Effects of Calcination Temperature on the Phase Composition, Photocatalytic Degradation, and Virucidal Activities of TiO2 Nanoparticles.

The application of TiO2 nanoparticles in the photocatalytic treatment of chemically or biologically contaminated water is an attractive, albeit unoptimized, method for environmental remediation. Here, TiO2 nanoparticles with mixed brookite/rutile phases were synthesized and calcined at 300-1100 °C to investigate trends in photocatalytic performance. The crystallinity, crystallite size, and particle size of the calcined materials increased with calcination temperature, while the specific surface area declined significantly. The TiO2 phase composition varied: at 300 °C, mixed brookite/rutile phases were observed, whereas a brookite-to-anatase phase transformation occurred above 500 °C, reaching complete conversion at 700 °C. Above 700 °C, the anatase-to-rutile phase transformation began, with pure rutile attained at 1100 °C. The optical band gaps of the calcined TiO2 nanoparticles decreased with rising calcination temperature. The mixed anatase/rutile phase TiO2 nanoparticles calcined at 700 °C performed best in the photocatalytic degradation of methylene blue owing to the synergistic effect of the crystallinity and phase structure. The photocatalytic virus inactivation test demonstrated excellent performance against the MS2 bacteriophage, murine norovirus, and influenza virus. Therefore, the mixed anatase/rutile phase TiO2 nanoparticles calcined at 700 °C may be considered as potential candidates for environmental applications, such as water purification and virus inactivation.

Introduction

Although industrial production is critical to our world economy, the wastewaters generated during various production processes can result in serious water pollution. Other sources of fouled water can be traced to biocontaminants such as viruses and bacteria.[1,2] To solve these growing threats to health and safety, the development of semiconductor-based photocatalysts that can effectively degrade organic or biological pollutants has been intensively studied in recent years.[3−8] Among these photocatalysts, titanium dioxide (TiO2) nanoparticles are fascinating materials with the desirable properties of chemical stability, nontoxicity, high photoreactivity, corrosion resistance, and cost-effectiveness.[9−12] TiO2 naturally occurs in three distinct crystalline phases with different physical and chemical properties: brookite (orthorhombic crystal structure), anatase (tetragonal crystal structure), and rutile (tetragonal crystal structure).[13,14] Under ambient conditions, bulk rutile is thermodynamically stable, whereas anatase and brookite are thermodynamically metastable.[15] Thus, these phases can be accessed through thermally driven phase transformations, as may occur during calcination. Among the three phases, anatase and rutile are widely used for photocatalytic applications because of their facile synthesis.[16] In contrast, brookite has been rarely investigated as a photocatalyst because it is not readily accessible in its pure form.[17−19] However, it has been reported that the photocatalytic performance of brookite is superior to those of anatase and rutile.[20,21]

Generally, the photocatalytic activity of TiO2 is affected by its phase structure, crystallite size, specific surface area, and pore structure.[22−28] Although rutile has a smaller band gap (3.0 eV) than anatase (3.2 eV), its photocatalytic activity is inferior to that of anatase because rutile exhibits a faster electron (e–) and hole (h+) recombination rate, larger grain size, and smaller specific surface area.[29−32] The lifetimes of electrons and holes generated upon photon absorption are longer for anatase than rutile, enhancing the surface chemical reaction rate of the photoexcited species in anatase. Although anatase has several advantages as a photocatalyst, its performance is limited by its fast e–/h+ recombination rate.

Several researchers have shown that phasic mixtures of TiO2 exhibit substantially higher photocatalytic performance than single-phase TiO2. The different band alignments of mixed TiO2 phases can facilitate charge transfer at the interface. This enhances e–/h+ separation and reduces e–/h+ recombination.[16,33−35] A well-known example is Degussa P25, a widely used commercial TiO2 photocatalyst containing 75% anatase and 25% rutile phases.[36] Upon irradiation of P25 with UV light, photoexcited electrons from the anatase phase are transferred to the rutile phase, which has a lower conduction band energy, thus inhibiting the recombination of electrons and holes.[37] However, the mixing ratios of the different TiO2 phases must still be explored to optimize photocatalytic performance.[38−40]

In this study, we investigate the effects of calcination temperature on the particle and crystallite sizes, phase transformations, and photocatalytic performance of as-prepared TiO2 nanoparticles. The samples before and after calcination were characterized using X-ray diffraction (XRD), Brunauer–Emmett–Teller (BET) specific surface area and Barrett–Joyner–Halenda (BJH) pore size analyses, field emission scanning electron microscopy (FE-SEM), high-resolution transmission electron microscopy (HR-TEM), and UV–vis spectrophotometry. To evaluate photocatalytic performance as a function of catalyst calcination temperature, the degradation of methylene blue dye as an organic pollutant surrogate was tested with TiO2 in aqueous solution. Finally, the photocatalytic virus inactivation capabilities of the catalysts were estimated against the MS2 bacteriophage, murine norovirus, and influenza virus using a TiO2/cotton fabric system.

