Article pubs.acs.org/JPCC
Preparation of Metal Oxide Nanofilms Using Graphene Oxide as a Template Sakae Takenaka,*,†,‡,§ Shuhei Miyake,† Shunsuke Uwai,† Hideki Matsune,† and Masahiro Kishida† †
Department of Chemical Engineering, Graduate School of Engineering, Kyushu University, 744 Moto-oka, Nishi-ku, Fukuoka 819-0395, Japan ‡ JST, PRESTO, 4-1-8 Honcho Kawaguchi, Saitama 332-0012, Japan § International Institute for Carbon Neutral Energy Research (I2CNER), Kyushu University, 744 Moto-oka, Nishi-ku, Fukuoka 819-0395, Japan S Supporting Information *
ABSTRACT: Two-dimensional (2D) materials have lately attracted significant attention, including nanosheets composed of metal oxide single crystallites that are typically prepared using a top-down synthesis method such as the exfoliation of layered metal oxides. In the present study, nanofilms composed of polycrystallites of metal oxides such as TiO2, ZrO2, Nb2O5, SnO2, and Ta2O5 were prepared from the corresponding metal alkoxides using graphene oxide (GO) as a template, applying a bottom-up method. In this process, dried GO powder was dispersed in cyclohexane containing metal alkoxides and then treated at 453 K in an autoclave, such that the GO was converted into reduced GO (rGO) and metal oxide nanofilms were deposited on the rGO. Free-standing metal oxide nanofilms were subsequently obtained by calcination of the rGO/metal oxide composites in air at temperatures above 723 K. When dried GO was dispersed in cyclohexane containing metal alkoxides, the alkoxides adsorbed on the oxygen-containing functional groups of the GO were hydrolyzed by adsorbed or intercalated water in the dried GO and/or by water generated during the reduction of the GO, to form metal oxide nanofilms on the rGO.
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intercalated with dodecyl sulfate.10 Although nanosheets composed of single crystallites of metal oxides are thus obtained by top-down methods, the specific metal oxide nanosheets that may be obtained by exfoliation methods are inevitably limited because these methods require layered compounds as starting materials. Thus, it would be beneficial to also be able to create 2D materials using bottom-up processes. Unfortunately, it is difficult to prepare 2D materials in this manner because components such as metal cations and oxide anions are seldom spontaneously arranged in a 2D manner.11,12 In the present study, this problem was addressed by preparing nanofilms composed of polycrystallites of metal oxides with a bottom-up method through the use of graphene as a template. Graphene, a 2D honeycomb carbon network, has been studied by researchers in various fields ranging from catalysis to electronics due to its excellent electrical and thermal conductivity, high chemical stability, and large surface area.13 Graphene is also used as a building block for the preparation of 2D materials with high functionalities.14 Typically, metals and metal oxides are supported on graphene, and such composites composed of graphene and metals or metal oxides have exhibited superior performance as catalysts, photocatalysts, and
INTRODUCTION Two-dimensional (2D) materials have recently been the focus of significant attention with regard to the development of highly functionalized substances.1,2 This has occurred because 2D materials are expected to exhibit specific physical and chemical properties as a result of their nanoscale thicknesses and essentially infinite planar lengths. In particular, metal oxide nanosheets have recently been utilized as catalysts, photocatalysts, electrocatalysts, gas sensors, and capacitors, among other applications.3,4 The majority of the elements contained in metal oxide nanosheets is exposed at the surface, and thus these nanosheets possess high specific surface areas along with many coordination-unsaturated metal species that typically show enhanced catalytic activity. In addition, certain atoms and ions will readily diffuse into the nanosheets due to their molecularlevel thicknesses. As an example, HTiNbO5 nanosheets have been found to function as powerful solid acid catalysts due to the availability of interlayer acidic OH groups, while the bulk HTiNbO5 layered oxide exhibits poor activity for the same acid catalytic reactions.5 Ruthenium oxide nanosheets show an extremely high specific capacitance compared with bulk ruthenium oxides, as a result of their large electrochemically active surface areas and the full utilization of surface-active sites for the redox reaction.6 Metal oxide nanosheets have typically been prepared by the exfoliation of layered metal oxides or metal hydroxides.7−9 Ni oxide nanosheets, for example, can be prepared via the exfoliation of layered Ni hydroxides © XXXX American Chemical Society
