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Chemical Engineering Journal 181– 182 (2012) 45– 55

Contents lists available at ScienceDirect

Chemical Engineering Journal

j ourna l ho mepage: www.elsev ier .com/ locate /ce j

anocomposite TiO2–SiO2 gel for UV absorption

ngkhana Jaroenworalucka,∗, Nuchanaporn Pijarna, Nudthakarn Kosachana, Ron Stevensb

National Metal and Materials Technology Center, 114 Thailand Science Park, Paholyothin Rd., Klong 1, Klong Luang, Pathumthani 12120, ThailandMechanical Engineering Department, University of Bath, BA2 7AY Bath, UK

r t i c l e i n f o

rticle history:eceived 5 April 2011eceived in revised form 8 August 2011ccepted 9 August 2011

eywords:

a b s t r a c t

Nano-sized particle TiO2-doped SiO2 gels have been synthesized for use as composites in which the UVabsorption efficiency, the major factor for UV protection, can be enhanced. SiO2 gels having a mesoporousmorphology have been synthesized via a sol–gel processing route using rice husk as the starting mate-rial and further treated using additions of TiO2 from two sources. The chemical purity of the SiO2 wasmeasured by X-ray fluorescence analysis (XRF). Typical samples of pure TiO2, SiO2 and their composites

iO2–SiO2 geliO2–fumed SiO2

esoporous SiO2 gelV absorptionnergy band gapice husk

were tested for surface characteristics using N2-sorption (BET surface area). All the different compositionsprocessed have been characterized by X-ray diffraction (XRD), Fourier Transform Infrared Spectroscopy(FT-IR), and imaged using Scanning and Transmission Electron Microscopy (SEM, TEM). The UV absorp-tion values which determine the degree of UV-protection, were measured and the results discussed. Theconcentration of TiO2 loading, the particle size, and the surface characteristics are shown to relate to thedegree of UV absorption and the measured energy band gap of the composites.

(((

. Introduction

Rice husk (RH) has been long recognized as one of the prime nat-ral sources for silicon (Si). Many studies describe how to recycleH to produce useful ceramic materials such as silica (SiO2), siliconarbide (SiC), silicon nitride (Si3N4), zeolites (alumino-silicate), etc.1]. However, when sophisticated technology methods are used,he economics of such processing needs to be considered when theroduct is intended for practical uses in various industries. Amonghese reformed materials, SiO2 is considered to be a viable mate-ial because of its cheaper and easier processing. Previous studiesave revealed not only amorphous SiO2 particles having a size inhe nanoscale range, but also the typical characteristics of the parti-les, specifically that a high surface area, could be generated by thenal process of heat treatment [2–4]. These characteristics are con-idered to be useful for further processing in that intimate mixingith other oxides is possible, generating higher value end-products

or more effective applications.Titania (TiO2), because of its attractive properties, has found

ide ranging applications; it is non-toxic, inexpensive, has goodtability, demonstrates superior photocatalytic property, etc. [5].n practice it can be used for anti-bacterial applications, water and

ir purification, self-cleaning surfaces, UV-screening of agriculturelms, as an opacifying agent in paint, and for UV-absorption in the

orm of sunscreen cosmetic products.

∗ Corresponding author. Tel.: +66 2 564 6500; fax: +66 2 564 6447.E-mail address: angkhanj@mtec.or.th (A. Jaroenworaluck).

385-8947/$ – see front matter © 2011 Elsevier B.V. All rights reserved.oi:10.1016/j.cej.2011.08.028

© 2011 Elsevier B.V. All rights reserved.

Modification of the TiO2 involving doping or coating with var-ious metal oxides to enhance properties has been carried out inorder to:

(i) increase its efficiency,ii) extend usage range from the UV to the visible light region,

iii) reduce the energy band gap for energy conversion applications,iv) retard or accelerate phase transformation, and finally,(v) modify its surface characteristics.

For UV protection, TiO2, as a fine powder has been mixedwith organic oils or suspended in water in sunscreen cosmet-ics, and large quantities are used in household paints to enhancebrightness, whiteness and preserve stability in sunlight. TiO2, hasbeen preferred to ZnO, for addition to sunscreens because it canabsorb a broad-spectrum of both the UVB (290–320 nm) and UVA(320–400 nm, UVA1: 320–340 nm, UVA2: 340–400 nm), whereasZnO can absorb well only in a wide range of UVA [6]. However,in UV protective sunscreen products, the absorption characteris-tics need to be optimized usually by employing a combination oforganic and inorganic UV absorbers to provide a board-spectrumformula for the range of natural UV [6–9]. The maximum amountof TiO2 that can be added is restricted since high levels of TiO2 cancause irritation of the skin, it becomes difficult to remove and theviscosity becomes excessive. During exposure to light, the visible

light is scattered, providing opacity while the UV components areabsorbed by the TiO2 crystal. Here, rutile is preferred to anatase[5] since the UV is absorbed more efficiently and it is the morestable phase. The latest cosmetics use nanosized-TiO2 in place of

46 A. Jaroenworaluck et al. / Chemical Engineering Journal 181– 182 (2012) 45– 55

F l and

(

mvZa

ptowShgouidseaa

2

2

ps

ig. 1. N2 adsorption–desorption isotherms of samples: (a) the synthesized SiO2 geP25)–SiO2 (gel).

icron-sized particles in the sunscreen formulations together witharious other metal oxides additives such as CeO2, Fe2O3, Al2O3,rO2 and SiO2, to optimize the combined properties of reflection,bsorption and transparency.

