SiO2 hybrid composite films

Journal of Catalysis 342 (2016) 117–124

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Enhanced photocatalytic activity through insertion of plasmonic nanostructures into porous TiO2/SiO2 hybrid composite films Irina Levchuk a,b,⇑, Mika Sillanpää a, Chantal Guillard c, Damia Gregori b, Denis Chateau b, Frederic Chaput b, Frédéric Lerouge b, Stephane Parola b,⇑ a b c

Laboratory of Green Chemistry, Lappeenranta University of Technology, Sammonkatu 12, 50130 Mikkeli, Finland Laboratoire de Chimie, Ecole Normale Supérieure de Lyon, CNRS, University Claude Bernard Lyon 1, University of Lyon, UMR 5182, 46 allée d’Italie, 69364 Lyon, France Institut de Recherches sur la Catalyse et l’Environnement, IRCELYON, CNRS – University of Lyon, 69100, France

a r t i c l e

i n f o

Article history: Received 19 June 2016 Revised 22 July 2016 Accepted 22 July 2016

Keywords: Gold nanoparticles Titanium dioxide Bipyramids Thin films Surface plasmon resonance Sol–gel Photocatalysis Porous materials

a b s t r a c t Composite TiO2/SiO2 porous thin films prepared using the sol–gel process were doped with dispersion of bipyramid-like gold nanoparticles (AuBP). The impact of the presence of the nanoparticles on the photocatalytic activity during degradation of formic acid in aqueous media was fully investigated and an improvement of the efficiency was observed. The films compositions and structures were characterized by means of UV–vis spectroscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM), inductively coupled plasma optical emission spectroscopy (ICP-OES), goniometry, and diffuse reflectance measurements. The photocatalytic activity was evaluated through decomposition of formic acid and correlated with the structure and the spectroscopic characterizations. The best results in terms of photocatalysis were obtained with thin films doped with 1 wt.% of AuBP; more specifically, the initial degradation rate of formic acid was nearly two times higher than with the undoped reference. It was observed that after three cycles the performance of the prepared photocatalyst does not undergo significant changes. No photocatalytic activity was detected for any of the synthesized thin films under visible light irradiation alone. The photocatalytic improvement was correlated with the lowering of the band gap of TiO2. Ó 2016 Elsevier Inc. All rights reserved.

1. Introduction Titanium dioxide is one of the best photocatalytic materials among semiconductors [1]. Also, TiO2 is known as nontoxic and relatively cheap, which makes it very attractive for large-scale applications in particular [2]. However, its main drawback is limited photocatalytic activity in the low visible region, and only the UV part of the spectrum can be exploited in the case of natural solar excitation. Therefore, different approaches such as doping TiO2 with metals and nonmetallic elements are generally used to extend its photocatalytic activity [1]. Modification of titanium dioxide with noble metals benefits from the relative stability of these metals under UV and aerated atmosphere [3]. As a result, TiO2 doping with noble metals is widely studied in order to extend

⇑ Corresponding authors at: Laboratory of Green Chemistry, Lappeenranta University of Technology, Sammonkatu 12, 50130 Mikkeli, Finland (I. Levchuk). Laboratoire de Chimie, Ecole Normale Supérieure de Lyon, CNRS, University Claude Bernard Lyon 1, University of Lyon, UMR 5182, 46 allée d’Italie, 69364 Lyon, France (S. Parola). E-mail address: [email protected] (I. Levchuk). http://dx.doi.org/10.1016/j.jcat.2016.07.015 0021-9517/Ó 2016 Elsevier Inc. All rights reserved.

the photocatalytic response of TiO2 into the visible area [1]. Among these metals, gold has attracted significant attention during the past two decades [4]. The unique properties and high potential of gold nanostructures lead to new applications in different areas such as optoelectronics, sensors, nanocatalysis, and nanomedicine [5–12]. Mostly, these properties are caused by collective oscillation of electrons when interacting with electromagnetic waves, known as surface plasmon resonance (SPR). The size and geometry of gold nanoparticles have critical effects on the wavelength and intensity of SPR. In this sense, the use of anisotropic gold nanoobjects presents a real advantage in term of the possibility of plasmon control up to IR wavelengths and of intense local fields when the structure exhibits tips, as for bipyramids or stars [13–24]. In most works devoted to the photocatalytic activity of TiO2 doped with gold nanoparticles, they are introduced with spherical shape [25–29]. However, recently the photocatalytic properties of TiO2 doped with gold nanorods [30,31] have been reported. Tips bearing nanoparticles were explored for fluorescence enhancement of dyes, which appeared to be more efficient when spherical shapes were used [32]. Moreover, they were shown to improve

