical stability are used in a range of applications such as catalyst supports,3-5 H2 storage,6,7 and lithium batteries.8-10 Methods for synthesizing titania/titanate nanostructures include chemical (template) synthesis, electrochemical synthesis (anodization of Ti metal), and the alkaline hydrothermal method.11 Kasuga et al.12,13 first reported using hydrothermal synthesis to prepare titanate nanotubes by reacting TiO2 particles and concentrated NaOH followed by acid washing. Moreover, the synthesis conditions could be adjusted to fabricate titanate as other structures, including nanotubes, nanosheets, nanorods/nanowires, and nanoribbons/nanobelts. For instance, nanotubes were produced within the temperature range 100180 °C,12-18 while nanoribbons/nanobelts were generated at temperatures above 180 °C.19-21 Huang et al.,22 however, recently reported that the hydrothermal temperature affected only the kinetic rate and did not govern product morphology. They suggested anatase TiO2 particles were always first transformed into nanosheets and then further transformed into nanotubes or nanowires depending on the hydrothermal treatment period. The nanostructures have been found to possess different crystal structures and components, including H2Ti3O7/ Na2Ti3O7/NaxH2-xTi3O7,8,20,23-26 H2Ti2O4(OH)2/Na2Ti2O4(OH)2/NaxH2-xTiO5(H2O),27 and HxTi2-x/4Ax/4O4 (x ∼ 0.7, A stands for vacancy).28 Conjecture also surrounds the role of acid washing on titania/titanate nanostructure formation. Some researchers have suggested the nanostructures are actually formed during the acid washing process, following the hydrothermal *To whom correspondence should be addressed. E-mail: r.amal@unsw. edu.au. pubs.acs.org/crystal
Published on Web 06/11/2010
reaction,12,13,24,29 whereas others have argued the structures are formed during the hydrothermal process, prior to the washing stage, and suggested the washing process serves merely to exchange Naþ with Hþ.15,23,27,30,31 Tsai and Teng32 proposed that differences in the severity of the hydrothermal conditions may also be a contributing factor. While the capacity of hydrothermal synthesis for preparing titanate nanostructures is known, there remains limited understanding regarding the mechanism of crystal growth as well as the role aspects of the synthesis conditions play in crafting the final structure. This study focuses on providing further insight into these two gaps in the knowledge, with a specific emphasis on fabricating the titanate nanoribbon structure. The role of cooling/aging, synthesis duration, and washing on structural features was assessed. 2. Experimental Section 2.1. Reagents. Titanium dioxide P25 (80% anatase, 20% rutile (Sigma-Aldrich)), sodium hydroxide (Ajax Finechem), and hydrochloric acid (32 vol %, Ajax Finechem) were used without further purification. 2.2. Titanate Nanoribbons Synthesis. Titanate nanoribbons were synthesized using the hydrothermal conditions reported by Kasuga et al.12,13 The synthesis stages and variations applied to the synthesis stages are depicted in Figure 1. Initially, 0.5 g of TiO2 and 20 mL of 10 M NaOH were stirred for 1.5 h in a plastic bottle. The slurry was transferred to a Teflon-lined autoclave, which was sealed and placed in an oven at 200 °C. The slurry was then hydrothermally treated for a set duration, cooled for a set duration, and then either washed and centrifuged or directly centrifuged before being dried for 72 h at 80 °C. Initial variations to the synthesis process (path A in Figure 1) comprised hydrothermally treating the sample for 24 h and then terminating the process by the following: (1) immersing the autoclave in an ice bath; (2) allowing the autoclave to stand at room temperature for 5 h; or (3) allowing the autoclave to stand at room temperature for 20 h. The products were washed first with 0.1 M HCl and then with water until the pH value of the wash solution reached approximately 6. Subsequent variations to the synthesis process involved altering the hydrothermal treatment time at 200 °C (path B in Figure 1). r 2010 American Chemical Society
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Figure 1. Schematic of variations to the experimental parameters during hydrothermal synthesis of titanate nanoribbons. Path A illustrates conditions assessed during the cooling stage; Path B illustrates the hydrothermal treatment periods and recovery steps employed. Hydrothermal treatment was varied over the time frame of 3-40 h with the fabricated particles then subject to either an acid wash and centrifuging or direct centrifuging prior to drying. 2.3. Sample Characterization. The crystal and structural characteristics of the products were investigated by powder X-ray diffraction (XRD) performed on a Philips X’pert Multipurpose X-ray diffraction system with monochromatized Cu KR radiation ( χ = 1.5418 A˚). Nanoribbon morphology was assessed by scanning electron microscopy (SEM, Hitachi S900) and transmission electron microscopy (TEM, JEOL 1400 and Phillips CM200 including selected area electron diffraction (SAED)). Raman spectroscopy was performed using a Renishaw Invia Raman microscope, with an excitation wavelength of 514 nm. The quantity of sodium was measured by inductively coupled plasma-atomic emission spectroscopy (ICP-AES, Perkin-Elmer ELAN 6100 ICP-MS and DRC2 ICP-MS).
