Introduction

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.

Experimental

Microporous silica nanoparticles were synthesized using CTAB as a structure-directing agent and TEOS as precursor as reported by (Cao et al. 2016) with a slight modification. The obtained particles were coded as MS-60, MS-75 and MS-90 according to reaction time, i.e., 60, 75 and 90 min, respectively. The hydrolysis and condensation of the precursor, i.e., TEOS on the edges of the template followed by calcination, yielded porous morphology. The proposed mechanism of the synthesis is shown in Scheme 1. The obtained microporous silica nanoparticles were functionalized using APTMS ((3-aminopropyl) trimethoxysilane) and were coded as MS-60-NH2. Curcumin crystalline, methylene blue, Congo red and direct green 26 dyes were used for adsorption analysis.

Scheme 1
scheme 1

Synthesis of microporous silica nanoparticles

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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 morphology of the obtained SiO2 nanoparticles was analyzed using a scanning electron microscope (SEM) and a high resolution transmission electron microscope (HRTEM) as shown in Figs. 1 and 2, respectively. The histograms of particle diameter analyzed using SEM is shown in Figure S1. SEM analysis revealed that MS-60 nanoparticles were of spherical shape with an average diameter of 277 ± 74 nm. Further increase in reaction time to 75 and then to 90 min led to formation of more uniform and spherical particles with decreased diameter of 86 ± 12 and 72 ± 6 nm, respectively. It has been observed that the longer reaction time causes the reduction in diameter because of highly basic condition which results in etching of the particles (Chiang et al. 2011). The etching of particles makes them more uniform in diameter.

Fig. 1
figure 1

Scanning electron micrographs of a MS-60, b MS-75, and c MS-90

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Fig. 2
figure 2

HRTEM micrographs of a MS-60, b MS-75 and c MS-90

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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

Particle size and UV–Vis spectroscopy analysis of triangular Ag NPs are shown in Fig. 3. UV–Vis spectra of the particles match with the literature, showing absorption maxima at 700 nm and peak corresponding to SPR at 329 nm. The triangular Ag NPs were found to have two peaks in particle size analysis, one at 3 nm and other at 50 nm (broad), respectively, indicating variation in particle diameter.

Fig. 3
figure 3

a UV–Vis spectra and b Particle size analysis of as synthesized Ag nanoparticles

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Adsorption of dyes with various functionalities on the surface of microporous silica

The dyes with different functionalities including curcumin crystalline, methylene blue, Congo red and direct green 26 were used for adsorption studies. The 0.02, 0.2 and 1 wt% of MS-60 nanoparticles were analyzed for adsorption, and the decrease in dye concentration was estimated using a UV–Vis spectrophotometer as shown in Fig. 4. It has been observed that the concentration of dye was decreased on addition of MS-60 nanoparticles. The decrease in UV–Vis spectra corresponds to adsorption (not degradation) of dye molecules by MS-60. The decrease (%) was calculated using UV–Vis absorption spectra and was found to vary linearly with concentration of MS-60. Subsequently, the extent of decrease (%) of adsorption was measured and observed to be in the following order: direct green > curcumin crystalline > methylene blue > Congo red as shown in Fig. 5. The adsorption behavior of dyes was observed to vary according to their functional groups. It may be inferred from the results that high porosity and large surface area of microporous silica facilitate the adsorption of all studied dyes.

Fig. 4
figure 4figure 4

Adsorption of dyes with various functionalities a direct green 26, b curcumin crystalline, c methylene blue and d Congo red

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Fig. 5
figure 5

Adsorption of dyes with various functionalities a direct green 26, b curcumin crystalline, c methylene blue, d Congo red

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Methylene blue is one of the most investigated dyes and therefore selected for photocatalysis. Fourier transform infrared spectroscopy (FTIR) analysis was carried out to investigate the interaction of methylene blue with microporous silica nanoparticles leading to its adsorption as shown in Fig. 6. The peaks at 800 and 3000–3700 cm−1 corresponding to symmetric stretching of Si–O–Si and –OH, respectively, were found to be broadened. Further, the peak corresponding to –C=C stretching was found to be shifted from 1600 to 1635 cm−1. The peak corresponding to asymmetric stretching of Si–O–Si at 1050–1150 cm−1 disappeared indicating the interaction of dye molecules with the MS-60 nanoparticles (Lu 2013). This interaction resulted in adsorption of dye by microporous silica nanoparticles.

