Advanced Materials Research Vol. 620 (2013) pp 179-185 (2013) Trans Tech Publications, Switzerland doi:10.4028/www.scientific.net/amr.620.179 XRD and EDXRF Analysis of Anatase Nano-TiO 2 Synthesized from Mineral Precursors MAHDI E. M. 1, 3, M. HAMDI 2, MEOR YUSOFF M. S. 3 and WILFRED P. 3 1 Advance Manufacturing and Material Processing Center, University Malaya, 50603, Kuala Lumpur, Malaysia 2 Department of Engineering Design and Manufacture, Faculty of Engineering, University Malaya, 50603, Kuala Lumpur 3 Material Technology Group, Industrial Technology Division, Malaysian Nuclear Agency, 43000, Kajang, Selangor mahdiezwan@nuclearmalaysia.gov.my Keywords: Nano-TiO 2, EDXRF, XRD, heat treatment, crystallinity Abstract. This work details the characterization of anatase nano-tio 2 particles synthesized from Malaysian mineral precursors using the XRD and EDXRF. The properties that were analyzed were its crystallite sizes, relative crystallinity, phases, and chemical composition. It was determined that the crystallite size was quite small (15.6 nm), although the crystallinity of the sample is relatively low. The anatase phase seems to be dominant (100%), although in some cases when the processing parameters were changed or heat treatment were conducted, the existence of rutile is detected. The chemical composition showed that TiO 2 is the majority compound in the sample (~96%), although some metallic and non-metallic impurities are present (Zr, Nb, and S). It is concluded that Malaysian mineral precursors are capable of producing relatively high quality nano-tio 2. Introduction Titania (TiO 2 ) is a compound that is both familiar and abundant, having seen many applications in diverse areas such as pigmentations, cosmetics, coatings and water purification. This attribute is mainly due to the flexibility of titania as a compound, where it comprise of many unique phases and crystal systems that is responsible for its behavior in certain conditions.titania comprises of eleven phases (some only exist in high pressure states), and four crystal systems (orthorhombic, monoclinic, tetragonal and cubic). Some common phases of titania are anatase (tetragonal), brookite (orthorhombic) and rutile (tetragonal). These phases occur naturally in minerals and are regularly extracted and separated from its ores. Sources of titania includes, but is not limited to, ilmenite (FeTiO 3 ), leucoxene ores, or rutile beach sand. Titania, as seen in its commercial form, is manufactured or processed from these sources using a myriad of methods, which includes the more common methods such as sol-gel method (widely used commercially), the hydrothermal and solvothermal methods, to the specialized and seldom used electrodeposition and the sonochemical method. The uniqueness of titania s attribute depends partly on its fabrication route, where we can see titania produced in different forms and shapes such as tubes (solvothermal) or rods (hydrothermal), segregated spheres (sol-gel), and smooth coatings (electrodeposition) when different fabrication methods are used. The size of titania particles are also paramount in determining its characteristics and potential application. The smaller the particle gets, the more diverse its potential application can be. With today s focus on nanotechnology, interest in how titania can play a role in this field is being pursued by many scientist and researchers. As a result of this fervor, we see nanosized titania being used in areas previously thought unfeasible, such as electrochromic devices, electronic sensors and photovoltaic cells. The inclusion of titania into these devices produces effects such as lengthening All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of TTP, www.ttp.net. (ID: 210.195.122.2-13/10/12,04:05:15)
180 Advanced X-Ray Characterization Techniques of process cycles and increased efficiency. The fabrication method mentioned above needs to be routinely modified to produce products that are deemed to be nano in size, with determining factors such as crystallite/grain size and thickness being given special attention (removed text)[1]. The determination of properties such as crystallite sizes, crystallinity and chemical composition of TiO 2 requires X-ray related methods due to the nature of the samples. Bulk samples, in powder form, are best analyzed with methods such as the X-Ray Diffraction and Energy Dispersive X-Ray Fluoroscence (EDXRF) due to its high energy and penetration, wide scan range, and the myriad of elements that is capable of being detected. The objective of this study is to use X-Ray methods to characterize nano-tio 2 particles synthesized using local minerals as its precursor. The characterization aims to compare the structural properties of anatase nano-tio 2 particles synthesized from the modified hydrothermal method and determine