Temperature-dependent thermal stability and dispersibility of SiO2–TiO2 nanocomposites via a chemical vapor condensation method

Powder Technology 267 (2014) 153–160

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Temperature-dependent thermal stability and dispersibility of SiO2–TiO2 nanocomposites via a chemical vapor condensation method Minsu Kim a,b, Eunseuk Park a, Hyounduk Jung a,b, Seong-Taek Yun b, Jongsoo Jurng a,b,⁎ a b

Center for Environment, Health and Welfare Research, Korea Institute of Science and Technology, Hwarangno 14 gil 5, Seoul, Seongbuk-gu 136-791, Republic of Korea Graduate School of Energy Environment Policy and Technology, Korea University, Anam-ro 145, Seoul, Seongbuk-gu 136-701, Republic of Korea

a r t i c l e

i n f o

Article history: Received 25 March 2014 Received in revised form 7 July 2014 Accepted 7 July 2014 Available online 13 July 2014 Keywords: Nanoparticles Nanocomposites Chemical vapor condensation (CVC) Heat treatment

a b s t r a c t Surface-modified TiO2 nanoparticles were prepared by a chemical vapor condensation method (CVC). The resultant nanocomposites were subjected to post-heat treatment with various temperatures. The sedimentation behavior of the surface-modified TiO2 nanoparticles in aqueous solution was investigated visually using a separation analyzer. The dispersion stabilities of the pure CVC-made TiO2 and the surface modified SiO2–TiO2 samples were enhanced due to low zeta potential values compared to a commercial TiO2 sample (P25). With increasing heat treatment temperature, the photocatalytic activity of the pure TiO2 samples (CVC–TiO2 and P25) suddenly dropped, which resulted from the increased particle size and the reduced anatase content. For the surface-modified TiO2 nanoparticles, the anatase-to-rutile phase transformation process did not occur even at 900 °C. Although the surface-modified TiO2 nanoparticles formed using the chemical vapor condensation method were thermally treated with high temperature, the photocatalytic activity was higher than that of the pure TiO2 samples. This is probably due to the fact that the surface-modified TiO2 nanoparticles had increased thermal stability, and SiO2 provides better adsorption sites in the vicinity of the TiO2. © 2014 Elsevier B.V. All rights reserved.

1. Introduction The number of studies on titanium dioxide (TiO2) has increased remarkably due to its wide applications in photocatalysis, solar cells, chemical sensors, cosmetics, and pigments. Among the various applications, TiO2 photocatalysts are of great interest due to its strong oxidizing activity in powder form, nontoxicity, and long-term photostability. It is well known that TiO2 has three phases: anatase (tetragonal), rutile (tetragonal), and brookite (orthorhombic). Among these phases, the anatase phase shows higher photocatalytic activity than other phases owing to its metastable structure. Also, the large surface area in anatase phase TiO2 is expected to lead to good photocatalytic activity since the reactions take place on the TiO2 surface. But, when the anatase phase TiO 2 is exposed to high temperature, it can be easily transformed into stable rutile phase, and ultrafine particles will agglomerate into larger ones, resulting in adverse effects on the photocatalytic activity [1–4]. Many studies have reported suppressing the anatase-to-rutile phase transformation. Doping cationic and/or anionic dopants is often used as an effective method, the effects of which have been attributed to mass transport inhibition by the dopant phase [4–8]. Among dopants, SiO2 is often one of the most effective [4,9–16]. Surface-modified TiO2 ⁎ Corresponding author. Tel.: +82 2 958 5688; fax: +82 2 958 6711. E-mail address: [email protected] (J. Jurng).

http://dx.doi.org/10.1016/j.powtec.2014.07.013 0032-5910/© 2014 Elsevier B.V. All rights reserved.

nanoparticles formed through SiO2 addition to TiO2 have improved photocatalytic activity as well as dispersibility in aqueous solution. This is influenced by the negatively charged particles tending to repel each other due to the surface charge change as a result of silica addition in TiO2 nanoparticles, resulting in a more stable dispersion in aqueous solution [17–19]. It is well known that the material properties of TiO2 nanoparticles strongly depend on the synthesis method, such as the sol–gel process [20], pyrolysis [21], electron beam evaporation [22], chemical vapor deposition [23], atomic-layer deposition [24] and the hydrothermal processes [25], as well as the post-treatment conditions, since they have a decisive influence on the chemical and physical properties of TiO2 nanoparticles. The chemical vapor condensation (CVC) method is an alternative method that features good photocatalytic activity with high specific surface area, and improved crystallinity [26–28]. The CVC method allows individual adjustments of the synthesis conditions, such as the synthesis temperature, precursor vapor residence time in the heating zone and precursor vapor concentration. Therefore, if the surface-modified TiO2 nanoparticles using the CVC method are synthesized, the thermal stability and dispersibility will be greatly improved, resulting in improved photocatalytic activity. It is necessary to elucidate the effect of the synthesis conditions and the post-heat treatment temperature on the particle morphology, crystalline phase and chemical and physical properties, which affect the photocatalytic activity, the thermal stability and dispersibility in aqueous solution.

