Preparation of TiO2 nanocrystals in anatase and rutile phase

Abstract. TiO2 nanocrystals were prepared by pyrolysis of titanium tetrachloride (TiCl4) as precursor in an HCl aqueous solution at 80 ºC. The morphology and crystal structure of the resulting materials were characterized using a transmission electron microscope (TEM), X-Ray diffraction (XRD) and Raman spectroscopy. The concentration of the HCl reaction medium and the following aging are the essential factors affecting the formation and phase composition of the resulting TiO2 nanocrystals. TiO2 nanoparticles in the form of pure anatase and the rutile phase can be extracted and segregated from the suspension and precipitate of the synthesizing medium.

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JOURNAL OF SCIENCE OF HNUE DOI: 10.18173/2354-1059.2015-00073 Chemical and Biological Sci. 2015, Vol. 60, No. 9, pp. 14-20 This paper is available online at Received November 11, 2015. Accepted December 2, 2015. Contact Nguyen Trong Tung, e-mail address: trongtung227@yahoo.com 14 PREPARATION OF TiO2 NANOCRYSTALS IN ANATASE AND RUTILE PHASE Nguyen Trong Tung1,2 and Duong Ngoc Huyen1 1 School of Engineering Physics, Hanoi University of Science and Technology, Hanoi 2 College of Television, Thuong Tin, Hanoi Abstract. TiO2 nanocrystals were prepared by pyrolysis of titanium tetrachloride (TiCl4) as precursor in an HCl aqueous solution at 80 ºC. The morphology and crystal structure of the resulting materials were characterized using a transmission electron microscope (TEM), X-Ray diffraction (XRD) and Raman spectroscopy. The concentration of the HCl reaction medium and the following aging are the essential factors affecting the formation and phase composition f he resulting TiO2 nanocrystals. TiO2 nanoparticles in the form of pure anatase and the rutile phase can be extracted and segregated from the suspension and precipitate of the synthesizing medium. Keywords: Nanocrystal, TiO2, anatase, rutile. 1. Introduction TiO2 (titanium dioxide) is a safe material, nontoxic to humans and widely used. TiO2 is considered to be a semiconductor with photocatalytic activities, chemical stability, long durability, nontoxicity and low cost which has great potential for application in igments, catalyst support, fillers, coatings, photoconductors, photocatalysts and environment purification [1-4]. For example, TiO2 nanoparticles absorb ultraviolet (UV) light and can therefore be used to filter out sunlight in cosmetics to protect the skin [5]. Photocatalytic properties of TiO2 can also be apply for antibacterial purposes, self-cleaning glass and water purification [3]. In the clean energy field, TiO2 is used to convert solar energy in dye- sensitized solar cells (DSSC), and producing energy from hydrogen fuel [5, 6]. TiO2 exists in three main structural configurations under normal conditions: rutile, anatase and brookite in which rutile is more stable while anatase and brookite are metastable at room temperature. The difference in morphology, crystalline and electronic structure results in different overall properties and potential for application. Furthermore, in nanostructures, TiO2 appears in unique forms such as nanotubes [7], nanowires [8], nanorods [9] and nanoparticles [10-13]. The combination of the quantum confinement effect and morphological structure could result in new features and applications. To prepare TiO2 nanost uctures, the hydrothermal method, solvothermal methods, sol-gel method, direct oxidation method, Preparation of TiO2 nanocrystals in anatase and rutile phase 15 chemical vapor deposition (CVD), electrodeposition, sonochemical method and microwave method have been used [10]. Synthesis conditions such as temperature, pH, pressure and aging time are considered to be critical to control and achieve TiO2 nanostructures of a desired size and morphology. The hydrothermal method is an easy route to prepare a well-formed crystalline oxide under moderate reaction conditions at low temperature with a short reaction time. Recently, TiO2 nanocrystalline particles with different structures and morphologies have been synthesized in hydrothermal medias using precursors such as TiCl4, TiCl3, amorphous TiO2 and P25 [9]. In this study, pyrolysis of TiCl4 in hydrochloric acid aqueous solutions with concentrations ranging from 0.0 - 1.0 M at a temperature of 80 ºC was used to synthesize TiO2 nanoparticles. The effect of HCl on anatase and rutile composition is presented and discussed. 