brookite ratio in TiO2–Ag porous nanocomposites on visible photocatalytic performances

Materials Chemistry and Physics 141 (2013) 234e239

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Weighting the influence of TiO2 anatase/brookite ratio in TiO2eAg porous nanocomposites on visible photocatalytic performances V. Iancu a, M. Baia a, *, L. Diamandescu b, Zs. Pap c, d, A.M. Vlaicu b, V. Danciu c, L. Baia a a

Babes-Bolyai University, Faculty of Physics & Interdisciplinary Research Institute on Bio-Nano-Sciences, 400084 Cluj-Napoca, Romania National Institute of Materials Physics, P.O. Box MG-7, 77125 Bucharest-Magurele, Romania c Faculty of Chemistry and Chemical Engineering, Babes-Bolyai University, Arany Janos 11, 400028 Cluj-Napoca, Romania d Research Group of Environmental Chemistry, Institute of Chemistry, University of Szeged, Tisza Lajos krt. 103, H-6720 Szeged, Hungary b


Synthesis of composites based on TiO2 aerogel and Ag nanoparticles.
Existence of mixed crystalline structure consisting of brookite and anatase phases.
Composites visible photocatalytic activity reduces as brookite phase content raises.
Ag nanoparticles addition enhances the visible photocatalytic activity.

g r a p h i c a l a b s t r a c t

CSA [μ M]

h i g h l i g h t s

C0

C1 C3 C2

Time [min]

a r t i c l e i n f o

a b s t r a c t

Article history: Received 28 September 2012 Received in revised form 26 April 2013 Accepted 1 May 2013

Nanocomposites based on TiO2 aerogel and Ag nanoparticles have been successfully obtained through different synthesis methods and their specific surface areas have been determined by N2 sorption (BET method). The photocatalytic potential for salicylic acid degradation has been evaluated. It was found that under visible light irradiation, all synthesized nanocomposites exhibit higher photocatalytic activity than the commercially available Aeroxide P25. By correlating the structural parameters with the photocatalytic performances, it has been found that the Ag nanoparticles and brookite phase presence alongside the anatase play important roles on the visible photocatalysts behavior. For the Ag containing samples with mixed anataseebrookite phases, it has been observed that the visible photocatalytic performance decreases with the increase in brookite crystalline phase content. On the other hand, the addition of Ag nanoparticles results, as expected, in a clear enhancement of the visible photocatalytic activity. Ó 2013 Elsevier B.V. All rights reserved.

Keywords: Composite materials Nanostructures Annealing Optical properties

1. Introduction New materials and methods of fighting pollution are constantly being sought out. One of the most studied topics is photocatalysis, a process that uses light to activate a photocatalyst, which determines the decomposition of toxic pollutants. New photocatalytic

* Corresponding author. Tel.: þ4 0264 405300; fax: þ4 0264 591906. E-mail address: [email protected] (M. Baia). 0254-0584/$ e see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matchemphys.2013.05.005

materials, which are active under visible light, are going to contribute to the issues of environmental pollution. Nanomaterials with a mesoporous structure should be more promising because light harvesting can be further enhanced due to their enlarged surface area. They can also facilitate better accessibility of reactants to the catalysts [1]. Aerogels are a class of materials suited for this task, having extremely low density, large open pores and a high specific surface area. Among the possible aerogels, the one based on TiO2 is the most extensively used in photocatalysis. It is biologically inert and

