Raman Spectroscopy: A New Approach to Measure the Percentage of

Mar 8, 2012 - Uma Tumuluri , Joshua D. Howe , William P. Mounfield , III , Meijun Li ..... Valeria Nicolosi , Karine Heuzé , Karl Mandel , Sofia Demb...
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Raman Spectroscopy: A New Approach to Measure the Percentage of Anatase TiO2 Exposed (001) Facets Fang Tian,† Yupeng Zhang,† Jun Zhang, and Chunxu Pan* School of Physics and Technology, Center for Electron Microscopy and MOE Key Laboratory of Artificial Micro- and Nano-structures, Wuhan University, Wuhan 430072, China ABSTRACT: Controlling the growth of high-activity anatase TiO2 exposed {001} facets improves greatly the adsorption and electronic and photocatalytic properties and has been attractive for various environmental and energy-related applications. In this paper, we introduced a novel approach for quantitatively measuring the percentage of exposed {001} facets in anatase TiO2 by using Raman spectroscopy. Comparing to XRD, Raman peaks originate from the vibration of molecular bonds, that is, vibrational mode Eg and A1g peaks, which are related to different crystal planes. Therefore, it provided a high sensitivity and accuracy for measuring the percentage of the exposed facets from the micro perspective of molecular bonding with less measurement errors. With the photocatalytic experiments, we found that 50% was the optimal percentage of the exposed {001} facets for the highest efficiency, which seemed more reasonable than the value of 70% obtained from XRD. For example, Han et al.8 and Xiang et al.9 improved the process by using tetrabutyl titanate (Ti(C4H9O)4) and HF as a capping agent under hydrothermal conditions and enhanced the photocatalysis efficiency 9 times higher than that of the commercial P25 particles. Their methods had advantages, such as being simple, convenient, controllable, and repeatable. Moreover, the other alternative processes focused on the preparation methods of the chemical route;10,11 shapecontrol;12,13 C, N, and S doping;14−16 semiconductor quantum dots sensitization;17 and so on. It has been realized that there is an optimal percentage of exposed {001} facets in anatase TiO2 for reaching the highest photocatalytic activity.9 Currently, X-ray diffraction (XRD) is the only approach for characterizing the percentage of exposed {001} facets in anatase TiO2. It is based upon the following principles: (1) The thickness of the TiO2 nanosheets in the [001] direction and the length in the [100] direction can be calculated from the full width at half-maximum of (004) and (200) diffraction peaks, respectively. (2) Theoretically, the shape of the TiO2 nanosheets is supposed to be a standard cuboid. (3) According to the geometrical relationships between thickness and side length, the surface area of the exposed {001} facets and the total area of the TiO2 nanosheets can be calculated. (4) At last, the percentage of the exposed {001} facets in anatase TiO2 is roughly calculated from the ratio of these two areas. By using the XRD approach, a large sampling volume is required and the sweep speed should be very slow. Raman spectroscopy is a widely used approach for characterizing material surface structures, spatial uniformity,

1. INTRODUCTION Since Fujishima and Honda found that TiO2 split water to generate hydrogen in 1972,1 TiO2 has been widely used in many promising applications, such as sensors, cell batteries, and photocatalysis, due to its nontoxic, high catalytic activity, strong antioxidation capability and stability of organisms, chemistry, and photoelectrochemistry. In general, TiO2 has three major crystal structures involving rutile, anatase, and brookite, in which anatase TiO2 has been paid more attention, because of its high photocatalytic activity. To narrow its energy band gap (3.2 eV) for improving absorption in the visible light range, and reduce recombination of electron hole pairs for increasing photocatalytic efficiency, many research works have been reported, including doping with metal ions (Co, Fe, Ni, Cu, etc.),2 nonmetallic elements (C, N, etc.),3,4 noble metal (Au etc.) deposition,5 and semiconductor compounds (CdS, etc.).6 Recently, it has been known that the {001} facet of anatase TiO2 exhibits the highest surface free energy with the largest photocatalytic activity, because the average surface free energies of anatase TiO2 are in the sequence {001} 0.90 J/m2 > {100} 0.53 J/m2 > {101} 0.44 J/m2. However, under the common conditions, the majority of the external surface of anatase TiO2 is {101} facets during growth due to its lowest surface energy, rather than {001} facets. Therefore, controllable synthesis of the {001} exposed facets has became a hot research direction for greatly improving the photocatalytic activity of anatase TiO2. In 2008, Lu et al.7 first prepared the micrometer-sized anatase TiO2 single crystals with 47% highly reactive (001) exposed facets by using hydrofluoric acid (HF) as a capping agent under hydrothermal conditions, which exhibited high photocatalysis and efficiency of splitting water for hydrogen. Hereafter, variant processes have been proposed for synthesizing a high percentage of exposed {001} facets in anatase TiO2. © 2012 American Chemical Society