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We have prepared surface-modified TiO2 nanoparticles using CVC. The resultant nanocomposites were subjected to post-heat treatment at various temperatures. This study provides an alternative synthesis method for producing high-performance SiO2–TiO2 nanocomposites, with increased thermal stability and dispersibility that will expand the application areas of the material, to areas such as nanocomposite coatings. 2. Material and methods 2.1. Catalyst preparation The SiO2–TiO2 nanocomposites were prepared using a CVC method, as shown schematically in Fig. 1 and reported elsewhere [26–28]. Titanium tetraisopropoxide ([(CH 3)2CHO]4Ti, TTIP, Aldrich, N 97%) and tetraethoxysilane ((C2H5O)4Si, TEOS, Samchun Chemicals, N98%) were used as the TiO2 and SiO2 precursors, respectively. Each precursor was heated by an oil bath, and the heating temperatures were fixed at 95 °C and 60 °C, respectively. TTIP and TEOS vapors were introduced into an alumina tube and decomposed thermally to produce TiO2 and SiO2 powder, respectively. The argon flow rate, which changes the TEOS vapor concentration, was fixed at 0.35 L min−1. The argon flow rate, which was used for the TTIP vapor, was fixed at 0.7 L min−1. The synthesis temperature and the air flow rate were fixed at 900 °C and 7.0 L min−1, respectively. Finally, the residence time of the precursor vapor was about 0.55 s. As shown in Fig. 1, the synthesized SiO2–TiO2 samples were sampled using a particle collection device, which could condense the produced vapor. Then, to investigate the thermal stability, the collected samples were heat-treated at 500, 700 and 900 °C for 1 h, respectively. The unmodified and modified samples were labeled as XP25, XCVC and XCVC–SiO2, respectively. X represents the heat treatment temperature. For example, the sample produced with heat treatment at 700 °C was labeled 700CVC–SiO2. For comparison, the commercial TiO2 (Degussa, P25) and CVC-made TiO2 without SiO2 were prepared and heat-treated at the same temperature. 2.2. Catalyst characterization XRD patterns were obtained with a focal spot size of 5 mm2 and a Cu rotating anode. The mean diameter, (dXRD), was obtained using the Scherrer equation: dXRD ¼

kα ; β cosθ

where β is the line broadening (β = βs − βo, where βs and βo are the half-widths of the XRD peak of the sample and a silicon standard), k is related to the crystallite shape (k = 0.9), and α and θ are the radiation

wavelength and Bragg angle, respectively. A single-crystal silicon standard (βo = 0.1058) was used for the calibration. The anatase content, fA, was determined from the integrated intensity of the anatase diffraction line, IA, and that of the rutile diffraction line, IR, using the following equation [29]: fA ¼

0:79IA : IR þ 0:79IA

The crystallite size and shapes were observed by transmission electron microscopy (TEM) (Philips; operated at 300 kV, image resolution b0.23 nm). The powder specific surface area (SSA, m2 g−1) was determined by nitrogen adsorption (N 99.999%) at 77 K on a Micromeritics Tristar 3000 apparatus using the Brunauer–Emmett–Teller (BET) method. Prior to analysis, the sample was heated (150 °C, 1 h) with N2 flow (N 99.999%) to remove the adsorbed water. Assuming monodisperse spherical primary particles, the BET-equivalent particle diameter (dBET) was calculated using the formula, dBET = 6 / (ρ × SSA), where ρ is the particle density. XPS measurements were made on a VG scientific ESCA Lab II Spectrometer (resolution 0.1 eV) with Mg Kα (1253.6 eV) radiation as the excitation source. All binding energies were referenced to the C1s peak at 285.0 eV for adventitious carbon. The dispersion stability of the prepared nanocomposites was evaluated in distilled water. The prepared materials were sonicated for 10 min and then stirred overnight. The dispersion was then allowed to stand for at least 1 week, after which the sedimentation behavior was visually examined. The zeta potential of the prepared samples was measured in a pH range of 3–7, using an Electrophoretic Light Scattering Spectrophotometer (ELS-8000). 2.3. Photoactivity measurement The photocatalytic activity of the prepared samples was characterized by measuring the rate of methylene blue (MB) degradation. Methylene blue was selected because of its strong adsorption to metal oxide surfaces, well-defined optical absorption and good resistance to light degradation. The photocatalytic experiments were carried out at an initial pH of 7.0. The catalyst (50 mg) was mixed with 500 cm3 of a methylene blue solution (~ 153 ppm) in a 500-cm3 beaker, and irradiated with a 1 × 4 W UV light (UVItec, LF-204) with intense emission lines at 365 nm. The slurry was stirred constantly to prevent settling and to ensure constant exposure of the catalyst to the UV radiation for 0–60 min. The MB remaining in the solution was measured from the absorbance at 600 nm using a spectrophotometer (Hach, DR2800). In the absence of a photocatalyst, MB was stable when exposed to UV radiation for extended periods. 3. Results and discussion