3. Results 3.1. Effect of the Hydrothermal Termination Procedure. The SEM micrographs in Figure 2 demonstrate the quality of the fabricated nanoribbons is influenced by the hydrothermal termination procedure. In particular, a more uniform and “regular” nanoribbon structure is generated when the hydrothermal reaction is quenched. Figure 2a indicates the nanoribbons grow to a range of lengths up to 10 μm and possess widths up to 200 nm. Allowing the system to cool and age at room temperature promoted the formation of particles as well as nanoribbons (Figure 2b and c). The lower magnification images show nanoribbons of similar dimensions to those of the quenched sample, and as higher magnifications are invoked, the presence of particles becomes more prevalent, particularly for the 5 h aging time.
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At the highest magnification, the particles in the samples aged for 5 h appear to comprise aggregates of smaller particles, 50-200 nm in diameter. These aggregates are not as apparent for the sample aged for 20 h. Based on these results, quenching was used to terminate the hydrothermal reaction in all subsequent experiments. 3.2. Influence of Acid Washing and Hydrothermal Treatment Time. 3.2.1. Without Acid Washing. Morphology evolution of the titanate nanoribbons after hydrothermal treatment at 200 °C for 3, 6, 15, 18, 21, 24, 27, 30, and 40 h and without acid washing is displayed in the TEM images of Figure 3. After 3 h the image shows the presence of a number of larger darker particles (up to 0.5 μm in diameter) within the aggregate. At 6 h evidence of nanoribbon formation is apparent, with what appears to be a mixture of long and short lengths in the sample. As hydrothermal treatment time continues, the nanoribbons and particles are present in varying sizes and to different extents. Evident in the 27 h image, and more so in the 30 h image, is the darker particles appear to be attached to the nanoribbons and do not exist as isolated entities. By 40 h the dark particles have essentially disappeared, with well-defined nanoribbon structures remaining. The crystal structure and composition of the nanoribbons and particles observed in Figure 3 were investigated by Raman spectroscopy, XRD, and SAED. The Raman spectra (Figure 4) show the presence of a sharp band at 1080 cm-1 irrespective of the treatment time. This can be ascribed to the stretching vibration of a Ti-O-Na bond33 and is indicative of the concentrated NaOH breaking the Ti-O-Ti bond to form a Ti-O-Na bond. The band regions of 100-200 cm-1 and 600-1000 cm-1 are related to Ti-O with different stretching vibrations in TiO6 octahedra.33-35 Noticeable in Figure 4, the vibration in edge-shared TiO6 at 600-700 cm-1 is shifted to lower values with increasing hydrothermal time. This is due to bending of the structure when forming the layers.36 The bands at 720-970 cm-1 are characteristic of titanate.35 At longer reaction times, a peak representing Ti-O-Ti bonding appeared at 250-500 cm-1.14 This may be indicative of increasing connection between the TiO6 frameworks to form the final structure. The presence of Ti-O-Na and Ti-O-Ti bonds in the nanoribbon structure is corroborated by the XRD profiles in Figure 5, whereby the diffraction peaks of all samples could be indexed to sodium titanate (Na2Ti3O7). The corresponding lattice constants of Na2Ti3O7 were determined as a = 15.13, b = 3.74, c = 9.15 A˚, and β = 99.30°. The complex XRD pattern at different hydrothermal treatment times highlights the disorder in the crystal phase of this sample. The 3 h profile has noticeably fewer peaks than the ensuing profiles, which may be attributed to the lack of nanoribbon formation at this time (illustrated in Figure 3). Additionally, comparing the XRD profiles of the 3 h sample and unreacted TiO2 P25 gave no evidence of TiO2 P25 after 3 h, indicating it had been consumed by this stage. The 6 h profile displays additional peaks compared to the 3 h profile, with new peaks appearing at 2θ = 26, 27, 47, and 48° demonstrating the formation of new crystal phases within the system. The XRD peak at 58° is evident until 21 h, with its disappearance at 24 h coinciding with the appearance of the diffraction peak at 11°. The peak at 11°, appearing after 24 h, corresponds to the [100] titanate crystal facet and is an indicator of growth in that direction after a set time frame.37 Loss of the peak at 58° indicates separation of Ti in edge
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Figure 2. SEM images of titanate samples terminated by (a) quenching in an ice bath; (b) aging at room temperature for 5 h; or (c) aging at room temperature for 20 h.