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nonlinear optical properties greatly when associated with dyes in sol–gel matrices, in a more efficient way than spheres or rods [33–35]. To the best of our knowledge, no work demonstrating effects of association of TiO2 with gold bipyramidal nanostructures on photocatalytic activity has been reported. The use of the sol–gel method for the preparation of efficient TiO2/SiO2 photocatalytic flexible coatings where an SiO2 matrix was used as a binder to stabilize dispersion of TiO2 nanoparticles was previously reported [36]. These films were porous, which is of particular interest for efficient photocatalytic activity due to surface of high catalyst/ organics interactions. A similar strategy was used here for the preparation of the composite. Hence, the aim of this work was to study the photocatalytic properties of TiO2/SiO2 composite films synthesized via a sol–gel method and doped with bipyramidlike gold nanoparticles (AuBP). For the study of the photocatalytic activity, formic acid degradation was performed under UV and/or visible light. 2. Experimental 2.1. Chemicals Methyltriethoxysilane (MTEOS) was purchased from ABCR, and tetrachloroauric acid trihydrate (99.9%) and silver nitrate were obtained from Alfa Aesar, while CTAB (cetyltrimethylammonium bromide, 99%), CTAC (cetyltrimethylammonium chloride solution, 25%, in water), 2-methyl-8-hydroxyquinoline (MHQL, 98%), and 8-hydroxyquinoline (HQL, 99%) were received from Aldrich. The 8-hydroxyquinoline was purified by dissolution in refluxing toluene, followed by hot filtration on a silica plug and crystallization. The formic acid was purchased from Acros Organics (99%). Titanium dioxide (P25, anatase 72%, rutile 28%) with surface area 50 m2 g
1 was purchased from Degussa-Hüls-AG. 2.2. Synthesis and material characterization 2.2.1. Hybrid silica sol preparation The preparation of silica sol was previously investigated and reported [36–39]. MTEOS (60 ml) was hydrolyzed in acidic medium (H2O at pH 3.5 using hydrochloric acid) in a round-bottomed flask equipped with a short distillation column. The water/precursor hydrolysis molar ratio was 14. The mixture was stirred for 1 h at room temperature and heated to 130 °C (oil bath temperature). The formed ethanol was distilled till the mixture became white, indicating phase separation between water and hybrid silica sol. Five minutes after, the oil bath was removed, 40 ml of ice cold water was introduced into the solution, and the mixture was cooled down while being stirred. Ten minutes later, 90 ml of diethyl ether was introduced slowly with strong stirring to extract the sol from the mixture. The stirring was stopped after 5 min, and the aqueous layer was discarded. Then diethyl ether was evaporated and replaced with ethanol so that the dried solid residue after evaporation of the solvents represented 30% by mass. This prepared sol was stored at
24 °C and kept for several months. 2.2.2. Preparation of gold bipyramids (AuBP) with LSP around 600–650 nm AuBP were prepared as described elsewhere [40]. Briefly, a growth solution was prepared by mixing 1200 ll of HAuCl4 (25 mM in water) and 40 ml of CTAB (47 mM in water). Then 1050 ll of aqueous silver nitrate (5 mM) was added, followed by 700 ll of 8-hydroxyquinoline (0.4 M in ethanol). Silver ions were essential for controlled growth of AuBP [40]. The growth solution turned light yellow. Then 4500 ll of 6–7 nm CTAC-capped gold