Particle collectiondevice 3.1. Catalyst characterization

Air (MFC)

Alumina tube Cooling water

Electric furnace Ar (MFC)

TTIP Temperature controller

TEOS

Oil bath Oil bath

Fig. 1. Schematic diagram of the synthetic device setup for CVC-made SiO2–TiO2 nanocomposites.

Zeta potential measurements can be used to determine the isoelectric point (IEP) and the nanocomposite stability of the prepared samples at a specific pH. Fig. 2 shows the pH dependence of zeta potentials for the prepared samples before and after the heat treatment at 900 °C. The IEP of P25 occurs at a pH of about 5.8, which has also been reported to be 6.0–7.0 by other researchers [17,30–32]. The IEP of SiO2 particles has been reported in the range of 1.8 [33] to 2.7 [34]. It is known that the particle surface is negatively charged at pH N IEP and positively charged at pH b IEP [17]. The surface hydroxyl groups on the TiO2 surface can become protonated or deprotonated according to Eqs. (1) and (2) [17,35,36] þ

þ

≡ Ti – OH2 ↔ B ≡ Ti – OH þ H

ð1Þ

M. Kim et al. / Powder Technology 267 (2014) 153–160

Zeta potential (mV)

20

turbid), and it is completed within 1 day. For CVC and 900CVC, the nanocomposites suddenly settle down after 1 day and 1 week, respectively. The SiO2-modified sample (900CVC–SiO2) is even more stable than 900CVC after 1 week. This suggests that the static electricity repellence and steric hindrance repulsion are caused by the silica addition, which leads to better particle dispersion, and also electrostatic repulsion of the nanoparticles, as revealed by zeta potential measurements [17]. XRD was used to examine the changes in the phase structure of the prepared nanocomposites at different heat-treatment temperatures. Fig. 4 shows the XRD patterns of different TiO2 samples, including modified and commercially available P25TiO2. For the P25 sample, a peak at 2θ = 25.3° was observed, corresponding to the (101) plane diffraction of anatase, (Fig. 4a) and its crystallite size was about 20.5 nm (Table 1). There was no major difference in the XRD

P25 CVC 900CVC CVC-SiO 2

0

900CVC-SiO2 -20

-40

-60

3

4

5

6

7

155

8

pH Fig. 2. Zeta potential profiles for the prepared samples before and after heat-treatment at 900 °C.

≡ Ti – OH↔ ≡ Ti – O

þ

þ H :

ð2Þ

The negatively charged particles tend to repel each other, resulting in a more stable dispersion in aqueous solution [17]. As seen in Fig. 2, the zeta potential of the 900CVC sample was lower than that of the CVC sample due to the thermal heat treatment. But, after SiO2 addition in the CVC sample, there were slight increases in the SiO2-modified CVC sample due to the stable thermal stability. As clearly seen in Fig. 3, the dispersion sedimentation of P25 and 900P25 begins rapidly after 1 h (the upper part of the dispersion is still slightly

Relative intensity (a.u.)

−

a) P25

A R

900oC

700oC

500oC

no heat-treatment 20

30

40

50

60

Degrees (2 theta) 900P25

900CVC

CVC

b) CVC Relative intensity (a.u.)

P25

900oC

700oC

500oC no heat-treatment 20

30

40

50

60

CVC-SiO2

900CVC-SiO2

Relative intensity (a.u.)

c) CVC-SiO2 900oC

700oC

500oC

no heat-treatment 0

1 hr

1 day 1 week

0

1 hr

1 day 1 week

Fig. 3. Visual observations of sedimentation for the prepared samples before and after heat-treatment at 900 °C.

20

30

40

50

60

Fig. 4. X-ray diffraction patterns of (a) P25, (b) CVC and (c) CVC–SiO2 heat-treated at 500, 700 and 900 °C. The symbols ‘A’ and ‘R’ indicate anatase and rutile, respectively.