TiO6, which is then connected to the layered titanate framework, represented as the peak at 11° after 24 h.38 SAED analysis (Figures 6 and 7) of the components (i.e., nanoribbons and nanoparticles) within the system provided further detail on crystal growth during synthesis. Analysis of the particles formed during the first 3 h of hydrothermal treatment indicated they were amorphous in nature, providing no diffraction pattern (Figure 6a). Similar amorphous diffraction patterns were obtained for the dark particles coexisting with the nanoribbons during longer treatment times (Figure 6b). SAED analysis of nanoribbon structures present in the 6-21 h period (Figure 7a) indicated growth of titanate in the [020] direction with a lattice spacing of 0.19 nm. Beyond 24 h appearance of the [100], [020], and [110] crystal facets occurred in the SAED pattern (Figure 7b), with it possessing a lattice spacing of 0.75 nm. This facet relates to the interlayer spacing of sodium titanate in the direction perpendicular to the ribbons axis and agrees well with the XRD pattern. It suggests nanoribbon width is expanded with continued hydrothermal treatment. The appearance of the [020] facet and the [100] facet in the diffraction patterns after
6 and 24 h, respectively, also coincides closely with the XRD findings. 3.2.2. With Acid Washing. As shown by the TEM images of Figure 8, after 3 h of hydrothermal treatment, titanate aggregates were present as well as evidence of nanoribbon formation at this time. After 6 h, a mixture of nanoribbons and the larger dark particles was observed. At 15 h and beyond, only nanoribbons were observed, although with increasing treatment times (>24 h) they appear to become increasingly fragmented. ICP-AES of the samples indicated up to 99% of the Naþ was exchanged by Hþ during the acid wash.39 This is reflected in the Raman spectra (Figure 9), where there is no evidence of the Ti-O-Na stretching vibration band at 1080 cm-1. The Raman spectra depict strong Ti-O-Ti vibration bands at 120-700 cm-1.40,41 Additionally, the band at 787 cm-1 corresponds to covalent Ti-O-H bonds relating to hydrogen titanate (H2Ti3O7).40 Peaks in the 3 h profile are not as intense as at later times, corresponding to the limited titanate nanoribbon formation observed in Figure 8.
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Figure 3. TEM images of titanate samples prepared hydrothermally at 200 °C for 3, 6, 15, 18, 21, 24, 27, 30, and 40 h without acid washing.
Figure 4. Raman spectra of unwashed titanate samples with increasing hydrothermal treatment period.