In this study, TiO2-doped SiO2 gels in the form of nanocompositearticles have been prepared. TiO2 was doped into the SiO2 gel par-icles with the intention that nanosized TiO2 particles would coatr deposit on the gel particle surfaces. This typical microstructureould be expected to relate to enhancement of UV absorption. The

iO2 gel used has been synthesized via a sol–gel route using riceusks as the starting material selecting a process route designed toive a mesoporous structure, unlike previous studies of TiO2–SiO2r SiO2–TiO2 nanocomposites where commercial chemicals weresed [10–20]. The energy band gaps of TiO2-doped SiO2 gel for var-

ous TiO2 loadings in the SiO2 gel has been measured, and the use ofifferent TiO2 sources is reported. UV absorption properties of theynthesized nanocomposites, intended for use as the active ingredi-nt for sunscreen, have been determined. The experimental resultsre discussed in terms of the phases present, the energy band gapsnd the microstructural changes.

. Experimental

.1. Materials

Rice husk without any prior chemical treatment or washingrocess was used as the starting source of the SiO2. The refluxolvent was sodium hydroxide, with the pH adjusted by addition

the fumed SiO2, (b) TiO2 (TIP) and TiO2 (P25), (c) TiO2 (TIP)–SiO2 (gel) and (d) TiO2

of sulfuric acid. Titanium (IV) isopropoxide (TIP) and TiO2 powder(AEROXIDE® TiO2 P25, Degussa, Germany) were used as the sourceof TiO2. The TiO2 (TIP) was dissolved in isopropyl alcohol whereasethyl alcohol was used to dissolve the commercial TiO2 (P25) pow-der. Fumed silica (SiO2), AEROSIL® 200, Degussa, Germany, wasused for comparison with the laboratory produced SiO2 gel.

2.2. Preparation of SiO2 gel

The rice husk was calcined at 650 ◦C for 6 h. A mixture of ricehusk ash (10 g) and 1 M NaOH (320 mL) was heated under a refluxfor 5 h. The reaction mixture was then filtered using No. 5 Whatmanfilter paper, and the carbon residue washed with 400 mL of heatedDI water, and then cooled to room temperature. The solution wastitrated with 1 M H2SO4 to pH 7. The sol–gel aged for 18 h was gentlyfragmented and centrifuged for 5 min at 6000 rpm. To wash outthe impurities, distilled water (400 mL) was added, gently swirledand the suspension again centrifuged. The washing process wasrepeated 6 times to clean the gel, which was then spread on a glassdish and dried in a vacuum oven at 80 ◦C for 24 h. The dried gel wasdry-milled using ZrO2 ball media in a HDPE plastic container

2.3. Preparation of TiO2-doped SiO2 gels

SiO2 gel powder prepared as described in Section 2.2 was addedto titanium (IV) isopropoxide (TIP) and dissolved in isopropyl alco-hol under constant stirring at room temperature. Water was thenadded to obtain a molar ratio of H2O:TIP = 4:1. The sol was allowed

A. Jaroenworaluck et al. / Chemical Enginee

Table 1Nomenclature for the powders and their composites.

Samples No. Powders and its composites Sample names

TiO2 SiO2 TiO2–SiO2

1 – Silica gel SiO2 (gel)2 – Fumed silica SiO2 (fume)3 TIP TiO2 (TIP)4 TIP Silica gel TiO2 (TIP)–SiO2 (gel)5 TIP Fumed silica TiO2 (TIP)–SiO2 (fume)6 P25 – TiO2 (P25)

tiUwaht

2

g

◦

F(

7 P25 Silica gel TiO2 (P25)–SiO2 (gel)8 P25 Fumed silica TiO2 (P25)–SiO2 (fume)

o age for 1 h at room temperature. The samples were then placedn an oven and heated from 30 to 110 ◦C for 0.5 h at 20 ◦C intervals.sing a similar processing route, a second composite was madehen SiO2 gel was added to TiO2 (P25) and suspended in ethyl

lcohol at room temperature with constant stirring, followed byeating. The TiO2 loading was 2.5, 6.5, 10.0 and 25.0 wt%, respec-ively.

.4. Characterization

The chemical purity of SiO2 produced from the synthesized SiO2el was evaluated by X-ray fluorescence analysis (XRF, a Phillips,

ig. 2. X-ray diffraction patterns: (a) uncalcined pure SiO2 and TiO2, (b) calcined pure STIP)–SiO2 (fume) composites.

ring Journal 181– 182 (2012) 45– 55 47

PW-2404, The Netherlands). The sample for the XRF analysis wasprepared by pressing 5.0 g powder with 1.0 g of boric acid (H3BO3)binder.

An X-ray diffractometer (XRD, JDX-3530, JEOL, Japan) employ-ing Cu K� radiation (� = 1.5418 A) at a scanning rate of 0.04 2� s−1

was used to measure crystallinity and determine the phases presentin the composites. The accelerating voltage and the applied currentused was 30 kV and 40 mA, respectively. The X-ray diffraction traceswere acquired in the 2� range 10–80◦. The diffraction peaks wereidentified with JCPDS data using Jade 7.5 software. The crystallitesize of the powders was calculated using the software and standardformulae based on the Scherrer equation [21]:

t = K�

B cos �B

where t is crystallite size, K is the Scherrer constant (0.94), � is thewavelength of X-radiation (1.5418 A), B is peak width (FWHM), and�B is the diffraction angle at which the half width is measured.

