Photocatalytic, Morphological and Structural Properties

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Journal of Surfaces and Interfaces of Materials Vol. 2, 305–310, 2014

N–TiO2–Ag Based Porous Structures: Photocatalytic, Morphological and Structural Properties M. Rusu1 , G. Kovács1 2 3 , C. Cotet2 , I. Fort2 , A. Vulpoi1 , L. Baia1 , Zs. Pap1 3 , and V. Danciu2 ∗ 2

1 Faculty of Physics, Babes-Bolyai University, M. Kog˘alniceanu 1, RO-400084, Cluj-Napoca, Romania Faculty of Chemistry and Chemical Engineering, Babes-Bolyai University, Arany János 11, RO-400028, Cluj-Napoca, Romania 3 Faculty of Science and Informatics, Department of Applied and Environmental Chemistry, University of Szeged, H-6720, Szeged, Rerrich Béla tér 1, Hungary

RESEARCH ARTICLE

TiO2 -based porous nanocomposites are gaining more and more attention due to their specific properties and large variability of synthesis pathways offering an alternative approach against traditional/commercially available photocatalysts. In our work, nitrogen containing TiO2 based aerogels were obtained and modified with different amounts of Ag nanoparticles. Their morphological and structural properties were investigated and further correlated with the data obtained from the photocatalytic experiments. Applying different techniques such as DRS, XRD, N2 adsorption and HRTEM, the nanoparticles and their composites’ morphological and structural details were successfully evaluated. Important differences were observed in the photocatalytic activity of the compositecatalysts in which only the concentration of the noble metal was changed.

Keywords: Titanium Dioxide, Aerogel, Silver Nanoparticles, Nitrogen Doping, Photocatalysis, Salycilic Acid.

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1. INTRODUCTION Creative combination of well-known materials, taking into account their functionalities and their properties is one of the most challenging research-field in materials chemistry. Considering the alarming level of pollution of the hydrosphere, more and more alternative pathways are investigated in order to eliminate organic pollutants from the environment. TiO2 is considered to be a promising material in the photocatalytic pathway of pollutant-removal, being non-toxic, relatively inexpensive and presents a notable UV light absorption and photocatalytic efficiency toward degradation of various model-contaminants, like phenol,1 2 oxalic acid,3 formic acid,4 salicylic acid5 6 and methyl orange.7 But, like everything in the balanced nature, it also has its own insufficiencies which are decreasing its efficacy. The most representative ones are the wide band-gap energy of the native titania, which only allows adsorption of the UV light of the solar spectrum and its relatively low active specific surface area (for example, in the case of commercial P25 it is around 50 m2 g−1 .8 In order to solve the above mentioned aspects titaniabased aerogels can be prepared by using the sol–gel method followed by supercritical drying and their morphology further optimized (surface area and porosity). ∗

Author to whom correspondence should be addressed.

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The main challenge after the “ageing” process is to avoid surface tension effects as the wet gel is dried so that the aerogel porous nanostructure is maintained. This can be achieved by freeze-drying, ambient pressure drying with the use of surfactants to lower the surface tension or by the mostly widely used approach, by supercritical drying, which can overcome the surface tension effects during drying process.9 A relatively large number of methodologies were already published regarding titania aerogels.10–15 These materials generally have higher surface areas than native titania and following a heat treatment, they can show high photocatalytic activity, due to the increase of the (photo)catalytically active surface area. First time the enhancement of the photocatalytic activity of nitrogen-doping over the TiO2 -based structures was described by Asahi et al. showing that the doped titania can exhibit higher absorption in the visible region.16 Further investigations have shown that the nitrogen-effect on TiO2 depends on many factors, like the synthesis pathway (sol–gel, ion implantation, oxidation of titanium nitrite, etc.), the location of nitrogen species in the semiconductors’ structure and the interaction between the N centers and oxygen vacancies.15 17 In the chase of obtaining TiO2 -based photocatalytic materials with higher activity, incorporation of various non-metallic18 and metallic elements19 was also attempted.

