SiO2 Nanocomposite

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One-Step Fabrication of Thermally Stable TiO2/SiO2 Nanocomposite Microspheres by Evaporation-Induced Self-Assembly J. Bahadur,*,† D. Sen,† S. Mazumder,† P. U. Sastry,† B. Paul,‡ H. Bhatt,§ and S. G. Singh∥ †

Solid State Physics Division, ‡Materials Processing Division, §High Pressure & Synchrotron Radiation Physics Division, and Technical Physics Division, Bhabha Atomic Research Centre, Mumbai 400085, India

∥

ABSTRACT: The evaporation-induced self-assembly of mixed colloids has been employed to synthesize microspheres of TiO2/SiO2 nanocomposites. Small-angle neutron/X-ray scattering and scanning electron microscopy experiments reveal the hierarchical morphology of the microspheres. Although the internal structure of the microspheres, consisting of solely silica nanoparticles, gets significantly modified with time because of the reduction in the high specific surface area by internal coalescence, the same for the composite microspheres remains stable over an aging time of 1 year. Such temporal stability of the composite microspheres is attributed to the inhibition of coalescence of the silica nanoparticles in the presence of titania nanoparticles. X-ray diffraction and thermogravimetric results show the improved thermal stability of the composite grains against the anatase-to-rutile phase transition. Such thermal stability is attributed to the suppression of the growth of titania nanoparticles in the presence of silica nanoparticles. The UV−vis results indicate the confinement effect of the TiO2 nanoparticles in the silica matrix. A plausible mechanism has been elucidated for the formation of microspheres with different morphology during self-assembly.

1. INTRODUCTION Nanostructured titania (TiO2) is used in a wide range of applications, such as photocatalysis, separations, sensor devices, paints, and dye-sensitized solar cells.1,2 In addition, it is an efficient photocatalyst for the detoxication of air and water pollutants and thus has received much research attention during the past two decades. 3 The properties of TiO2 nanoparticles (NPs) depend on their crystal structure, size, and morphology.4−6 For practical applications, it is important to design a TiO2-based functional material having improved efficiency as far as its photocatalytic properties are concerned. TiO2 exists in three main phases: anatase, brookite, and rutile. It is reported that the anatase form has a higher photoactivity than those in the rutile form.7−10 Under ambient conditions, macrocrystalline rutile is thermodynamically stable in comparison to macrocrystalline anatase or brookite.11 However, the thermodynamic stability depends on the particle size. For particle diameter below ∼14 nm, the anatase phase is more stable than the rutile phase.11 If TiO2 nanocrystals are heated, then crystal growth leads to the alteration of phase stabilities, and ultimately, the conversion of anatase to rutile takes place. Such polymorphic transformation of the anatase phase to the rutile phase is considered to be one of the drawbacks that limits the photocatalytic activity. Another drawback is that bulk TiO2 with a high photocatalytic activity usually has a relatively low surface area and pore volume, leading to its low adsorption © 2012 American Chemical Society

capability for organic pollutants. To overcome this problem, two strategies have been developed: one is the synthesis of mesoporous TiO2 with a higher specific surface area12,13 and the other is the combination of TiO2 with absorbent.14,15 As an absorbent, silica (SiO2) is a potential candidate because of the ease of synthesizing it with a large specific surface area and pore volume. Furthermore, it has no absorption in the UV range. To get a TiO2-based photocatalyst having improved properties, a method is desired to develop meso/macroporous TiO2/SiO2 micrometric grains. In the past, mesoporous TiO2 composites have been synthesized by the sol−gel method.16 However, this method results in an inhomogeneous phase distribution due to macroscopic reactions, leading to nonoptimal performance. Furthermore, this method generally needs a relatively long time for both reaction and posttreatment such as in surfactant/solvent removal and calcination. Therefore, it is essential to develop a process for the rapid and controlled preparation of (TiO2/SiO2)-based nanocomposites. Evaporation-driven self-assembly17−22 is a relatively new technique for synthesizing TiO2/SiO2 composites. The meso/ macroporous micrometric grains can be prepared from the spray drying of micrometric droplets having nanosized TiO2 Received: June 6, 2012 Revised: July 13, 2012 Published: July 15, 2012 11343

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Figure 1. Micrographs of assembled grains consisting of (a) only SiO2 NPs, with the inset showing the cross-sectional view of the hollow grains and (b) only TiO2 NPs, with the inset showing doughnut-type grains.

