Surface & Coatings Technology 202 (2008) 6113–6119

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Surface & Coatings Technology

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Porous TiO2 coating using the solution precursor plasma spray process

Dianying Chen a,⁎, Eric H. Jordan b, Maurice Gell a

a Department of Chemical, Materials and Biomolecular Engineering, Institute of Materials Science, University of Connecticut, Storrs, CT 06269-3136, United Statesb Department of Mechanical Engineering, University of Connecticut, Storrs, CT 06269-3136, United States

⁎ Corresponding author. Tel.: +1 860 486 9068; fax: +E-mail address: chendy@ims.uconn.edu (D. Chen).

0257-8972/$ – see front matter © 2008 Elsevier B.V. Aldoi:10.1016/j.surfcoat.2008.07.017

a b s t r a c t

a r t i c l e i n f o

Article history:

Porous titania coatings wer Received 29 March 2008Accepted in revised form 18 July 2008Available online 25 July 2008

Keywords:Solution precursor plasma sprayAnataseRutilePorous coatingTitania

e deposited from aqueous solution containing titanium isopropoxide using thesolution precursor plasma spray (SPPS) process. Effects of plasma power on coating microstructure and phasecomposition were investigated. X-ray diffraction and Raman spectra analyses indicated that the amount ofanatase and rutile phases in the as-sprayed coatings can be adjusted by simply changing the plasma power.With the increase of plasma power, the coating anatase content decreases and the rutile content increases.Scanning electron microscope characterization shows that all of the as-sprayed coatings are very porous.

© 2008 Elsevier B.V. All rights reserved.

1. Introduction

Extensive studies on titanium dioxide (TiO2) for photoelectricaland photocatalytical applications have been carried out in the lastdecades. TiO2 is one of the most important photocatalysts and hasa great potential for applications such as purification and treatmentof water and air, which are concerned in the protection of the envi-ronment [1,2]. For photocatalytical applications, the titanium dioxidecan be used as powders or coatings. However, the use of conventionalpowder catalysts results in disadvantages of stirring during thereaction and of separation of powders from the liquid state used inwater treatment [3,4]. Preparation of the catalysts coated as thin filmor coating will make it possible to overcome these disadvantagesand to extend the industrial applications [4]. The photocatalyticactivity of TiO2 depends strongly on the phase structures and porosityof the thin films/coatings [5–8]. A porous structure is highlydemanded because it provides a much larger surface area than thedense one.

Porous TiO2 thin films/coatings have been deposited by varioustechniques such as sol–gel [8–12], hydrothermal [6], chemical vapordeposition [13], spray pyrolysis [7,14] and plasma-spray deposition[15], etc.

Recently, a solution precursor plasma spray (SPPS) process hasbeen developed for the deposition of highly durable 7YSZ thermalbarrier coatings [16–18] as well as the deposition of dense and hardcoatings [19,20]. In the SPPS process, liquid-precursor solutions areinjected directly into the plasma jet. The atomized droplets undergo aseries of physical and chemical reactions prior to deposition on the

1 860 486 4745.

l rights reserved.

substrate as a coating. The SPPS method for the deposition of ceramiccoatings offers several advantages over the conventional plasma-spray methods, such as circumvention of the powder-feedstockpreparation step, better control over the chemistry of the depositand deposition of nanostructured coatings.

In this research, porous TiO2 coatings were deposited using thesolution precursor plasma spray process. Effects of plasma power oncoating phase composition and microstructure were investigated.

2. Experimental procedure

2.1. Precursor preparation

The solution precursor was prepared by hydrolysis and condensa-tion of titanium isopropoxide (97%, Alfer Aesar) in H2O in the presenceof HCl. The resultant TiO2 transparent solution precursor has a molarconcentration of 0.375 M with pH value of ∼2.0. The precursorsolution was then dried on a hot plate at ∼100 °C. To study the phaseevolution of the solution precursor, the dried precursor powders werethen heated at various temperatures with a heating rate of 10 °C/min,and then holding for 1 h.

