Effect of Calcination Temperature and Environment on Photocatalyticand Mechanical Properties of Ultrathin Sol–Gel Titanium Dioxide Films

Murat Erdem Kurtoglu,z,y Travis Longenbach,z Patricia Reddington,z and Yury Gogotsiw,z

zDepartment of Materials Science and Engineering, A. J. Drexel Nanotechnology Institute, Drexel University,Philadelphia, Pennsylvania 19104

yArtCraft Glassware Inc., Kutahya 43001, Turkey

Effects of calcination environment (air and nitrogen) andtemperature (4001–10001C) on the structure, optical, photocat-alytic, and mechanical properties of ultrathin sol–gel titaniumdioxide (TiO2) films were studied. X-ray diffraction and Ramananalysis of the films have shown preferred rutile formation athigh calcination temperatures and low oxygen pressures. Filmscalcined under nitrogen flow were optically absorbing andcontained significant amounts of carbon due to the carboniza-tion of the organic precursor, which also resulted in a porousstructure. Although photocatalytic activity of the films calcinedin different atmospheres showed similar temperature dependen-cies, nitrogen-calcined films performed better at high calcinationtemperatures and showed improved visible light activity. On theother hand, the mechanical properties of films calcined in airwere superior to those calcined under nitrogen.

I. Introduction

HETEROGENEOUS photocatalysis by metal oxide semiconduc-tors has been one of the hottest research areas in the past

decades because of its potential applications in water and airpurification and disinfection systems, solar cells, and hydrogenproduction by water dissociation.1,2 Titanium dioxide (TiO2), anabundant material with a high photocatalytic efficiency andchemical stability, is undoubtedly the most widely studied andused among all photocatalytic materials. Although TiO2 can beused in powder form for some applications, its immobilizedform (film) is necessary for many applications including but notlimited to self-cleaning and bactericidal surfaces,3,4 antifoggingglasses,5 and advanced filters.6 Several methods have beenreported for the preparation of TiO2 films: sol–gel,7 chemicalvapor deposition,8 magnetron sputtering,9 spray pyrolysis,10

direct deposition,11 and layer-by-layer coating.12 In particular,sol–gel is an attractive method because of the capability to coatmaterials with various shapes and ease of control over the com-position of the films with relatively simple and inexpensiveequipment.7 Sol–gel preparation conditions and parameterssuch as the type and amount of the precursor and solventused,13 concentration of water in the solution,14 pH,15 andchelating agents16 have dramatic effects on the structure andfunctionalities of TiO2 films. On the other hand, morphologyand the mechanical stability of the films are almost entirelydetermined by the postdeposition thermal treatment, or calci-nation.17 In many cases, good mechanical integrity needs to becoupled with good photocatalytic and optical properties inorder to achieve sufficient lifetime of the coatings, because their

functionalities can be lost if the films are deteriorated due towear or environmental degradation. There is little work reportedon the effect of calcination temperature15,18,19 and environ-ment20 on the structural and photocatalytic properties of TiO2

films prepared by wet chemical methods. Yu et al.18 have re-ported on the photocatalytic activity of TiO2 films with respectto calcination temperature in air, whereas TiO2 films wereprepared by the liquid phase deposition method, which is inher-ently different than a sol–gel process. Ahn and colleagues15 havereported on the structural and optical properties of TiO2 filmsbut not on the resulting functionalities, such as photocatalyticactivity. In addition, influence of calcination environment andtemperature on the mechanical properties of such films has notbeen reported, although films showing good photocatalyticbehavior may appear to be weak mechanically and have nouse because of poor adhesion and weak mechanical properties.In this study, structural, photocatalytic, optical, and mechanicalproperties of ultrathin sol–gel TiO2 films were investigated withrespect to calcination conditions.