V. Iancu et al. / Materials Chemistry and Physics 141 (2013) 234e239

corrosion resistant, it requires less post-processing making it inexpensive. Moreover, it is a nontoxic material with high chemical stability and photosensitivity. Due to the high photocatalytic response, TiO2 aerogels are widely used in manufacturing materials for decontaminating water, air and soil [2e4]. Additionally, due to their properties conferred by the very small physical dimensions, TiO2 nanocrystallites that build up the titania network of the aerogels exhibit higher photocatalytic efficiency [5e8]. Also, in order to enhance the photocatalytic performances of TiO2, a loading of noble metal nanoparticles into its network is necessary, a procedure which has been extensively described in the literature [9e19]. It was proven that it directly influences the intrinsic properties of TiO2. Furthermore, the noble metal nanoparticles could extend the titania aerogels photoresponse into the visible domain. In the particular case of Ag nanoparticles, it was shown that their presence inside the TiO2 network can increase the efficiency of the photocatalysis [20], because Ag nanoparticles show a very intense localized surface plasmon absorption band in the near-UV region [21]. TiO2 of anatase phase is a known semiconductor with a band-gap of approximately 3.2 eV [22], so nearUV irradiation can generate electronehole pairs. Recently [23], we have shown that the visible light activity of TiO2 is dependent on the crystalline phase content, and it increases concomitantly with the brookite particle size. However, to our best of knowledge there is no paper discussing the synergic effect of anatase and brookite phases on the photocatalytic performances of Ag loaded TiO2 under visible light irradiation. The samples discussed in the herein paper do not consist entirely in just one crystalline phase, they being a mixture of anatase and brookite crystalline phases. For this reason, the synergic effect of the two phases on the photocatalytic performances of TiO2eAg nanocomposites must be taken into consideration. Thus, in the present study, we propose to determine the influence of brookite nanocrystallites presence alongside the anatase and Ag nanoparticles on the visible photocatalytic performances and to identify the structural modifications that lead to the identified changes. 2. Experimental section 2.1. Samples preparation The silver colloidal suspension was prepared as follows: 30 mL of 5  103 M AgNO3 solution was brought to a boil under vigorous stirring on a magnetic stirring hot plate. 90 mL of 2  103 M aqueous solution of NaBH4 was added drop-wise to the AgNO3 solution. Stirring and boiling was continued for further 60 min. The TiO2 gel was obtained by solegel method using Ti [OCH(CH3)2]4, HNO3, C2H5-OH and H2O applying them in 1/0.08/21/ 3.675 molar ratio. The gels were allowed to age for 40 days. All of the gels were supercritically dried with liquid CO2 (T > 35  C and p > 1200 psi) using SAMDRI-PVT 3D (Tousimis) equipment. The nanocomposites were obtained by three different methods as follows: i) The TiO2 aerogel was introduced in the colloidal suspension where it was maintained for two days. After impregnation, the resulted composite was filtered and then dried in an oven at 100  C. This sample was denoted C1. ii) The TiO2 gel was introduced in the colloidal suspension where it was maintained for two days. The gels impregnated with colloidal nanoparticles were washed with plenty of C2H5-OH and supercritically dried with liquid CO2. This sample was denoted C2. iii) The nanocomposite was obtained by adding the silver colloidal suspension in the synthesis mixture of the TiO2 gel.