Received: March 13, 2015 Revised: May 13, 2015
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DOI: 10.1021/acs.jpcc.5b02447 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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The Journal of Physical Chemistry C electrocatalysts. TiO2−graphene composites, for example, can be used as highly active photocatalysts, in which electrons generated by photoirradiation of the composite are transferred from TiO2 to graphene, which retards the recombination of electron−hole pairs in the TiO2.15−17 The Co3O4−graphene composites also show high activity for the oxygen reduction reaction18 since the strong interaction between Co3O4 and graphene leads to excellent catalytic performance. These composites composed of metal oxides and graphene are often prepared by the hydrolysis of metal alkoxides or of various metal oxide precursors under hydrothermal conditions.19,20 Under these preparation conditions, spherical, relatively large metal oxide particles are often stabilized on the graphene surfaces since the metal oxide precursors, such as metal alkoxides and metal chlorides, are readily hydrolyzed and polycondensed under hydrothermal conditions. The integration of graphene with thinner metal oxides having 2D structures, rather than with spherical metal oxide particles, should lead to strong interactions between the two phases due to the increase in their contact area, potentially resulting in highly sophisticated functionality through a synergistic enhancement. In the present study, we prepared composites of graphene and metal oxides with 2D structures, using graphene oxide (GO) as a template. Free-standing nanofilms composed of polycrystalline metal oxides could also be prepared by removing graphene from the composites.
solution (60%) at 393 K for 8 h in order to form oxygencontaining functional groups on its surface. TiO2 samples were prepared using these carbon materials as templates using a method similar to that applied during the preparation of metal oxide nanofilms, as described above. Characterization of the Samples. Transmission electron microscopy (TEM) images of the samples were acquired with an FEI Tecnai-20, operating at 200 kV. Specimens were prepared by ultrasonic dispersion in 2-propanol, after which an aliquot of the suspension was deposited on a carbon-coated copper grid. Atomic force microscopy (AFM) images were obtained using a Shimadzu scanning probe microscope (SPM9600) in tapping mode. In this process, MOx/rGO samples were dispersed in 2-propanol, and an aliquot of the suspension was deposited on a Si substrate. Following the acquisition of AFM images, the MOx/rGO specimens were heated at 723 K in air for 2 h to remove graphene, and the specimens thus obtained were observed again with AFM. Thermogravimetric analysis (TGA) was conducted with a Shimadzu DTG60 using a Pt pan sample holder in air, with a heating rate of 5 K min−1. Fourier-transform infrared (FTIR) spectra were obtained with a Jasco FT/IR-4200 spectrometer. The IR specimens were thoroughly ground with KBr and pressed into thin disks prior to measurements. Raman spectra for the MOx/rGO and MOx nanofilms were obtained with a Jusco NRS-3100 (excitation wavelength = 532 nm). Powder X-ray diffraction (XRD; Rigaku Ultima IV) patterns of the samples were measured using Cu Kα radiation (λ = 1.5406 Å). Zr K-edge and Ta LIII-edge X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) data were acquired at the Photon Factory of the Institute of Materials Structure Science for High Energy Accelerator Research Organization (KEK), Japan (proposal no. 2013G529). XANES/EXAFS spectra of the samples were obtained in the transmission mode at room temperature using the NW-10A beamline with a Si(311) twocrystal monochromator. Analysis of EXAFS spectra was performed using the REX EXAFS program (Rigaku Co.). Fourier transformation of the k3-weighted EXAFS data was conducted over the k range of 4−15 Å−1.