Experimental Methods

Materials

Titanium tetrachloride (TiCl4, 99%), nitric acid (HNO3, 60%), and methylene blue were purchased from Junsei Chemical Co. Ltd., Japan. Sodium hydroxide pellets (NaOH, 98%) were obtained from Dae-Jung Chemicals Ltd., South Korea. Poly(ethylene glycol) (PEG) with a molecular mass of 600 was purchased from Merck, Germany. Commercially bleached cotton fabric was employed in this study. All chemicals were of analytical grade and used as received without further purification.

Synthesis Process

TiCl4 was used as a starting material to prepare TiO2 powder. A stock solution of TiOCl2 (4.0 M in Ti4+) was prepared by the careful dropwise addition of aqueous HNO3 (1.0 M) into vigorously stirred TiCl4 at 0 °C. To prepare TiO2, the stock solution was diluted with aqueous HNO3 (5.0 M) and heated at 80 °C for 15 h. After cooling to room temperature, the pH of the solution was adjusted to pH 7 by adding NaOH solution (1.0 M). The white precipitates were collected on a membrane filter and washed, first with dilute HNO3 solution (1.0 M) to eliminate titanium hydroxide and then with distilled water. After drying at 100 °C, samples were calcined at 300, 500, 700, 900, and 1100 °C (heating rate of 9 °C/min) for 2 h.

Characterization of TiO2 Photocatalysts

The phase compositions of the synthesized TiO2 nanoparticles were identified using a Philips X’pert 3 X-ray diffractometer (Eindhoven, Netherlands) with Cu Kα radiation and an accelerating voltage and current of 40 kV and 30 mA, respectively. The 2θ scanning range was 20–80° and the step size was 0.01°. The crystallite sizes of the anatase, brookite, and rutile phases were estimated using Scherrer’s equation (D = Kλ/β cos θ), where K = 0.93, λ = 0.154059 nm, β = full width at half-maximum (FWHM) in radians, and θ = the Bragg angle.[41] The characteristic peaks of the phases (anatase (101) peak at 2θ = 25.3°, brookite (121) peak at 2θ = 30.8°, and rutile (110) peak at 2θ = 27.5°) were used to calculate the crystallite sizes in the samples.[9,42] The weight fraction of each phase was obtained using the following eq (41,43)where WA, WB, and WR represent the weight fractions of the anatase, brookite, and rutile phases of TiO2, respectively. AA, AB, and AR are the integrated intensities of the anatase (101), brookite (121), and rutile (110) peaks of the TiO2 nanoparticles, respectively. The terms KA and KB are coefficients with values of 0.886 and 2.721, respectively.[41,43] Raman analysis was conducted by a Raman spectrometer (JASCO Co., NRS-5100, Tokyo, Japan) at room temperature with a solid-state laser at 532 nm with a notch filter grating of 1800 g/mm.

The BJH pore size distributions and BET surface areas were obtained from nitrogen adsorption–desorption isotherms (Quantachrome, Autosorb-iQ & Quadrasorb SI, Boynton Beach, FL). The surface morphologies of the TiO2 nanoparticles were analyzed by FESEM using a Carl Zeiss SUPRA 40VP instrument (Oberkochen, Germany). The microstructures of the TiO2 nanoparticles were observed by HR-TEM (FEI Co., TALOS F200X, Hillsboro, OR). The TiO2 nanoparticles were dispersed in ethanol and then mounted on a copper grid with formvar. The particle size of each sample was also measured by the particle size distribution histogram of HR-TEM.

