Gram-scale synthesis of UV–vis light active plasmonic photocatalytic nanocomposite based on TiO2/Au nanorods for degradation of pollutants in water

https://doi.org/10.1016/j.apcatb.2018.11.002Get rights and content

Highlights

  • Nanocomposite based on TiO2 nanoparticles and plasmonic Au nanorods have been prepared.

  • Calcination temperature affects morphology and structure of the nanocomposites.

  • The plasmonic domain beneficial for the photocatalytic performance.

  • TiO2/Au nanorods calcinated at 450 °C up to 13 times more photoactive than unmodified TiO2.

  • TiO2/Au nanorods calcinated at 450 °C up to 2.5 times more photoactive than TiO2 P25.

Abstract

Semiconductor/metal nanocomposites based on anatase TiO2 nanoparticles and Au nanorods (TiO2/AuNRs) were prepared by means of a co-precipitation method and subsequently calcinated at increasing temperature (from 250° to 650 °C) obtaining up to 20 g of catalysts. The structure and the morphology of the obtained nanocomposite material were comprehensively characterized by means of electron microscopy (SEM and TEM) and X-ray diffraction techniques. The photocatalytic performance of the TiO2/AuNRs nanocomposites was investigated as a function of the calcination temperature in experiment of degradation of water pollutants under both UV and UV–vis irradiation, Photocatalytic experiments under UV irradiation were performed by monitoring spectrophotometrically the decolouration of a target compound (methylene blue, MB) in aqueous solution. UV–vis light irradiation was, instead, used for testing the photocatalytic removal of an antibiotic molecule, Nalidixic acid, by monitoring the degradation process by HPLC-MS analysis. Interestingly, TiO2/AuNRs calcined at 450 °C was up to 2.5 and 3.2 times faster than TiO2P25 Evonik, that is a commercially available reference material, in the photocatalytic degradation of the Methylene Blue and the Nalidixic Acid, under UV and visible light, respectively. The same nanocomposite material showed a photocatalytic degradation rate for the two target compounds up to 13 times faster than the bare TiO2-based catalysts.

The obtained results are explained on the basis of the structure and morphology of the nanocomposites, that could be tuned according to the preparative conditions. The role played by the plasmonic domain in the heterostructured materials, either under UV and UV–vis illumination, is also highlighted and discussed.

The overall results indicate that the high photoactivity of TiO2/AuNRs in the visible range can be profitably exploited in photocatalytic applications, thanks also to the scalability of the proposed synthetic route, thus ultimately envisaging potential innovative solution for environmental remediation.

Introduction

In the quest to solve environmental remediation and solar energy conversion issues, plasmonic heterostructures composed of noble metals in combination with nanostructured semiconductors have been attracting tremendous attention [1,2]. Nanostructured TiO2 has been generally regarded as benchmark material for photocatalysis based on metal oxide semiconductors and its role in such a relevant area has been substantially studied for decades [3]. Nevertheless, bare TiO2 nanoparticles (NPs) are characterized by a wide band gap that limits its photoabsorption to the UV region (λ < 390 nm), being UV light just a small portion (4%) of the whole solar energy spectrum [4]. Modification of titanium (IV) oxide photocatalyst has often been used to enhance its photocatalytic activity and to extend its absorption wavelength range to the visible region. Many research groups demonstrated the beneficial effect of the integration of a noble metal in TiO2-based photocatalysts. Such an approach generally involves the dispersion of noble metal NPs (mostly Au and Ag, with sizes in the order of tens to hundreds of nanometers) into semiconductor photocatalysts and aims at enhancing the photoreactivity under UV and/or visible light irradiation [3,5]. Compared with the bare semiconductor, noble metal modified photocatalysts, also known as plasmonic photocatalysts, possess two distinct features: (i) a Schottky junction and (ii) localized surface plasmon resonance (LSPR) tuneable in the Vis-NIR range, both beneficial for the photocatalytic activity [6].

