Uniform deposition of ultra-thin TiO2 film on mica substrate by atmospheric pressure chemical vapor deposition: Effect of precursor concentration

Ming Liu,Ying Li,Rui Wang,Guoqiang Shao,Pengpeng Lv,,Jun Li,Qingshan Zhu,3,

1 State Key Laboratory of Multiphase Complex Systems,Institute of Process Engineering,Chinese Academy of Sciences,Beijing 100190,China

2 School of Chemical & Environmental Engineering,China University of Mining and Technology-Beijing,Beijing 100083,China

3 School of Chemical Engineering,University of Chinese Academy of Sciences,Beijing 100049,China

Keywords:Chemical vapor deposition TiO2 thin film Nucleation reaction Precursor concentration Pearlescent pigment

ABSTRACT The performance of pearlescent pigment significantly affected by the grain size and the roughness of deposited film.The effect of TiCl4 concentration on the initial deposition of TiO2 on mica by atmospheric pressure chemical vapor deposition(APCVD)was investigated.The precursor concentration significantly affected the deposition and morphology of TiO2 grains assembling the film.The deposition time for fully covering the surface of mica decreased from 120 to 10 s as the TiCl4 concentration increased from 0.38%to 2.44%.The grain size increased with the TiCl4 concentration.The AFM and TEM analysis demonstrated that the aggregation of TiO2 clusters at the initial stage finally result to the agglomeration of fine TiO2 grains at high TiCl4 concentrations.Following the results,it was suggested that the nucleation density and size was easy to be adjusted when the TiCl4 concentration is below 0.90%.

1.Introduction

The pearlescent pigment is essential materials applied in additives,including car paint,cosmetics,and textile.The worldwide demand of titanium pearlescent pigment was about 4.5 million tons per year [1],which doubtlessly increased with the development of automobile industry and cosmetics.In substrate-based pearlescent pigments,the lustrous and angle-dependent optical effects are enhanced because of the big difference in refractive index between the substrate and the coating layer.Consequently,the substrate-based pearlescent pigments are gaining popularity in various applications.Mica coating titanium dioxide is the most universal substrate-based pearlescent pigments because of the transparency,which have minimum light loss and enhance the color of pigment [2].

In order to improve the optical performance,pearlescent pigment is coated by a layer of TiO2thin film on the substrate bases[3].According to the optical principle,the deposited TiO2film on mica substrate should be smooth with the grain size of less than 40 nm [4,5].The main method for preparation of pearlescent pigment is liquid phase deposition (LPD) method due to its low production cost.However,the pearlescent pigment produced by the LPD process is a batch process,which is not suitable for large scale production of pearlescent pigment.In addition,the removal of chlorine in production of pearlescent pigment and the waste water management during the LPD process become big problems for the sustainable development.

Fluidized bed-chemical vapor deposition (FB-CVD),which has been successfully applied to coating nuclear material and electrode material [6-8],would be a promising method to produce pearlescent pigment due to its high production efficiency,simple absorption of hydrogen chloride to decrease the exhaust and direct calcination of pigment powder to remove chlorine.For FB-CVD preparing coated nuclear-fuel particle,the microspheric UO2fuel kernel was coated four micrometer scaled layers [9].The main problems for coating nuclear fuel particles are high collection efficiency,high particle density and uniform coating microstructure[10],which were solved by investigating the uniform distribution of reactant gas in spouted-bed reactor [7,11,12].For the preparation of polysilicon,the major challenge lies in the generation of fine powder because of the self-nucleation [13].The optimization of pyrolysis temperature and reactant ratio proved the decreased amount of fine powder [14-16].Apart from the coating of nuclear fuel particles and preparation of polysilicon,FB-CVD method also was experimentally applied in coating electrode material,surface modificationetc.[8,17-19].However,for the pearlsecent pigment production,controllable deposition of nanometer scaled TiO2film is needed [4,5],thus the morphology regulation is important for the preparation of pearlescent pigment.

