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Cite this: DOI: 10.1039/c3dt53389a

Received 2nd December 2013,Accepted 22nd March 2014

DOI: 10.1039/c3dt53389a

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A novel route for synthesis of UV-resistanthydrophobic titania-containing silica aerogels byusing potassium titanate as precursor†

Wei Wei, Xiaomeng Lü, Deli Jiang, Zaoxue Yan, Min Chen and Jimin Xie*

Developing a novel and facile way to synthesize composite aerogels plays an important role in the appli-

cations of aerogels. UV-resistant hydrophobic titania-containing silica aerogels are prepared for the first

time using potassium titanate as precursor by a modified ambient pressure drying method. The well

established silica–titania networks, which can be tuned from 10 to 30 nm by adjusting the precursor

content in the preparation process, provide effective confinement of spherical solid clusters. The UV-

resistant hydrophobic composite aerogels show excellent photocatalytic dye degradation activity under

visible light irradiation. This can be ascribed to the insert of suitable titania into the silica organizational

structure. The present work gives a promising method of one pot synthesis and surface modification of

aerogel composite structures, which have a broader application as photocatalyst.

Introduction

Nanostructured titania–silica composite aerogels have receivedenormous attention due to their promising high activity inheterogeneous reactions.1–9 High porosity and large specificsurface area of aerogels prepared by the sol–gel method arecharacteristic properties that have made them very attractive incatalysis.3 Since Wang et al. studied the properties of mixedaerogels of TiO2–SiO2 with the incorporation of transitionmetal ions,4 various synthesis routes have been extended todiscover new visible light photocatalyst formation for photo-catalysis applications. Ismail and co-workers reported optimumconditions for preparation of TiO2–SiO2 aerogels as photo-catalyst for degradation of CN−.5 Furthermore, Kim et al. syn-thesized TiO2–SiO2 aerogel powders with desirable propertiesfor decolorization of organic pollutants using a less expensivesilica source and titanium oxychloride as a titania precursor.6,7

In all these cases, it is important to insert a suitable light har-vesting material into silica in order to introduce photocatalyticactivities.8 To date, numerous scientific efforts of aerogelresearch have been conducted to find a novel synthesismethod for composite binary aerogel photocatalysts.10–15

Meanwhile, using a less expensive source and a simple drying

operation in nanostructured titania–silica composite aerogelsare also highly desirable.

As is well known, conventional titania–silica compositeaerogels synthesis incorporates the guest component into thesilica structure at the stage of silicon precursor formation.9–16

It is accepted that the aerogel structure is formed under suit-able conditions determined by the reaction rates of binaryprecursors in cluster forming and enlarging.1,3,11 When thenucleation rate is not slower than or comparable to the growthrate, mixed separation precipitation will occur.12 Only byadjusting the reaction times of different precursors separ-ately,13,17 researchers can obtain composite aerogels when thebinary precursors are mixed at an appropriate time.18

Obviously, this way of preparing binary composite aerogelswith a certain structure is uncontrollable and inconclusive.Thus, it becomes a challenge to find a facile, cost-effectivemethod for preparation of binary composite aerogels with acertain artificial structure.

Low-temperature synthetic techniques such as ion-exchangereaction called “chimie douce” have resulted in major develop-ments in the field of soft-chemical of transition metaloxides.19,20 Ion-exchange reaction is an exchange of ionsbetween two electrolytes or between an electrolyte solutionand a complex. This process can be unselective or havebinding preferences for certain ions or classes of ions, depend-ing on their chemical structures.21,22 Chen et al. first preparedmesoporous titania synthesized from potassium titanate(K2Ti2O5) by hydration, ion exchange and calcination.23 Later,Lü’s group found a special way to prepare titanium oxide(TiO2–B nanofibers) by using tetratitanate (H2Ti2O5·xH2O or

†Electronic supplementary information (ESI) available. See DOI: 10.1039/c3dt53389a

School of Chemistry and Chemical Engineering, Jiangsu University, Zhenjiang

212013, P.R. China. E-mail: westwater@vip.qq.com, xiejm391@sohu.com;

Fax: +86 11 88791800; Tel: +86 11 88791708

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H2Ti4O9·xH2O) as precursor, which was derived from potass-ium tetratitanate (K2Ti2O5 or K2Ti4O9) via exchange.24 Mean-while, Li and co-workers reported potassium titanate(K2Ti6O13) could be fully transferred to pure TiO2 through ahydrothermal reaction in acidic solution.25 These studies showthat process conditions such as ion-exchange reaction and pot-assium tetratitanate have a significant influence on the mor-phology and size of the as-synthesized titania products.