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N–TiO2 –Ag Based Porous Structures: Photocatalytic, Morphological and Structural Properties A N-TiO2-Ag 1% R

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Noble metals are highly exploited in this point of view, in noble metal (gold,20 platinum21 22 and silver23 -titania composites. Silver is a favorable metal for TiO2 -based composites, due to its remarkable electric, optical and catalytic characteristics.24 In the meantime, Ag nanoparticles have been reported to exhibit high bactericidal activity and biocompatibility compared to other nanoparticles. The synergistic coupling of Ag and TiO2 can be attributed to the Schottky-barrier that is formed giving the possibility for Ag nanoparticle to act as an electron trap which can inhibit the recombination of photogenerated electron– hole pairs, enhancing in this way the overall photocatalytic activity of the composites.25 Based on the facts described above, the aim of the present work was to synthesize N–TiO2 –Ag-based aerogels, to investigate the Ag nanoparticles additions’ effect on the titania matrix from the perspective of the morphological and structural properties by using various methods (diffuse reflectance spectroscopy-DRS, transmission electron microscopy-TEM and N2 sorption), and, last but not the least, to correlate the obtained results with those acquired from the photocatalytic experiments, in which a model-pollutant was used.

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the quantities were added in double units, with exception of ethanol (8.45 mL) (sol C).

Solution C was added in solution B and the obtained mixture was added dropwise to solution A; the jellification process started with the addition of the last drops of (B+C) mixture. The obtained gel was left for maturation for 14 days in polyethylene recipient. After maturation, the gels were washed three times with ethanol and dried in supercritical conditions (using liquid CO2 and thermally treated at 600 C for 2 hours (heating speed 4 C/min) in order to obtain a higher crys2. EXPERIMENTAL DETAILS tallinity grade, and -in parallel-higher photocatalytic effi2.1. Synthesis of TiO2 -Based Aerogels ciency caused by the appearance of anatase-phase in the structure previously of amorphous Publishing Technology to: University Szeged aerogels.26 For preparation of N–TiO2Delivered –Ag-basedbyaerogels titanium IP: 160.114.58.213 On: Sun, 22 Feb 2015 16:46:44 isopropoxide was used as precursor (Sigma-Aldrich) with American Publishers and ethanolic solu- Scientific diethylamine (Sigma-Aldrich), HNO3 Copyright: 2.2. Morpho-Structural Characterization tion of AgNO3 (50 mM). The final theoretic concentration X-ray diffraction (XRD) measurements were performed of Ag and N was varied (0.5 and 1%) changing the conusing a BRUKER D8 Advance X-ray diffractometer, using centration of the appropriate precursor. Cu K radiation ( = 1543 Å) and the step-scanning mode In order to be able to compare the results from the varwere of 0.02 (2. PowderCell program enabled a quanious synthesis pathways, pure TiO2 and N–TiO2 aerogels titative phase (volume fractions) analysis method by comwere prepared, as follows: parison of the different scattering powers of the component —to obtain “pure” TiO2 gel, solution A (obtained by materials. addition of 10 mL of titania precursor dropwise to The adsorption–desorption isotherms of the samples 23.66 mL of ethanol) was added to the mixture resulting were recorded using a Sorptomatic 1990, Thermo. from solution B (13.33 mL of ethanol and 10 L of water) Previously to the adsorption-isotherms measurements, the and solution C (2.2 mL of distilled water and 0.168 mL calcined samples were heated at 120 C for 2 hours. The of HNO3 cc . specific surface area (SBET of the samples was calcu—to obtain N–TiO2 gels, the amount of the precursors lated by BET method. The mesopores size distribution and the synthesis methodology was the same as for the and mesopores volume were determined using the Barrett, N–TiO2 –Ag gels, with the exception that the solution of Joyner, Halenda method applied on the desorption branch AgNO3 was replaced with the same volume of ethanol. of the isotherm. For the synthesis of the N–TiO2 –Ag aerogels was used three different solutions, as follows: Table I. Morphological, optical and photocatalytic data as obtained —10 mL of titania precursor was added dropwise to from the investigated materials. 23.66 mL of ethanol under vigorous stirring (sol A) Crystallite Specific —2.2 mL of distilled water was added to 0.168 mL of kapp × 103 mean size surface area Egindirect HNO3 cc (sol B) 2 Sample (nm) (m /g) (eV) (min−1 ) —in order to obtain aerogel with 0.5% Ag content, TiO2 15 54 3.02 1.8 2.44 mL of AgNO3 EtOH , 25 L of diethylamine, 5 L of 12 57 2.94 8.8 N–TiO2 –Ag 0.5% distilled water and 10.89 mL of ethanol were mixed under – 42 2.88 9.4 N–TiO2 –Ag 1% vigorous stirring and for reaching the 1% content of Ag, 306

