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Phase transformation of nanostructured titanium dioxide from anatase-to-rutile via combined ultrasound assisted sol–gel technique
Ultrasonics Sonochemistry 17 (2010) 409–415
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Ultrasonics Sonochemistry journal homepage: www.elsevier.com/locate/ultsonch
Phase transformation of nanostructured titanium dioxide from anatase-to-rutile via combined ultrasound assisted sol–gel technique Krishnamurthy Prasad a, D.V. Pinjari b, A.B. Pandit b, S.T. Mhaske a,* a b
Polymer Engineering and Technology and Surface Coating Technology Division, Institute of Chemical Technology, Matunga, Mumbai-400 019, India Chemical Engineering Division, Institute of Chemical Technology, Matunga, Mumbai-400 019, India
a r t i c l e
i n f o
Article history: Received 12 June 2009 Received in revised form 27 August 2009 Accepted 8 September 2009 Available online 12 September 2009 Keywords: Sol–gel Acoustic cavitation (ultrasound) Titanium dioxide (TiO2) Anatase-to-rutile phase transformation
a b s t r a c t An effort was made to synthesize nanostructured TiO2 via sol–gel technique to obtain a 100% rutile polymorph of nanostructured TiO2. The sol–gel synthesis technique was suitably modified by incorporating ultrasound to study the effect of cavitation on the phase transformation, crystallite size, crystallinity and morphological (scanning electron microscopy) properties of the obtained nano-TiO2. It was observed that using ultrasound, yield of the nano-TiO2 was improved from 86.35% to 95.078%. The phase transformation of anatase-to-rutile of TiO2 was studied for both (ultrasound assisted and conventional) the processes. Complete phase transformation of the TiO2 was observed as expected with and without the use of ultrasound but the marked reduction in the required calcination temperature for obtaining 100% phase transformation with ultrasound was the major achievement in the present study, leading to 70% energy savings during calcination. Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction
TiAOR þ H2 O ! TiAOH þ RAOH Hydrolysis TiAOH þ ROATi ! TiAOATi þ RAOH Alcohol condensation
The sol–gel process is a versatile solution process for making ceramic and glass materials. The sol–gel process, as the name implies, involves the evolution of inorganic networks through the formation of a colloidal suspension (sol) and gelation of the sol to form a network in a continuous liquid phase (gel). A sol is a dispersion of the solid particles, with diameter of 1–1000 nm, in a liquid where only the Brownian motions kept particles in suspension. While a gel is a state where both liquid and solid are dispersed in each other, which presents a solid network filled with liquid components. The precursors for synthesizing these colloids consist of a metal or metalloid element surrounded by various reactive ligands. Metal alkoxides are most popular because they react readily with water. The most widely used metal alkoxides are the alkoxysilanes, such as tetramethoxysilane (TMOS) and tetraethoxysilane (TEOS). However, other alkoxides such as aluminates, titanates, and borates are also commonly used in the sol–gel process, often mixed with TEOS [1]. Three main reactions that occur during the sol–gel process are: hydrolysis, alcohol condensation, and water condensation. The reaction is as follows [2];
* Corresponding author. Tel.: +91 22 4145616; fax: +91 22 4145614. E-mail address: [email protected] (S.T. Mhaske). 1350-4177/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.ultsonch.2009.09.003
TiAOH þ HOATi ! TiAOATi þ H2 O Water condensation Titanium dioxide (TiO2) is an inorganic metal oxide which is widely used in the surface coating and the polymer industry as a pigment and a filler. It is also noteworthily used as an additive in sunscreen and food coloring. TiO2 mainly exists in two crystalline polymorphs viz.: anatase and rutile. In the anatase form, particularly, TiO2 is a photocatalyst under ultraviolet (UV) light, generating radical species that cause degradation of surface which, it is in contact. If it is used as a pigment or filler in exterior application, the materials can be degraded, which ultimately result in a marked reduction in the material’s properties. Sol–gel synthesis has been used widely for synthesizing nanoparticles of TiO2 [3–5], WO3 [6], SiO2 hybrids [7,8] and other metallic oxides. The sol–gel process is of interest in preparing these materials due to its mild conditions such as low temperature and pressure requirements. This process provides a convenient route to combine inorganic and organic components as a homogeneous hybrid material [9]. In these hybrid materials, organic and inorganic components can be chemically bonded or just physically mixed. A significant feature to enhance compatibility in hybrid materials is the formation of covalent bonding between organic–inorganic polymers and inorganic components [10,11]. In addition, the uses of organicallymodified precursors provide unique opportunities to tailor the
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physical and chemical properties of the hybrid materials and have the potential to incorporate chemically tailored features. A technique, which in some ways combines the aspects of different solution precipitation techniques, is the hydrodynamic cavitation process. Nanocrystalline oxide ceramic particles in the range 100 nm to a few microns have been produced by hydrodynamic processing in a microfluidizer. The method of producing oxide nanoparticles by the hydrodynamic cavitation process begins with the co-precipitation of the metal oxide components. The precipitated slurry stream is then drawn into a device where it is immediately elevated to high pressures within a small volume. The precipitated gel experiences ultrashear forces and cavitational heating. These two aspects lead to the formation of nanophase particles and high-phase purity in complex metal oxides [12]. Further research was done in the area of cavitation for synthesizing mesoporous nanoparticles upon the discovery that ultrasound could play a role in generating cavitation in a medium. Cavitation is the phenomenon of sequential formation, growth and collapse of millions of microscopic vapour bubbles (voids) in the liquid. The collapse or implosion of these cavities creates high localized temperatures roughly of about 5000–10,000 K and a pressure of about 1000–2000 atm and results into short-lived, localized hot spot in cold fluid. [13,14]. The chemical and physical effects of ultrasound in a fluid medium were first systematically reported by Suslick et al. [15] who stated that the chemical effects of ultrasound do not derive from a direct coupling of the acoustic field with chemical species on a molecular level. Instead, sonochemistry and sonoluminescence derive principally from acoustic cavitation, the formation, growth, and implosive collapse of bubbles in liquids irradiated with high-intensity ultrasound. The chemical effects of ultrasound have been well-explained as the consequence of localized hot spots created during bubble collapse [13] The collapse of bubbles caused by cavitation produces intense local heating and high pressures, with very short lifetimes. Cavitation can create extraordinary physical and chemical conditions in otherwise cold fluid [13–15]. The sonochemical preparation of nanophased oxides