Facile synthesis of microporous SiO2/triangular Ag composite nanostructures for photocatalysis
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In this article, we present a novel fabrication of microporous SiO2/triangular Ag nanoparticles for dye (methylene blue) adsorption and plasmon-mediated degradation. Microporous SiO2 nanoparticles with pore size <2 nm were synthesized using cetyltrimethylammonium bromide as a structure-directing agent and functionalized with APTMS ((3-aminopropyl) trimethoxysilane) to introduce amine groups. Amine-functionalized microporous silica was used for adsorption of triangular silver (Ag) nanoparticles. The synthesized microporous SiO2 nanostructures were investigated for adsorption of different dyes including methylene blue, congo red, direct green 26 and curcumin crystalline. Amine-functionalized microporous SiO2/triangular Ag nanostructures were used for plasmon-mediated photocatalysis of methylene blue. The experimental results revealed that the large surface area of microporous silica facilitated adsorption of dye. Triangular Ag nanoparticles, due to their better charge carrier generation and enhanced surface plasmon resonance, further enhanced the photocatalysis performance.
KeywordsPhotocatalysis Microporous SiO2 Surface plasmon resonance Composite nanostructures
Dyes are vital chemicals used in several applications such as textiles, food, furniture and paint. However, dumping or accidental discharge of dye-contaminated water into the environment creates considerable environmental and health hazards. Dye, once dissolved in water, becomes stable and hence non-biodegradable. The chemical treatment of dyes is not economically viable. Biological treatment is a better solution but it is quite tedious due to requirement of a large area of land and cumbersome operation (Huang et al. 2011). Use of adsorbents, is a simple and effective way for decontaminating dye pollutants (Ghorai et al. 2014). Adsorption process is influenced by numerous factors including surface area of adsorbent, and extent and type of interaction of dye with the absorbent solids (Wu et al. 1997).
Mesoporous silica due to its large surface area, high hydrothermal stability, and diverse surface functionality, has been explored extensively in biomedical applications, including cell imaging, diagnosis, biosensing, intracellular drug delivery (Argyo et al. 2013; Bharti et al. 2015; Chen 2016; Huang et al. 2014; Sharma et al. 2014; Sharmiladevi et al. 2016; Tang and Cheng 2013; Zhang et al. 2015) and controlled pesticide release (Cao et al. 2016). The morphology and the particle size of mesoporous silica strongly influence the absorption and release of drug/pesticide (Lu et al. 2009). Usually mesoporous silica is synthesized by polycondensation of a silica precursor such as tetraethylorthosilicate (TEOS) or tetramethylorthosilicate (TMOS) in the presence of surfactants which act as structure-directing agents (Giraldo et al. 2007; Wei et al. 2010). Various ionic (cetyltrimethylammonium bromide, CTAB) and non-ionic surfactants (amphiphilic triblock copolymers) can be used for obtaining mesoporous silica with distinctive pore structure and morphology (Wei et al. 2010). The particle size, shape and porosity of particles can be tuned by controlling synthesis parameters, such as pH, reaction time, and temperature. Microporous silica owing to large number of pores with pore size <2 nm, provides ample surface area for materials to get adsorbed on it. The presence of silanol group further ensures better adsorption of dye molecules on its surface (Krysztafkiewicz et al. 2002). Researchers have investigated photocatalytic activity of various nanoparticles (NPs) including TiO2 (Schneider et al. 2014), ZnO (Bandekar et al. 2013; Elmolla and Chaudhuri 2010), MnO2 (Li et al. 2008) and silver (Badr and Mahmoud 2007), etc. The Ag/SiO2 nanostructures with Ag as core material have also been used for sensing (Aslan et al. 2007; Nimrodh Ananth et al. 2011), antibacterial (Alimunnisa et al. 2017) and surface-enhanced Raman scattering (SERS) (Wang et al. 2009) applications. Ag NPs have recently been proposed as a novel photocatalyst for degradation of organic pollutants due to their surface plasmon resonance (SPR) absorption in the visible range. Ag NPs, when excited with photon, energize conduction electrons of 5sp bands and excite them to higher energy states. These excited electrons participate in chemical reactions (Chen et al. 2010). Additionally, holes left in the 5sp bands possess strong oxidizing power and hence act as driving force for photocatalysis. SPR frequency of Ag is a function of its particle size and shape, and triangular silver nanoparticles are reported to have better photocatalytic activity. Non-porous SiO2/Ag core shell nanostructures consisting of yellow Ag NPs prepared using Stober method have been investigated for plasmon-mediated photocatalysis (Chen et al. 2012). However, microporous SiO2/blue Ag composite nanostructures have not been reported to the best of our knowledge. On account of better charge carrier generation, blue Ag NPs are likely to improve photocatalytic performance of SiO2/blue Ag nanostructures in a significant manner.
