Chinese Physics Letters, 2017, Vol. 34, No. 11, Article code 116101 Synthesis of Different TiO$_{2}$ Nanostructures and Their Physical Properties T. Hoseinzadeh1, Z. Ghorannevis2, M. Ghoranneviss1**, M. K. Salem1, A. H. Sari1 Affiliations 1Department of Physics, Science and Research Branch, Islamic Azad University, Tehran, Iran 2Department of Physics, Karaj Branch, Islamic Azad University, Karaj, Iran Received 11 August 2017 **Corresponding author. Email: ghoranneviss@gmail.com Citation Text: Hoseinzadeh T, Ghorannevis Z, Ghoranneviss M, Salem M K and Sari A H 2017 Chin. Phys. Lett. 34 116101 Abstract Titanium dioxide (TiO$_{2}$) nanosheet, nanorod and nanotubes are synthesized using chemical vapor deposition (CVD) and anodizing processes. TiO$_{2}$ nanosheets are grown on Ti foil which is coated with Au catalyst in CVD, TiO$_{2}$ nanorods are synthesized on treated Ti foil with HCl by CVD, and TiO$_{2}$ nanotubes are prepared by the three-step anodization method. Scanning electron microscopy shows the final TiO$_{2}$ structures prepared using three processes with three different morphologies of nanosheet, nanorod and nanotube. X-ray diffraction verifies the presence of TiO$_{2}$. TiO$_{2}$ sheets and rods are crystalized in rutile phase, and TiO$_{2}$ tubes after annealing turn into the anatase crystal phase. The optical investigations carried out by diffuse reflection spectroscopy reveal that the morphology of TiO$_{2}$ nanostructures influencing their optical response and band gap energy of TiO$_{2}$ is changed for different TiO$_{2}$ nanostructures. DOI:10.1088/0256-307X/34/11/116101 PACS:61.05.-a, 61.46.Np, 61.46.Km © 2017 Chinese Physics Society Article Text Titanium dioxide is one of the most multifunctional semiconductors, used in a wide range of research activity. Various physical and chemical properties such as wide band gap, high chemical stability and low cost, make it applicable in photocatalysis, photovoltaics, sensors, lab-on-a-chip devices and blood-compatible implants.[1-7] In most of the applications, the morphology of TiO$_{2}$ is known to play an important role in performance of devices. For instance, in the case of dye and quantum dot solar cells, structures of the TiO$_{2}$ strongly affect the cell performance. Different morphology parameters such as roughness factor, grain boundaries, electron lifetime and crystallinity were found to be essential parameters in solar cell efficiencies.[8-13] Moreover, mesoporous structures based on TiO$_{2}$ nanoparticles were observed to be suitable for dye solar cells. On the other hand, the surface area of the mesoporous TiO$_{2}$ is not important in heterojunction quantum dot solar cells and it was found that one-dimensional structures show better electron transport and higher efficiencies in the case of these cells.[14] Therefore, in the present study, three different TiO$_{2}$ nanostructures of nanotubes, nanorods and nanosheets are synthesized under optimum conditions by different methods. We examine morphological aspects of the TiO$_{2}$ nanostructures on the basis of x-ray diffraction (XRD), scanning electron microscopy (SEM) and diffuse reflection spectra (DRS) experimental data. Chemical vapor deposition (CVD) process is used to synthesize TiO$_{2}$ nanosheets. For removing the surface contamination, the Ti foils (0.25 thickness, 99.9% pure, Sigma Aldrich) were ultrasonically cleaned with ethanol, acetone and deionized water for 20 min, respectively, and then a thin film of Au is sputtered on the Ti foil surface. A horizontal tube furnace is used to oxidize the Ti foils and then the Ar gas is introduced into the furnace tube at a flow rate of 200 sccm for 4 h. Three different flows of gasses are used to study their effects on the morphologies of synthesized films. For the first sample, the furnace was heated to 850$^{\circ}\!$C with the rate of 30$^{\circ}\!$C$\cdot$min$^{-1}$ and then oxygen gas at a constant flow rate of 100 sccm was introduced into the tube. The furnace temperature was held for 1.5 h at 850$^{\circ}\!$C and then naturally cooled to room temperature under Ar stream. For the second sample, after the system is purged with a pure Ar stream for 4 h, when the furnace is heated up to 850$^{\circ}\!