started long ago and is still continuing [16,17]. Titania [17], MnOx [18], etc. were prepared by employing ultrasound radiation. Precursors such as titanium isopropoxides [17], MnSO4, MnCl2, Mn(NO3)2, Mn(OAc)2, Mn(II) acetylacetonate, and MnCO3 [18], were used for the sonochemical fabrication of the nanooxides. Awati et al. [19] studied the advantages of sonication over the conventional method of preparing titanium dioxide. It was found that the more uniform distribution or dispersion of the nanoparticles, a marginally higher surface area, better thermal stability, and phase purity, are some of the advantages of the preparation of nanocrystalline titania by the ultrasonication method [19]. Cavitation has wide spread uses in the nanoparticle industry owing to its dual advantages in improving both the chemical reaction kinetics and helping in reducing the particle size to nanometer level with relative ease. Products obtained are sometimes nano-amorphous and in other cases nanocrystalline. This depends on the temperature in the fluid region where reaction takes place [13]. In this paper, nanostructured titanium dioxide was synthesized using sol–gel method and it is combined with ultrasound for a hybrid technique (ultrasound assisted sol–gel method). The effect of acoustic cavitation on calcination temperature and phase transformation in titanium dioxide from anatase-to-rutile was studied. 2. Materials and methods 2.1. Materials Titanium Tetraisopropoxide (TTIP) precursor was obtained from Spectrochem Pvt. Ltd., India. 2-propanol (99.9% pure) was pur-
chased from s. d. Fine Chemicals Ltd, Mumbai, India and glacial acetic acid solution was ordered from Merck Ltd., Mumbai, India. 2.2. Preparation of TiO2 by conventional sol–gel method (non-ultra sound assisted (NUS) sol–gel) 5 mL of Titanium Tetraisopropoxide (TTIP) precursor was added dropwise to 30 mL of 2-propanol. The resulting clear solution was added dropwise under stirring to 5 mL of glacial acetic acid solution. The sol thus obtained was left at ambient temperature for 24 h. For getting nearly 100% conversion (i.e. for ensuring complete hydrolysis of TTIP precursor), the solution needed to be kept as such for approximately 1 day. The sol obtained after 24 h was then converted into a gel by adding it to 30 mL of distilled water under stirring. The obtained gel was then dehydrated in an air circulating oven at 110 °C for 3 h. The dehydrated material was then calcined in a muffle furnace at various temperatures (450–850 °C) for a further 3 h. The white powder obtained after calcinations was cooled, ground and weighed to check for yield of the process. 2.3. Preparation of TiO2 by ultrasound assisted (US) sol–gel method The mixture of TTIP precursor, 2-propanol and glacial acetic acid solution was subjected to sonication using an Ultrasonic Horn (ACE 22 kHz) at 40% amplitude actually delivering 29.2 W of power for 10 min with a 5 s pulse and 5 s relaxation cycle. Thus, the sol obtained after keeping it for 24 h was again ultrasonically irradiated by keeping same parameters (output power and pulse and relaxation cycle). The remaining procedure including the volume of the reactants was kept the same (as reported in Section 2.2). The temperature rise in the system was restricted only to a few degree centigrade on account of spontaneous high energy collapse of the cavitation bubbles. The duration of this hot spot is of microsecond duration and cannot transfer enough heat during such a short duration. On a whole the system temperature was maintained at RT by making use of a cooling ice bath. Local and spontaneous high temperature has negligible effect on the chemical reaction. The use of a dual sonication can be explained as follows: The pre-hydrolysis sonication was done to ensure an efficient micromixing of the various reactants in the reaction system. This was done, therefore, to ensure that the reactants were contacted properly in the highly adverse conditions exerted onto the system during the 10 min of pre-hydrolysis sonication. The post-hydrolysis sonication was intended to encourage formation of nanoparticles (as nuclei) with smaller average particle sizes. This also ensured that during calcination the heat was transferred more efficiently into the bulk of the material thereby ensuring that all of the particles in the bulk were subjected to the highest possible temperature in the lowest possible time. After calcination the TiO2 powder (white in color) obtained was cooled, ground, checked for yield and characterized using the same techniques as that used for the NUS sol–gel synthesized samples. The calcinations temperatures that the samples were subjected to were 450 °C, 550 °C, 650 °C, 750 °C and 850 °C. Accordingly, the NUS sol–gel synthesized TiO2 samples were labeled as TiO2 NUS 450, TiO2 NUS 550, TiO2 NUS 650, TiO2 NUS 750 and TiO2 NUS 850 while the US sol–gel synthesized TiO2 samples were labeled as TiO2 US 450, TiO2 US 550, TiO2 US 650, TiO2 US 750 and TiO2 US 850. Muffle furnace (Expo Hi Tech Mumbai, India, Power input = 2 kW and Length Breadth Height = 30 10 10 cm) was used for calcination. Keeping in mind, heating rate for NUS and US sol–gel samples were kept same. The above mentioned samples were exposed to calcination when the mentioned temperature was reached.
K. Prasad et al. / Ultrasonics Sonochemistry 17 (2010) 409–415
3. Characterization The TiO2 samples were first characterized by studying their Xray Diffraction patterns on a Rigaku Mini-Flex X-ray Diffractometer. Crystallite sizes were determined using the Debye–Scherrer equation. Scanning Electron Microscopy of the samples was carried out on a JEOL JSM 680LA 15 kV SEM to estimate the surface characteristics of the sample. Together the XRD and SEM methods provide exact knowledge regarding the crystallite size and crystalline characteristics of the synthesized TiO2. 4. Results and discussions 4.1. X-ray diffractometer (XRD) study 4.1.1. Phase transformation of TiO2 in conventional (NUS) sol–gel method The XRD patterns of the TiO2 samples prepared by non-ultrasound (NUS) sol–gel method are shown in Fig. 1. The anatase form of TiO2 shows a peak at 2h 25° while the presence of rutile form is indicated by a peak at 2h = 28°. In the case of NUS sol–gel synthesized samples, TiO2 NUS 450 and TiO2 NUS 550 show predominantly anatase peaks i.e. percentage of rutile in these samples is for all practical purposes zero. In samples TiO2 NUS 650 and above there is a marked presence of rutile phase shown by the appearance of Diffraction peaks at 28° and so on. % Rutile was calculated by the following non-linear equation [20,21]:
%R ¼ 100=½ðA=RÞ 0:884 þ 1
ð1Þ
where:% R = Rutile content in percent.A = Peak Intensity of the peak at 2h = 25°.R = Peak Intensity of the peak at 2h = 28°. The % R in the samples continuously increases from TiO2 NUS 450 to TiO2 NUS 850 and consequently the sample TiO2 NUS 850 calcined at 850 °C consists of 100% Rutile phase, devoid of the presence of any anatase TiO2 form. The phase transformation was also evident from the peak pattern from 2h = 35° to 2h = 45°. TiO2 NUS 450 possesses a single peak at 2h = 37.86° corresponding to the anatase phase. The peak is whole with no shouldered peaks present around it thus indicating zero rutile influence. The same can be said for the TiO2 NUS 550 sample because the shouldering though present is insignificant enough to indicate no influence of rutile phase.
