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A novel route for synthesis of UV-resistant hydrophobic titania-containing silica aerogels by using potassium titanate as precursor†
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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 applications 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 UVReceived 2nd December 2013, Accepted 22nd March 2014
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
DOI: 10.1039/c3dt53389a
structure. The present work gives a promising method of one pot synthesis and surface modification of
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aerogel composite structures, which have a broader application as photocatalyst.
Introduction Nanostructured titania–silica composite aerogels have received enormous attention due to their promising high activity in heterogeneous reactions.1–9 High porosity and large specific surface area of aerogels prepared by the sol–gel method are characteristic properties that have made them very attractive in catalysis.3 Since Wang et al. studied the properties of mixed aerogels of TiO2–SiO2 with the incorporation of transition metal ions,4 various synthesis routes have been extended to discover new visible light photocatalyst formation for photocatalysis applications. Ismail and co-workers reported optimum conditions for preparation of TiO2–SiO2 aerogels as photocatalyst for degradation of CN−.5 Furthermore, Kim et al. synthesized TiO2–SiO2 aerogel powders with desirable properties for decolorization of organic pollutants using a less expensive silica source and titanium oxychloride as a titania precursor.6,7 In all these cases, it is important to insert a suitable light harvesting material into silica in order to introduce photocatalytic activities.8 To date, numerous scientific efforts of aerogel research have been conducted to find a novel synthesis method for composite binary aerogel photocatalysts.10–15 Meanwhile, using a less expensive source and a simple drying
School of Chemistry and Chemical Engineering, Jiangsu University, Zhenjiang 212013, P.R. China. E-mail:
[email protected],
[email protected]; Fax: +86 11 88791800; Tel: +86 11 88791708 † Electronic supplementary information (ESI) available. See DOI: 10.1039/ c3dt53389a
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operation in nanostructured titania–silica composite aerogels are also highly desirable. As is well known, conventional titania–silica composite aerogels synthesis incorporates the guest component into the silica structure at the stage of silicon precursor formation.9–16 It is accepted that the aerogel structure is formed under suitable conditions determined by the reaction rates of binary precursors in cluster forming and enlarging.1,3,11 When the nucleation rate is not slower than or comparable to the growth rate, mixed separation precipitation will occur.12 Only by adjusting the reaction times of different precursors separately,13,17 researchers can obtain composite aerogels when the binary precursors are mixed at an appropriate time.18 Obviously, this way of preparing binary composite aerogels with a certain structure is uncontrollable and inconclusive. Thus, it becomes a challenge to find a facile, cost-effective method for preparation of binary composite aerogels with a certain artificial structure. Low-temperature synthetic techniques such as ion-exchange reaction called “chimie douce” have resulted in major developments in the field of soft-chemical of transition metal oxides.19,20 Ion-exchange reaction is an exchange of ions between two electrolytes or between an electrolyte solution and a complex. This process can be unselective or have binding preferences for certain ions or classes of ions, depending on their chemical structures.21,22 Chen et al. first prepared mesoporous 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
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H2Ti4O9·xH2O) as precursor, which was derived from potassium tetratitanate (K2Ti2O5 or K2Ti4O9) via exchange.24 Meanwhile, Li and co-workers reported potassium titanate (K2Ti6O13) could be fully transferred to pure TiO2 through a hydrothermal reaction in acidic solution.25 These studies show that process conditions such as ion-exchange reaction and potassium tetratitanate have a significant influence on the morphology and size of the as-synthesized titania products. As is well known, K2Ti6O13 as potassium tetratitanate with high chemical and thermal stability has a potential application as photocatalyst for water decomposition.26 Zhang also reported an easy way to prepare a series of nanostructured K2Ti6O13 whisker-doped SiO2 composite aerogels.27 However, Lee et al. reported that more than 0.02 mol L−1 titanium was extracted at [H+] > 0.05 mol L−1, indicating that the microstructure of K2Ti6O13 was destroyed.28 Therefore, K2Ti6O13 can be used with negligible leaching in a solution with pH > 2.0. Besides, Li’s group has found the transformation mechanism from H/K-titanate to TiO2 at lower H+ concentration would proceed with a dissolution and nucleation mechanism, the surface of K2Ti6O13 gradually decomposes and produces Ti– (OH)4 fragments.29 In this paper, we have successfully developed a facile, lowcost synthetic method to prepare UV-resistant hydrophobic titania-containing silica aerogels by using potassium titanate as a titania source. A simple and one-pot sol–gel method with an ion-exchange reaction was employed in the synthesis of titania-containing silica aerogels with controlled morphology and structure. In addition, the composite aerogels can float on water due to their hydrophobic surface in waste-water treatment. It is found that UV-resistant hydrophobic titania-containing silica aerogels exhibited higher photocatalytic activity and were easier to recycle from solution, suggesting a potential application in waste-water treatment. The combined effect between the structure and titania content on improving the photocatalytic activity of titania–silica oxide aerogels were explored in detail.
