Preparation and Photoelectrochemical Activity of Cr-Doped TiO2

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J. Phys. Chem. C 2010, 114, 2873–2879

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Preparation and Photoelectrochemical Activity of Cr-Doped TiO2 Nanorods with Nanocavities Hong Zhu, Jie Tao,* and Xiang Dong College of Material Science and Technology, Nanjing UniVersity of Aeronautics and Astronautics, Nanjing 210016, P.R. China ReceiVed: September 5, 2009; ReVised Manuscript ReceiVed: December 18, 2009

Cr-doped TiO2 nanorods with nanocavities were synthesized by a facile hydrothermal treatment and heating in air. The samples were characterized respectively by means of X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM), and X-ray photoelectron spectroscopy (XPS). XRD patterns indicated that all the samples were anatase crystalline. HRTEM results and the electron diffraction patterns illustrated that the TiO2 nanorods possessed the single-crystalline structure. TEM images confirmed that there were different types of nanocavities inside the nanorods, such as a circle, hexagon, and rectangle. XPS results suggested that Cr elements were successfully doped into the TiO2 nanorods after hydrothermal and most Cr congregated on the surface in the form of Cr2O3 after heating. The optical properties of the samples were studied with a UV-vis spectrometer. The photoelectrochemical activity of the Cr-doped TiO2 nanorods thin film was better than that of the commercial anatase TiO2 particulate thin film. The high photoelectrochemical activity of the synthesized Cr-doped TiO2 nanorods could be attributed to three factors: the doped Cr, onedimensional nanostructure of the nanorod, and the increased light-harvesting abilities. Introduction Titania, in particular, anatase TiO2, has been extensively used in photoelectrochemical systems, such as photocatalysts,1 solar cells,2 electrochromic devices,3 and sensors.4 It is well-known that one-dimensional nano-TiO2 has a significant influence on its photoelectric properties.5 In recent years, many researchers have devoted themselves to the synthesis of TiO2 with different morphologies for special applications, such as nanotubes,6 nanorods,7 and nanowires.8 However, the main disadvantage of TiO2 as a photoanode is related to poor absorption of sunlight due to its wide band gap (ca. 3.2 eV). Therefore, many groups are investigating how to narrow the band gap of TiO2 to maximize the utilization efficiency of solar energy and increase the yield of the electron-hole pair which is the key factor for the TiO2 applications. One effective approach is to dope different elements into TiO2, including metal9 or nonmetal elements.10 Some researchers have reported that Cr-doped TiO2 nanotubes can improve its visible light response.11 It is well-known that hydrothermal treatment is a cheap and facile method to prepare Cr-doped one-dimensional nano-TiO2.12 Furthermore, theoretical calculations predict a significant enhancement of the optical absorbance in a thin silicon film with nanocavities,13 which indicates another route to increase the photoreactivity of semiconductors. Nanocavities are isolated entities inside a solid and are very different from nanopores, which (often being irregular and made from amorphous material) connect together and are open to the surface.14 Han et al. reported the preparation of TiO2 nanorods with nanocavities by simply heating of the H2Ti3O7 nanorods in air and found that these dense nanocavities can significantly enhance the optical absorption coefficient of TiO2 in the near-ultraviolet region, thereby providing a new approach to increase the photoreactivity * Corresponding author. Tel.: +86-25-84895378. Fax: +86-25-52112626. E-mail: [email protected].

of the TiO2 nanorods, which can be used in applications related to absorbing photons.15 In this work, Cr-doped TiO2 nanorods with nanocavities were prepared by a facile hydrothermal method, followed by heating in air. The morphologies, structures, and optical properties of the doped TiO2 nanorods were also investigated. Experimental Section Commercial anatase TiO2 particles (0.75 g) (Shanghai HuZheng NaNo Tech Co., Ltd.) were mixed uniformly with 10 M NaOH (70 mL) in a 100 mL Teflon-lined stainless steel autoclave. Then different amounts of Cr(NO3)3 · 9H2O were added (the molar ratios of Cr/Ti are 1%, 3%, 5%, and 10%, respectively). The autoclave was heated to 180 °C for 48 h in the oven, subsequently followed by HNO3 washing. After being dried at 80 °C, the product was heated in air at 650 °C for an hour. The crystalline structure of the products was analyzed using powder X-ray diffraction (Bruker D8 Advance) with Cu KR radiation. The microstructures of the products were explored with high-resolution transmission electron microscopy (HRTEM, JEM2100). Simultaneously, the UV-visible light absorption spectra were obtained by using a UV-vis spectrometer (UV-vis, Shimadzu UV-2550). X-ray photoelectron spectroscopy (XPS) analysis of the microspheres was carried out through the Thermo ESCALAB 250 System. The photoelectrochemical properties of the synthesized TiO2 nanorods and commercial anatase TiO2 particles were further investigated in a three-electrode system with the aid of electrochemical workstation CHI (660B), using the FTO covered by the samples thin film, platinum electrode, and saturated calomel electrode as the working electrode, auxiliary electrode, and reference electrode, respectively. The Hg lamp (∼8 mW) was used for UV illumination. The sample size was 1.0 × 1.0 cm2.