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It can be seen that from TiO2 NUS 650, the phase transformation starts taking shape. The peak at 2h = 37.66° that was only one in the first two (NUS 450 and NUS 550) cases now gets cleaved into four peaks corresponding to the presence of both anatase and rutile phases. This also coincides with the existence, now, of a peak at 2h = 28° that is in particular indicator of Rutile TiO2, starts appearing for NUS 650. In the case of TiO2 NUS 750 and TiO2 NUS 850 there is an increase in the influence of rutile in the sample (Table 1). The anatase peaks start reducing and finally vanish totally in the TiO2 NUS 850 sample confirming complete phase transformation of anatase-torutile. 4.1.2. Phase transformation of TiO2 in Ultrasound (US) assisted sol–gel method The XRD patterns of the TiO2 samples prepared by ultrasound (US) assisted sol–gel method are shown in Fig. 2. The US sol–gel synthesized nano-TiO2 XRD patterns are similar in principle to the one obtained for the NUS samples. However, the impact of acoustic cavitation was evident in not only the increased yields (86.35% for NUS compared to 95.08 % for US) but also the reduction in average crystallite size (Table 1). The yield of the reaction was estimated on the basis of initial raw materials weight taken and final weight of product (weighed accurately up to 0.001 g) obtained after the calcination. i.e. on the basis of stoichiometry. Hundred percent conversion was assumed to have occurred since sufficient time was given for the sol reaction to complete. The proper contacting of the reactants by the pre-hydrolysis sonication is evidenced by the subsequent high yields of the ultrasound assisted process. The major impact of cavitation on the sol–gel reaction was observed in the sample TiO2 US 750. In NUS synthesized TiO2 total phase transformation from anatase-to-rutile, confirmed by total absence of the peak at 25° occurred at or above 850 °C (TiO2 NUS 850). In the case of US sol–gel synthesized TiO2 100% phase transformation occurs at 750 °C – a whole 100 °C reduction in the required calcination temperature required for complete phase transformation. However, the onset of phase transformation starts again at 650 °C and above. This is due to reduced crystallite size using ultrasound (Fig. 4B) and reaching calcining temperature earlier.
Fig. 1. XRD patterns of NUS sol–gel synthesized nano-TiO2 at various calcination temperatures.
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Table 1 Crystallite size, % rutile and % crystallinity of synthesized titanium dioxide using conventional or non-ultrasonic (NUS) sol–gel method and ultrasound (US) assisted sol–gel method. Process
Sample
Particle size (nm)
Predominant phase
% Rutile (R)
% Crystallinity
% Yield
Conventional or non-ultrasonic (NUS) sol–gel method
NUS 450 NUS 550 NUS 650 NUS 750 NUS 850 US 450 US 550 US 650 US 750 US 850
10 14 26 37 26 8 10 28 30 28
Anatase Anatase Anatase Rutile Rutile Anatase Anatase Anatase Rutile Rutile
0 0 29.53 71.04 100 0 0 16.49 100 100
22.56 26.94 38.29 43.26 40.11 21.19 22.94 40.05 43.21 42.67
84.9 85.5 86.7 86.38 86.27 95.3 95.43 95.2 95.12 94.34
Ultrasound (US) assisted sol–gel method
Fig. 2. XRD Patterns of US sol–gel synthesized nano-TiO2 at various calcination temperatures.
4.1.3. % Crystallinity and crystallite size of NUS and US sol–gel method From the XRD patterns (Figs. 1 and 2), it is also possible to compute the % crystallinity and crystallite size. The amorphous content of the sample may be determined by taking the ratio of the amorphous area of the X-ray diffractogram to the total area. By amorphous area we mean that area of the diffractogram not contained by any diffraction peak. The process involves: 1. Smoothing the diffraction graph by a suitable smoothening method (Savitzky–Golay method is preferred). 2. Creating a baseline for the diffractogram whereby all the peaks shall be essentially starting at a common base. This is done by making use of the Sonnefield–Visser method obtained as part of the software. 3. Computing Integral area which is nothing but the crystalline fraction of the material. 4. Now the total area of the diffractogram is computed by carrying out this entire operation except that the baseline is not created. 5. Now 100 times the ratio of crystalline area to the total area is the % crystallinity. Crystallite size of the TiO2 samples may be determined using the Debye–Scherrer equation. Chandramouleeswaran et. al. [22] had used the Debye–Scherrer equation for measurement of size of nano ZnO. All the obtained values including %yields of the reaction, crystallinity and crystallite size were tabulated in Table 1. The Debye–Scherrer equation is used frequently in X-ray analysis of
materials, particularly powder diffraction of metal oxides. It relates the peak breadth of a specific phase of a material to the mean crystallite size of that material. It is quantitative equivalent of saying that the larger the material’s crystallites are, the sharper are the XRD peaks will be. The equation takes the form:
Bhkl ¼ Kk=ðDhkl cos hhkl Þ
ð2Þ
where: B is the width of the peak at half maximum intensity of a specific phase (hkl) in radians. D2 equals (Bo2 bo2), where Bo is the ‘full width half maximum’ (FWHM) of the XRD peaks and bo is the natural width of the XRD spectrometer. K, is a constant that varies with the method of taking the breadth (0.89 < K < 1). Here, in this work, for calculation purpose K = 0.9. k, is the wavelength of incident X-rays, = 1.54 Å for Cu Ka. h is the center angle of the peak. D is the crystallite length or primary crystallite size. The crystallinity of all the samples is considerable because of the inherent characteristics of the sol–gel process. Even in the case of the US sol–gel synthesized TiO2 the adverse environments created during cavitation did not affect the crystallinity of the TiO2 sample. The TiO2 sample calcined at 750 °C possesses the maximum crystallinity amongst both the NUS and US sol–gel synthesized TiO2 samples. The crystalline properties of the TiO2 calcined at 750 °C is similar in both cases (NUS and US) but the crystallite
K. Prasad et al. / Ultrasonics Sonochemistry 17 (2010) 409–415
size of the TiO2 US 750 sample is 18.5% lower than that of the TiO2 NUS 750. Thus a considerable reduction in crystallite size and that too not at the cost of crystallinity is obtained by the US sol–gel process. At 850 °C the heat energy provided to the TiO2 samples via calcinations had a disruptive effect on the crystallinity. This is attributed to the high energy provided by the increased temperature which in turn increases the randomness of the random/stochastic motion of the TiO2 molecules. Even though individual TiO2 molecules may not exist in the crystallites (agglomerated TiO2 molecules) which can also exhibit random motion, especially during cavitation induced acoustic streaming or thermal convective currents. This reduces the ability of the molecules to remain in a stable lattice positions for long leading to the lowering of crystallinity. The crystallite sizes of rutile are typically greater than anatase and hence an increase in primary crystallite size served as an indicator for changing rutile content.