seeds (0.25 mM in Au0) was introduced into the growth solution under stirring. The mixture was gently stirred for 20 s and directly put into an oven at 40–45 °C for 45 min. After 1 h of cooling at room temperature, the solution was centrifuged at 8000 rpm for 30–60 min. The supernatant was discarded and replaced with a 1 mM CTAB solution. The same operation was repeated one more time and the particles were dispersed in half of the initial suspension volume of Milli-Q water. The resulting concentrated solution contained approximately 0.3 g l
1 of AuBP. 2.2.3. Preparation of TiO2/AuBP/hybrid silica sol A quantity of 1 g of TiO2 was dispersed in 50 ml of water under sonication and vigorous stirring. Then, the calculated volume of AuBP suspension was added. The dispersion was stirred for 5 min and sonicated. The mixture was centrifuged at 10,000 rpm for 20 min and the supernatant was discarded. A maximum amount of water was removed. Then 5 ml of ethanol were added to disperse the precipitate, followed by 5 g of 20 wt.% MTEOS sol. The mixture was sonicated to give a bluish milky suspension ready for deposition. The reference sol was prepared in similar way except addition of AuBP. Thin films were deposited using a dip-coating technique on Si wafers (3  3 cm). The Si wafers were cleaned using the following procedure: wafers were wiped with acetone, immersed in TFD4 detergent for 10 min, rinsed with Milli-Q water, immersed in absolute ethanol, and dried with air. Coating was performed at dipping and retrieval speed 50 mm min
1 and dip duration 10 s. After deposition, thin films were dried in a Memmert 100–800 oven at 120 °C for 20 h. 2.2.4. Material characterization UV–visible measurements were performed using a Perkin– Elmer Lambda 750 UV–vis–NIR spectrometer. TEM images were obtained using a TOPCON EM002B transmission electron microscope operating at 200 kV to estimate the size and shape of gold nanoparticles. Sample preparation for TEM analysis consisted of scraping of prepared thin films and direct deposition of fragments onto a copper microscopy grid coated with a holey carbon film. A Zeiss SUPRA 55 VP scanning electron microscope was used for observation of prepared thin film morphology. Concentrations of immobilized TiO2 were measured using an ‘‘Activa” Jobin Yvon inductively coupled plasma optical emission spectrometer (ICP-OES). Sample preparation was done by dissolving TiO2/SiO2 and TiO2/SiO2-Au NP coatings in a mixture of H2SO4, HNO3, and HF at 120 °C. The values of the contact angle before and after UVC pretreatment were obtained using a GBX Digidrop goniometer connected with a digital camera. Milli-Q water droplets with volume 20 lL were deposited on different places on coatings and recorded immediately. Mean values were calculated using from five to seven measurements per sample. The band gaps of the prepared materials (in the form of thick films) were evaluated from diffuse reflectance measurements done using the an AvaSpec-2048 (Avantes) fiber UV–vis spectrometer equipped with AvaLight-DHS deuterium (UV) and halogen lamps (vis region). Barium sulfate was used as a reference. Evaluation of band gaps for all prepared samples was performed [41,42]. 2.3. Photocatalytic experiment To test the photocatalytic activity of the prepared materials, a model solution of formic acid (FA) with concentration 1.09 mM was utilized (pH 3.4). The formic acid was chosen as a pollutant because no stable intermediates are formed during the photocatalytic degradation of this compound [43].

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The presence of AuBP in the 1% doped films is evidenced by the presence of longitudinal SPR around 610 nm and the transverse SPR band around 520 nm. Interestingly, compared with the AuBP solutions (Fig. 1b), the transverse contribution of the SPR was red-shifted while the longitudinal contribution was blue-shifted. According to earlier studies [45,46], in the case where the morphology of nanoparticles is similar, the shift of SPR could be correlated with differences in the optical properties of the matrix. The red shift of the transversal SPR band could probably be explained by an increase of the refractive index of thin films (about 1.8) in comparison with a solution of AuBP (about 1.3) [46]. It should be noted that longitudinal SPR is very sensitive to oxidation of AuBP tips, which could probably explain the blue shift [23]. Fig. 2a shows a TEM picture of the prepared gold nanobipyramids and Fig. 2b shows an Au bipyramid surrounded by TiO2 particles inside the silica matrices in the 1% AuBP doped composite film. The presence of Au BPs can be distinguished because of relatively high electron density and specific shape. As can be seen from Fig. 2, the size of Au BPs is similar to that of TiO2 NPs. The average size of Au BPs was estimated to be about 33 nm. The morphology of the thin films was similar for all samples. Hence, for the sake of brevity, only SEM images for TiO2/SiO2 doped with 0.1% AuBP are presented in this work (Fig. 3). As shown in Fig. 3, the thin films showed a porous structure as previously reported using a similar procedure [36]. According to the literature [42], the most accepted method for semiconductor band gap determination is plotting the square root of the Kubelka–Munk function multiplied by photon energy versus photon energy, and extrapolating the linear part of the rising curve to zero (indirect band gap). Results are presented in Table 1. As can be seen from Table 1, a decrease of the indirect band gap was observed with an increase of the Au nanoparticles concentration. This fact could be attributed to the introduction of energy levels into the band gap of TiO2/SiO2. Similar results were reported earlier when a semiconductor was doped with transition metal ions [26,42,47]. In the work of Navio and Hidalgo, ZrO2 synthesized using a sol–gel method was doped with Fe3+, leading to materials with highly disordered structure. The observed decrease of the band gap with increased iron content was attributed to defects and impurities in the synthesized materials, leading to introduction of energy levels into the semiconductor [47]. A decrease of the band gap of TiO2 doped with Au nanospheres relative to TiO2 due to introduction of energy levels into the semiconductor was reported by Centeno et al. [26]. Loading of gold nanoparticles