10.1021/jp9085987  2010 American Chemical Society Published on Web 01/29/2010

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Figure 1. XRD patterns of (a) undoped TiO2 nanorods, (b) 1 mol % Cr/TiO2 nanorods, (c) 3 mol % Cr/TiO2 nanorods, (d) 5 mol % Cr/ TiO2 nanorods, and (e) 10 mol %Cr/TiO2 nanorods after hydrothermal treatment before heating treatment (A) and after heating at 650 °C (B). Figure 2. TEM images: (a) undoped TiO2 nanorods; (b,c) 3 mol % Cr/TiO2 nanorods after hydrothermal treatment before heat treatment.

Results and Discussion Figure 1 displays the XRD patterns of pure and Cr-doped TiO2 nanorods samples before and after heating treatment, with Cr content that ranges from 1 to 10 mol %. After hydrothermal treatment and washing with HNO3, the TiO2 precursor is transformed into titanate (H2Ti3O7), as indicated by the XRD pattern in Figure 1A. The XRD pattern in Figure 1A shows obvious diffraction of the peaks at 2θ ) 11.2°, 24.4°, and 48.4°, which respectively correspond to the characteristic diffraction of the (200), (110), and (114) planes of the titanate. There is no diffraction line of chromium or ion related oxide phases in all Cr-doped samples before heating treatment. It can be concluded that chromium ions have inserted into the lattice of TiO2 and locate at substitutional sites.16 After heating treatment, all the XRD patterns in Figure 1B match the anatase crystalline phases, having diffraction lines of (101), (004), (200), (105), (211), and (204) planes at 2θ values of 25.3°, 37.8°, 48.0°, 53.9°, 55.1°, and 62.7°, respectively, which coincide with the value of JCPDS card no. 21-1272. For the samples with 1-5 mol % Cr, the quantity and size of Cr2O3 particles are too small compared to those of TiO2 nanorods, which is proved by TEM and XPS results, and XRD cannot detect the diffraction peak of Cr2O3. However, when Cr-loading concentration reaches 10 mol %, two weak diffraction peaks appear at 2θ ) 33.6° and 36.2°, which respectively correspond to the characteristic

diffraction of the (104) and (110) planes of Cr2O3. Moreover, there is a small peak at around 44° in Figure 1A(a) and Figure 1B(a,b), which matches the carbon crystalline phase with the value of JCPDS card no. 50-1083. The small quantity of carbon may come from the pollution of the experimental process. Figure 2 shows the TEM images of undoped and 3 mol % Cr-doped TiO2 nanorods after hydrothermal treatment. The undoped and 3 mol % Cr-doped nanorods were 20-50 nm in diameter and 200 nm to 1 µm in length in Figure 2a,b. A small amount of short and thin nanotubes existed in the products in Figure 2b and no additional particles appear on the surface of the nanorods, so it can be assumed that chromium ions have inserted into the lattice of TiO2 after hydrothermal treatment. Figure 2c shows a HRTEM image of a Cr-doped H2Ti3O7 nanorod and also the corresponding electron diffraction patterns (inset) with lattice spacings of 0.78 nm, which corresponds to the (200) crystal plane. The diffraction pattern revealed that the H2Ti3O7 nanorod with Cr-doped possessed the single-crystalline structure. Figure 3 presents the TEM images of undoped and 3 mol % Cr-doped TiO2 nanorods after heating at 650 °C. There were many nanocavities inside both the undoped and doped TiO2 nanorods shown in Figure 3a,d. In Figure 3b,e, the sizes of the

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Figure 3. TEM images: (a-c) undoped TiO2 nanorods; (d-f) 3 mol % Cr/TiO2 nanorods after heating at 650 °C.

