1 Introduction

Persistent organic pollutants are an increasing urgency that alter the composition of aquatic environments [1,2,3]. In addition to the well-known polycyclic aromatic hydrocarbons [4, 5] other contaminants, derived from pharmaceuticals, health-care products, drugs and pesticides [6,7,8], are increasingly being detected in watercourses and their presence can severely impact on the ecosystem.

This necessity fosters the development of sustainable methodologies to remove these pollutants.

One promising strategy, that has attracted considerable interest, is based on photocatalysts [9,10,11], i.e. materials able to induce the degradation of a wide range of organic pollutants once triggered by radiation [12,13,14].

Several semiconductors (TiO2, SnO2, ZrO2, Fe2O3, ZnO, ZnS, CdS) nanoparticles have been studied as photocatalysts [15, 16] and, among them, TiO2 and ZnO nanomaterials have been the most investigated due to their good photocatalytic performances [17,18,19].

Silica nanomaterials (SiO2 NPs) represent an appealing alternative because they guarantee lower environmental impact and an easily processable surface for post-synthesis functionalization [20,21,22]. However, the use of silica-based materials as photocatalysts has been strongly limited by their electronic properties. Indeed, crystalline silica has a very wide bandgap (approximately 9 eV [23]) which makes it transparent to UVB, UVA and visible radiations, making difficult their photoexcitation in natural irradiation conditions.

However, the presence of structural defects turns silica photoactivable with UV radiation; several defects have been studied and characterized for amorphous silica nanoparticles [24, 25], such as non-bridging oxygen hole centers and neutral deficient oxygen centers. These defects possess optical absorption covering the UVB and UVA regions, which are able to enhance catalytic properties of SiO2 NPs under UV irradiation [26, 27].

However, to make the photocatalytic systems even more appealing and efficient, there is the need to extend the catalyst activation in the visible range, to take advantage of the sunlight radiation which is mainly composed of visible photons (40–50%) [28].

One feasible strategy is given by the combination of semiconductor nanomaterials with metal nanoparticles, whose surface plasmon resonance (SPR) absorption can be shifted from UV to IR range [29]. The plasmon excitation leads to an enhanced photocatalytic ability of the nanocatalyst, mainly for two reasons: firstly, the hot-carrier injection from the metal to the semiconductor can enhance the abundance of charge carriers within the nanomaterial, thus increasing the number of catalytic active sites [30, 31]. Secondly, the photogenerated hot electrons can be collected by the species adsorbed on the semiconductor surface, such as dyes, water or O2, generating highly reactive radicals [32].

For instance, He and coworkers [33] demonstrated that the hot-carrier injection was possible with nickel–TiO2 heterostructure. They found that both the hot electrons and hot holes, generated by the plasmon excitation of nickel, migrated from Ni to TiO2. The transferred hot electrons tend to occupy the oxygen vacancies on TiO2, while the hot holes are located on the surface oxygens of the semiconductor. Moreover, they pointed out that hot electrons dominantly contributed to the degradation of methylene blue. Saravanan and coworkers [34] revealed that the conjugation of silver to TiO2 induce structural/electronic perturbation that alters the bandgap of the semiconductor and prevents the charge-carrier recombination. These findings result in good photocatalytic efficiency under visible light for the degradation of methyl orange and for the photoinduced hydrogen production through water splitting. Raji et al. [35] reported the successful photodegradation of sulforhodamine B under sunlight with ZnO/Ag nanorods as catalyst; in particular, the increase of adsorbed Ag led to an enhanced photocatalytic activity, which was assigned to the improved production of reactive oxygen species. Indeed, the formation of reactive oxygen species (ROS) has been demonstrated to be relevant in photocatalytic degradation of organic pollutants in aqueous media [36]. Therefore, the development of efficient and sustainable photocatalysts can be based in growth of efficient ROS photosensitizers [36]; their photosensitization efficiency is an important parameter to be quantitatively determined.

Mesoporous silica has been widely used as a support or templating material [37,38,39], but its photocatalytic performances have been barely investigated [40, 41].