II. Materials and Experimental Procedure

TiO2 coatings were prepared by a sol–gel method as follows:120 g of ethyl alcohol (200-proof, ElectronMicroscopy Sciences,Hatfield, PA) and 15 g of Ti-(O-i-C3H7)4 (Alfa-Aesar, WardHill, MA) were mixed and subsequently stirred on a magneticstirrer for 30 min. Then, a mixture of 0.96 g of deionized water(18 MO), 0.52 g of HCl (38%, Fisher Scientific, Pittsburgh, PA),1 g of poly (ethylene glycol) (MW5 2000, Sigma-Aldrich, St.Louis, MO), and 6.3 g of ethyl alcohol were added drop-wise tothe main solution. The solution was stirred for 2 h. Then, 9.6 gof acetylacetone (Sigma-Aldrich) and 5 mL of deionized waterwere added to the prepared sol and stirred for an additionalhour.

Films were prepared by dip coating (MTI Dip CoaterHL-01, MTI Corporation, Richmond, CA); for this purpose,Si (100) substrates were dipped in the prepared solution andwithdrawn at a constant speed of 100 mm/min. Si (100) waferswere used because they provide the same SiO2 surface as glass,but their flatness, purity, and relatively small thermal expansionmismatch with TiO2 make them ideal model substrates. Thehumidity was varied between 20% and 30% during coating.Following the coating procedure, samples were taken to a tubefurnace for calcination at temperatures between 4001 and10001C (in 1001C steps) either in static air or under nitrogenflow (270 cc/min) for 2 h with a heating rate of 251C/min.

The crystal structure of the films was analyzed via X-raydiffraction (XRD) using a Siemens D500 (Siemens Corporation,Munich, Germany) with nickel filtered CuKa radiation (40 kV,30 mA) between 2y5 201 and 2y5 401. Raman spectra weremeasured using a 514.5 nm Ar ion laser as the excitation source(Renishaw RM1000, Renishaw plc, Gloucestershire, U.K.).Optical spectra were collected using a UV-Vis Spectrophotom-

B. Dunn—contributing editor

This work was financially supported in part by the ArtCraft Glassware.wAuthor to whom correspondence should be addressed. e-mail: gogotsi@drexel.edu

Manuscript No. 28111. Received May 30, 2010; approved September 27, 2010.

Journal

J. Am. Ceram. Soc., 94 [4] 1101–1108 (2011)

DOI: 10.1111/j.1551-2916.2010.04218.x

r 2010 The American Ceramic Society

1101

eter (Evolution 600, Thermo Scientific, Franklin, MA) equippedwith a diffuse reflectance accessory. The SEM images of thefilms were collected using a Zeiss Supra 50 VP (Carl Zeiss SMTAG, Germany). Film thickness was measured with a spectro-scopic ellipsometer at an incident angle of 701. Thickness of thenative oxide layer of silicon wafers was measured separately onthe similarly heat-treated uncoated wafers. Photocatalytic activ-ity of the films was evaluated by methylene blue (MB) degra-dation. For this, each sample (2.5 cm� 2.5 cm) was placed in apetri dish containing 15 mL of 2 ppm aqueous MB solution andilluminated with UVA light (16 W, 360 nm) for 5 h. A similarsetup was used under a fluorescent lamp (8 W, CoolWhite,Philips, Eindhoven, the Netherlands, 1.7% o380 nm, 97.5%380–700 nm, 0.8% 4700 nm) to measure the visible light pho-tocatalytic activity. Representative samples (1 mL) were takenevery hour from the solutions and the dye concentration wasmeasured using UV-Vis spectrophotometry. Tests were repeatedtwice producing no more than 5% scatter in the data.

Mechanical properties of the films were tested by nanoinden-tation (MTS NanoIndenter XP, MTS Systems Corporation,Eden Prairie, MN) using the continuous stiffness measurementmode. Each sample was indented up to 50 nm depths six timeswith a 5 mm radius spherical tip. The maximum drift rate andharmonic displacement target were set at 0.05 nm/s and 2 nm,respectively.