Fig. 6
figure 6

FTIR spectra of a MS-60 and b methylene blue dye and c dye adsorbed MS-60 nanoparticles

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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.

Adsorption of triangular Ag nanoparticles because of their enhanced SPR is likely to further improve photocatalytic behavior. To achieve better photocatalytic activity, SiO2/Ag nanoparticles were synthesized by treatment of SiO2 with APTMS followed by adsorption of blue silver nanoparticles on their surface as shown in Scheme 2.

Scheme 2
scheme 2

Functionalization of microporous silica nanoparticles and synthesis of SiO2/Ag

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The FTIR-analyzed spectra of amine-functionalized microporous silica nanoparticles are shown in Fig. 7. The stretching vibrations of hydrogen-bonded Si–OH and –OH of physically adsorbed water molecules and –NH2 groups were integrated and as broad peak at 3425 cm−1. The new peaks at 2959, 2852 and 1633 cm−1 were attributed to asymmetric and symmetric C–H stretching of aminopropyl and –NH2 bending vibrations, respectively (Castruita-de León et al. 2015). Splitting of Si–O–Si symmetric stretching peak at 772 cm−1 further confirmed interaction of Si–O–Si with –NH2 groups. These MS-60-NH2 silica nanoparticles were used for adsorption of triangular silver nanoparticles.

Fig. 7
figure 7

FTIR spectra of a MS-60 and b MS-60-NH2 nanoparticles

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The HRTEM analysis of triangular Ag NPs and MS-60-NH2/Ag composite nanostructures is shown in Fig. 8. The micrographs revealed triangular shape of blue silver nanoparticles with an average size of 20 ± 10 nm. The adsorbed nanoparticles on the surface of mesoporous SiO2 are clearly visible in the HRTEM micrograph and the presence of Ag was further confirmed by energy-dispersive X-ray (EDX) analysis (Fig. 9).

Fig. 8
figure 8

HRTEM micrographs of a blue Ag nanoparticles, b MS-60-NH2/Ag composite nanostructures

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Fig. 9
figure 9

a HRTEM micrographs and b EDX spectrum of SiO2/Ag composite nanostructures

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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.

As shown in Fig. 10, the dye degradation was observed in both MS-60-NH2 microporous silica and MS-60-NH2/Ag nanostructures. However, the extent of decrease of the dye absorption was higher with MS-60-NH2/Ag nanostructures (i.e., 97.3%) as compared to MS-60-NH2 (i.e., 61.0%). The reduction in MS-60-NH2 is due to adsorption of dye by microporous silica. Interestingly, the dye was found to decolorize within 1 min of exposure using MS-60-NH2/Ag nanoparticles. Large surface area and porosity of microporous silica nanoparticles will make them good adsorbing material. The addition of Ag nanoparticles significantly improved the photocatalytic efficiency of MS-60-NH2/Ag because of their plasmon-mediated photocatalysis. The expected mechanism of photodegradation is shown in Scheme 3. To quantitatively compare the photocatalytic performance of MS-60-NH2 microporous silica and MS-60-NH2/Ag nanostructures, the apparent rate constant for MB photodegradation (kMB) was computed using the pseudo-first-order approximation. The kMB values of 0.94 and 4.2 min−1 were obtained for MS-60-NH2 and MS-60-NH2/Ag nanostructures, respectively. The proposed MS-60-NH2/Ag nanostructures offer applications in the area of waste water from textile industries.

Fig. 10
figure 10

UV–Vis spectra showing degradation of methylene blue using MS-60-NH2 and MS-60-NH2/Ag

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Scheme 3
scheme 3

Photodegradation mechanism of methylene blue using MS-60-NH2/Ag composite nanostructures

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Conclusions

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.