Here, we explore the photocatalytic properties of silica colloids and the possibility to exploit the system under visible irradiation upon conjugation of silver nanoparticles; we determined the efficiency of various silica-based nanomaterials in the degradation of 9-anthracenecarboxylic acid (9ACA), which is used as model aromatic compound for persistent organic pollutants. Bare silica nanoparticles have been prepared by a sol–gel method and then functionalized with a post-synthesis process with amino groups to investigate the role of surface chemistry on the aromatic compound photodegradation. Finally, after the conjugation of silver nanoparticles to silica colloids, the photocatalytic performance under visible light has been measured. Moreover, to achieve insight on the potential use of the proposed systems, the irradiation with a common white light source (WL) has been performed as well. To disclose the photocatalytic mechanism, the analysis and quantification of ROS generation have been carried out.

2 Experimental section

2.1 Materials

9-Anthracenecarboxylic acid (9ACA, 99%), tetraethylorthosilicate (TEOS, 98%), (3-aminopropyl) triethoxysilane (APTES, ≥ 98%), cetyltrimethylammonium tosilate (CTATos, 95%), triethanolamine (TEAH3, > 98%), 1,3-diphenylisobenzofuran (DPBF, 97%), rose bengal (RB, ≥ 95%), silver nitrate (AgNO3, ≥ 99.9%), sodium borohydride (NaBH4, ≥ 99%) and 3-mercaptopropionic acid (MPA, ≥ 99%) were all purchased from Sigma-Aldrich and used as received. Nanopure water (≤ 15.0 MΩ) from a Millipore Milli-Q gradient system and ethanol (96%) from Sigma-Aldrich were used as solvents.

2.2 Silica nanoparticles synthesis

Silica nanoparticles (SiO2 NPs) are synthesized with a procedure described elsewhere, using the sol–gel method in the presence of a surfactant as templating agent [42].

Briefly, in a 100 mL flask equipped with a magnetic bar, 0.95 g of cetyltrimethylammonium tosilate (CTATos), 120 µL of triethanolamine (TEAH3) and 50 mL of water are added and stirred at 80 °C. After 1 h, 7.69 mL of tetraethylorthosilicate (TEOS) are quickly added, and the mixture is left under stirring for 2 additional hours at 80 °C. At the end of the reaction, the mixture is cooled in a cold water bath for about 10 min. The nanoparticles are recovered by centrifugation at 3000g for 30 min and then washed several times with ethanol. Finally, the incorporated (or pore occluding) organic surfactants are removed through calcination at 550 °C for 6 h.

Post-synthesis treatment to graft amino groups on the surface of SiO2 NPs is performed in ethanol with a procedure described elsewhere [43], using SiO2 NPs in concentration of 2.0 mg mL−1 and 8 µL of APTES per mL of ethanol.

2.3 Silver nanoparticles synthesis

25.0 mL of a 2.0 mM aqueous solution of sodium borohydride (NaBH4) are stirred in an ice bath. Then, 0.8 mL of a 1.2 mM solution of silver nitrate (AgNO3) are added dropwise. After 10 min, 5.0 mL of an aqueous solution 0.050 mM of 3-mercaptopropionic acid (MPA) are added to 5.0 mL of the yellow colloidal solution and stirred for 5 additional minutes. The as-synthesized silver nanoparticles (Ag NPs) are stored in dark at room temperature.

2.4 Adsorption procedure of silver nanoparticles onto silica nanoparticles surface

10 mL of Ag NPs colloidal solution are added in a 25 mL flask and put it in an ultrasound bath; then 10 mg of NH2-SiO2 NPs are added very slowly. After the formation of a fine suspension, the system is transferred to a magnetic stirrer and kept under vigorous stirring for 2 h. The silver-silica nanoparticles (Ag-SiO2 NPs) are collected through centrifugations steps, to separate the Ag-SiO2 NPs from the non-adsorbed Ag NPs; the centrifugation is repeated until the surnatant contained silver nanoparticles. Finally, the obtained pellet is washed three times with ethanol and then dried under a gentle flux of nitrogen. The amount of adsorbed Ag NPs is derived by the analysis of the extinction spectrum recorded before (starting colloidal solution) and after (supernatants) the anchoring procedure.

