Article pubs.acs.org/crystal
Synthesis of Anatase Nanosheets with Exposed (001) Facets via Chemical Vapor Deposition Woo-Jin Lee and Yun-Mo Sung* Department of Materials Science & Engineering, Korea University, Seoul 136-713, South Korea S Supporting Information *
ABSTRACT: High-density anatase titanium dioxide (TiO2) nanosheets with high-energy (001) surfaces were successfully synthesized on silicon and silicon-coated substrates via chemical vapor deposition (CVD). Randomly oriented nanosheets and aligned nanosheets were synthesized depending upon gas flow conditions, and different growth mechanisms were proposed for each structure. To prevent anatase-to-rutile phase transformation, the substrate temperature was maintained as low as 450 °C, and instead, hydrogen (H2) autoignition was induced to provide additional heat and pressure to the substrates in a moment. It is obvious that silicon vapor can suppress the growth of anatase crystals into a [001] orientation, resulting in the formation of two-dimensional (001) nanosheets. This strategy of passivating specific crystal facets using silicon can be simply extended to the tailoring of other nanosheet structures that are impossible to be obtained via general crystal growth approaches. surface passivation by F−, (001) planes became more stable than (101) and finally the nanosheets with exposed (001) planes could be synthesized. After this breakthrough, various wet chemical approaches have been introduced for the synthesis of anatase nanosheets using fluorine adsorption.12−16 The anatase nanosheets could grow regardless of substrates. This approach, however, reveals critical drawbacks. F and its compounds have been known as very hazardous chemicals. Moreover, these reports are focused only on hydrothermal/ solvothermal synthesis. These methods require high pressure and long reaction times, and therefore, some equipment such as an autoclave is needed. From this point of view, a different approach is needed for safer and simpler synthesis of anatase nanosheets with exposed (001) facets. Although CVD is one of the most well-established and effective methods for the growth of nanocrystals,17 only a few results have been reported on the CVD growth of anatase nanosheets. For instance, Wu et al.16 reported anatase nanowalls that are a combination of several nanorods, and Yuan et al.18 reported anatase nanosheets using zinc oxide (ZnO) templates. However, the exposed facets of anatase nanosheets could not be controlled, and only polycrystals could be obtained from those approaches. Perhaps the best solution for these problems is to synthesize single crystalline anatase nanosheets directly. In this paper, we report the direct synthesis of anatase (001) nanosheets via a CVD method. It was newly demonstrated that silicon (Si) could function as a capping
1. INTRODUCTION Because of the versatility in photocatalytic, water splitting, photovoltaic, gas sensing, and lithium storage characteristics, titanium dioxide has been intensively researched for the past decades.1−6 Also, titanium dioxide demonstrates superior photochemical stability compared to other compound semiconductors, such as CdSe, CdS, ZnSe, ZnS, ZnO, etc., and thus reliable and long-term use is possible for the various applications. It is well-known that anatase shows the best photochemical performance among three crystalline phases of titanium dioxide due to its conduction band position being slightly higher than the hydrogen generation level. To further enhance its surface photochemical reactivity, a variety of nanostructures, including nanoparticles, nanorods, nanowires, nanotubes, nanosheets, nanocomposites, etc., have been exploited. Recently, special attention has been paid to the surface engineering of anatase nanosheets to obtain more photoreactive surfaces. Although there have been controversial reports on the photoreactivity of each anatase surface,7,8 it is obvious that high-energy facets are necessary to maximize reactivity. However, it is hard to obtain the high-energy facets of anatase, such as (010) and (001), because anatase (101) is thermodynamically more stable compared to other planes. That is, the average surface energies of anatase TiO2 are 0.44, 0.53, and 0.90 J m−2 for {101}, {100}, and {001}, respectively.9,10 Therefore, it generally forms octahedra consisting of eight equivalent {101} family. Yang et al.11 numerically calculated and experimentally proved that fluorine adsorption can suppress the crystal growth of anatase TiO2 along the [001] direction. Initially, fluorine atoms were effectively adsorbed to (001) facets with relatively high surface energy. Because of the © 2012 American Chemical Society
Received: September 10, 2012 Revised: October 12, 2012 Published: October 12, 2012 5792
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agent for anatase (001) instead of F. Moreover, a simple method was investigated to obtain aligned nanosheets. Morphological features and crystal structures were investigated using SEM, XRD, and TEM, and crystal growth mechanisms were discussed in detail.