XRD analysis (Figure 10) indicates the acid washed samples are hydrogen titanate (H2Ti3O7), consolidating the Raman spectra results. On comparing the XRD pattern between samples with and without acid treatment, the acid washed samples display a much simpler peak array, suggesting that acid treatment has an effect on the sample crystal phase. The disordered phase is rearranged into a higher order phase with more uniform orientation. This is reflected in the
peak intensity representing the major facet of the acid washed nanoribbons (at 11°) being more pronounced than that for the unwashed sample. The three distinct peaks located at 2θ ≈ 11°, 25°, and 48° correspond to the [200], [110], and [020] crystal facets, respectively. As evident from the XRD pattern, there is a delayed appearance of the peak at 2θ ≈ 11° (corresponding to the [200] crystal facet) with a small peak discernible after 6 h that has become distinct by 15 h. The spectra indicate at 3 h the samples comprise predominantly [110] and [020] crystal facets, with establishment of the [200] crystal facet occurring after 6 h. SAED analysis of the 3 h acid washed sample (Figure 11a) confirms the presence of the [110] and [020] facet observed in the XRD spectra (at 2θ ≈ 24.4° and 49°), possessing d-spacings of 0.37 and 0.19 nm, respectively. SAED analysis of the samples treated for 6 h or longer (Figure 11b) confirms the appearance of the [200] facet, giving a lattice spacing of 0.79 nm. 4. Discussion The sequence of TEM images corresponding to nanoribbon formation and growth with time (Figure 3) without acid washing have provided insight into the mechanism leading to final structure formation. It is anticipated the presence of TiO2 in concentrated NaOH at elevated temperatures will initially lead to its dissolution. As the TiO2 dissolves, it releases Ti4þ
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Figure 5. XRD patterns of unwashed titanate samples with increasing hydrothermal treatment period. Included is the XRD pattern for Degussa P25 and the peak reference for sodium titanate (Na2Ti3O7).
Kiatkittipong et al.
Figure 7. TEM images of the unwashed titanate samples hydrothermally treated for (a) 6-21 h or (b) 24-40 h. Corresponding SAED patterns showing the crystallography of the nanoribbon at 6-21 h and formed at 24-40 h.
ions which are surrounded by an octahedron of six O2- ions. The highly concentrated NaOH breaks the Ti-O-Ti bonds, replacing them with amorphous Ti-O-Na bonds to form a disordered TiO6 octrahedral framework.42 This manifests itself as the dark, amorphous particles observed in the TEM images and from SAED analysis. In our system, essentially complete conversion of Ti-O-Ti to amorphous Ti-O-Na is implied due to the lack of a TiO2 anatase phase, as was highlighted by the XRD and Raman spectra as reported earlier. Sodium titanate (Na2Ti3O7) is then constructed from the TiO6 frameworks, comprising Na between interlayers of dimensional frameworks and represented by the diffraction peak at 2θ = 11°. Formation of the sodium titanate can be described by eq 1. 3TiO2 þ 2NaOH f Na2 Ti3 O7 þ H2 O
Figure 6. TEM images of the unwashed titanate samples hydrothermally treated for (a) 3 h or (b) 6-30 h. Corresponding SAED patterns showing the crystallography of the amorphous particles at 3 h and formed at 6-30 h.
ð1Þ
Under hydrothermal synthesis, amorphous Ti-O-Na was detected during the first 3 h, while appearance of TiO6 frameworks and the interlayer structure, due to formation of the two-dimensional framework, was observed after 6 and 24 h, respectively. The difference between the rates implies that, for nanoribbon formation without acid washing, the step of TiO-Ti bond breaking by NaOH to produce Ti-O-Na is rapid and may not be the rate determining step. These findings illustrate that the complete formation of a layered nanoribbon framework without amorphous Ti-O-Na can be achieved with a sufficient NaOH hydrothermal treatment period and no acid wash. The sodium titanate nanoribbon formation stages without acid washing are depicted in Figure 12. Included are
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Figure 8. TEM images of titanate samples prepared hydrothermally at 200 °C for 3, 6, 15, 18, 21, 24, 27, 30, and 40 h with acid washing.