2.5 Photocatalytic degradation of 9-anthracenecarboxylic acid by various silica nanocomposites

Photodegradation analysis of 9-anthracenecarboxylic acid (9ACA) is carried out by varying either the concentration of silica nanocomposites or the irradiation wavelength, while every other experimental setup is kept constant. The time-dependent decrease of 9ACA concentration under irradiation is followed through fluorimetric assays; indeed, the photocatalytic pathway generates non-fluorescent products [44] which makes it easy to follow the degradation of 9ACA through the decrease of the relative fluorescence emission.

In a typical experiment, 200 μL of 9ACA solution (1.1 × 10–4 M in water) are placed in a cuvette (with 1 cm optical path length) and diluted with 2 mL of an aqueous suspension of silica (each nanocomposite is in concentration of 1.0 mg mL−1). Then, the overall system is irradiated with a mercury lamp or a WL under continuous stirring, while fluorescence spectra are acquired at specific time intervals, up to 120 min of irradiation.

The photocatalytic efficiency of the nanocomposites and the kinetic profiles of the photodegradation of 9ACA, are calculated similarly to Selvaggi et al. [45]. All the reported data are mean values of three independent experiments.

Similarly, a control experiment with AgNPs was performed. The AgNPs were diluted to obtain an extinction value between 0.2 and 0.25 at 400 nm. To this 200 μL of 9ACA solution (1.1 × 10–4 M in water) were added before irradiation.

2.6 Quantitative analysis of the photosensitized reactive oxygen species

The photo-induced quantum efficiency of ROS formation is determined through a widely used ROS trap, 1,3-diphenylisobenzofuran (DPBF). The data are analyzed with an already reported model [43] (see also Supporting Information). Since a white light source is adopted, instead of monochromatic irradiation, the absorbance values of the standard and the sample under investigation are integrated into the whole UV–VIS range [46].

The used sample concentration (Ag-SiO2 NPs) is 0.5 mg mL−1 in all the experiments and DPBF concentration is fixed to ca. 50 µM. The experiment is performed in ethanol.

2.7 Instrumentation

2.7.1 Optical characterization

Diffuse reflectance UV–VIS spectra of powder samples are recorded with a Varian Cary 4000 spectrophotometer equipped with a 150-mm integration sphere; a barium sulfate tablet is used as reference. The absorption and extinction spectra of solutions and nanoparticle suspensions are collected through a Cary 8454 UV–VIS Diode Array spectrophotometer. The photocatalytic degradation analysis is performed by acquiring the fluorescence emission spectra of 9ACA (λexc = 363 nm) on a Fluorolog-2 (Spex, F112AI) fluorimeter.

2.7.2 Morphological and structural characterization

A Philips 208 transmission electron microscope (TEM) with 80 kV of beam acceleration is used to acquire the nanoparticle images by depositing one drop of each suspension on a 300 square mesh Formvar-coated copper grid. The relative histograms are derived from ImageJ (Rasband, W.S., ImageJ, U. S. National Institutes of Health, Bethesda, Maryland, USA, http://imagej.nih.gov/ij/, 1997–2016), and the mean diameters are calculated from the best Gaussian fit of the experimental data.

Energy dispersive X-ray microanalysis (EDX) spectroscopy supported by a field emission scanning electron microscope (FE-SEM) (FEG LEO 1525) is used to analyze both the elemental composition and the morphology of Ag-SiO2 sample.

2.7.3 Radiation sources for photocatalysis experiments

The irradiation experiments are performed using two different lamps. The first one is a mercury-vapor lamp operating at high pressure (500 W) equipped with bandpass filters at 313 ± 15 nm (for the excitation of silica defects) or 405 ± 15 nm (for the photoactivation of Ag NPs plasmon); the second source is a white-light fluorescent lamp (WL) for indoor lighting (Beghelli FLUO Tli8 TRIMAX 18 W). From the manufacturer's specifications, it is estimated a WL irradiance of about 33 W/m2. The irradiance spectra of both lamps are reported in the supporting info (Figs. S13 and S14).

3 Results and discussion

3.1 Morphological and optical characterization of silica nanocomposites

Following the procedures described in the experimental section, silica nanoparticles are prepared. The hydrolysis of tetraethylorthosilicate (TEOS) at pH ~ 7 and using cetyltrimethylammonium tosilate (CTATos) as templating agent [42, 47], results in silica nanoparticles (SiO2 NPs) with mesoporous channel-like structure. The tight control of the synthesis conditions enables to obtain SiO2 NPs with uniform shape and dimensions presenting an average diameter of about 120 ± 9.7 nm (Figs. 1a and S1). The mesoporous structure of silica increases the exposed surface area, as documented for similar materials, where the formation of channel-like structure on silica, more than doubled the material surface area [48] and hence its contact area.