$C, ethanol was carried into the furnace tube by an Ar stream passing through a bottle containing ethanol, and the temperature was again held for 1.5 h. Finally the system under the Ar stream was cooled down to the room temperature. For the third sample, the system was purged and heated the same as the second sample, but the Ar stream passed through a bottle of acetone instead of ethanol. According to the SEM images shown in Fig. 1, comparing samples with three different annealing processes, different structures are obtained. The oxidation has a remarkable effect on the morphology of TiO$_{2}$ nanostructures. The use of the mixture of Ar with low concentration oxygen yielded a microcrystalline TiO$_{2}$ film, whereas the use of ethanol carried by Ar gave a randomly oriented nanowall and the use of acetone carried by Ar produced dense well-aligned nanosheet arrays. CVD process is used to synthesize TiO$_{2}$ nanorods. For removing the surface contamination, the Ti foils (0.25 thickness, 99.9% pure, Sigma Aldrich) were ultrasonically cleaned with ethanol, acetone and deionized water for 20 min, respectively. Later the samples are etched in 30 wt% HCl at 80$^{\circ}\!$C for 10 min to remove the oxide layer on the surface, then these are washed with distilled water and finally dried in a vacuum oven at 40$^{\circ}\!$C. A horizontal tube furnace was used to oxidize Ti foils and then the Ar gas was introduced into the furnace tube at a flow rate of 200 sccm for 4 h. To investigate the effects of these gasses during the films' synthesis on their morphologies, three different gasses were used. For the first sample, the furnace was heated up to the 850$^{\circ}\!$C with rate of 30$^{\circ}\!$C$\cdot$min$^{-1}$ and then oxygen at a constant flow rate of 100 sccm was introduced into the tube. The furnace temperature was held for 1.5 h at 850$^{\circ}\!$C and then naturally cooled to room temperature under the Ar stream. For the second sample, after the system was purged with the Ar stream for 4 h, when the furnace was heated up to 850$^{\circ}\!$C, ethanol was carried into the furnace tube by an Ar stream passing through a bottle containing ethanol, and the temperature was again held for 1.5 h. Finally the system was cooled to the room temperature under the Ar stream. For the third sample, the system was purged and heated the same as the previous sample, but the Ar stream passed through the bottle of acetone instead of ethanol. According to the SEM images shown in Fig. 2, the sample grown under Ar/acetone stream leads to the development of TiO$_{2}$ nanorods with more uniform features.
cpl-34-11-116101-fig1.png
Fig. 1. SEM images of the nanosheets synthesized in CVD at 850$^{\circ}\!$C for 1.5 h: (a) under stream of Ar and O$_{2}$ flow, (b) under stream of Ar and C$_{2}$ H$_{6}$O (ethanol) flow, and (c) under stream of Ar and C$_{3}$ H$_{6}$O (acetone) flow.
cpl-34-11-116101-fig2.png
Fig. 2. SEM images of the nanorods synthesized in CVD at 850$^{\circ}\!$C for 1.5 h: (a) under Ar and O$_{2}$ flow, (b) under Ar and C$_{2}$ H$_{6}$O (ethanol) flow, and (c) under Ar and C$_{3}$ H$_{6}$O (acetone) flow.
In last part, three steps of anodization process were used to synthesize TiO$_{2}$ nanotubes. For removing the surface contamination, the Ti foils (0.25 thickness, 99.9% pure, Sigma Aldrich) were ultrasonically cleaned with ethanol, acetone and deionized water for 20 min in order and were dried by an N$_{2}$ stream. The electrolytic solution contained ethylene glycol (C$_{2}$H$_{6}$O$_{2}$), ammonium fluoride salt (NH$_{4}$F) and deionized water (DI). Electrochemical setup and the anodizing parameters such as concentration of electrolyte, process temperature and anodizing time were kept the same for all of the processes. Different voltages of 35, 45 and 55 V were applied to study the effect of the applied voltage on the TiO$_{2}$ grown samples. The Ti foil was anodized for 1 h in the first step. The hexagonal patterns on Ti foil surface were observed by ultra-sonication of the TiO$_{2}$ nanotube samples in methanol. The second step that the uniform TiO$_{2}$ nanotubes were grown was performed for 3 h in the same solution. The samples were ultra-sonicated in methanol again to remove the top of the tube, after the second anodization, and then dried by the N$_{2}$ stream. The TiO$_{2}$ nanotubes that were grown on the surface of the Ti foils are annealed in ambient air at 450$^{\circ}\!$C for 1 h with a heating ramp of 2.5$^{\circ}\!$C$\cdot$min$^{-1}$. After annealing, the third step of the anodization process was carried out for 1 h and finally dried by the N$_{2}$ stream. The effective approach, three-step anodization process, develops highly ordered crystallized and top open channel TiO$_{2}$ nanotubes. The optimum anodization voltage to grow top open and compacted arrays in electrolyte solution contained (2 v%) deionized water and (0.3 wt.%) NH$_{4}$F was about 55 V, reported in our previous work.[15]
cpl-34-11-116101-fig3.png
Fig. 3. SEM images of the nanotubes anodized in electrolyte solution containing (2 v%) deionized water and (0.3 wt.%) NH$_{4}$F and annealed at 450$^{\circ}\!$C for 1 h for different applied volts of (a) 35 V, (b) 45 V and (c) 55 V.
