Effect of pressure on TiO₂ crystallization kinetics using in‐situ high‐temperature synchrotron radiation diffraction

Abstract

The phase transformation behavior of TiO 2 sol-gel synthesized nanopowder heated in a sealed quartz capillary from room temperature to 800°C was studied using in-situ synchrotron radiation diffraction (SRD). Sealing of the capillary resulted in an increase in capillary gas pressure with temperature. The pressures inside the sealed capillary were calculated using Gay-Lussac's Law, and they reached 0.36 MPa at 800°C. The as-synthesized material was entirely amorphous at room temperature, with crystalline anatase first appearing by 200°C (24 wt% absolute), then increasing rapidly in concentration to 89 wt% by 300°C and then increasing more slowly to 97 wt% by 800°C, with there being no indication of the anatase-to-rutile transformation up to 800°C. The best estimate of activation energy for the amorphous-to-anatase transformation from the SRD data was 10 (2) kJ/mol, which is much lower than that observed when heating the material under atmospheric pressure in a laboratory XRD experiment, 38(5) kJ/mol. For the experiment under atmospheric pressure, the anatase crystallization temperature was delayed by~200°C, first appearing after heating the sample to 400°C, after which crystalline rutile was first observed after heating to 600°C. The estimated activation energy for the anatase-to-rutile transformation was 120(18) kJ/mol, which agrees with estimates for titania nanofibers heated under atmospheric pressure. Thus, heating the nanopowders material under pressure promoted the amorphous-to-anatase transformation, but retarded the anatase-to-rutile transformation. This behavior is believed to occur in an oxygen-rich environment and interstitial titanium is also expected to form when the material is heated under high gas pressure. This suggests that atmospheric oxygen appears to accelerate the amorphousto-anatase transformation, whereas interstitial titanium inhibits TiO 2 structure relaxation, which is required for the anatase-to-rutile transformation. K E Y W O R D STiO 2 nanopowders, anatase, rutile, sol-gel method, in-situ synchrotron radiation diffraction

Thermal expansion coefficient determination of pure, doped, and co-doped anatase nanoparticles heated in sealed quartz capillaries using in-situ high-temperature synchrotron radiation diffraction

“…The effect of doping and co-doping on anatase TEC was studied based on pure, In-doped, Al-doped, co-doped Ag–Cr, and co-doped In–Cr titania nanoparticles. Information on pure/undoped anatase nanopowder and that doped with In and (In–Cr) has been provided previously [ 33 , 35 , 36 ]. A sol–gel approach was taken using titanium (IV) isopropoxide (C 12 H 28 O 4 Ti, molecular weight of 284.22 g/mol, Sigma-Aldrich, New South Wales, Australia).…”

“…The transformation of anatase-to-rutile has been studied for 400–1200 °C [ 24 , 25 , 26 , 27 , 28 , 29 , 30 , 31 , 32 , 33 , 34 , 35 , 36 ], but limited titania thermal-expansion data exist near the transition temperature. Titania thermal expansion has not been studied in detail and the effects of form, crystal structure, synthesis, doping, particles size, surface area, and atmospheric reactions from a high to a transition temperature are unknown.…”

Dynamic Diffraction Studies on the Crystallization, Phase Transformation, and Activation Energies in Anodized Titania Nanotubes

“…The thermal treatment of amorphous titania at elevated temperature yields, anatase, rutile, or a mixture of these crystalline structures as the most common titania phases, whereas brookite formation is rare [ 10 ]. Titania phase transformation is influenced by several conditions, such as calcining temperature, heating rate, calcination time, synthesis method, dopants, the level of impurities, grain size, and atmospheric condition and type [ 11 , 12 , 13 , 14 , 15 , 16 , 17 ].…”

The Role of Nb Addition in TiO2 Nanoparticles: Phase Transition and Photocatalytic Properties

“…Over the last years, several works have studied the kinetic of structural transformation (anatase‐to‐rutile) in TiO 2 compound synthesized via different methodologies . In these studies, it has been observed that the stability of the anatase phase as well as the phase transition could be controlled by some parameters, such as precursor materials, annealing temperature, particle size, morphology, surface chemistry, and addition and concentration of impurities (dopants or caping agents) . According to literature, both the catalytic properties were proved to strongly depend on the crystalline structure, making the control of anatase‐to‐rutile transformation process becomes a key parameter for the development of devices operating with maximum performance …”

“…[35,[41][42][43][44][45][46][47][48][49][50] In these studies, it has been observed that the stability of the anatase phase as well as the phase transition could be controlled by some parameters, such as precursor materials, annealing temperature, particle size, morphology, surface chemistry, and addition and concentration of impurities (dopants or caping agents). [16,25,35,[44][45][46][48][49][50][51][52][53][54][55] According to literature, both the catalytic properties were proved to strongly depend on the crystalline structure, [23,31,34,36,42] making the control of anatase-to-rutile transformation process becomes a key parameter for the development of devices operating with maximum performance. [27,31,56] The addition of transition metals ions (e.g., Fe, Co, and Nb) into the TiO 2 lattice has been an efficient way to enhance the (photo) catalytic activity.…”