4.2. Scanning Electron Microscopy (SEM) From the SEM micrographs (Fig. 3) of the various samples some observation have been made, that supported the results observed from the XRD patterns. We have specifically taken the image results of the TiO2 calcined at 450 °C for NUS (Fig. 3A) and US (Fig. 3B) at the same magnification of 2000. The US sol–gel synthesized TiO2 show not only comparatively smaller crystallite size but also considerably less agglomeration. The micrographs of the
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samples calcined at 850 °C are also displayed at the same magnification of 10,000. A building block sort of structure (fairly organized) is observed in the US samples (Fig. 3D) that is absent in the NUS samples (Fig. 3C). Lesser agglomeration and sharply defined structures are observed in the case of the US sol–gel samples in general. The SEM study of the uncalcined material also proves to be an aid in understanding the increased efficiency of the US assisted process compared to the NUS process for synthesizing TiO2. In Fig. 4, the uncalcined US synthesized material (Fig. 4B) shows considerably less aggregation and agglomeration and also much reduced crystallite size as compared to the uncalcined NUS synthesized material (Fig. 4A). Use of Cavitation as a reaction aid has had its influence in the pre-calcination step by considerably increasing the packing of the particles and reducing their crystallite size. This not only helps in improving the effect of the subsequent calcinations on the material by increasing the effective surface area reducing the phase transformation temperatures earlier and complete phase transformation at only 750 °C but also aids in increasing the crystallinity of the synthesized TiO2.
5. Efficacy of energy utilization For the comparison of the energy based performance of the two types of sol–gel methods (conventional and ultrasound assisted) to obtain 100% rutile TiO2 nanoparticles, the analysis is reported in Appendix I. The energy (kJ) taken into consideration is the total en-
Fig. 3. SEM Micrographs of Synthesized TiO2. (A) NUS 450 at 2000, (B) US 450 at 2000, (C) NUS 850 at 10,000 and (D) US 850 at 10,000.
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Fig. 4. SEM Micrographs of Uncalcined product. (A) NUS at 0.03 and (B) US at 0.03.
ergy supplied for calcination of TiO2 and also the energy supplied by the ultrasonic horn during irradiation. It is already explained in Sections 4.1.2 and 4.1.3 that 100% rutile TiO2 nanoparticles were obtained at calcination temperature of 750 °C for ultrasound assisted sol–gel process and 850 °C for conventional sol–gel process. Total energy required to achieve the temperature of 750 °C in furnace is 2100 kJ plus the energy used during sonication which is 10.51 kJ while total energy required to achieve 850 °C in same furnace is 7200 kJ. Thus, a total of 2110.5 kJ of energy is used in US assisted sol–gel process as against 7200 kJ without US treatment (conventional sol– gel process) bringing above a saving of more than 5000 kJ in the US assisted sol–gel process to synthesize 100% rutile grade TiO2.
2. Energy required for calcination Power input in Muffle furnace = 2 kW = 2000 W (measured current Amp; voltage 440 V) Time required to reach a temperature at 750 °C = 35 min (1050 s) Time required to reach a temperature at 850 °C = 60 min (3600 s) 2.1 US assisted sol–gel process Energy delivered (kJ) to reach a temperature at 750 °C = Power input (kW) in Muffle furnace Time required to reach a temperature at 750 °C
¼ 2 kW 1050 s ¼ 2000 J=s 1050 s
6. Conclusions Phase transformation from anantase to rutile of nanostructured titanium dioxide was achieved. 100% rutile polymorph of nanostructured TiO2 was achieved using sol–gel technique. Further, sol–gel method was modified by incorporating sonication and its effect on the properties of the obtained nano-TiO2 was examined. Using sonic energy, yield of the process was increased by 10% (85–95%). The phase transformation of anatase-to-rutile of TiO2 was studied during calcination for both the mentioned processes (NUS and US sol–gel method). Complete phase transformation of the TiO2 was achieved for both the processes but the marked reduction (by 100 °C) in the required calcination temperature of US sol–gel method was the major achievement of this study. This resulted into a substantial (almost 70%) savings of the energy for calcination. Appendix I. Energy calculations 1. Energy delivered during sonication Electrical energy delivered during ultrasonication using horn for 20 min (indicated by the power meter) = 35.040 kJ Efficiency of Horn taken for the calculation = 30% (estimated independently using calorimetric studies) Actual Energy delivered by horn during sonication = Energy delivered during sonication using horn in 20 min Efficiency of Horn
¼ 35:040 30=100 ¼ 10:512 kJ
¼ 2; 100; 000 J ¼ 2100 kJ
Total Energy gained by the TiO2 before calcination at 750 °C = Energy delivered (KJ) to reach a temperature at 750 °C (B) + Actual Energy delivered by horn during sonication (A)
¼ 2100 kJ þ 10:51 kJ ¼ 2110:51 kJ
ðCÞ
2.2 Sol–gel process without US Energy delivered (kJ) to reach a temperature at 850 °C = Total Energy gained by the TiO2 before calcination at 850 °C = Power input (kW) in Muffle furnace Time required to reach a temperature at 850 °C
¼ 2 kW 3600 s ¼ 2000 J=s 3600 s ¼ 7; 200; 000 J ¼ 7200 kJ
ðDÞ
3. Energy saved Net energy saved = (Energy delivered (KJ) to reach a temperature at 850 °C for non US sol gel process (D)) (Total Energy gained by the TiO2 before calcination at 750 °C (C) for US sol gel process
¼ 7200 kJ 2110:51 kJ ðAÞ
ðBÞ
¼ 5089:49 kJ
K. Prasad et al. / Ultrasonics Sonochemistry 17 (2010) 409–415
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Ultrasonics Sonochemistry journal homepage: www.elsevier.com/locate/ultsonch
Phase transformation of nanostructured titanium dioxide from anatase-to-rutile via combined ultrasound assisted sol–gel technique Krishnamurthy Prasad a, D.V. Pinjari b, A.B. Pandit b, S.T. Mhaske a,* a b
Polymer Engineering and Technology and Surface Coating Technology Division, Institute of Chemical Technology, Matunga, Mumbai-400 019, India Chemical Engineering Division, Institute of Chemical Technology, Matunga, Mumbai-400 019, India
a r t i c l e
i n f o
Article history: Received 12 June 2009 Received in revised form 27 August 2009 Accepted 8 September 2009 Available online 12 September 2009 Keywords: Sol–gel Acoustic cavitation (ultrasound) Titanium dioxide (TiO2) Anatase-to-rutile phase transformation