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N–TiO2 –Ag Based Porous Structures: Photocatalytic, Morphological and Structural Properties N-TiO2-Ag 0.5%

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RESEARCH ARTICLE

Delivered by Publishing Technology to: University of Szeged IP: 160.114.58.213 On: Sun, 22 Feb 2015 16:46:44 Copyright: American Scientific Publishers Fig. 2.

TEM images of the N–TiO2 –Ag systems. Individual and aggregated Ag nanoparticles can be observed.

JASCO-V650 spectrophotometer was used for measuring the DRS spectra of the samples ( = 300–800 nm). The band-gap of the photocatalysts was determined via the Kubelka-Munk method. TEM/HRTEM images were obtained with a FEI Tecnai F20 field emission, high resolution Transmission Electron Microscope (TEM) operating at an accelerating voltage of 200 kV and equipped with Eagle 4 k CCD camera. 2.3. Photocatalytic Measurements Photocatalytic activity of the obtained aerogels was evaluated using a Pyrex-reactor, illuminated with a mercury lamp (medium pressure TQ, 150 W), kept at 25 C using thermostat. For measurements, solution of salicylic acid (5 ∗ 10−4 M) was photodegradated with concentration of 1 g/L catalyst, following the degradation of the mentioned model contaminant with spectrophotometer ( = 297 nm).

3. RESULTS AND DISCUSSION 3.1. XRD Measurements X-ray diffraction was used in order to investigate the crystallinity of the samples. As shown in Figure 1, anatase is the dominant crystalline phase.27 For the bare TiO2 sample, J. Surf. Interfac. Mater. 2, 305–310, 2014

the additional peak observed at 27 (2 indicates that the rutile crystalline structure is also present, perhaps as a result of the chosen temperature for the thermal treatment. By taking into consideration the changes induced in the XRD patterns by the presence of Ag and N besides titania, one can observe that not only rutile, but the anatase phase is also progressively inhibited. This trend can be correlated with the decrease of crystallite mean size, as determined from the Scherrer formula26 (Table I). In the study of other similar Ag–TiO2 –N systems prepared by a modified method, Zhang et al. correlated this effect with the formation of Ag2 O layers on the surface of TiO2 grains.28 Such layers could restrain the growth mechanisms of crystalline particles by modifying their surface energy. 3.2. TEM Investigations The HR-TEM images for sample TiO2 –N–Ag are presented in Figure 2. After evaluating the size distribution (15–25 nm), larger nanoparticles were evidenced in contrast with the XRD data. Furthermore, one can see that not all particles exhibit well-defined crystalline planes. This also supports that the crystallinity was hindered in N and Ag containing samples. In the meantime, the formation 307

N–TiO2 –Ag Based Porous Structures: Photocatalytic, Morphological and Structural Properties