In a Schottky junction charge carriers flow from the semiconductor to the metal creating a spatial charge separation across the semiconductor–metal junction that hinder the recombination of photogenerated electron/hole pairs (e/h+). As a consequence, combination with noble metal NPs is expected to inhibit the electron–hole pair recombination by trapping electrons and to facilitate the transfer of holes on the TiO2 surface. Subramanian et al. demonstrated that, in the case of Au/TiO2 NPs, the Fermi level energy shifts toward potential more negative than that of bare TiO2, that results in an increase of the charge storage efficiency and an enhancement of charge separation, and, overall a higher reducing power of the nanocomposite system. Moreover, the particle size has been found to affect the shift of apparent Fermi level (EF*), as the smaller are the Au particles the larger is the shift induced in EF* [7]. Interestingly, while the effect of the enhanced charge separation can be observed upon UV irradiation, a significant contribution of the metal NPs to the overall photocatalytic efficiency of composite catalysts, also in the visible region, is expected due to their peculiar optical properties. Their plasmon band, which is tunable in the Vis-NIR region, has been found to enhance the TiO2 photoactivity under visible light or solar light irradiation [8], demonstrating the potential of such anisotropic metal NPs [2,9]. The surface plasmon resonance (SPR) absorption of metal nanorods (NRs) is featured by two bands, corresponding to the collective oscillations of the free conduction band electrons along the longitudinal and transverse axis of the NRs, respectively. The transverse mode shows a resonance signal around 520 nm, somehow in the same position of the SPR band of spherical particles of similar diameter, while the spectral position of the longitudinal mode is positioned at higher wavelength progressively red-shifting in the Vis-NIR range as the aspect ratio (defined as the long to short axis ratio) of NRs increases [10]. These phenomena can be ascribed to the charge accumulation along the rod axis (longitudinal plasma) and along a perpendicular direction (transversal plasma), respectively, being the charge accumulation higher for the latter. As the restoring force is proportional to such charge accumulation, less intense forces and consequently smaller resonance frequencies are required for exciting longitudinal plasmon resonance [11]. The absorption properties of metal nanostructures are extremely sensitive to their size, shape, chemical environment, and mutual distance among metal NPs [12,13]. Owing to their tuneable longitudinal plasmon band and their anisotropic shape, cylindrical Au NRs are among the most studied types of plasmonic NPs. AuNPs are usually preferred over other metals such as Ag or Cu, thanks to their lower tendency to oxidation that ensure the long-term photochemical stability of the resulting photocatalytic material [14].

The present work focuses on the synthesis of TiO2/Au NRs nanocomposites based on pre-synthesized Au NRs as nucleation substrate for the synthesis of TiO2 NPs by a simple and easily up-scalable co-precipitation procedure followed by calcination. The resulting nanocomposite showed a peculiar morphology featured by Au nanodomains embedded in a TiO2 sub-micrometric aggregates composed of anatase NPs that can be prepared in scale up to tens of grams.

The composite nanostructures obtained under different experimental conditions, controlling calcination temperature, were tested to evaluate the effect of the materials structure and morphology on the photocatalytic performance both under UV and visible light. Namely, UV irradiation was used in experiments aiming at decolouration of a model dye, the Methylene Blue (MB), while visible light was irradiated for the photocatalytic degradation of Nalidixic Acid (NA). The photocatalytic performance of the prepared nanocomposites was evaluated as a function of the different calcination temperature and compared with that recorded for the bare TiO2 prepared under the same experimental conditions, calcinated at the corresponding temperature values in order to investigate the possible effect of the plasmonic domain in the nanocomposites and of the calcination temperature. A commercially available photocatalyst, TiO2 P25 Evonik, was also used as a reference material.