Experimental Chemicals Tetraethoxysilane (TEOS), anhydrous ethanol (C2H5OH), n-hexane (≥95%), oxalic acid, aqua ammonia (25 wt%, NH3·H2O) and trimethylchlorosilane (TMCS) were purchased from Sinopharm Chemical Reagent Co., Ltd, China while potassium titanate (K2Ti6O13), whiskers were obtained from Shanghai Composites Whisker Manufacturing, Shanghai, P. R. China. All chemicals were used as received without further purification. Synthesis of titania-containing silica aerogels (TSA) The superhydrophobic silica aerogels were synthesized by a sol–gel method as has been described in our previous reports.30,31 The TSA were synthesized by the following procedure: firstly, TEOS was mixed with C2H5OH and then oxalic
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acid was added as acidic catalyst under vigorous stirring for 10 min. Secondly, a particular amount of potassium titanate as a titanium source was added directly into the above solution. The resulting mixture was stirred for 12 h at 60 °C. Then ammonia water solution was added as the basic catalyst. The molar ratio of TEOS : C2H5OH : oxalic acid (0.008 M) : ammonia water solution (0.5 M) was set at 1 : 8 : 4 : 0.1. After gelation, the water in the alcohol gels was exchanged with C2H5OH three times in 36 h and then with n-hexane three times in 36 h. After the washing, TMCS was applied to the gels. The gels were modified by 20% TMCS in hexane for 24 h at rt. The unreacted TMCS was washed with n-hexane three times in 36 h. The produced from solvent surface modification step and carried over from the unreacted chemicals, were removed three times with n-hexane washing. Then, the solvent was decanted and the gels were covered with an aluminium foil and dried at 60 °C/4 h, 80 °C/2 h, 120 °C/2 h, 200 °C/1 h, respectively. Finally, the precipitates of Ti-containing silica aerogels with different theoretical mass percentages (0.5, 1, 2, 4%) of K2Ti6O13 to initial silica aerogels (denoted as TSA-1, TSA-2, TSA-3, TSA-4, respectively) were collected after cooling down to rt. These aerogel samples were also annealed to improve the crystallinity at different temperatures ranging from 400 to 800 °C. (The calcined samples were dubbed TS3-calcination temperature. For example, the TSA-3 powder annealed at 600 °C was denoted as TS3-600.) Characterization The phase of the as-synthesized products was characterized using X-ray diffraction (XRD, Bruker D8 Advance diffractometer) with Cu Kα radiation (λ = 1.5406 Å) at a scanning rate of 0.02° s−1 in the 2θ range from 10 to 80°. The morphology of as-prepared samples were examined by scanning electron microscopy (SEM) using a field emission scanning electron microscope (JEOL JSM-7001F, Japan), transmission electron microscopy (TEM) and high resolution transmission electron microscopy (HRTEM) (JEOL-JEM-2010, Japan) operating at 200 kV. Selected area electron diffraction (SAED) analysis was also recorded on a JEOL-JEM-2010. The chemical states of the samples were recorded in the NEXUS-470 FT-IR apparatus (Thermo Nicolet, USA). Elemental analysis was conducted by inductively coupled plasma-atomic emission spectroscopy (ICP-AES, Optima 2000DV, USA). Brunauer–Emmett–Teller (BET) and Barrett–Joyner–Halenda (BJH) methods on the nitrogen adsorption apparatus (Quantachrome Instruments, USA) were applied to determine the specific surface area, pore volume and pore size distribution. The contact angle (θ) was measured by a contact angle meter (OCA20 contact angle analyzer; Data Physics, San Jose, CA). The UV-vis diffuse reflectance spectra of the as-prepared samples were measured using a UV-vis spectrophotometer (UV-2450, Shimadzu, Japan). Photocatalytic performance measurements The photocatalytic activity of the photocatalyst was tested in the 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 positioned 20 cm away from the reactor (GHX-2, luminous flux = 15 lm W−1, Yangzhou University Technology Co., Ltd, China). For a typical photocatalytic experiment, a total of 0.1 g catalyst powders was added to 100 mL of the above RhB solution in the quartz tube. Prior to irradiation, the suspensions were magnetically stirred in the dark for 30 min to ensure establishing an adsorption/desorption equilibrium. The above suspensions were kept under constant air-equilibrated conditions before and during irradiation. At given time intervals, about 2 mL aliquots were sampled and centrifuged to remove the particles. The filtrates were analyzed by measuring the maximum absorbance of RhB at 550 nm using an UV-vis spectrophotometer.