The surface area, pore size and pore volume of the SiO2 gel andfumed SiO2, were measured using nitrogen (N2) adsorption (Quan-

tachrome Autosorp-1, USA). The samples were degassed at 250 Cfor 660 min before testing. Surface area, pore size and pore vol-ume of as-received TiO2 (P25), calcined TiO2 (TIP), TiO2 (TIP)–SiO2gel TiO2 (P25)–fumed SiO2 including typical TiO2–SiO2 compos-

iO2 and TiO2, (c) calcined TiO2 (TIP)–SiO2 (gel) composites, and (d) calcined TiO2

48 A. Jaroenworaluck et al. / Chemical Engineering Journal 181– 182 (2012) 45– 55

Table 2Surface characteristics of samples determined by the N2 adsorption–desorption method.

Samples BET surfacearea (m2 g−1)

Total porevolume(cm3 g−1)

Average porediameter (nm)

SiO2 (gel) 353.04 0.6743 7.64SiO2 (fume) 170.14 0.8685 20.42TiO2 (TIP) 57.09 0.1664 11.66TiO2 (P25) 55.42 0.8335 60.162.5 wt% TiO2 (TIP)–SiO2 (gel) 301.75 0.6394 8.486.5 wt% TiO2 (TIP)–SiO2 (gel) 385.52 0.6003 6.2310.0 wt% TiO2 (TIP)–SiO2 (gel) 378.56 0.5436 5.7425.0 wt% TiO2 (TIP)–SiO2 (gel) 349.77 0.5287 6.052.5 wt% TiO2 (P25)–SiO2 (gel) 211.55 0.6432 12.166.5 wt% TiO2 (P25)–SiO2 (gel) 230.68 0.6546 11.3510.0 wt% TiO2 (P25)–SiO2 (gel) 154.10 0.6239 16.20

0.8055 13.531.3090 39.571.5080 61.83

is

tmtada0

oJtpsdsw1mf

3

3

ifdpp

tmtpwamsac2uS

25.0 wt% TiO2 (P25)–SiO2 (gel) 238.13

25.0 wt% TiO2 (TIP)–SiO2 (fume) 132.34

25.0 wt% TiO2 (P25)–SiO2 (fume) 97.56

tes were measured using the same methods as used for analysingurface characteristics of the SiO2 gel and fumed SiO2.

Infrared spectra were collected on a FT-IR spectrometer (Sys-em 2000, Perkin-Elmer, England) at 20 ◦C using the standard KBr

ethod. Sample spectra were recorded for the chemically func-ional groups at wavelengths in the region 4000–400 cm−1. The UVbsorption properties of the composites were measured using aiffused UV–vis spectrophotometer (UV-2550, Shimadzu, Japan),t wavelengths in the range 200–800 nm, at 0.1 step scan speed,.1 sampling interval and 1.0 slit width.

A scanning electron microscope (SEM, JSM-6310F, JEOL, Japan)perated at 20 kV and a transmission electron microscope (TEM,SM 2010, JEOL, Japan) operated at 200 kV were used to examinehe morphology and microstructure of each of the TiO2–SiO2 com-osites. Samples for SEM investigation were dispersed in ethanol,onicated in an utrasonic bath for 10 min, dropped onto brass stubs,ried in a vacuum chamber at room temperature and coated withputtered gold prior to examination. Samples for TEM examinationere dispersed in ethanol and sonicated in an ultrasonic bath for

0 min, and then fished out onto carbon films supported on 200esh Cu grids. The grids were dried and placed in the microscope

or microstructural observation.

. Results and discussion

.1. Composite characteristics and phases present

A list of samples prepared using the present synthesis routes shown in Table 1. The TiO2–SiO2 gels were made using TiO2rom two sources; TIP and P25. Fumed SiO2 was used as a stan-ard for comparison with the synthesized SiO2 gel. The laboratoryrocessed SiO2 gel and TiO2–SiO2 gels are all in the form of whiteowders. High purity SiO2 gel (98.79% SiO2 from XRF analysis).

Fig. 1(a) shows the N2 adsorption–desorption isotherms ofhe fumed SiO2 and SiO2 gel. A characteristic hysteresis loop of

esopores of the SiO2 gel isotherm is seen and considered to behe so-called type IV which has been attributed to “ink-bottle”ores [22] while the isotherm of the fumed SiO2 appears some-hat different. The SiO2 gel isotherm may relate to localization of

gglomerated particles. Unlike pure fumed SiO2, the gel can adsorboisture because of its highly porous structure, defined by a BET

pecific surface area of 353.04 m2 g−1, a pore diameter of 7.64 nm,nd a mesoporous structure. Fumed SiO2 has a lower BET spe-

ific surface area of 170.14 m2 g−1, with a larger pore diameter of0.42 nm (see the inset table in Fig. 1). It is noted that the pore vol-me of the synthesized SiO2 gel is lower than that of the fumediO2, which may be related to its agglomerated morphology.

Fig. 3. (a) FT-IR spectrograms of the composites: TiO2 (TIP)–SiO2 (gel), TiO2

(TIP)–SiO2 (fume), TiO2 (P25)–SiO2 (gel) and TiO2 (P25)–SiO2 (fume) at 10 wt% TiO2

loading (a) before and (b) after calcination at 400 ◦C for 10 h.

Pure, white TiO2 can be obtained from TIP, a colorless liquidthat rapidly hydrolyses, readily evaporates and easily forms a solid(powder) residue. Fig. 1(b) shows the N2 adsorption–desorptionisotherms of the calcined TiO2 (TIP) and as-received TiO2 (P25).

A. Jaroenworaluck et al. / Chemical Engineering Journal 181– 182 (2012) 45– 55 49

Table 3Crystal size of uncalcined and calcined sample powders based on XRD analysis.

