Synthesis of New Hollow Nanocomposite Photocatalysts: Sunlight Applications for Removal of Gaseous Organic Pollutants
Introduction
Industrial activities and continuous gas emissions from automobile cause numerous environmental pollution problems and destruction of several ecosystems due to accumulation of serious pollutants such as volatile organic compounds (VOCs) [1,2]. Consequently, there is a need to develop new materials for VOCs detection. Semiconductor photocatalysts have widely used in the air pollution treatment and remediation. These are due to the generation of electron-hole pairs in the excited state after the photon absorption with energy larger than the band gap energy [3]. Photocatalysts TiO2, a well-known photocatalytic material has been widely studied for the organic pollutant decomposition from air and water under only ultraviolet irradiation [4]. One of the biggest limitations of TiO2 is its wide band gap energy (>3 eV) and a very low catalytic efficiency in environmental control under solar radiation which absorbs only 5% or less of the solar light [5]. Another basic disadvantage of TiO2 is the charge carrier recombination [6]. Therefore, the development of efficient photocatalysts having strong sunlight absorption and effectively overcoming the drawbacks that restrict the photocatalytic activity has gradually become an important topic in the research fields. Distinctive promising systems had been developed for improving both light absorption and utilization of electron-hole pairs. Among of them are surface modifications, and doping with different elements, as reported in the several studies [4,[6], [7], [8]].
Modification of photocatalysts using diverse methods involving coupling with narrow band gap semiconductors, sensitized surface by organic dyes, deposition of noble metals, and the way of elemental doping can easily address these limitations. Moreover, the photocatalyst separation from the treated systems can limit their applications. Coupling of photocatalysts with magnetic materials could help their recovering by means of external magnetic field [9]. Loading of co-catalysts and lastly introduces of hetero- or homo-junctions having multi-components such as BiVO4/WO3 [10] and ZnS/CuS/CdS [11] are also promising systems. Hetero structured photocatalysts have many advantages in visible light absorption, like quick charge separation, effective co-catalyst and stability [12]. Recently, all applications of photocatalysts require controlled size and shape of nanocomposites to improve their performance due to high light-absorption efficiency and fast mobility of charge carriers [13,14]. Furthermore, the photocatalytic nanocomposites have been largely investigated to overcome the poor sunlight absorbance of TiO2 by determining the properties of each component. The action of a composite photocatalyst depends upon various parameters; the band gaps of semiconductors, charge separation efficiency, the capacity of charge carrier transfer between the composite components, and the presence of co-catalysts [15]. Hollow spheres structured composites is another promising material for the applications in the field of photocatalytic processes [16]. The most obvious feature of hollow nanospheres is that their internal spaces are large thus, empower both the external and inner surface areas towards the reactants and then augment its performance, due to various scattering of the exciting visible light into the interior cavities and can then solve the problem of electron−hole pair and the retention time that make them more efficient in many applications [17], [18], [19].
Recently, carbon spheres as templates have been broadly investigated. Their surface possesses a high density of hydrophilic groups (–OH and C=O groups) so that they can absorb easily and uniformly different metal ions without any further modification. This is the critical superiority when carbon spheres are used as templates [20].
Another tactic to enhance the photocatalysts is through doping TiO2 with different noble metals such as Pt, Au, and Ag, since these metals could act as electron carriers to effectively transfer the photo-generated currents, thus augmenting the photocatalytic performance [21], [22], [23], [24]. Furthermore, the presence of Pt/TiO2 doped structure effectively augments the absorption of visible light, surface area, and charge separation [25]. Noble metal deposition on TiO2 can introduce donor and/ or acceptor energy levels in the wide band gap of TiO2, shifting the light absorption in the visible region and then lead to a reduction in the electron-hole recombination [26]. Introducing of Pt into TiO2 requires temperatures, thus, the homogeneous dispersion of dopants brings down the temperature [27]. Besides, the presence of metal cations (Mn, Cd, Fe, Cu, etc.) as stabilizer maintains the surface area and offers a favorable preference to the Pt-doped TiO2 photocatalysts. In addition, the doped cations as co-catalysts fundamentally generate the electron - hole pair and inhibit its recombination rate by serving as a temporal trapping sites of photo-induced currents [28]. Several semiconductors, including Cu2O, SiC, CdS, PbS, Bi3O4Cl, BiOI/AgBr, CoWO4 and CdSe have been combined with TiO2 to form hetero nanocomposite structures [29,30].