Investigations on the morphology regulation of TiO2thin film by sol-gel method reported that slow hydrolysis rate of the precursor leads to narrow size distribution and the surface of the TiO2thin film is smooth [20].The roughness of the thin film also is related to the substrates.It was reported that the roughness of thin film deposited on Si is lower than that deposited on indium tin oxide (ITO) [21].Moreover,the roughness of TiO2film deposied on Si and ITO both increased with the film thickness.In the preparation of TiO2thin film by APCVD,studies on controlling surface morphology is scare.It was found that most works achieved nanometer scaled morphology using sol-gel approache and atomic layer deposition (ALD).

Nucleation process is origin of chemical vapor deposition,which is the key to decide the uniform growth of TiO2grains forming requested nanometer scaled morphology.The chemical vapor deposition of graphene and MoS2demonstrated that the grain size decrease with the increase of nucleation density [22-25].Therefore,the controllable nucleation of TiO2is important for the deposition of expected grain size and smooth film.The nucleation is a complex process especially for nonideal deposition conditions.The nucleation density is releated to the deposition temperatutre[22],supersaturation [26],substrate [27,28],gas flow rate[29,30],deposition time [31] and pressure [22].Studies on the CVD of TiO2mainly are the crystal facet engineering and photocatalytic instead of the morphology control[32-34].The properties of substrate and precursor radically affect the deposition characteristics[35,36].In addition,the nucleation of TiO2on mica by chemical vapor deposition using TiCl4as precursor was scarcely studied.Therefore,it is important to investigate the nucleation process in respect to specific substrate and precursor,which are used for the preparation of pearlsecent pigment.

The controllable deposition process promises the stability of the film properties,which means the deposition time should be long enough for the operation of APCVD.In this work,the initial deposition stage of TiO2on mica substrate by APCVD using water vapor and TiCl4as the precursor was studied to understand the nucleation of TiO2on mica.This study focuses on the effect of precursor concentration on nucleation process,which is one of the key factors affecting the deposition process.The nucleus morphology and nucleation density evolved with TiCl4concentration and deposition time were analyzed to realize the controllable deposition.Moreover,the mechanism of the nucleation characteristics under different conditions was revealed.This study supplies the theoretical foundation for the uniform growth of TiO2grains on mica surface by simple APCVD method.

2.Experimental

2.1.Materials

The substrate used in this study is mica powder from RIKA technology Co.,Ltd (Yichun,China).The shape and the size of mica plate were analyzed by scanning electron microscope as shown in Fig.1.It was found that the diameter of the mica plate was about 10-60 μm and there are many gullies on the surface of mica.The elemental analysis was analyzed by AXIOS max X-ray fluorescence(XRF,PANalytical B.V.,Netherlands)as shown in Table 1.The main chemical composition of mica powder is SiO2,Al2O3and K2O.The X-ray diffraction analysis (XRD,X’Pert PRO MPD,Netherlands)results are shown in Fig.2.It can be found that the mineral phase of mica powder is fluorophlogopite.

Fig.1.The size distribution and surface characteristics analysis of mica plate.

Fig.2.XRD analysis of fluorophlogopite powder.

Table 1 XRF analysis of mica powder

2.2.Chemical vapor deposition experiments

The hydrolysis reaction of TiCl4(99.9%,Shanghai Macklin Biochemical Co.,Ltd,China) to deposit TiO2can be simplified to Eq.(1).The stoichiometric ratio of TiCl4and H2O reactant is 1:2.The volume concentration of TiCl4and H2O in carrier gas can be calculated according to Dalton’s Law of Partial Pressures [37].The partial pressure ratio of H2O and TiCl4in carrier gas at the same temperature range of 0-50 °C was in the range of 2.27-2.3 as shown in Fig.S1 in Supplementary Material.The deposition process was performed in fixed bed reactor,a quartz tube (650 mm in length and 30 mm in diameter),which was placed in furnace(450 mm in height and the heating zone was 250 mm) as shown in Fig.3.The mica powder was dispersed in deionized water and dripped on sapphire pieces.The solutions on pieces were dried in an oven and the mica plates was separated on sapphire pieces.The sapphire pieces were placed in a basket,which was fixed on the holder in the quartz tube as shown in Fig.3.The carrier gas of TiCl4and H2O is Ar (99.9% in purity) and flow rate of carrier was fixed at 100 ml.min-1both for H2O and TiCl4.The temperature of TiCl4and H2O can be controlled by the assistance of heating tape.Therefore,the TiCl4concentration in carrier gas can be adjusted by changing the heating temperature as shown in Fig.S1 (in Supplementary Material).The deposition temperature was fixed at 300 °C to grow TiO2grains.