Photocatalytic Degradation Tests

The photocatalytic performance of the calcined TiO2 nanoparticles was evaluated by decomposing methylene blue (MB). Calcined TiO2 nanoparticles (0.5 wt %) were added to aqueous MB solution (20 ppm, 20 mL, pH 7), and the suspension was ultrasonicated and stirred for 60 min under dark conditions. The suspension was then irradiated under UV light (8 W × 4 lamps) at room temperature. During the process of degradation, solution aliquots (2 mL) were withdrawn via a syringe at 30 min intervals up to 180 min. UV–vis spectra were collected for these samples in the 300–800 nm range, measuring the absorbance with a single-beam UV–vis spectrophotometer (Shimadzu, model UV-1280, Kyoto, Japan). The photocatalytic performance was calculated according to eq .where C0 is the initial absorbance and C is the absorption at a certain irradiation time t of the MB solution (λ = 665 nm). All of the analyses were conducted using a quartz cuvette as the sample holder.

Photocatalytic Virus Inactivation Test

The dip-padding method was used to coat TiO2 nanoparticles onto the cotton fabric (area: 100 mm × 100 mm) to evaluate the photocatalytic virus inactivation performance. To remove residual impurities, the cotton fabric sample was boiled successively for 30 min with sodium carbonate solution (2.0 g/L), followed by sodium dodecylbenzene sulfonate (2.0 g/L), and then washed with distilled water and air-dried at room temperature. A PEG–TiO2 sol was prepared by mixing TiO2 nanoparticles (0.3 g) and PEG-600 (50 mL) in an ultrasonic mixer for 5 h. The pretreated fabric was immersed in the prepared PEG–TiO2 sol for 1 min and then passed through a two-roller laboratory padding machine at a nip pressure of 4 bar to ensure a constant amount of TiO2 on the fabric. After padding, the fabric was immediately dried at 100 °C for 5 min in a preheated oven and finally cured at 120 °C for 3 min.

The virus inactivation experiments were conducted in a deep Petri dish using phosphate buffer solution (20 mL, pH 7.0, 10 mM) with the target virus (MS2 bacteriophage, murine norovirus, or influenza virus) and a TiO2/cotton fabric pad (50 mm × 100 mm). The initial populations of the virus in the disinfection experiments were controlled to about 106 plaque forming units (PFU)/mL. In the UV light experiments, illumination was provided by 3 Blacklight Blue lamps (BLB, 4 W, Philips Co.; light intensity: 1.8 × 10–6 Einstein/l s), which emitted in the 300–400 nm range.

Typically, three samples (1.0 mL) were collected over 40 min to measure the viable virus; each sampled solution was diluted to 1/10 and 1/100. Three replicate plates were used at each dilution. All disinfection experiments were repeated three times and their averaged values with statistical deviations were used for the data analysis. The same experiments were also carried out with para-chlorobenzoic acid (pCBA), a well-known OH radical probe, to investigate the role of OH radicals in virus inactivation for the TiO2/cotton fabric system. The concentration of pCBA (initial concentration: 300 ppb) was analyzed by HPLC (Waters Co.). A reverse-phase C18 column (XTerra Rp-18, 5 μm, 150 mm × 2.1 mm) was used with a UV detector (UV–vis 151, Gilson Co.) at 230 nm.

Murine norovirus-1 strain CW1 (MNV1) was propagated in confluent monolayers using the murine macrophage cell line RAW264.7, which was cultured in HyClone Dulbecco’s modified Eagle’s medium (DMEM)/high glucose (GE Healthcare Life Sciences, Logan, UT).[44] The culture was supplemented with 10% fetal bovine serum (FBS, Invitrogen, Carlsbad, CA) containing penicillin/streptomycin and incubated at 37 °C in a 5% CO2 chamber. The virus was incubated for 2 days, subjected to 3 freeze–thaw cycles, and finally harvested by low-speed centrifugation at 1000g for 30 min. To concentrate the virus, the supernatant was filtered using an Amicon Ultra-15 centrifugation unit (Merck Millipore, Ireland).

The influenza A (H3N2) virus was obtained from the Korea Bank for Pathogenic Viruses (KBPV, Seoul, South Korea). The virus was propagated in Madin–Darby canine kidney (MDCK) cells (American Type Culture Collection (ATCC), Manassas, VA) and assessed by plaque titration. Cells were maintained in DMEM supplemented with 10% FBS containing 100 U/mL penicillin, 100 μg/mL streptomycin (HyClone), 0.2% bovine serum albumin (BSA, Gibco, Waltham, MA), and 25 mM HEPES (Gibco) in a humidified atmosphere containing 5% CO2 at 37 °C.