2. EXPERIMENTAL SECTION

and SiO2 colloids. The grain size, composition, morphology, and hence the catalytic performance can be tailored by manipulating precursor properties as well as process parameters. TiO2 nanocrystals as the effective photocatalysts were efficiently utilized by selectively distributing them in the SiO2 matrix by the evaporation-driven self-assembly of these nanocolloids inside the micrometer-sized droplets, resulting in high photocatalytic efficiency. During the drying of a colloidal droplet, various physicomechanical processes take place, which in turn decide the morphology of the resulting assembled grain.23−26 A dimensionless parameter, the Peclet number (Pe), can be defined to demarcate two drying regimes. The Peclet number Pe is defined as the ratio of mixing time of the NPs in the droplet (τmix = R2/D, where R is the droplet radius and D is the diffusion coefficient of the NPs in the droplet) to the drying time of the droplet (τdry). If the Peclet number Pe ≫1, then the drying process is regarded as fast and there is the possibility of forming hollow or doughnut/crumpled grains. However, if Pe ≪ 1, then the drying process is regarded as slow and the droplet shrinks isotropically throughout the drying process, resulting in uniformly jammed spherical grains. Recently, spray drying experiments have been carried out in both regimes, slow and fast, which led to spherical27 and doughnut28−30 grains, respectively. It has been shown recently that tailored morphologies such as the buckball-type structure31 and porous microcapsules can be achieved by modifying the physicochemical parameters during drying.32 However, the fabrication of complex composite assemblies from drops containing a mixture of two or more different types of colloids is not well studied, in particular, for the case of colloids consisting of two different species with different sizes and polydispersities of the primary particles. In the present work, mesoporous TiO2/SiO2 micrometric grains have been synthesized by the evaporation-induced selfassembly of the mixed SiO2 and TiO2 colloids in order to achieve an improved phase stability for TiO2. The mesoscopic characterization has been carried out by scanning electron microscopy and small-angle neutron/X-ray scattering. The phase stability of the TiO2 nanocrystals in the amorphous SiO2 matrix has been probed by X-ray diffraction. Thermogravimetric analysis and differential thermal analysis have also been performed on the composites to follow the weight loss and possible phase transitions. Infrared spectroscopy has been carried out to investigate the functional groups in the composites. The confinement effect of the TiO2 nanocrystals in the amorphous matrix has been investigated by UV−vis spectroscopy.

The SiO2 and TiO2 dispersions were obtained from Sigma-Aldrich. The zeta potential for HS-40 LUDOX dispersion was found to be ∼−48 mV. The pH values of all of the dispersions were nearly same (∼9.5). The typical density of the silica NPs is ∼2.0 g/cm3, and that of the TiO2 NPs is ∼4.0 g/cm3. Spray-drying experiments have been performed on 2 wt % SiO2 and 2 wt % TiO2 dispersions using an LU 228 spray dryer33 having two fluid atomizer nozzles. The SiO2 and TiO2 dispersions were mixed in ratios of 0.5, 1.0, and 2.0 wt %, keeping the overall solid content constant. It is important to mention at this juncture that the ratio of the number of SiO2 particles to the number of TiO2 particles is a more important parameter than the ratio of the weight fraction of SiO2 and to that of TiO2 during the assembly process of these mixed colloids. The volume fraction (φv) of the colloids may be estimated from weight fractions (φm) of the colloids as follows φv = φm/d, where d is the density of the colloid. For 2 wt % SiO2 (i.e., φm = 0.02), (φv)SiO2 is estimated to be 0.01. For 2 wt % TiO2 (i.e., φm = 0.02), (φv)TiO2 is estimated to be 0.005. Thus, for equal weight fractions of the silica and titania colloids, the ratio of the volume fraction of silica to titania is (φv)SiO2/(φv)TiO2 = 2. Thus, a proper scaling can be applied to obtain the ratio of the volume fractions of the colloids in place of their weight fractions. The mixed dispersions have been found to be stable during spray drying. The atomization pressure for the droplet generation was kept at 2.0 kg/cm2, and inlet temperature during drying was fixed at 170 °C. The solution feed rate was maintained at 2 mL/min, and the aspiration rate was fixed at 50 m3/h. Poly(ethylene glycol) (PEG) with an average molecular weight of 1000 was obtained from Sigma-Aldrich. A different concentration of PEG was mixed into the mixed dispersion of SiO2 and TiO2 in order to modify the morphology of the assembled grain. The evaporation-driven assembly of SiO2 and TiO2 and its mixed dispersion has been performed by using a spray-drying technique. The specimens obtained from the spray drying of 2 wt % SiO2 and TiO2 colloids were named SI and TI, respectively. The specimens obtained after the spray drying of mixed dispersions having TiO2/SiO2 weight ratios of 2.0, 1.0, and 0.5 were named TiSi2, TiSi1, and TiSipt5, respectively. The specimens obtained after the drying of mixed dispersions of TiO2, SiO2, and PEG in weight ratios of 1.0/1.0/ 0.5, 1.0/1.0/1.0, and 2.0/1.0/1.0 were designated as 1Ti1Si0.5PEG, 1Ti1Si1PEG, and 2Ti1Si1PEG, respectively. Mesoscopic characterization of the samples has been carried out by scanning electron microscopy (SEM). The virgin dispersions of SiO2 and TiO2 have been characterized by small-angle X-ray scattering (SAXS) in the q range of 0.1 to 2 nm−1. Preliminary measurements with medium-resolution small-angle neutron scattering (MSANS)34 on freshly prepared composite grains have indicated two levels of structural hierarchy in the grains. Further SANS experiments have been performed on KWS135 and KWS336 instruments at the JCNS facility at high-flux reactor FRM-II, Germany in order to collect data over a wide scattering-vector (q) range with improved statistics as well as resolution. The SAXS experiments have also been performed on a 11344

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Figure 2. (a) Micrographs of assembled grains consisting of mixed colloids in a weight ratio of TiO2/SiO2 = 2.0. The inset shows the edge of a magnified grain, and the rough surface of the grains indicates the presence of larger NPs on the surface. (b) TiO2/SiO2 = 1.0. (c) TiO2/SiO2 = 0.5.

