Research On The Mechanism Of Carbon-Induced Crystallization And Phase Transition In Titanium Dioxide

Titanium dioxide is a widely used metal oxide semiconductor material with two main crystal structures:anatase and rutile.Due to the superior photocatalytic performance of the mixed-phase titanium dioxide composed of anatase and rutile,it is necessary to regulate the process of transformation from anatase to rutile to obtain the mixed-phase titanium dioxide.The introduction of dopants is one of the main methods to regulate the crystal structure of titanium dioxide,and carbon is an ideal dopant for controlling the crystal structure of titanium dioxide due to its easy removal without affecting the crystal structure.Currently,carbon doping is mainly used to improve the photoelectric properties of anatase,while little attention has been paid to the effect of carbon on the crystallization of anatase and the impact of carbon outgassing on the transformation of anatase to rutile.Based on the experimental boundary of dopant ion radius and valence state on the promotion and inhibition of titanium dioxide phase transformation,carbon is exactly on the experimental boundary line,so the effect of carbon on the transformation of anatase to rutile is uncertain from the perspective of doping.When carbon outgases as CO or CO₂,it is predicted to promote the phase transformation because it forms oxygen vacancies in titanium dioxide.However,in practice,carbon is generally used as a high-temperature stabilizer for anatase,and the transformation temperature from anatase to rutile in carbon-doped titanium dioxide is above 700℃,while the transformation temperature is generally believed to be 600℃.From the experimental results,carbon inhibits the phase transformation of titanium dioxide.To address the inconsistency between theoretical and experimental results,we plan to comprehensively explore the influence of carbon on the whole process of titanium dioxide from crystallization to phase transformation by quantitatively analyzing the effect of carbon on the phase transformation of titanium dioxide.In this paper,we aim to quantitatively study the influence of carbon on the transformation from amorphous to rutile and from rutile to anatase under different atmospheres by stepwise calcination.The method involves two steps:low-temperature carbonization in argon will transform the titanium dioxide dried gel into amorphous titanium dioxide and convert its organic components into carbon;calcination in air at different times can yield amorphous titanium dioxide with varying carbon contents.Finally,the amorphous titanium dioxide with a fixed and determined carbon content will be crystallized at high temperature in argon.XRD,Raman spectroscopy,and TG-MS are used to comprehensively and quantitatively analyze the different influences of carbon on the crystallization and phase transformation stages of titanium dioxide.The main research contents of this paper are as follows:（1）The inhibitory effect of carbon on the crystallization of rutile titanium dioxide.It was found through TG-DSC and XRD that the crystallization temperature increased with the increase of carbon content,while the crystallinity and particle size of rutile decreased with the increase of carbon content.The TG-MS results showed that the oxygen vacancies formed by the evaporation of carbon in amorphous titanium dioxide would hinder the sharing of edges or vertices of adjacent Ti O₆ octahedra,thus affecting the newly formed small anatase crystals during the calcination process.Carbon is not conducive to the crystallization of amorphous titanium dioxide into anatase.The refined XRD data showed that as the carbon content increased,the a and b axis parameters of the rutile crystal cell remained unchanged,but the c axis parameter became smaller.This suggested that the presence of carbon would compress the anatase crystal cell,increase the activation energy required for the aggregation and growth between the rutile crystal cells,and thus delay the growth of anatase.The above research results indicate that carbon impedes the crystallization and growth of anatase during the transformation from amorphous titanium dioxide to anatase.（2）The promoting effect of carbon on the transformation of anatase to rutile.a.XRD data revealed that compared with the anatase-rutile transformation temperature of 625℃in air,the temperature of crystal phase transformation could be reduced by 100℃to 525℃with a small amount of carbon in argon,indicating that carbon is beneficial to phase transformation in argon.TG-MS data indicated that after500℃in argon,the evaporation of carbon would form a small amount of oxygen vacancies,which would assist the sharing of edge or vertex oxygen atoms of adjacent Ti O₆ octahedra,and thus promote the transformation of loosely piled anatase to densely piled rutile.b.The crystal type proportions of titanium dioxide surface and bulk were tested by 325 nm UV and 633 nm visible Raman spectroscopy,respectively.It was found that the formation of anatase on the surface was far slower than that in the bulk,indicating that the transformation of rutile to anatase occurred first in close-packed bulk that needed sufficient rutile connections to occur.Therefore,the crystallinity of anatase would affect the transformation of anatase to rutile.In addition,theoretical studies found that 14 nm is the critical size for the transformation of anatase to rutile,so it is necessary to study the influence of crystal degree and size of anatase on the transformation.c.XRD analysis showed that anatase in titanium dioxide would only transform into rutile when the particle size of the anatase is larger than 14 nm,the crystallinity is greater than 30%,and the calcination temperature reaches 525 degrees.The inhibitory effect of carbon mainly manifested as reducing the crystallinity and grain size of anatase,making it unable to reach the crystallinity and size required for crystal type transformation,thus affecting the subsequent transformation of anatase to rutile.Therefore,under high carbon concentration conditions,carbon will show inhibition on the transformation of anatase to rutile,which is not the effect of carbon itself on the transformation.（3）The influence of different crystal parameters of titanium dioxide on its photocatalytic performance.a.Based on the research results of 1 and 2,the inhibitory effect of carbon on the crystallization of titanium dioxide and the promoting effect of carbon on crystal type transformation depend on whether the size and crystallinity of anatase meet the conditions required for its transformation,induced by the changes in carbon content.Using this dual effect of carbon,a sample library of mixed phase titanium dioxide was constructed by calcining amorphous titanium dioxide with different carbon content in different temperatures and times under argon.b.The proportions of rutile,as well as the size of anatase and rutile crystals,are interrelated in previous studies,and changing one parameter will result in changes of the other two.Through this sample library,titanium dioxide with one or two crystal parameters unchanged and the other changed can be selected for independent studies on the influence of different crystal parameters on the photocatalytic performance of titanium dioxide.Taking the photocatalytic degradation of gaseous acetone by mixed phase titanium dioxide as an example,the influence of crystal type proportion or crystal size on the reaction rate was analyzed independently.The research results showed that the optimal proportion of rutile for mixed phase titanium dioxide catalyst was about16%,the size of anatase crystal was about 24.9 nm,and the size of rutile crystal was about 40 nm.Our work hopes to draw attention to the different stages where the same dopant may show different effects on inducing the crystallization and phase transformation of metal oxides,especially the non-metal dopants like carbon and nitrogen,which will inevitably be brought into metal oxides to affect their crystallization and phase transformation.We also hope to expand the method of stepwise calcination under different atmospheres to the use of more non-metal dopants,such as carbon and nitrogen,to regulate the crystal structure of titanium dioxide and other functional metal oxides,in order to design new metal oxide catalysts for application in more fields.