As is well known, K2Ti6O13 as potassium tetratitanate withhigh chemical and thermal stability has a potential applicationas photocatalyst for water decomposition.26 Zhang alsoreported an easy way to prepare a series of nanostructuredK2Ti6O13 whisker-doped SiO2 composite aerogels.27 However,Lee et al. reported that more than 0.02 mol L−1 titanium wasextracted at [H+] > 0.05 mol L−1, indicating that the microstruc-ture of K2Ti6O13 was destroyed.28 Therefore, K2Ti6O13 can beused with negligible leaching in a solution with pH > 2.0.Besides, Li’s group has found the transformation mechanismfrom H/K-titanate to TiO2 at lower H+ concentration wouldproceed with a dissolution and nucleation mechanism, thesurface of K2Ti6O13 gradually decomposes and produces Ti–(OH)4 fragments.29

In this paper, we have successfully developed a facile, low-cost synthetic method to prepare UV-resistant hydrophobictitania-containing silica aerogels by using potassium titanateas a titania source. A simple and one-pot sol–gel method withan ion-exchange reaction was employed in the synthesis oftitania-containing silica aerogels with controlled morphologyand structure. In addition, the composite aerogels can float onwater due to their hydrophobic surface in waste-water treat-ment. It is found that UV-resistant hydrophobic titania-con-taining silica aerogels exhibited higher photocatalytic activityand were easier to recycle from solution, suggesting a potentialapplication in waste-water treatment. The combined effectbetween the structure and titania content on improving thephotocatalytic activity of titania–silica oxide aerogels wereexplored in detail.

ExperimentalChemicals

Tetraethoxysilane (TEOS), anhydrous ethanol (C2H5OH),n-hexane (≥95%), oxalic acid, aqua ammonia (25 wt%,NH3·H2O) and trimethylchlorosilane (TMCS) were purchasedfrom Sinopharm Chemical Reagent Co., Ltd, China whilepotassium titanate (K2Ti6O13), whiskers were obtained fromShanghai Composites Whisker Manufacturing, Shanghai,P. R. China. All chemicals were used as received withoutfurther purification.

Synthesis of titania-containing silica aerogels (TSA)

The superhydrophobic silica aerogels were synthesized by asol–gel method as has been described in our previousreports.30,31 The TSA were synthesized by the following pro-cedure: firstly, TEOS was mixed with C2H5OH and then oxalic

acid was added as acidic catalyst under vigorous stirring for10 min. Secondly, a particular amount of potassium titanate asa titanium source was added directly into the above solution.The resulting mixture was stirred for 12 h at 60 °C. Thenammonia water solution was added as the basic catalyst. Themolar ratio of TEOS : C2H5OH : oxalic acid (0.008 M) : ammoniawater solution (0.5 M) was set at 1 : 8 : 4 : 0.1. After gelation, thewater in the alcohol gels was exchanged with C2H5OH threetimes in 36 h and then with n-hexane three times in 36 h. Afterthe washing, TMCS was applied to the gels. The gels weremodified by 20% TMCS in hexane for 24 h at rt. The unreactedTMCS was washed with n-hexane three times in 36 h. The pro-duced from solvent surface modification step and carried overfrom the unreacted chemicals, were removed three times withn-hexane washing. Then, the solvent was decanted and thegels were covered with an aluminium foil and dried at60 °C/4 h, 80 °C/2 h, 120 °C/2 h, 200 °C/1 h, respectively.Finally, the precipitates of Ti-containing silica aerogels withdifferent theoretical mass percentages (0.5, 1, 2, 4%) ofK2Ti6O13 to initial silica aerogels (denoted as TSA-1, TSA-2,TSA-3, TSA-4, respectively) were collected after cooling down tort. These aerogel samples were also annealed to improve thecrystallinity at different temperatures ranging from 400 to800 °C. (The calcined samples were dubbed TS3-calcinationtemperature. For example, the TSA-3 powder annealed at600 °C was denoted as TS3-600.)

Characterization

The phase of the as-synthesized products was characterizedusing X-ray diffraction (XRD, Bruker D8 Advance diffracto-meter) with Cu Kα radiation (λ = 1.5406 Å) at a scanning rateof 0.02° s−1 in the 2θ range from 10 to 80°. The morphology ofas-prepared samples were examined by scanning electronmicroscopy (SEM) using a field emission scanning electronmicroscope (JEOL JSM-7001F, Japan), transmission electronmicroscopy (TEM) and high resolution transmission electronmicroscopy (HRTEM) (JEOL-JEM-2010, Japan) operating at 200kV. Selected area electron diffraction (SAED) analysis was alsorecorded on a JEOL-JEM-2010. The chemical states of thesamples were recorded in the NEXUS-470 FT-IR apparatus(Thermo Nicolet, USA). Elemental analysis was conducted byinductively coupled plasma-atomic emission spectroscopy(ICP-AES, Optima 2000DV, USA). Brunauer–Emmett–Teller(BET) and Barrett–Joyner–Halenda (BJH) methods on the nitro-gen adsorption apparatus (Quantachrome Instruments, USA)were applied to determine the specific surface area, porevolume and pore size distribution. The contact angle (θ) wasmeasured by a contact angle meter (OCA20 contact angle ana-lyzer; Data Physics, San Jose, CA). The UV-vis diffuse reflec-tance spectra of the as-prepared samples were measured usinga UV-vis spectrophotometer (UV-2450, Shimadzu, Japan).