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EXPERIMENTAL SECTION Preparation of Metal Oxide Nanofilms. A 1 wt % dispersion of GO in water was obtained from the Mitsubishi Gas Chemical Co. The dispersion was dried to obtain GO powder. A 0.1 g portion of the dried GO was subsequently dispersed in 30 mL of cyclohexane (super dehydrated, Wako Pure Chemicals), after which 3.0 mL of the desired metal alkoxide (Ti(OnC4H9)4, Zr(OnC4H9)4, Nb(OnC4H9)5, Sn(OnC4H9)4, or Ta(OnC4H9)5) was added to the suspension. The suspension was stirred at room temperature for several days until the dried GO powder was homogeneously dispersed, following which the products were removed by centrifugation and washed several times with cyclohexane. The samples thus obtained are denoted as M(OBu)x/GO (M = Ti, Zr, Nb, Sn, and Ta). Each M(OBu)x/GO product was again dispersed in cyclohexane (40 mL) and transferred to a 100 mL Teflon-lined stainless steel autoclave in which it was heated at 453 K for 6 h. The resulting products were centrifuged and dried at 303 K, and the samples thus obtained are denoted as MOx/rGO (M = Ti, Zr, Nb, Sn, and Ta). The MOx/rGO composites were calcined in air at temperatures above 723 K for 2 h to form freestanding metal oxide nanofilms by removal of carbon atoms through combustion. The calcination temperatures of the composites of the metal oxides and rGO are provided in parentheses after the sample name. As an example, the TiO2 nanofilm obtained by calcination of TiOx/rGO at 723 K in air is referred to as the TiO2 nanofilm (723 K). TiO2 was also prepared using carbon black (Vulcan XC72), carbon black that had been treated with concentrated HNO3, GO that had been reduced with hydrazine, and GO with preadsorbed water molecules as templates. To achieve the reduction of GO, hydrazine monohydrate was added to a dispersion of GO in water after which the suspension was heated at 453 K in an autoclave.21 GO with preadsorbed water was obtained by adding distilled water (0.1 g) onto dried GO (0.1 g). Carbon black was treated in a concentrated HNO3
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RESULTS AND DISCUSSION Figure 1 shows TEM images and selected area electron diffraction (SAED) patterns for both GO and TiOx/rGO specimens. The TEM images of GO are highly transparent with some visible wrinkles, indicating that the GO sample was very thin (Figures 1a and b). The lateral dimensions of the GO sample ranged from 2 to 5 μm. A well-defined 6-fold symmetry was observed in the SAED pattern for GO (inset to Figure 1a), and this pattern was consistent with that reported for exfoliated graphene.22 As noted, dried GO was dispersed in 2-propanol prior to TEM observations. In contrast, dried GO powder was observed by scanning electron microscopy (SEM) without dispersion in 2-propanol. As shown in Figures S1a and b (Supporting Information), the graphene sheets in the dried GO powder were heavily stacked, thus the dried GO powder was used as a template for the preparation of metal oxide nanofilms. In addition, a strong diffraction peak was observed at 2θ = 10.2° in the XRD pattern of dried GO (Figure S1c, Supporting Information). This diffraction peak is assigned to the (002) reflection of stacked GO. Thus, the nanosheets in dried GO were regularly stacked. The (002) interlayer spacing for dried GO was evaluated from the XRD pattern as 0.87 nm, which is much larger than that for pristine graphite (0.34 nm). OxygenB
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Figure 2. (a) TEM image and elemental mapping images of (b) C, (c) O, and (d) Ti for TiOx/rGO.
Spherical particles were not evident in the TEM image of TiOx/rGO (Figure 2a), while the positions of the bright spots indicating the presence of Ti atoms (Figure 2d) coincided with the locations of carbon and oxygen atoms (Figures 2b and c). Titanium oxides in the TiOx/rGO were thus highly dispersed over the entire surface of the graphene sheets, even though they could not be observed clearly in the TEM images. The thickness of the TiOx/rGO specimen was evaluated based on its AFM images. Figure 3 shows representative AFM Figure 1. TEM images and SAED patterns obtained from (a and b) GO and (c, d, e, and f) TiOx/rGO.