a b s t r a c t An effort was made to synthesize nanostructured TiO2 via sol–gel technique to obtain a 100% rutile polymorph of nanostructured TiO2. The sol–gel synthesis technique was suitably modified by incorporating ultrasound to study the effect of cavitation on the phase transformation, crystallite size, crystallinity and morphological (scanning electron microscopy) properties of the obtained nano-TiO2. It was observed that using ultrasound, yield of the nano-TiO2 was improved from 86.35% to 95.078%. The phase transformation of anatase-to-rutile of TiO2 was studied for both (ultrasound assisted and conventional) the processes. Complete phase transformation of the TiO2 was observed as expected with and without the use of ultrasound but the marked reduction in the required calcination temperature for obtaining 100% phase transformation with ultrasound was the major achievement in the present study, leading to 70% energy savings during calcination. Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction
TiAOR þ H2 O ! TiAOH þ RAOH Hydrolysis TiAOH þ ROATi ! TiAOATi þ RAOH Alcohol condensation
The sol–gel process is a versatile solution process for making ceramic and glass materials. The sol–gel process, as the name implies, involves the evolution of inorganic networks through the formation of a colloidal suspension (sol) and gelation of the sol to form a network in a continuous liquid phase (gel). A sol is a dispersion of the solid particles, with diameter of 1–1000 nm, in a liquid where only the Brownian motions kept particles in suspension. While a gel is a state where both liquid and solid are dispersed in each other, which presents a solid network filled with liquid components. The precursors for synthesizing these colloids consist of a metal or metalloid element surrounded by various reactive ligands. Metal alkoxides are most popular because they react readily with water. The most widely used metal alkoxides are the alkoxysilanes, such as tetramethoxysilane (TMOS) and tetraethoxysilane (TEOS). However, other alkoxides such as aluminates, titanates, and borates are also commonly used in the sol–gel process, often mixed with TEOS [1]. Three main reactions that occur during the sol–gel process are: hydrolysis, alcohol condensation, and water condensation. The reaction is as follows [2];
* Corresponding author. Tel.: +91 22 4145616; fax: +91 22 4145614. E-mail address: [email protected] (S.T. Mhaske). 1350-4177/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.ultsonch.2009.09.003
TiAOH þ HOATi ! TiAOATi þ H2 O Water condensation Titanium dioxide (TiO2) is an inorganic metal oxide which is widely used in the surface coating and the polymer industry as a pigment and a filler. It is also noteworthily used as an additive in sunscreen and food coloring. TiO2 mainly exists in two crystalline polymorphs viz.: anatase and rutile. In the anatase form, particularly, TiO2 is a photocatalyst under ultraviolet (UV) light, generating radical species that cause degradation of surface which, it is in contact. If it is used as a pigment or filler in exterior application, the materials can be degraded, which ultimately result in a marked reduction in the material’s properties. Sol–gel synthesis has been used widely for synthesizing nanoparticles of TiO2 [3–5], WO3 [6], SiO2 hybrids [7,8] and other metallic oxides. The sol–gel process is of interest in preparing these materials due to its mild conditions such as low temperature and pressure requirements. This process provides a convenient route to combine inorganic and organic components as a homogeneous hybrid material [9]. In these hybrid materials, organic and inorganic components can be chemically bonded or just physically mixed. A significant feature to enhance compatibility in hybrid materials is the formation of covalent bonding between organic–inorganic polymers and inorganic components [10,11]. In addition, the uses of organicallymodified precursors provide unique opportunities to tailor the
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physical and chemical properties of the hybrid materials and have the potential to incorporate chemically tailored features. A technique, which in some ways combines the aspects of different solution precipitation techniques, is the hydrodynamic cavitation process. Nanocrystalline oxide ceramic particles in the range 100 nm to a few microns have been produced by hydrodynamic processing in a microfluidizer. The method of producing oxide nanoparticles by the hydrodynamic cavitation process begins with the co-precipitation of the metal oxide components. The precipitated slurry stream is then drawn into a device where it is immediately elevated to high pressures within a small volume. The precipitated gel experiences ultrashear forces and cavitational heating. These two aspects lead to the formation of nanophase particles and high-phase purity in complex metal oxides [12]. Further research was done in the area of cavitation for synthesizing mesoporous nanoparticles upon the discovery that ultrasound could play a role in generating cavitation in a medium. Cavitation is the phenomenon of sequential formation, growth and collapse of millions of microscopic vapour bubbles (voids) in the liquid. The collapse or implosion of these cavities creates high localized temperatures roughly of about 5000–10,000 K and a pressure of about 1000–2000 atm and results into short-lived, localized hot spot in cold fluid. [13,14]. The chemical and physical effects of ultrasound in a fluid medium were first systematically reported by Suslick et al. [15] who stated that the chemical effects of ultrasound do not derive from a direct coupling of the acoustic field with chemical species on a molecular level. Instead, sonochemistry and sonoluminescence derive principally from acoustic cavitation, the formation, growth, and implosive collapse of bubbles in liquids irradiated with high-intensity ultrasound. The chemical effects of ultrasound have been well-explained as the consequence of localized hot spots created during bubble collapse [13] The collapse of bubbles caused by cavitation produces intense local heating and high pressures, with very short lifetimes. Cavitation can create extraordinary physical and chemical conditions in otherwise cold fluid [13–15]. The sonochemical preparation of nanophased oxides started long ago and is still continuing [16,17]. Titania [17], MnOx [18], etc. were prepared by employing ultrasound radiation. Precursors such as titanium isopropoxides [17], MnSO4, MnCl2, Mn(NO3)2, Mn(OAc)2, Mn(II) acetylacetonate, and MnCO3 [18], were used for the sonochemical fabrication of the nanooxides. Awati et al. [19] studied the advantages of sonication over the conventional method of preparing titanium dioxide. It was found that the more uniform distribution or dispersion of the nanoparticles, a marginally higher surface area, better thermal stability, and phase purity, are some of the advantages of the preparation of nanocrystalline titania by the ultrasonication method [19]. Cavitation has wide spread uses in the nanoparticle industry owing to its dual advantages in improving both the chemical reaction kinetics and helping in reducing the particle size to nanometer level with relative ease. Products obtained are sometimes nano-amorphous and in other cases nanocrystalline. This depends on the temperature in the fluid region where reaction takes place [13]. In this paper, nanostructured titanium dioxide was synthesized using sol–gel method and it is combined with ultrasound for a hybrid technique (ultrasound assisted sol–gel method). The effect of acoustic cavitation on calcination temperature and phase transformation in titanium dioxide from anatase-to-rutile was studied. 2. Materials and methods 2.1. Materials Titanium Tetraisopropoxide (TTIP) precursor was obtained from Spectrochem Pvt. Ltd., India. 2-propanol (99.9% pure) was pur-