Figure 9. Raman spectra of hydrogen titanate with different hydrothermal treatment periods.
representations of the growth direction present during each phase of the growth, as interpreted from the XRD spectra and SAED analyses. The oval shape at 3 h without HCl treatment represents the amorphous particle structure, with the randomly arranged lines within the particle representing its disordered phase. The rectangular shapes represent the
nanoribbon structure. The major (ordered) facets and minor (disordered) facets of the nanoribbons are represented by the black and gray lines, respectively. The particles exhibit some degree of crystallinity, although it is low with a disordered array of crystal facets. As the presence of nanoribbons becomes more prevalent, additional facets appear, including the [020] sodium titanate facet, which represents growth along the nanoribbon axis. The 24 h growth period marks the appearance of the [110] and [100] titanate crystal facets, representing growth perpendicular to the nanoribbon axis in the [001] zone axis. The interlayer spacings of [020], [110], and [100] facets are 0.19, 0.37, and 0.75 nm, respectively. HCl acid washing promoted total exchange from TiO-Na to Ti-O-H in the 3 h sample, as demonstrated by Raman, XRD, and ICP-AES. However, there was no evidence of the interlayered titanate structure at this time. The crystallinity of the 3 h sample may result from the growth of TiO6 octahedra in an ordered direction. Edge sharing of TiO6 octahedra with Hþ present between the layers became evident from 6 h onward, with the appearance of the XRD feature at 2θ = 11°, which was much quicker when compared with the hydrothermal reaction without acid washing (24 h). The shorter hydrothermal time needed to produce the interlayered architecture in the acid washed case may derive from interlayer formation of the intermediate in the hydrogen bond
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Figure 10. XRD patterns of acid washed titanate samples with increasing hydrothermal treatment period. Included is the peak reference for hydrogen titanate (H2Ti3O7).
Figure 12. Schematic view of the sodium titanate and hydrogen titanate with different hydrothermal treatment periods (unit nm).
to the Ti-O-Ti anatase bond breaking step. Acid washing the sample can be described by eq 2. Na2 Ti3 O7 þ 2HCl f H2 Ti3 O7 þ 2NaCl
ð2Þ
The resulting hydrogen titanate nanoribbons had a more clearly defined crystal structure than their unwashed counterparts (Figure 12), with the [020] and [110] directions present after 3 h and the [200] direction initially evident after 6 h. The growth and structure of the acid washed samples is schematically shown in Figure 12. The interlayer spacings of [020], [110], and [200] facets correspond to 0.19, 0.37, and 0.79 nm, respectively. The main crystal growth direction of hydrogen titanate (H2Ti3O7) is similar to that of sodium titanate (Na2Ti3O7), which is in good agreement with the reports of Kolen’ko et al.43 and Zhang et al.44 Interestingly, as the synthesis time was increased to beyond 24 h, the acid washed nanoribbons became increasingly fragmented. Others23,45,46 have also found HCl treatment not to enhance formation of their titanate nanostructures, instead damaging the morphology and generating a higher number of defects. Figure 11. TEM images of the acid washed titanate samples hydrothermally treated for (a) 3 h or (b) 6-40 h. The insets are the corresponding SAED patterns showing the crystallography of nanoribbons formed at 3 h or nanoribbons formed at 6-40 h.
array (e.g., Ti-O-H) being faster than that of the sodium bond array (e.g., Ti-O-Na) under milder conditions (e.g., room temperature). That is, for acid washing, the step of interlayer nanoribbon formation becomes more comparable
5. Conclusions Titanate nanoribbons were synthesized via the hydrothermal reaction of TiO2 P25 and 10 M of NaOH solution at 200 °C. Immediate quenching of the hydrothermal reaction provided well-defined nanoribbons with diameters in the range 100-200 nm and lengths on the order of 10 μm. Nanoribbon evolution occurred via the formation of intermediate
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amorphous Na2Ti3O7 particles. The final nanoribbons were determined to possess a layered framework comprising Naþ cations sandwiched between titanate sheets. Acid washing the sample after hydrothermal treatment exchanged the Naþ with Hþ and accelerated nanoribbon formation. This acceleration was attributed to the comparatively faster rate of Ti-O-H bond formation during acid washing compared with TiO-Na bond formation when acid washing was not employed. In both instances, the nanoribbon structures were elongated in the [020] and [110] facets with the [100] or [200] facets present in the direction perpendicular to the axis for the unwashed and acid washed materials, respectively. The sodium titanate structure also possessed a secondary disordered array of facets. Acid washing samples hydrothermally treated for 24 h or greater promoted defect formation and fragmenting of the structure. Acknowledgment. The authors would like to thank Dr. Aaron Dodd for his assistance with the SAED and Dr. Yu Wang for his support with XRD measurements
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