2.2. Coating deposition

The coatings were deposited using the solution precursor plasmaspray process, details of which can be found in previous publications[19–21]. An atomizing nozzle attached to the plasma torchwas used toinject solution precursor mist into the plasma jet. The direct current(DC) plasma torch used here was the Metco 9MB (Sulzer Metco,Westbury, NY), which was attached to a six-axis robotic arm. Ar andH2 were used as the primary and the secondary plasma gases,

Table 1Conditions of solution precursor plasma spray TiO2 coatings

Sampleno.

Plasmapower(KW)

Atomizing gaspressure(MPa)

Liquidflow rate(ml/min)

Sprayingdistance(mm)

Gun traversespeed(mm/s)

T1 21 0.14 25 50 1000T2 25 0.14 25 50 1000T3 30 0.14 25 50 1000T4 36 0.14 25 50 1000T5 45.5 0.14 25 50 1000

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respectively. Coatings were deposited on stainless steel substrates(disks 25 mm diameter, 3 mm thickness), surfaces of which werepreviously roughened by grit blasting (Al2O3 grit of #30 mesh size).The coating deposition parameters are listed in Table 1.

2.3. Characterization

Both differential thermal analysis (DTA) and thermal gravimetricanalysis (TGA) experiments were performed simultaneously on theas-dried TiO2 precursor powders using a SDT-Q6000 thermal analyser(TA Inc., New Castle, DE). Thermal analysis was performed in flowingair by heating the sample from room temperature to 1000 °C at a rateof 10 °C/min. A12O3 powder was used as a reference material.

Fourier-transform infrared spectrum (FTIR) (MAGNA-IR 560,Nicolet, Madison, WI) was used to identify structural features of theprecursor and the dried powders. Measurements were conductedin a wave number range of 4000–400 cm−1. All samples for FTIRmeasurement were well mixed with KBr in a weight ratio of 1:20 andthen pressed into translucent pellets.

The crystalline phase composition of all the samples was carriedout using X-ray diffraction (XRD, Cu Kα radiation; D5005, Bruker AXS,Karlsruhe, Germany). The XRD patterns were collected in a 2θ rangefrom 20° to 80° with a scanning rate of 2°/min. The average crystallitesize was estimated based on XRD peak broadening using the Scherrerformula [22]:

Dhkl ¼0:9λ

βhkl cosθ

where Dhkl is the average dimensions of crystallites, λ is the wave-length of the X-ray radiation (λ=0.15405 nm) and θ is the Bragg angleof reflection of specific crystalline plane, βhkl is full width of halfmaximum (FWHM) of the peak intensity after correction for theinstrumental line broadening. There are several ways to take the

Fig. 1. XRD of TiO2 powders calcined at various temperatures.

instrumental line broadening into account. One suggested by Taylor[23] and used in this study is:

βhkl ¼ffiffiffiffiffiffiffiffiffiffiffiffiffi

B2−b2p

where B is the FWHM of the peak and b is the instrumental linebroadening. The instrumental line broadening was obtained fromFWHM of peaks in the same angular region for polycrystalline TiO2

with crystal size N1 μm.Raman spectrum (Renishaw™ Ramascope 2000. Renishaw, Glou-

chestershire, UK) was also used to measure the phase composition ofthe coating. An Ar-ion 514-nmwavelength laser was used to excite theRaman scattering.

An environmental scanning electron microscope (ESEM 2020,Philips Electron Optics, Eindhoven, The Netherlands) and a JEOL JSM-6335F field emission scanning electron microscope (FESEM, Tokyo,Japan) were used to characterize the coating microstructure.

3. Experimental results

3.1. Thermal and crystallization behaviors of the as-dried solution precursor

Fig. 1 shows the XRD patterns of the gel powders calcined atdifferent temperatures for 1 h. The gel powders calcined at 250 °C arestill amorphous. When the temperature is increased to 350 °C, theanatase TiO2 crystalline phase begins to form. However, the broad andweak peaks imply poor crystallinity and fine grains. When thecalcination temperature is increased from 350 °C to 500 °C, the peaksbecome sharper and stronger, which indicates increased crystallinityof the anatase TiO2 powders. At 500 °C, rutile TiO2 crystalline peakbegins to appear. With increasing the temperature from 500 °C to

Fig. 2. FTIR of dried TiO2 precursor powders heat-treated at various temperatures.

