Environ Sci Pollut Res DOI 10.1007/s11356-014-2624-2

PHOTOCATALYSIS: NEW HIGHLIGHTS FROM JEP 2013

Synthesis of BiVO4/TiO2 composites and evaluation of their photocatalytic activity under indoor illumination Giulia Longo & Fernando Fresno & Silvia Gross & Urška Lavrenčič Štangar

Received: 15 November 2013 / Accepted: 4 February 2014 # Springer-Verlag Berlin Heidelberg 2014

Abstract BiVO4/TiO2 composites with different weight ratios have been prepared by coprecipitation-based reactions followed by either thermal or hydrothermal treatment with the aim of evaluating the TiO2 photosensitization by BiVO4. The obtained materials present in all cases the desired monoclinic phase of BiVO4 and anatase phase of TiO2. Visible light absorption increased with increasing amount of bismuth vanadate. XPS results reveal the surface enrichment of Ti with respect to the bulk composition in samples characterised by a higher content of BiVO4. The photocatalytic activity of the prepared materials was tested for the degradation of isopropanol in the gas phase under indoor illumination conditions. Although none of the composites was able to improve the activity of TiO2, the low BiVO4 containing samples appear as more suitable for further synthesis tuning. Keywords Photocatalysis . Titania . Bismuth vanadate . Photosensitization . Coupled photocatalysts . Composite photocatalysts

Introduction The lack of absorption of visible light due to a large band gap, which matches the UV light region, is probably the main drawback of TiO2 as a photocatalyst (Fresno et al. 2014;

Responsible editor: Philippe Garrigues G. Longo : F. Fresno (*) : U. L. Štangar Laboratory for Environmental Research, University of Nova Gorica, Vipavska 13, 5000 Nova Gorica, Slovenia e-mail: [email protected] G. Longo : S. Gross IENI-CNR, Dipartimento di Scienze Chimiche, Università degli Studi di Padova, via Marzolo 1, 35131 Padova, Italy

Pelaez et al. 2012). This is particularly true when dealing with indoor applications, like indoor air depollution or selfcleaning surfaces, where the presence of UV light is even lower than in the case of sunlight. Several strategies have been followed in order to extend the photocatalytic activity of titania to the visible range. Cationic and anionic doping are classical pathways followed to achieve this goal, by which varying results have been obtained (Fresno et al. 2014; Pelaez et al. 2012). Another interesting strategy is the photosensitization of TiO2, i.e. its coupling with another species able to absorb visible light and transfer the excitation to titania. This species can be either an organic molecule or another semiconductor. The first type has been successfully exploited in the socalled dye-sensitised solar cells (Grätzel 2003) and, although to a lesser extent, organically sensitised visible-light-promoted photocatalytic degradation of pollutants and water splitting have been reported too (Pei and Luan 2012). Nevertheless, and despite the fact that a very fast and efficient charge transfer can be attained with these systems, the lack of stability of the sensitizer under the conditions of a photocatalytic reaction makes their use as efficient and durable photocatalysts an open challenge (Youngblood et al. 2009). A possible alternative is the use of a second inorganic semiconductor as photosensitizer for TiO2. Although some charge transfer efficiency may likely be lost with respect to organic molecules, inorganic sensitizers would provide the system with long-term stability under a wide range of reaction conditions (Kubacka et al. 2012). A classical example of this strategy is the combination of TiO2 with metal sulphide semiconductors absorbing light in the visible range, and particularly the case of CdS has been known for many years (Serpone et al. 1984). Interesting results have been obtained with this system under purely visible light, but under UV-Vis irradiation, an improvement of the photocatalytic activity of TiO2 by the CdS/TiO2 couple is not so clear (Robert 2007). In this respect, an improvement of the TiO2 activity under the latter

