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Amorphous TiO2 nanostructured thin films (TNFs) were deposited by the glancing angle deposition technique (GLAD) with an electron gun. All of the prepared TNFs were composed of discrete nanoscale columns, characterized by scanning electron microscope. With the annealing treatment, the pure anatase phase was transformed from amorphous TNFs. The morphological, structural and optical properties of TNFs under the annealing treatment were measured, and the evolvement mechanism was further analyzed. The optimum annealing temperature for the pure anatase precipitation of TNFs is about 400°C. The variation of morphology, chemical state and crystallization of TNFs also resulted in the shift of the transmittance spectra. The results show that appropriate post-annealing treatment can build fine nanostructures and pure anatase precipitation, which can support the applications and researches about the anatase phase TNFs powerfully.
© 2015 Optical Society of America
Corrections
Bin Wang, Hongji Qi, Hu Wang, Yanyan Cui, Jiaoling Zhao, Jialu Guo, Yun Cui, Youchen Liu, Kui Yi, and Jianda Shao, "Morphology, structure and optical properties in TiO2 nanostructured films annealed at various temperatures: publisher’s note," Opt. Mater. Express 5, 2545-2545 (2015)https://opg.optica.org/ome/abstract.cfm?uri=ome-5-11-2545
13 October 2015: A correction was made to the author listing.
1. Introduction
Titanium dioxide (TiO2), as an important semiconductor, has drawn more and more attention in many applications such as photo-electrochemical water splitting [1], dye-sensitized solar cells [2], phosphonic acid self-assembled monolayers [3], anode of lithium-ion batteries [4]. TiO2 has three types of crystallographic structures: anatase, rutile and brookite [5,6],in which the rutile is the most stable phase, and the anatase and brookite phase are metastable and unstable respectively. The anatase phase can be synthesized at relatively low temperatures and can be transformed into the rutile phase over 700-800°C [7]. Nevertheless, comparing with the rutile phase TiO2, the anatase phase TiO2 has attracted great research interests in various fields due to its better photocatalytic activity and wider band gap [8]. Traditional method to fabricate the anatase phase TiO2 thin film by heating the substrate is inefficient, complicated and costly. Takahiro Nakamura and Testsu Ichitsubo [7,9] proposed the post annealing technique for the anatase formation from amorphous phase. The anatase phase formation of TiO2 films deposited on unheated substrates was observed by Hitosugi and A. Ueda [10] with post annealing technique at around 350°C. Then the annealing treatment is widely adopted to fabricate the anatase phase TiO2 thin films [11–13].
Nanostructured materials exhibit large surface-to-volume ratio and unique properties [14]. Many studies have focused on unique properties of TiO2 nanostructured thin films (TNFs), such as photo-catalytic performance [15], transparent conductor [16]. The glancing angle deposition (GLAD) is a versatile nanofabrication technique based on the self-shadowing effect during a thin-film deposition process, which can be used for fabricating nanostructured thin films, such as titled nanorods array, zigzag nanorods and helical structure [17–21]. Compared with other nanofabrication techniques, GLAD offers the unique features of morphology sculpture, hetero-nanostructure design and composition tenability [17]. Using the GLAD, the nanorods height, diameter and the length are determined by the rotation speed and the deposition rate [17–20]. Thus, one can accurately generate nanorods with different controllable morphologies and nanostructures. As we know, the obvious features of chemical synthesis technique are easy operation, high chemical homogeneity and rapid film forming [1–4]. Compared with chemical synthesis techniques, GLAD can accurately control the morphologies and nanostructures better than chemical synthesis. As an indispensable nanofabrication technique [1–4,8], GLAD has received more and more attention [18–21]. Ryan T. Tucker [16], Zhengcao Li [22] and X. D. Xiao [23] fabricated and studied the nanostructured TiO2 films by GLAD. However, the pure anatase phase was not obtained in their researches. The anatase TNFs have not been obtained through annealing for amorphous TNFs deposited by GLAD. Meanwhile, the influence of the annealing on the morphological, structural and optical properties of TNFs is still vague.
In this paper, we have prepared amorphous TNFs by GLAD with electron beam deposition technology. The evolution of morphology, structure, chemical state and optical properties TNFs under annealing treatment have been characterized in detail. The amorphous structure successfully transformed into pure anatase phase through annealing process. The evolvement mechanisms have been discussed further. The optimum annealing condition was presented for fabricating pure anatase phase TNFs. Combined annealing progress with GLAD, one can accurately obtain the fine anatase TNFs.