containing functional groups, such as carboxylic, hydroxyl, and epoxide groups, on both sides of the GO nanosheets increase the interlayer spacing of graphite. TEM images of TiOx/rGO were also transparent, similarly to the GO images, as shown in Figures 1c and d. There were no particulates observed in the TEM images of the TiOx/rGO, even though TiO2 was present in the TiOx/rGO at a level of 27 wt %. Titanium oxide crystallites were hardly observed in the high-resolution TEM (HRTEM) image (Figure 1e). Several layers of stacked graphene were observed at the edges of the TiOx/rGO (Figure 1f). On the other hand, neither crystallized titanium oxides nor stacked GO were observed in the XRD pattern for TiOx/rGO (Figure S1c, Supporting Information), while dried GO as a starting material was heavily stacked. During the preparation of TiOx/rGO, dried GO is difficult to be dispersed in cyclohexane due to its hydrophilic property. When Ti(OC4H9)4 is added to the suspension, the titanium alkoxides adsorb on the functional groups on the GO, which results in the change of the surface property of the GO from hydrophilic to hydrophobic. Thus, GO sheets were highly dispersed in the TiOx/rGO. It therefore appears that the titanium oxides in the TiO x /rGO were both highly concentrated and uniformly dispersed on graphene sheets a few layers thick. The structure of titanium oxides in the TiOx/ rGO was amorphous. Elemental mapping images of Ti, O, and C in the TiOx/rGO were acquired in order to assess the dispersion of titanium oxide species on the GO, with the results presented in Figure 2.
Figure 3. (a and c) AFM images and (b and d) height profiles along the lines indicated in the AFM images for TiOx/rGO.
images and height profiles along the corresponding lines in the AFM images. The 2D nature of the material was evident in these images; the thickness of the TiOx/rGO was relatively uniform and ranged from 2 to 3 nm. This TiOx/rGO was obtained by impregnation with Ti(OC4H9)4 in cyclohexane followed by heating at 453 K in an autoclave, hence both sides of the graphene sheets were covered with very thin titanium oxide layers. The extent of titanium oxide loading in the TiOx/rGO was evaluated with TGA, and Figure 4a presents the TGA profiles C
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Figure 4. (a) TG profiles of GO and TiOx/rGO acquired under air and (b) FT-IR spectra of (i) GO, (ii) Ti(BuO)x/GO, and (iii) TiOx/rGO.
for GO and TiOx/rGO. In the TGA profile for GO, mass loss due to the desorption of water and the decomposition of functional groups was observed at approximately 300 and 470 K, respectively, before the combustion of carbon was initiated around 700 K.23 The GO was subsequently completely burned off by heating at 900 K in air. In contrast, neither the desorption of water nor the decomposition of functional groups was identified, although mass loss due to graphene combustion started at approximately 500 K in the TGA profile of TiOx/ rGO, indicating the reduction of GO to rGO during the treatment of Ti(BuO)x/GO at 453 K in the autoclave. The loading of titanium oxides in the TiOx/rGO was found to be 27 wt % from the TGA data. FT-IR spectra also showed the reduction of GO to rGO during the preparation of TiOx/rGO, based on the results presented in Figure 4b. The FT-IR spectrum of GO contained peaks assignable to O−H deformation, CO stretching, and O−H stretching at approximately 1620, 1740, and 3400 cm−1, respectively. In addition, small peaks due to C−OH stretching and the deformation of the symmetrical epoxy ring were observed between 1100 and 1400 cm−1.24,25 These peaks resulting from the functional groups in the GO were also found in the FT-IR spectrum of Ti(OBu)x/GO, although the peak positions were slightly shifted after the adsorption of Ti(OnC4H9)4 on the GO surfaces. Ti(OnC4H9)4 thus appears to be stabilized on the functional groups of GO during the impregnation of GO with cyclohexane containing Ti(OnC4H9)4. In contrast, the O−H deformation, CO stretching, and O−H stretching peaks disappeared, and a new absorption band assignable to the skeletal vibration of graphene appeared in the vicinity of 1560 cm−1 following heating of Ti(OBu)x/GO at 453 K in the autoclave. Therefore, the GO was reduced to rGO during the treatment of Ti(OBu)x/GO at 453 K. Metal oxides other than titanium oxides were deposited on the rGO by the impregnation of dried GO with the corresponding metal alkoxides in cyclohexane followed by treatment at 453 K in an autoclave. Figure 5 shows representative TEM images of ZrO x/rGO, NbOx/rGO, TaOx/rGO, and SnOx/rGO. The TGA profiles for these composites indicated that the concentrations of ZrO2, Nb2O5, Ta2O5, and SnO2 were 31, 30, 38, and 32 wt %, respectively (Figure S2, Supporting Information). Although TGA showed that the metal oxides were stabilized on the rGO in all the MOx/rGO composites, the presence of spherical metal oxide particles could not be confirmed in the TEM images. In addition, well-crystallized materials could not be found in the
Figure 5. TEM images of (a and b) ZrOx/rGO, (c and d) NbOx/rGO, (e and f) TaOx/rGO, and (g and h) SnOx/rGO.