Prior to each test, the samples were treated for 1600 min under UVC (4.2 mW/cm2) according to a previously reported procedure to ensure final stabilization of the microstructure of the films [36,44]. All photocatalytic tests were carried out at room temperature (22 ± 2 °C) utilizing an experimental 100 ml batch glass reactor with quartz optical window. The diameter of the optical window was 4 cm. Thin films deposited on 3  3 cm Si wafers were located inside the reactor and covered with a glass substrate, preventing sample floating. The volume of solution used in all tests was 30 ml; sampling volumes of 0.3 ml were conducted each 30 min. Changes in formic acid concentration were monitored with a Varian ProSTar high-performance liquid chromatograph (HPLC) equipped with column COREGEL-87H3 (300  7.8 mm) and UV– vis detector (k = 210 nm). As a mobile phase, sulfuric acid with concentration 5  10
3 mol l
1 and flow rate 0.7 ml min
1 was used. In order to reach adsorption–desorption equilibrium, the first 30 min of each photocatalytic experiment was conducted in darkness. Photolysis and adsorption tests were performed to confirm the photocatalytic degradation of formic acid in the presence of the composite material. A high-pressure mercury lamp (Philips HPK 125 W) was used for all experiments except photocatalytic tests in the visible region only. The lamp was stabilized for at least 30 min before each test. UVC, UVB, and infrared beams did not reach the reactor due to a circulation water cell with Pyrex and O-52 cutoff filters. For experiments in the visible region, an LED lamp was used (400–800 nm) with maximum peaks at 555 and 447 nm with intensity 156 mW cm
2. A VLX-3W radiometer equipped with CX-365 (UVA) and CX-312 (UVB) was used before and after each experiment for radiant flux measurements at the bottom of the reactor. The UVA irradiance at the bottom of the reactor was about 7 ± 0.2 mW cm
2. At the level of the thin film location, UVA was 5.2 ± 0.2 mW cm
2.

3. Results and discussion 3.1. Material characterization The AuBP-doped composite films were first analyzed by UV–vis spectroscopy. Since P25 is rather polydisperse and easily forms aggregates, significant light diffusion occurred. Therefore, the films were analyzed by reflectance and compared with a reference film without gold nanoparticles (Fig. 1).

1,0

Normalized absorbance

94

90 0 Difference (%)

Reflectance (%)

92

88

86 1% AuBP No AuBP

-1 -2

450

400

500

600

700

550

600

Wavelength (nm)

(a)

0,4

0,2

0,0

Wavelength (nm)

500

0,6

-3

84 400

0,8

650

700

750

450

Film AuBP 1% - Reference AuBP solution 500

550

600

650

700

750

Wavelength (nm)

(b)

Fig. 1. (a) Reflectance spectra of reference film and doped film with 1% AuBP. Insert shows the difference of reflectance between the two films. (b) Comparison of AuBP solution spectra and the difference of absorbance between reference film and 1% AuBP doped film.

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Fig. 2. (a) TEM photograph of pure AuBP. (b) TEM photograph of the 1% AuBP doped film. The white arrow points to the gold bipyramid.

200 nm

100 nm

Fig. 3. SEM images of TiO2/SiO2 doped with 0.1% AuBP thin film, showing the porosity of the films.

Table 1 Band gap energy evaluation.

Table 2 Contact angle of thin films before and after UVC pretreatment.