235

The resulted gel was allowed to age and then dried under supercritical conditions. This sample was denoted C3. All samples have been heat treated at 500  C for 2 h, in air, by using a heating rate of 4  C min1. 2.2. Sample measurements The samples surface area was determined by the Brunauere EmmeteTeller (BET) method, in a partial pressure range of 0.05 < p/ po<0.3. The nitrogen adsorption was carried out at 77 K with a Sorptomatic 1990 equipment. SEM images were collected with a JEOL JSM5510LV scanning electron microscope. Elemental mapping was performed with energy dispersive X-ray analysis (EDX), the silver amount representing the average value of the data obtained after performing measurements on four areas of the same samples. One should note that the difference between the recorded values was smaller than 6% in comparison with the average value. A JEOL JEM1010 TEM operating at an accelerating voltage of 100 kV equipped with a MegaView III CCD camera was utilized to obtain TEM micrographs. The X-ray diffraction patterns were recorded using a DRON Xray powder diffractometer linked to data acquisition and processing facilities; CuKa radiation (l ¼ 1.5406 Å) and a graphite monochromator were used. The average size of the crystallites was calculated from the diffraction peaks by using the Scherrer equation [24]. The photocatalytic activity of the composites was established from the degradation rate of salicylic acid that was used as standard pollutant molecule as elsewhere reported [25]. The decrease in time of the salicylic acid concentration (the intensity of the band located at 295 nm) was monitored using a JASCO V-530 UVevis spectrophotometer (C0 ¼ 5  104 M for all investigated samples). The composites immersed in salicylic acid solution were irradiated with six visible light lamps (6 W each). The working temperature was of 24e26  C and the solution pH was of 5.3. For the photodegradation experiments an amount of powder of 0.1 g of each composite was introduced in a photoreactor containing 100 mL of salicylic acid. The distance between the lamp and the photoreactor was of approximately 2 cm. One should also mention that before visible irradiation measurements, the mixture with the sample was stirred for 15 min in order to achieve the equilibrium of the adsorptionedesorption process. The salicylic acid adsorption on the aerogels surface is indicated by a bright yellow color of their surface (the solution is colorless) originating from the formation of surface charge transfer complexes [26,27]. The photodecomposition reaction follows pseudo-first order kinetics and consequently, the apparent rate constant was calculated by plotting ln (C0/C) vs. time. The slope of the plot given by the linear fit represents the apparent rate constant. Throughout the photodecomposition process no shift of the salicylic acid band located at 295 nm was observed. The band-gap energy of the composites was calculated from their diffuse reflectance spectra (DRS) that were recorded with a JASCO V-530 UVevis spectrophotometer in the wavelength range of 800e200 nm. The values were obtained by applying a Kubelkae Munk transformation. 3. Results and discussion The porous character of the prepared nanocomposites was evidenced by SEM measurements. A selection of images is presented in Fig. 1. The specific surface areas of the samples are presented in Table 1. It can be seen that the highest recorded value for Ag loaded

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V. Iancu et al. / Materials Chemistry and Physics 141 (2013) 234e239

Fig. 1. Selected SEM images of the porous composites as indicated.

nanocomposites, was obtained for sample C2 (157 m2 g1) and the lowest one for C3 (129 m2 g1), suggesting a relatively strong influence of the preparation method on the sample’s morphology. One should emphasize that all composites have a specific surface area higher than that of the blank aerogel sample C0 (121 m2 g1). In order to get information about the samples structure, XRD measurements have been performed. The patterns illustrated in Fig. 2 reveal the existence of a mixed crystalline structure consisting of brookite and mainly, anatase phases. The strongest diffraction peaks can be seen at 25 , 37, 48 , 54 and 62 , and are attributed to (101), (112), (200), (105) and (204), respectively, crystallographic planes. These features can be undoubtedly indexed to anatase with tetragonal crystalline structure (JCPDS no. 211272). On the other hand, the peak at 31, corresponding to (211) crystallographic plane, can be attributed to brookite with orthorhombic crystalline structure (JCPDS no. 76-1937). No characteristic peaks associated with the rutile phase or impurities were detected. The crystalline phase content was also determined, by Rietveld refinement [28], and the obtained results are shown in Table 1. It is remarkable that brookite is present alongside anatase in all investigated samples, excepting C2, whose structure consists of 100% anatase phase. One should also note that the brookite content is higher in the composites as compared with that of the C0 sample. The mean size of the TiO2 anatase nanocrystallites ranges from 10 to 15 nm, while for the brookite crystallites it was found to be 9 nm for the Ag loaded composites and 13 nm for the blank sample C0 (see Table 1). TEM images provided additional information related to the dimension of the Ag nanoparticles, found to be around 8.5  2.5 nm for all investigated samples (see Fig. 3). Photocatalytic behavior of the synthesized nanocomposites was investigated during the photodegradation reaction of salicylic acid under visible irradiation by monitoring the decrease in time of the salicylic acid concentration as illustrated in Fig. 4. The obtained