The prepared visible light active plasmonic nanocomposite photocatalyst, obtained by calcination at 450 °C demonstrated the highest photocatalytic performance under UV and visible light irradiation. A degradation rate 2.5 faster than that found for TiO2 P25 Evonik and 6.4 times higher than that of the bare TiO2 calcined at the same temperature of the nanocomposite was obtained for the UV-assisted decolouration of MB. In addition, under visible light irradiation the NA degradation rate assisted by the plasmonic nanocomposite was found 3.2 and 13 times faster than that catalysed by using TiO2 P25 Evonik and the bare TiO2 calcinated at 650 °C, respectively.

Moreover, the possibility of achieving gram scale amounts of photocatalytic nanocomposite further support the great potential and the accessibility of the synthetic procedure towards their application on a real scale.

Section snippets

Materials

All chemicals were used as received without further purification. Titanylsulfate (TiOSO4, 29% TiO2, 17% H2SO4), ammonium bicarbonate (NH4HCO3 99%), sodium borohydride (NaBH4, 99%), l-ascorbic acid (99%), hydrogen tetrachloroaurate(III) trihydrate (HAuCl4·3H2O,99.9%), silver nitrate (AgNO3, 99.9999%), Methylene Blue (3,7-bis(Dimethylamino)-phenazathionium chloride, MB) and all solvents were purchased from Aldrich Chemical reagent. All solvents used were of analytical grade. TiO2P25 Evonik (TiO2

Results and discussion

In recent years, the possibility to embed AuNRs in a TiO2 matrix opened up the way to a new class of functional materials. Several methods have been proposed to integrate metal NPs in semiconductor-based nanostructured materials, including chemical and thermal methods, photodeposition, sputtering, core–shell nanostructure fabrication [3]. Herein, we propose a co-precipitation procedure followed by a calcination step, in order to exploit pre-synthesized AuNRs as nucleation substrate for the

Conclusions

The preparation, characterization and investigation of the photocatalytic properties of a novel nanocomposite photocatalyst based on TiO2 and Au NRs is described and its impact on photocatalytic degradation under UV and visible light of two organic compounds was demonstrated.

AuNRs modified TiO2 nanocomposites were prepared on the scale up to tens of grams by a general co-precipitation method and the calcination temperature was investigated in order to elucidate the role of such a parameter on

Acknowledgements

“This Special Issue is dedicated to honor the retirement of Dr. John Kiwi at the Swiss Federal Institute of Technology (Lausanne), a key figure in the topic of photocatalytic materials for the degradation of contaminants of environmental concern.” This work was partially supported by the EC-funded project Innovaconcrete (H2020; Grant No. 760858), by the Italian Regional Network of Laboratories “Sens&Micro” and “VALBIOR” projects (POFESR 2007-2013), by Apulia Region funded FontanApulia (WOBV6K5)

References (35)

  • Y. Ben-Shahar et al.

    Hybrid semiconductor–metal nanorods as photocatalysts

    Top. Curr. Chem.

    (2016)
  • S.T. Kochuveedu et al.

    A study on the mechanism for the interaction of light with noble metal-metal oxide semiconductor nanostructures for various photophysical applications

    Chem. Soc. Rev.

    (2013)
  • A. Truppi et al.

    Visible-light-active TiO2-based hybrid nanocatalysts for environmental applications

    Catalysts

    (2017)
  • Z. Xuming et al.

    Plasmonic photocatalysis

    Rep. Prog. Phys.

    (2013)
  • V. Subramanian et al.

    Catalysis with TiO2/gold nanocomposites. Effect of metal particle size on the fermi level equilibration

    J. Am. Chem. Soc.

    (2004)
  • A. Bumajdad et al.

    Understanding the superior photocatalytic activity of noble metals modified titania under UV and visible light irradiation

    Phys. Chem. Chem. Phys.

    (2014)
  • U. Banin et al.

    Hybrid semiconductor–metal nanoparticles: from architecture to function

    Chem. Mater.

    (2014)
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