its viability as nanoparticle processing method. Materials and Methodology Fabrication of nano-tio 2. Nano-TiO 2 was prepared using a modified hydrothermal method. Commercially available synthetic rutile, derived from Malaysian ilmenite was used as a precursor in this process. The ilmenite is a by product of Tin mining, and it is converted to synthetic rutile via hydrometallurgy by Tor Minerals Sdn. Bhd., a mining company in Lahat, Perak, Malaysia. Fig. 1 demonstrates the relationship between the ilmenite and nano-tio 2. Ilmenite Hydrometallurgy Synthetic rutile Modified Nano-TiO 2 Hydrothermal Fig. 1. Relationship between the ilmenite and nano-tio 2 A 100 g of synthetic rutile and 200 g of Sodium Hydroxide were thoroughly mixed and heated in a furnace at 550 o C for 3 hours. The product was then washed with deionized water and its precipitate collected. This process was repeated until the impurities were removed from the precipitate. The cleaned precipitate was leached with sulfuric acid (3M) at 80 o C for 4 hours. After leaching, the precipitate was dried in a furnace at 200 o C for 1.5 hours. It is then grinded with mortar and pestle. Fig. 2 briefly summarizes this process. Fusion With NaOH, 550 o C for 3 hours Washing Leaching Drying With Distilled H 2O, 500 rpm, 2 hours With 3M H 2SO 4, at 80 o C for 4 hours 150 o C for 1.5 hours Grinding/Milling Fig. 2. Process flow chart for the fabrication of nano-tio 2 from ilmenite
Advanced Materials Research Vol. 620 181 Characterization. The X-Ray Diffraction (XRD) analysis was performed using a PANalytical PW3040/60 X Pert PRO apparatus. The voltage and anode current used were 40 kv and 30 ma, respectively. The CuK α = 0.15406 nm and the scanning range was from 20 o to 80 o. The XRD analysis is conducted in conjunction with the Electron Dispersive X-Ray Fluoroscence (EDXRF) analysis for all samples mentioned above. All measurements were carried out under the vacuum condition, using an EDAX International DX-95 EDXRF spectrometer with a Mo target, equipped with a liquid- nitrogen-cooled Si(Li) detector. The incident and take-off angles were 451, with a Be window thickness of 12.5 mm. The distance between the sample (exposed diameter of 22 mm) and the detector was 4.5 cm. The energy resolution was 0.16 kev. The concentrations of elements from Na to U were measured. In order to verify the results from the X-Ray methods, the Scanning Microscope (SEM) and Particle Size Analyzer were also used. The SEM used was FEI Quanta 4000, while the particle size analyzer used was MicroTrac X100. Results and Discussion Being a geographically dependant mineral, the exact content of ilmenite varies, although most ilmenite will commonly have Ti, Fe, S, Mg, Ca, U and Th, and their oxides. The separation, extraction and purification of these individual elements are the key to most ilmenite processing methods [2,3]. Fig. 1 demonstrates the relation between the mineral ilmenite and its final product, while Fig. 2 details the conversion and purification process of an intermediate product of ilmenite processing. Table 1 summarizes the chemical composition of Malaysian ilmenite, synthetic rutile and the nano-tio 2. Table 1. Elemental composition in wt% of ilmenite, intermediate and final product of the modified hydrothermal method Elements (in wt %) Sample Ti Zr Nb S Fe Si Ilmenite 67.94 NA 0.92 NA 29.00 NA Synthetic Rutile 93.80 0.34 1.05 0.08 2.32 2.09 Nano-TiO 2 95.07 0.49 0.36 2.83 0.09 NA The hydrometallurgy process, described by R.C.M. Mambote [4], extracts TiO 2 from ilmenite in the form synthetic rutile. The hydrothermal method was modified to accommodate the usage of synthetic rutile as a precursor in producing nano-tio 2. Two amendments were made to the conventional hydrothermal method; first, the utilization of natural precursors, and second, the elimination and simulation of the autoclave from the process. These modifications, by itself, significantly reduced the cost of the process, as synthetic rutile is rather cheap (USD 2/kg), and the elimination of an autoclave results in power consumption reduction during the process. The hydrometallurgy method employed by Tor Minerals Sdn. Bhd. extracted TiO 2 from ilmenite, along with other elements such as Fe 2 O 3, SiO 2, Zr, Nb. However, Table I indicates that Ti/TiO 2 still forms the majority in synthetic rutile (93.796%). XRD analysis in Fig. 3 confirms this while also showing the TiO 2 in the rutile phase; a structurally and chemically stable, non-volatile phase of TiO 2. It is surmised that the rutile phase in synthetic rutile is highly crystalline; this is confirmed by the low value of its Full Width Half Maximum (FWHM) value. This is also indicative of a large crystallite size (41.6 nm); calculated by applying FWHM values of the dominant peaks in Fig. 3 to the Scherrer Equation (Eq. 1).