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Table 1 Physicochemical properties of each sample. Sample label

Synthesis conditions

P25 500P25 700P25 900P25 CVC 500CVC 700CVC 900CVC CVC–0.35SiO2 500CVC–0.35SiO2 700CVC–0.35SiO2 900CVC–0.35SiO2 a b c

TEOS carrier gas flow rate (L min−1)

Thermal treatment temperature (°C)

0.00 0.00 0.00 0.35 0.35 0.35 0.35

– 500 700 900 – 500 700 900 – 500 700 900

SBETa (m2 g−1)

dBETb (nm)

dXRDc (nm)

fA (−)

55.2 54.5 43.1 6.9 95.8 54.9 35.7 4.4 80.7 74.3 74.1 68.9

27.6 28.0 32.8 204.6 16.0 27.9 42.4 319.5 19.1 20.7 20.7 22.3

20.5 23.1 77.7 116.1 11.6 12.2 33.9 130.3 13.1 13.3 15.3 17.4

0.889 0.887 0.083 0.032 0.948 0.960 0.796 0.022 0.976 0.975 0.975 0.950

Elemental analysis from EDX. Particle size by equation using BET surface area. Crystallite size from XRD using Scherrer formula.

peak for the sample thermally treated at 500 °C. As the thermal treatment temperature increased, the anatase XRD peaks gradually decreased, but the rutile peaks increased, indicating that the anatase content became reduced. The crystallite size and the BET-equivalent particle size both gradually increased. At 700 °C, the anatase content suddenly dropped to 0.083, indicating that the anatase-to-rutile phase transformation process occurred. Furthermore, most of the anatase phase disappeared after thermal treatment at 900 °C. For the CVC-made TiO2 samples without SiO2, there was no major difference in the sample heat-treated at 500 °C, as shown in Fig. 4b. But, as the thermal treatment temperature was gradually increased, the crystallite size and the BET-equivalent particle size also increased.

a

The anatase content was also reduced, as listed in Table 1. As the thermal treatment temperature became larger, the anatase XRD peak was gradually enhanced, indicating that the anatase crystallinity increases. The corresponding crystallite size also becomes larger (Table 1) [4]. As the thermal treatment temperature was continually increased, the anatase content was gradually reduced. After thermal treatment at 900 °C, there was only rutile phase. It is well known that the anatase phase composition and the crystallite size are directly related to the photocatalytic activity. Therefore, as the thermal treatment temperature increases, the reduced anatase phase composition and the increased crystallite size can have a negative effect on the photocatalytic activity.

500oC

700oC

900oC

no heat-treated

500oC

700oC

900oC

no heat-treated

500oC

700oC

900oC

no heat-treated

70 nm

b

c

Fig. 5. TEM images of (a) P25, (b) CVC and (c) CVC–SiO2 heat-treated at 500, 700 and 900 °C.

a

500P25

P25 Ti-O

Ti-O

Ti-O

Ti-O

-OH

-OH

-OH

-OH

500CVC

CVC

700CVC

900CVC

Ti-O

Ti-O

Ti-O

-OH

-OH

c

-OH

CVC-SiO2 Ti-O

CVC-700SiO2

CVC-500SiO2 Ti-O

Ti-O

CVC-900SiO2 Ti-O

M. Kim et al. / Powder Technology 267 (2014) 153–160

b

900P25

700P25

Si-O -OH

-OH

Si-O

Si-O Si-O

526

528

530

532

Binding energy (eV)

534

536

526

528

530 526 532 Binding energy (eV)

534

536

528

-OH 530

-OH 532

534

536

Binding energy (eV)

526

528

530

532

534

536

Binding energy (eV)

Fig. 6. XPS spectra of the O1s region for (a) P25, (b) CVC and (c) CVC–SiO2 heat-treated at 500, 700 and 900 °C.

157

158

M. Kim et al. / Powder Technology 267 (2014) 153–160

Table 2 Results of curve-fitting of XPS spectra for the O1s region of P25, CVC and CVC–SiO2 heattreated at 500, 700 and 900 °C. Sample label

Ti–O

P25 500P25 700P25 900P25 CVC 500CVC 700CVC 900CVC CVC–0.35SiO2 500CVC–0.35SiO2 700CVC–0.35SiO2 900CVC–0.35SiO2

OH

Si–O

Eb (eV)

ri (%)a

Eb (eV)

ri (%)a

Eb (eV)

ri (%)a

528.3 528.3 528.3 528.3 528.3 528.3 528.3 528.3 528.3 528.3 528.3 528.3

85.7 88.7 89.4 85.5 67.8 78.1 88.3 80.9 51.7 58.5 47.5 45.3

529.8 529.9 530.0 529.7 529.5 529.7 530.3 530.3 529.8 529.9 530.0 529.4

14.3 11.3 10.5 14.5 32.2 21.9 11.7 19.1 37.4 23.4 21.0 9.6

531.3 530.8 530.7 530.7

10.9 18.1 31.5 45.1

a Area percentage (ri) in the results of XPS spectra curve fitting in the O1s region for the prepared samples.