photodegradation rate constant values are summarized in Table 1. It is worth mentioning that for the samples containing Ag, the visible photodegradation rates were clearly higher than that of the blank sample C0 and even higher than that of the commercial product Aeroxide P25. Out of the aerogel samples, the one that exhibits the highest photodegradation rate is C2, and the one with the lowest is C0. By looking at the Ag loaded samples data from Table 1, one can notice that sample C3 contains approximately 89 wt.% anatase and 11 wt.% brookite and its photodegradation rate is lower than that of C2, whose structure is 100 wt.% built up of TiO2 anatase. Composite C1 with the lowest photodegradation rate contains 76.7 wt.% anatase and 23.3 wt.% brookite, meaning that an increase of the brookite content leads to a decrease in photocatalytic performance under visible light exposure (see Fig. 5). The role of the brookite phase content is not yet entirely clarified in the literature, although there are a few studies [29,30] in which brookite was proven to positively influence the photocatalytic activity under UV light irradiation. In the case of our study, for visible light irradiation, it was observed that the brookite phase content negatively influences the photocatalytic behavior. An important part in the photocatalytic performance of the obtained nanocomposites when exposed to visible light is played by the imbedded silver. When irradiated with visible light, the samples containing silver nanoparticles exhibit a better photocatalytic activity than C0, meaning that the addition of the silver leads to a better photocatalytic performance. This is an expected result since the Ag nanoparticles absorption band given by the surface plasmon resonances is in the visible region [31]. Moreover, the Ag nanoparticles, located inside the TiO2 porous network, induce important changes of the structure and morphology during the preparation process due to the contact between titania and Ag

Table 1 Results derived from the N2 sorption measurements and XRD analysis together with photodegradation rate values (kapp) for the prepared composites and the commercial product Aeroxide P25. Sample

C0 C1 (1.0Ag wt.%) C2 (0.54Ag wt.%) C3 (0.2Ag wt.%) Aeroxide P25 Errors

SBET (m2 g1)

121 135 157 129 50 6

Crystalline phase content (%)

kapp  103 (min1)

Crystallites size (nm)

Anatase

Brookite

Rutile

Anatase

Brookite

Rutile

Visible light

92.2 76.7 100.0 88.9 90.0 0.5

7.8 23.3 e 11.1 e 0.5

e e e 10.0 0.5

11 10 10 15 25 1.5

13 9 e 9 e 1.5

e e e 30 1.5

3.8 8.2 10.2 9.1 2.3 0.2

V. Iancu et al. / Materials Chemistry and Physics 141 (2013) 234e239

(101)

(200)

(211)

(105)

(204)

C3

CSA [μ M]

1.00

(112)

Intensity

237

Aeroxide P25

0.95

C0 0.90

C2

C1

C1 C3 C2

0.85

C0 0 20

30

40

2θ

ο

50

60

70

20

40

60

80

100

120

140

160

180

200

Time [min] Fig. 4. Photodegradation data analysis of the porous composites as indicated.

Fig. 2. X-ray patterns of the composites as indicated.

nanoparticles [18]. If a correlation is made between the photocatalytic results, the crystalline phase content and the silver amount, one can assume that Ag nanoparticles further enhance the photocatalytic performance when connected to the anatase TiO2 crystallites. The higher the anatase phase content the better the photodegradation rates are under visible light irradiation. Moreover, photocatalytic performances can very well be influenced by the crystallites size. Small particles produce high surface areas and shorten the route on which an electron from the conduction band migrates to its surface. In our particular case, all samples containing Ag nanoparticles, excepting C2, have brookite crystallites of z9 nm, thus no difference in the photoactivity can be associated with the size change of the brookite nanocrystallites. Anatase grains from the sample C2, which exhibited the best visible light photocatalytic activity, have 10 nm in size as those belonging to composite C1, which exhibited the lowest photocatalytic performance out of the Ag loaded samples. Under visible light irradiation, the obtained results demonstrate that the composites with crystallites of 10 nm in size do not always give the best photocatalytic response. One can thus infer that the anatase crystallites size alone is not a decisive parameter in the case of the investigated samples, the presence of brookite in the phase content having a much higher influence on the photodegradation rate under visible light. By analyzing the N2 adsorption measurements, one would expect that the samples with the highest surface area to exhibit the best photocatalytic activity since a high surface area of the