Fig. 1
figure 1

TEM images of: a SiO2 NPs (scale bar 200 nm) and b Ag-SiO2 NPs (scale bar 100 nm); inset magnification of a single Ag-SiO2 NP (the red arrows indicate the AgNPs). c SEM image and d relative EDX spectrum of Ag-SiO2 NPs (the complete EDX spectrum is displayed as an inset)

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The outer surface of SiO2 NPs is then modified by grafting 3-aminopropyltriethoxysilane (APTES), to obtain amino-functionalized nanoparticles, namely NH2-SiO2 NPs. The successful surface functionalization with APTES is demonstrated by FTIR spectra of NH2-SiO2 NPs and SiO2 NPs (Fig. S1); the presence of additional peaks in the spectrum of NH2-SiO2 NPs at 2850 and 2920 cm−1 assigned to the asymmetric and symmetric CH2 stretching, respectively, are due to propyl chain of grafted APTES, while the weak peaks at 1555 and 1537 cm−1 are due to N–H stretching. The peak at 1740 cm−1 could be tentatively assigned to the CO2 adsorbed on the amino-functionalized silica surface.

The presence of amino groups modifies the silica surface charge distribution, as indicated by zeta potential values measured for similar materials (− 62 mV for bare mesoporous silica [47] and + 12 mV after the amino functionalization [48]), but also affects the optical properties of silica powders, as evidenced by the reflectance spectra of the samples (Fig. 2a), likely altering the electronic structure of the silica surface. The grafting procedure of amino groups does not affect the morphology and the dimension of the particles, as indicated by the analysis of TEM images (see Figs. S2–S4 in supporting information for the size distribution analysis).

Fig. 2
figure 2

a Reflectance spectra of SiO2 NPs (black line) NH2-SiO2 NPs (red line) and Ag-SiO2 NPs (blue line) powders. b Extinction spectra of aqueous NH2-SiO2 NPs (red line) and Ag-SiO2 NPs (blue line) suspensions (0.5 mg/mL)

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Previously prepared silver nanoparticles have been anchored on the silica surface, taking advantage of the amino-functionalized surface. Silver colloids (Ag NPs), with a mean diameter of 10 ± 3 nm (Fig. S4) have been formerly synthesized through a selective wet reduction (see experimental section) using 3-mercaptopropionic acid (MPA) as a stabilizer. Ag NPs are chemi-adsorbed on the surface of NH2-SiO2 NPs by contacting the colloidal suspensions. The substantial dissociation of MPA (pKa = 4.3 for the carboxylic group), at neutral pH conditions, drives the establishment of electrostatic interactions with the amino groups on the silica surface, forming silica-silver nanocomposites (Ag-SiO2 NPs). The spectrophotometric analysis of the silver suspension, before and after the contact with the aminated silica, enables to estimate an anchoring efficiency equal to 85% (Fig. S5). Therefore, the estimated amount of adsorbed silver is \({1}{\text{.7 }}\mu g_{Ag} /mg_{{SiO_{2} }}\). Moreover, it is possible to confirm the adsorption of Ag NPs on silica with TEM and SEM imaging, as shown in Fig. 1b, c. The elemental composition through EDX analysis (Fig. 1d) confirms the presence of silver throughout the analyzed sample.

The reflectance spectra of the silica samples in powder form (Fig. 2a) display the lowest energy bands located in the 200–350 nm range [49], likely due to the presence of surface defects [45].

The presence of Ag NPs on the silica surface significantly broadens the visible response of Ag-SiO2 NPs colloids (Fig. 2a, b) for the attendance of surface plasmon resonance band, centered at 405 nm.

The extinction spectrum of an aqueous suspension of Ag-SiO2 NPs is monitored for 72 h (Fig. S5); the spectra display only a slight red-shift of the absorption maximum, from 405 to 407 nm, thus indicating the substantial stability of the nanocomposite in suspension.