As the TEOS vapor was introduced to the alumina tube, as seen in Fig. 4c, there is a possibility of amorphous phase existing in the samples due to a slight change in the background of the XRD patterns around 2θ = 20–25° [9]. It can be seen in Fig. 4c that the surface-modified TiO2 nanoparticles made using the CVC method exhibited high anatase thermal stability indicated by the lack of anatase-to-rutile phase transformation. Even though slight rutile peaks were detected at 700 and 900 °C, they did not affect the photocatalytic activity since rutile contents were negligible. For example, the anatase phase composition of the CVC sample was 94.8%, and that of the 900CVC–SiO2 sample was 95.0%. The TEM images of the prepared samples are shown in Fig. 5. It can be seen that all the samples have similar spherical shape for the individual particles, and polyhedral structure. For the P25 sample (Fig. 5a), there was no big difference after heat treatment at 500 °C. After continually increasing the thermal treatment temperature, the particle size suddenly increased. The particle size at 900 °C was greater than 100 nm, and the polyhedral structure was destroyed. It is known that the size of the primary particles is determined not only by the rate of coalescence, but also by the rate of coagulation. Coagulation is the process by which two or more particles collide and bond together

Rate constant (min-1

1.5

1.2

0.9

0.6

900CVC-SiO2

700CVC-SiO2

500CVC-SiO2

CVC-SiO2

900CVC

700CVC

500CVC

CVC

900P25

700P25

500P25

0.0

P25

0.3

Fig. 7. Rate constants for the photocatalytic decomposition of methyl blue on the prepared samples.

to form a new, larger particle. Coalescence refers to the fusion of two or more particles to form a single spherical particle [26]. The large particle size for P25 was due to coagulation and coalescence at high temperature. The unmodified CVC sample (Fig. 5b) also grew greatly with large particle size at 900 °C. The unmodified TiO2 samples begin to agglomerate greatly after 700 and 900 °C, demonstrating that the phase transformation processes are accompanied by aggregations of nanoparticles, which consequently increases the particle size markedly and decreases the surface area [4]. In comparison, the surface-modified TiO2 nanoparticles made using the CVC method effectively inhibit the particle growth and maintain the polyhedral structure. XPS is a highly sensitive surface analysis technique, and an effective method for examining the surface composition and chemical states of all solid samples. Fig. 6 shows the O1s region for P25, CVC and CVC– SiO2 heat-treated at 500, 700 and 900 °C. The O1s XPS spectra were wide and asymmetric, demonstrating at least three O chemical states according to the binding energy range from 524.0 to 536.0 eV. The main contribution was attributed to Ti–O in TiO2. The other two kinds of oxygen contributions were assigned to the OH on the TiO2 surface [37–40] and Si–O in SiO2. The O1s XPS spectra were fitted to the three chemical states using Origin software with a Gaussian rule. Hydroxyl radical groups are powerful oxidants, leading to enhanced photocatalytic reactions, and are a significant factor for photocatalytic oxidation. Table 2 lists the results of curve fitting of XPS spectra for P25, CVC and CVC–SiO2 heat-treated at 500, 700 and 900 °C. It can be seen from Table 2 and Fig. 6 that the hydroxyl content of the prepared samples gradually decreased with increasing thermal treatment temperature, resulting in decreased photocatalytic activity. Yu et al. [38] suggested that the hydroxyl content of the surface-modified TiO2 nanoparticles prepared by liquid phase deposition decreased with increasing calcination temperature, which was due to the fact that there was a reaction on the surface of TiO2 films during the calcination process: Ti–OH + HO–Ti → Ti–O–Ti + H2O. They also mentioned that the TiO2 films easily adsorb water vapor in the air, leading to the formation of hydroxyl on the films. As listed in Table 2, the amount of Si–O in the prepared samples increased with the heat treatment. This was probably due to the fact that Si element on the surface of the heat treatment device diffused to the surface-modified TiO2 nanoparticles [38]. Although the thermal treatment temperature increased for the TiO2 nanocomposites with SiO2 added, there was no major difference in the hydroxyl content. As the thermal treatment temperature increased, the hydroxyl content was gradually reduced. But, in comparison with the unmodified P25 and CVC samples, the SiO2-modified CVC samples exhibited high anatase thermal stability, resulting in increased photocatalytic activity. The rate constants for the photocatalytic decomposition of each sample were evaluated by photocatalytic decolorization of MB aqueous solution, as shown in Fig. 7. It is generally known that the photocatalytic activity of the TiO2 sample is directly affected by the anatase content. In addition, it is commonly accepted that a smaller particle size corresponds to more powerful redox ability, because a smaller particle size corresponds to a larger band gap [28,41]. During the photocatalysis process, the separation and recombination of photo-induced charge carriers are competitive processes, and the photocatalytic reaction is effective only when photo-induced electrons and holes are separated [2]. As seen in Fig. 7, with increasing heat treatment temperature, the photocatalytic activities of the P25 and CVC samples suddenly dropped due to the increased particle size and the decreased anatase content. The 900P25 and 900CVC samples had no photocatalytic activities due to high particle sizes and low anatase contents. This suggests that the photocatalytic activities of the pure TiO2 samples can disappear after high-temperature heat treatment. The photocatalytic activities of the CVC samples were higher than those of the P25 samples at each heat treatment temperature due to smaller particle size and higher anatase content. In addition, the hydroxyl content could affect the photocatalytic