Active species trapping experiments According to our previous work,33 in order to detect the active species during the photocatalytic process, some sacrificial agents, 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 hydroxyl radical (OH•) scavenger, respectively. This method was similar to the former photocatalytic experiment with an addition of 1 mmol of quencher in the presence of RhB.
SiðOC2 H5 Þ4 þ TiðOHÞ4 ! ðOHÞ3 Ti–O–SiðOC2 H5 Þ3 þ C2 H5 OH
ð4Þ
TiðOHÞ4 þ ðOHÞ4 Si ! ðOHÞ3 Ti–O–SiðOHÞ3 þ H2 O
ð5Þ
SiðOHÞ4 þ ðOHÞ4 Si ! ðOHÞ3 Si–O–SiðOHÞ3 þ H2 O
ð6Þ
In a typical way, the surface of K2Ti6O13 gradually decomposed to Ti(OH)4 fragments, to react with acid under stirring condition at higher temperature according to eqn (1). Then, Ti(OH)4 was rearranged through a dehydration reaction between Ti–OH and HO–Ti in an edge-sharing manner,29 while tetraethoxysilane (TEOS) in ethanol was added with a defined amount of water to start the hydrolysis [eqn (2)] and condensation [eqn (3)] reactions. Condensation already occurs even if not all of the OC2H5 groups were hydrolyzed. Small clusters were initially formed by the condensation reactions [eqn (4)–(6)] and then the sol particles which eventually form the oxidic gel network. However, all intermediate species still contain Si–OH, Ti–OH, Si–OC2H5 groups. Hydrolysis therefore takes place parallel to condensation during all steps of the sol– gel process.33 Then, the gels were exchanged with a solvent and silylation in modification condition after the composite gel was formed. As has been reported,30,35 the silylated composite gels were washed with solvent prior to ambient pressure drying. Phase structures and surface states
Results and discussion Titania-containing silica aerogels were formed by the ionexchange with the alkoxysilanes hydrolysis–condensation process, as illustrated in Scheme 1. It is expected that there are 6 main chemical reactions existing during the whole process, as follows: Hx K2x Ti6 O13 þ 11H2 O þ ð2 xÞHþ ! 6TiðOHÞ4 þ ð2 xÞKþ ð1Þ SiðOC2 H5 Þ4 þ 4H2 O ! SiðOHÞ4 þ 4C2 H5 OH
ð2Þ
SiðOC2 H5 Þ4 þ ðOHÞ4 Si ! ðOHÞ3 Si–O–SiðOC2 H5 Þ3 þ C2 H5 OH
Scheme 1
ð3Þ
As shown in Fig. 1A-a, the XRD pattern of potassium titanate reveals two distinct diffraction peaks at 11.48 and 43.35°, which can be indexed to the (200) and (313) diffraction planes of the monoclinic potassium titanium oxide, and match well the data (space group C2/m, JCPDF card no. 40-0403).29,35 Fig. 1A-(b–d) provides XRD patterns of the samples obtained in the presence of different amounts of potassium titanium oxide as precursor. From Fig. 1b–d, no peaks of potassium titanate can be detected, indicating the amorphous structure of the obtained products. However, unreacted potassium titanate can be detected when the level number of potassium titanium oxide was up to 4% (Fig. 1A-e). In view of the low crystallinity, the TSA samples were also calcined to improve crystallinity at different temperatures, ranging from 400 to 800 °C. Fig. 1B presents XRD patterns of the as-calcined products of TSA
Schematic illustration of TSA composites with different titania mass ratios.
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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 calcined samples TSA3-400, TSA3-600, TSA3-800 and TSA4-800.