chased from s. d. Fine Chemicals Ltd, Mumbai, India and glacial acetic acid solution was ordered from Merck Ltd., Mumbai, India. 2.2. Preparation of TiO2 by conventional sol–gel method (non-ultra sound assisted (NUS) sol–gel) 5 mL of Titanium Tetraisopropoxide (TTIP) precursor was added dropwise to 30 mL of 2-propanol. The resulting clear solution was added dropwise under stirring to 5 mL of glacial acetic acid solution. The sol thus obtained was left at ambient temperature for 24 h. For getting nearly 100% conversion (i.e. for ensuring complete hydrolysis of TTIP precursor), the solution needed to be kept as such for approximately 1 day. The sol obtained after 24 h was then converted into a gel by adding it to 30 mL of distilled water under stirring. The obtained gel was then dehydrated in an air circulating oven at 110 °C for 3 h. The dehydrated material was then calcined in a muffle furnace at various temperatures (450–850 °C) for a further 3 h. The white powder obtained after calcinations was cooled, ground and weighed to check for yield of the process. 2.3. Preparation of TiO2 by ultrasound assisted (US) sol–gel method The mixture of TTIP precursor, 2-propanol and glacial acetic acid solution was subjected to sonication using an Ultrasonic Horn (ACE 22 kHz) at 40% amplitude actually delivering 29.2 W of power for 10 min with a 5 s pulse and 5 s relaxation cycle. Thus, the sol obtained after keeping it for 24 h was again ultrasonically irradiated by keeping same parameters (output power and pulse and relaxation cycle). The remaining procedure including the volume of the reactants was kept the same (as reported in Section 2.2). The temperature rise in the system was restricted only to a few degree centigrade on account of spontaneous high energy collapse of the cavitation bubbles. The duration of this hot spot is of microsecond duration and cannot transfer enough heat during such a short duration. On a whole the system temperature was maintained at RT by making use of a cooling ice bath. Local and spontaneous high temperature has negligible effect on the chemical reaction. The use of a dual sonication can be explained as follows: The pre-hydrolysis sonication was done to ensure an efficient micromixing of the various reactants in the reaction system. This was done, therefore, to ensure that the reactants were contacted properly in the highly adverse conditions exerted onto the system during the 10 min of pre-hydrolysis sonication. The post-hydrolysis sonication was intended to encourage formation of nanoparticles (as nuclei) with smaller average particle sizes. This also ensured that during calcination the heat was transferred more efficiently into the bulk of the material thereby ensuring that all of the particles in the bulk were subjected to the highest possible temperature in the lowest possible time. After calcination the TiO2 powder (white in color) obtained was cooled, ground, checked for yield and characterized using the same techniques as that used for the NUS sol–gel synthesized samples. The calcinations temperatures that the samples were subjected to were 450 °C, 550 °C, 650 °C, 750 °C and 850 °C. Accordingly, the NUS sol–gel synthesized TiO2 samples were labeled as TiO2 NUS 450, TiO2 NUS 550, TiO2 NUS 650, TiO2 NUS 750 and TiO2 NUS 850 while the US sol–gel synthesized TiO2 samples were labeled as TiO2 US 450, TiO2 US 550, TiO2 US 650, TiO2 US 750 and TiO2 US 850. Muffle furnace (Expo Hi Tech Mumbai, India, Power input = 2 kW and Length Breadth Height = 30 10 10 cm) was used for calcination. Keeping in mind, heating rate for NUS and US sol–gel samples were kept same. The above mentioned samples were exposed to calcination when the mentioned temperature was reached.
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3. Characterization The TiO2 samples were first characterized by studying their Xray Diffraction patterns on a Rigaku Mini-Flex X-ray Diffractometer. Crystallite sizes were determined using the Debye–Scherrer equation. Scanning Electron Microscopy of the samples was carried out on a JEOL JSM 680LA 15 kV SEM to estimate the surface characteristics of the sample. Together the XRD and SEM methods provide exact knowledge regarding the crystallite size and crystalline characteristics of the synthesized TiO2. 4. Results and discussions 4.1. X-ray diffractometer (XRD) study 4.1.1. Phase transformation of TiO2 in conventional (NUS) sol–gel method The XRD patterns of the TiO2 samples prepared by non-ultrasound (NUS) sol–gel method are shown in Fig. 1. The anatase form of TiO2 shows a peak at 2h 25° while the presence of rutile form is indicated by a peak at 2h = 28°. In the case of NUS sol–gel synthesized samples, TiO2 NUS 450 and TiO2 NUS 550 show predominantly anatase peaks i.e. percentage of rutile in these samples is for all practical purposes zero. In samples TiO2 NUS 650 and above there is a marked presence of rutile phase shown by the appearance of Diffraction peaks at 28° and so on. % Rutile was calculated by the following non-linear equation [20,21]:
%R ¼ 100=½ðA=RÞ 0:884 þ 1
ð1Þ
where:% R = Rutile content in percent.A = Peak Intensity of the peak at 2h = 25°.R = Peak Intensity of the peak at 2h = 28°. The % R in the samples continuously increases from TiO2 NUS 450 to TiO2 NUS 850 and consequently the sample TiO2 NUS 850 calcined at 850 °C consists of 100% Rutile phase, devoid of the presence of any anatase TiO2 form. The phase transformation was also evident from the peak pattern from 2h = 35° to 2h = 45°. TiO2 NUS 450 possesses a single peak at 2h = 37.86° corresponding to the anatase phase. The peak is whole with no shouldered peaks present around it thus indicating zero rutile influence. The same can be said for the TiO2 NUS 550 sample because the shouldering though present is insignificant enough to indicate no influence of rutile phase.