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lighting conditions would be realistically more interesting from a practical point of view. On the other hand, the toxicity of Cd can be a serious limitation for an extensive use of these composites, especially taking into account that photocorrosion of the sulphide is not prevented by the interfacial charge separation scheme of this system. In this respect, visible light absorbing metal oxides, especially those not undergoing photocorrosion, would be very interesting inorganic sensitizers. An example of this would be represented by the WO3/TiO2 system (Miyauchi et al. 2002). Great interest has recently been devoted to visible light absorbing metallates, which have been studied not only for water splitting, but also for decontamination processes (Hernández-Alonso et al. 2009), and their high absorption in the visible region makes them potential candidates for inorganic TiO2 sensitization. In this respect, BiVO4 is a visible light absorbing material presenting a high mobility of photogenerated charge carriers that makes it a good candidate for photocatalytic applications, either on its own or coupled with other semiconductors (Coronado et al. 2013). Regarding the first situation, it has been mainly used for water oxidation reactions, but also for pollutant degradation, although its performance in the latter type of processes is not fully assessed (Hernández-Alonso et al. 2009). It has also been proposed recently as a visible light sensitizer of TiO2 for environmental applications following an interfacial charge transfer mechanism (Hu et al. 2011, Min et al. 2011). In this work, the evaluation of BiVO4/TiO2 composites as photocatalysts for pollutant abatement under indoor illumination conditions is proposed. For that purpose, crystalline phase-controlled composites with different BiVO4:TiO2 weight ratios were prepared by coprecipitation-based synthetic routes coupled with either hydrothermal or thermal posttreatment. The former route (combination of coprecipitation and hydrothermal route) had, to the best of the authors’ knowledge, not yet been explored in the literature for this coupled system. BiVO4 exists in four polymorphs: orthorhombic pucherite, tetragonal dreyerite and monoclinic clinobisvanite, plus a tetragonal scheelite-like phase, reversibly formed from the latter at 528 K. It is generally obtained in the laboratory as the second and third forms of this list, being the thermodynamically stable clinobisvanite phase the one with the best photoelectrochemical behaviour (Coronado et al. 2013). Regarding TiO2, it is well known that the anatase phase shows a better photocatalytic performance in most applications (Carp et al. 2004). Therefore, the first aim of the synthesis was to obtain the coupled photocatalysts in the clinobisvanite/anatase form. To confirm this point and to assess other physicochemical and structural properties of the obtained materials, a basic characterisation was performed. Finally, their activity was evaluated for the photocatalytic oxidation of isopropanol in the gas phase under unfiltered indoor illumination.

Experimental Catalyst preparation Single-phase catalysts For the preparation of BiVO4 powders (Zhang et al. 2006), two solutions were prepared: solution A containing 5 mmol of Bi(NO3)3·5H2O in 10 ml of 4 M HNO3, and solution B, with 5 mmol of NH4VO3 in 10 ml of 2 M NaOH. Solution B was poured into solution A, forming an intense orange solution. The pH of the solution was raised to 7 using 2 M NaOH and controlling the value with a pH-meter. Through the basification, a yellow suspension was formed. Two routes were then followed. In the hydrothermal procedure, this suspension was left stirring for 30 min, and then transferred into teflon-lined stainless steel autoclaves and heated in a convection oven at 453 K for 24 h. The final yellow solid was separated by centrifugation, thoroughly washed with H2O and EtOH, and dried in air at room temperature for 12 h, then at 373 K for 2 h. In the thermal route, the yellow suspension was left under stirring for 15 h, then separated, washed and dried and finally calcined at 773 K for 3 h with a heating ramp of 12 K min−1. For the preparation of TiO 2 powders, 1.48 ml of titanium(IV) isopropoxide was peptized in 17.76 ml of bidistilled water with 0.127 ml of concentrated HNO3 at 313 K for 24 h. After this period, the obtained sol was basified to pH 7 using 2 M NaOH and a pH-meter. The resulting white suspension was left stirring for 30 min. Two routes were then followed. In the hydrothermal procedure, the sol was poured into Teflon-lined stainless steel autoclaves and heated in a convection oven at 453 K for 24 h. The final powder was separated by centrifugation, thoroughly washed with bidistilled water and EtOH and dried in air at room temperature for 12 h and at 373 K for 2 h. In the thermal route, the TiO2 suspension was left under stirring for 15 h, then separated, washed and dried and finally calcined at 773 °C for 3 h with a heating ramp of 12 K min−1.