182 Advanced X-Ray Characterization Techniques (1) where K* is a constant (ca. 0.9), λ the X-ray wavelength (1.5418 Å), θ B the Bragg angle and θ 0.5 the pure diffraction broadening of a peak at half height, due to crystallite dimensions. The amount of TiO 2 increased by 25.9% from ilmenite to synthetic rutile, and increased again by 1.27% from synthetic rutile to nano-tio 2. This is confirmed by both EDXRF and XRD analysis, showcased in Table 1 and Fig. 3, respectively. XRD and EDXRF analysis also shows the reduction, and finally the elimination, of Fe (Fe 2 O 3 ) and Si (SiO 2 ) during the processes, and the incorporation of Zr and Nb throughout the modified hydrothermal process. Two criteria need to be fulfilled before the product is considered a nano-tio 2 in this case, namely, the crystallite size and its phase. Generally, its size needs to be less than 100 nm, in most forms. Also, the phase of the product needs to be in the anatase phase, as synthetic rutile, with its rutile composition, is far too stable and inert for it to be of use for any complex applications such as photocatalysis or self-cleaning coatings. The first step in this process involves fusing synthetic rutile with NaOH. This process breaks Ti-O-Ti bonds, and fuses these broken bonds with Na and OH, forming Ti-O-Na and Ti-OH. The recreation of new bonds will release impurities that have embedded themselves in the Ti-O matrix, making it possible to remove them via a simple physical process such as washing. It will also simultaneously decrease the crystallite size of the product by recreating smaller matrices of Ti- O. The next step in the process, washing and filtration, simply removes the access Na and impurities that have been dispersed from the matrix, increasing the purity of the fusion product. Repeating this process will ensure that most impurities are removed. This fusion product, at this point, is still in the rutile phase. Generally, the leaching process is dependent on three or more factors, but in this particular study, only molarity, kinetics, and temperature will be considered [5,6]. Acid molarity provides the protons and SO 4 2- to the reaction; higher acidity increases the reaction rate in the system, while lower acidity decreases it. Kinetics involves keeping the solution during leaching constantly stirred, ensuring maximum interaction between the fusion product and the acid. A standard speed for the magnetic stirrer during leaching would be 250-300 rpm. Minimal interaction between the acid and the fusion product will cause it to remain in the rutile phase. Finally, a constant temperature (80 o C) is vital to ensure the reaction is not heat dominated, where the heat is merely complements and enhance the reaction between the TiO 2 and H 2 SO 4. When the temperature gets too high and heat dominates the reaction, crystallization will proceed rapidly, and the precipitate will remain in the rutile phase, although its crystallite size will be relatively larger, defeating our purposes. The leaching process induces three events; recrystallization, removal of impurities, and crystallite size reduction. The H 2 SO 4 reacts with the TiO 2 crystallites in the fusion product and with inducement in the form of heat, kinetics and pressure to encourage interaction, initiates crystallization. During crystallization, the minute amount of impurities present in the fusion product will be dispersed into the acid, making it susceptible to removal via repeated washing. However, despite repetitive washing, impurities such as Zr, Nb and Fe remain in the product, as shown in Table 1. These impurities will usually embed themselves in the Ti-O matrix in the form of subsitutional impurities, altering properties such as mechanical strength, band gaps and reactivity [4-6]. Fig. 3 details the XRD of all three elements, and their subsequent transformation from one phase to another.
Advanced Materials Research Vol. 620 183 2θ (Deg) Fig. 3. XRD Diffraction Peaks of ilmenite synthetic rutile and nano-tio 2 particles The crystallization process initiated during leaching will involve breaking and reforming the particles, and subsequently crystallites, and it continues until the end of the process. Once it is over, the overall crystallite size of the product will be greatly reduced; the crystallite size in this case was reduced by 62.5% from synthetic rutile to nano-tio 2, while its particle size was reduced by 98.62% (Table 2). The product from the modified hydrothermal process is a nano-tio 2 powder with minute amount of impurities (Zr, Nb, and Fe). These powders and its commercial counterpart purchased from American Elements (99.9% TiO 2 ) are examined using the SEM, and it was discovered that the nano-tio 2 is in particulate form. These particles measures 100 nm per particle, and are evenly distributed with minimal agglomeration. It is also significantly smaller than the particles that made up the commercial nano-tio 2 particles. The SEM micrograph of the samples and the commercial sample are shown in Fig. 4. The SEM shows that despite the tendency of nano-tio 2 formed by the hydrothermal process to curl and form tubes due to a more stable energy configuration in this form, the samples in this case remained in particulate form, due to its already stable energy state. a) b) c) Fig. 4. SEM of the a) commercial nano-tio 2 and b), c) nano-tio 2 particles produced by modified hydrothermal method taken at 40kx magnification There are two immediate benefits from the modified hydrothermal process; reduced processing costs and an environmentally friendly process. As mentioned previously, the cost of the precursor materials is quite low. The process itself is considered a clean, environmentally b) process, as it uses synthetic rutile, a stable and inert compound processed from a non-toxic mineral, and also sulfuric acid, which requires minimal caution during handling. The common chemicals/precursor in the hydrothermal process is usually chemicals that are deemed hazardous and expensive such as titanium alkoxide, titanium butoxide, as detailed in Titanium (IV) Butoxide: Material Safety Data Sheet. Due to the relatively low toxicity of the precursors and reagants, the process can be conducted in a simple fume hood, instead of having to construct deliberate mechanisms and systems to handle the release of toxic gases [6,7].