Figure 3. Micrograph of assembled grains consisting of mixed colloids in weight ratios of (a) 1.0: 1.0: 0.5 TiO2/SiO2/PEG, (b) 1.0:1.0:1.0 TiO2/ SiO2/PEG, and (c) 2.0:1.0:1.0 TiO2/SiO2/PEG. freshly prepared specimen of SiO2 and aged specimens of SiO2 and TiO2 and their composite grains. The thermal stability of the assembled grains has been monitored by thermogravimetric analysis (TGA) and differential thermal analysis (DTA). The DTA data were recorded in air under ambient conditions at a uniform heating/cooling rate of 5 °C min−1 up to a maximum temperature of 1000 °C in each experiment. The SETARAM TG-92 system was employed for this purpose. The TGA signal was recorded using a microgram scale having 0.1 μg as the smallest value for the same temperature range. The microscopic characterization of the composite grains has been performed using a diffractometer using a Cu Kα source in θ−θ geometry with a 2°/min scanning speed. IR spectroscopy has been performed on assembled grains in order to probe the nature of functional groups in composite grains. Infrared spectra of the powders were recorded in the spectral range of 400−4000 cm−1 using a Bruker Vertex 80 V FTIR spectrometer under vacuum conditions at an apodized resolution of 2 cm−1. The spectrometer was configured with a globar source, a KBr beamsplitter, and a deuterated triglycine sulfate (DTGS-mid IR) detector. For the recording of the infrared spectrum, powders have been pelletized with samples dispersed in a KBr matrix. A background spectrum using a bare KBr pellet has been divided in each case to obtain the absorbance spectrum, denuded from instrumental and environmental profiles. UV−visible spectroscopy has been performed on the grains in transmission mode by dispersing powders in toluene.

grains with large shell thicknesses are evident from the fractured grains. It is discernible from the micrograph (Figure 1b) that TiO2 grains are somewhat doughnut-shaped. The inset of Figure 1b shows one of the representative doughnut grains. At this juncture, it is interesting that the SEM micrograph is a projection of 3D objects, which may result in insufficient information, in particular, about anisotropic objects. For example, in Figure 1b, some of the grains appear to be spherical, which may not be the actual morphology. Micrographs of grains obtained for a mixed dispersion of TiO2 and SiO2 are shown in Figure 2. The grains obtained from a mixed dispersion of TiO2 and SiO2 possess spherical morphology, as is observed from Figure.2. Micrographs obtained for the mixed dispersion of TiO2, SiO2, and PEG, are depicted in Figure 3. It is also evident from Figure 3 that the morphology of the grains, obtained from the PEG-modified dispersion, remains spherical for 1Ti1Sipt5PEG and 1Ti1Si1PEG specimens. However, a morphological transformation is clearly observed for 2Ti1Si1PEG specimens. 3.2. Charterization of Dispersions. To estimate the size of SiO2 and TiO2 NPs in the dispersions, SAXS experiments have been performed individually on 2 wt % SiO2 and TiO2 dispersions (Figure 4). SAXS experiments have also been performed on mixed dispersions. It is observed from scattering experiments that the interaction between SiO2 and TiO2 NPs in the mixed dispersion remains insignificant. It is apparent from Figure 4 that the SAXS profile of the mixed dispersion is mimicked by adding individual SAXS profiles of SiO2 and TiO2 NPs with appropriate weighting factor without invoking any extra structure factor.

3. RESULTS AND DISCUSSION 3.1. Morphology of the Grains: Electron Microscopy. The SEM measurements reveal the overall morphology of the grains for different specimens. The micrograph of SI and TI grains are depicted in Figure 1a,b, respectively. It is observed from SEM micrographs (Figure 1a) that grains of the SI specimen possess spherical morphology. Hollow 11345

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SiO2 NPs is relatively narrow and does not possess a broad tail, in contrast to the size distribution of TiO2 NPs. 3.3. SAXS/SANS Characterization of Self-Assembled Grains. To obtain information on the internal morphology of the grains, SANS/SAXS experiments have been carried on the assembled grains. The SANS profiles for different specimens are shown in Figure 6. The inset of Figure 6 depicts the scattering profiles in the Porod representations (I(q)q4 vs q).

Figure 4. SAXS profiles of the SiO2 and TiO2 dispersion. SAXS profiles of the mixed dispersion having SiO2 and TiO2 in equal proportions. The lines represent the model fit to the data.

The SAXS profiles for the dispersion have been analyzed by assuming an ensemble of polydisperse spheres. The scattering intensity for an ensemble of polydisperse spheres may be written as37 2

I(q) = n(ρp − ρs )

∫ P(q , r) D(r) v(r)

2

dr

Figure 6. SANS profiles of the assembled composite grains. The inset shows the profiles in the Porod representation.