Received: February 8, 2012 Revised: March 7, 2012 Published: March 8, 2012 7515

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Figure 1. TEM (a) and HRTEM (b) of TiO2 nanosheets (sample TF15). The inset shows the corresponding image of vertical TiO2 nanosheets that show the thickness of the nanosheets.

mercury lamp was used as a light source, which generated light in the 350−450 nm range with a maximum intensity at 365 nm. The lamp was placed 25 cm above the liquid surface. An amount of 100 mg of photocatalyst was put into 100 mL of a 12 mg/L MB solution. The mixed solution was stirred incessantly, and after every 30 min, 3 mL of the solution was extracted to test the residual concentration of methylene blue, which was evaluated by measuring the change of maximum absorbance in the UV−vis spectrometry.

chemical compositions, phase transformation, crystallinity and defects, etc. It has been applied to study oxygen deficiency caused by the surface reconstruction of highly reactive {001} TiO2,18 Eg vibrational mode shifting to low wavenumber due to element S doping,19 and Raman shift and peak broadening along with the thickness increase of the high-energy {001} TiO2 nanosheets.20 In this paper, we find that Raman spectroscopy is also a simple, efficient, and accurate alternative approach for measuring the percentage of exposed {001} facets in anatase TiO2. Compared to XRD, it needs only a small sampling volume and is more convenient and easier to process.

3. RESULTS AND DISCUSSION Figure 1 shows typical TEM and HRTEM micrographs of sample TF15. It can be observed that a large amount of nanosheets exhibited the size of 28 nm in the side length and 3 nm in thickness, and they were well-dispersed. The flank of TiO2 nanosheets shows that the lattice spacing parallel to the top and bottom facets was 0.235 nm, corresponding to the (001) surface of anatase TiO2, which indicated that the top facets of the nanosheets were the (001) surface. Figure 2 illustrates XRD patterns for the anatase TiO2 with different volumes of hydrofluoric acid (HF). Obviously, along

2. EXPERIMENTAL DETAILS Sample Preparation. Anatase TiO2 nanosheets with a variant percentage of exposed {001} facets were synthesized by a hydrothermal route as described in ref 8. The detailed process was as follows: (1) A 50 mL portion of Ti(OC4H9)4 and 10 mL of deionized (DI) water were mixed with an appropriate amount of HF in a Teflon-lined autoclave with a capacity of 100 mL and then kept at 180 °C for 24 h. (2) After hydrothermal reaction, the white precipitates were collected, washed with DI water and ethanol three times, and then dried in an oven at 80 °C for 6 h. To adjust the percentage of the exposed {001} facets, the HF content was changed from 5 to 10 to 15 mL and named as TF0, TF5, TF10, and TF15, respectively. As a comparison, the sample TF15 was further mixed with NaOH solution for removing F− ions from the surface and named as TF15-F. Characterizations. Crystal structures of the samples were measured by using an X-ray diffractometer (XRD) (AXS D8 Advanced XRD, Germany) with Cu Kα radiation. The microstructures were observed by using a high-resolution transmission electron microscope (HRTEM) (JEOL JEM2010FEF, Japan). Raman measurement was carried out using a Raman spectroscopy (HORIBA Jobin Yvon LabRAM HR, France). The power of the laser was 10 mW, and the laser excitation was 488 nm. Scans were taken on an extended range (1000−3000 cm−1), and the exposure time was 2 s. The photoluminescence (PL) emission spectra of samples were detected with a fluorescence spectrophotometer (Hitachi F4600, Japan) using a 325 nm line from a xenon lamp. Photocatalytic Experiments. Photocatalytic properties of all the samples were examined by measuring the decomposition rate of methylene blue (MB) solution with the presence of the photocatalysts. In the experiment, a 250 W high-pressure