Photocatalytic performance measurements

The photocatalytic activity of the photocatalyst was tested inthe degradation of Rhodamine B (RhB) aqueous solution(10 mg L−1) under irradiation by using a 350 W Xenon arc

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lamp with a UV-cutoff filter (420 nm) and was positioned20 cm away from the reactor (GHX-2, luminous flux = 15 lmW−1, Yangzhou University Technology Co., Ltd, China). For atypical photocatalytic experiment, a total of 0.1 g catalystpowders was added to 100 mL of the above RhB solution inthe quartz tube. Prior to irradiation, the suspensions weremagnetically stirred in the dark for 30 min to ensure establish-ing an adsorption/desorption equilibrium. The above suspen-sions were kept under constant air-equilibrated conditionsbefore and during irradiation. At given time intervals, about2 mL aliquots were sampled and centrifuged to remove theparticles. The filtrates were analyzed by measuring themaximum absorbance of RhB at 550 nm using an UV-visspectrophotometer.

Active species trapping experiments

According to our previous work,33 in order to detect the activespecies during the photocatalytic process, some sacrificialagents, such as ammonium oxalate (AO), 1,4-benzoquinone(BQ) and tert-butanol (t-BuOH) were used as the hole (hvb

+)scavenger, superoxide radical (O2

•−) scavenger and hydroxylradical (OH•) scavenger, respectively. This method was similarto the former photocatalytic experiment with an addition of1 mmol of quencher in the presence of RhB.

Results and discussion

Titania-containing silica aerogels were formed by the ion-exchange with the alkoxysilanes hydrolysis–condensationprocess, as illustrated in Scheme 1. It is expected that there are6 main chemical reactions existing during the whole process,as follows:

HxK2�xTi6O13 þ 11H2Oþ ð2� xÞHþ ! 6TiðOHÞ4 þ ð2� xÞKþ

ð1ÞSiðOC2H5Þ4 þ 4H2O ! SiðOHÞ4 þ 4C2H5OH ð2Þ

SiðOC2H5Þ4 þ ðOHÞ4Si ! ðOHÞ3Si–O–SiðOC2H5Þ3þ C2H5OH ð3Þ

SiðOC2H5Þ4 þ TiðOHÞ4 ! ðOHÞ3Ti–O–SiðOC2H5Þ3þ C2H5OH ð4Þ

TiðOHÞ4 þ ðOHÞ4Si ! ðOHÞ3Ti–O–SiðOHÞ3 þH2O ð5ÞSiðOHÞ4 þ ðOHÞ4Si ! ðOHÞ3Si–O–SiðOHÞ3 þH2O ð6Þ

In a typical way, the surface of K2Ti6O13 gradually decom-posed to Ti(OH)4 fragments, to react with acid under stirringcondition at higher temperature according to eqn (1). Then,Ti(OH)4 was rearranged through a dehydration reactionbetween Ti–OH and HO–Ti in an edge-sharing manner,29

while tetraethoxysilane (TEOS) in ethanol was added with adefined amount of water to start the hydrolysis [eqn (2)] andcondensation [eqn (3)] reactions. Condensation already occurseven if not all of the OC2H5 groups were hydrolyzed. Smallclusters were initially formed by the condensation reactions[eqn (4)–(6)] and then the sol particles which eventually formthe oxidic gel network. However, all intermediate species stillcontain Si–OH, Ti–OH, Si–OC2H5 groups. Hydrolysis thereforetakes place parallel to condensation during all steps of the sol–gel process.33 Then, the gels were exchanged with a solventand silylation in modification condition after the compositegel was formed. As has been reported,30,35 the silylated compo-site gels were washed with solvent prior to ambient pressuredrying.

Phase structures and surface states

As shown in Fig. 1A-a, the XRD pattern of potassium titanatereveals two distinct diffraction peaks at 11.48 and 43.35°,which can be indexed to the (200) and (313) diffraction planesof the monoclinic potassium titanium oxide, and match wellthe data (space group C2/m, JCPDF card no. 40-0403).29,35

Fig. 1A-(b–d) provides XRD patterns of the samples obtainedin the presence of different amounts of potassium titaniumoxide as precursor. From Fig. 1b–d, no peaks of potassium tita-nate can be detected, indicating the amorphous structure ofthe obtained products. However, unreacted potassium titanatecan be detected when the level number of potassium titaniumoxide was up to 4% (Fig. 1A-e). In view of the low crystallinity,the TSA samples were also calcined to improve crystallinity atdifferent temperatures, ranging from 400 to 800 °C. Fig. 1Bpresents XRD patterns of the as-calcined products of TSA

Scheme 1 Schematic illustration of TSA composites with different titania mass ratios.