3.2 Photocatalytic degradation of 9ACA through monochromatic light

The effect of surface functionalization of the colloids has been explored by monitoring their photocatalytic activity on the degradation of 9ACA, irradiating at 313 nm or 405 nm, to selectively photo-activate the silica or silver, respectively. The 9ACA concentration is tracked by following the change of its fluorescence emission intensity at 409 nm at different irradiation times (Figs. 3 and S7). The reduction of the emission intensity is correlated to the decrease of 9ACA concentration since the photodegradation pathway generates non-fluorescent products [44]. As a control experiment, the same irradiation setup is used on the 9ACA solution without the colloids; the changes of 9ACA fluorescence intensity in the presence of silica colloids is, within the experimental errors, negligible when the samples are stored in the dark.

Fig. 3.
figure 3

9ACA photodegradation kinetics performed in water, in absence of nanoparticles (Black box) and presence of SiO2 NPs (Orange circle), NH2-SiO2 NPs (Blue Triangle), and Ag-SiO2 NPs (Purple Down Triangle) in concentration of 1.0 mg mL−1 a λirr = 313 nm; b λirr = 405 nm. The dotted lines are the linear fits of the data

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The time evolution data of 9ACA upon irradiation at 313 nm, in the different experimental conditions, are collected in Fig. 3a; Table 1 summarizes the obtained results, in terms of process efficiency (in the used experimental conditions), photocatalytic rate constant and half-life t1/2 of 9ACA during degradation. As a reference, the results in the absence of the catalyst are reported as well. The data evidence that 9ACA in aqueous solution (in the absence of any colloids) and even in the presence of SiO2 NPs, is stable and the irradiation at 313 nm does not produce any decrease of the anthracene concentration. Interestingly, when NH2-SiO2 NPs are used as a catalyst, a reduction of 9ACA concentration up to 30% is measured after two hours of irradiation. Using Ag-SiO2 NPs under 313 nm irradiation, a remarkable degradation of 9ACA is found; in two hours almost 70% of 9ACA is photodegraded (see Table 1), which is than more than double the values reached with NH2-SiO2 NPs.

Table 1 Parameters for the photodegradation of 9ACA (λirr = 313 nm)
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In these experimental conditions, the 313 nm radiation is mainly absorbed by the silica core of the nanomaterials (see Fig. S8 for 9ACA absorption spectrum), making the photocatalysts active to induce the photodegradation of 9ACA.

On the bases of our results, we ascribe the silica catalytic performances to the activation of non-bridging oxygen hole centers (\(\equiv Si-{O}^{\bullet }\)) surface defects which show \(\sim {\text{305 nm }}\left( { \sim {\text{4 eV}}} \right)\) absorption contribution [24, 25, 50], comparable to the absorption band reported in Fig. S5. The excitation of this kind of defect leads to a charge-transfer process from Si–O bonding orbital to a non-bonding orbital of non-bridging oxygen [50]. This charge separated state has a typical duration of ca. 10–20 μs [15, 36], which is long enough to favor the charge transfer to the surrounding molecules rather than the direct relaxation of this excited state.

However, to make the photodegradation process effective, it is essential that interactions between the catalyst surface and 9ACA occur, as demonstrated by our results. The absence of any photocatalytic activity in bare SiO2 NPs can be related to the establishment of repulsive interactions between the particles and the anthracene derivative, since both species are negatively charged in water. Instead, the amino groups, grafted on the silica surface, are able to establish attractive interactions with 9ACA, thus allowing the organic compound to approach the surface of the semiconductor and enhancing the photocatalytic efficiency up to 30%, as previously observed for the photodegradation of acridine orange in water [45].

The electron-acceptor character of the metal nanoparticles can account for the higher photocatalytic performance of Ag-SiO2 NPs. Indeed, once the silica defects are photoactivated by 313 nm radiation and form charge separated species, the hot electrons can migrate towards the metal. This can extend even further the lifetime of silica separated charged species, thus increasing the probability of the interactions with 9ACA [30, 31].