Fig.3.Schematic of the in-situ experiments of chemical vapor deposition.

2.3.Characterization of the TiO2 film

The nucleation morphology was analyzed by a MultiMode 8 atomic force microscope (AFM,Bruker,Germany) using ScanAsyst mode and a Gemini 300 scanning electron microscope(SEM,Zeiss,Germany).The TiO2grain structure was analyzed by a JEM-F200 transmission electron microscope (TEM,JEOL,Japan).

3.Results and Discussion

3.1.Analysis of TiO2 deposition process on mica by APCVD

The aim of this work is to control the nucleation density and the surface roughness.The nucleation density and grain size both evolved with the deposition time.The deposition characteristics evolved with deposition time also reflected the deposition rate,which decided the controllability of deposition process.Therefore,the evolution of surface characteristics with deposition time was analyzed.The TiO2deposition process on mica at the concentration of 1.51% in the initial stage is shown in Fig.4.Fig.4(a) shows that the surface of mica is smooth in small region.Actually,mica is stratified structure and there exist stages on the surface of mica as analyzed in Fig.1(b).Fig.4(b) shows that TiO2grains can be found on the surface of mica when the deposition time reaches 10 s.The height of the deposited grain was about 1 nm.The number of TiO2grains evidently increased when the deposition time is 20 s.The grains showed a non-uniform distribution on mica.The height of the grains still is less than 1 nm.As the reaction proceed to 30 s,a compact TiO2granular layer can be observed as shown in Fig.4(d).It was found that the formation of evident TiO2grains on mica substrate occurs in merely 10 s.The surface of the granular layer was smooth,which is ideal for the preparation of pearlescent pigment.According to the classical nucleation theory,it can be inferred that the size of nucleus was less than the critical nucleation radius and discomposed when the source of TiO2was cut off in the first 10 s.When the deposition time reached 20 s,only some large nuclei that larger than the critical nucleation radius can stably exist on the surface of mica substrate as shown in Fig.4(c).At the same time,it can be found that the vertical distance of grains still is about 1 nm in different deposition times.It was inferred that the deposited TiO2particles occurred epitaxial growth in the initial deposition stage.The evolution of film characteristics with deposition time decides the controllability of the growth of granular layer.Hence,the deposition characteristics of the granular layer with different times deposited at different TiCl4concentrations was compared.

Fig.4.Evolution of the nucleation process with deposition time when the TiCl4 concentration was 1.51% at the deposition temperature of 300 °C (the horizontal scale is 500 nm × 500 nm).

The deposition process at the TiCl4concentration of 1.51%showed that the granular layer formed in about 10 s from less nuclei on mica.Even the surface showed a uniform deposition,the growth process was difficult be controlled in APCVD experiments.Therefore,the deposition process at lower TiCl4concentration was further studied.The TiO2deposition characteristics at different times when the TiCl4concentration was 0.90% was studied as shown in Fig.5.The deposition process showed that the formation of TiO2nuclei was the dominant reaction.The TiO2nuclei rapidly grew up to large TiO2grains when the deposition time increased from 50 s to 60 s.The nuclei and the grains at different times both showed uniform distribution and the surface of the granular layer also was smooth.In this case,the nucleation process can be controlled by changing the deposition time and the surface of the granular also was smooth.Apart from the surface roughness,the grain size in the granular layer also is important for the properties of pearlescent pigment.

Fig.5.Evolution of the nucleation process with deposition time when the TiCl4 concentration was 0.90% at the deposition temperature of 300 °C (the horizontal scale is 500 nm × 500 nm;the vertical scale bar is 2 nm).