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(TSA-3 as the representative product). It displays a XRD patternof the samples by calcination treatment at 400 and 600 °C,indicating an amorphous phase. However, TSA-3 annealed at800 °C showed peaks for anatase TiO2 crystals at 25 and 38°while the TSA-4 showed significant peaks for anatase phase(JCPDS 21-1272). As is well known, it is commonly acceptedthat high temperature calcination, at least at 400 °C, isrequired to prepare anatase TiO2. Through this comparison we

found the high titania constant of TSA (TSA-3 and TSA-4) attemperatures ranging from 400 to 800 °C influences the amor-phous to anatase phase. These results were also proven bymany researchers with reference to calcined TiO2–SiO2 hybridaerogels6,30 and TiO2 samples by K-titanate as titaniumprecursor.24,25

In order to ascertain the elemental composition of the pre-pared samples ICP-AES analysis was carried out. As shown inTable 1, the amount of titanium present in the samples, TSA,varied with the amount of Ti-precursor used in the content.The concentrations of Ti detected in the samples were 0.78,1.34, 3.02, 6.01, and 2.76 mg L−1 for TSA-1 (0.5%), TSA-2 (1%),TSA-3 (2%), TSA-4 (4%) and TSA3-600 (2%), respectively. Thiswas slightly lower than the actual amount used in content. Thedifference may have resulted during washing of the preparedphotocatalyst. However, the potassium ion concentration inICP analysis can be detected for TSA-4 to clarify this samplecontains potassium titanate further. Thus, the ICP analysisfurther confirms the existence of Ti in the TSA compositesystem prepared in this research.

The FTIR spectra based on samples are shown in Fig. 2.The absorption peaks indexed to symmetrical and asymmetricalstretching of Si–O–Si can be seen at 830 cm−1 and 1094 cm−1,respectively.36 The peak spanning from 2896 to 2985 cm−1 iscontributed to both the CH2 and CH stretchings of the CH3

stretching of the surface modification reagent.37 The presenceof these alkyl groups made the modified aerogels hydrophobic.The peak located at 848 cm−1 for the Si–C bond is a clearindication of the presence of a surface modification reagent,while the broadband of 3300 to 3500 cm−1 is accounted forphysically adsorbed water by the aerogel composites.36 Andabove all, the presence of a heterolinkage Ti–O–Si band at935–950 cm−1 indicates incorporation of TiO2 into SiO2 toform binary TiO2–SiO2 systems.38,39

Fig. 3 shows the Raman spectrum for samples (TSA-1,TSA-3, TSA-4 and K2Ti6O13). A similar spectrum was observedfor sample (TSA-2) but is not shown. These bands at 610 and490 cm−1 can be attributed to the D1 and D2 bands of the silicaaerogels support,40 respectively, confirming that the structureof mesoporous silica was not destroyed during addition ofK2Ti6O13. The band at 800 cm−1 is assigned to the network Si–O–Si symmetric bond stretching, further proving that the struc-

Fig. 1 (A) XRD patterns for the as-prepared samples: (a) K2Ti6O13, (b)TSA-1, (c) TSA-2, (d) TSA-3, and (e) TSA-4; (B) XRD of the calcinedsamples TSA3-400, TSA3-600, TSA3-800 and TSA4-800.

Table 1 Physiochemical properties of titania-containing silica aerogels

Physiochemical properties

Sample name

TSA-1 TSA-2 TSA-3 TSA-4 TS3-600

Mass of K2Ti6O13 (mg) 13.5 27.4 54.1 108.2 54.1Volume of TEOS (mL) 10 10 10 10 10Potassium ion concentration (mg L−1) — — — 2.26 —Titanium ion concentration (mg L−1) 0.78 1.34 3.02 6.01 2.76Si/Ti 210 105 52 26 52Specific surface area (m2 g−1) 628.52 380.77 210.05 103.52 111.19Pore diameter (nm) 9.634 9.509 9.86 12.31 17.62Pore volume (cm3 g−1) 2.658 1.276 0.67 0.25 0.923Reaction rate constant k (min−1) 0.00702 0.01074 0.01384 0.00421 0.0047Correlation coefficient R 0.9418 0.9708 0.9907 0.9219 0.9990

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ture inside particles of TSA is that of silica glass.41 Furthermore,the band at 1079 cm−1 can be assigned to Ti–O–Si bonds. Theappearance of the band indicates that the titanium element isbonded to the SiO2 support. The characteristic bands (K–O–Ti:201 cm−1, 400 cm−1; Ti–O–Ti: 650 cm−1; Ti–O: 858 cm−1) forK2Ti6O13 are observed evidently with the increase of potassiumtitanate addition when it was 4%,42 These results indicate thatTSA-4 is a mixture of titania-containing silica aerogels and par-ticles of K2Ti6O13. However, the as-prepared samples (TSA-1,TSA-2 and TSA-3) contained some amount of methyl, whichindicated by the bands between 2900 and 3000 cm−1 in spec-trum c of Fig. 3.42 It is further proven that TSA has the advan-tage of high oil absorbence due to the hydrophobic group.Therefore, the existence of titanium in these samples (TSA-1,TSA-2 and TSA-3) is incorporation of TiO2 into SiO2 supportaccording to the experimental results.