Composite catalysts Three BiVO4/TiO2 composites with different nominal weight ratios were produced by using either the hydrothermal or the thermal route (see Table 1). In the first route, a titania sol was obtained by peptization of Ti(OiPr)4 as mentioned above. A BiVO4 precursor was produced as mentioned, mixing solutions A and B. Then the TiO2 sol was poured into the vessel containing the BiVO4 precursor in the appropriate relative amounts to reach the targeted weight ratios. Afterwards, the whole mixture was basified to pH 7, left stirring for 30 min, then transferred into autoclaves and heated at 453 K for 24 h. The final solids were washed

Environ Sci Pollut Res Table 1 Prepared photocatalysts, band gap energies obtained from DR-UV-vis spectra and BET areas from N2 adsorption isotherms Synthetic route

Material

Hydrothermal

BiVO4 80 % BiVO4–20 50 % BiVO4–50 20 % BiVO4–80 TiO2 BiVO4 80 % BiVO4–20 50 % BiVO4–50 20 % BiVO4–80 TiO2

Thermal

% w/w TiO2 % w/w TiO2 % w/w TiO2

% w/w TiO2 % w/w TiO2 % w/w TiO2

Short name

Band gap (eV)

BET surface area (m2 g−1)

BiVO4-ht 80Bi-20Ti-ht 50Bi-50Ti-ht 20Bi-80Ti-ht TiO2-ht BiVO4-c 80Bi-20Ti-c 50Bi-50Ti-c 20Bi-80Ti-c TiO2-c

2.42 2.42 2.41 2.42 2.98 2.44 2.46 2.44 2.44 2.92

1.42 41.4 98.2 178.4 46.4 1.24 13.0 40.9 61.3 62.9

In the composite samples, the band gap value corresponds to the absorption of BiVO4

with bi-distilled water and EtOH, left drying in air at room temperature for 12 h and finally dried at 373 K for 2 h. In the second route, hydrothermally prepared BiVO4 was added to a sol of titania in the appropriate weight ratio. The pale yellow suspension was basified to pH 7 with NaOH 2 M, and was left stirring for 15 h, then washed with bi-distilled H2O and EtOH and left drying in air at room temperature for 12 h. The dry composites were calcined for 3 h at 773 K with the usual heating rate.

(Briggs and Seah 1990). Survey scans were obtained in the 0– 1350 eV range (pass energy 187.5 eV, 1.0 eV/step, 25 ms/ step). Detailed scans (29.35 eV pass energy, 0.1 eV/step, 50– 150 ms/step) were recorded for the O1s, C1s, Ti2p, TiKLL, V2p, Bi4f and N1 regions. The atomic composition, after a Shirley-type background subtraction, was evaluated using sensitivity factors supplied by PerkinElmer (Shirley 1972). Charge effects were partially compensated by using a charge neutralizer (flood gun). Peak assignment was carried out according to literature data.

Characterisation techniques Photocatalytic reactions X-ray powder diffractograms were recorded in the 10–60° 2θ range on a PANalytical X-ray diffractometer using Cu Kα radiation, a step size of 0.017° and a scan step time of 51 s. Crystallographic analyses were carried out with the PANalytical X’pert High Score Plus software, using the ICDD Powder Diffraction Files 00-014-0688 (clinobisvanite BiVO4) and 01-083-2243 (anatase TiO2) as references. Average crystallite sizes were estimated by means of the Scherrer equation. UV-Vis spectra were recorded in the diffuse reflectance mode on a PerkinElmer Lambda 650S spectrometer equipped with an integrating sphere using a scan interval of 1 nm and BaSO4 as reference. Surface areas were determined by the Brunauer-EmmetTeller (BET) method from N2 adsorption isotherms obtained at 77 K by a Micromeritics TriStar 3000 instrument after degassing the samples at 383 K. The powders were investigated by XPS with a PerkinElmer Φ 5600ci instrument using standard Al-K α radiation (1,486.6 eV) operating at 350 W. The working pressure was

TiO2 composites and evaluation of their photocatalytic activity under indoor illumination.

BiVO4/TiO2 composites with different weight ratios have been prepared by coprecipitation-based reactions followed by either thermal or hydrothermal tr...
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