cpl-34-11-116101-fig4.png
Fig. 4. SEM images of the TiO$_{2}$ nanostructures of (a) nanosheet synthesized in CVD at 850$^{\circ}\!$C for 1.5 h with Au catalyzer under Ar and C$_{3}$ H$_{6}$O (acetone) flow, (b) nanorod synthesized in CVD at 850$^{\circ}\!$C for 1.5 h with HCl treatment under Ar and C$_{3}$ H$_{6}$O (acetone) flow, and (c) nanotubes anodized at 55 V in electrolyte solution containing (2 v%) deionized water and (0.3 wt.%) NH$_{4}$F and annealed at 450$^{\circ}\!$C for 1 h.
TiO$_{2}$ nanorod, TiO$_{2}$ nanosheet and TiO$_{2}$ nanotubes were obtained in each part. The main purpose here was to study the effect of the morphological properties of TiO$_{2}$ nanostructures on the physical properties. The morphological observations such as length, width, diameter and wall thicknesses of the TiO$_{2}$ nanostructures are evaluated using field emission scanning electron microscopy (FESEM). Structural properties of the samples are studied by an x-ray diffractometer (XRD) using Cu $K\alpha$ radiation ($\lambda =1.5418$ Å). Moreover, diffusion reflection spectra (DRS) are also obtained in the wavelength range 300–950 nm to calculate band gap energy of the samples using the optical absorption spectra. The FE-SEM images of the TiO$_{2}$ nanorod, nanosheet and nanotubes are shown in Fig. 4. As seen in Figs. 4(a) and 4(b), oxidation of terminated Ti foil using acetone as the oxygen source provides an excellent and simple method to fabricate well-aligned TiO$_{2}$ nanostructure arrays on a Ti substrate. The TiO$_{2}$ nanosheet in Fig. 4(a) was grown in different orientations without any perfect growth orientation but each sheet growing is controlled by the other neighboring sheets. The average thickness and width of the nanosheet are about 60 nm and 700 nm, respectively. These nanosheets are not parallel to each other but perpendicular to the substrate. In Fig. 4(b), TiO$_{2}$ nanorod arrays of large scale are formed on the Ti substrate. The nanorod length is 4 μm. The Ti oxidation process is described by several different models. Badescu and Momirlan[16] proposed that oxygen diffusion through the grain boundaries of the oxide layer and oxidation site is predominant at a temperature in the range of 800–1000$^{\circ}\!$C. On the other hand, Czerwinski and Szpunar[14] showed that the TiO$_{2}$ grown at 900$^{\circ}\!$C is controlled by the surface diffusion of Ti on the sidewalls of pyramidal grains. In our case, the nanosheet and nanorod grown using acetone source had different morphologies from the samples grown in pure oxygen. When acetone is used for oxidation, the amount of oxygen for Ti oxidation process is much lower than the other growing condition like mixture of Ar with a low concentration of oxygen. Thus using acetone during the Ti oxidation caused diffusion of Ti species to the oxide surface as the predominant process and forming different nanostructures. As was seen, if the oxygen abundance is much higher than acetone, like pure oxygen, diffusion of oxygen will be more than Ti diffusion and cause formation of thin polycrystalline.[17] Finally, the SEM image of Fig. 4(c) shows the top open and highly oriented TiO$_{2}$ nanotube. The diameters and wall thicknesses of tubes measured from SEM images are about 65 nm and 8 nm, respectively. Moreover, the growing of TiO$_{2}$ nanotubes can be explained through these processes:[18] (i) By applied anodization voltage on Ti alloy, stable compact oxide layer of Ti, TiO$_{2}$, is formed on the surface of the metal by electric field that is assisted between electrodes according to $$ {\rm Ti}+2{\rm H}_{2}{\rm O}_{2}\rightarrow {\rm TiO}_{2}+2{\rm H}^{+}+2{\rm e}^{-}. $$ (ii) The percentage of fluoride ions in electrolyte is responsible for the chemical dissolution and pores are formed on oxide–electrolyte interface to produce soluble Ti(F$_{6}$)$^{2-}$ complex, $$ {\rm TiO}_{2}+4{\rm H}^{+}+6{\rm F}^{-}\rightarrow {\rm Ti} ({\rm F}_{6})^{2-}+2{\rm H}_{2}{\rm O}_{2}. $$ (iii) When these processes, chemical dissolution and oxidation, achieve a stable condition then TiO$_{2}$ nanotubes will be formed. The crystal structure and phase of different nanostructures of TiO$_{2}$ nanorod, nanosheet and nanotubes are determined by XRD. The crystal phases of these structures are important for studying their different properties such as photoelectrochemistry and photocatalysis of the TiO$_{2}$.[19-21] The nanosheet and nanorod were heated in a mixture of Ar and acetone flow at 850$^{\circ}\!