Comparing the TiO2deposition process at two different TiCl4concentrations,it was found that the nucleation process was fast at the TiCl4concentration of 1.51%.The nuclei distribution was non-uniform but the granular layer was smooth.The nucleation process and the growth rate were slow for the TiCl4concentration is 0.90%.The nuclei and granular layer both showed uniform distribution.In respect to the controllability of deposition process,the nucleation process was ideal when the TiCl4concentration is 0.90% because of the slow deposition rate.At low deposition rate,the film thickness and the grain density both can be adjusted by controlling the deposition time.

3.2.Morphology evolution of TiO2 granular layer with TiCl4 concentrations

Following the above analysis,a series of experiments at different TiCl4concentrations were compared to reveal the mechanism of the morphology evolution.The concentration of precursor directly decides the material source for nucleation and crystal growth.In this case,the concentration of TiCl4significantly affect the nucleation and growth process during the deposition process.It is easy to be understand that the deposition time to fully cover the surface of mica substrate increased with the decrease of TiCl4concentration.Fig.6 shows the deposition characteristics when the surface of mica substrate was fully covered at different TiCl4concentrations at different reaction times.The TiCl4concentration in the reactor ranged from 0.38%to 2.44%in volume and the deposition time to fully cover the surface also ranged from 120 to 10 s.Large and bright grains can be evidently observed on the surface of mica and there are also very fine grains on the surface as shown in Fig.6(a).It was indicated that the fine grains have fully covered the surface and new grains start to grow on fine TiO2granular layer.It was noted that the particle size evidently increased with the TiCl4concentration.The AFM analysis show that the TiO2grains uniformly distributed on the surface of mica.The vertical scale bar shows the height of the deposition layer.It was found that the height of granular layer was uniform for Fig.6(a)-(c).However,the deposited grain exists height difference and the bottom layer also was granular layer instead of substrate surface,which indicates there are more than one granular layer when the concentration was above 2.44%.The deposited grains were non-uniformly distributed at high concentrations.The height difference of the granular layer was up to 10 nm in different regions.The aggregation of TiO2grains can be directly observed by AFM analysis as shown in Fig.6(d) at higher concentration.Therefore,there exist surface defect on mica substrate surface and bare mica surface was exposed as show in Fig.6(d).The same phenomenon also was found in diamond deposition process [38].As introduced above,the surface of mica was non-uniform in large scale and the roughness of the selected region in granular layer was analyzed as shown in Fig.7(a).The roughness of the TiO2granular layer were 0.53,0.43,0.58 and 4.85 nm for the film deposited at the concentration of 0.38%,0.90%,1.51% and 2.44%,respectively.The rootmean-square roughness(RMS)results showed that the film roughness evidently increased when the TiCl4concentration was above 2.44%.It was suggested that the film deposited below the concentration of 1.51%is better than that deposited above the concentration of 2.44%.

Fig.6.Evolution of the nucleation process with different TiCl4 concentrations at different deposition times: (a) 120 s,(b) 60 s,(c) 30 s and (d) 10 s.The scale is 500 nm × 500 nm for (a)-(c) and 1 μm × 1 μm for (d).

Fig.7.The evolution of (a) root-mean-square roughness of deposited granular layer,(b) apparent particle size and theoretical density with TiCl4 concentration when the substrate was fully covered by TiO2 grains at the deposition temperature of 300 °C.

The apparent average particle size at different concentrations was analyzed by NanoScope analysis software and the theoretical surface grain density was calculated and the results are shown in Fig.7(b).The theoretical grain density ρ was calculated as:

whereLis the surface area anddpis the average particle size.The apparent average TiO2particle sizes were 9.1,10.7,17.3 and 26.7 nm that grown at the TiCl4concentration of 0.38%,0.90%,1.51% and 2.44%,respectively.The apparent particle size evidently increased with the TiCl4precursor concentration.The apparent grain density decreased from 3013 to 350 N.μm-2with the TiCl4concentration increased from 0.38% to 2.44%.The nucleation densityn*is expressed as Eq.(3) when there arenatoms in contact with the substrate surface [39]:

whereknis the nucleation rate constant andmis the reaction index to supersaturation.In this study,the evolution of apparent grain density with the increase of precursor concentration is contrast to the classical nucleation theory,which indicates that the nucleation density increased with the degree of supersaturation.It also should be noted that the apparent grain density is different from the nucleation density.Therefore,we carried out the following analysis to further explain this phenomenon.