2. Content 2.1. Experiment Titanium tetrachloride (TiCl4) of 99.9% purity (Aldrich Chemical Co.) was used as a starting material to produce TiO2 nanoparticles using the hydrothermal method. All precursors were kept refrigerated at 3 - 5 oC in order to minimize heat effect. An aqueous solution with a TiCl4concentration of 0.4 M and a HCl concentration ranging from 0.0-1.0 M were used as starting materials. This aqueous TiCl4 solution was poured into a reactor with a lid and placed in an oven at 80 oC. When the solution changed to a milk-like emulsion, that indicated that the TiO2 segregated out and precipitated in the reaction medium. After 3.0 h the solution was cooled to room temperature and kept in refrigerator to age. After an unknown period of time, the TiO2 nanoparticles were collected by filtration, washed with distilled water and analyz d for crystalline structure. The pH of the aqueous TiCl4 solution before and during the synthesis was monitored by a Hanna HI 2221. The structures of the TiO2 were surveyed using XRD and Raman spectra. XRD experiments were carried out in a D8 Advance Bruker diffractometer using Cu Kα radiation,  = 0.154056 nm. The morphology and particle size were determined using a JEOL JEM - 1010 Transmission Electron Microscopy (TEM). Raman spectra were carried out in LabRAM HR800 (Horiba) using a 632.8 nm excitation laser at a resolution of 1.0 cm-1. 2.2. Results and discussions Experimental results of the TiO2 crystalline phase as determined using X-ray diffraction and Raman spectra show the formation of a TiO2 crystalline structure. The phase composition depends on the concentration of HCl and aging time. The XRD patterns of the formation of TiO2 crystals structure after being aged for a week are shown in Figure 1. The TiO2 crystals exist in both phases: anatase and rutile in XRD pattern of TiO2. All f the XRD peaks in Figure 1 indicate the formation of crystalline TiO2 with the anatase phase structure being dominant. However, there also appeared XRD peaks of the rutile phase, but with an intensity weaker than those of the anatase phase peaks. The relative peak intensity refracted at the (110) plane of the rutile phase can be used to estimate the variation of rutile content as synthesis conditions change. As can be seen in Figure 1, the concentration of HCl affects TiO2 crystal structure formation. When the concentration of HCl was gradually increased from 0.0 to 0.5 M, the TiO2 rutile phase appeared, reaching a maximum in the 0.5 M HCl aqueous solution. When the concentration of HCl was greater than 0.5 M, it can be seen from the XRD pattern that the rutile phase was reduced while the anatase phase became dominant. When the Nguyen Trong Tung and Duong Ngoc Huyen 16 HCl concentration ranged from 0.5 to 1.0 M, the (121) peak of the brookite phase appears and is at a maximum at around 0.7 M HCl. The brookite phase is considered to be an intermediate transition from anatase to rutile. After being aged for six months, the solution stored in the refrigerator becomes more transparent due to the greater amount of TiO2 colloidal particles that have precipitated out and now lie on the bottom of the flask. Extracting the TiO2 and determining the crystalline structure, the XRD patterns (Figure 2) show that the TiO2 crystall ne structures changed markedly. The TiO2 crystal in 0.0 M HCl shows peaks of XRD rutile phase clearer than peaks of anatase phase. The XRD peaks of TiO2 rutile and anatase phase appeared in pattern TiO2 particles from solution at a concentration of 1.0 M HCl. At a concentration of 0.5 M HCl, the TiO2 is completely transformed into a pure rutile phase structure. 20 40 60 (1 0 1 )R HCl 1 M HCl 0.7 M HCl 0.5 M HCl 0.2 M (0 0 2 )R (2 1 1 )R (2 0 0 )A (0 0 4 )A (1 0 1 )R (1 1 0 )R (1 0 1 )A In te n si ty ( a .u ) 2Degree HCl 0 M 20 40 60 0 200 400 600 1 M 0.5 M (3 01 )R (0 02 )R (2 20 )R (2 11 )R (1 11 )R(1 11 )R (1 01 )R (1 10 )R (2 00 )A (0 04 )A(1 01 )A In te ns it y (a .u ) 2Degree 0 M Figure 1. XRD patterns of TiO2 particles obtained from aqueous TiCl4 solution after a week hydrothermal Figure 2. XRD patterns of TiO2 particles obtained from aqueous TiCl4 solution after six months hydrothermal 20 30 40 50 60 70 0 20 40 60 80 100 120 140 160 0.5 M (1) In te n si ty ( a. u ) 2Degree 0.5 M (2) 20 30 40 50 60 70 0 0M (2) In te n si ty ( a. u ) 2Degree 0M (1) Figure 3. XRD patterns of TiO2 in 0.5 M HCl: (1) suspension, (2) precipitation after one month hydrothermal Figure 4. XRD patterns of TiO2 in 0 M HCl: (1) suspension, (2) precipitation after six months hydrothermal In order to further clarify the effect of the reaction solution on phase transformation, the TiO2 nanoparticle in a 0.5 M HCl solution after one month of hydrothermal was extracted from the two parts: the suspension and the precipitate. After separation, phase structure of the materials was determined by XRD. As can be seen from Figure 3, the colloid in the Preparation of TiO2 nanocrystals in anatase and rutile phase 17 suspension solution yields TiO2 crystals in the anatase phase while the precipitate is a mix of rutile and anatase with rutile content dominating. The TiO2 nanoparticles in the HCl 0 M solution which was stored for 6 months are also extracted and separated into two parts: the suspension and the precipitate patterns. XRD results in Figure 4 shows a separation of the anatase phase and the rutile phase. The XRD precipitation pattern shows that the spectrum peak of the anatase phase has a lower intensity than the rutile phase. In order to confirm the phase of the material, Raman spectra were used to determine the properties of TiO2 crystallites. Figure 5 shows the Raman spectrum after aging for one week, the spectrum showing vibrational modes that represent anatase peaking at around 155. 399, 513 and 634 cm-1 [14]. Among of Raman spectra, only the TiO2 pattern in HCl 0.5 M has the peak shift, that peak being 399 cm-1 to 466 cm-1 [14], this peak shift showing the formation of the rutile phase in the receiving pattern. After being stored 6 months, TiO2 patterns were examined using Raman spectrum and the results are shown in Figure 6. Of the three patterns, the one in 0.5 M has the specific appearance of the vibrational mode of rutile phase while the other two patterns have vibrational modes of both rutile and anatase phase. The intensity of vibrational mode of the pattern in 1.0 M HCl is greater than that in the 0.0 M HCl solution. Comparing the XRD patterns and Raman spectra to present a brief conclusion, the measurements show that the rutile content in the aging TiO2 gradually increases over time indicating a transformation from anatase phase to rutile phase during the aging. The enhancement of the rutile phase growth [15, 16] and phase transformation in this experiment is attributed to the HCl-rich environment. 0 500 1000 513399 In te n si ty ( a .u ) Raman shift (cm -1 ) HCl 1M 155 634 In te n si ty ( a .u ) HCl 0M In te n si ty ( a .u ) HCl 0.2M In te n si ty ( a .u ) HCl 0.5M In te n si ty ( a .u ) HCl 0.7M 200 400 600 800 634 513 Raman shift (cm -1 ) HCl 1M 399 In te n si ty ( a. u ) HCl 0M HCl 0.2M HCl 0.5M HCl 0.7M Figure 5. Raman spectrums of TiO2 after 7 days of hydrothermal 0 200 400 600 800 1M 0.5M In te ns it y (a .u ) Raman shift (cm -1 ) 0M 0.5M 1M 155 446 610 0M 0 1000 2000 3000 In te ns it y (a .u ) Raman shift (cm -1 ) Figure 6. Raman spectrum of TiO2 after 6 months of hydrothermal Nguyen Trong Tung and Duong Ngoc Huyen 18 TiO2 morphology is observed through TEM images. As known from XRD and Raman measurements, TiO2 in 0.0 M HCl after 6 months of hydrothermal is in suspension in the anatase phase and is a precipitate in the rutile phase. As shown in Figure 7a for the TiO2 suspension and Figure 7b for TiO2 precipitation, the TiO2 crystallites in anatase phase are particles 2- 5 nm in size while the TiO2 precipitate (rutile phase) is crystallized in a bar-like form 20 nm in width. The TiO2 in 1.0 M HCl after 6 months of aging is a mixture of both anatase and rutile phase, its structure shown in Figure 8. Many particles 20 - 30 nm in size (rutile) are formed and agglomerated to form big clusters and there are elements 2–5 nm in size (anatase) on the surface of them. The formation of TiO2 crystals in the rutile phase in the synthesis process can be explained by free energy and the size limit of elements in solution [17]. The smaller TiO2 crystals have a higher free energy. Anatase phase formation occurs when the TiO2 nanoparticle size is 2- 7 nm because of free energy of anatase phase lower. These crystals get bigger when TiO2 crystals are bigger than 10 nm while the rutile phase is formed as the free energy becomes lower. A solvent environment with the presence of HCl creates Ti4+ ion converting solvent and the formation of TiO2 crystals, which makes the split of crystals in the anatase and rutile phase easier. Figure 7. TEM images of TiO2 in HCl 0 M in 6 months: a) suspension, b) precipitation Figure 8. TEM image of TiO2 in HCl 1 M after 6 months ab a b