III. Results and Discussion

The film thickness exhibited by ellipsometric measurements onsamples calcined at 9001C was approximately 30–35 nm for bothair and nitrogen heat-treated samples.

Figure 1 shows the XRD pattern of films calcined in (a) airand (b) nitrogen. No peaks were detected from the films calcinedat 4001C regardless of the calcination atmosphere, whereas onlynitrogen calcined films at 5001C did not show any peaks. Yuet al.18 have reported the absence of XRD detectable crystalli-nity in liquid-phase-deposited TiO2 films calcined in air below5001C and they attributed this behavior to the more difficultcrystallization of liquid-phase-deposited films as opposed to sol–gel prepared films in which crystallization occurs around 4001C.However, as it will be shown later, Raman spectroscopy wouldbe a better choice for the analysis of such films, as conventionalXRD technique is likely to fail in detecting crystallinity in suchthin films with low crystallinity, due to the high penetrationdepth of X-rays (12mm for rutile), which results in a low signal/noise ratio. This is particularly relevant to TiO2 films becausestrongest peaks of anatase (25.40) and rutile (27.45) are situatedaround the amorphous broad hump (24–26) coming from theamorphous silica. Air-calcined films were composed of anataseup to 8001C, after which rutile peaks were detected along withanatase. At 10001C, transformation to rutile was completed.Rutile was the only crystal phase detected from nitrogen-calci-ned films at all temperatures. Preferred formation of rutile afternitrogen annealing has been reported for TiO2 films.21 Note thatrutile films obtained at 10001C in both atmospheres showedpreferred orientation (110) as evident from the lack of the (101)peak. Substrate-dependent orientation of sol–gel rutile films hasbeen reported before, in which (101) planes of rutile wereparallel to the substrate (on Si (100) and fused silica).22 In ourexperiments, however, rutile was preferentially oriented along[101]. Because the preferred orientation was similar both in air-and nitrogen-calcined films, this discrepancy most probably isrelated to the difference in the heating rates (251C/min in thisstudy versus 11C/min in Selvaraj et al.22).

Crystal size of the films calculated from their respective XRDpatterns by Scherrer equation is reported in Table I. It is im-portant to note that there is only a small change in the crystalsize up to 9001C with the film thickness being the most probablelimiting factor for grain growth. A significant growth is shownby a drastic increase in the peak intensity and decrease in back-ground noise at 10001C, when formation of rutile occurs. Note

that the Scherrer equation is not applicable to strongly aniso-tropic films with an average lateral crystal size larger than thefilm thickness.

Raman spectra of the films are shown in Fig. 2. Films calci-ned in air showed anatase bands at 144 cm�1 (B1g), 198 cm�1

(B1g, A1g), 398 cm�1 (B1g), and 640 cm�1 (Eg)23 beginning from

the lowest calcination temperature of 4001C. Contrary to theRaman analysis, XRD was unable to detect crystallinity forthe film calcined at 4001C in air, which can be explained by theparticular sensitivity of Raman spectroscopy to nanocrystals

Fig. 1. X-ray diffraction diagrams of the films calcined at differenttemperatures under (a) air and (b) nitrogen. Amorphous films are notshown. �: anatase, & : rutile.

Table I. Summary of XRD Crystal Size Measurements

Temperature (1C)

Crystal size (nm)

Air N2

400 w

500 16 w

600 22 18700 23 27800 23 27900 27 281000 w w

wAmorphous film or Scherrer equation is not applicable.