The harvested visible-light due to the presence of various semiconductors and efficient charge carrier suppression of recombination due to the formation of p-n- heterojunction are the predominant parameters affecting the photocatalytic activity [31].
Metal sulfides were considered as new photocatalytic materials in recent years owing to their narrow band gap, potent light absorption characteristics, and high photocatalytic performance [32]. Particularly, copper sulfide and cadmium sulfide are transition metals that had been observed to be very efficient for photocatalytic application because of their high stability, little cost, and higher catalytic activity [33,34]. The Pt sulfides exhibit also much higher activity than that of Pt-TiO2 composites [35], [36], [37]. The presence of Pt as a co-catalyst in CdS-TiO2 composites substantially enhanced their photocatalytic efficiency [36].
Our strategy was for coupling the small band gap of metal sulfides with Pt- TiO2, which permit to use the visible light and even close IR parts of the sunlight to obtain excellent photocatalytic activity. Despite this, just a little of data about such study of metal sulfide hollow nanospheres photocatalysts was investigated. Along these lines, the improvement of novel and proficient nanocomposites photocatalysts demonstrates a predominant system under solar irradiation that is a considerable challenge.
This work presents novel research in the improvement of sunlight-driven sulfide hollow nanosphere composite as photocatalysts with excellent sunlight response and charge separation effectiveness. It can perform at normal temperature and pressure, safe to the environment and human body, economic, completely effective to organic decomposition.
This study aims to synthesize a new type of hollow CuS/Pt–TiO2 and CuS-CdS/Pt–TiO2 nanocomposites in which Pt and CuS-CdS act as redox co-catalysts instead of the cupper and cadmium oxide used by the conventional methods. The contribution of Cu and Cd species is serving as stabilizers and co-catalysts. The new photonanocatalyst was further examined and applied for isopropanol gas decomposition under solar irradiation as a model organic pollutant to test its performance activity. The strategy to synthesize these multi-heterojunction photocatalysts can give an innovative perspective to design highly efficient systems for solving environmental pollution problems using TiO2 under visible light.
Section snippets
Chemicals
Sucrose (C12H22O11, ACS grade, ≥ 99.0 %), hexachloroplatinic acid hexahydrate (H2PtCl6.6H2O, ACS grade, ≥37.50% Pt basis), cupper nitrate (Cu (NO3)2•xH2O, ACS grade, ≥99.99%), cadmium nitrate (Cd(NO3)2 •4H2O, ACS grade, 99.997%), tetraethyl titanium butoxide (Ti(OCH2CH2CH2CH3)4, ACS grade, ≥98.0%) and isopropyl alcohol (CH3CHOHCH3, ACS Grade, ≥ 99.5%) were used. All reagents were purchased from Sigma-Aldrich and used without further purification.
Hollow material synthesis
The novel hollow structure photocatalysts were
Scanning electron microscopy
Fig. 2 displays typical scanning electron microscopy (SEM) images of the CuS /Pt– TiO2 and CuS-CdS /Pt– TiO2 samples before calcinations (a and b, respectively) and CuS-CdS /Pt– TiO2 after calcination (c). All of the synthesized nanospheres samples have quite uniform nanospheric structure with an average diameter size of 500 nm to 2 mm. The hollow structured sphere was obtained as a result of combusting the colloidal spheres of carbon at 680°C of calcination. Some broken particles were noted
Conclusion
In this study, a new hetero-photonanocatalyst hollow structure was developed. The hollow nano-photocatalysts indicate incredible guarantee as competitors for use in proficient photocatalytic systems. This new photocatalyst is a promising system for the development of future photocatalysts due to diverse reasons; its higher surface area, which is restricted by conventional one; the high surface area between the individual parts, which is an essential factor for reducing charge recombination;
Declaration of Competing Interest
The authors declare that they have no conflict.
Acknowledgments
The authors wish to express their sincere gratitude to Air Pollution Department, National Research Centre, Egypt and Department of Chemical Engineering, Laval University, Quebec, G1V 0A8, Canada for Funding and laboratory facilities.
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