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DOI: 10.1021/acs.jpcc.5b02447 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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The Journal of Physical Chemistry C HRTEM images for ZrOx/rGO, NbOx/rGO, TaOx/rGO, and SnOx/rGO (Figure S3, Supporting Information). Neither metal oxide crystallites nor stacked graphene sheets were observed in the XRD patterns for ZrOx/rGO, NbOx/rGO, TaOx/rGO, and SnOx/rGO (Figure S4, Supporting Information). Thus, the metal oxides in the composites were highly and uniformly dispersed on the rGO surfaces a few layers thick, similarly to the titanium oxides in TiOx/rGO. The MOx/rGO composites were calcined at 723 K in air to remove the rGO. Raman spectra for the samples thus obtained showed the removal of GO. Two bands were observed at around 1350 and 1580 cm−1 in the Raman spectra for TiOx/ rGO and ZrOx/rGO (Figure S5, Supporting Information). The peak at 1580 cm−1 (denoted as the G band) can be attributed to the in-plane carbon−carbon stretching vibrations of graphite layers, and the peak at 1350 cm−1 (denoted as the D band) is assignable to the structural imperfection of graphite.26,27 The relative intensity values of the D band to the G band for the TiOx/rGO and ZrOx/rGO were larger than that for the GO. The results also indicated that the GO was reduced into the rGO during the treatment of M(BuO)x/GO at 453 K in the autoclave.28,29 These bands due to the rGO completely disappeared after calcination of TiOx/rGO and ZrOx/rGO at 723 K in air, which indicated the removal of the rGO from MOx/rGO by combustion. As described earlier, XRD patterns showed that rGO nanosheets in the MOx/rGO are not heavily stacked but are highly dispersed. In addition, the rGO nanosheets in the MOx/rGO are in contact with metal oxides, which have catalytic activity for the oxidation of carbons with molecular oxygen. Thus, the rGO was removed by the treatment of the MOx/rGO at 723 K for 2 h in air. Figure 6 shows TEM images of the samples obtained by calcination of the MOx/rGO at 723 K in air. Interestingly, the 2D structure of the MOx/rGO was maintained after the removal of the rGO, and only very few spherical metal oxide particles appear in the TEM images, indicating the formation of free-standing metal oxide nanofilms by calcination of the MOx/rGO in air. The lateral sizes of these metal oxide nanofilms were similar to those of the initial MOx/rGO composites. The thickness of each metal oxide nanofilm also appeared to be very uniform from the TEM images. In addition, the SAED patterns for the TiO2 nanofilm (723 K) and SnO2 nanofilm (723 K) showed the polycrystalline nature of anatase TiO2 and rutile SnO2, whereas the crystallization degree of metal oxides in ZrO2 nanofilm (723 K), Nb2O5 nanofilm (723 K), and Ta2O5 nanofilm (723 K) was very poor. XRD patterns and HRTEM images for the TiO2 nanofilm (723 K) and SnO2 nanofilm (723 K) also showed the polycrystalline nature of these samples (Figure S6, Supporting Information). The XRD patterns for the TiO2 nanofilm (723 K) and SnO2 nanofilm (723 K) were well consistent with those for anatase TiO2 and rutile SnO2, respectively (Figure S6a, Supporting Information). The visible lattice fringes correspond to a spacing of 0.35 nm for ZrO2 nanofilm (723 K) (Figure S6b, Supporting Information) and 0.36 nm for SnO2 nanofilm (723 K) (Figure S6c, Supporting Information), which match with the expected d spacing of the (101) plane of anatase TiO2 and the (110) plane of rutile SnO2, respectively. These results demonstrated that free-standing nanofilms consisting of metal oxide polycrystallites could be prepared by calcination of the MOx/rGO composites in air. The thicknesses of the metal oxide nanofilms were evaluated by AFM. Figure 7 shows the AFM images obtained for TiO2, ZrO2, Nb2O5, and Ta2O5 nanofilms generated by calcination of
Figure 6. TEM images and SAED patterns of nanofilms of (a and b) TiO2, (c and d) ZrO2, (e and f) Nb2O5, (g and h) Ta2O5, and (i and j) SnO2 obtained by calcination of the corresponding MOx/rGO composites at 723 K in air.