1102 Journal of the American Ceramic Society—Kurtoglu et al. Vol. 94, No. 4

in thin films.24 This shows the necessity of using complemen-tary techniques for characterizing such thin films. Position ofthe lowest frequency anatase band (144 cm�1) is an importantindicator of the particle size in TiO2 films due to phonon con-finement effect.25 Oxygen deficiency and stress levels can alsoaffect the position of Raman bands, although the contributionof the former should be negligible in our samples because theywere calcined in an oxygen-rich atmosphere (air). The lowestfrequency anatase band positions for air-calcined samples areshown in Fig. 2(d). Smaller blue shifts were observed with anincreased calcination temperature due to an increased particlesize. At the maximum calcination temperature (10001C), onlyrutile bands 442 cm�1 (Eg) and 610 cm�1 (A1g)

23 were detected,in agreement with the XRD findings. For films calcined undernitrogen, it was difficult to separate the rutile peaks at lowcalcination temperatures because of the overlapping positions ofthe rutile bands with the weak Si bands coming from thesubstrate (433 and 621 cm�1). At the maximum calcinationtemperature (10001C), signals coming from rutile were stronger,showing rutile bands at 439 and 610 cm�1. Slight red-shift of theEg mode of rutile in nitrogen-calcined films can be explained bythe disruption of the planar O–O interactions due to oxygenvacancies.26

As evidenced by the XRD and Raman spectra, calcinationenvironment has a dramatic impact on the structure of TiO2

films. Calcination under nitrogen hindered the crystallization ofthe films, possibly due to the incomplete decomposition of or-ganic titanium compounds (e.g., acetylacetonates). For samplescalcined at 6001 and 7001C, organic matter had seeminglydecomposed into carbon, for which D and G bands27 weredetected (Fig. 2(c)). Lack of carbon above 7001C in nitrogen-calcined films can be explained by the fast decomposition oflow-carbon-yield organics before densification of the film

occurs. Another possibility is a reaction between carbon andTiO2 leading to oxygen-deficient films. While the exact reason isnot clear, loss of carbon at higher calcination temperatures un-der nitrogen for carbon-coated TiO2 has been reported.28 Pre-ferred occurrence of rutile in nitrogen-calcined samples can beexplained by two reasons: (a) since the anatase to rutile trans-formation requires shrinkage of the lattice, removal of oxygenatoms facilitates this transformation,29 and (b) partial hydroge-nation of the TiO2 (due to the delayed decomposition of acidicprecursor), which further promotes rutile formation.30 No car-bon peaks have been observed after calcination in air, suggestingcomplete burn off of the organic material. Carbon formed innitrogen-calcined films could act as a reducing agent in the pres-ence of nitrogen,31 creating oxygen-deficient TiO2 films.

SEM inspection of the films (Fig. 3) has shown differences inmorphology between air- and nitrogen-calcined samples. Air-calcined films were typically composed of closely packed TiO2

grains (ca. 20 nm) at 6001C, which began sintering at 9001Cforming a mix of small (ca. 40 nm) and large (ca. 200 nm)particles and finally forming large flat grains (ca. 0.5 mm in x–ydirection) at 10001C. Formation of textured (as shown by XRD)and separated rutile crystals at the maximum calcinationtemperature can be explained by the dissolution of some ofTiO2 in the SiO2.

32 Films calcined under nitrogen were porouswith a smaller average grain size at all temperatures with graingrowth that was less dramatic compared with the air-calcinedsamples, which was an important finding given that rutile hadbeen shown to grow faster than anatase in many cases.33 On theother hand, crystal growth in thin films is significantly affectedby the underlying substrate. This effect can be clearly seen inFig. 4, where crystals were significantly larger around the edgeof a micro-crack compared with the crystals in the middle ofthe film. At the maximum calcination temperature (10001C),

Fig. 2. Raman spectra of the films calcined at various temperatures in (a) air and (b) nitrogen. Carbon range of the Raman spectra of nitrogen-calcinedfilms is shown in (c). Spectra in (c) have been normalized using the peak of Si at 960 cm�1. (d) The lowest frequency anatase band positions for air-calcined samples with respect to calcination temperature.

April 2011 Ultrathin Sol—Gel Titanium Dioxide Films 1103

porosity disappeared, but the crystal size remained small.TEM analysis performed on the samples calcined at 9001C isshown in Fig. 5. It can be seen that the air-calcined film hadwell-developed crystals, whereas at the same temperature, nitro-

gen-calcined film had much smaller crystals with rough crystalfaces, possibly due to reaction with carbon.