The MS2 bacteriophage (ATCC 15597) was quantified by the soft agar overlay (double-agar layer) plaque assay method[45] with Escherichia coli C3000 as host bacteria, cultured using 10 g/L tryptone, 1 g/L glucose, 1 g/L yeast extract, 8 g/L NaCl, and 0.8 g/L CaCl2. The top and bottom agars for the plaque assays contained 7 and 15 g/L agar, respectively.

Results and Discussion

Phase Structure

The XRD patterns of the TiO2 nanoparticles before and after calcination at temperatures from 300 to 1100 °C are shown in Figure . The samples were designated as as-prepared, HTi-1, HTi-2, HTi-3, HTi-4, and HTi-5 according to calcination conditions (untreated, 300, 500, 700, 900, and 1100 °C, respectively). Each sample displays the main characteristic peaks of the anatase, brookite, and rutile phases at 25.3, 30.8, and 27.5°, respectively, as referenced from the Joint Committee on Powder Diffraction Standards (JCPDS card numbers: anatase, 21-1272; brookite, 29-1360; and rutile, 21-1276). At low calcination temperatures (<500 °C), broad XRD peaks are observed owing to the amorphous structure of the TiO2 nanoparticles. With increasing calcination temperature (>500 °C), the XRD peaks steadily narrow and sharpen. This may be attributed to the elimination of grain boundary defects during calcination at high temperatures, which therefore increases the crystallinity of the TiO2 nanoparticles.[46] Further, the crystallite size of all of the samples increases with calcination temperature, as shown in Table . The crystallite size of each phase was calculated using Scherrer’s equation, and the weight fractions of the anatase, brookite, and rutile phases were estimated using eq . A mixture of brookite and rutile phases is observed for the as-prepared sample and HTi-1 (Figure a and Table ). Since the anatase (101) diffraction peak at 25.3° overlaps the brookite (120) diffraction peak at 25.3°, Raman spectroscopy analysis is needed to check the potential presence of anatase. Identification of the composition of each phase was conducted by comparing the observed vibration modes with the literature.[47−50] In Figure b, HTi-1 shows characteristic signals of brookite at 153 (A1g) and 247 (A1g) cm–1 and characteristic signals of rutile at 447 (Eg) and 612 (A1g) cm–1. Meanwhile, HTi-2 presents characteristic signals of brookite and rutile as well as the signals of anatase at 399 (B1g), 516 (A1g), and 639 (Eg) cm–1. Upon increasing the calcination temperature from 300 to 500 °C, the brookite phase is transformed into the anatase phase (HTi-2). At 700 °C (HTi-3), the characteristic brookite (121) peak at 30.8° disappears, indicating the complete phase transformation from brookite to anatase. With further increases in calcination temperature (to 1100 °C), the rutile phase content increases from 21 to 100%, with the higher thermal stability of the rutile phase driving the transformation at high temperatures.[51] These data suggest that an increasing calcination temperature leads to higher crystallinity, a larger crystallite size, and phase transformation in the TiO2 nanoparticles.

Figure 1

(a) XRD patterns and (b) Raman spectra of TiO2 nanoparticles: as-prepared and calcined at different temperatures.

Table 1

Physical Properties of TiO2 (HTi) Nanoparticles as a Function of Calcination Temperature

		anatase		brookite		rutile
sample no.	calcination temperature (°C)	size (nm)	content (%)	size (nm)	content (%)	size (nm)	content (%)
as-prepared	0			6.35	81	4.80	19
HTi-1	300			10.10	81	10.70	19
HTi-2	500	17.01	39	13.45	41	18.17	20
HTi-3	700	28.19	79			26.69	21
HTi-4	900	46.10	12			51.90	88
HTi-5	1100					77.60	100

SEM and TEM Analyses

FE-SEM and HR-TEM analyses were performed to investigate the effect of calcination temperature on the morphology and particle size of the TiO2 nanoparticles. Figure shows the FE-SEM images of the calcined TiO2 nanoparticles. Clearly, the calcination temperature does not influence the morphology of the nanoparticles, which retain their irregular spherical shapes after calcination. However, the particle size increases with calcination temperature, as shown in Table and Figure .

Figure 2

FE-SEM images of TiO2 nanoparticles calcined at different temperatures: (a) HTi-1, (b) HTi-2, (c) HTi-3, (d) HTi-4, and (e) HTi-5.