the corresponding MOx/rGO materials in air at 723 K. These images were acquired both before and after calcination of the MOx/rGO supported on Si substrates at 723 K in air. The same nanofilms on Si substrates were assessed before and after E
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can be prepared from metal alkoxides using GO templates. On the basis of the thicknesses of these nanofilms, the original MOx/rGO samples must have been composed of metal oxide nanofilms with single-layer thicknesses on the rGO. The distribution seen in the thicknesses of the MOx/rGO specimens would have therefore resulted from the stacking quantity of rGO layers. We have previously reported that the thicknesses of free-standing TiO2 and ZrO2 nanofilms were found to range from 2 to 3 nm based on AFM images, values that are slightly higher than those observed for the metal oxide nanofilms in the present study.30 These former free-standing TiO2 and ZrO2 nanofilms were prepared by calcination of the corresponding MOx/rGO powders (loaded in an alumina boat) at 723 K in air. The samples thus obtained were deposited on a Si substrate to allow for the AFM measurements, while the samples used to obtain the AFM images in the present study were prepared by calcination of the MOx/rGO supported on a Si substrate in air at 723 K. The Si substrate is believed to have prevented sintering of the metal oxide nanofilms during calcination of the MOx/rGO in air. The MOx/rGO composites were calcined at different temperatures in air to examine the stability of the metal oxide nanofilms. Figure 8 shows TEM images of ZrO2 and Ta2O5 nanofilms obtained by calcination of the corresponding MOx/ rGO composites at 823 and 923 K in air. The TEM images of the nanofilms obtained by calcination at 723 K were previously presented in Figure 6. When the calcination temperatures for the ZrOx/rGO and TaOx/rGO were increased to 823 and 923 K, the 2D structures of both metal oxides did not change. The lateral sizes of the metal oxide nanofilms obtained by calcination of MOx/rGO at temperatures above 823 K were similar to those of the MOx/rGO. In contrast, the crystallinity of the metal oxides was improved with increasing calcination temperature. Although the diffraction rings in the SAED patterns for the ZrO2 nanofilm (723 K) were very broad (Figures 6c), the rings became sharper and clearer with increasing calcination temperature for ZrOx/rGO (Figures 8a and c). The SAED patterns for the ZrO2 nanofilms were consistent with that for tetragonal ZrO2. XRD patterns for the ZrO2 nanofilms also showed the improvement of crystallinity of ZrO2 with calcination temperatures of ZrOx/rGO (Figure S8, Supporting Information). Any diffraction lines could not be observed in the XRD pattern for ZrO2 nanofilm (723 K); however, diffraction lines due to tetragonal ZrO2 appeared in the XRD pattern for the ZrO2 nanofilm (823 K), and their intensity became stronger with the calcination temperatures of the ZrOx/rGO. Lattice fringes with a spacing of 0.30 nm were visible in the HRTEM images for ZrO2 nanofilm (923 K) (Figure S9a, Supporting Information). The fringes match well with the expected d spacing of the (101) plane of tetragonal ZrO2. The crystallinity of metal oxides in the nanofilms of TiO2, Nb2O5, and SnO2 was also improved with the calcination temperatures of the corresponding MOx/rGO in air. In contrast, Ta oxides in Ta2O5 nanofilms were not easily crystallized during the treatment of TaOx/rGO in air up to 923 K. No diffraction pattern was observed in the SAED (Figures 8e and g) and XRD patterns (Figure S8b, Supporting Information) for the Ta2O5 nanofilms. Thus, lattice fringes could not be found in the HRTEM images for the Ta2O5 nanofilm (923 K) (Figure S9b, Supporting Information). However, an increase in the calcination temperatures for the TaO x/rGO to 1023 K led to the formation of Ta 2O 5
Figure 7. (a, c, e, and g) AFM images and (b, d, f, and h) height profiles along the indicated lines in the AFM images for nanofilms of (a and b) TiO2, (c and d) ZrO2, (e and f) Nb2O5, and (g and h) Ta2O5 obtained by calcination of the MOx/rGO composites on Si substrates at 723 K in air.