Figure 2. XRD patterns of the TiO2 nanosheets prepared with varying RF .

with increasing the volume of HF, the intensity of (004) diffraction peaks was reduced and its full width at halfmaximum was broadened, which indicated that the thickness of the TiO2 nanosheets in the [001] direction was decreased with adding the HF. In addition, the intensity of the (200) Bragg peaks was enhanced and its full width at half-maximum became 7516

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narrowed, which revealed that the side length of the TiO2 nanosheets in the [100] direction was increased with adding the HF. Therefore, according to these two peaks, the thickness in the [001] direction and the side length in the [100] direction of TiO2 nanosheets can be obtained, and the percentage of the exposed (001) facets can be calculated.7 Table 1 lists the Table 1. Structural Information of the TiO2 Nanosheets Synthesized at Different Reaction Conditions samples

average thickness (nm)

average length (nm)

percentage of {001}

TF0 TF5 TF10 TF15

10 10 4 3

8 19 21 28

11% 38% 68% 80%

Figure 4. Structural models of the anatase TiO2(101) surface (a) and (001) surface (b).

{001} facets, the number of the symmetric stretching vibration modes of O−Ti−O becomes less when the exposed {001} facets exist. That is to say, the higher the percentage of {001} facets are exposed, the less the number of the symmetric stretching vibration modes of O−Ti−O will be, and correspondingly, the intensity of the Eg peaks in the Raman spectra becomes decreased. In contrast, when the exposed {001} facets exist, the number of the symmetric bending vibration and the antisymmetric bending vibration of O−Ti−O increase. Similar to as discussed above, the higher the percentage of {001} facets that are exposed, the more the number of the symmetric bending vibration and the antisymmetric bending vibration of O−Ti−O, accordingly, and the intensity of the A1g and B1g peaks in the Raman spectra become increased. According to the above experimental results and theoretical analysis, clearly, the percentage of exposed anatase TiO2 {001} facets has a relationship with the intensity variations of the Raman vibrational mode Eg peak and A1g peak. Therefore, we proposed an alternative approach to quantitatively obtain the percentage of exposed {001} facets in anatase TiO2 by measuring the peak intensity ratio of the Eg and A1g peaks. Table 2 lists the results for the present samples.

experimental results for the samples. That is to say, along with the increasing of volume of HF, the thickness of the TiO2 nanosheets was decreased from 10 to 3 nm and the side length was increased from 19 to 28 nm, and correspondingly, the percentage of the exposed {001} facets was augmented from 38% to 80%, which was in accordance with the literature.9 When Raman spectroscopy was applied to characterize the TiO2 nanosheets, all samples were of the similar peaks appearing at 144, 394, 514, and 636 cm−1, as shown in Figure 3. It is indicated the typical anatase TiO2 phase and was also consistent with the XRD results. However, it is noticed that, along with the increasing of volume of HF, the intensity of the Eg peaks at 144 and 636 cm−1 reduced gradually, while the intensity of the B1g peak at 394 cm −1 and the A 1g peak at 514 cm −1 increased simultaneously. It has been known that the Eg peak is mainly caused by symmetric stretching vibration of O−Ti−O in TiO2, the B1g peak is caused by symmetric bending vibration of O− Ti−O, and the A1g peak is caused by antisymmetric bending vibration of O−Ti−O. For the regular TiO2 nanosheets without exposed {001} facets, the bonding modes on the surface are mainly saturated 6c-Ti and 3c-O modes, and unsaturated 5c-Ti and 2c-O. However, for the highly active TiO2 nanosheets with exposed {001} facets, only the unsaturated 5c-Ti and 2c-O bonding modes are distributed on the surface.7 The different vibration models of anatase TiO2 (101) and (001) surfaces are shown in Figure 4. As can be illustrated, compared to the nonexposed