M. Kim et al. / Powder Technology 267 (2014) 153–160

activities of the P25 and CVC samples, respectively, as shown in Fig. 6 and Table 2. It was reported that a mixed oxide of the surface-modified TiO2 nanoparticles was a more efficient photocatalyst than TiO2 alone in aqueous solution. The increase in efficiency was attributed to the presence of an adsorbent (SiO2). The adsorbent phase increased the concentration of methyl orange or R-6G near the TiO2 sites relative to the solution concentration [38,42]. Yu et al. [38] reported that the increasing amount of SiO2 in the TiO2 film can form an adsorption center for methyl orange. The TiO2 behaves as a photoactive center, e.g., generating hydroxyl radicals under UV irradiation, while the SiO2 provides better adsorption sites in the vicinity of the TiO2. As seen in Fig. 7, the surface-modified TiO2 nanoparticles had high photocatalytic activities compared to the pure TiO2 samples at each heat treatment temperature. This suggests that the surface-modified TiO2 nanoparticles are thermally stable and have advantages in photocatalytic activity 4. Conclusions Surface-modified TiO2 nanoparticles were prepared by the CVC method. The as-prepared samples were subjected to post-heat treatment at various temperatures. The prepared samples listed in order of zeta potential are CVC–SiO2 N CVC N P25 at 900 °C of heat treatment temperature. The surface-modified TiO2 nanoparticles have thermal stability as well as dispersibility. This was attributed to enhanced repulsion forces caused by the surfaces being modified by SiO2. For higher heat treatment temperatures, the photocatalytic activities of the pure TiO2 samples suddenly decreased. The pure TiO2 samples have no photocatalytic activity after heat treatment at 900 °C. In aqueous solutions of the prepared samples, with the addition of SiO2, the photocatalytic activities were increased though high-temperature heat treatment. This was attributed to the increased surface area, maintained anatase content, and the presence of an adsorbent (SiO2). This study provides an alternative synthesis method for producing high-performance SiO2–TiO2 nanocomposites. The increased thermal stability and the dispersibility will expand the application areas of the material to areas such as nanocomposite coatings. Acknowledgments This work was supported by the National Research Foundation of Korea Grant funded by the Korean Government (Ministry of Science, ICT & Future Planning) (2014, University-institute cooperation program 2N38812), The Korea institute of Science and Technology (KIST) Institutional Program (2E24652). References [1] Michael R. Hoffmann, Scot T. Martin, Wonyong Choi, D.W. Bahnemannt, Environmental applications of semiconductor photocatalysis, Chem. Rev. 95 (1995) 69–96. [2] Guohui Tian, Fu. Honggang, Liqiang Jing, Baifu Xin, K. Pan, Preparation and characterization of stable biphase TiO2 photocatalyst with high crystallinity, large surface area, and enhanced photoactivity, J. Phys. Chem. C 112 (2008) 3083–3089. [3] D. He, F. Lin, Preparation and photocatalytic activity of anatase TiO2 nanocrystallites with high thermal stability, Mater. Lett. 61 (2007) 3385–3387. [4] Chuanhong Kang, Liqiang Jing, Tong Guo, Hucheng Cui, Jia Zhou, H. Fu, Mesoporous SiO2-modified nanocrystalline TiO2 with high anatase thermal stability and large surface area as efficient photocatalyst, J. Phys. Chem. C 113 (2009) 1006–1013. [5] S. Vargas, R. Arroyo, E. Haro, R. Rodrıguez, Effects of cationic dopants on the phase transition temperature of titania prepared by the sol–gel method, J. Mater. Res. 14 (1999) 3932–3937. [6] K.T. Ranjit, I. Willner, S.H. Bossmann, A.M. Braun, Lanthanide oxide-doped titanium dioxide photocatalysts: novel photocatalysts for the enhanced degradation of p-chlorophenoxyacetic acid, Environ. Sci. Technol. 35 (2001) 1544–1549. [7] D.J. Reidy, J.D. Holmes, C. Nagle, M.A. Morris, A highly thermally stable anatase phase prepared by doping with zirconia and silica coupled to a mesoporous type synthesis technique, J. Mater. Chem. 15 (2005) 3494. [8] K.V. Baiju, C.P. Sibu, K. Rajesh, P.K. Pillai, P. Mukundan, K.G.K. Warrier, W. Wunderlich, An aqueous sol–gel route to synthesize nanosized lanthana-doped titania having an increased anatase phase stability for photocatalytic application, Mater. Chem. Phys. 90 (2005) 123–127.