Samples TiO2 SiO2

Anatase Rutile Before After

Before After Before After

SiO2 (gel) – – – – 18.10 21.60SiO2 (fume) – – – – 18.70 19.30TiO2 (TIP) 10.87 19.60 – – – –TiO2 (P25) 23.20 24.40 35.40 38.10 – –2.5 wt% TiO2 (TIP)–SiO2 (gel) ND ND – – ND ND6.5 wt% TiO2 (TIP)–SiO2 (gel) ND ND – – ND ND10.0 wt% TiO2 (TIP)–SiO2 (gel) ND 11.10 – – ND ND25.0 wt% TiO2 (TIP)–SiO2 (gel) ND 13.10 – – ND ND2.5 wt% TiO2 (P25)–SiO2 (gel) 16.00 15.80 ND ND ND ND6.5 wt% TiO2 (P25)–SiO2 (gel) 18.43 18.50 ND ND ND ND10.0 wt% TiO2 (P25)–SiO2 (gel) 19.50 19.40 22.90 22.90 ND ND25.0 wt% TiO2 (P25)–SiO2 (gel) 20.20 21.30 29.10 31.90 ND ND2.5 wt% TiO2 (TIP)–SiO2 (fume) ND ND – – ND ND6.5 wt% TiO2 (TIP)–SiO2 (fume) ND ND – – ND ND10.0 wt% TiO2 (TIP)–SiO2 (fume) ND ND – – ND ND25.0 wt% TiO2 (TIP)–SiO2 (fume) ND 11.20 – – ND ND2.5 wt% TiO2 (P25)–SiO2 (fume) ND ND ND ND ND ND6.5 wt% TiO2 (P25)–SiO2 (fume) ND ND ND ND ND ND10.0 wt% TiO2 (P25)–SiO2 (fume) 18.40 19.50 23.80 25.20 ND ND25.0 wt% TiO2 (P25)–SiO2 (fume) 19.70 19.90 28.20 31.10 ND ND

Note: –: none, ND: not detectable because of the analyzed limitation.

Fig. 4. UV–vis reflectance spectra of (a) pure SiO2 and TiO2, (b) TiO2 (TIP)–SiO2 (gel), (c) TiO2 (TIP)–SiO2 (fume), and (d) all composites doped with 10 wt% TiO2.

50 A. Jaroenworaluck et al. / Chemical Enginee

F(

Diis

aTctgs

The XRD results indicate the presence of the crystalline phases of

ig. 5. The energy band gap (Eg) of TiO2 (TIP)–SiO2 gel, TiO2 (TIP)–fumed SiO2, TiO2

P25)–SiO2 gel and TiO2 (P25)–fumed SiO2 versus wt% TiO2 present.

ifferent profiles can be observed which suggest that the TiO2 (P25)s not agglomerated and there is no mesoporous characteristic, ass present in the TiO2 (TIP) formed by agglomeration of individualmall particles.

Fig. 1(c) and (d) show for comparison profiles of the N2dsorption–desorption isotherms of the TiO2 (TIP)–SiO2 gel andiO2 (P25)–SiO2 gel, respectively. The mesoporous characteristicsan be obvious for each composition. The isotherm characteris-

ics of the TiO2 (TIP)–SiO2 gel is similar to the TiO2 (P25)–SiO2el although the BET surface area of both types composites showignificant differences.

Fig. 6. SEM micrographs of the pure oxides, (a) SiO2 gel

ring Journal 181– 182 (2012) 45– 55

The BET surface area of TiO2 (P25)–SiO2 gel has a lower valuethan the TiO2 (TIP)–SiO2 gel composites, as summarized in Table 2,while the different values for BET surface area of TiO2 (P25) andTiO2 (TIP) are not significant. This indicates a different characteris-tic which can imply that the TiO2 (P25) particles covering the SiO2gel surfaces decrease the overall composites’ surface area whereasthis has not occurred in TiO2 (TIP)–SiO2 gel. It is possible that theTiO2 (P25) particles are not agglomerated and can distribute read-ily on the gel particle surfaces which could well reduce the surfacearea of the gel. However, the TiO2 (TIP) particles which are agglom-erated can be distributed locally on the gel surfaces. It is possiblethat the gel surfaces can be exposed without the TiO2 being totallycovered by particles. It is also interesting that the TiO2–SiO2 gelcomposites have higher BET values than the TiO2–fumed SiO2. Thisshould be related to the surface area of the SiO2 gel which is higherthan that of the fumed SiO2 itself. The number of TiO2 particlesattaching onto the substrate surfaces is possible to be higher.

Fig. 2(a) and (b) shows a comparison of phase present in theuncalcined and calcined samples from pure TiO2 and the synthe-sized and commercial SiO2, respectively. The calcined temperatureand holding time in this study was fixed to control the phase presentin the TiO2–SiO2 composites. XRD profiles show clearly that boththe SiO2 gel and fumed SiO2 have an amorphous structure with adiffuse intensity peak at 2� ∼ 22◦.

For analysis of the XRD profiles of pure TiO2 (P25), JCPDS no. 21-1272 and 21-1276 were used, the highest peak intensity of (1 0 1)and for (1 1 0) were measured to calculate the phases present andto determine the crystallite size of anatase and rutile, respectively.

anatase and rutile for the as-received and calcined TiO2 (P25). How-ever the ratio of anatase:rutile phases changed from approximately90:10 to 80:20 after the TiO2 (P25) had been calcined. This is a clear

, (b) fumed SiO2, (c) TiO2 (TIP), and (d) TiO2 (P25).

A. Jaroenworaluck et al. / Chemical Engineering Journal 181– 182 (2012) 45– 55 51

F TiO2 ((

ie

caoJor(awTcspIpr(

cat(lit

ca

ig. 7. SEM micrographs of the calcined composites doped with 10 wt% TiO2: (a)

P25)–SiO2 (fume).

ndication that the calcination condition used in this study had anffect on the phase transformation process.