Total reflectance spectra of the films are shown in Fig. 6.Overall, films calcined under nitrogen were significantly less re-flective compared with the air-calcined films. This behaviorshould be arising from (a) the presence of optically absorbingcarbon, (b) rougher surface, and (c) entrapment of light withinthe porous structure. Films calcined between 4001 and 7001C inair had similar spectra, whereas at 8001 and 9001C, there is asignificant red-shift with an increased reflectivity, which is ex-pected because of the higher refractive index and smaller bandgap of rutile compared with anatase. At 10001C, incompletecoverage of the Si wafers due to a dramatic increase in the grainsize in lateral dimension (as evident from the SEM inspection)and the possible growth of silica under layer caused a differentpattern. Nitrogen-calcined films became more absorbing withthe increased calcination temperature until 7001C due to for-mation of carbon in the film. At 8001 and 9001C, the films wereslightly more reflective, owing to the disappearance of carbonfrom the film and the densification of the structure. At 10001C,the film was significantly more reflective due to disappearingporosity. Reflection maximums (absorption minimum or edge)of the films are plotted in Fig. 7 in order to better understand thedensification behavior and electronic structure of TiO2 films.Reflection maximum of the films shifted to lower energies, i.e.

Fig. 3. Morphological evolution of the films calcined at 6001C (a and d), 9001C (b and e), and 10001C (c and f). (a), (b), (c) belong to the films calcinedunder air and (d), (e), (f) to the ones calcined in air.

Fig. 4. Scanning electron microscopic image around a microcrackshowing the extensive growth of TiO2 particles on the edge.

1104 Journal of the American Ceramic Society—Kurtoglu et al. Vol. 94, No. 4

longer wavelengths, indicating a narrowing of the band gapand an increase in refractive index due to densification. At lowcalcination temperatures (4001–6001C), films calcined in air hadsmaller reflection maximums due to their better crystallinity.Upon crystallization, nitrogen-calcined films had lower valuescompared with air-calcined samples, which can be explained bytheir higher rutile content, which has a higher refractive indexcompared with anatase. There is almost a linear decrease in thereflection maximum of air-calcined films between 7001 and10001C, which can be attributed to the increased rutile content.For nitrogen-calcined films, the decrease in the reflection max-imum is probably related to the formation of oxygen vacanciesand densification of the rutile film.

Photocatalytic activity of the films under UV light and visiblelight is shown in Fig. 8. MB was chosen as a model pollutant toassess the photocatalytic activity. It should be emphasized thatdegradation pathways of MB are different under UV and visiblelight irradiation, in which the former one proceeds by photoox-idation, whereas the latter involves photosensitized decomposi-tion and photooxidation depending on the nature of thecatalyst.34

Degradation of MB in low concentrations (few ppm) followsa first-order reaction rate given by34:

lnC0

C

� �¼ k � t (1)

where C, C0, k, and t are the MB concentration, initial MBconcentration, rate constant, and time, respectively. Calculatedrate constants are given in Table II. Reported values are some-what different than the ones reported by Yu et al.35 (ca. 1.1 h�1);however, we believe the differences are due to the different in-tensity illumination sources (16 W in this study vs 125 W) andthickness (30 nm in this study vs 150 nm).