Fig. 3. Typical TG-DTAcurvesof driedTiO2precursorpowders ataheating rate of 10 °C/min.

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700 °C, the peak intensity of rutile increases and at the same time, thepeak intensity of anatase decreases. At 700 °C, the anatase almostcompletely transforms to rutile.

Fig. 4. Microstructure of single scan deposits on room temperature substrate usin

Fig. 2 shows the FTIR spectrum of the TiO2 gel powders calcined atdifferent temperatures in the wave number range between 400 and4000 cm−1. In the FTIR spectrum of as-dried precursor, a broad absorp-tionpeak appearing at 3100–3600 cm−1 is assigned to the fundamentalstretching vibration of hydroxyl groups (O–H) [24]. The absorptionband at 1620 cm−1 is caused by bending vibration of coordinated H2O[25]. The absorption peak localized at 1380 cm−1 is assigned to stretch-ing vibrations of the C–H group. After annealing at 250 °C, the peakattributed to C–H group becomes weak, indicating the decompositionof precursor, while vibration of hydroxyl groups is still observed atabout 3200 cm−1. After annealing at 400 °C, the peaks of C–Hgroup andhydroxyl group (–OH) disappear. These results provide evidence thatprecursor pyrolysis occurs at temperatures below 400 °C.

The weight loss of the precursor as a function of temperature in airwas measured using a TG-DTA instrument after the precursor wasdried at 100 °C in air. Typical TG-DTA curves for the crystallization ofTiO2 precursor obtained at a heating rate of 10 °C/min in air are shownin Fig. 3. The sample weight decreases with increasing temperaturecontinuously from room temperature to ∼350 °C, and the total weightloss is about 25 wt.%. The endothermic peak in the DTA curve around100 °C can be ascribed to water evaporation. The wide exothermicpeak at 400 °C can be ascribed to the crystallization of TiO2 from theamorphous to the anatase phase. Another wide exothermic peak at550 °C can be ascribed to the phase transformation of anatase to rutile.

g plasma powers: (a) 21 KW; (b) 25 KW; (c) 30 KW; (d) 36 KW; (e) 45.5 KW.

Fig. 5. Microstructure of single scan deposits on 450 °C substrate using the 45.5 KW plasma power at different magnifications.

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The onset temperatures for anatase crystallization and rutile phaseformation are at 350 °C and 500 °C, respectively, which are consistentwith the XRD results.

3.2. Single scan results

Since the coating is built up by the accumulation of deposits fromthe multiple single-scans, the examination of deposits microstructurefrom single scan can give us more information on coating formation.Thus, single scan experiments were carried out. The plasma torch was

Fig. 6. Surface morphologies of the as-sprayed SPPS TiO2 coatings using pla

programmed to scan one pass across the polished substrate at itsmaximum speed (∼1.0 m/s).

Fig. 4(a)–(e) shows the microstructures of single scan deposits onroom temperature substrates using plasma powers of 21, 25, 30, 36and 45.5 KW, respectively. When the plasma power is below 30 KW,the deposits are mainly composed of gel-like smooth films and fewspherical particles (Fig. 4a, b). At higher plasma power, semi-pyrolyzed hollow-shell structures are observed besides the gel-likesmooth films and fine particles (Fig. 4c, d, e). No splats are observed forall the single scan deposits even under the high plasma power.

sma powers: a) 21 KW; (b) 25 KW; (c) 30 KW; (d) 36 KW; (e) 45.5 KW.

Fig. 7. Typical polished cross-section of SPPS TiO2 coating using the 45.5 KW plasmapower.

Fig. 9. Ramanspectraof the as-sprayed coatingsusingplasmapowers: (a) 21KW; (b)25KW;(c) 30 KW; (d) 36 KW; (e) 45.5 KW.

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To understand the effect of substrate temperature on depositmorphologies, single scan experiments were also performed on 450 °Csubstrate, which is higher than the precursor solvent evaporation andpyrolysis temperature. The single scan experiments on 450 °Csubstrate were carried out using 21 KW and 45.5 KW plasma power,respectively. The microstructures of deposits collected on 450 °Csubstrates using the 21 KW and 45.5 KW plasma power are verysimilar. Fig. 5 shows the morphologies of deposits collected on 450 °Csubstrate using the 45.5 KW plasma power. The microstructuralfeatures of deposits collected at 450 °C are totally different from thatcollected at room temperature (Fig. 4e). The deposits are composed ofporous, sponge-like splats, as well as small amounts of solid spheres.