For the synthesized TiO2 (TIP), for both the uncalcined andalcined powder, characteristic peaks of anatase were found. Inddition a high intensity peak at 2� ∼ 30.8◦, the characteristic peakf the (1 2 1) plane of brookite was clearly apparent. Based on theCPDS file no. 29-1360, this peak is the second highest intensity peakf brookite. The first and the third are at 2� ∼ 25.3◦ and 2� ∼ 25.7◦,espectively, which overlap the major characteristic peak of anatase2� ∼ 25.3 of (1 0 1) plane). For the uncalcined powder, this char-cteristic peak of anatase appears to be split at its top, all ofhich suggests that a brookite phase is present with the anatase.

hese particular characteristic peaks could also be found in the cal-ined powder but the peaks of the calcined powder are noticeablyharper. It may well be that the brookite formation takes place by ahase transformation from anatase during the calcination process.

t is clear that the majority of the crystalline phase in the TiO2 (TIP)owder is anatase and that for the calcination condition used, theutile phase could not be identified in any of the samples of TiO2TIP).

Fig. 2(c) shows XRD profiles of the calcined TiO2 (TIP)–SiO2 gelomposites. With less than 10.0 wt% TiO2, the crystalline phase ofnatase cannot be clearly identified. The base line of all the XRDraces includes a signal due to the amorphous phase. For TiO2TIP)–fumed SiO2 composites, the trend of the XRD profiles is simi-ar to that of the calcined TiO2 (TIP)–SiO2 gel composites, as shownn Fig. 2(d). However the brookite phase could be seen in the XRD

races of the 25 wt% TiO2 (TIP)–fumed SiO2 composites.

For all the TiO2 (P25)–SiO2 composites, their XRD profiles indi-ate the presence of crystalline phases of anatase and rutile beforend after calcination. Table 3 summarises the calculated crystal size

TIP)–SiO2 (gel), (b) TiO2 (TIP)–SiO2 (fume), (c) TiO2 (P25)–SiO2 (gel), and (d) TiO2

of pure SiO2, TiO2 and their composites. The amorphous nature ofSiO2 had a marked effect on the base lines of the XRD profiles whichlimited the process of identifying the crystalline size by this tech-nique. However, the crystallite size of the calcined SiO2 gel and TiO2(TIP) is in the same size range as the commercial fumed SiO2 andTiO2 (P25). The estimated size of the TiO2 in the TiO2 (TIP)–SiO2composites is smaller than that measured for the pure TiO2 (TIP) orfor the composites of TiO2 (P25)–SiO2.

3.2. Chemical functional groups

FT-IR spectra of the composites, 10.0 wt% TiO2 (TIP) and TiO2(P25) for both fumed SiO2 and SiO2 gel before and after calcination,have been determined and are compared in Fig. 3(a) and (b), respec-tively. The spectral peaks near 3500 and 1640 cm−1 present in allsamples are attributed to the stretching mode of water hydroxylbonds [12,14,23]. There are strong bands at 470, 800 and 1100 cm−1

ascribed to the bending modes of Si–O–Si, to symmetric vibrationof Si–O–Si and to asymmetric stretching of Si–O–Si, respectively[10,12,19,20]. A weak band at ∼900–950 cm−1 is attributed toSi–OH stretching frequency for the silanol groups [24,25].

The samples exhibit a slightly more intense broad band at3500 cm−1 prior to calcination. The FT-IR results in the region ofthe peaks at ∼3500 cm−1 demonstrate that the SiO2 gel shows ahigher adsorbed water content than fumed SiO2. The compositesof the SiO2 gel have a higher intensity peak than that of the fumedSiO2 around 3500 cm−1. This large broad band is attributed to the

presence of the O–H stretching frequency of the silanol groups andalso to any of the remaining adsorbed water [26]. Thus it wouldappear that the composites based on the SiO2 gel can adsorb waterrather more effectively than those of fumed SiO2. Furthermore, the

52 A. Jaroenworaluck et al. / Chemical Engineering Journal 181– 182 (2012) 45– 55

iO2 ge

TcT

tgaAbttm

3

atSSf

Fig. 8. TEM bright field images of pure oxides, (a) S

iO2 (TIP)–SiO2 gel shows higher adsorbed water content than thealcined sample and has a higher adsorbed water content than theiO2 (P25)–SiO2 gel or the uncalcined sample.

From the FT-IR spectra at 450–750 cm−1, only one absorp-ion peak for anatase is present at ∼500 cm−1 whereas the rutileives two peaks at 510 cm−1 and 640 cm−1. The spectral peaksre ascribed to the stretching vibrations of the Ti–O bond [10,23].s the calcined temperature reaches 400 ◦C, a broad absorptionand appears at 500–650 cm−1, which indicates the formation ofhe rutile phase, probably on or near the surface of anatase crys-als. Thus the phase transformation from anatase to rutile can be

onitored by the relative intensity of these characteristic peaks.

.3. UV absorption properties

Fig. 4(a) shows the UV absorption spectra of pure samples. Thebsorption edge is absent in the two lowest curves corresponding to

he pure fumed SiO2 and pure SiO2 gel, respectively. It is clear thatiO2 itself does not have good absorption properties, however, theiO2 having the high porosity may be used as a support mediumor the TiO2 particles. The results indicate that the absorption

l, (b) fumed SiO2, (c) TiO2 (TIP), and (d) TiO2 (P25).

spectrum for pure calcined TiO2 (TIP) shows the largest shift toa longer wavelength and higher absorption than TiO2 (P25).