3.3.Growth mechanism and controllability of granular layer on mica

The deposition characteristics at typical different TiCl4concentrations and reaction times were analyzed before the surface was fully covered with TiO2grains as shown in Fig.8.Fig.8(a) shows the surface analysis and vertical distance of the selected grain was marked in AFM graph.The TiCl4concentration in the mixed gas was 0.90% and the deposition time was 50 s.It can be found that the height of the grain was about 1.1 nm and the horizontal distance (the length of the grain) was about 5 nm.This result showed that the TiO2grain on mica surface was a tabular shape.The AFM analysis of TiO2grains deposited on mica surface at the concentration of 1.51% for 20 s is shown in Fig.8(b).it was found that the diameter to height ratio was up to 20.The vertical distance of TiO2grain is about 0.7 nm,which is approximate to that grew at the concentration of 0.90%.However,the horizontal distance was measured to be 14 nm.The long horizontal distance indicated that the TiO2clusters aggregated and attached on mica surface at high concentrations.In this case,the tabular clusters would grow to be large aggregated TiO2particles.

Fig.8.Comparison of particle size under different concentrations at the initial deposition stage.The analyzed grains were marked using red line as shown in AFM images.The scale of the AFM images is 500 nm × 500 nm.

The observed granular layer assembled by single grain also was demonstrated as shown in Fig.9(a).The TiO2grains also showed uniform distribution on mica surface as shown in TEM graph.Moreover,the size of TiO2grains was measured and marked in the right of Fig.9(a).It was found that the diameter of the TiO2grains was approximate to that counted by AFM analysis as shown in Fig.7.Besides,the AFM analysis of the granular layer also is shown in Fig.9(b).It was measured that the particle size was 10.49 nm in the assembled uniform granular layer.This result demonstrated that the TiO2granular layer was assembled by single TiO2grain instead of the aggregate as deposited at high concentrations.The grain density directly reflected the nucleation density at the TiCl4concentration of 0.90%.Therefore,the mechanism of TiO2nucleation on mica and assembly of granular at different concentrations was disparate.

Fig.9.(a)TEM and(b)AFM analysis of the nuclei aggregation at high concentration of TiCl4 precursor with 0.90%.The horizontal scale of the AFM images is 500 nm.

TiO2grains grown on mica surface at higher concentration of 1.51% was analyzed by TEM and AFM analysis.The TEM analysis indicated that fine TiO2grains showed a uniform distribution as shown in Fig.10(a).The thickness of the mica plate was about 100 nm,which was difficult to be transmitted to observe the boundary of fine TiO2grains.It can be found that the size of a single grain is around 4 nm.The ultra-fine grains uniformly distributed on mica surface.However,the AFM analysis showed that the marked TiO2particle size was about 15.48 nm.Some grains united and enlarged the particle size even though the granular layer is uniform.These results reflected that the observed large TiO2grains on mica surface was the aggregate of fine grains.The size of separated TiO2grain deposited at the concentration of 0.90% was larger than that of fine TiO2grains in the aggregate deposited at the concentration of 1.51%.Moreover,the nuclei already aggregated at the initial deposition stage as analyzed in Fig.8(b).The growth of TiO2grains was based on the nuclei and the aggregation of fine TiO2grains still could be found in TEM analysis.The apparent grain density was different from the nucleation density because of the nuclei aggregation at the TiCl4concentration of 1.51%.The surface of the TiO2granular layer was non-uniform because of the fast deposition reaction.The aggregation of the nuclei and grains would increase the crystal boundary and increase the surface roughness,which is not beneficial to the preparation of pearlescent pigment.

Fig.10.(a) TEM and (b) AFM analysis of the nuclei aggregation at high concentration of TiCl4 precursor with 1.51%.The horizontal scale of the AFM images is 500 nm.