Table 2. Peak Intensity and the Ratio of the Raman Vibrational Modes between Eg and A1g samples

peak intensity of Eg (144 cm−1)

peak intensity of A1g (514 cm−1)

percentage of {001}

TF0 TF5 TF10 TF15

1869.68 1502.07 754.74 625.75

149.64 299.26 394.70 488.21

8% 20% 53% 78%

Figure 3. Raman spectra in the range of 100−800 cm−1 for the (a) TiO2 nanosheets prepared with varying volumes of HF and (b) with and without surface fluorine termination. 7517

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Figure 5. Comparison of photodecomposition of MB with different photocatalysts under the irradiation of UV−vis light.

the F− on the surface of the highly reactive TiO2 nanosheets with exposed {001} facets, and they had the same peaks shift with the sample TF0. The photocatalytic experiments for the above samples revealed that there existed an optimal percentage of exposed {001} facets in anatase TiO2. That is to say, according to the Raman method, the photocatalysis efficiency researched the highest, when the HF added was 10 mL, and the corresponding percentage of the exposed {001} facets was around 50%. For example, in the case for degradation of methylene blue (MB) solutions, the photocatalytic activity was in the order: TF10 > TF15 > TF15-F > TF5 > TF0, as shown in Figure 5, which is similar to the other report.9 The fluorescence spectrum can provide information, including carrier capture, migration, conversion, separation, etc., and has been used for measuring the separation of the photogenerated electron−hole pair.22 The emission signals in the fluorescence spectrum are mainly from the recombination of the photogenerated electron−hole pairs, and the lower fluorescence intensity relates to the higher photocatalytic efficiency.23 Figure 6 shows the fluorescence spectra of the

It was evident that the trends of the percentage of exposed {001} facets were similar between XRD and Raman results. But compared with XRD, the data from Raman spectra seemed a little smaller. Owing to the absence of more accurate methods to measure the percentage of exposed {001} facets in anatase TiO2, therefore, the most precise method, XRD or Raman, can only be analyzed from their measuring principles and measurement errors. XRD measurement is based on the X-ray diffraction effect of periodical arrangements of atoms in a crystal and gives statistical average data. XRD has been widely used for precisely measuring lattice constant and crystal structures. However, the precision is associated with the crystal quality. For example, due to the nanosized effects, the defects in the TiO2 nanoparticles increase, which results in the expansion of the crystal and affects the width and shape of XRD peaks. Therefore, it will make possible measurement errors when measuring the height and the full width at half-maximum of the diffraction peaks. In addition, the TiO2 nanosheets are assumed to be a perfect geometry for calculating the percentage of exposed {001} facets by using XRD, which obviously is a discrepancy with actual situations. However, in the case of measurement by using microconfocal laser Raman spectroscopy, Raman peaks are originated from the vibration of molecular bonds, which is of high measuring sensitivity. In principle, the molecular bonds are in different ways on different crystal planes, and therefore, when the exposed facets varied in different percentages, the intensity of the corresponding Raman vibration modes will also exhibit different values. Actually, the Raman spectrum provides an approach to measure the percentage of anatase TiO2 exposed {001} facets from the micro perspective of molecular bonding, which is much closer to the real states. In addition, only the peak height is involved in the computational process, and the measurement error is smaller than that by using the XRD method. Thus, we believe that the measurement by the Raman method is more accurate, and it also has advantages, such as less sample volume requirement, measuring fast, more convenient, etc. As shown in Figure 3a, we also found that the Eg peaks (144 and 636 cm−1) of the TiO2 nanosheets tended to shift toward the low wavenumber with HF increasing, which was due to the changes in the thickness of the TiO2 nanosheets or the phonon confinement effect of nanoparticles caused by the nano effect.21 Figure 3b shows that Raman peaks of samples TF15 and TF15F are identical. It indicated that the peaks' position and intensity of the Raman vibration modes were not influenced by