Fig. 4(b) and (c) show for comparison the absorption spectra ofpure SiO2 gel, fumed SiO2 and their composites formed with 2.5,6.5, 10.0 and 25.0 wt% TiO2 (TIP) The degree of absorption appearsto be increased with the increase in wt% TiO2 and shift to longerwavelength. For the TiO2 (TIP)–SiO2 (gel) composites, the high-est absorption is found at 6 wt% TiO2 (TIP). The higher additions ofTiO2 may show a reduced absorption as a result of particle agglom-eration. It is apparent that the absorption edges shift to a higherwavelength (red shift) as the wt% TiO2 (TIP) increases. The resultsindicate a trend in that the increasing absorption value depends onthe wt% TiO2 (TIP) added. In Fig. 4(d), the TiO2 (TIP)–SiO2 compos-ites are seen to have higher absorption values when compared tothe commercial TiO2 (P25). The shift of the absorption edge towardslonger wavelength, the so called red shift, could be observed. Theobserved red shift of the absorption edge for the TiO2–fumed SiO2

and TiO2–SiO2 gel compared to pure TiO2 (P25) and TiO2 (TIP) issuggested to be due to size effects typical of small particles. UVabsorption of TiO2–SiO2 gel is higher than for the TiO2–fumed SiO2for both the TiO2 from P25 and TIP. This may well relate to the

A. Jaroenworaluck et al. / Chemical Engineering Journal 181– 182 (2012) 45– 55 53

Table 4Absorption edge and Eg values of pure and composite samples.

Sample Absorption edge (nm) Eg (kV)wt% TiO2 wt% TiO2

2.5 6.5 10.0 25.0 100.0 2.5 6.5 10.0 25.0 100.0

TiO2 (TIP) – – – – 385 – – – – 3.22TiO2 (P25) – – – – 390 – – – – 3.18TiO2 (TIP)–SiO2 (gel) 350 370 370 385 – 3.54 3.35 3.35 3.22 –TiO2 (P25)–SiO2 (gel) 390 395 400 400 – 3.18 3.14 3.10 3.10 –TiO2 (TIP)–SiO2 (fume) 358 367 367 386 – 3.47 3.38 3.38 3.21 –TiO2 (P25)–SiO2 (fume) 380 385 390 402 – 3.26 3.22 3.18 3.09 –

F )–SiO2

(

mt

3

a

E

ig. 9. TEM bright field images of the calcined 10 wt% TiO2 composites: (a) TiO2 (TIPfume).

esoporous structure of SiO2 gel allowing the TiO2 to deposit onhe gel surfaces and particularly to the smaller pore size of the gel.

.4. Energy band gap

The energy band gap can be calculated by substituting theppropriate wavelength value (�) into the equation [27]:

= hc

�

(gel), (b) TiO2 (TIP)–SiO2 (fume), (c) TiO2 (P25)–SiO2 (gel), and (d) TiO2 (P25)–SiO2

where E is the Energy gap, h is Planck’s constant (6.625 × 10−34 J s),c is the velocity of light (3.0 × 108 m s−1) and � is the wavelength(nm), respectively. The wavelengths were determined from theabsorbance curves at the point of contact with the x-axis of theabsorbance-wavelength plots. This method was used to evaluate

energy band gaps (Eg) after a trial and error calculation for theenergy band gap of TiO2 (P25). The energy band gap of TiO2 (P25)obtained using this method is closer to the theoretical value whencompared to the two methods:

5 ngineering Journal 181– 182 (2012) 45– 55

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(i) the (F(R) · h�)1/2 versus h� plot, in term of the Kubelka–Munkfunction, where F(R) = (1 − R)2/(2R) and R is reflectance (%) [13]and

ii) differential reflectance (dR/d�) versus � plot [28].

The relationship between Eg and wt% TiO2 doped in SiO2 is sum-arized and plotted in Table 4 and Fig. 6, respectively.For crystalline TiO2, the theoretical band gap between the

alence band and the conduction band is 3.2 eV for anatase7,10,18,23] and 3.0 eV for rutile [7,8,10,18,23]. From this study,he energy band gap was determined ∼3.18 eV for the TiO2 (P25)onsisting of mixed anatase and rutile phases. The trend in val-es of Eg for mixed TiO2 nano-crystals of both rutile and anatasehases decreases continuously with the increase in rutile content.he energy gap in the TiO2 (P25)–SiO2 gel and TiO2 (TIP)–SiO2 gel isigher than that for TiO2 (P25)–fumed SiO2 and TiO2 (TIP)–fumediO2 for all the different TiO2 additions. The results in Fig. 5 showhat the highest value for Eg is given for the 2.5 wt% TiO2 (TIP)–SiO2el. The band gap decreases for all the samples with increase in theiO2 content. Again, the crystalline form of the samples affects thehange in band gap with the TiO2 (P25)–SiO2 gel giving the lowestalue and the TiO2 (TIP)–SiO2 gel giving the highest value. The lackf sensitivity of the energy band gap to small additions of dopantTiO2) strongly suggests that the energy change is due to small bondength changes induced by nearest neighbor crystal chemistry.

However for additions >10.0 wt%, the energy band gapecreases. Consequently, it would appear that 10.0 wt% TiO2 (P25)ddition is the optimum composition for the TiO2 (P25)–SiO2 gel.he energy band gap decreases with the amount of TiO2 (TIP) added.t 25.0 wt% TiO2 addition, the highest absorption and the lowestalue for the energy band gap was found. The absorption increasesith the wt% TiO2 (TIP) increase up to 10.0 wt%, but above this value

he absorption decreases, the 10 wt% TiO2 (TIP)–SiO2 gel havinghe highest absorption. Conversely, the energy band gap increaseshen the wt% TiO2 (TIP) decreases. The energy band gap values for

.5 and 10.0 wt% TiO2 (TIP)–SiO2 gel are not significantly differentrom that of the 25.0 wt% addition which has the lowest energyand gap in the series. However for ease of preparation of the TiO2TIP)–SiO2 gels, the most suitable concentration is considered to be10.0 wt% of TiO2 (TIP).