Photocatalytic activity of TiO2 films depend on several factorsincluding but not limited to crystal structure,36 particle size,37

film thickness,35 oxygen vacancies,38 optical absorption, surfacearea, and surface chemistry.39 At low calcination temperatures(4001–6001C), better crystallization of air-calcined samples en-sured a higher photocatalytic response compared with the ni-trogen-calcined samples, as amorphous TiO2 is known to be apoor photocatalyst.40 At 7001C and above, nitrogen-calcinedsamples had slightly better photocatalytic activity comparedwith air-calcined samples. Although films calcined under nitro-gen between 7001 and 9001C had similar particle size, opticalabsorption, and the same crystal structure, the one calcined at7001C exhibited the most photocatalytic activity. This behaviorcan possibly be attributed to the beneficial effect of carbon onthe adsorption of MB.28 Photocatalytic activity of the films wassignificantly degraded when the calcination temperature reached10001C in both atmospheres. This can be explained by the de-creased surface area and {110} orientation of the rutile crystalsas the {110} face of rutile is an active reducer, whereas oxidationreactions preferentially occur on the {011} face.41

Fig. 5. TEM images of the films calcined at 9001C in (a) air and (b)nitrogen.

Fig. 6. Optical spectra of TiO2 films calcined in (a) air and (b) nitrogen.Sample photographs are shown in (c).

April 2011 Ultrathin Sol—Gel Titanium Dioxide Films 1105

Degradation of MB under visible light occurs by photosensi-tization for nonvisible-light-active photocatalysts (i.e., air-calcinedfilms in this study), whereas it is a combination of photooxida-tion and photosensitization for visible-light-active photocatalyst(nitrogen-calcined films in this study). Degradation of MB byTiO2 under UV light can generally be expressed as

MB �!hn MB�þ (2)

MB �!hnþTiO2MB�þ (3)

where reactions (2) and (3) are the photolysis of MB by UV ir-radiation and the photooxidation driven by TiO2, respectively.Reaction (2) was ruled out by normalization of the data withrespect to a blank sample irradiated with UV light in the absenceof TiO2. On the other hand, visible light bleaching, in additionto the reaction (3), involves the sensitization of MB to its tripletstate (reaction (4)) followed by electron injection to TiO2 con-duction band (reaction (5))42:

MB �!hn MB� (4)

MB� þ TiO2 !MB�þ þ TiO2ðe�Þ (5)

Visible light degradation of MB is driven mainly by reactions(3) and (5), where reaction (3) is nonexistent in nonvisible-light-active catalysts. Reaction (5) strongly depends on the absorptionbehavior of MB on the catalyst in order to provide an efficientelectron injection.43 Because a significant change in the conduc-tion band position is not expected in our films, the differences inthe visible light activity of films can be attributed to the visiblelight photooxidation (reaction (3)) and/or an increased absorp-tion of MB to the film surface.

Visible light photocatalytic activity of most of the sampleswas low compared with their activity under UV light, with theexception of the sample calcined at 9001C under nitrogen, whichproduced a significant visible light photocatalytic activity. Theimproved visible light photocatalytic activity was attributed tothe oxygen-deficient structure of TiO2

38,44 in nitrogen-calcinedfilms. As explained before, the increased activity can be attrib-uted to either reaction (2) or (5). A higher content of OH groupscan be attained by nonstoichiometric TiOx (0oxo2),45 whichimproves the rate of reaction (5) due to an increased adsorptionof a cationic dye such as MB. However, we have not detectedany difference in their adsorption behavior between films calci-ned at 7001, 8001, and 9001C (MB concentrations were mea-sured after films were kept in dark for 1 h to equilibrateadsorption before illumination); yet, there are significant differ-ences in their respective photocatalytic activity under visiblelight. Therefore, the difference should be coming from reaction(3), i.e. photooxidation process by TiO2. Although rutile hasbeen reported to have higher visible light photocatalytic activitycompared with anatase,46 we do not think this was the reasonfor this behavior as other films with rutile did not perform aswell. On the other hand, the absorption edge of the film calcinedat 9001C under nitrogen was more red-shifted compared withothers (Figs. 5(b) and 6), which can explain its higher photo-catalytic activity in the visible light. Similarly, the sudden de-crease in the visible light photocatalytic activity of the filmcalcined at 9001C under air can be explained by its very highreflectivity in the visible spectrum. However, the exact mecha-nism for this behavior still requires further study.