nanocavities are about 10-20 nm in diameter and their are different shapes of these nanocavities, such as circle, hexagon, and rectangle. Figure 3c,f presents HRTEM images of TiO2 nanorods with 0.35 nm lattice spacings corresponding to the (101) crystal plane of anatase, which was confirmed by the XRD results. The electron diffraction patterns shown in Figure 3c,f indicate that both the undoped and doped anatase TiO2 nanorods possess the single-crystalline structures. Moreover, there are some small aggregates on the surface of the TiO2 nanorods in Figure 3d and it can be speculated that they were aggregates of small Cr2O3 particles.12 However, the quantity and the size of the Cr2O3 particles were much smaller than those of TiO2 nanorods and XRD did not detect the diffraction peak of Cr2O3 in the sample with 3 mol % Cr. Figure 4 shows the EDX result of 3 mol % Cr-doped TiO2 nanorods after heat treatment, which indicates that Cr element exists in the TiO2 nanorods. Table 1 lists the content of the elements in all the samples and it can be calculated that the molar ratio of Cr/Ti was 0.009, 0.023, 0.030, and 0.078 for 1%, 3%, 5%, and 10% Cr-doped TiO2 nanorods, respectively. It can be concluded that a small quantity of Cr species ran off in the experimental process. We present the XPS spectra of 3 mol % Cr-doped TiO2 nanorods in Figure 5. Carbon was used as the energy reference material, and its characteristic peak appeared at around 285 eV in Figure 5a1,a2. Figure 5b1 shows two peaks of Ti 2p at 458.6

and 464.5 eV, which correspond to Ti 2p3/2 and Ti 2p1/2, respectively. The binding energies were shifted toward the higher energy side compared to those of pure TiO2 (458.4 and 464.2 eV for Ti 2p3/2 and Ti 2p1/2, respectively). Thus, we assumed that Cr doping had affected the chemical states of TiO2 by means of Cr substitution for Ti.16 The peaks at 577.5 and 586.7 eV in Figure 5c1 were identified as Cr 2p3/2 and Cr 2p1/2, respectively, indicating the presence of Cr3+ in the sample. After heating, the two Ti 2p peaks in Figure 5b2 were located at 458.4 and 464.2 eV, respectively, which were the same as those of pure TiO2. This suggested that Cr did not occupy the lattice of TiO2 and congregated on the surface. Figure 5c2 presents the two peaks of Cr 2p at 577.2 and 586.6 eV, which correspond to Cr 2p3/2 and Cr 2p1/2, respectively. The binding energies agreed with those of Cr2O3. Therefore, we can assume that Cr2O3 particles were distributed on the surface of the TiO2 nanorods after heating. During the heating process, Cr atoms obtained enough energy and they diffused to the surface through oxygen vacancies by a vacancy diffusion mechanism and at last the Cr atoms on the surface were oxidized. Table 2 presents the molar ratio of Cr/Ti in all the samples analyzed by XPS measurement. After hydrothermal treatment, the molar ratios of Cr/Ti for 1%, 3%, 5%, and 10% Cr-doped TiO2 nanorods were 0.008, 0.024, 0.042, and 0.078, respectively. Thus, we assumed that most of the chromium elements had been successfully doped into the TiO2. Furthermore, after heating,

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Figure 4. EDX of 3 mol % Cr-doped TiO2 nanorods after heating at 650 °C.

TABLE 1: Content of the Elements in All the Samples Analyzed by EDX Measurement sample (Cr/Ti)