Sample

Indirect band gap, eV

TiO2/SiO2 TiO2/SiO2/AuBP 0.1% TiO2/SiO2/AuBP 0.5% TiO2/SiO2/AuBP 1%

3.11 3.02 2.92 2.90

varied from 0.11 to 1.26%; however, no direct dependence between Au loading and indirect band gap was observed, probably due to significant variations of Au NP size in samples with different loading. Interestingly, the material with the smallest band gap (0.25% Au/TiO2, 2.8 eV) demonstrated the highest activity for phenol degradation in an aquatic environment. The hydrophilicity of TiO2/SiO2 composites prepared by a sol– gel method after heat treatment at 500 °C was reported earlier [49,50], as well as the photoinduced hydrophilicity of TiO2 [51]. In this study, the wettability of obtained coatings was checked after heat treatment and after UVC pretreatment and the results are summarized in Table 2. Obviously, after UVC treatment, all samples exhibit hydrophilic properties, while they were hydrophobic before the irradiation. This can be explained by the presence of organic groups in the thin films left after the synthesis, since the temperature of the treatment (120 °C) is not high enough to remove them. However, after

Sample name

Mean contact angle before UVC pretreatment

Mean contact angle after UVC pretreatment

TiO2/SiO2 TiO2/SiO2/AuBP 0.1% TiO2/SiO2/AuBP 0.5% TiO2/SiO2/AuBP 1%

117.9° 132.2° 131.2° 127.8°

9.0° 8.1° 4.9° 4.9°

UVC treatment, most of the groups were removed through photocatalytic decomposition. This parameter was rather important, since hydrophilicity was required to obtain efficient photocatalytic activity. 3.2. Photocatalytic activity It is well known that initial photocatalytic reaction rates are directly proportional to the mass of the photocatalyst up to a certain limit, which depends on the reactor geometry and operational conditions [52]. Hence, it is important to determine the influence of the photocatalyst mass on the initial reaction rate. This was achieved for pure TiO2/SiO2 thin films (Fig. 4). Pure reference TiO2/SiO2 thin films with different loadings of TiO2 were prepared and pretreated as mentioned above. Initial disappearance rates of FA for all samples were measured for 90 min of contact time.

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1.20

1.6

1.00

TiO2/SiO2/AuBP1% Photolysis Adsorpon TiO2/SiO2

UV on

1.4

C(FA), mM

initial FA disappearence rate, uM min -1 cm-2

2 1.8

1.2 1 0.8 0.6

0.80 0.60 0.40 0.20

0.4

0.00 0.2

0

10

20

30

40

50

60

70

80

90

Time, min

0

0

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

0.09

Fig. 6. Photocatalytic degradation of FA with 1% TiO2/SiO2/Au BP (highest photocatalytic activity) and TiO2/SiO2 (reference) coating.

TiO2, mg cm-2 Fig. 4. Influence of mass of deposited TiO2/SiO2 on initial disappearance rate of FA.

As can be seen from Fig. 6, negligible changes in FA concentration were detected during photolysis and adsorption tests. Significant differences in degradation of FA were observed for TiO2/SiO2 and TiO2/SiO2/AuBP films doped with 1 wt.% Au. For instance, after 60 min under UVA + vis irradiation, 58% of FA was decomposed when AuBP doped film was used, while for the reference coating this value was only 24%. Enhanced photocatalytic activity of thin films doped with AuBP could possibly be explained by the fact that gold nanoparticles act as electron traps and promote electron–hole separation [25,30,31,54]. It is of great interest to compare the photocatalytic activity of TiO2/SiO2/AuBP with that of similar coatings doped with spherical gold nanoparticles. The analogous TiO2/SiO2 coatings modified with Au nanospheres were tested in an earlier study [55]. In the strict sense, direct comparison of TiO2/SiO2 films modified with spherical and bipyramidlike Au NPs is not possible, due to differences in experimental conditions. Major parameters of the experimental conditions of this and an earlier study [55] are presented in Table 3. As can be seen from Table 3, type of reactor, lamp, model compound and its initial concentration, and pretreatment of coatings were similar. However, such important parameters as irradiance intensity, type of UV light, and TiO2 areal loading varied in these studies. Thus, critical effects of these parameters can be observed when the initial degradation rate of FA is compared for reference TiO2/SiO2 films, which were identical in both works. As expected,