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It can be seen that from TiO2 NUS 650, the phase transformation starts taking shape. The peak at 2h = 37.66° that was only one in the first two (NUS 450 and NUS 550) cases now gets cleaved into four peaks corresponding to the presence of both anatase and rutile phases. This also coincides with the existence, now, of a peak at 2h = 28° that is in particular indicator of Rutile TiO2, starts appearing for NUS 650. In the case of TiO2 NUS 750 and TiO2 NUS 850 there is an increase in the influence of rutile in the sample (Table 1). The anatase peaks start reducing and finally vanish totally in the TiO2 NUS 850 sample confirming complete phase transformation of anatase-torutile. 4.1.2. Phase transformation of TiO2 in Ultrasound (US) assisted sol–gel method The XRD patterns of the TiO2 samples prepared by ultrasound (US) assisted sol–gel method are shown in Fig. 2. The US sol–gel synthesized nano-TiO2 XRD patterns are similar in principle to the one obtained for the NUS samples. However, the impact of acoustic cavitation was evident in not only the increased yields (86.35% for NUS compared to 95.08 % for US) but also the reduction in average crystallite size (Table 1). The yield of the reaction was estimated on the basis of initial raw materials weight taken and final weight of product (weighed accurately up to 0.001 g) obtained after the calcination. i.e. on the basis of stoichiometry. Hundred percent conversion was assumed to have occurred since sufficient time was given for the sol reaction to complete. The proper contacting of the reactants by the pre-hydrolysis sonication is evidenced by the subsequent high yields of the ultrasound assisted process. The major impact of cavitation on the sol–gel reaction was observed in the sample TiO2 US 750. In NUS synthesized TiO2 total phase transformation from anatase-to-rutile, confirmed by total absence of the peak at 25° occurred at or above 850 °C (TiO2 NUS 850). In the case of US sol–gel synthesized TiO2 100% phase transformation occurs at 750 °C – a whole 100 °C reduction in the required calcination temperature required for complete phase transformation. However, the onset of phase transformation starts again at 650 °C and above. This is due to reduced crystallite size using ultrasound (Fig. 4B) and reaching calcining temperature earlier.
Fig. 1. XRD patterns of NUS sol–gel synthesized nano-TiO2 at various calcination temperatures.
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Table 1 Crystallite size, % rutile and % crystallinity of synthesized titanium dioxide using conventional or non-ultrasonic (NUS) sol–gel method and ultrasound (US) assisted sol–gel method. Process
Sample
Particle size (nm)
Predominant phase
% Rutile (R)
% Crystallinity
% Yield
Conventional or non-ultrasonic (NUS) sol–gel method
NUS 450 NUS 550 NUS 650 NUS 750 NUS 850 US 450 US 550 US 650 US 750 US 850
10 14 26 37 26 8 10 28 30 28
Anatase Anatase Anatase Rutile Rutile Anatase Anatase Anatase Rutile Rutile
0 0 29.53 71.04 100 0 0 16.49 100 100
22.56 26.94 38.29 43.26 40.11 21.19 22.94 40.05 43.21 42.67
84.9 85.5 86.7 86.38 86.27 95.3 95.43 95.2 95.12 94.34
Ultrasound (US) assisted sol–gel method
Fig. 2. XRD Patterns of US sol–gel synthesized nano-TiO2 at various calcination temperatures.
4.1.3. % Crystallinity and crystallite size of NUS and US sol–gel method From the XRD patterns (Figs. 1 and 2), it is also possible to compute the % crystallinity and crystallite size. The amorphous content of the sample may be determined by taking the ratio of the amorphous area of the X-ray diffractogram to the total area. By amorphous area we mean that area of the diffractogram not contained by any diffraction peak. The process involves: 1. Smoothing the diffraction graph by a suitable smoothening method (Savitzky–Golay method is preferred). 2. Creating a baseline for the diffractogram whereby all the peaks shall be essentially starting at a common base. This is done by making use of the Sonnefield–Visser method obtained as part of the software. 3. Computing Integral area which is nothing but the crystalline fraction of the material. 4. Now the total area of the diffractogram is computed by carrying out this entire operation except that the baseline is not created. 5. Now 100 times the ratio of crystalline area to the total area is the % crystallinity. Crystallite size of the TiO2 samples may be determined using the Debye–Scherrer equation. Chandramouleeswaran et. al. [22] had used the Debye–Scherrer equation for measurement of size of nano ZnO. All the obtained values including %yields of the reaction, crystallinity and crystallite size were tabulated in Table 1. The Debye–Scherrer equation is used frequently in X-ray analysis of
materials, particularly powder diffraction of metal oxides. It relates the peak breadth of a specific phase of a material to the mean crystallite size of that material. It is quantitative equivalent of saying that the larger the material’s crystallites are, the sharper are the XRD peaks will be. The equation takes the form:
Bhkl ¼ Kk=ðDhkl cos hhkl Þ
ð2Þ
where: B is the width of the peak at half maximum intensity of a specific phase (hkl) in radians. D2 equals (Bo2 bo2), where Bo is the ‘full width half maximum’ (FWHM) of the XRD peaks and bo is the natural width of the XRD spectrometer. K, is a constant that varies with the method of taking the breadth (0.89 < K < 1). Here, in this work, for calculation purpose K = 0.9. k, is the wavelength of incident X-rays, = 1.54 Å for Cu Ka. h is the center angle of the peak. D is the crystallite length or primary crystallite size. The crystallinity of all the samples is considerable because of the inherent characteristics of the sol–gel process. Even in the case of the US sol–gel synthesized TiO2 the adverse environments created during cavitation did not affect the crystallinity of the TiO2 sample. The TiO2 sample calcined at 750 °C possesses the maximum crystallinity amongst both the NUS and US sol–gel synthesized TiO2 samples. The crystalline properties of the TiO2 calcined at 750 °C is similar in both cases (NUS and US) but the crystallite
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size of the TiO2 US 750 sample is 18.5% lower than that of the TiO2 NUS 750. Thus a considerable reduction in crystallite size and that too not at the cost of crystallinity is obtained by the US sol–gel process. At 850 °C the heat energy provided to the TiO2 samples via calcinations had a disruptive effect on the crystallinity. This is attributed to the high energy provided by the increased temperature which in turn increases the randomness of the random/stochastic motion of the TiO2 molecules. Even though individual TiO2 molecules may not exist in the crystallites (agglomerated TiO2 molecules) which can also exhibit random motion, especially during cavitation induced acoustic streaming or thermal convective currents. This reduces the ability of the molecules to remain in a stable lattice positions for long leading to the lowering of crystallinity. The crystallite sizes of rutile are typically greater than anatase and hence an increase in primary crystallite size served as an indicator for changing rutile content.