O

Ti

Cr

1% 3% 5% 10%

65.67 64.62 65.62 68.04

34.03 34.58 33.37 29.66

0.30 0.80 1.01 2.30

the surface molar ratio of Cr/Ti increased to 0.026, 0.083, 0.151, and 0.229, respectively. Thus, we could also calculate that the content of Cr2O3 on the surface of TiO2 nanorods was about 1.3, 4.1, 7.5, and 11.4 mol %, respectively. The molar ratios of Cr/Ti for all the samples were about 3 times larger than those before heating. This suggested that the surface content of Cr was higher than the body; that is, Cr congregated on the surface in the form of Cr2O3 after heating. Figure 6 is the UV-vis spectrum of the different samples. In a comparison to the commercial anatase TiO2 particles, the undoped and Cr-doped TiO2 nanorods exhibited a stronger absorption in the range of 250-400 nm, which can be attributed to the backscattered light (Rayleigh scattering for small nanocavities, and the gradual transition from Rayleigh scattering to diffraction phenomena with the size of the nanocavities varying from small to large) effect induced by the nanocavities.13 Furthermore, from Figure 6, we can see that the absorption edges of anatase, undoped, 1% Cr-doped, 3% Cr-doped, 5% Cr-doped, and 10% Cr-doped TiO2 nanorods are 550, 700, 730, 755, 600, and 500 nm, respectively. The absorption edge of the TiO2 nanorods with 1% and 3% Cr shifted to the greater wavelength in the visible range, but the 5% and 10% Cr-doped samples presented blue shift compared to the undoped TiO2 nanorods. It can be concluded that appropriate Cr doping resulted in narrowing of the band gap of the anatase TiO2. The narrower band gap should be attributed to the introduction of the dope levels, which would induce the excitation of an electron from the valence band to the dope levels.11 Also, Cr doping could form an electron-capture trap to prevent the electron-hole pairs from recombination.17However, excessive Cr doping would form deep dope levels, which are considered as the recombination centers and they can accelerate the recombination of the electron-hole pairs. Therefore, excessive Cr doping caused the band gap to broaden.

The photoelectrochemical properties of the TiO2 nanorods thin film were studied by means of linear sweep voltammetry. For comparison, commercial anatase TiO2 particles were used as the benchmark to assess the photoelectrochemical properties of the TiO2 nanorods thin film. Figure 7 shows the photocurrent density vs applied potential curves of the anatase TiO2 particulate film, the undoped TiO2 nanorods film, and 3 mol % Crdoped TiO2 nanorods film under UV illumination. The electrolyte used in this photoelectrochemical study was 0.5 M Na2SO4 solution. Under UV illumination, the photocurrents are less than 25 µA/cm2 for all three films and almost stay at a constant value from -0.25 to 1.1 V and the photocurrent of the commercial anatase TiO2 particles is the smallest (12.5, 16, and 20 µA/cm2 for commercial anatase TiO2 particles, undoped TiO2 nanorods, and 3 mol % Cr-doped TiO2 nanorods, respectively). The photocurrent of the 3 mol % Cr-doped TiO2 nanorods film begins to increase sharply from 1.1 V. The photocurrents of the undoped TiO2 nanorods film and the anatase TiO2 particulate film begin to increase from 1.2 and 1.25 V, respectively. At 1.5 V, the photocurrents increase to 25, 60, and 225 µA/cm2 for commercial anatase TiO2 particles, undoped TiO2 nanorods, and 3 mol % Cr-doped TiO2 nanorods, respectively. This suggests that the band gap of the 3 mol % Cr-doped TiO2 nanorods is the smallest, followed by the undoped TiO2 nanorods, and then commercial anatase TiO2 particles, which agrees with the UV-vis spectrum results. Meanwhile, the photocurrent transient measurement method was used to further examine the photoelectrochemical properties of the three kinds of thin films and the results are shown in Figure 8. Obviously, the photocurrent is strongly dependent on the applied potential and time. When the UV light is on, the photocurrents rise to a steady state immediately. As the UV light is off, the change trends are different due to different bias potentials. At the 0 V bias potential, the photocurrents of all three samples decrease to a constant value in a few seconds. But at the 1.5 V bias potential, the photocurrent of the 3 mol % Cr-doped TiO2 nanorods decreases slowly. After 10 s of illumination, the photocurrent density of the 3 mol % Cr-doped TiO2 nanorods is the largest of the three samples (24 and 45 µA/cm2 at 0 and 1.5 V, respectively), which is confirmed by the result of linear sweep voltammetry. This reveals that Cr

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Figure 5. XPS spectra: (a1) survey, (b1) Ti 2p,and (c1) Cr 2p before heating; (a2) survey, (b2) Ti 2p, and (c2) Cr 2p after heating.

TABLE 2: Molar Ratio of Cr/Ti Analyzed by XPS Measurement before hydrothermal treatment

after hydrothermal treatment

after heat treatment

0.01 0.03 0.05 0.10

0.008 0.024 0.042 0.078

0.026 0.083 0.151 0.229

doping can induce the separation of the electron and hole and prevent the electron-hole pairs from recombination, thus extending the life of the charge carriers.