mesoporous TiO2 can provide more active sites, and consequently, adsorb more substrate molecules. The recorded data show this to be true, and that the existence of the highest specific surface area does in fact ensure the obtaining of the highest photodegradation rate under visible light irradiation. From the long series of factors that could influence the photocatalytic performances, crystalline phase content, particle sizes and surface areas can be interrelated with the differences observed in the evaluation of the composites photodegradation activity. In order to give a further insight for the obtained photocatalytic results, the UVevis specular reflectivity spectra were recorded (see Fig. 6(a)). By using the KubelkaeMunk absorption model and the Tauc plot [32], the band-gap energy of the samples was estimated for both direct and indirect transitions. The values are as follows: Eg(C0) ¼ 3.41 eV, Eg(C1) ¼ 3.14 eV, Eg(C2) ¼ 3.24 eV, Eg(C3) ¼ 3.25 eV, Eg(Aeroxide P25) ¼ 3.49 eV for direct transitions, and Eg(C0) ¼ 3.17 eV, Eg(C1) ¼ 2.7 eV, Eg(C2) ¼ 2.98 eV, Eg(C3) ¼ 2.98 eV, Eg(Aeroxide P25) ¼ 3.18 eV for indirect transitions. By analyzing the experimental results several inconsistencies between the photodegradation data and the derived Eg values can be seen. For instance, the Eg values obtained, when direct transitions were considered, correspond to wavelengths located in the UV spectral region, result that, at first sight, indicates the missing of any visible photoactivity of the silver composites. By considering the photocatalytic data, one can see that this is not true, all the silver composites acting as visible light photocatalysts. It becomes obvious that the majority of the transitions in our TiO2 composites are indirect (see Fig. 6(b)), thus, the application of the Kubelkae

Fig. 3. Selected TEM images of selected samples.

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V. Iancu et al. / Materials Chemistry and Physics 141 (2013) 234e239 10.5

-1

kapp x 10 (min )

10.0

enhanced near-field amplitudes of localized surface plasmon can generate more pairs of electrons and holes, if the band-gap of the composite has a value corresponding to a wavelength of approximately 410 nm. This behavior will lead to a higher photocatalytic performance. Sample C1 has the lowest band-gap of 2.7 eV, corresponding to a wavelength of 459 nm well into the visible region. However, one should emphasize that this smaller value of the Eg can be explained when it is considering the energy value where the surface plasmon resonances of Ag aggregates occur (their presence being further evidenced). These resonances give a signal located at wavelengths higher than 450 nm that in our case is convoluted by the titania electronic absorption. Thus, the charge transfer between silver and titania nanoparticles is less favored in the case of the composite C1 because a reduced number of contacts are expected to exist comparing with the other composites, where the Ag nanoparticles are better dispersed inside the TiO2 porous network although the silver amount is substantially higher. As for samples C2 and C3, they have the same band-gap energy of 2.98 eV corresponding to a wavelength of 413 nm, being very close to the surface plasmon absorption band of silver. Thus, even though their band-gap energy is higher than that of C1, their photocatalytic performance is improved because more pairs of electron and holes are excited, a similar behavior being relatively recent reported [20]. Even though they have the same band-gap energy, a small difference in the photodegradation rate of samples C2 and C3 has occurred under visible light irradiation. This difference could be attributed to the brookite crystalline phase content, which diminishes the photocatalytic activity. However, in the assessment of such composite system one should keep in mind the important role played by the charge

C2 A(100%)

A - Anatase B - Brookite

9.5

-3

C3 A(88.9%)-B(11.1%) 9.0

8.5

C1 A(76.7%)-B(23.3%) 8.0

75

80

85

90

95

100

Crystalline phase content (%) Fig. 5. Relationship between photocatalytic activity and anatase/brookite ratio.