According to Fig. 4, the initial reaction rate rises with increased TiO2 loading up to a maximum value obtained for about 0.04 mg cm
2. In the range from about 0.04 to 0.09 mg cm
2 no significant changes were observed. It should be noted that due to slight changes in the properties of TiO2/SiO2 sols after Au doping, it was extremely important to measure the amount of TiO2 deposited for all prepared samples in order to compare the photocatalytic activity properly. The TiO2 loading for thin films doped with AuBP varies in the range from 0.044 to 0.088 mg cm
2 of TiO2. In this interval, the same initial reaction rates were observed when reference samples (undoped with AuBP) were tested. Finally, the initial reaction rates of the FA decomposition for TiO2/SiO2/ AuBP and TiO2/SiO2 thin films were correlated with TiO2 loading in the range from 0.04 to 0.09 mg cm
2 of TiO2. The mean values of the initial FA disappearance rate determined for TiO2/SiO2/AuBP thin films are presented in Fig. 5. The results shown in Fig. 5 demonstrate that with an increase of AuBP loading, the rate rose and photocatalytic activity was improved. The highest initial FA disappearance rate was attributed to TiO2/SiO2/AuBP 1 wt.%, which was about two times higher than for the reference sample with a similar TiO2 loading. Similar observations were reported for degradation of acetic acid in an aquatic environment under UV irradiation, which was increased by a factor of 1.7 when TiO2 (P25) modified with AuNP (2 wt.%) was compared with bare TiO2 [53]. The photocatalytic degradation of FA when TiO2/SiO2/AuBP 1 wt.% and TiO2/SiO2 (with TiO2 loading 0.063 mg cm
2) films were used is shown in Fig. 6.

BP 1% BP 0.5% BP 0.1% Reference

0

0.2

0.4

r, µM

0.6

min-1cm-2

0.8

1

1.2

1.4

under UVA + Vis

Fig. 5. Initial disappearance rate of FA for TiO2/SiO2/AuBP films with three different doping ratios and the reference without Au nanoparticles.

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Table 3 Comparison of major experimental parameters of studies with Au BPs and Au nanospheres. Parameter

TiO2/SiO2/Au spheres [54]

TiO2/SiO2/AuBP

Light source Type of irradiance UVA irradiance at the level of the film Reactor type Initial concentration of FA Pretreatment Initial degradation rate of FA using TiO2/SiO2 thin films (TiO2 areal loading about 0.085 mg TiO2/cm2) Initial degradation rate of FA using TiO2/SiO2 modified with Au NPs (AuBP—0.5%; Au spheres—0.5%) Initial degradation rate of FA using TiO2/SiO2 modified with Au NPs (AuBP—1%; Au spheres—1.25%)

HPK lamp 125 W UVB + UVA + vis 6.5 ± 0.2 mW cm
2 Peak reactor 1.09 mM 1600 min under UVC (4.2 mW cm
2) 0.9 lM min
1 cm
2

HPK lamp 125 W UVA + vis 5.2 ± 0.2 mW cm
2 Peak reactor 1.09 mM 1600 min under UVC (4.2 mW cm
2) 0.54 lM min
1 cm
2

1.07 lM min
1 cm
2 (TiO2 areal loading 0.112 mg TiO2 cm
2) 1.1 lM min
1 cm
2 (TiO2 areal loading 0.12 mg TiO2 cm
2)

1.095 lM min
1 cm
2 (TiO2 areal loading with TiO2 loading 0.04–0.09 mg cm
2) 1.3 lM min
1 cm
2 (TiO2 areal loading with TiO2 loading 0.063 mg cm
2)