Morphological investigations

SEM micrographs of TSA-1–TSA-4 (Fig. 4a–f ) with variousK2Ti6O13 mass concentrations are shown in Fig. 4. It can beclearly seen that the aerogels possessed aggregates with spheri-cal solid clusters. The dense aggregates of particles wereobserved with increasing potassium titanate mass concen-tration in the same preparation situation. An increase in theK2Ti6O13/TEOS molar ratio from 0.00078 in TSA-1 (Fig. 4b)sample to 0.0031 in the TSA-3 (Fig. 4d) sample results in theaggregation of the clusters and small voids among them.43 TheTSA-4 gelation at highest K2Ti6O13/TEOS molar ratio of 0.0063causes the numbers of K2Ti6O13 whisker precursors among theTSA clusters. Preparation at low K2Ti6O13/TEOS molar ratioresulted in slow aggregation and small and bulky clusters withsmall voids within the obtained structure. Individual silicaaerogels with no K2Ti6O13 whisker added have a compactnetwork with a particle size of about units digit of nanometerswhile that of the titania-containing silica aerogels with various

K2Ti6O13 mass concentration is as large as several tens ofnanometers.

However, K2Ti6O13 whisker was observed (Fig. 4e) with theincrease of potassium titanate addition when it was up to 4%.These results also indicate that TSA-4 is a mixture of Ti-con-

Fig. 3 Raman spectrum of samples in the frequency ranges (a)50–3400 cm−1, (b) 50–1500 cm−1 and (c) 2800–3200 cm−1.

Fig. 2 FT-IR spectra of various samples: (a) TSA-1, (c) TSA-2, (d) TSA-3,and (e) TSA-4.

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Fig. 4 SEM, EDX spectra of as-prepared aerogels: SEM image of (a) silica aerogel, (b) TSA-1, (c) TSA-2, (d) TSA-3, (e) TSA-4, (f ) TSA-4; EDX patternof (g) TSA-3, (h) TSA-4.

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taining silica aerogels and K2Ti6O13 whisker. The K2Ti6O13

whisker was dispersed within the skeletal silica nanoparticles.When the TSA-4 part was magnified (Fig. 4f), the nanosizedagglomeration solid clusters were known to exist.42 Therefore,according to the quantitative analysis of EDX (Fig. 4h), themolar ratio of K to Ti was found as 1 : 2.73, which was consist-ent with the stoichiometric K2Ti6O13 within experimentalerror. Moreover, only three kinds of elements (Si, Ti and O)can be detected in a single TSA-3 system by energy spectrumanalysis in Fig. 4g. The conclusion is that the skeletal silicananoparticles have been coated conformally with Ti–O–Tibands according to the above experimental results.

TEM images of the obtained silica aerogels and TSA-1–TSA-3 are also presented in Fig. 5. These images clearly

demonstrate the effect of K2Ti6O13/TEOS molar ratio on theformation of TSA network structure. In the silica aerogelssample, in the presence of sufficient TEOS, uniform andregular growth of small silica particles and pores lead to ahomogeneous silica network.43 Fig. 5g–i show that at highK2Ti6O13/TEOS molar ratio, a uniform net work structure withlarger clusters as compared to the silica aerogel is formedwhich can be related to irregular growth of the particles andTi–O–Si network. As can be seen, the structure of the silicaaerogels is very dense with close packing of silica particles,and there is a porous structure in the TSA sample. Comparedwith the TSA sample, it can be observed that silica aerogels(Fig. 5a–c) exhibit a porous network structure which includes<10 nm spherical solid clusters and pores below 20 nm

Fig. 5 Representative TEM micrographs of (a–c) silica aerogels; (d–f ) TSA-1; (g–i) TSA-2; ( j–l) TSA-3.

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between them, which is consistent with previous studies.30

The composite aerogels (Fig. 5d–l) consisting of titanium showa large quantity of irregular nanoparticles and the diametersof these nanoparticles are in the range of 20–25 nm. Thespherical titania-containing SiO2 particles with a size of a fewtens of nanometers form a three-dimensional network contain-ing homogeneous pores (size in the range of 50–80 nm). Theresulting networks of all these samples are characterized bysmall particles and pores (called “polymeric” gel). Theseresults are owing to the sol–gel method such as long gelationtimes and relative rates of hydrolysis and condensation.33 Con-sequently, the high concentration of titania molecules pro-duces a dense network with close packing of the particles.This may be due to the increased content of titania in the aero-gels.34 In order to study the crystalline structures of TSA at thenanoscale, we choosed a TSA-3 as the detection target. A HRTEMimage of the spheres with obscure lattice fringes is presented inFig. S1,† confirming its low crystallinity. Further evidence of theexistence of an amorphous phase is provided by the SAEDpattern. The broad, diffusive ring suggests a typical amorphousstructure. This is in accordance with the XRD results.