When the colloids under investigation (NH2-SiO2 NPs and Ag-SiO2 NPs) are irradiated at 405 nm, degradation of 9ACA is observed (Fig. 3b), although to different extents; the photodegradation kinetic parameters, determined from fluorescence measurements, are summarized in Table 2. In the presence of Ag-SiO2 NPs, 60% photodegradation of 9ACA is achieved after two hours of irradiation (Table 2) with monochromatic light at 405 nm, where it is essentially absorbed by metal nanostructures. Only a slight decrease of 9ACA concentration is observed in the presence of NH2-SiO2 NPs (about 6% of the degradation observed with Ag-SiO2 NPs) and no reduction is detected in the absence of particle.

Table 2 Parameters for the photodegradation of 9ACA (λirr = 405 nm)
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This VIS-activated photocatalytic efficiency is undoubtedly related to the surface plasmon resonance (SPR) of Ag NPs [51]. The high extinction coefficient and the broad absorption of SPR allow the most effective and wider-wavelength activation of the nanocomposite.

The hot-carriers, generated by the plasmon excitation, can directly react with adsorbed molecules (that can be the anthracene substrates, O2 or water molecules), generating extremely reactive radical species able to degrade 9ACA. However, the involvement of the silica is supported by the lack of observing relevant changes in the 9ACA concentration when irradiated in the presence of simple Ag-colloids (Fig. S15).

3.3 Photocatalytic degradation of 9ACA under white light irradiation

To evaluate the relevance of these materials as photocatalysts, their photocatalytic performances are tested also under irradiation with a white light (WL) lamp, mimicking indoor illumination, which displays a higher emission component in the visible range. The results are reported in Table 3 and Fig. S8.

Table 3 Parameters for the photodegradation of 9ACA under irradiation through white light
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As expected, SiO2 NPs do not display any significant photocatalytic activity, whereas, the silver-silica nanocomposite is the most efficient catalyst, with a degradation rate constant doubled compared to NH2-SiO2 NPs. Indeed, NH2-SiO2 NPs absorb only the UV portion of the lamp radiation, while the Ag- SiO2 NPs absorb also in the visible range, thus a larger percentage of the photons emitted by the WL are able to activate the metal-functionalized photocatalyst. The smaller efficiencies measured with WL are related to the lower photon flux of the used source compared to the monochromatic irradiation.

3.4 Efficiencies of ROS production under white light irradiation

It has already been reported that under the proper illumination the silica defects can be activated [26] and generate hydroxyl radical (OH•) or superoxide radical anion (O2•−) [45], which are extremely reactive against organic chemicals adsorbed on the surface of the catalyst.

Our photodegradation experiments indicate that the activation of both silica defects and silver SPRs can generate charge separated carriers, whose interaction with the surrounding water or oxygen molecules may bring to the production of reactive oxygen species (ROS). However, the quantum efficiency of ROS photosensitization has not been determined.

Therefore, the ability of Ag-SiO2 NPs to photo-induce the formation of ROS is evaluated using 1,3-diphenylisobenzofuran (DPBF, see experimental section) which reacts selectively with O2•− and 1O2 resulting in colorless products [52].

The time evolution of DPBF concentration in the presence of Ag-SiO2 NPs under irradiation is spectrophotometrically monitored. Control experiments are performed as well, to ensure the absence of side-reactions.

Negligible bleaching of DPBF absorption is detected when DPBF solution is kept in dark both in the absence and in presence of Ag-SiO2 NPs (Fig. S9), thus proving that the reaction is triggered by light. Moreover, the direct irradiation of a DPBF solution without Ag-SiO2 NPs for 1 h demonstrates the substantial photostability of DPBF in the considered time-range (Fig. S10).

Instead, the exposure of DPBF to the light in presence of Ag-SiO2 NPs shows considerable bleaching of the scavenger absorption (ca. 50% in 1 h, see Fig. 4); the kinetic analysis of the DPBF bleaching results in a constant rate of 11.2 × 10–3 min−1. The quantum efficiency of ROS production (ΦROS) by Ag-SiO2 NPs is estimated analysing the DPBF oxidation data, following the procedure described in literature (see also the Supporting information) and using Rose Bengal as a standard (ΦROS = 0.76 [53], Fig. S11). Thus, a value of 17% has been determined for ΦROS by Ag-SiO2 NPs. The reported observations and this value indicate that white light can activate Ag-SiO2 NPs and photo-induce the O2•− and 1O2 formation, likely through a charge transfer process between the nanocomposite and the species adsorbed on the silica, as already reported for other materials [54].