The electrolyte used in this photoelectronchmical study was 0.5 M Na2SO4 solution, and the reaction investigated here is thus the water-splitting reaction:

2H2O f O2 v + 2H2v

(1)

The photowater splitting activity of the Cr-doped TiO2 nanorods thin film is superior to that of the anatase TiO2 particulate thin film, as shown in Figures 7 and 8. The enhanced photocatalytic activities of the Cr-doped TiO2 nanorods can be attributed to

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Figure 6. UV-visible absorption spectra of samples after heating at 650 °C.

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Figure 8. Photocurrent response curves of the samples at different applied electrode potentials in 0.5 M Na2SO4 under Hg lamp pulsed irradiation (∼8 mW/cm2): (a) Commercial anatase TiO2 (0 V); (b) undoped TiO2 nanorods (0 V); (c) 3 mol % Cr-doped TiO2 nanorods (0 V); (d) commercial anatase TiO2 (1.5 V); (e) undoped TiO2 nanorods (1.5 V); (f) 3 mol % Cr-doped TiO2 nanorods (1.5 V).

charge recombination through experiencing the less frequent trapping/detrapping events.19Therefore, more electrons surviving from the charge recombination can lead to the increase of photocurrent. The other is the increased light-harvesting abilities caused by nanocavities. As discussed before, the nanocavites can induce backscattered light effect, leading to improvement of the light-harvesting abilities. Spontaneously, the more photons are absorbed, the more photocarriers are produced, resulting in larger photocurrent. Conclusions

Figure 7. Variation of photocurrent density vs applied electrode potential for the samples in 0.5 M Na2SO4 under Hg lamp irradiation (∼8 mW/cm2). (a) Commercial anatase TiO2; (b) undoped TiO2 nanorods; (c) 3 mol % Cr-doped TiO2 nanorods.

the combined effect of several factors: the Cr doping, the onedimensional nanostructure, and the increased light-harvesting abilities. As the fact discussed before, Cr element can induce more effective light harvesting, including visible light, which is caused by photoexcitation of the extrinsic absorption band of the catalyst. The extrinsic absorption comes from the photoionization of original or newly formed defects and the excitation of surface states. Furthermore, the extrinsic absorption can create oxygen vacancies to enhance the photocatalyst ability of the Cr-doped TiO2.18 Such absorption requires less energy to activate. Thus, Cr-doped TiO2 probably generates more free charge carriers to induce surface chemical reactions than pristine TiO2 under irradiation (visible light or UV light). Therefore, the photocurrent from the Cr-doped TiO2 nanorods thin film is larger than that of the anatase TiO2 particulate thin film in our study. However, the unique one-dimensional nanostructure of the nanorods also contributes to its high photocatalytic ability. Nanorods are believed to have exceptional electron-transport properties and have been considered as alternatives to nanoparticles. It has been reported that TiO2 nanorods can reduce intercrystalline contacts between grain boundaries and its stretched grown structure to the specific directionality makes a slightly favorable contribution to the electron transport and the significant improvement of electron lifetime by the degraded

(1) The Cr-doped single-crystalline anatase TiO2 nanorods with nanocavities of various shapes were successfully prepared by a hydrothermal method with the assistance of the heating process. Nanocavities could enhance the optical absorption of TiO2 in the near-ultraviolet region. Appropriate Cr-doped TiO2 nanorods presented red shift and excessive Cr-doped TiO2 nanorods presented blue shift compared to the undoped TiO2 nanorods. (2) The 3 mol % Cr-doped TiO2 nanorods thin film presented photoelectrochemical properties superior to those of the undoped TiO2 nanorods and the commercial anatase TiO2 particles. The doped Cr, the one-dimensional nanostructure, and the increased light-harvesting abilities were key impacting factors for the superior photoelectrochemical characteristics of the Cr-doped TiO2 nanorods. (3) With the outstanding photoelectrochemical characteristics, the Cr-doped TiO2 nanorods with nanocavities could be used for solar cells, photocatalysts, electronic devices, sensors, and so on. Furthermore, the approach described in this work provides a low-cost method to synthesize Cr-doped TiO2 nanorods with nanocavities on a large scale. Acknowledgment. The project was supported by the Natural Science Foundation of Jiangsu province (BK2004129) and the Aeronautical Science Foundation of China (04H52059). References and Notes (1) Negishi, N.; Iyoda, T.; Hashimoto, K.; Fujishima, A. Chem. Lett. 1995, 9, 841–842. (2) Bach, U.; Lupo, D.; Comte, P.; Moser, J. E.; Weissortel, F.; Salbeck, J.; Spreitzer, H.; Gratzel, M. Nature 1998, 395, 583–585.

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