(TSA-3 as the representative product). It displays a XRD pattern of the samples by calcination treatment at 400 and 600 °C, indicating an amorphous phase. However, TSA-3 annealed at 800 °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 accepted that high temperature calcination, at least at 400 °C, is required to prepare anatase TiO2. Through this comparison we Table 1
found the high titania constant of TSA (TSA-3 and TSA-4) at temperatures ranging from 400 to 800 °C influences the amorphous to anatase phase. These results were also proven by many researchers with reference to calcined TiO2–SiO2 hybrid aerogels6,30 and TiO2 samples by K-titanate as titanium precursor.24,25 In order to ascertain the elemental composition of the prepared samples ICP-AES analysis was carried out. As shown in Table 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. This was slightly lower than the actual amount used in content. The difference may have resulted during washing of the prepared photocatalyst. However, the potassium ion concentration in ICP analysis can be detected for TSA-4 to clarify this sample contains potassium titanate further. Thus, the ICP analysis further confirms the existence of Ti in the TSA composite system prepared in this research. The FTIR spectra based on samples are shown in Fig. 2. The absorption peaks indexed to symmetrical and asymmetrical stretching 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 is contributed to both the CH2 and CH stretchings of the CH3 stretching of the surface modification reagent.37 The presence of these alkyl groups made the modified aerogels hydrophobic. The peak located at 848 cm−1 for the Si–C bond is a clear indication of the presence of a surface modification reagent, while the broadband of 3300 to 3500 cm−1 is accounted for physically adsorbed water by the aerogel composites.36 And above all, the presence of a heterolinkage Ti–O–Si band at 935–950 cm−1 indicates incorporation of TiO2 into SiO2 to form 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 observed for sample (TSA-2) but is not shown. These bands at 610 and 490 cm−1 can be attributed to the D1 and D2 bands of the silica aerogels support,40 respectively, confirming that the structure of mesoporous silica was not destroyed during addition of K2Ti6O13. The band at 800 cm−1 is assigned to the network Si– O–Si symmetric bond stretching, further proving that the struc-
Physiochemical properties of titania-containing silica aerogels
Sample name Physiochemical properties
TSA-1
TSA-2
TSA-3
TSA-4
TS3-600
Mass of K2Ti6O13 (mg) Volume of TEOS (mL) Potassium ion concentration (mg L−1) Titanium ion concentration (mg L−1) Si/Ti Specific surface area (m2 g−1) Pore diameter (nm) Pore volume (cm3 g−1) Reaction rate constant k (min−1) Correlation coefficient R
13.5 10 — 0.78 210 628.52 9.634 2.658 0.00702 0.9418
27.4 10 — 1.34 105 380.77 9.509 1.276 0.01074 0.9708
54.1 10 — 3.02 52 210.05 9.86 0.67 0.01384 0.9907
108.2 10 2.26 6.01 26 103.52 12.31 0.25 0.00421 0.9219
54.1 10 — 2.76 52 111.19 17.62 0.923 0.0047 0.9990
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Fig. 2 FT-IR spectra of various samples: (a) TSA-1, (c) TSA-2, (d) TSA-3, and (e) TSA-4.
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. The appearance of the band indicates that the titanium element is bonded 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) for K2Ti6O13 are observed evidently with the increase of potassium titanate addition when it was 4%,42 These results indicate that TSA-4 is a mixture of titania-containing silica aerogels and particles of K2Ti6O13. However, the as-prepared samples (TSA-1, TSA-2 and TSA-3) contained some amount of methyl, which indicated by the bands between 2900 and 3000 cm−1 in spectrum c of Fig. 3.42 It is further proven that TSA has the advantage 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 support according to the experimental results. Morphological investigations SEM micrographs of TSA-1–TSA-4 (Fig. 4a–f ) with various K2Ti6O13 mass concentrations are shown in Fig. 4. It can be clearly seen that the aerogels possessed aggregates with spherical solid clusters. The dense aggregates of particles were observed with increasing potassium titanate mass concentration in the same preparation situation. An increase in the K2Ti6O13/TEOS molar ratio from 0.00078 in TSA-1 (Fig. 4b) sample to 0.0031 in the TSA-3 (Fig. 4d) sample results in the aggregation of the clusters and small voids among them.43 The TSA-4 gelation at highest K2Ti6O13/TEOS molar ratio of 0.0063 causes the numbers of K2Ti6O13 whisker precursors among the TSA clusters. Preparation at low K2Ti6O13/TEOS molar ratio resulted in slow aggregation and small and bulky clusters with small voids within the obtained structure. Individual silica aerogels with no K2Ti6O13 whisker added have a compact network with a particle size of about units digit of nanometers while that of the titania-containing silica aerogels with various
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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.
K2Ti6O13 mass concentration is as large as several tens of nanometers. However, K2Ti6O13 whisker was observed (Fig. 4e) with the increase of potassium titanate addition when it was up to 4%. These results also indicate that TSA-4 is a mixture of Ti-con-
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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 pattern of (g) TSA-3, (h) TSA-4.
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demonstrate the effect of K2Ti6O13/TEOS molar ratio on the formation of TSA network structure. In the silica aerogels sample, in the presence of sufficient TEOS, uniform and regular growth of small silica particles and pores lead to a homogeneous silica network.43 Fig. 5g–i show that at high K2Ti6O13/TEOS molar ratio, a uniform net work structure with larger clusters as compared to the silica aerogel is formed which can be related to irregular growth of the particles and Ti–O–Si network. As can be seen, the structure of the silica aerogels is very dense with close packing of silica particles, and there is a porous structure in the TSA sample. Compared with the TSA sample, it can be observed that silica aerogels (Fig. 5a–c) exhibit a porous network structure which includes