In the present work, we have made an attempt to synthesis microporous silica/blue Ag composite nanostructures. The adsorption behavior of different dyes using microporous silica (MS) has been investigated. Amine-functionalized MS (MS-NH2) was used for adsorption of blue silver nanoparticles on their surface. The synthesized MS-NH2/Ag composite nanostructures were compared with MS-NH2 nanoparticles for their photocatalytic behavior against methylene blue dye. MS-NH2/Ag composite nanostructures are expected to have better adsorption of dye due to large surface area and porosity of microporous silica along with improved dye degradation facilitated by plasmon-assisted photocatalysis of Ag nanoparticles.
Silver nanoparticles were synthesized using AgNO3 as precursor (Dong et al. 2010; Kelly et al. 2012) as mentioned in the supporting information. The obtained silver nanoparticles were characterized using a UV spectrophotometer (Lambda 35, PerkinElmer, USA), a particle size analyzer (Malvern Zetasizer Nano ZS, USA) and a high resolution transmission electron microscope (HRTEM). The blue Ag NPs were adsorbed on the surface of amine-functionalized microporous silica nanoparticles. Microporous SiO2/blue Ag composite nanostructures were coded as MS-60-NH2/Ag. Further, MS-60-NH2 and MS-60-NH2/Ag were compared for their photocatalysis behavior using 0.01 wt% methylene blue. Further experimental details of synthesis and characterization are provided in supporting information.
Results and discussion
Morphology of microporous SiO2 nanoparticles
The porous nature of the silica was confirmed by HRTEM micrographs. The estimated pore size of the particles was found to be in the microporous range, i.e., 1.2 ± 0.5 nm.
Characterization of silver nanoparticles
Adsorption of dyes with various functionalities on the surface of microporous silica
Morphology of SiO2/blue Ag composite nanostructures and their photocatalytic activity
SiO2 offers large surface area due to its microporous nature and therefore acts as a good photocatalytic agent.
Thus, 0.2% (w/v) of MS-60-NH2 microporous silica and MS-60-NH2/Ag composite nanostructures were compared for their photocatalytic activity. The nanoparticles were dispersed in dye solution (0.01% w/v) followed by exposure to sunlight. The samples were placed in sunlight and visually observed for decoloration of dye and analyzed using UV–Vis spectrophotometer for photocatalytic behavior.
Microporous silica nanoparticles were successfully synthesized using CTAB as a structure-directing agent and varied reaction time. SEM and HRTEM analyses revealed that as the reaction time was increased from 60 to 75 and further to 90 min, the diameter was decreased from 277 ± 74 to 86 ± 12 nm and then to 72 ± 6 nm, respectively. HRTEM confirmed the microporous morphology of silica with pore size of 1.02 ± 0.5 nm. Adsorption of different dyes including curcumin crystalline, methylene blue, Congo red and direct green 26 was analyzed for 0.02, 0.2 and 1 wt% of MS-60 nanoparticles. Dyes exhibited different extent of adsorption according to their functional groups in the order of direct green > curcumin crystalline > methylene blue > Congo red. Adsorption methylene blue dye, owing to its interaction with microporous silica nanoparticles, was confirmed using FTIR analysis. Synthesis of SiO2/triangular Ag composite nanostructure was carried out by adsorbing synthesized blue Ag NPs on amine-functionalized microporous silica nanoparticles. The presence of Ag NPs on the surface of SiO2 was evidenced by HRTEM and EDX analysis. Photon-mediated photocatalysis of SiO2/Ag composite nanostructures was investigated and compared with pristine SiO2 microporous nanoparticles. The UV–Vis spectra revealed a substantial improvement in degradation of methylene blue from 61% in SiO2 microporous nanoparticles to 93% in SiO2/Ag composite nanostructures, resulting in a colorless solution within 1 min of exposure. Large surface area and high porosity of microporous silica nanoparticles in combination with surface plasmon resonance of triangular Ag contributed to significant improvement in photocatalysis of SiO2/Ag composite nanostructures.
The authors would like to thank Bhaskaracharya College of Applied Sciences for providing the lab facilities and University of Delhi for financial support.
Compliance with ethical standards
Conflict of interest
The authors declare no competing financial interest.
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