159

[9] Masanori Hirano, Keisuke Ota, H. Iwata, Direct formation of anatase (TiO2)/silica (SiO2) composite nanoparticles with high phase stability of 1300 °C from acidic solution by hydrolysis under hydrothermal condition, Chem. Mater. 16 (2004) 3725–3732. [10] C. Xie, Z. Xu, Q. Yang, B. Xue, Y. Du, J. Zhang, Enhanced photocatalytic activity of titania–silica mixed oxide prepared via basic hydrolyzation, Mater. Sci. Eng. B 112 (2004) 34–41. [11] Ying Li, C.-Y. Wu, Role of moisture in adsorption, photocatalytic oxidation, and reemission of elemental mercury on a SiO2–TiO2 nanocomposite, Environ. Sci. Technol. 40 (2006) 6444–6448. [12] K. Inumaru, T. Kasahara, M. Yasui, S. Yamanaka, Direct nanocomposite of crystalline TiO2 particles and mesoporous silica as a molecular selective and highly active photocatalyst, Chem. Commun. (2005) 2131–2133. [13] G. Calleja, D.P. Serrano, R. Sanz, P. Pizarro, Mesostructured SiO2-doped TiO2 with enhanced thermal stability prepared by a soft-templating sol–gel route, Microporous Mesoporous Mater. 111 (2008) 429–440. [14] Weiyang Dong, Yaojun Sun, Chul Wee Lee, Weiming Hua, Xinchun Lu, Yifeng Shi, Shicheng Zhang, Jiamin Chen, D. Zhao, Controllable and repeatable synthesis of thermally stable anatase nanocrystal-silica composites with highly ordered hexagonal mesostructures, J. Am. Chem. Soc. 129 (2007) 13894–13904. [15] S. Perathoner, P. Lanzafame, R. Passalacqua, G. Centi, R. Schlögl, D.S. Su, Use of mesoporous SBA-15 for nanostructuring titania for photocatalytic applications, Microporous Mesoporous Mater. 90 (2006) 347–361. [16] J. Yang, J. Zhang, L. Zhu, S. Chen, Y. Zhang, Y. Tang, Y. Zhu, Y. Li, Synthesis of nano titania particles embedded in mesoporous SBA-15: characterization and photocatalytic activity, J. Hazard. Mater. 137 (2006) 952–958. [17] S. Pazokifard, S.M. Mirabedini, M. Esfandeh, M. Mohseni, Z. Ranjbar, Silane grafting of TiO2 nanoparticles: dispersibility and photoactivity in aqueous solutions, Surf. Interface Anal. 44 (2012) 41–47. [18] Gang Li, Renvi Bai, X.S. Zhao, Coating of TiO2 thin films on the surface of SiO 2 microspheres: toward industrial photocatalysis, Ind. Eng. Chem. Res. 47 (2008) 8228–8232. [19] H. Tada, O. Nishio, N. Kubo, H. Matsui, M. Yoshihara, T. Kawahara, H. Fukui, S. Ito, Dispersion stability of TiO2 nanoparticles covered with SiOx monolayers in water, J. Colloid Interface Sci. 306 (2007) 274–280. [20] N. Arconada, A. Durán, S. Suárez, R. Portela, J.M. Coronado, B. Sánchez, Y. Castro, Synthesis and photocatalytic properties of dense and porous TiO2-anatase thin films prepared by sol–gel, Appl. Catal. B Environ. 86 (2009) 1–7. [21] R. Campostrini, M. Ischia, L. Palmisano, Pyrolysis study of sol–gel derived TiO2 powders part II. TiO2-anatase prepared by reacting titanium(IV) isopropoxide with oxalic acid, J. Therm. Anal. Calorim. 71 (2003) 1011–1021. [22] P. Eiamchai, P. Chindaudom, A. Pokaipisit, P. Limsuwan, A spectroscopic ellipsometry study of TiO2 thin films prepared by ion-assisted electron-beam evaporation, Curr. Appl. Phys. 9 (2009) 707–712. [23] G. Li Puma, A. Bono, D. Krishnaiah, J.G. Collin, Preparation of titanium dioxide photocatalyst loaded onto activated carbon support using chemical vapor deposition: a review paper, J. Hazard. Mater. 157 (2008) 209–219. [24] M. Rooth, R.A. Quinlan, E. Widenkvist, J. Lu, H. Grennberg, B.C. Holloway, A. Hårsta, U. Jansson, Atomic layer deposition of titanium dioxide nanostructures using carbon nanosheets as a template, J. Cryst. Growth 311 (2009) 373–377. [25] K. Byrappa, T. Adschiri, Hydrothermal technology for nanotechnology, Prog. Cryst. Growth Charact. Mater. 53 (2007) 117–166. [26] S. Chin, E. Park, M. Kim, J. Jurng, Photocatalytic degradation of methylene blue with TiO2 nanoparticles prepared by a thermal decomposition process, Powder Technol. 201 (2010) 171–176. [27] S. Chin, E. Park, M. Kim, J. Jeong, G.-N. Bae, J. Jurng, Preparation of TiO2 ultrafine nanopowder with large surface area and its photocatalytic activity for gaseous nitrogen oxides, Powder Technol. 206 (2011) 306–311. [28] S. Chin, E. Park, M. Kim, G.N. Bae, J. Jurng, Synthesis and photocatalytic activity of TiO2 nanoparticles prepared by chemical vapor condensation method with different precursor concentration and residence time, J. Colloid Interface Sci. 362 (2011) 470–476. [29] R.A. Spurr, H. Myers, Quantitative analysis of anatase-rutile mixtures with an X-ray diffractometer, Anal. Chem. 29 (1957) 760–762. [30] K. Ravindranathan Thampi, P. Ruterana, M. Gratzel, Low-temperature thermal and photoactivation of TiO2-supported Ru, Rh, and Cu catalysts for CO–CO reaction, J. Catal. 126 (1990) 572–590. [31] K. Mogyorósi, N. Balázs, D.F. Srankó, E. Tombácz, I. Dékány, A. Oszkó, P. Sipos, A. Dombi, The effect of particle shape on the activity of nanocrystalline TiO2 photocatalysts in phenol decomposition. Part 3: the importance of surface quality, Appl. Catal. B Environ. 96 (2010) 577–585. [32] X. Wang, S.O. Pehkonen, A.K. Ray, Photocatalytic reduction of Hg(II) on two commercial TiO2 catalysts, Electrochim. Acta 49 (2004) 1435–1444. [33] S. Veeramasuneni, M.R. Yalamanchili, j.D. Miller, Interactions between dissimilar surfaces in high ionic strength solutions as determined by atomic force microscopy, Colloids Surf. A Physicochem. Eng. Asp. 131 (1998) 77–87. [34] Kavitha Subramaniam, Sotira Yiacoumi, C. Tsouris, Copper uptake by inorganic particles — equilibrium, kinetics, and particle interactions: experimental, Colloids Surf. A Physicochem. Eng. Asp. 177 (2001) 133–146. [35] I.A. Siddiquey, T. Furusawa, M. Sato, K. Honda, N. Suzuki, Control of the photocatalytic activity of TiO2 nanoparticles by silica coating with polydiethoxysiloxane, Dyes Pigments 76 (2008) 754–759. [36] S. Farrokhpay, A review of polymeric dispersant stabilisation of titania pigment, Adv. Colloid Interf. Sci. 151 (2009) 24–32. [37] Liqiang Jing, Baifu Xin, Fulong Yuan, Lianpeng Xue, Baiqi Wang, H. Fu, Effects of surface oxygen vacancies on photophysical and photochemical processes of