(1)

where n is the number density of NPs and ρp and ρs are the electron densities of NPs and solvent, respectively. P(q, r) is the spherical form factor, and D(r) is assumed to be the log-normal size distribution. The volume-weighted radius distributions of NPs in 2 wt % TiO2 and SiO2 dispersions are shown in Figure 5.

It is evident from the figure that the assembled grains possess hierarchical length scale structure. The existence of two Porod levels is evident in the inset of Figure 6. The first Porod level corresponds to the surface area of the overall grains, and the second Porod level corresponds to the surface area of the interstices between the jammed NPs. The low-q part of each SANS profile has also been analyzed in light of the Guinier approximation in order to have some idea of the assembled grain size of different specimens. To see the variation in the correlation peak due to the jammed SiO2 NPs, SANS data of the composite grains have also been depicted in the enlarged q range of 0.1−0.9 nm−1 (Figure 7). The correlation of the SiO2 NPs in the assembled grains is manifested in the scattering profile at q ≈ 0.45 nm−1 as the peak. Because of the narrow polydispersity of the SiO2 NPs, the oscillations in the form factor are also visible in the scattering

Figure 5. Bimodal size distribution of the TiO2 NPs. The inset shows the SiO2 radius distribution.

It is observed that TiO2 NPs possess a bimodal-type size distribution. The average radius of the smaller TiO2 NPs has been found to be 3.1 nm with a polydispersity of 0.8 nm. Similarly, the average radius of the larger NPs is 8.5 nm with a 4.5 nm polydispersity. The SiO2 NP size distribution has also been estimated by fitting the SAXS profile using eq 1. The radius distribution of the SiO2 NPs is depicted in the inset of Figure 5. The average radius of the SiO2 NPs is found to be 8.2 nm with a polydispersity of 1.0 nm. It important to mention here that the size distribution of the TiO2 NPs possesses an extensive tail at higher radius. However, the size distribution of

Figure 7. Magnified view of the SANS profiles of the assembled composite grains. 11346

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profile of the SiO2 grains. It is also evident that the scattering profile of the TI specimens does not show any prominent peak because of the significant polydisperisty in the size of the TiO2 NPs. The correlation peak corresponding to SiO2 NPs gets broadened for composite grains with the addition of TiO2 NPs. The broadening of the correlation peak is related to the volume fraction of the SiO2 NPs in composite grains. The greater the broadening of the peak, the lower the packing fraction. This observation can be understood in terms of the homogeneous mixing of the TiO2 and SiO2 NPs. The local packing fraction of SiO2 NPs is reduced by the presence of TiO2 NPs in the vicinity, which is also reflected in the slight shift in the correlation peak toward low q. The broadening of the correlation peak increases with the increase in the concentration of TiO2 NPs. The SAXS profiles for the freshly prepared SiO2 grains (SI_0) and aged SiO2 grains (SI_1year) and TiO2 and its composite grains have been depicted in Figure 8 in the Porod representation.

Table 1. Parameters Obtained from SANS/SAXS Analysis sample SI_t = 0 SI_t = 1 year TI TISIpt5 TISI1 TISI2

specific surface area (m2/g)

silica NP correlation peak position (nm−1)

Guinier radius of the grains (nm)

118.8 62.6

0.445 0.420

281 ± 6

33.8 85.7 70.0 52.7

0.399 0.399 0.399

340 258 316 293

± ± ± ±

7 7 11 7

reduced as compared to that for the fresh specimen. This gives crucial information regarding the stability of the assembled grains of SiO2 NPs. This observation indicates that smaller SiO2 NPs coalesced into larger NPs under ambient condition over a long period of time in order to reduce the specific surface area. From Table 1, it is evident that the specific surface area of the TiO2/SiO2 composite grains lies between the specific surface area of the TI and SI_t = 0 specimens. It is interesting that the surface area of the TiSipt5 and TiSi1 specimens is higher than that of the aged SI specimen. This indicates that the aging effect (i.e., coalescence of the SiO2 NPs) on the composite grain is not significant. The coalescence of the SiO2 NPs is inhibited by the presence of TiO2 NPs. The SANS data, collected for freshly prepared SI and TI specimens, have been compared to the corresponding SAXS data of the aged specimens (Figure 9).

Figure 8. SAXS profiles of the assembled grains in the Porod representation. The inset shows a magnified view of the correlation peak.