$C and the nanotubes were annealed at 450$^{\circ}\!$C in ambient air before XRD measurement. It can be seen that all the samples are polycrystalline. For the nanosheet and nanorod samples, the diffraction peaks (110), (101), (111), (211), (002) and (112) are attributed to the tetragonal TiO$_{2}$ rutile phase that the lattice parameters of rutile TiO$_{2}$ are $a=b=0.458$ nm and $c=0.295$ nm (JCPDS file No. 20494). These XRD results show that nanorod and nanosheet samples have the same phase, rutile, and the intensities of these diffraction peaks for nanorod are significantly larger than the nanosheet intensity peaks. On the other hand, for the TiO$_{2}$ nanotubes anodized in (2 v%) DI water and (0.3 wt.%) NH$_{4}$F solution, the anatase phase is a dominant phase and the peaks corresponding to (101), (004), (105) and (220) planes appear only in the tubes sample patterns (JCPDS file No. 20387). Moreover, Ti peaks can also be seen only in the XRD spectra of tubes sample, which come from the Ti substrate used as the anode electrode in anodization and the two other samples do not show any Ti peaks. These phases changing for TiO$_{2}$ nanostructures may be caused by different annealing temperatures. It is expected that the growth process of TiO$_{2}$ nanostructures depends on the temperature and would affect the structure of the TiO$_{2}$ film. This is in good agreement with other researchers' motions that the TiO$_{2}$ anatase phase would appear at low temperatures, and rutile would appear at higher temperatures.[22,23]
cpl-34-11-116101-fig5.png
Fig. 5. XRD pattern of the TiO$_{2}$ nanostructures: (TNS) nanosheet, (TNR) nanorod and (TNT) nanotube.
One of the most useful optical observations is DRS to obtain electronic transitions of the material. The typical variation of diffuse reflectance of TiO$_{2}$ nanostructures is recorded in the wavelength range 300–950 nm, as shown in Fig. 6. As can be seen, sensitization of different TiO$_{2}$ nanostructures such as nanosheet, nanorod and nanotube shows different absorption edges since nanosheet spectra have the most shifting to red region. Different absorption edges can be related to their different crystal phases. Moreover, it shows that the intensity of nanotube reflection spectra is much greater than the other structures. The highest intensity of nanotube spectra may be caused by its highly ordered tube morphology. As is seen in SEM images of Fig. 4, nanosheets and nanorods are grown in random directions without any preferred orientations but nanotube arrays are grown compacted and highly ordered. A highly ordered one-dimensional channel structure can provide a direct path for electrons, can promote the electron transport velocity, and can also cause reduction of the charge recombination.[22]
cpl-34-11-116101-fig6.png
Fig. 6. DRS of the TiO$_{2}$ nanostructures: (TNS) nanosheet, (TNR) nanorod and (TNT) nanotube.
cpl-34-11-116101-fig7.png
Fig. 7. Band gap energy of the TiO$_{2}$ nanostructures: (TNS) nanosheet, (TNR) nanorod and (TNT) nanotube.
Band gap energy was obtained using DRS from the Wood–Tauc equation as[23] $$ \alpha h\vartheta=\beta(h\vartheta-E_{\rm g})^m,~~ \tag {1} $$ where $h$ is a plank constant, $\vartheta$ is a frequency, $\alpha$ is an absorption coefficient, $\beta$ is a proportionality constant, and $m$ is a parameter for semiconductor direct transition and this value can be considered as 2. The band gap energies of TiO$_{2}$ nanostructures are estimated by plotting the tauc plot $\sqrt{\alpha h\vartheta}$ versus $h\vartheta$ with extra plotting line to the $x$-axis in Fig. 7. The band gap energies of the nanosheet, nanorod and nanotube samples are determined to be about 3.6, 3.45, 3.4 eV, respectively. The differences among the band gap energies of these different structures of TiO$_{2}$ can be related to their crystal phases that can be obtained from the XRD patterns. In summary, TiO$_{2}$ nanostructures with different morphologies have been synthesized by changing the experimental conditions such as growth methods, using different gases and process temperatures. It is observed that three different nanostructures of sheet, rod and tube are achievable. In comparison of the physical properties of these three nanostructures such as structural, morphological and optical, it is found that morphology, crystal structures and optical response can change correspondingly, which lead to the use of these nanostructures in different applications.