Figure shows the morphology, d-spacing values, and selected area electron diffraction (SAED) patterns of the calcined TiO2 nanoparticles subsequent to their HR-TEM analyses. The particle and crystallite sizes of the nanoparticles increase with calcination temperature. Figure a shows nanoparticles with both spherical and rod shapes, comprising a mixture of brookite and rutile phases. The d-spacing values between the fringes are estimated as 0.290 and 0.321 nm, which are close to the (121) lattice spacing of brookite and the (110) lattice spacing of rutile, respectively. With increasing calcination temperature (to 500 °C, Figure b), a mixture of anatase (101), brookite (121), and rutile (110) phases with d-spacings of 0.352, 0.290, and 0.326 nm, respectively, is observed. Figure c,d shows mixtures of anatase (101) and rutile (110) phases with d-spacings of 0.351 and 0.323 nm, respectively. Figure e shows the single rutile (200) phase with d = 0.229 nm. The SAED patterns of the calcined TiO2 nanoparticles confirm the mixtures of brookite and rutile phases, as shown in Figure a. After calcination at 500 °C, a triphasic mixture of anatase (101), brookite (121), and rutile (110) phases is observed. The SAED patterns shown in Figure c,d indicate mixtures of anatase (101) and rutile (110) phases. Finally, after calcination at 1100 °C, only the rutile (200) phase is observed, again confirming complete transformation of the anatase phase. Moreover, from the XRD results, the crystallinity increases as a result of crystallite growth with thermal energy input. Therefore, different phase transformations occur depending on the calcination temperatures.

Figure 3

HR-TEM images and SAED patterns of TiO2 nanoparticles calcined at different temperatures: (a) HTi-1, (b) HTi-2, (c) HTi-3, (d) HTi-4, and (e) HTi-5.

BET and BJH Analyses

Figure shows the nitrogen adsorption–desorption isotherms of the calcined TiO2 nanoparticles. According to the IUPAC classification, the obtained isotherms can be described as type IV[52] indicating the presence of well-developed mesopores associated with capillary condensation of the adsorbent. Adsorption isotherms of mesoporous TiO2 nanoparticles are also described by hysteresis loops (H1–H4) that indicate the distribution and shapes of the pores within the materials.[53] The calcined TiO2 nanoparticles present H3-type hysteresis loops, which represent isotherm curves that increase slowly and then rise sharply at high relative pressure P/P0 (P: the balance pressure; P0: saturation pressure).[54] These results indicate that the pores are irregular with parallel, slit-like, and open-ended-tubular shapes.

Figure 4

Nitrogen adsorption–desorption isotherms of the TiO2 nanoparticles calcined at different temperatures.

Table shows the surface areas and pore volumes of the TiO2 nanoparticles obtained at various calcination temperatures. The surface areas of the calcined TiO2 nanoparticles are 101.24, 45.68, 17.61, 5.32, and 3.25 m2/g, respectively. Thus, with increasing calcination temperature, the surface areas, and pore volumes decrease due to the crystallization of the TiO2 nanoparticles.[55,56]

Table 2

BET Surface Areas and Pore Volumes of TiO2 (HTi) Nanoparticles Calcined at Various Temperatures

sample no.	surface area (m2/g)	pore volume (cc/g)
HTi-1	101.24	0.268
HTi-2	45.68	0.210
HTi-3	17.61	0.132
HTi-4	5.32	0.026
HTi-5	3.25	0.015

Optical Analysis

The optical properties of the calcined TiO2 nanoparticles were investigated before the photocatalytic performance tests because the UV–vis absorption edge is associated with the energy band of the semiconductor. The UV–vis absorption spectra of the TiO2 nanoparticles in Figure a clearly show that the UV–vis absorption edge is shifted toward higher wavelengths with an increase in calcination temperature.

Figure 5

(a) UV–vis absorption spectra of TiO2 nanoparticles calcined at different temperatures. (b) Direct and (c) indirect Tauc plots demonstrating the band gaps of TiO2 nanoparticles calcined at different temperatures.

