Abstract
Titanium dioxide (TiO2) nanoparticle was achieved in an alternative sol-gel route, as involved in 1 M acidic solution: HCl-tetrahydrofuran (HCl-THF), HNO3-tetrahydrofuran (HNO3-THF), and ClHNO2-tetrahydrofuran (ClHNO2-THF) solution. Resultant TiO2 nanoparticle was further investigated in a systematic analytical approach. Nanoscale TiO2 structure was observed at a moderate hydrolysis ratio (). Particle size range was much narrower in an aprotic HNO3-THF medium, as compared to a differential HCl-THF medium. Biphasic TiO2 structure was detected at a certain hydrolysis ratio (). Even so, relative anatase content was rather insignificant in an aprotic HCl-THF medium, as compared to a differential HNO3-THF medium. Tetragonal TiO2 structure was observed in the entire hydrolysis ratio (). Interstitial lattice defect was evident in an aprotic HNO3-THF medium but absent in a differential ClHNO2-THF medium.
1. Introduction
Titanium dioxide (TiO2) nanoparticles are applicable to high performance technologies, as evident in current innovative applications [1]. TiO2 nanoparticles are therefore considered as superior functional materials [2, 3]. Stoichiometric TiO2 nanoparticles are categorized as active dielectric () materials; nonstoichiometric nanoparticles are categorized as intrinsic semiconductor materials [4].
TiO2 nanoparticles are proven effective in numerous optoelectronic applications. Such predominant phenomena are related to superior optoelectronic properties: high dielectric constant, high transmission coefficient, and high breakdown strength [5]. TiO2 nanoparticles are also beneficial to several photovoltaic applications [6]. Such predominant phenomena are related to sufficient photoreactive properties [7].
TiO2 nanoparticles are prevalent in three distinct metastable configurations: tetragonal, monoclinic, and orthorhombic [8]. In general, anatase TiO2 nanostructures are more reactive than other polymorphic configurations. Such preferences are attributed to distinctive crystallographic properties [9]. Even so, anatase TiO2 nanostructures are rather difficult to synthesize, as susceptible to phase transformation [10].
In the recent past, anatase TiO2 nanostructures are often derived from sol-gel reaction [11]. Resultant internal nanostructures are more dependent on hydrolysis ratio. Spherical monodisperse nanostructures are achieved at stoichiometric ratio (). Resultant internal nanostructures are also dependent on hydrolysis catalyst [12, 13]. Dense microporous nanostructures are achieved in acidic catalysis (); loose mesoporous nanostructures are achieved in basic catalysis.
In the present investigation, TiO2 nanoparticles were achieved in sol-gel synthesis: HCl-THF, HNO3-THF, and ClHNO2-THF catalysis. Resultant TiO2 nanoparticles were then subjected to systematic characterization.
2. Methods
2.1. TiO2 Nanoparticles Preparation
TiO2 sol solutions were obtained at a specific H2O/alkoxide molar ratio (Table 1). TiO2 sol solutions were stirred at ambient room temperature (2 hours). Resultant sol particles were then dried in microwave oven.
2.2. TiO2 Nanoparticles Characterization
DLS analyses were conducted on a Malvern Zetasizer Nano ZS DLS spectrometer (Malvern Instruments GmbH, Herrenberg, Germany). DLS data were acquired in the backscattered mode. DLS data were collected at a fixed detection angle (°).
SEM analyses were conducted on a Carl Zeiss LEO 1450 VP microscope (Carl Zeiss AG, Oberkochen, Germany). SEM micrographs were acquired in the backscattered electron (BSE) mode. SEM micrographs were taken at a moderate acceleration voltage (15–20 kV).
XRD analyses were conducted on a Bruker D8 Advance diffractometer (Bruker AXS GmbH, Karlsruhe, Germany). XRD patterns were acquired in the symmetrical Bragg-Brentano configuration. XRD patterns were recorded in the 2-theta () range (15–60°; step size 0.02°).
FTIR analyses were conducted on a Perkin-Elmer Spectrum 400 spectrometer (Perkin-Elmer Ltd, Buckinghamshire, UK). FTIR spectra were acquired in the attenuated total reflectance (ATR) mode. FTIR spectra were recorded in the mid-infrared range (4000–650 cm−1; spectral resolution 4 cm−1).
3. Results and Discussion
3.1. Particle Size Distribution and Morphological Characterization
Particle size data was presented in graphical form (histogram), as shown in Figure 1.
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Broad particle size distribution was observed at high hydrolysis ratio (). Broad particle size distribution is attributed to rapid condensation reaction, as commenced before hydrolysis completion.
Particle growth process is accelerated at high condensation rate [14]. In such a case, particle growth process is pertained to monomers (Ti-OH) addition. Particle growth rate is dependent on interface reaction. In effect, rapid growth rate is expected at high monomers concentration. Dense aggregate structure () is predominant in monomers addition.
Narrow particle size distribution was observed at moderate hydrolysis ratio (). Narrow particle size distribution is attributed to more efficient hydrolysis reaction [15].
Distinct growth process is anticipated at high hydrolysis rate (Figure 2). More specific, particle growth process is pertained to clusters (Ti-OH-Ti) aggregation [16]. Particle growth rate is dependent on Brownian motion. In effect, rapid growth rate is expected at high effective collision. Fractal aggregate structure () is predominant in clusters aggregation.
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Broad particle size distribution was observed in HCl-THF medium. Broad particle size distribution is attributed to concurrent hydrolysis and condensation reactions. Such critical phenomenon is correlated to insignificant acidification.