calcination in order to evaluate the thickness change associated with the removal of the rGO layer. The AFM images for ZrOx/ rGO, NbOx/rGO, and TaOx/rGO prior to calcination are shown in Figure S7 (Supporting Information), whereas the TiOx/rGO image before calcination is presented as Figure 3a. Figure 7 shows the variations in the height profiles along the lines indicated in the AFM images for the MOx/rGO samples before and after calcination in air. From the AFM images in Figure S7 (Supporting Information) it is evident that 2D materials were produced in the case of the ZrOx/rGO, NbOx/ rGO, and TaOx/rGO specimens. Although the thicknesses of the MOx/rGO samples varied with the metal oxide, from 3 to 8 nm, variations in film thickness were also observed within the same MOx/rGO sample (panels b, d, f, and h in Figure 7). Following calcination of the MOx/rGO composites in air, the formation of 2D materials on the Si substrates was again confirmed, as shown in panels a, c, e, and g of Figure 7, indicating the formation of metal oxide nanofilms by calcination of MOx/rGO in air. This result is consistent with the TEM images shown in Figure 6. The thickness of the MOx/rGO specimens was seen to decrease during the calcination process; the thickness of each metal oxide nanofilm obtained by calcination of MOx/rGO at 723 K was estimated to be approximately 1 nm, while the thicknesses of the original MOx/ rGO composites ranged from 2 to 8 nm. Therefore, we conclude that metal oxide nanofilms approximately 1 nm thick F
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Figure 9. (a) Zr K-edge XANES spectra, (b) k3-weighted EXAFS spectra, and (c) Fourier transforms of the k3-weighted EXAFS data for ZrOx/rGO, ZrO2 nanofilms, and a ZrO2 reference. Legend: i, ZrOx/ rGO; ii, ZrO2 nanofilm (723 K); iii, ZrO2 nanofilm (923 K); iv, yttrium (8 mol %)-stabilized ZrO2.
removed from the ZrOx/rGO specimen during calcination at 723 K to form the ZrO2 nanofilm. In contrast, the EXAFS spectrum for the ZrO2 nanofilm (923 K) more closely resembled that for the tetragonal ZrO2 reference sample. Thus, the structure of the zirconium oxides in the ZrOx/rGO specimen did not change appreciably during removal of rGO by calcination at 723 K. As described earlier, Zr oxides with 2D structures were formed by calcination of ZrOx/rGO at 723 K. Thus, the Zr oxides in the ZrOx/rGO also had a 2D structure. A further increase of the calcination temperature for the ZrOx/ rGO from 723 to 923 K resulted in the crystallization of ZrO2 into a tetragonal phase. In the Fourier transforms of the EXAFS for all samples, two strong peaks due to scattering from O and Zr atoms were observed in the vicinity of 1.7 and 3.2 Å, respectively. It should be noted that the peak intensities due to scattering from Zr atoms for the ZrO2 nanofilms and the ZrOx/ rGO were much lower than that observed in the case of the tetragonal ZrO2 reference sample, although the peak intensity due to Zr−O bonds was almost the same for all samples. These results imply that the Zr oxides in the ZrOx/rGO and in both the ZrO2 nanofilms had 2D structures.31 Therefore, it is apparent that ZrO2 nanofilms were integrated with the rGO through the impregnation of the dried GO with Zr(OC4H9)4 in cyclohexane followed by treatment at 453 K in the autoclave. In addition, free-standing ZrO2 nanofilms could be prepared by calcination of ZrOx/rGO composites at temperatures higher than 723 K. The structural changes of the tantalum oxide species in TaOx/rGO during calcination were also examined through analysis of their Ta LIII-edge XANES and EXAFS spectra, with the results shown in Figure 10. The XANES spectrum for TaOx/rGO was similar to those obtained for the Ta2O5 nanofilms and is also consistent with that for the Ta2O5 reference sample, having an orthorhombic structure. Thus, the tantalum species in the TaOx/rGO and in the Ta2O5 nanofilms were stabilized in the Ta5+ state. The EXAFS spectrum for the TaOx/rGO sample did not change after
Figure 8. TEM images and SAED patterns for nanofilms of (a to d) ZrO2 and (e to h) Ta2O5 obtained by calcination of the corresponding MOx/rGO composites in air at (a, b, e, and f) 823 and (c, d, g, and h) 923 K.