The optical band gap energies of the samples were calculated on the basis of Tauc plots. The band gap values can be determined using the following equation[57,58]where α is the absorption coefficient, hv is the photon energy, B is a constant relative to the material, Eg is the energy gap, and n is a value that depends on the nature of the transition (n = 2 for a direct allowed transition, 2/3 for a direct forbidden transition, and 1/2 for an indirect allowed transition). The band gap values were calculated by extrapolating the plots of (αhv) versus photon energy for direct (n = 2; Figure b) and indirect (n = 1/2; Figure c) transitions. The direct and indirect band gap values are depicted in Table . The estimated direct band gap values for the calcined TiO2 nanoparticles range from 3.05 to 2.98 eV, whereas the estimated values for the indirect band gap vary from 2.99 to 2.91 eV. The band gap of rutile TiO2 (HTi-5) is smaller than that of bulk rutile TiO2 (3.02 eV).[59] For both direct and indirect transitions, the optical band gap clearly decreases with calcination temperature, and the direct band gap values are greater than the corresponding indirect band gap values. These results can be explained on the basis of particle size, which affects the optical band gap: for most semiconductors, a decrease in band gap with an increase in particle size leads to a red shift of the optical absorption edge.[30,60]

Table 3

Direct and Indirect Band Gap Values of TiO2 Nanoparticles Calcined at Different Temperatures

	band gap (eV)
sample no.	direct (αhν)2	indirect (αhν)1/2
HTi-1	3.05	2.99
HTi-2	3.03	2.98
HTi-3	3.02	2.96
HTi-4	3.01	2.94
HTi-5	2.98	2.91

Photocatalytic Degradation

The photocatalytic performance of the calcined TiO2 nanoparticles was evaluated in the degradation of MB under UV light irradiation. The degradation of a dye indicates that a photochemical reaction has occurred. Figure a,b presents the changes in MB absorption over time in the presence of HTi-3 (calcined at 700 °C) and commercial Degussa P25 as a reference, respectively. The maximum peak at 665 nm decreases progressively with elapsed UV irradiation time, and the rate of decrease is more rapid with HTi-3 compared to P25. This can also be observed in Figure a,b, which shows the photocatalytic performance of the calcined TiO2 nanoparticles and P25 in MB degradation. It is well known that the adsorption on the surface of the catalyst affects the photocatalytic performance. Therefore, the adsorption capacity under dark conditions of the sample was investigated before the sample was exposed to the light irradiation. In Figure a, it was observed that 7.5, 6.1, 5.8, 4.1, 2.8, 1.8, and 0.5% of MB readily adsorbed onto P25, as-prepared, HTi-1, HTi-2, HTi-3, HTi-4, and HTi-5, respectively, during the adsorption process (in dark conditions). The lower adsorption capacity of MB on the samples calcined at higher temperatures could be ascribed to the decrease of the surface area of HTi samples (Table ). The calcination temperature of the TiO2 nanoparticles also has an impact on the degradation kinetics, as shown by the constants in Table . The first-order rate constants were calculated using ln(C0/Ct) = kt, where k is the first-order constant, C0 is the initial concentration, and Ct is the concentration of the dye after the photocatalytic reaction for time t. The order of MB degradation rate constants is HTi-3 > P25 > HTi-2 > HTi-4 > HTi-1 > as-prepared > HTi-5. Although HTi-3 (anatase/rutile = 79:21) has a similar phase composition to that of P25 (anatase/rutile = 75:25), HTi-3 shows the best photocatalytic performance, with a rate constant of 3.11 × 10–2 min–1. Generally, the photocatalytic performance of TiO2 nanoparticles depends on many factors, including phase composition, crystallinity, crystallite size, and surface area.[22−24] In our study, HTi-3 shows the best photocatalytic performance because of its high crystallinity and anatase/rutile phase composition. Further, anatase has an indirect band gap structure, while brookite and rutile have direct band gap structures. The indirect band gap structure provides longer e–/h+ lifetimes, leading to the lower recombination of e–/h+ pairs. Despite the larger surface areas in HTi-1 and HTi-2 compared to HTi-3, lower photocatalytic performance is observed, with k values of 3.68 × 10–2 and 1.06 × 10–2 min–1, respectively. This is consistent with the study of Ozawa et al.,[34] which reported that photocatalytic performance was enhanced as WA increased and WB decreased. HTi-5 delivers the lowest photocatalytic performance with k = 1.59 × 10–3 min–1 because this material consists of only single-phase rutile and has the largest crystallite size (77.6 nm) and smallest surface area (3.25 m2/g). Moreover, rutile has a direct band gap structure, which leads to the fast recombination of e–/h+ pairs. These results are clearly observed in Figure c,d, wherein both P25 and HTi-3 completely decompose MB after 180 min UV irradiation, while HTi-5 hardly degrades MB at all.