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of Ag nanoparticles having dimensions between 15 and 20 nm was confirmed. As seen from Figure 2, the Ag nanoparticles were found in both cluster-like distributions and fine dispersions. 3.3. N2 Adsorption/Desorption Data The adsorption/desorption isotherms are depicted in Figure 3. According to IUPAC convention, they were classified as type IV isotherms, due to their hysteresis like behavior and to the linear behavior of the adsorption curve at intermediate pressures. These features are characteristic to mesoporous systems. According to the data obtained from the BJH method, the mesopore size distribution had 308

a Gaussian shape and the size of the pores was between 2 and 50 nm. The presence of Ag nanoparticles did not influence the mesoporous nature of the samples, nevertheless an increase in pore size and a 25% decrease in surface area (Table I) was noticed for the sample with the highest doping concentration. In N–TiO2 –Ag systems, prepared via hydrothermal treatment,29 the observed decrease in BET area was explained in terms of mesopore site blocking by well adhered Ag nanoparticles. 3.4. DRS Measurements In order to investigate the powdered material’s spectral response, diffuse reflectance UV-Vis spectroscopy was J. Surf. Interfac. Mater. 2, 305–310, 2014

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TiO2 lattice, increasing in this way their photocatalytic activity. The presence of such energy levels will permit electrons to be excited into the band-gap by absorbing visible light.30

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The materials obtained through the studied synthesis route conserve their mesoporous nature even when other precursors such as diethylamine and AgNO3 are used. Nevertheless, lower surface areas were observed. After the thermal treatments, anatase crystalline sites were detected in all of the studied samples. The doping of TiO2 hindered the crystallite growth and phase transition to rutile. The increase in photocatalytic activities was correlated with the shift of the material’s optical transitions to lower energies due to the plasmonic interactions between the TiO2 lattice with Ag nanoparticles and to the presence of permitted energy levels induced by N sites.

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further used. The absorbance spectra are presented in Figure 4. When comparing the bare TiO2 sample with the Ag doped samples, one can observe an increase in absorbance at higher wavelengths. This broad signal can be associated with the plasmonic response of both singuAcknowledgments: This work was supported by the lar nanoparticles (in-between 400 and 480 nm) and Ag Grants of the Romanian National Authority for Scienclusters (above 480 nm). The indirect band gaps were tific Research, MNT ERA-NET nr. 7-065/26.09.2012 and determined using Kubelka-Munk transformations applied PNII-ID-PCE-2011-3-0442. G. Kovács wants to acknowlto the reflectance curves and Tauc plots. As presented in edge The research by carried out by G. Kovács underlyTable I, Ag and N doping red-shifted the energy gaps proing the publication was realised in the framework of the gressively with the increase of the dopant concentration. TÁMOP 4.2.4.A/2-11-1-2012-0001 “National Excellence Furthermore, in other studies performed on N–TiO2 –Ag Programme—Elaborating and operating an inland student systems, when only Ag precursor concentration varied, it Delivered by Publishing Technology to: University of Szeged and support system convergence proOn: Sun, 22researcher Feb 2015personal 16:46:44 was also observed that at higherIP: Ag160.114.58.213 content, the absorpgramme.” The project was subsidised by the European 29 Copyright: American Scientific Publishers tion in visible light was enhanced. This is also valid in Union and co-financed by the European Social Fund. the present study. It can be concluded that TiO2 aerogels achieved a better response in visible light by integrating Ag nanoparticles and N impurities into the system. References and Notes 3.5. Photoactivity Results The photocatalytic activity tests performed in UV light on a salicylic acid solution suggest a correlation between the material’s energy gap red-shift, as revealed from the presented spectral response, and the increase of the photocatalytic degradation constant (Table I). Even so, other parameters are also essential for improving the degradation efficiency. Firstly, this would refer to the increase in numbers and strength of the adsorption sites. Secondly, better photocatalytic degradation would be achieved if oxidant species are to be generated in higher numbers. The last case is essential for degradation mechanisms of nonadsorbed pollutant molecules. In order to increase the concentration of oxidant species, the excited charge carriers must be effectively transferred from titania to O2 and H2 O molecules instead of suffering recombination reactions. It is known that Ag, under optimal concentrations,29 creates electron accepting energy levels, thus integrating Ag nanoparticles would inhibit electron–hole recombinations. On the other hand, N-doping also creates permitted energy levels inside the energy gap of TiO2 , together with the increased adsorption in the visible region sensitizes the J. Surf. Interfac. Mater. 2, 305–310, 2014

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Received: 2 November 2014. Accepted: 8 December 2014.

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