polycrystallites with orthorhombic structure as shown in Figure S10 (Supporting Information). The chemical states and local structures of the Zr species in the ZrOx/rGO and ZrO2 nanofilms were examined by obtaining Zr K-edge XANES/EXAFS spectra, with the results shown in Figure 9. The XANES spectrum for ZrOx/rGO was very similar to those for the ZrO2 nanofilms obtained at 723 and 923 K. The threshold for the XANES spectra for both the ZrOx/rGO and the ZrO2 nanofilms, at approximately 18 000 eV, was consistent with that for a reference sample of tetragonal ZrO2. Thus, the Zr species in the ZrOx/rGO and the ZrO2 nanofilms were present as Zr4+. The features of the k3-weighted EXAFS data and the corresponding Fourier transform of the ZrO2 nanofilm (723 K) were also compatible with those obtained for ZrOx/rGO, although the rGO template was G
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Article
The Journal of Physical Chemistry C
Figure 10. (a) Ta LIII-edge XANES spectra, (b) k3-weighted EXAFS spectra, and (c) Fourier transforms of the k3-weghted EXAFS data for TaOx/rGO, a Ta2O5 nanofilm, and a Ta2O5 reference. Legend: i, TaOx/rGO; ii, Ta2O5 nanofilm (723 K); iii, Ta2O5 nanofilm (923 K); iv, Ta2O5 reference.
calcination at 723 K in air, although the rGO in the sample was removed during this treatment. In addition, the EXAFS spectrum for the Ta2O5 nanofilm did not change with calcination temperatures of TaOx/rGO in air, indicating that the local structure of Ta oxides remained unchanged during the calcination of TaOx/rGO. As noted above, the Ta2O5 nanofilm (723 K) had a 2D structure, and its thickness was approximately 1 nm based on its AFM image. Thus, the TaOx/rGO was composed of rGO with a Ta2O5 nanofilm. Finally, we evaluated the role of GO as a template for the formation of metal oxide nanofilms. As described earlier, metal oxide nanofilms could be prepared by calcination of MOx/rGO in air. The MOx/rGO composites were prepared by the impregnation of dried GO in cyclohexane containing the metal alkoxides, followed by treatment at 453 K in an autoclave. In contrast, no solid products were obtained by heating cyclohexane containing the metal alkoxides in an autoclave at 453 K in the absence of dried GO. In addition, TiO2 samples were prepared using templates consisting of carbon black (Vulcan XC72), carbon black treated with concentrated HNO3, GO reduced with hydrazine, and GO with preadsorbed water. The composites thus obtained were calcined in air at 923 K. TEM images of these composites and the TiO2 samples obtained by calcination of the composites are shown in Figure 11. Carbon black particles of 30−40 nm in diameter are observed in the TEM images of the composites prepared from carbon black, but TiO2 particles do not seem to have been deposited on the carbon black (Figure 11a). In addition, the TiO2 loading in the composites was found to be very low (