Figure 6

Absorption spectra changes of methylene blue under UVA light irradiation for different time periods: (a) HTi-3 (700 °C) and (b) P25.

Figure 7

(a) Photocatalytic degradation curve and (b) corresponding degradation kinetics for degradation of methylene blue by calcined TiO2 nanoparticles and P25. Images of methylene blue photodegradation by calcined TiO2 nanoparticles and P25: (c) initial (t = 0 min) and (d) final (180 min).

Table 4

Kinetics Constants k for the Degradation of Methylene Blue

sample no.	k (min–1)
P25	2.99 × 10–2
as-prepared	2.97 × 10–3
HTi-1	3.68 × 10–3
HTi-2	1.06 × 10–2
HTi-3	3.11 × 10–2
HTi-4	1.02 × 10–2
HTi-5	1.59 × 10–3

Photocatalytic Virus Inactivation

Since TiO2 has been shown to be capable of inactivating a broad range of microorganisms, photocatalytic virus inactivation against MS2 bacteriophage, murine norovirus, and influenza virus was evaluated for the possible antimicrobial applications in public settings, such as cruise ships, nursing homes, hospitals, daycare centers, etc. Figure shows photocatalytic virus inactivation profiles obtained for the HTi-3-coated cotton fabric system, which were estimated from MS2 bacteriophage, murine norovirus, and influenza virus inactivation kinetics in aqueous conditions during the 40 min BLB irradiation. In control experiments (without BLB irradiation), no inactivation of the targets is observed on the time scale of this study. As OH radicals have been shown to be major participants in microbial inactivation for TiO2 systems in a previous study,[61] the degradation of the well-known OH radical probe pCBA was also assayed (Figure offset) under the same experimental conditions.

Figure 8

Photocatalytic virus inactivation performance of calcined TiO2 nanoparticles (HTi-3 with cotton fabric) with MS2 bacteriophage, murine norovirus, and influenza virus.

For the prepared fabrics, significant inactivation of the viruses is observed. The HTi-3-coated cotton requires 40 min to inactivate 99.9% (3 log) of the influenza virus under BLB irradiation. From the radical probe experiment in Figure (offset) and our previous assumptions[61] (kexp = kOH radical, pCBA [OH radical]ss (kexp = 0.126 s–1, kOH radical, pCBA = 5 × 109 M–1 s–1)), the steady-state concentration of OH radicals could be calculated as 2.5 × 10–11 M (=4.3 × 10–7 mg/L). Considering that the times required for 3 log inactivation of the MS2 bacteriophage, murine norovirus, and influenza virus are 36, 32, and 40 min, respectively, the required CT (disinfectant concentration × reaction time) values for 3 log inactivation of the targets can be calculated as 1.5 × 10–5, 1.4 × 10–5, and 1.7 × 10–5 mg/L min, respectively. The results clearly demonstrate that the HTi-3-coated cotton effectively inhibits the MS2 bacteriophage, murine norovirus, and influenza virus.

Conclusions

In this study, as-prepared TiO2 nanoparticles were calcined at temperatures ranging from 300 to 1100 °C to investigate the effect of calcination temperature on photocatalytic performance. The calcination temperature affected the crystallite size, crystallinity, particle size, and phase transformation in the as-prepared TiO2 nanoparticles according to XRD, SEM, and TEM analyses. The XRD patterns revealed that the calcined TiO2 nanoparticles were composed of brookite/rutile phases (300 °C calcination temperature), anatase/brookite/rutile phases (500 °C), anatase/rutile phases (700 and 900 °C), and rutile phase (1100 °C). Further, a reduction in surface area from 101.24 to 3.25 m2/g was observed with increasing calcination temperature. The optical band gaps of the calcined TiO2 nanoparticles were determined from the UV–vis absorption spectra; the direct and indirect band gaps decreased from 3.05 to 2.98 eV and from 2.99 to 2.91 eV, respectively, with increasing calcination temperature. The mixed anatase/rutile-phase TiO2 nanoparticles calcined at 700 °C delivered the highest photocatalytic performance, superior to that of the commercial photocatalyst Degussa P25, because of the synergistic effects of crystallinity and phase structure. The photocatalytic virus inactivation tests with the MS2 bacteriophage, murine norovirus, and influenza virus also demonstrated excellent performance. Based on these results, TiO2 nanoparticles calcined at 700 °C appear to be promising candidates for environmental applications.