.5. Microstructure investigations

.5.1. SEM observationFig. 6(a)–(d) are SEM images showing the microstructures of

alcined SiO2 gel, fumed SiO2, as-received TiO2 (P25), and calcinediO2 (TIP), respectively. It is clear that particles of TiO2 (TIP) andiO2 gel have agglomerated while particles of TiO2 (P25) and fumediO2 are randomly dispersed. The image revealed that pure SiO2el consists of both large and small agglomerated particles. Thegglomerated mass has broken up into small pieces revealing thene individual gel particles on the fractured surfaces.

The microstructure of all the composites observed differs some-hat. Fig. 7(a)–(d) are selected as being representative of theicrostructures of the 10.0 wt% TiO2–SiO2 composites. The images

f the 10.0 wt% TiO2 (TIP)–SiO2 gel show a structure consistingf agglomerations of both large and small particles which mayhemselves be formed of even smaller agglomerates covering theirurfaces. However the images of the 10.0 wt% TiO2 (TIP)–fumediO2 show well dispersed nanoparticles having a narrow size distri-

ution. From examination of the TiO2 (P25)–fumed SiO2, particlesf TiO2 and SiO2 are observed to be dispersed uniformly (seeig. 7(c)). In the course of the SEM observations, it was not pos-ible to distinguish between the TiO2 and SiO2 particles which aref a similar size and shape and have similar brightness and contrast.

Fig. 10. HR-TEM images of (a) 10 wt% TiO2 (TIP)–SiO2 gel and (b) 10 wt% TiO2

(TIP)–fumed SiO2 after calcination at 400 ◦C for 10 h.

3.5.2. TEM observationBright field images of the pure SiO2 and TiO2 samples are shown

in Fig. 8(a)–(d). The SiO2 gel particles are agglomerated whereasthe grains of the fumed SiO2 are well dispersed and have a fairlynarrow size distribution. The TiO2 (TIP) particles are smaller thanTiO2 (P25) but these smaller particles tend to have agglomerated.The particle characteristics of the TiO2 (P25) and the fumed SiO2are similar, as shown in Fig. 8(b) and (d).

Fig. 9(a)–(d) shows the microstructures of the composites,which can be readily compared. For the composite made up of TiO2(TIP)–SiO2 (gel), the crystalline TiO2 grains are agglomerated andattached onto the SiO2 gel surface as is shown in Fig. 9(a). Howeverwith the sample of TiO2 (TIP)–fumed SiO2, individual TiO2 (TIP)particles could be observed on the surfaces of the fumed SiO2 (the

dark spots in Fig. 9(b)). In the TiO2 (P25)–SiO2 gel composite it canbe seen that the TiO2 (P25) particles have attached to each otherand some are completely surrounded on the gel surface. The TiO2

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P25)–fumed SiO2 composite shows different behavior in that bothypes of particles are intimately mixed. Grains of the TiO2 (P25)nd the SiO2 gel particles have different structural forms, beingrystalline and amorphous respectively.

HRTEM images, Fig. 10(a) and (b) reveal agglomeration of crys-alline TiO2 (TIP) on SiO2 gel and fume SiO2. It is noted that the sizef the individual particles in the composite is smaller when mea-ured from calibrated micrographs than the values obtained fromRD data and the Scherrer equation, ∼7 nm (see Table 3 and the fig-res). It is suggested that the TiO2 particles in the composites are

imited in their ability to grow easily since they become occludedy the SiO2 grains and pores. A detailed investigation reveals that

ndividual TiO2 (TIP) particles having a crystalline structure arettached onto the fumed SiO2 surfaces, a feature which may beelated to the larger pore diameters compared to those present inhe SiO2 gel.

The TEM images allow conclusions to be made particularlyegarding the influence of the nanostructure in that the UV absorp-ion properties of the TiO2–SiO2 gel or TiO2–fumed SiO2 depend onhe particle size of the active TiO2 phase in the composites and theurface properties of the supporting media (SiO2 gel or fumed SiO2),elating directly to the particle distribution of the components.

. Conclusions

A mesoporous SiO2 gel was synthesized utilising recycled riceusk as a precursor. The gel was doped with various amounts ofiO2 using TIP and P25 and these were characterized and mea-ured for UV absorption for evaluation of their energy band gapy comparing with TiO2–fumed SiO2 composites. At the temper-tures and conditions used for calcination, the synthesized TiO2repared from the TIP forms a well crystallised pure anatase phase.iO2 gel particles are preferable to agglomerates and the TiO2 (TIP)articles are best dispersed on the SiO2 gel with its mesoporoustructure. However, with the TiO2 (P25)–SiO2 gel, the TiO2 (P25),articles are preferable to agglomerates. The UV absorption prop-rties have been related to the level of TiO2 doping and the phase,ize and surface characteristics of the support media (SiO2 gel orumed SiO2).

cknowledgement

This work is financially supported by the National Metal andaterials Technology Center (MTEC) under funded no. MT-B-48-

ER-07-190-I and MT-B-51-CER-07-206-I.

eferences

[1] L. Sun, K. Gone, Silicon-based materials from rice husks and their applications,

Ind. Eng. Chem. Res. 40 (2001) 5861–5877.