Narrow particle size distribution was observed in HNO3-THF medium. Narrow particle size distribution is attributed to sequential hydrolysis and condensation reactions. Such critical phenomenon is correlated to significant protonation.
Particle formation process is preferred in high acidic medium. In particular, mechanism is involved in high acidic medium. Local equilibrium behavior is dependent on ionic strength gradient.
Metal alkoxide (Ti-OR) group is protonated in the initial step. In such a case, Ti-OR+∙ species is susceptible to hydrolysis reaction (Scheme 1).
Reactive Ti-OH+∙ species can react further in the subsequent step. Ti-O-Ti network structure is achieved in oxolation reaction; M-OH-M network structure is achieved in olation reaction (Scheme 2).
Smallest particle size data () is subjected to SEM measurement. DLS particle size data is based on hydrodynamic parameter, and therefore, resultant particle size distribution is much broader than SEM result. Similar observation was also reported in several previous studies [17].
Small aggregate particles were discerned in uniform bright contrast. Each aggregate particle does not exceed 50 nm in size. Small aggregate particles are attributed to attractive interparticle interactions, electrostatic attractive force, and covalent interactive force.
3.2. Phase Identification
Diffraction pattern was presented in the 2-theta range (), as shown in Figure 3.
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Broad diffraction hump was obtained at low hydrolysis ratio (). Broad diffraction hump was observed at . Such diffraction hump is attributed to random lattice orientation. In this particular case, amorphous TiO2 nanostructure is encountered in sixfold octahedral coordination but no fourfold tetrahedral coordination.
Broad intense diffraction band was observed at high hydrolysis ratio (). Preferable anatase diffraction band was detected at 2θ = 25.3° (101), 37.9° (004), 48.1° (200), 54.3° (105), and 62.5° (204).
More specific, anatase TiO2 nanostructure is achieved in gradual titanium-oxygen lattice rearrangement, that is, Ti-O bonds disproportionation (Figure 4). In literature, anatase TiO2 nanostructure is arranged in fourfold coordination: each Ti atom is linked to six oxygen atoms; each oxygen atom is linked to three Ti atoms [18].
Less intense diffraction band was obtained in HCl-THF medium. In such a case, phase transformation process is restricted in HCl-THF catalysis.
Broad intense diffraction band was obtained in HNO3-THF medium. In such a case, phase transformation process is preferred in HNO3-THF catalysis. Phase transformation process is mediated at low ambient supersaturation. Such critical phenomenon is attributed to rapid hydrolysis reaction.
Phase transformation process is spontaneous in aqueous acidic medium. Phase transformation process can occur in multiple distinct stages: nucleation, growth, and further coalescence [19]. Nucleation process is achieved in successive monomers condensation. Nucleation process is preferred above critical crystallite size, that is, 12 Ti-O bonds dimension. In general, nucleation rate is dependent on monomers concentration.
Crystallite growth process is achieved in monomers (Ti-OH) addition. Crystallite growth process is based on interface reaction, as involved in mass transfer (or heat transfer). In effect, crystallite growth rate is dependent on thermodynamic properties.
3.3. Structural Characterization
IR absorption spectrum was presented in the mid-infrared region (4000–650 cm−1), as shown in Figure 5.
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TiO2 characteristic band was obtained in HCl-THF and ClHNO2-THF medium. Ti-O absorption band was observed at 950 and 850 cm−1. Such absorption band is assigned to tetrahedral [TiO4]4− units. In such a case, Ti-O network structure is encountered in tetrahedral [TiO4]4− coordination.
Ti-OH absorption band was observed at 3800–3750 cm−1, but not obvious. In such a case, subsequent condensation reaction is continued till tetrahedral [TiO4]4− formation.
Board hydroxyl absorption band was observed at 3600–3000 cm−1. Broad absorption band is attributed to intermolecular interactions (e.g., H2O⋯H2O and/or Ti-OH⋯H2O interactions) (Figure 6). In general, surface hydroxyl group is resided at octahedral surface. Surface hydroxyl group is beneficial in photocatalytic activities.
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Surface H2O absorption band was observed at 1620 cm−1. Surface H2O absorption process is beneficial to heat generation, as consequence in Ti4+-OH2 bonds formation. In effect, phase transformation process is possible in ambient operational condition.
N-TiO2 characteristic band was obtained in observed in HNO3-THF medium. Ti-O-N absorption band was observed at 1550, 1300, and 1050 cm−1. Such absorption band is corresponded to interstitial nitrogen sites. In such a case, Ti-O-N network structure is encountered in interstitial coordination, rather than substitutional coordination (Figure 7).
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Nitrate (NO3) absorption band was observed at 1400 cm−1. Nitrate anion is chelated in bidentate fashion, and therefore, nitrate anion is rather difficult to eliminate.
4. Conclusions
Resultant TiO2 nanostructure was dependent on chemical reaction parameters. Narrower size range was obtained at a moderate hydrolysis ratio (). Substantial anatase phase was achieved in a certain hydrolysis ratio (). Even more, interstitial lattice defect was developed in an aprotic HNO3-THF medium.
Competing Interests
The authors declare that they have no competing interests.
Acknowledgments
O. L. Kang would like to extend his sincere appreciation to Universiti Kebangsaan Malaysia for financial support through University Research Grants (1) NND/NM(2)/TD11-046, (2) ERGS/1/2013/TK07/UKM/02/4, and (3) FRGS/1/2014/ST01/UKM/02/1. O. L. Kang would also like to extend his sincere appreciation to the CRIM for technical support. U. A. Rana would like to extend his sincere appreciation to King Saud University for financial support through Prolific Research Group, Project no. PRG-1436-18.