2. EXPERIMENTAL SECTION Aligned nanosheets (ANSs) and randomly oriented nanosheets (RNSs) were synthesized using two conventional tube furnaces. The two tube furnaces were connected to each other in series (one near the gas inlet denoted as furnace 1 and the other near the gas outlet denoted as furnace 2). One inch diameter and 0.24 in. diameter quartz tubes were used as an outer and inner tube, respectively. Substrates of silicon, alumina, and tungsten with a size of 1.5 × 2.5 cm were loaded at the center of the furnace 2. A titanium tetrachloride (TiCl4) bubbler was prepared under argon protection to prevent hydrolysis and was connected to the inner tube. A valve was mounted at the inlet of the bubbler (denoted as Ti-valve). For the synthesis of RNSs, argon (75 sccm) and oxygen (5 sccm) were provided through the outer tube. Furnaces 1 and 2 were heated to 800 and 450 °C, respectively. Subsequently, the TiCl4 bubbler was heated to 60 °C. A small amount of H2 (5 sccm) was then introduced through the bubbler and inner tube to secure H2 in a 800 °C region. It should be noted that H2 MFC was set to 50 sccm, and the flow was limited by partially opening the Ti-valve. When the temperature and gas flow became stabilized, the Ti-valve was opened entirely, and a flash by hydrogen autoignition was observed. The reaction time was 20 s. For the synthesis of ANSs, the experimental condition was identical to that of RNSs except for the H2 flow. H2 MFC was set to 50 sccm, and the flow was limited to 10 sccm by the Ti-valve. When the temperature and gas flow were stabilized, as mentioned in the RNS experiment, the Ti-valve was closed, and the bubbler was quenched to 20 °C. This decreased the bubbler pressure, and backflow was induced from the quartz tube to the bubbler. The Ti-valve was then opened entirely, and the reaction occurred during 20 s. Detailed information for the experimental setup is provided in the Supporting Information (Scheme S1).
Figure 1. Schematic diagram showing the gas flow condition for RNS growth (left) and SEM images of RNSs with different magnifications (right; a, b).
with a layered morphology were noticed at the beginning stage (Figure S1b, Supporting Information). Considering the experimental condition, either titanic acid (TiOx(OH)4−2x) or titanium oxychloride (TiOCl) seems to be the layered intermediate (Scheme S2, Supporting Information). It is wellknown that such layered molecules can be produced by hydrolysis or oxidation of TiCl4.19−23 Detailed molecular structures are provided in the Supporting Information (Scheme S2). Although water vapor was not provided, it would be easily produced from the reaction between H2 and O2. Moreover, in this experiment, only limited elements, H2, O2, and TiCl4, were used, ruling out other possibilities. However, it is hard to define its molecular structure exactly, mainly because those molecules are generally unstable and readily form TiO2 at this reaction temperature. Although the molecular structure was not defined, it is obvious that hydrolysis or oxidation of TiCl4 could affect the orientation of nanosheets. In addition, this means that aligned nanosheets could be synthesized by promoting those reactions. To confirm this hypothesis, backflow from the quartz tube to the bubbler was induced to provide O2 to the H2/TiCl4 mixture, right before H2 autoignition, as mentioned in the Experimental Section. As a consequence, ANSs could be obtained, as shown in Figure 2. The size of ANSs was smaller than that of RNSs, and thus the density of the nanosheets was higher (∼20−30/μm2) compared with that of RNSs. To confirm the crystal growth orientation, the nanosheets were observed using TEM. Figure 3a,b shows TEM and HRTEM images of a nanosheet with different magnifications. Figure 3b shows a fringe image and d-spacing corresponding to anatase (200) and (020). Because anatase has a tetragonal structure, this observation indicates that the exposed facet is (001). The (001) anatase nanosheets were almost free of crystalline defects, such as stacking faults. To further analyze the crystallinity in detail, XRD was conducted for the nanosheets. Figure 4 shows XRD patterns of RNSs (blue) and ANSs (red). Excluding the diffraction peaks from the sample holder (inset), all the peaks are in good agreement with the JCPDS standard diffraction patterns (No. 84-1285) of the anatase phase. XRD patterns indicate that anatase nanosheets are highly crystalline, and the XRD background increase in the range of 20−30° comes not from the sample but from the
3. RESULTS AND DISCUSSION Efforts were made to obtain anatase nanosheets with exposed (001) facets via the CVD process. Instead of fluorine, Si vapor produced from Si substrates by hydrogen autoignition was used for the passivation of (001) surfaces. The low substrate temperature of 450 °C could maintain the anatase phase, preventing anatase-to-rutile phase transformation. Figure 1a,b shows SEM images of anatase RNSs with different magnifications. The thickness of the nanosheets varied from 50 to 100 nm, and the density of the nanosheets was ∼10/μm2. The width largely varied because each nanosheet grew with a random orientation and was blocked by the growth of nearby nanosheets. Sometimes double- or triple-layered nanosheet structures were observed in this experiment (Figure S1a, Supporting Information). Although they were observed intermittently and locally, it was an abnormal phenomenon considering the experimental condition. As mentioned in the Experimental Section, H2 autoignition occurred in a quartz tube; therefore, the overall gas flow should be turbulent. In this condition, the supply of precursor cannot be steady, and nanosheets should grow with random orientations. On the basis of this consideration, we hypothesized that layered intermediates existed and functioned as precursors for the growth of anatase nanosheets. On the basis of the assumption that a layered structure would appear at the beginning stage if layered intermediates existed, a series of experiments were conducted for decreased reaction time (1−2 s). As a result, nanosheets 5793