184 Advanced X-Ray Characterization Techniques For comparison purposes, both the nano-tio 2 and the commercial nano-tio 2 were analyzed for properties such as crystallite size and morphology. Fig. 5 shows the XRD results that compares the commercial product with the nano-tio 2. 2θ (Deg) Fig. 5. XRD results of a) commercial grade nano-tio 2 and b) nano-tio 2 (anatase), with * indicating the presence of sulfur The immediate difference between the two samples can be seen in the FWHM and the intensity of the peaks, which represents the crystallite size and crystallinity, respectively. The FWHM of the nano-tio 2 is larger, indicating a smaller crystallite size, with lower overall crystallinity, while the FWHM of the commercial nano-tio 2 is smaller, sharper and more defined, representing larger, more uniform crystallites in terms of size and distribution, with the values detailed in Table 2. The commercial sample is clearly more crystalline compared to the fabricated nano-tio 2. Its crystallite size, however, is smaller by 61%. It can be seen from both Figs. 3 & 5 that there is a small, noticeable presence of a Sulfur peak in the nano-tio 2 sample. This is due to sulfuric acid being used as a leaching agent during the hydrothermal process, and the remaining sulfate and proton was not fully removed during the subsequent washing and filtration. The presence of sulfur, although relatively significant, did not adversely affect the overall mechanical properties of the nano-tio 2 [8]. Table 2: Comparison of crystallite size and surface area of the samples at various forms and stages Samples Crystallite sizes (nm) FWHM (a.u.) Particle Size (µm) Ilmenite 29.5 0.3897 232.5 Synthetic Rutile 41.6 0.2922 298.2 Nano-TiO 2 15.6 0.4546 4.1 Commercial nano-tio 2 40.0 0.2273 27.9 Table 2 confirms that the crystallites of nano-tio 2 is smaller than its commercial counterpart, although, its crystallinity is lower, as per the FWHM value. However, in this regard, it can be argued that the crystallinity of both samples are similar, however, due to the smaller crystallites, it is more disperse and less agglomerated, and XRD analysis, at powder diffraction settings, might be incapable of comprehensively analyzing the samples. Particle Size Analyzer (PSA) results supports this theory, where it is seen that the average particle size and distribution of the nano-tio 2 particle is 4.1 um, significantly lowered from the particle sizes of ilmenite and synthetic rutile. Even when compared to commercial nano-tio 2 particles, the samples are still superior in terms of average particle sizes and distribution.
Advanced Materials Research Vol. 620 185 Conclusion It is concluded via EDXRF and XRD analysis that synthetic rutile derived from ilmenite is a viable precursor in nano-tio 2 particle production. The methods and parameters need to be modified to suit the nature of the precursor, however, the resulting product from this modified techniques are in some ways superior to a commercial product. Acknowledgement We would like to thank the Malaysian Nuclear Agency (NM-R&D-11-09) and University Malaya (PV058/2011A) for funding this project, as well as colleagues in the Material Technology Group and Advance Manufacturing and Materials Processing Center for their assistance and diligence in completing this project. References [1] X. Chen and S. Mao: Chem. Rev. Vol. 107, (2007), p. 2891-2959. [2] C. Li, B. Liang, L.-h. Guo and Z.-b. Wu: Minerals Engineering Vol. 19 (2006), p. 1430 1438. [3] T. Chernet: Miner. Eng. Vol. 12 (1999), p.485-495. [4] R.C.M. Mambote, M.A. Reuter, P.Krijgsman and R.D. Schuling: Miner. Eng. Vol. 13 (2000), p. 803-822. [5] E. Şayan and M. Bayramoğlu: Hydrometallurgy Vol. 57 (2000), p. 181-186. [6] E. Şayan and M. Bayramoğlu: Trans. IChemE. Vol. 79(b) (2001), p. 291-296 [7] Y. Zhao, C. Li, X. Liu, F. Gu, H.L. Du and L. Shi: Appl. Catal., B: Environmental Vol. 79 (2008), p. 208-215. [8] M. Hamadanian, A. Reisi-Vanani and A. Majedi: Mater. Chem. and Phys. Vol. 116 (2009), p. 376-382.