4.2. Scanning Electron Microscopy (SEM) From the SEM micrographs (Fig. 3) of the various samples some observation have been made, that supported the results observed from the XRD patterns. We have specifically taken the image results of the TiO2 calcined at 450 °C for NUS (Fig. 3A) and US (Fig. 3B) at the same magnification of 2000. The US sol–gel synthesized TiO2 show not only comparatively smaller crystallite size but also considerably less agglomeration. The micrographs of the
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samples calcined at 850 °C are also displayed at the same magnification of 10,000. A building block sort of structure (fairly organized) is observed in the US samples (Fig. 3D) that is absent in the NUS samples (Fig. 3C). Lesser agglomeration and sharply defined structures are observed in the case of the US sol–gel samples in general. The SEM study of the uncalcined material also proves to be an aid in understanding the increased efficiency of the US assisted process compared to the NUS process for synthesizing TiO2. In Fig. 4, the uncalcined US synthesized material (Fig. 4B) shows considerably less aggregation and agglomeration and also much reduced crystallite size as compared to the uncalcined NUS synthesized material (Fig. 4A). Use of Cavitation as a reaction aid has had its influence in the pre-calcination step by considerably increasing the packing of the particles and reducing their crystallite size. This not only helps in improving the effect of the subsequent calcinations on the material by increasing the effective surface area reducing the phase transformation temperatures earlier and complete phase transformation at only 750 °C but also aids in increasing the crystallinity of the synthesized TiO2.
5. Efficacy of energy utilization For the comparison of the energy based performance of the two types of sol–gel methods (conventional and ultrasound assisted) to obtain 100% rutile TiO2 nanoparticles, the analysis is reported in Appendix I. The energy (kJ) taken into consideration is the total en-
Fig. 3. SEM Micrographs of Synthesized TiO2. (A) NUS 450 at 2000, (B) US 450 at 2000, (C) NUS 850 at 10,000 and (D) US 850 at 10,000.
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Fig. 4. SEM Micrographs of Uncalcined product. (A) NUS at 0.03 and (B) US at 0.03.
ergy supplied for calcination of TiO2 and also the energy supplied by the ultrasonic horn during irradiation. It is already explained in Sections 4.1.2 and 4.1.3 that 100% rutile TiO2 nanoparticles were obtained at calcination temperature of 750 °C for ultrasound assisted sol–gel process and 850 °C for conventional sol–gel process. Total energy required to achieve the temperature of 750 °C in furnace is 2100 kJ plus the energy used during sonication which is 10.51 kJ while total energy required to achieve 850 °C in same furnace is 7200 kJ. Thus, a total of 2110.5 kJ of energy is used in US assisted sol–gel process as against 7200 kJ without US treatment (conventional sol– gel process) bringing above a saving of more than 5000 kJ in the US assisted sol–gel process to synthesize 100% rutile grade TiO2.
2. Energy required for calcination Power input in Muffle furnace = 2 kW = 2000 W (measured current Amp; voltage 440 V) Time required to reach a temperature at 750 °C = 35 min (1050 s) Time required to reach a temperature at 850 °C = 60 min (3600 s) 2.1 US assisted sol–gel process Energy delivered (kJ) to reach a temperature at 750 °C = Power input (kW) in Muffle furnace Time required to reach a temperature at 750 °C
¼ 2 kW 1050 s ¼ 2000 J=s 1050 s
6. Conclusions Phase transformation from anantase to rutile of nanostructured titanium dioxide was achieved. 100% rutile polymorph of nanostructured TiO2 was achieved using sol–gel technique. Further, sol–gel method was modified by incorporating sonication and its effect on the properties of the obtained nano-TiO2 was examined. Using sonic energy, yield of the process was increased by 10% (85–95%). The phase transformation of anatase-to-rutile of TiO2 was studied during calcination for both the mentioned processes (NUS and US sol–gel method). Complete phase transformation of the TiO2 was achieved for both the processes but the marked reduction (by 100 °C) in the required calcination temperature of US sol–gel method was the major achievement of this study. This resulted into a substantial (almost 70%) savings of the energy for calcination. Appendix I. Energy calculations 1. Energy delivered during sonication Electrical energy delivered during ultrasonication using horn for 20 min (indicated by the power meter) = 35.040 kJ Efficiency of Horn taken for the calculation = 30% (estimated independently using calorimetric studies) Actual Energy delivered by horn during sonication = Energy delivered during sonication using horn in 20 min Efficiency of Horn
¼ 35:040 30=100 ¼ 10:512 kJ
¼ 2; 100; 000 J ¼ 2100 kJ
Total Energy gained by the TiO2 before calcination at 750 °C = Energy delivered (KJ) to reach a temperature at 750 °C (B) + Actual Energy delivered by horn during sonication (A)
¼ 2100 kJ þ 10:51 kJ ¼ 2110:51 kJ
ðCÞ
2.2 Sol–gel process without US Energy delivered (kJ) to reach a temperature at 850 °C = Total Energy gained by the TiO2 before calcination at 850 °C = Power input (kW) in Muffle furnace Time required to reach a temperature at 850 °C
¼ 2 kW 3600 s ¼ 2000 J=s 3600 s ¼ 7; 200; 000 J ¼ 7200 kJ
ðDÞ
3. Energy saved Net energy saved = (Energy delivered (KJ) to reach a temperature at 850 °C for non US sol gel process (D)) (Total Energy gained by the TiO2 before calcination at 750 °C (C) for US sol gel process
¼ 7200 kJ 2110:51 kJ ðAÞ
ðBÞ
¼ 5089:49 kJ
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