Biosilica from diatomaceous earth as support to CdS-mediated photocatalysis in dry and aqueous phase

Highlights

• Thermolysis of Cd-thiourea complex formed CdS nanoparticles.

• CdS nanocrystals were deposited onto diatomite biosilica.

• CdS diameters were 14.2, 44.8 and 71.8 nm respectively after 3.3, 11 and 30 min.

• Photocatalyst activities were 96.5% and 95% in aqueous and dry phase respectively.

Abstract

This work reports the use of diatomite biosilica as support to deposition of homogeneously dispersed CdS nanoparticles by aqueous reaction. Time evolution of the process revealed that 30 min are sufficient to obtain well dispersed hexagonal CdS nanoparticles, without formation of external particles. Characterization by UV/visible reflectance spectroscopy allowed estimating nanocrystal size as ranging from 4 to 6 nm, while TEM and STEM images showed that those primary CdS nanocrystals formed aggregates with 14.2–71.8 nm depending on the reaction time. Photocatalysis in flow setup through glass column under solar light, under UVA irradiation and adsorption in dark conditions showed that the process under solar light was the most efficient. Photocatalytic tests in dry phase under solar light revealed that the presence of CdS favors dye degradation when compared to diatomite containing the adsorbed dye. Finally, photocatalysis of aqueous suspensions under solar light was also more efficient than adsorption or photolysis, following pseudo-first order kinetics with a half time of 46.2 min.

Introduction

Diatomite is a siliceous material formed by the fossil residues of diatom shells and has unique porous architectures with good mechanical strength [1]. The silica shells not only provide mechanical strength to the diatoms but also their porous architectures act as mini-photonic devices [2]. As a result, diatoms are among the most efficient known photosynthetic systems owing to their intricate geometric patterns. Previous works have evaluated in detail the optical properties of diatomite, whose silica cell wall can be regarded as a photonic crystal slab [3]. Authors demonstrated that light can be coupled into the waveguide and that there are some photonic resonances in the visible spectral range.

Owing to the combined mechanical and optical properties, several works report the use of diatomite biosilica as support for photocatalysts, in an approach involving modification of diatomite into various materials while maintaining its hierarchical structures [4], [5], [6]. The strategy can be particularly useful for photocatalysts as shown by Frost and co-workers in a comparative study of TiO₂ catalysts supported onto different porous amorphous silica minerals [7]. The special pore structure and good adsorption capacity of diatomite make significant contribution to the excellent activity of TiO₂/diatomaceous earth (DE) photocatalyst.

Among a wide variety of photocatalyst materials, TiO₂ has become the most important due to its relatively large quantum yield and high corrosion and photo-corrosion resistance in aqueous media [8], [9], [10]. Efforts to prepare nanoparticulate photocatalysts [11] – including TiO₂ – were motivated by the increase in surface area and consequently a potential increase in the photocatalytic activity. However, suspensions of nanoparticles may exhibit disadvantages such as agglomeration, low quantum efficiency, low specific surface area, costly and difficult separation at the end of the process, motivating studies of supported photocatalysts. Photocatalysts supported onto different substrates such as carbon nanotubes [12], graphene [13] silica-gel [14], montmorillonite [15] among others were reported.

Recently a series of works describe different strategies to prepare diatomite-supported TiO₂ and evaluation of photocatalytic activity toward dye degradation. Methods used include sol-gel from titanium isopropoxide [16], hydrolysis precipitation from TiCl₄ [17], [18] and sol-gel from tetrabutyltitanate to obtain undoped TiO₂ [19] and vanadium-doped TiO₂ [9].

It is clear that most of the studies on semiconductor photocatalysts are concentrated on TiO₂. However, TiO₂ presents a serious drawback related to the mismatch between its band gap energy and the sunlight spectra, which limits its application under solar irradiation. This has motivated studies of alternative photocatalysts with absorption in the visible range in order to be used in sustainable technologies based on solar energy [20]. In this context, the CdS band gap (2.42 eV) is narrower than that of TiO₂ (3.2 eV), favoring its application under visible light, and making CdS a competitive candidate as photocatalyst [21]. Here we report the use of diatomite biosilica as support for the preparation of CdS nanocrystals, an approach that is still incipient for this semiconductor [22], to application in the photocatalytic degradation of methylene blue (MB) under sunlight. This material can be potentially advantageous considering the response of CdS to visible light, the possibility of improving the surface area of the semiconductor for pollutant adsorption. Moreover, as the electron-hole recombination decreases the photocatalytic activity, the presence of a high surface area support may contribute to keep charge separation favoring the photocatalysis process [23].