3.3. Microstructure and phase composition of the as-sprayed coatings

In the solution precursor plasma spray process, the coatings weresprayed using the processing parameters listed in Table 1. Thesubstrate was only subjected to the heat treatment from plasma jetduring coating deposition. Fig. 6(a)–(e) presents the surface morphol-ogies of TiO2 coatings sprayed at various plasma powers. The coatingsare characterized by rough surface with high porosity. The coatingsare mainly composed of the aggregates and fine spherical particles,which are totally different from that of single scan deposits. At plasmapower of 21 KW, gel-like precursors are observed on the coatingsurface (arrow indicated in Fig. 6a).

All of the coatings sprayed at various plasma powers have thesimilarmicrostructures. Fig. 7 shows the typical polished cross-section

Fig. 8. XRDpatternsof the as-sprayedcoatingsusingplasmapowers: (a) 21KW; (b) 25KW;(c) 30 KW; (d) 36 KW; (e) 45.5 KW.

of TiO2 coating sprayed at plasma power of 45.5 KW. The coating isquite porous with thickness of ∼25 μm.

Fig. 8 shows theXRDpatterns of the as-sprayed coatings at differentplasma powers. The as-sprayed coatings using plasma powder below36 KW are composed of anatase and rutile mixtures and with theincrease of plasma power, the peak intensity of rutile increases. At45.5 KWplasma power, anatase disappears and only rutile exists. Fig. 9shows the Raman spectra of the as-sprayed coatings. The peaks around395, 517, 639 cm−1 are assigned to fundamental vibration modes ofanatase with the symmetries of B1g, A1g, B1g, respectively and thepeaks around 448, 612 cm−1 are assigned to Eg and A1g modes of therutile [26,27]. With the increase of plasma power, the peak intensity ofanatase decreases and the peak intensity of rutile increases, whichclearly reflects that the anatase content decrease with the increase ofplasma power. Those results are very consistent with the XRD analysis.

Rutile content (fR) in the as-sprayed coatings is determined by theequation [28].

fR ¼ 11þ 0:8IA

IR

IA and IR are the XRD peak intensity of anatase (101) at 2θ=25.4°and rutile (110) at 2θ=27.4°. The calculated anatase and rutile contents

Fig. 10. Anatase and rutile contents as a function of plasma power.

Fig. 11. Anatase and rutile grains size as a function of plasma power.

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are plotted in Fig. 10. In the as-sprayed coating using the plasmapower of 21 KW, there are 45.8% rutile and 54.2% anatase; with theincrease of plasma power, the rutile content increases. The coatingsprayed using the plasma power of 45.5 KW is composed of 100%rutile.

The average crystallite sizes of anatase and rutile estimated usingthe Scherrer formula are plotted in Fig. 11. In the coating sprayed usingthe 21 KW plasma power, the average grain sizes of anatase and rutileare 26 nm and 50 nm, respectively. With increasing the plasma powerfrom 21 KW to 36 KW, the anatase grain size gradually increases from26 nm to 39 nm, while the rutile grain size has a little increase from50 nm to 56 nm. With further increase of the plasma power to45.5 KW, the anatase completely transforms to rutile and the rutile hasan average grain size of 75 nm. Moreover, the grain size of rutile isalways larger than that of anatase in all of the as-sprayed coatings.

4. Discussion

4.1. Phase formation

As indicated in the XRD and TG-DTA analysis (Fig. 1 and Fig. 3), thesolution precursor will undergo solvent vaporization, precursorpyrolysis and crystallization. The anatase and rutile formation tem-peratures are 350 °C and 500 °C, respectively. Single scan experimentindicated that the deposits are mainly composed of unpyrolyzed gel-like films and hollow-shell structures even under high plasma power.These results indicate that the droplets didn't undergo the pyrolysisand melting process in the plasma jet. XRD and Raman analysesshow that the coatings are composed of crystalline anatase and rutilein the as-sprayed coatings. Since the coating is built up by the accu-mulation of deposits from the multiple single-scans, the crystallineanatase and rutile in the as-sprayed coatings must be the result ofcrystallization of these unpyrolyzed gel-like precursors after arrivingat the substrate.