References Titanium Dioxide Nanomaterials: Synthesis, Properties, Modifications, and ApplicationsA review on highly ordered, vertically oriented TiO2 nanotube arrays: Fabrication, material properties, and solar energy applicationsTiO 2 Nanostructures: Recent Physical Chemistry AdvancesFacile Synthesis of Monodisperse Mesoporous Zirconium Titanium Oxide Microspheres with Varying Compositions and High Surface Areas for Heavy Metal Ion SequestrationModified TiO 2 For Environmental Photocatalytic Applications: A ReviewTunable superomniphobic surfaces for sorting droplets by surface tensionHemocompatibility of Superhemophobic Titania SurfacesNovel nanostructures for next generation dye-sensitized solar cellsHierarchically structured photoelectrodes for dye-sensitized solar cellsRecombination in Quantum Dot Sensitized Solar CellsRecent Advances in Sensitized Mesoscopic Solar CellsDye-Sensitized Solar CellsQuantum-Dot-Sensitized Solar CellsSelf-Assembled, Nanowire Network Electrodes for Depleted Bulk Heterojunction Solar CellsEffect of different electrolyte concentrations on TiO2 anodized nanotubes physical propertiesStatistics of TiO2 crystal growth in air on a metallic surface heated at temperatures in the range of 900–1100°CAnodization Fabrication of Highly Ordered TiO 2 NanotubesAtomic force microscopy imaging of the growth features on the surface of rutileTitanium dioxide photocatalysisRecent topics in photoelectrochemistry: achievements and future prospectsHeterogeneous photocatalysis: state of the art and present applications In honor of Pr. R.L. Burwell Jr. (1912–2003), Former Head of Ipatieff Laboratories, Northwestern University, Evanston (Ill).Effect of calcination temperature on morphology and photoelectrochemical properties of anodized titanium dioxide nanotube arraysWeak Absorption Tails in Amorphous Semiconductors
[1] Chen X and Mao S S 2007 Chem. Rev. 107 2891
[2] Mor G K, Varghese O K, Paulose M, Shankar K and Grimes C A 2006 Sol. Energy Mater. Sol. Cells 90 2011
[3] Kamat P V 2012 J. Phys. Chem. C 116 11849
[4] Chen D and Caruso R A 2012 Adv. Funct. Mater. 22 1966
[5] Daghrir R, Drogui P and Robert D 2013 Ind. Eng. Chem. Res. 52 3581
[6] Movafaghi S et al 2016 Lap Chip 16 3204
[7] Movafaghi S et al 2017 Adv. Healthcare Mater. 6 1600717
[8] Tétreault N and Gratzel M 2012 Energy Environ. Sci. 5 8506
[9] Zhang Q F and Cao G Z 2011 J. Mater. Chem. 21 6769
[10] Mora-Ser I, Gimnez S, Fabregat-Santiago F, Gmez R, Shen Q, Toyoda T and Bisquert J 2009 Acc. Chem. Res. 42 1848
[11] Grätzel M 2009 Acc. Chem. Res. 42 1788
[12] Boschloo G, Hagfeldt A, Sun L, Kloo L and Pettersson H 2010 Chem. Rev. 110 6595
[13] Rühle S, Shalom M and Zaban A 2010 ChemPhysChem 11 2290
[14] Lan X, Bai J, Masala S, M Thon S, Ren Y et al 2013 Adv. Mater. 25 1769
[15] Hoseinzadeh T, Ghorannevis Z and Ghoranneviss M 2017 Appl. Phys. A 123 436
[16] Badescu V and Mormirlan M 1996 J. Cryst. Growth 169 309
[17] Li S, Zhang G, Guo D, Ligang Yu and Zhang W 2009 J. Phys. Chem. C 113 12759
[18] Czerwinski F and Szpunar J A 1998 Micron 29 201
[19] Fujishima A, Rao T N and Tryk D A 2000 J. Photochem. Photobiol. C 1 1
[20] Tryk D A, Fujishima A and Honda K 2000 Electrochim. Acta 45 2363
[21] Herrmann J M 2005 Top. Catalysis 34 49
[22] Yu J G and Wang B 2010 Appl. Catal. B 94 295
[23] Wood D and Tauc J 1972 Phys. Rev. B 5 3144