The initial deposition characteristics of TiO2on mica was different at different TiCl4concentrations.The comparison of the deposition characteristics at low and high concentrations with time evolution are shown in Fig.11.It was concluded that the TiO2molecular clusters was separated and uniformly distributed on mica surface at the TiCl4concentration below 0.90%.Therefore,the TiO2grains on mica substrate was uniform and the size was small as shown in Fig.11(a).Besides,the nucleation process can be controlled by changing deposition time because of the slow deposition rate.However,at high precursor concentrations,the TiO2molecular clusters aggregated and attached on mica surface showing a tabulate shape as shown in Fig.11(b).In short deposition times,the aggregated TiO2grains with large particle size was observed on mica substrate.The size of the single TiO2grains in the aggregates was merely 4 nm,which unites forming large TiO2particles with the particle size of 15 nm.The granular layer was rough because of the fast deposition rate at high precursor concentrations.Based on the granular layer,new nucleation and growth of TiO2grains would deposited on the surface to form the coating layer.Thus,the effect of different smooth granular layer on the growth of new granular layer would be further studied in future work.

Fig.11.Analysis of the cluster aggregation and grain growth with deposition time at different concentrations.

As mentioned above,the nucleation process decides the particle size and grain density.The grains size and granular layer roughness significantly affect the performance of pearlescent pigment.It was important to control the nucleation characteristics (nucleation density and nucleation size) and achieve uniform TiO2grains distribution in APCVD.The evolutions of the surface roughness,characteristics of TiO2grains and the process controllability with TiCl4concentration are shown in Fig.12.This study shows that the nucleation process was difficult to be controlled because of the fast deposition rate when the TiCl4concentration was above 0.90%.The granular layer forming time only needs about 10 s at the TiCl4concentration of 1.51%.With the increase of TiCl4concentration,the TiO2grains unite and form rough surface.The granular layer roughness also was up to 4.85 nm when the TiCl4concentration was 2.44%.

Fig.12.The characteristics of TiO2 grains assembling the film at different TiCl4 concentrations.

4.Conclusions

The initial deposition characteristics of TiO2on mica substrate to realize the growth controllability of granular layer were studied by APCVD using H2O and TiCl4precursor.It was found that the deposition time evidently decreased from 120 to 10 s when the TiCl4concentration increased from 0.38% to 2.44% to fully cover the substrate surface.The observed particle size also increased from 9.1 to 26.7 nm and the apparent grain density decreased from 3013 to 350 N.μm-2.The TiO2cluster deposited on mica surface at typical TiCl4concentrations was analyzed before TiO2grains formed.The height to diameter ratio showed that the cluster was tabular shape when the TiCl4concentration was above 1.51%,which indicates the aggregation of TiO2clusters.Furthermore,results showed that the large grains formed at higher TiCl4concentrations consists of fine grains,while it is separated at lower TiCl4concentrations.The particle size of the fine grains in large grains was merely about 4 nm.The size of separated grain formed at the lower concentration with 0.90%was about 10 nm.The aggregation of molecular clusters results in the aggregation of fine grains and the increase of apparent particle size.The nucleation process is controllable because of the slow deposition reaction when the TiCl4concentration was below 0.90%to achieve the demand of particle size and density in pearlescent pigment.The smoothness of the granular layer was ideal when the TiCl4concentration was below 1.51%.

Outlook: based on the uniform smooth granular layer in this work,the control of crystal growth need to be further studied to satisfy the microstructure demand of pearlescent pigment.The grain boundaries should be less in anatase crystals and the grains size should be controlled in the range of 30-40 nm.Our studies also found that the mechanism of new nucleation changed because of the deposition on TiO2granular layer instead of mica surface.

Data Availability

Data will be made available on request.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

The authors would like to acknowledge the support from National Natural Science Foundation of China (22208355,22178363 and 21978300)and the financial support and mica samples from Changzi Wu and RIKA technology CO.,LTD.

Supplementary Material

Supplementary data to this article can be found online at https://doi.org/10.1016/j.cjche.2023.01.008.