The higher the plasma power is, the higher the temperature itproduces. With the increase of plasma power, the plasma jet tem-perature and length increases. It is observed the plasma jet length is∼50mmat 45.5 KWplasma power, which is the place of the substrateslocated. Since all the coatings are deposited at the same standoffdistance, the heat treatment of plasma jet on the coatings surface willbe stronger with the increase of plasma power. When the unpyrolyzedprecursors arrive at the substrates, they will experience in situpyrolysis and crystallization on the coating surface due to the plasmajet heat treatment effect. The effect of plasma jet heat treatment on theanatase to rutile phase transformation increases with the increase of

plasma power. Thus, the rutile content increases with the increase ofplasma power. The coating front-face temperature measured bypyrometer is approximately 770 °C using the plasma power of45.5 KW, which is higher than the anatase to rutile transformationtemperature. Therefore, the anatase completely transforms to rutile inthe coating sprayed using the plasma power of 45.5 KW.

4.2. Single scan deposits

When the plasma power is below 30 KW, only gel-like smoothfilms are observed in the single scan deposits. Because the plasma jettemperature produced by the low plasma power is so low, the dropletseven did not experience the solvent evaporation and solute surfaceprecipitation process. When the liquid droplets impact the substrate,they will spread and form the smooth films.

When the plasma power is above 30 KW, some semi-pyrolyzedhollow-shell structures are observed besides the gel-like smooth filmsand fine particles. The formation of semi-pyrolyzed hollow-shellstructure is the result of solute surface precipitation. Previous studieson the effect of solution concentration on zirconia splat formation andcoating microstructure indicated that the high concentration pre-cursor is far more likely to experience volume precipitation leading toa fully melted splat microstructure; while low concentration pre-cursor generally leads to shell formation and a majority of thematerials delivered to the substrate as semi-pyrolyzed materials [21].The solution precursor concentration used in the present experimentis 0.375 M, which is a dilute solution compared to that (1.5 M) used inthe deposition of dense TiO2 coatings by the SPPS process [20]. Thesolute surface precipitation will occur. The surface precipitation willinduce the formation of a shell on the droplets surface (Fig. 4c, d, e).

Regardless of the plasma power, when the unpyrolyzed precursordroplets impact the 450 °C substrate, which is higher than theprecursor solvent evaporation and pyrolysis temperature (Fig. 3), thegel-like smooth film and the semi-pyrolyzed precursor will in situevaporate, pyrolyze and leave the pores on the deposits and finallyform the porous, sponge-like deposits (Fig. 5).

4.3. Porous coating formation

It is noted that the coating surface microstructures (Fig. 6) aredifferent from the single scan deposits morphologies on roomtemperature substrates (Fig. 4). The gel-like smooth film and semi-pyrolyzed hollow-shell structures disappear in all of the as-sprayedcoatings. Because the high temperature plasma jet will repeatedlyscan over the substrate during the plasma-spray process, it has astrong heat treatment effect on the coating surface; therefore, the gel-like deposits, semi-pyrolyzed hollow-shell structures will in situpyrolyze on the coating surface under the plasma jet heat treatmentand form the aggregates and porous coatings.

5. Conclusions

The SPPS method has been used to deposit porous TiO2 coatings.Single scan experiments indicate that the droplets didn't undergo thepyrolysis process in the plasma jet and they form the gel-like depositson the substrates. The formation of crystalline anatase and rutile in theas-sprayed coatings is the result of plasma jet heat treatment on thegel-like deposits. Plasma power has a significant effect on the coatingphase composition. With the increase of plasma power, the coatinganatase content decreases and the rutile content increases. All of theas-sprayed coatings are very porous.

Acknowledgement

This work is supported by U.S. Office of Naval Research under GrantNo. N00014-02-1-0171 managed by Dr. Lawrence Kabacoff.

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