The silica correlation peak due to local SiO2 NPs in the assembled grains is shown in the inset of Figure 8. The observations from the SAXS results corroborate with the SANS results discussed above. The SAXS profiles have been analyzed to get the specific surface area. The expression for the specific surface area may be written as37

Figure 9. SANS/SAXS profiles of the self-assembled grains of TiO2 and SiO2. Scattering profiles of assembled grains at two different times to compare the effect of aging on the grains.

lim (I(q)q 4)

Sv =

SANS profiles have been scaled in order to match the SAXS profiles. It is evident from the figure that the corresponding profiles of the fresh and aged TI specimens match reasonably well in the overlapping q regime. This indicates that there is no aging effect on the TiO2 grains. The temporal stability of the TiO2 grains may be attributed to the small specific surface area of the grains. Furthermore, SAXS data of the aged SI specimens do not match the SANS/SAXS profile of freshly prepared SI specimens. However, SAXS and SANS profiles of freshly prepared SI specimens match quite well in overlapping q regimes. These observations may be understood in terms of the aging effect of the SI grains due to the high specific surface area of the SI grains as discussed above. The SANS profiles of fresh composite grains have also been compared to the corresponding SAXS profiles of aged composite grains. The SANS profiles

q →∞

2

2π (Δρ) d

(2) 4

where limq→∞(I(q)q ) indicates the Porod level where the scattering intensity follows q−4 behavior. Δρ is the electron density difference between assembled NPs and air, and d is the density of the composites. It is important to note that the electron density for TiO2 is approximately 2 times that of SiO2. This results in a higher Porod level for TI grains as compared to that for SI grains. However, the specific surface area of SI specimens is higher than that of TI grains as estimated (Table 1) using eq 2. SAXS experiments on the freshly prepared SI specimen (SI_t = 0) and 1-year-aged SI specimens (SI_t = 1 year) reveal that the specific surface area for the aged SI specimen is significantly 11347

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be related to some crystalline phase transition of TiO2. However, no exothermic peak is observed for TiO2/SiO2 composite grains. To confirm the above results, structural characterization has been carried out by X-ray diffraction and will be discussed in the following paragraphs. The X-ray diffraction patterns for TiO2 assembled grains at room temperature are shown in Figure 12.

have been scaled in order to match the SAXS profiles (Figure.10).

Figure 10. SANS/SAXS profiles for the TiO2/SiO2 composites.

It has been observed that the profiles do not match at lower q in the overlapping q regime. The difference in SANS and SAXS profiles of the composite grains is due to a sharp difference in the contrast (Δρ)2 of SiO2 and TiO2 for neutrons and X-rays. The ratio of the contrast factor for SiO2 and TiO2 for neutrons is approximately 1.50. The same ratio for SiO2 to TiO2 in the case of X-rays is approximately 0.25. This results in a difference in the functionality of scattering profiles of composite grains with respect to that of X-rays and neutrons. 3.4. Thermal Stability of the Self-Assembled Grains. To probe the thermal stability of the grains, thermogravimetric and XRD experiments have been carried out. The thermogravimetric results of the composite grains have been depicted in Figure 11. The weight loss curves of the grains show that there is no significant weight change up to 1000 °C in all cases. The slight weight loss (∼2 wt %) is due to the evaporation of physically absorbed water from the assembled grains during heat treatment. The heat flow curves for all of the composite grains are mostly identical except for the presence of an additional small exothermic peak for TI specimens at ∼750 °C, and it may

Figure 12. X-ray diffraction pattern of the a- synthesized assembled TiO2 grains. The vertical lines are the peak positions of the anatase and rutile phases of TiO2.

The observed diffraction profiles have been compared to the anatase and rutile phases of TiO2 corresponding to JCPDS card nos. 21-1272 and 21-1276, respectively. It is observed that the TiO2 NPs exist in a mixed phase composed of both anatase and rutile phases. Diffraction experiments have also been performed on the composite grains (Figure.13). The diffraction profiles of the composite grains have been found to be additive as far as the diffraction profiles of TiO2 and amorphous SiO2 are concerned with the appropriate weight factor. The temperature-dependent X-ray diffraction experiments have also been performed on the TI and TiSipt5 grains. Hightemperature XRD on TI specimen reveals an anatase-to-rutile phase transformation below 850 °C (Figure 14). It shows that the assembled grains of only TiO2 NPs are not thermally stable as far as their crystalline phase is concerned.

Figure 11. Heat flow and weight loss curves for composite grains with different TiO2/SiO2 ratios. 11348

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The weight fractions of anatase (WA) and rutile (WR) in both specimens were calculated from the Spurr equations38 1 WA = I 1 + 1.26 IR

()

(3)

A

1

WR =

()

1 + 0.8

IA IR

(4)

where IA and IR are the intensities of peaks corresponding to the (101) plane of the anatase phase and the (110) plane of the rutile phase, respectively. The fractions of anatase and rutile phases, at different temperatures, have been reported in Tables 2 and 3 for both specimens. The mean crystallite size for Table 2. Parameters Obtained from the Analysis of XRD Patterns for TI Specimens

Figure 13. X-ray diffraction profiles of the specimens having SiO2, TiO2, and PEG in different concentrations.

weight percent temperature (°C)

anatase

rutile

27 400 600 850

85.2 84.5 83.9 92.6

14.8 15.4 16.1 7.4

average anatase crystalline size (nm)

average rutile crystallite size (nm)

19.4 19.3 19.0

23.5 23.1 22.7 21.5

Table 3. Parameters Obtained from the Analysis of XRD Patterns for TISIpt5 Specimens weight percent

Figure 14. X-ray diffraction pattern of the TI specimens at different temperatures.