2. Experiments
Synthesis of TiO2 nanostructured films
Amorphous TNFs were deposited with GLAD technique combined in electron beam deposition system that can be found in the literatures [23,24]. The tilted angle and rotation of substrate were automatically controlled by two step-motors. The distance between TiO2 source material and the center of fused silica was kept 27cm. The fused silica substrates (Φ30mm × 3mm) prior to deposition were ultrasonically cleaned in acetone and ethanol for 30min. The base pressure of the vacuum chamber was <1.5 × 10−5Torr. TNFs were deposited in a 2 mTorr pure oxygen atmosphere facilitated by a gas flow rate of 200 sccm. The deposition angle α between the incident flux and the normal of substrate was fixed to 70° for TNFs, without rotation and heating the substrate. Firstly, the granular TiO2 material (purity 99.99%) was premelted completely with the electron beam heating. Then, the shutter was turned to open and TiO2 material was evaporated to the surface of the substrate. The deposition rate was kept at around 0.3nm/s, monitored by quartz crystal oscillator.
Annealing treatment of TNFs
The as-deposited TNFs were annealed to improve crystallization. The annealing treatment was carried out in the muffle furnace in air. In order to set the contrast experiment, four samples were prepared for heat treatment at different temperatures. TNFs were annealed under 300, 400 and 500°C for 2 hours, respectively.
Characterizations
The morphologies of TNFs were measured by field emission scanning electron microscopy (FE-SEM, Hitachi S-4700) (Auriga, Carl Zeiss) with coated thin Cr layer for improving the conduction. The crystallographic structures of TNFs were characterized via X-ray diffraction (XRD, Empyrean, PANalytical), adopting a Rigaku D/MAX-2550 with Cu Kα (λ = 1.5408 Å). Measurements of Raman spectra were performed on a Renishaw Invia Raman Spectrometer taken a blue line (488nm) as the excitation source. The elemental analysis of nanostructured films was measured by X-ray photoelectron spectroscopy (K-Alpha, Thermo Scientific), the XPS spectra were acquired using the Al Kα emission line at 1486.6ev. For XPS spectra of TNFs, the pass energy was 10ev. The XPS spectra were analyzed by commercial software (Casa XPS). Binding energies of all elements in the XPS spectrum were calibrated by taking carbon C1s peak (284.6ev) as the reference. Optical transmission spectra of films were measured by a Lambda 900 spectrophotometer with integrating sphere from 300nm to 800nm. The back reflection of substrate was not subtracted.
3. Results and discussion
The XRD diffraction patterns of TNFs are shown in Fig. 1. As-deposited TNFs are amorphous, in order to form the well-separated nanorod arrays fabricated by GLAD, the self-shadowing effect shall dominate the growth process, while the adatom diffusion needs to be minimized with limited mobility of deposited atoms at the room deposition [17]. Therefore, the crystallinity of GLAD thin films is poor due to the limited atom diffusion. With the increase of annealing temperature, the amorphous TNFs begin to transform to (101) preferred oriented anatase phase at 400 and 500°C. The average crystal sizes and interplanar spacing are calculated with the Scherrer’s formula for the peak width, B, corrected for instrument broadening according to D = Kλ/Bcosθ, where K is the constant equal to 0.89, λ is equal to 1.54059Å, and θ is the reflection angle. The corresponding parameters sharp peaks in samples are listed in Table 1. The average crystal sizes of samples with 400 and 500°C annealing are 173 and 247.8Å. The higher temperature promotes the larger crystal size.
Table 1. The corresponding data of sharp peaks in samples with 400 and 500°C annealing.
Compared with the previous result [23], the crystallinity of anatase phase is significantly higher. Zhengcao Li et al. [22] prepared TiO2 columnar structures as the Ti films deposited by GLAD and accomplished the phase transition from Ti to rutile TiO2 with the 550°C annealing. In view of experiments in the reference [22], the rutile phase of Ti films deposited by GLAD prevented the anatase phase transformation. Besides, Ryan T. Tucker et al. [16] fabricated the TiO2 nanostructured thin films on substrate titled 30°with 330°C heating, the as-deposited thin films with poor nanostructures. Further, Ryan T. Tucker adopting 500°C post-annealing with 300 sccm 5% H2/Ar forming gas, the multi-phase samples have been acquired. Considering the experimental details in the reference [16], the high deposition temperature and H2/Ar forming gas prevented the fine nanostructures and anatase phase precipitation. However, all of them did not obtain the pure anatase phase TNFs. In our experiments, both amorphous as-deposited TNFs and the appropriate annealing condition that 300~500°C for 2 hours within air atmosphere operation can be contributed to the pure anatase precipitation. The detailed description of crystallization transformation mechanism can be found in the literature [7]. The valuable results of pure anatase phase precipitation of TNFs provide powerful support for applications of TNFs, such as photocatalytic activity and transparent conductors.