[2] R. Conradt, P. Pimkhaokham, U. Leela-Adisorn, Nano-structured silica from ricehusk, J. Non-Cryst. Solids 145 (1992) 75–79.

[3] T.H. Liou, Preparation and characterization of nano-structured silica from ricehusk, Mat. Sci. Eng. A: Struct. A364 (2004) 313–323.

[

ring Journal 181– 182 (2012) 45– 55 55

[4] M. Estevez, S. Vargas, V.M. Castano, R. Rodriguez, Silica nano-particles producedby worms through a bio-digestion process of rice husk, J. Non-Cryst. Solids 355(2009) 844–850.

[5] A. Fujishima, X. Zhang, D.A. Tryk, TiO2 photocatalysis and related surface phe-nomena, Surf. Sci. Rep. 63 (2008) 515–582.

[6] P. Kullavanijaya, H.W. Lim, Photoprotection, J. Am. Acad. Dermatol. 52 (2005)937–958.

[7] N. Serpone, D. Dondi, A. Albini, Inorganic and organic UV filters: their roleand efficacy in sunscreens and suncare products, Inorg. Chim. Acta 360 (2007)794–802.

[8] T.A. Egerton, N.J. Everall, J.A. Mattinson, L.M. Kessell, I.R. Tooley, Interaction ofTiO2 nano-particles with organic UV absorbers, J. Photochem. Photobiol. A 193(2008) 10–17.

[9] S. Yabe, T. Sato, Cerium oxide for sunscreen cosmetics, J. Solid State Chem. 171(2003) 7–11.

10] S.I. Seok, J.H. Kim, TiO2 nanoparticles formed in silica sol–gel matrix, Mater.Chem. Phys. 86 (2004) 176–179.

11] E. Pabón, J. Retuert, R. Quijada, A. Zarate, TiO2–SiO2 mixed oxides prepared bya combined sol–gel and polymer inclusion method, Microporous MesoporousMater. 67 (2004) 195–203.

12] Z. Li, B. Hou, Y. Xu, D. Wu, Y. Sun, W. Hu, F. Deng, Comparative study of sol–gelhydrothermal and sol–gel synthesis of titania–silica composite nanoparticles,J. Solid State Chem. 178 (2005) 1395–1405.

13] J. Aguado, R. van Grieken, M.J. López-Munoz, J. Marugán, A comprehen-sive study of the synthesis, characterization and activity of TiO2 and mixedTiO2/SiO2 photocatalysts, Appl. Catal. A: Gen. 312 (2006) 202–212.

14] A.A. Ismail, H. Matsunaga, Influence of vanadia content onto TiO2–SiO2 matrixfor photocatalytic oxidation of trichloroethylene, Chem. Phys. Lett. 447 (2007)74–78.

15] 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.

16] E. Beyers, E. Biermans, S. Ribbens, K. de Witte, M. Mertens, V. Meynen, S. Bals,G. van Tendeloo, E.F. Vansant, P. Cool, Combined TiO2/SiO2 mesoporous photo-catalysts with location and phase controllable TiO2 nanoparticles, Appl. Catal.B: Environ. 88 (2009) 515–524.

17] H.E. Byrne, W.L. Kostedt IV, J.M. Stokke, D.W. Mazyck, Characterization of HF-catalyzed silica gels doped with Degussa P25 titanium dioxide, J. Non-Cryst.Solids 355 (2009) 525–530.

18] B. Llano, G. Restrepo, J.M. Marín, J.A. Navío, M.C. Hidalgo, Characterisation andphotocatalytic properties of titania–silica mixed oxides doped with Ag and Pt,Appl. Catal. A: Gen. 387 (2010) 135–140.

19] A. Nilchi, S. Janitabar-Darzi, A.R. Mahjoub, S. Rasouli-Garmarodi, New TiO2/SiO2

nanocomposites—phase transformations and photocatalytic studies, ColloidsSurf. A 361 (2010) 25–30.

20] J. Zou, J. Gao, H2O2-sensitized TiO2/SiO2 composites wit high photocatalyticactivity under visible irradiation, J. Hazard. Mater. 185 (2011) 710–716.

21] B.D. Cullity, Element of X-ray Diffraction, second ed., Addison-Wesley Publish-ing Company, United States of America, 1978.

22] T. Allen, Particle Size Measurement, 5th ed., Kluwer Academic Publishers, TheNetherlands, 1999.

23] W. Jun, Z. Gang, Z. Zhaohong, Z. Xiangdong, Z. Guan, M. Teng, J. Yuefeng, Z.Peng, L. Ying, Investigation on degradation of azo fuchsine using visible lightin the presence of heat-treated anatase TiO2 powder, Dyes Pigments 75 (2007)335–343.

24] J.A.A. Sales, C. Airoldi, Calorimetric investigation of metal ion adsorption on3-glycidoxypropyltrimethylsiloxane + propane-1,3-diamine immobilized onsilica gel, Thermochim. Acta 427 (2005) 77–83.

25] Y.M. Wang, S.W. Liu, Z. Xiu, X.B. Jiao, X.P. Cui, J. Pan, Preparation and photocat-alytic properties of silica gel-supported TiO2, Mater. Lett. 60 (2006) 974–978.

26] J. Estella, J.C. Echeverría, M. Laguna, J.J. Garrido, Effects of aging and dryingconditions on the structural and textural properties of silica gels, MicroporousMesoporous Mater. 102 (2007) 274–282.

Chichester, 1991.28] M. Radecka, A. Trenczek-Zajac, K. Zakrzewska, M. Rekas, Effect of oxygen non-

stoichiometry on photo-electrochemical properties of TiO2−x , J. Power Sources173 (2007) 816–821.