In general, all of the films have shown good adhesion to thesubstrate with resistance to peeling off by a scotch tape. No filmbulging or delaminating was observed after indentation andscratch testing. Nanoindentation results (Fig. 9) have shownthat films calcined in air had higher overall hardness and mod-ulus compared with the ones calcined under nitrogen. The lowerhardness of nitrogen-calcined samples was possibly as a result ofthe porous nature and a lower crystallinity of the films as evi-denced by the SEM images. Also, films calcined at low temper-atures (o8001C) had better mechanical properties while analmost complete elastic recovery was observed for amorphousfilms. Nanoindentation results of thick anatase and rutile filmsprepared by magnetron sputtering have been reported before,where hardness of anatase and rutile were found to be 8 and 17GPa, respectively.47 Mechanical properties of the films calcinedat lower temperatures are close to that of anatase. Rutile wasreported to have a significantly higher modulus (260 GPa) com-pared with anatase (170 GPa). However, because the films in

Fig. 7. Reflection maximums of films with respect to the calcinationtemperature and environment.

Fig. 8. Photocatalytic activity of the films as obtained from methyleneblue decomposition test.

Table II. Calculated First-Order Photocatalytic RateConstants of the Films

Calcination temperature (1C)

Rate constant (h�1)

UV–Air UV–N2 Vis–Air Vis–N2

400 0.097 0.077 0.029 0.026500 0.128 0.071 0.024 0.022600 0.248 0.197 0.027 0.028700 0.154 0.340 0.034 0.036800 0.205 0.192 0.036 0.049900 0.166 0.196 0.020 0.0881000 0.075 0.076 0.036 0.008

1106 Journal of the American Ceramic Society—Kurtoglu et al. Vol. 94, No. 4

this study were very thin (30 nm), the results reflect the substrateinfluence where reported modulus and hardness values for Si(100) are around 165–178 and 12–13.3 GPa, respectively.48

Deterioration of mechanical properties in films calcined un-der nitrogen may also be attributed to the tensile stresses, whicharose due to the oxygen vacancies,49 which also explains theporous nature of the films. Presence of soft graphitic carbon inthese films (Fig. 2(c)) may provide another reason for lowermechanical properties. Large residual inelastic deformation ofthe film calcined at 9001C under nitrogen (Fig. 9(d)) can be at-tributed to the high porosity. Partial recovery of the film hard-ness after calcination at 10001C in nitrogen can be explained bythe formation of a sintered nanocrystalline film with less poros-ity (Fig. 3(f)). Thus, calcination at lower temperature leads toimproved mechanical properties and should be used when abra-sion resistance is of critical importance (e.g., window glass).

IV. Conclusions

The effect of calcination temperature and environment on thephotocatalytic, optical, structural, and mechanical properties ofsol–gel prepared TiO2 films was investigated. It was found thatcalcination under nitrogen promotes the rutile phase formationand increases the crystallization onset temperature comparedwith an oxygen-rich atmosphere (air). Delayed decompositionof carbon in films calcined under nitrogen hindered the crystalgrowth and increased the porosity. However, oxygen-deficientTiO2 structure gave rise to improved visible light photocatalyticactivity. Overall, the film calcined under nitrogen at 9001Cshowed improved photocatalytic activity under both visibleand UV light while possessing inferior mechanical properties.Films calcined at 4001–5001C show the highest mechanical prop-

erties and films calcined in air maintained a high hardness to7001C, while all films have shown good adhesion to the sub-strate. Thus, depending on potential application, a compromisebetween photocatalytic and mechanical properties should befound.

Acknowledgments

Authors are thankful to the Centralized Research Facility of Drexel Universityfor use of SEM, TEM, XRD, and Raman spectrometers. P. R. was supported by aGAANN Fellowship from the U. S. Department of Education. We also thank Dr.Caroline Schauer and Keith Fahnestock (both Drexel University) for help with theellipsometric measurements.

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