in the case when UVA intensity was higher (study with Au spheres), the initial FA degradation rate of TiO2/SiO2 was about 1.7 times higher. Interestingly, photoactivity of TiO2/SiO2 coatings doped with 0.5% of Au nanospheres and Au BPs was very similar. However, the initial FA degradation rate of TiO2/SiO2/Au BP 1% is higher than that of TiO2/SiO2/Au spheres 1.25%. Based on such commensuration, it can be suggested that photocatalytic activity of TiO2/SiO2 films modified with Au BPs is higher than that of TiO2/SiO2 modified with Au nanospheres. In the work of Okuno et al. [31], photocatalytic activity of TiO2– SiO2 doped with Au nanospheres (diameter 7 nm) and nanorods (diameter 7 nm, length 10–400 nm) was compared. Enhancement of photocatalytic activity was observed for the materials doped with Au nanorods. This fact was explained by increased charge separation between TiO2 and Au with increasing nanoparticle size [31]. Interestingly, in the study of Kaur and Pal [30], where photocatalytic activity of TiO2 doped with Au NPs of different shape and size was examined, the activity of TiO2 doped with Au nanorods (average length 23.6 nm, aspect ratio 2.8) was lower than that of TiO2 doped with Au nanospheres (3.5 nm). The value of the indirect band gap of the sample with the highest photocatalytic activity suggests the possibility of expanding the photocatalytic response into the area of visible light. Hence, TiO2/ SiO2/AuBP doped with 1 wt.% of Au were tested under visible light alone. No photocatalytic activity was detected under this irradiation conditions. Similar behavior (enhanced activity in the UV region and no activity under visible light) for TiO2 composites doped with gold nanoparticles was reported in recent works [31,56]. It was suggested that the increase of TiO2/Au photocatalytic activity in the UV region does not require electronic contact between nanoparticles of TiO2 and Au [56]. Visible-light-induced photocatalytic activity of TiO2 modified with AuNP was also reported [53]. It should be mentioned that AuNP were attached to the surface of TiO2, leading to the occurrence of electronic contact between them. The mechanism for photocatalytic activity of TiO2/Au under visible light suggested by Kowalska et al. [53] included absorption of photons by Au NPs through LSPR excitation, injection of electron from Au NPs into CB of titania, and reduction of molecular oxygen adsorbed on the surface of titania; as a result, electron-deficient gold oxidized organic pollutants. In the present study, no photocatalytic activity under visible light was detected, possibly due to the absence of a crystalline TiO2/AuBP interface, which is quite reasonable considering the differences of concentration between TiO2 and AuBP particles (over 103–104 for samples with 1 wt.% AuBP). The durability of the prepared materials was tested for the sample of TiO2/SiO2/AuBP 1 wt.% during three cycles of photocatalytic tests. Between each two tests, the samples were rinsed with water and dried in air. No decrease of photocatalytic activity was observed.

Table 4 Apparent quantum yields. Sample

Apparent QY, %

TiO2/SiO2 TiO2/SiO2/AuBP 0.1% TiO2/SiO2/AuBP 0.5% TiO2/SiO2/AuBP 1%

2.5 2.6 3.5 4.7

The apparent quantum yields were calculated for the tested samples according to the equation

n

dNi dt

uapp;in ¼ R k2 k1

o

in ; RAirr kdk hc

ð1Þ

where R is the irradiation intensity (W m
2 nm
1); Airr is the irradiated area (m2); h is Planck’s constant (6.62  10
34 J s
1); c is the speed of light in vacuum (2.997  108 m s
1); k1 is the lower wavelength of the spectrum in the range of interest (300 nm) and k2 the higher wavelength of the spectrum in the range of interest n o i (400 nm); and dN is the initial rate of photoconversion of i spedt in

cies molecules. The values of apparent quantum yields obtained for samples are shown in Table 4. As can be seen from Table 4, an increase of apparent QY was observed for TiO2/SiO2 thin films doped with AuBP. These values were in agreement with earlier reported results [56]. 4. Conclusions In this study, porous TiO2/SiO2 thin films doped with bipyramidlike gold nanoparticles were successfully synthesized by a sol–gel method and their photocatalytic activity was investigated. The initial rate of formic acid photocatalytic degradation under UV light was evaluated for samples with different loadings of gold. An increase of gold loading led to an improvement of initial rates of formic acid decomposition. The highest photocatalytic activity was attributed to the samples with TiO2/SiO2/AuBP 1 wt.%, which is about twice as efficient as the reference. Three cycles of photocatalytic tests were performed using the sample exhibiting the highest photocatalytic activity, and no changes in the initial rates of formic acid degradation were observed. Visible irradiation had no significant effect on the photocatalytic properties of the obtained materials. Acknowledgments Support from the hybrid membrane process for water treatment (HYMEPRO) project and a Maa-ja vesitekniikan tuki grant

I. Levchuk et al. / Journal of Catalysis 342 (2016) 117–124

are gratefully acknowledged. Felix Vadcard (ENS Lyon) is acknowledged for help with the TEM characterization.

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