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(105) peaks were broad compared with (101) and (200) peaks, indicating a small crystal size along ⟨001⟩ orientations. As a mechanism of this nanosheet formation, we propose that silicon adsorption suppressed the [001] growth of TiO2. Yang et al.11 calculated that the surface energy of anatase (001) is lower than that of (101) under fluorine and silicon adsorption, providing a theoretical foundation to this mechanism. To understand clearly, more attention should be paid to the experimental condition. First, nanosheets could be synthesized on a silicon substrate only. On the other substrates, such as alumina and tungsten, only bipyramidal or randomly agglomerated crystals were observed. When we use siliconcoated alumina and tungsten substrates, the nanosheets could be synthesized again (Figure S2, Supporting Information). For more definite evidence, an alumina plate was partially coated with silicon and used as a substrate. As a result, nanosheets were selectively grown on the coated region, as shown in Figure S3 (Supporting Information). Second, nanosheets were synthesized only when H2 autoignition occurred. Because silicon hardly evaporates at 450 °C, it is reasonable that an additional heat source is necessary. Moreover, the temperature of the H2 flame is 2000−2400 °C, which is near the boiling point of silicon (2355 °C). On the basis of these considerations, the possible mechanism is proposed. At the initial stage, H2 autoignition occurs. Because of the H2 flame, the surface of the silicon substrate was heated momentarily and Si evaporation occurred. Subsequently, Ti and O2 were provided, and TiO2 crystals formed on a silicon substrate. As for ANSs, this stage should be divided into two steps. Layered intermediates could be produced first by backflow of gas, and then precursors were supplied. While precursors were supplied from the top of the substrate, TiO2 crystals could be surrounded by silicon vapor. Because H2 autoignition could increase the pressure in the quartz tube, the adsorption rate of silicon on TiO2 would increase too. As a result, the precursor supply to the anatase [001] orientation could be effectively inhibited, and nanosheets were synthesized with exposed (001) facets. Since silicon is the most frequently used substrate for nanocrystal growth, it is inexpensive compared to other substrates, such as sapphire, SiC, AlN, etc., and it is a safe element to handle; this approach using Si together with H2 autoignition can be widely extended to the growth of other nanosheets with wanted facets. The proposed growth procedure is depicted in Scheme 1.
Figure 2. Schematic diagram showing the gas flow condition for ANS growth (left) and SEM images of ANSs with different magnifications (right; a, b).
Figure 3. (a) TEM and (b) HRTEM images of a synthesized anatase (001) nanosheet and corresponding FFT pattern (inset).
4. CONCLUSIONS To conclude, high-density anatase nanosheets showing exposed (001) facets were successfully synthesized via chemical vapor deposition. From TEM and XRD measurements, the crystallographic structure of the nanosheets was identified. Using two connected tube furnaces at different temperatures of 800 and 450 °C, H2 autoignition could be induced below its autoignition temperature (536 °C). This could not only enable evaporation of silicon but also prevent anatase−rutile phase transformation. This paper presents a distinct improvement in the synthesis of anatase nanosheets. First, this work demonstrates that another element, such as Si, can be used as a morphology-directing agent instead of F. Second, a simple route to obtain aligned nanosheet structures is proposed, showing a much higher density of nanosheets. Third, comparing with other approaches, the reaction time was remarkably decreased for anatase nanosheet formation. Although there have been plenty of reports on the synthesis
Figure 4. XRD patterns of random nanosheet (upper) and aligned nanosheet (lower) samples. Here, “A” denotes the anatase phase.
sample holder. The diffraction peaks from (103), (004), and (105) planes that are parallel or quasi-parallel to the (001) plane were relatively small or not observed. On the other hand, the intensity of (200) and (211) peaks was relatively large, indicating preferential crystal growth along the ⟨200⟩ orientation. Furthermore, in the aligned sample, (004) and 5794
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Scheme 1. Schematic Diagram Showing the Proposed Procedure of Anatase Nanosheet Formation: Silicon Evaporation, Precursor Supply, Nanosheet Growth under Si Adsorption, and Final Product (Clockwise)a
a
Broken lines and upward arrows in the third stage indicate the (001) nanosheet growth by Ti and O2 supply from the top.
of anatase nanosheets, this study shows a strategy and progress for effective controlling of the nanocrystal growth orientation. Also, this strategy can be easily extended to the synthesis of other nanosheets with certain exposed facets that cannot be achieved using conventional routes.
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ASSOCIATED CONTENT
S Supporting Information *
Detailed information of experimental setup and method, additional SEM image of nanosheets, and schematic diagram of the layered molecules. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the National Research Foundation of Korea (NRF) grants funded by the Korean Government (2011-0011205). The authors thank Dr. H. S. Baik of the Korea Basic Science Institute (KBSI) for providing the access to their HRTEM.
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REFERENCES
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