The XRD profiles of TiSipt5 specimens at different temperatures (Figure 15) show an interesting phenomenon. An anatase-to-rutile phase transformation is not observed below 850 °C, in contrast to that for only the TI case.

temperature (°C)

anatase

rutile

average anatase crystalline size (nm)

average rutile crystallite size (nm)

27 200 400 600 700 850

82.4 84.0 83.0 83.4 83.1 81.2

17.6 16.0 17.0 16.6 16.9 18.8

18.6 19.7 20.2 18.5 17.7 16.3

26.7 24.6 25.3 22.4 24.0 23.0

anatase and rutile phases was calculated from the broadening of the X-ray reflection (101) crystallographic plane for anatase and (110) crystallographic plane for rutile using the Scherrer formula39 Dp =

kλ β cos(θ)

(5)

where Dp is the linear dimension of a particle, k is the Scherrer constant (0.9), λ is the wavelength of the incident X-rays, θ is the Bragg’s angle, and β is the line broadening (fwhm). It is evident from the above tables that the average weight fractions of anatase and rutile for TI specimens are approximately 85 and 15%, respectively. The crystallite size estimated from the XRD analysis shows that the average sizes of anatase and rutile crystallites are typically 19 and 24 nm, respectively. It is interesting that the size distribution of TiO2 NPs has a tail toward higher size. The average size of the NPs is ∼16 nm, which is closer to the anatase crystallite size. However, the tail part of the size distribution contains larger particles with a typical size of 25 nm. Thus, the tail part of the distribution (higher size) possesses rutile NPs in contrast to average NPs, which are in the anatase phase.

Figure 15. X-ray diffraction pattern of the TiSipt5 specimens at different temperatures. The * marked peaks originate from the Pt sample holder and should not be considered in the present perspective. 11349

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3.5. Infrared Spectroscopy of Composite Grains. To understand the nature of the SiO2/TiO2 composite grains, IR experiments have been performed. IR spectra of the grains, in the range of 400−1500 cm−1, are shown in Figure 16.

Figure 18. UV−visible spectra of the composites.

The band gap of TiO2 has been estimated for various specimens. The band gap energy for TI specimens is found to be 3.2 eV, which is closer to that of the bulk anatase phase.42 The band gap for the rutile phase is typically 3.0 eV.43 It is observed that absorption peaks in the spectra are quite broad because of the higher polydispersity of the NPs in addition to the phase polydispersity. A blue shift is observed in the TiO2 band gap for TiO2/SiO2 composites. The blue shift in the absorption spectra may be attributed to the confinement of TiO2 NPs in the amorphous silica matrix. The band gap energies for TiSi1 and TiSi2 are calculated to be 3.7and 3.5 eV, respectively. 3.7. Self-Assembly Mechanism. In the following section, the mechanism of the formation of such a novel composite via EISA will be explained. The Peclet number Pe (= R2/Dτdry) for the present drying condition and for the 2 wt % SiO2 dispersion can be estimated as follows.27 Diffusion coefficient (D) may be calculated from the Einstein−Stokes relation, D = kBT/6πηr, where kB is the Boltzmann constant, η is the viscosity, and r represents the radius of the colloids. For T = 40 °C, r = 8 nm (for the silica colloids as observed from SAXS), and η = 6.53 × 10−4 Pa s (assuming the viscosity of pure water at 313 K), D works out to be 4.386 × 10−11 m2/s. The drying time (τ) can be calculated from the tube geometry (20 cm diameter, 60 cm length) and the gas flow rate (50 m3/h). In the present case, the velocity of the droplets was calculated to be ∼0.442 m/s (from the ratio of the volume of gas entering the tube per second and the cross section of the drying tube), and hence the drying time in a 60cm-long drying chamber is ∼1.3 s. From these data, Pe becomes equal to 3.6 (for a 15 μm radius of the initial liquid droplets). This suggests that the present drying condition is in a relatively fast drying regime. It is to be noted that all of the physical conditions were kept invariant during the drying of TiO2 and mixed dispersions. The Peclet number for the drying of a 2 wt % TiO2 dispersion remains nearly the same because the average sizes of TiO2 and SiO2 NPs are comparable. However, it is important to mention here that the Peclet number is inversely proportional to the diffusion coefficient of the NPs inside the drying droplet. Thus, the Peclet number for larger TiO2 NPs that falls in the tail of the size distribution turns out to be higher than for the smaller NPs. The formation of a shell of jammed NPs, during the drying of a droplet, plays an important role in deciding the final morphology of self-assembled grains. For the slow drying regime, the mixing time τmix of the NPs is shorter than the drying time τdry of the droplet. The NPs diffuse in the

Figure 16. IR spectra of the composite grains.

This gives information about functional groups arising from both SiO2 and TiO2. Interestingly, a Ti−O−Si mode at approximately 970 cm−140,41 is also observed, in addition to the individual functional groups of SiO2 and TiO2. The relative contribution of Ti−O−Si increases with increasing fraction of TiO2 in the composite. This is attributed to the interaction of unsaturated bonds at the surfaces of SiO2 and TiO2 NPs. The presence of the Ti−O−Si mode also gives an idea of the jamming behavior of the SiO2 and TiO2 NPs. The SiO2 and TiO2 NPs become neighbors after self-assembly/jamming and interaction among the unsaturated groups at the surfaces of the SiO2 and TiO2 NPs come into the picture. The IR modes in the range of 1500 to 3900 cm−1 are shown in Figure 17.