Hydrophobic properties

The hydrophobicity of the samples was measured by thecontact angles measurements of the surface using a waterdroplet as an indicator (Fig. 6a). The contact angles of theaerogel samples were established with the modificationprocess to be 145 ± 5°. As Rao reports,44 if the aerogel samplehas lower hydrophilicity (more hydrophobic), due to conden-sation of OH groups on the surface of the gel, the gel col-lapses, resulting in more shrinkage and hence dense gels, asshown in Fig. 5a for silica aerogels, whereas in the case of tita-nium-containing silica aerogels due to adequate silylation withinactive Si–CH3 groups, the impact factor of spring back ishigh enough to retain its original volume, resulting in aporous network, as seen in Fig. 5e. So, the influence of con-struction on the aerogel samples may depend on severalsurface states of the samples. Under the same silylation con-ditions, the titania-containing silica aerogel samples showmore hydrophobicity than the original samples (silica aero-gels). When the whisker is added in the preparation process,this process is obviously limited by the amount of gelationwater remaining adsorbed on the skeletal framework, which inturn can be controlled to a certain extent by the number ofpre-crosslinking washes with fresh solvent.33 However, TSA-4shows less hydrophobicity than silica aerogels due to lots ofunreacted K2Ti6O13 in the mixture matter. In conclusion,titania-containing aerogels are more hydrophobic than nativesilica aerogels. The hydrophobicity of the TSA3-600 was alsomeasured by the contact angles measurements (Fig. S2†).From the pictures of water contact angle on samples, thehydrophobicity of TSA-3 vanished entirely after heat-treatmentat 600 °C. Hydrophobicity turns into hydrophilicity due to thedisappearance of –CH3 groups on the surface by oxidation athigh temperature. Shi et al. have reported the pore size of aero-

gels decreases and hydrophobicity turns into hydrophilicityover certain high temperature (500 °C).46

It is more attractive to uncover the UV-durable hydrophobicproperty of titania-containing silica aerogels. As shown inFig. 6b, hydrophobic TSA-3 could maintain water contactangles after exposure under 50 W Hg lamp (λmax ∼ 265 nm) for180 min. Considering that the intensity of UV irradiation isstronger than that in sunlight, these results clearly demon-strate that titania-containing silica aerogels have ratherdurable hydrophobicity and may have potential applications inself-cleaning coatings in our daily life. Moreover, UV-resistanceis an important property to prove the stability of TSA in thephotocatalysis process. TSA did not lose their hydrophobicityunder illumination with UV light or visible light. Furthermore,TSA can still float on water and this can be used for waste-water treatment in the photocatalysis process. This interestingphenomenon provides us with a new angle to use the TSA asrecyclable material.

N2-sorption studies

To consider the porous structure and specific surface area ofTSA, N2 absorption analysis is carried to characterize the as-

Fig. 6 (a) Evolution of the water contact angle on aerogel samples. (b)Stability of TSA-3 in terms of contact angle to water. Inset is the shapeof a water droplet on the surface of aerogel samples.

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prepared samples. Fig. 7 shows the isotherm for TSA samples,which exhibit characteristic Type IV isotherms, signifying thepresence of a mesoporous structures.7,34 The hysteresis loopsof TSA samples seem to be very close to that of type H1 accord-ing to the IUPAC classification systems. All of these samplesshow hysteresis loops with a triangular shape at relative press-ures P/P0 ≥ 0.75 and relatively steep desorption attributed tothe pore connectivity effect.44 It can be found that the preparedTSA-1 shows a relatively large specific surface area (628.52 m2

g−1). Compared with TSA-2, TSA-3, and TSA-4 , it exhibitsdecreased BET surface areas due to the incorporation of a tita-nium composite which has a low BET surface area. Typically,TSA-1 specific surface area is almost 6.07 times that with TSA-4due to the more interlaced K2Ti6O13 on the surface of titania-containing silica aerogels (103.52 m2 g−1) structure. Thereduction in surface area resulting from titania-containingconfinement can be attributed to the occupation of titaniamaterial in the pores of the aerogels, as mentioned earlier.Meanwhile, the nitrogen adsorption–desorption isotherm ofthe calcined product (TSA3-600) is presented in Fig. S3.† TheTSA3-600 displayed a lower surface area (111.19 m2 g−1) ascompared to the TSA-3 (210.05 m2 g−1). There was a drasticdecrease of surface areas upon calcination as a result ofcrystallization.47,48 It is noticeable that the calcined sampledisplayed large pore sizes than the raw sample after removalof –CH3 groups on the surface through calcination. The pore-size distribution of the samples is also estimated usingthe Barrett–Joyner–Halenda (BJH) process from thedesorption branch of the isotherm, as shown in the inset ofFig. 7. The calculated pore size distribution using BJHmethod indicated that the size of mesopores is not uniformranging from 8 to 50 nm, which is consistent with TEMobservations. The specific surface area, pore volume and poresize distribution of different samples are also summarized inTable 1.