Figure 2 shows the Raman spectra of TNFs. The as-deposited and 300°C annealed TNFs shows no Raman peaks except the Raman spectra of the substrate [25], which indicates the amorphous structures of as-deposited and 300°C annealed TNF. Along with the annealing temperature rise, the Raman lines appear at 142, 398, 515 and 638 cm−1 which can be assigned as the Eg, B1g, B1g and Eg of the pure anatase phase [26]. The occurrence of 142 cm−1 mode indicates that the TNFs with 400 and 500°C annealing possess a certain degree of long-range order of the anatase phase [27]. Although a single anatase phase was confirmed by the XRD and Raman in the TNFs with 400°C and 500°C annealing, high-frequency Raman lines (398, 515 and 638 cm−1) exhibit a weak and broad feature, which indicates these nanocrystals are imperfect and the crystallization needs to be improved further. The low frequency Eg mode is known to be related to the grain size [27]. From the results of XRD, the average crystal sizes of sample with 500°C annealing is larger than that of sample with 400°C annealing. However, the 142cm−1 mode intensity of TNF with 400°C annealing is higher than that of TNF with 500°C. As we know, XRD usually reveals the long-range order of materials and give average structural information, while the Raman scattering as a local probe is very sensitive to the crystallinity and microstructures of materials [28]. Therefore, we can deduce that the microstructure of TNF with 500°C annealing degrades due to the high temperature. The structural imperfections and defects due to high temperature cause the relative weak 142cm−1 Raman peak. The microstructures degradation of TNFs with annealing treatment is discussed in the following.
Although the pure anatase phase of TNFs was obtained with the post-annealing technique, it could also lead to morphology degradation of GLAD films with high temperature treatment [29]. Study the annealing effect for morphology of TNFs is necessary and significant. Cross-sectional and top-view of TNFs with different annealing temperatures are shown in Fig. 3 and 4. As seen from the Fig. 3(a), the as-deposited TNFs shows oblique and porous nanostructures with nano-columns separated with 45° tilted angle. With the annealing treatment at 300°C for 2 hours, the nano-columns begin to aggregate, which yields the increasing densification and larger radius of nano-columns. Meanwhile, the oblique angle of nano-columns changes to 41°, as shown in Fig. 3(b). The nanostructures of TNFs after the 300°C annealing treatment become coarse. With the annealing treatment at 400°C for 2 hours, the nanostructures of TNFs have been transformed into conspicuous nano-columns. The oblique angle of nano-columns has changed to 30° as shown in Fig. 3(c). Besides, the coalition of nano-columns has been further expanded with larger voids on the surface of TNFs as shown in Fig. 4(c). Along with the temperature further increasing to 500°C, the nano-columns in TNFs have been collapsed totally, shifting towards “nanorod network” [29] as shown in Fig. 3(d). The thin film gets denser with less and smaller voids, shown in Fig. 4(d). The influence of annealing is conspicuous. After annealing, nanostructured GLAD films exhibit clumping and wilting, however, the overall porosity and tilted angle is still correlated with the initial deposition angle. Based on the experimental results above, the optimum annealing for evolution of morphology and pure anatase precipitation of TNF is about 400 °C.
Fig. 3 Cross-sectional images of TNFs with different annealing temperatures: (a) as-deposition (b) 300°C, 2h (c) 400°C, 2h, (d) 500°C, 2h.
Fig. 4 Top-view images of TNFs with different annealing temperatures: (a) as-deposition (b) 300°C, 2h (c) 400°C, 2h, (d) 500°C, 2h.
Combined annealing progress with GLAD technique, one can accurately obtain the fine anatase TNFs. Further, the nanostructures and crystallization can be precisely adjusted through setting annealing temperature.
The Ti 2p XPS spectra of TNFs are shown in Fig. 5. A carbon 1s peak (284.6ev) is taken as the calibrated reference. The presence of C1s is related to organic surface contamination, corresponding to the fact that the samples are exposed to air. The XPS spectra for TNFs were obtained after a 30s sputter for removing the surface contaminants. Only the Ti4+ peak (458.7ev) is observed in the Ti 2p XPS spectrum of the as-deposited TNF. An additional component at 456.97ev appears after 300°C annealing. Another obvious peak at 454.90ev appears after 400°C annealing. The components at 454.9 and 456.97ev are attributed to Ti2+ and Ti3+ [30]. The existence of Ti3+ and Ti2+ should be attributed to the existence of oxygen vacancies due to high temperature, which is also be found and discussed in the literatures [31,32]. The Ti2+ peak in the as-deposited and 300°C annealed TNFs is hardly existence. The Ti 2p3/2 XPS spectra of TNFs with 400 and 500°C annealing broaden obviously, the increase of Ti3+ and Ti2+ is remarkable, which indicates the occurrence of chemical shift and reduction reaction. It is understood that as-grown TNF could be preferred to transform into Ti2O3 rather than TiO at a high temperature annealing [32].