Figure 17. IR spectra of the composite grains.

Broad bending and stretching modes of OH groups appear at 1635 and 3600 cm−1, respectively, in addition to that for the Ti−O mode. These modes arise from the presence of the bound/free water and the OH groups at the surface of the NPs. 3.6. Confinement Effect of TiO2: UV−Visible Spectroscopy. To probe the confinement of the TiO2 nanoparticle in the SiO2 matrix, UV−vis measurements have been performed on the grains. UV−vis absorption spectra of the TiO2 and TiO2/SiO2 grains are shown in Figure 18. 11350

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The effective NP size distribution becomes narrower as compared to that of only the TiO2 dispersion. The relative shell thickness formed during the drying of mixed colloidal droplets will be higher. Thus, droplets of mixed dispersions do not buckle during drying and lead to the spherical morphology of the grains. The addition of PEG in a small weight fraction does not affect the grain morphology as in the case of the 1Ti1Si0.5PEG and 1Ti1Si1PEG specimens. However, the morphology of the grain for 2Ti1Si1PEG specimens gets significantly modified. It is important that an addition of PEG to a TiO2 dispersion alone leads to sticky grains during the drying experiment and the grains do not remain free-flowing. However, the addition of PEG to SiO2 dispersions leads to dried grains. It may be inferred from the above observation that the PEG molecule does not interact significantly with TiO2 NPs, which may lead to free PEG, which is quite viscous. However, the PEG molecule interacts significantly with SiO2 NPs, as is also shown in our previous work.31 Thus, the PEG molecules when added to a mixed dispersion of SiO2 and TiO2 gets preferentially absorbed onto the SiO2 NPs. Thus, in this case, free PEG molecules do not exist and resulting grains are not sticky. For 1Ti1Si0.5PEG and 1Ti1Si1PEG dispersions, there are a number of SiO2 NPs that are large enough to absorb the added PEG molecules. It has been reported that the role of free PEG during the drying of a droplet plays an important role. Buckling of the drying droplet containing NPs and free PEG may lead to anisotropically deformed grains.31 In the case of a 2TI1SI1PEG dispersion, the number of SiO2 NPs is not large enough to absorb all of the PEG molecules and may lead to some amount of free PEG. In addition to the presence of free PEG, a higher relative weight fraction of TiO2 in the dispersion is also responsible for the origin of deformed grains. It is to be noted that because of the above-mentioned EISA process for the stable mixed dispersion of SiO2 and TiO2 of comparable sizes, uniform composite grains have been fabricated except for the surface layer of the grains. On the surface layer of the grains, larger TiO2 NPs are present in a dominant way, which is evident from inset of Figure 2a. A schematic of the EISA mechanism of the single colloidal droplet and mixed colloidal droplet is shown in Figure 19. It is interesting that the assembly of colloidal particles during

droplet faster to make the concentration of droplets homogeneous. Thus, in a slow drying regime, the formation of a packed shell of NPs is unlikely. However, in a fast drying regime, the drying time τdry of the droplet is smaller than the mixing time τmix of the NPs. In this case, during the drying of the droplet, evaporation brings the NPs into the liquid−water interface where the concentration grows. In contrast, the volume fraction of the NPs in the core of the droplet remains approximately equal to its initial value. This accumulation thus leads to the formation of a boundary layer through which the concentration changes. As time passes, two effects occur. The particle concentration at the shell increases, and the boundary layer becomes thicker. When the concentration reaches the gel concentration value (∼0.6), a characteristic of the randomly close-packed particles, a gelled shell forms at the surface. As evaporation continues, the drop progressively flattens. As water evaporates, tiny menisci form in the gaps between particles at the surface of the shell.44 The pressure difference across the menisci is 2γ/rM, where rM is the local radius of curvature. The capillary pressure drives the deformation of the shell. However, a critical pressure pc = 4Y0(h/Rc)2 is required to induce the buckling of a homogeneous shell,45 where h/Rc is the relative shell thickness and Y0 is the Young’s modulus of the shell. If the stress on the shell due to capillary pressure exceeds the critical pressure, then the shell buckles. The relative shell thickness plays an important role in deciding the morphology of the drying droplet and hence the grain morphology. In the case of a SiO2 dispersion, the size distribution of NPs is quite narrow with a mean radius of 8.2 nm. During the drying of a 2 wt % SiO2 dispersion, a shell forms and gets thickened as drying proceeds. The relative shell thickness h/Rc is large enough and thus requires a higher critical pressure. Capillary pressure, acting on the shell, is not sufficient to overcome the critical pressure required for buckling. However, the formation of hollow grains is a possibility if the water evaporates through the porous shell of jammed NPs. In the present experiment, a hollow spherical silica grain has been observed and is well understood by the aforementioned mechanism. EISA during the drying of the 2 wt % TiO2 dispersions is different because of the higher polydispersity. The diffusion of the larger TiO2 NPs is smaller as compared to that of the smaller NPs. During the shrinkage of the drying droplet, the larger NPs are less mobile toward the core of the droplet. This leads to the accumulation of NPs, leading to shell formation in an early stage as compared to the drying of the droplet containing SiO2 colloids. This leads to a decrease in the relative shell thickness. The critical pressure required to deform a shell having a lower relative thickness is smaller. The capillary pressure acting on the shell is sufficient to deform it. It is important to mention at this juncture that the polydispersity of the NPs inside the drying droplet may also play a crucial role. It has been shown that the fluid phase during the jamming process of spheres may transform into a crystalline phase depending on the polydispersity of the spheres. The fluid phase never transforms into a crystalline phase under the jamming of the spheres if the polydispersity in their shape or size is beyond 10%.46 It has been shown that the packing of the polydisperse spheres may be represented by the random close packing (RCP) fraction ΦRCP(Δ),47 which equals ∼0.64 for the monodisperse (Δ = 0) system and increases very slowly with Δ.48 However, it is expected that the packing fraction of the NPs having a larger polydispersity reaches ΦRCP earlier as compared to the less-polydisperse NPs. However, for mixed colloids of SiO2 and TiO2, the fraction of smaller NPs increases.