Munk model for indirect transitions is recommended because of the highly accurate results obtained with the TiO2 composites. Under visible light exposure, the enhanced photocatalytic activity of the composites C1, C2, and C3, can be attributed to the existence of a smaller band-gap energy as a result of the contact between titania and Ag nanoparticles [18]. The reason for the bandgap narrowing of Ag loaded TiO2 is attributed to the shift of upper most valence band and lowest conduction band [33], thus inducing the visible light absorption in the 450e800 nm spectral range (see Fig. 6(c)). The intense localized surface plasmon absorption band of Ag is around 410 nm in the near-UV region, and, for this reason, the

Aeroxide P25

C2

c)

R[%]

C0 0 .3

C3 C1

a) 4 00

5 00

6 00

7 00

800

Wavelength (nm)

Absorbance

300

0 .2

C1 C3 0 .1

C2 [ F(r)hν ]0 .5

C0 0 .0 400

C2 C0 Aeroxide P25

500

600

700

800

Wavelength (nm)

C1 C3

2.0

b) 2.5

3.0

3.5

h ν [e V ] Fig. 6. UVevis diffuse reflectance spectra of the composites as indicated (a). The band-gap evaluation of samples from the Tauc representation of the UVevis spectra using the KubelkaeMunk absorption model (b). UVevis absorbance spectra of the composites as indicated (c).

V. Iancu et al. / Materials Chemistry and Physics 141 (2013) 234e239

transfer occurred in the nanostructures interface in which TiO2 is involved, as recently demonstrated [34]. In our particular case the charge transfer between titania and silver nanoparticles could certainly leads, alongside the Ag presence, to the obtained improvement of visible photocatalytic performances. In order to separate the role played by this effect a more simplified and controllable TiO2eAg nanoparticles system, i.e. that enable a close characterization of the contacts between titania and silver, should be designed and evaluated from both experimental and theoretical point of view. The percentage of silver loaded into the TiO2 porous network is listed in Table 1. As one can see, the sample C1 has the highest Ag content, while the smallest one was determined for the composite C3. By again analyzing the photocatalytic data of the investigated samples (see Table 1) one can observe that although the silver addition considerably improves the visible photocatalytic performances, its presence in a higher amount does not necessarily mean an increased photodegradation rate. In the case of sample C1 with the lowest photodegradation rate, the silver nanoparticles are not uniformly dispersed inside the porous titania matrix and form agglomerates inside the aerogel pores, fact clearly evidenced in the UVevis reflectance spectra (Fig. 5(a)). This is because the silver nanoparticles were added into the TiO2 aerogel matrix after the supercritically drying step. Only in this spectrum the Ag aggregate surface plasmon resonances signal can be seen in the 500e580 nm range (see arrow in Fig. 5(a)). As for samples C2 and C3 with the better photocatalytic response, the silver was loaded before the TiO2 gel was supercritically dried, thus allowing a better dispersion of the silver nanoparticles. This result further supports the more important role of the anatase/brookite ratio on the photocatalytic performances under visible irradiation rather than the silver content.

[13] [14] [15] [16] [17] [18] [19] [20]

4. Conclusions

[21] [22]

Nanocomposites based on TiO2 aerogel and Ag nanoparticles have been successfully obtained through three synthesis methods. The mesoporous character of the samples has been assessed by N2 sorption measurements and their structure has been investigated by XRD. The photocatalytic potential for salicylic acid degradation has been evaluated and it has been found that all nanocomposites exhibit a photocatalytic activity higher than that of the commercial product Aeroxide P25. The Ag loaded samples containing anatasee brookite crystalline phases show a dependency of the photocatalytic performance on the content of the brookite phase. By correlating the morpho-structural parameters with the photocatalytic activity it has been found that under visible light irradiation the photocatalytic performance increases with the decrease of

239

the brookite phase content. These results give essential information for the design and synthesis of highly active visible light photocatalysts. Acknowledgements This work was supported by CNCSIS-UEFISCSU, project number PN II-RU TE 81/2010, and MNT-ERANET, project number SMARTPACKe7-065/2012. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12]

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