Figure 6. PL spectra of the TiO2 nanosheets prepared with varying RF.

samples in a wavelength range of 350−550 nm. It can be seen that the peaks were in similar shapes, and the three main characteristic peaks at 398, 451, and 468 nm corresponded to the energy levels of 3.12, 2.75, and 2.65 eV, respectively. Moreover, Figure 6 also shows a decrease in emission intensity from the samples TF0 to TF10. This indicated that an appropriate amount of F− ions could significantly reduce the irradiative recombination rate of photogenerated electrons and holes in TiO2. The sample TF15, however, exhibited a 7518

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(14) Liu, G.; Yang, H. G.; Wang, X.; Cheng, L.; Pan, J.; Lu, G. Q.; Cheng, H. M. J. Am. Chem. Soc. 2009, 131, 12868−12869. (15) Yu, J.; Dai, G.; Xiang, Q.; Jaroniec, M. J. Mater. Chem. 2011, 21, 1049−1057. (16) Xiang, Q.; Yu, J.; Jaroniec, M. Phys. Chem. Chem. Phys. 2011, 13, 4853−4861. (17) Qi, L.; Yu, J.; Jaroniec, M. Phys. Chem. Chem. Phys. 2011, 13, 8915−8923. (18) Liu, G.; Yang, H. G.; Wang, X.; Cheng, L.; Lu, H.; Wang, L.; Lu, G. Q.; Cheng, H. M. J. Phys. Chem. C 2009, 113, 21784−21788. (19) Liu, G.; Sun, C.; Smith, S. C.; Wang, L.; Lu, G. Q.; Cheng, H. M. J. Colloid Interface Sci. 2010, 349, 477−483. (20) Yang, X. H.; Li, Z.; Liu, G.; Xing, J.; Sun, C.; Yang, H. G.; Li, C. CrystEngComm 2011, 13, 1378−1383. (21) Bersani, D.; Lottici, P. P. Appl. Phys. Lett. 1998, 72, 73−75. (22) Zhang, W. F.; Zhang, M. S.; Yin, Z.; Chen, Q. Appl. Phys. B: Lasers Opt. 2000, 70, 261−265. (23) Yu, J. C.; Yu, J.; Ho, W.; Jiang, Z.; Zhang, L. Chem. Mater. 2002, 14, 3808−3816.

significant increase in the emission intensity, when compared with TF10. This was due to the introduction of new defect sites (or recombination centers), which enhanced the recombination of photogenerated electrons and holes. Our PL measurement results mean that the photocatalytic activity was the order TF10 > TF15 > TF15-F > TF5 > TF0, which are consistent with the consequence of degradation of methylene blue water.

4. CONCLUSIONS In theory and experiment, it has been demonstrated that Raman spectroscopy provides a simple and more accurate approach for measuring the percentage of anatase TiO2 exposed {001} facets. That is: (1) Both XRD and Raman methods exhibited the similar variation tendency with the volume of HF addition. (2) Photocatalytic experiments revealed that there existed an optimal percentage of exposed {001} facets in anatase TiO2. The value of 50% from the Raman method seems more reasonable than the value of 70% from the XRD method. (3) The Raman method exhibits more advantages, such as only a small sampling volume requirement, more convenience, and easier processing.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +86-27-68752481, ext. 5201. Author Contributions †

These authors contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by the National Basic Research Program of China (973 Program) (No. 2009CB939705), the National Natural Science Foundation of China (No. 11174227), and the Chinese Universities Scientific Fund.

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REFERENCES

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