Figure 19. Schematic of the formation of assembled grains of TiO2 NPs and composite grains. 11351

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evaporation-induced assembly can be predicted by minimizing the second moment of the particle locations for the ratio of large to small sizes of less than 3 for very slow drying for binary colloids.55 However, it is to be noted that in the present work the droplets dried in fast drying regime, where assembly proceeds via a nonequilibrium phenomenon. In the nonequilibrium self-assembly process, the minimum second moment of the particle location configuration may not be achieved. Also, the colloids are quite polydisperse (i.e., the size of the colloids varies continuously in a certain size range), unlike the situation for only two types of colloids. Thus, the prediction of the assembled grain morphology using the principle of minimizing the second moment of the particle locations is quite cumbersome and requires further attention. The UV−vis results also confirm the nature of the composite grains. A blue shift of the characteristic absorption band of TiO2 NPs has been observed as a result of the confinement effect of TiO2 NPs in the SiO2 matrix. It is clear from the XRD analysis that rutile is the stable phase for larger TiO2 NPs; however, for smaller TiO2 NPs, the anatase phase is more favorable.49 At relatively small particle dimensions, the surface energy is an important part of the total energy, and it has been found that the surface energy of anatase is lower than that of rutile.50,51 The anatase-to-rutile polymorphic phase transformation follows a nucleation−growth mechanism.52 Upon thermal treatment, anatase crystallites grow until they reach a critical nuclei size. Once the crystallite size overcomes the critical nuclei size, rutile nucleation starts53 and involves the breaking of the anatase Ti−O bonds, followed by the cooperative motion of these atoms.54 The anatase-to-rutile transformation of the NPs in TI specimens may be well understood by the above mechanism. However, the anatase-torutile phase transformation has not been observed in the case of composite grains. It may be understood by the mesoscopic morphology of composite grains. Because the TiO2 NPs are uniformly jammed with SiO2 NPs in the grains, the growth process of the TiO2 NPs is inhibited and vice versa. Thus, the size and hence the anatase phase of TiO2 NPs remain constant with temperature in the composite grains.

Article

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are thankful to Mr. V. B. Jayakrishnan, SSPD, for his help with the XRD measurements. We are also thankful to Mr. P. Jha of TPD for his help with the UV−vis measurments. We thank Dr. Henrich Frielinghaus and Dr. Günter Goerigk, JCNS, Germany, for their help with the SANS experiments at KWS-1 and KWS-3, respectively. J.B. and D.S. thankfully acknowledge the financial support (DST(5)/AKR/P087/10-11/673) received from the Department of Science and Technology, India, through the S. N. Bose National Centre for Basic Science, Kolkata, India, for neutron-scattering work at JCNS, Germany.

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REFERENCES

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4. CONCLUSIONS TiO2/SiO2 mesoporous microspheres have been synthesized by the evaporation-induced self-assembly of mixed colloids. Mesoscopic characterization has been carried out by SEM, SAXS, and SANS. The origin of different morphologies of SiO2 and TiO2 and their composite grains has been related to the average size and polydispersity of the nanoparticles. The SAXS and SANS experiments reveal that composite grains do not show aging effects in contrast to only SiO2 grains, which is related to the specific surface area of the grains. X-ray diffraction experiments show that the composite grains are thermally stable against the anatase-to-rutile phase transition. This is attributed to the inhibition of the growth of the TiO2 NPs resulting from the confinement in the SiO2 matrix and is also confirmed by the UV−vis experiments. The interactions of the unsaturated groups on the TiO2 and SiO2 NPs are evident from the presence of the Ti−O−Si functional group. Thus, mesoporous TiO2/SiO2 grains, synthesized by the evaporation induced self-assembly of the mixed colloids, possess improved properties and may be considered to be a potential candidate for photocatalytic application. 11352

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