Photocatalytic activity and possible mechanism of TSAsamples in degradation of Rhodamine B (RhB)

Fig. 8a shows the UV-visible diffuse reflectance spectra of TSAsamples. As shown in Fig. 8a the band at 550 nm is observedfor all materials except for TSA-1 which presents a clear shiftto a lower wavelength. The enhancement of visible lightabsorption for TSA should be attributed to the amount of Ti-content in the SiO2 structure. For silica based photocatalyticsystems, bands at 210, 250 and 296 nm can be easily recog-nized in the system. The 250 nm band corresponds to hydratedtetrahedral Ti species.39 As is well known, transition metalsoxides are very good candidates for absorption in the visiblerange of the spectrum, as orbital energy transfer usually lies inthe visible range. Herein, to achieve visible light activity, it isnecessary to insert a suitable light harvesting material intosilica in order to introduce photocatalytic activities.8

Rhodamine B (RhB) is one of the commonly used organicdyes and has been widely applied in industrial production,which often contaminates the environment. In a photocatalytictest of TSA-3, RhB was selected as a pollutant to evaluate thephotocatalytic activity of the as prepared samples. As shown inFig. 8b, the maximum spectral peak of RhB solution shiftsfrom 550 to 500 nm. This blue shift is a common phenomenonin the de-ethylation process which indicates RhB is degradedin a stepwise manner.45 The peak intensity of RhB graduallydecreases with the irradiation time extended, which demon-strates that almost all dye molecules in the solution have beendecomposed in 150 min.

Furthermore, TSA-1–TSA-4 and the direct non-catalyticphotolysis of RhB were also tested under the same conditions.It can be found that the photolysis of RhB was negligible. Fur-thermore, a photocatalytic properties contrast experiment ofpure silica aerogels was carried out to clarify the adsorption orphotocatalytic activity of pure silica aerogels. As shown inFig. S4,† the maximum spectral peak of RhB solution doesn’tshift anymore. This appearance of test may demonstrate thatthe pure silica aerogels show minimal dye degradation and theadsorption of dye process in the photocatalytic test. The photo-catalytic activities of titania-containing silica aerogels (TSA-1,84.33%; TSA-2, 87.9%; TSA-3, 94.42%; TSA-4, 80.21%) wereenhanced as compared to those of our previous work on silica/titania aerogels,31 which suggest that TSA is an excellent com-posite photocatalyst under visible light. It is clear that thecontent of hydrated tetrahedral Ti species dramatically affectsthe photocatalytic activities of TSA even though the content ofhydrated tetrahedral Ti species was very low. When the theore-tical molar percentage of potassium titanate to silica aerogelwas 2%, the optimal photocatalytic degradation efficiency ofRhB was reached at 94.42%. This means that TSA-3 exhibitssuperior photocatalytic activity to other samples. Meanwhile,P25 photolysis of RhB was carried out under the same con-ditions. P25 as a reference sample only displayed 15.44%degradation rate after irradiation. Generally, the decolorizationrates of different samples were in the order: noncatalyst < P25< silica aerogels < TSA3-600 < TSA-4 < TSA-1 < TSA-2 < TSA-3.

Fig. 7 N2 adsorption–desorption isotherms of as-synthesized titania-containing silica aerogels (inset: pore size distribution of the as-syn-thesized samples).

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Therefore, the BET surface area, nanostructures and titaniacontent are the principal factors that influenced the photo-catalytic property. Moreover, Fig. 8d shows that photocatalyticdegradation of RhB on different catalysts fits pseudo-first-order kinetics, ln(C0/C) = kt, where C is the concentration ofthe RhB at a given irradiation time, C0 is the initial concen-tration of RhB solution (when adsorption equilibrium isreached), and slope k is the reaction rate constant. The resultimplies that the k value (0.01384 min−1) for TSA-3 isthe highest among all titania-containing silica aerogels(summarized in Table 1). We attribute the high visible-lightphotocatalytic activity not only to the appropriate contentof titania-containing structure but also to the unique

“polymeric” morphology. TSA-3 with high absorption abilitycould absorb more pollutant and provide more active sitesfor oxidative reactions. It is thought that the presence of crys-tallinity, BET surface area, hydrophobicity and particle sizesinfluenced the photocatalytic properties of hybrid aerogelsystems.