The changes of morphology and chemical state must influence the optical properties of TNFs. Figure 6 shows the optical transmission spectra of the as-deposited and annealed TNFs. The transmittance is as high as 90% due to the multi-aperture nanostructures in TNFs. As seen from the insert of Fig. 6, the transmittance edge of TNFs gets red-shifted with the increase of annealing temperature. The similar phenomenon can be found in the literature [5,31,32]. The phenomenon can be ascribed to scattering and absorption increase in TNFs after annealing treatment. According to the XPS results, with annealed at high temperature, oxygen of TNFs would release to environment so that the oxygen vacancy content would increase in TNFs during the annealing process [31]. According to researches of D. J. Won [31] and Guanglei Tian [32], the existence of oxygen vacancy, Ti3+ and Ti2+ decreases the transmittance, which indicates the presence of Moss−Burstein effect due to the filling of the conduction band by free carriers. Another reason is the higher scattering loss posed by surface roughness rise with the increase of annealing temperature. In Fig. 3 and 4, TNFs exhibit clumping and wilting after annealing, increasing the surface roughness. As a result, the scattering is seriously strengthened near the cutoff edge. Therefore, the cutoff edge shift to longer wavelength and transmission decreasing may result from the slight changes of Ti charge states and the scattering loss.
The transmission spectra of TM and TE waves for TNFs have been also measured, as shown in Fig. 7(a). From Fig. 7(a), the transmission spectra of TM and TE polarized waves in TNF are distinctively different. Thin films fabricated by GLAD can be also used for the optical components due to the anisotropic property of TNFs. The detailed mechanism analysis for the difference of transmission spectra between TM and TE polarized waves in TNFs has been described in our previous work [21,24]. The difference of transmission spectra between TM and TE polarized waves could response morphological change of nanostructures in TNFs. The refractive indices of two orthogonal polarized waves, nTM and nTE, were extracted using the envelop method. The linear birefringence Δn (nTE - nTM) of as-deposited TNFs at 633nm is 0.062, as shown in Fig. 7(b). After 300°C annealing, the separated nano-columnar structures are amalgamated and degenerated, so the larger densification and spacing among nano-columns appear. Therefore, the linear birefringence Δn would be enlarged. It can be seen that birefringence increases from 0.062 to 0.083 with the 300°C annealing process. As we know, the nano-columns with larger densification and more conspicuous tilted nano-columns in the TNFs are attributed to a larger birefringence. After 400°C annealing, TNFs exhibit a greater degree of clumping and wilting only with the 30° tilted angle. As a deciding factor, the degenerating tilted nano-columns with the 400°C annealing process cause the lower linear birefringence (Δn = 0.067) compared with Δn of TNFs under the 300°C annealing. After the 500°C annealing, the birefringence value decreases to 0.066 close to Δn (0.067) of TNFs with 400°C annealing. The change of Δn in TNFs with 500°C annealing should be connected with the morphological change of nanostructures. As seen from Fig. 3(d) and Fig. 4(d), the nano-columns in TNFs with 500°C annealing have been collapsed shifting towards “nanorod network”. Meanwhile, the collapsed nano-columns with the weak sloping trend have less and smaller voids than that of TNFs under 400°C annealing. In view of the weak sloping trend, the 0.066 birefringence value in TNFs with 500°C annealing should be contributed to the larger density, according the analysis above. The birefringence of TNFs is higher than that of the common bulk materials, such as quartz (Δn = 0.009) and MgF2 (Δn = 0.012). The GLAD technique may offer an effective method to obtain large birefringence, necessary for the retardation plate and polarizer. The transmission spectra and Δn is consistent with the evolution of morphology in TNFs with annealing treatments.
Fig. 7 Transmission spectra of TM and TE polarized waves and Δn for TNFs with different thermal treatment conditions.
4. Conclusion
In this paper, we have successfully fabricated the TNFs with GLAD technique. After that, different temperature annealing treatments have been adopted for study the effect of thermal treatment for the morphology, crystallization, chemical state and optical property in TNFs, characterized with SEM, XRD, Raman, XPS and spectrophotometer. Further, the evolvement mechanisms have been investigated thoroughly. The preferential formation of the pure anatase in amorphous TNFs under the annealing treatment has been reported, and the optimum thermal treatment temperature is about 400°C. The results in this paper can provide useful information and powerful support for producing anatase phase TNFs and application research.
Acknowledgment
This work was supported by the National Natural Science Foundation of China under Grant No. 61205211.
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