160

M. Kim et al. / Powder Technology 267 (2014) 153–160

Zn-doped TiO2 nanoparticles and their relationships, J. Phys. Chem. B 110 (2006) 17860–17865. [38] Yu. Jia-Guo, Yu. Huo-Gen, Bei Cheng, Xiu-Jian Zhao, Jimmy C. Yu, W.-K. Ho, The effect of calcination temperature on the surface microstructure and photocatalytic activity of TiO2 thin films prepared by liquid phase deposition, J. Phys. Chem. B 107 (2003) 13871–13879. [39] X. Qin, L. Jing, G. Tian, Y. Qu, Y. Feng, Enhanced photocatalytic activity for degrading Rhodamine B solution of commercial Degussa P25 TiO2 and its mechanisms, J. Hazard. Mater. 172 (2009) 1168–1174.

[40] J. Yu, H. Yu, B. Cheng, M. Zhou, X. Zhao, Enhanced photocatalytic activity of TiO2 powder (P25) by hydrothermal treatment, J. Mol. Catal. A Chem. 253 (2006) 112–118. [41] D.S. Kim, S.J. Han, S.Y. Kwak, Synthesis and photocatalytic activity of mesoporous TiO(2) with the surface area, crystallite size, and pore size, J. Colloid Interface Sci. 316 (2007) 85–91. [42] A.J. Carl Anderson, Bard, improved photocatalytic activity and characterization of mixed TiO2/SiO2 and TiO2/Al2O3 materials, J. Phys. Chem. B 101 (1997) 2611–2616.