In the durability tests, the TSA-3 photocatalyst was alsoreused in three successive photoreaction experiments by cen-trifugation. Fig. 8e shows the results of three cycles of run ofphotodegradation of RhB using the reclaimed catalyst whilekeeping the experimental conditions unchanged. It can beseen that titania-containing silica aerogels do not show signifi-cant loss of photocatalytic activity in three successive runs. It

Fig. 8 (a) UV-vis spectrum of the samples; (b) UV-Vis spectral changes of RhB dye (sample: 100 mg; RhB: 10 mg L−1); (c) adsorption and photo-catalytic degradation of RhB by different samples; (d) ln(C0/C) − T for photocatalytic degradation of RhB; (e) recycling test on TSA-3 for RhB undervisible light irradiation; (f ) trapping experiment of active species during photodegradation of RhB over TSA-3 under visible light irradiation.

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indicates that the photocatalytic activity of Ti-containing silicaaerogels is readily reused.

Based on previous investigations,32 the decolorization ofdye solution can be supposed to occur due to formation of theactive radicals (superoxide radical or hydroxyl radicals) gener-ated during the photochemical process in the presence ofphotocatalysts. Shao et al. have reported that the TiO2–SiO2

composites aerogels can produce hydroxyl radical though theradical-trapping photoluminescence equipment.3,6,7 In orderto ascertain the active species in the photodegradation processof TSA-3, sacrificial agents such as AO, BQ and t-BuOH wereused as the hole (hvb

+) scavenger, superoxide radical (O2•−) sca-

venger and hydroxyl radical (OH•) scavenger in the photo-catalytic degradation of RhB, respectively. As shown in Fig. 8f,with 1 mmol AO as the hvb

+ scavenger added to the photode-gradation process, the rate of RhB degradation over TSA-3slightly decreased, indicating that little holes (hvb

+) areinvolved in the degradation of RhB. However, with addition of1 mmol BQ, degradation of RhB was also significantlydepressed, suggesting that the O2

•− pathways have played animportant role in the process of RhB oxidation. On the otherhand, 1 mmol t-BuOH was added into the solution to removehydroxyl radicals. In the absence of hydroxyl radicals, theremoval rate of RhB decreased from 94.42 to 68.36%. There-fore, hydroxyl radicals have also played an important role inRhB removal.

According to the effects of different scavengers and earlierreports,6,7 the photooxidations mechanism of titania-contain-ing silica aerogels system may involve indirect reaction withOH• radicals, indirect reaction with O2

•− radicals, or a dualmechanism involving both surface O2

•− and OH• radicals.These results indicate that titania-containing silica aerogelsystem photocatalysts exhibited excellent in situ photodegrada-tion of the organic dye. The pathway of photocatalytic degra-dation can be described throughout the following equations:

TSA þ hν ! TSA ðe� þ hνbþÞ ð7Þ

OH� þ hνbþ ! OH• ð8Þ

e� þ O2 ðairÞ ! O2•� ð9Þ

H2Oþ hνbþ ! OH• þHþ ð10Þ

OH• þ dye ! degradation products ð11Þ

O2•� þ dye ! degradation products ð12Þ

Due to the hydrophobicity of TSA, the samples can float onwater and can contact with more air (O2). When light illumina-tion is present, the electrons (e−) accumulated on the surfaceof titania-containing silica aerogels are then scavenged byoxygen on the surface of the catalyst to form super oxide radi-cals (O2

•−), which react with RhB dye.

Conclusions

In summary, the potassium titanate precursor prepared by anion-exchange reaction with an alkoxysilanes hydrolysis–condensation process, together with surface modification andambient pressure drying processes, was utilized to fabricatetitania-containing silica aerogels for efficient photocatalyticdegradation of RhB. The resulting composite aerogels possesshigh specific surface areas, UV-durable hydrophobic propertiesand the adjusted nanostructures exhibited enhanced visible-light-driven photocatalytic activity in the degradation of RhBin aqueous solution. The optimum photocatalytic activity of2% titania-containing silica aerogels (TSA-3) for the degra-dation of RhB was attributed to the appropriate structure andtitania content. Moreover, it is found that the photodegrada-tion of RhB molecules is mainly attributed to indirect reactionwith OH• radicals, the oxidation action of O2

•− radicals, or adual mechanism involving both surface O2

•− and OH• radicals.This work indicates a novel and one pot method to preparecomposite aerogels. This convenient preparation and facileconstruction of aerogels structures via an ambient pressuredrying process are great advantages for expanding the aerogels’practical applications.

Acknowledgements

This work was partly supported by the National NaturalScience Foundation (21003065, 21306067), the Natural ScienceFoundation of Jiangsu Province (BK2010044), the IndustryHigh Technology Foundation of Jiangsu Province (BE2013090)and the Science & Technology Foundation of Zhenjiang(SH2010025, SH2011005, SH2012011 and GY2012048),China. A. M. Showkot Hossain is acknowledged for his help inthe modification of the English language in this manuscript.

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