Fig. 4
figure 4

a DPBF absorption spectra and b decays at 410 nm upon irradiation of Ag-SiO2 NPs with WL. The dotted line in panel b represents the linear fit of the data analyzed through the Equation reported in SI

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In a photocatalytic process in water media, the main ROS species generated are likely OH• and O2•−. The redox potentials (EH, respect to the normal hydrogen electrode) for their redox couple, H2O/OH•, O2/O2•−, are 2.2 V and − 0.2 V, respectively. In general, a photocatalyst is able to generate these ROS species [55, 56] when: (i) the valence band is at lower energy than EH of OH•, thus the photoexcited holes can oxidize H2O, (ii) the conduction band presents a higher energy value than the EH of O2/O2•−, thus the photoexcited electrons own enough reductive power to reduce O2.

Although the irradiation conditions we have used cannot afford the transition from the valence to the conduction band of the silica materials (9 eV) [55], the data indicate that the necessary charge separated species are formed through the activation of silica defects by UV light, as reported elsewhere [57].

Thus, it is likely that defect sites involve intermediate energy states (ca. 4 eV), located between the valence and conduction band of silica and it is possible to hypothesize that the energies of these charge separated levels are able to satisfy the conditions discussed above in points (i) and / or (ii).

Previously reported experimental evidences demonstrated that, through the excitation of Ag NP plasmons, the direct sensitization of OH• and O2•− occurs [56]. However, since DPBF is a selective ROS scavenger, the Ag-SiO2 NPs most probably produces O2•− radicals [52]. Moreover, as discussed in the photodegradation section, the hot carrier generated by the SPR excitation can activate the silica defects, through electron transfer processes, thus creating a situation similar to that under UV irradiation, but with a less energetic irradiation source.

Therefore, using irradiation sources with both UV and visible components, such as solar or artificial light illumination, the photocatalytic efficiency can be enhanced by the combined sensitizing activity of the silver-silica nanocomposite. The capability of Ag-SiO2 NPs to form highly oxidative species upon exposure to UV–visible irradiation makes the nanocomposite an excellent candidate for environmental remediation based on controlled photocatalysis.

4 Conclusion

We demonstrate the successful application of silver-silica nanocomposites for water photocatalytic remediation.

Mesoporous silica nanoparticles (SiO2 NPs) are successfully synthesized with a mean diameter of 120 nm and a regular channel-like meso-structure, and subsequently functionalized with NH2 groups (NH2-SiO2 NPs). Silver nanoparticles of 10 nm diameter are anchored on the surface of amino-grafted silica nanoparticles (Ag-SiO2 NPs). The three nanocomposites show electronic transitions in the UV region, due to surface defects of silica, whereas for Ag-SiO2 NPs the broadening of the extinction spectrum in the visible region is due to the surface plasmon resonance of the metal nanostructures.

A different set of experiments are carried out to determine the photodegradation efficiency of the prepared materials, using 9-anthracenecarboxylic acid (9ACA) as a model compound of aromatic pollutants.

Upon irradiation at 313 nm, SiO2 NPs do not show any detectable degradation of 9ACA; NH2-SiO2 NPs induced a degradation up to 30% of anthracene derivative and the effect is even improved when Ag-SiO2 NPs are used as catalysts (66%). Interestingly, also exposure to the 405 nm radiation successfully activates Ag-SiO2 NPs photocatalyst, providing photodegradation of 60%.

The use of different materials and irradiation sources is functional to discern the photocatalytic role of the different components of the nanocomposites and to obtain insight on the photocatalysis mechanism.

The amino groups on the silica surface are fundamental for the photodegradation of 9ACA, since the electrostatic interactions help the negatively charged aromatic molecules to be closer to the positively charged silica surface, a crucial condition for an efficient photocatalysis. It is worth to underline that the presence of Ag NPs enhances the photocatalytic efficiencies even under visible radiation, and very interestingly Ag-SiO2 NPs can be activated also by common white fluorescence lamp.

To achieve insight into the catalytic mechanism, the quantum efficiency to generate reactive oxygen species (ROS) is determined. The irradiation of Ag-SiO2 NPs suspensions leads to a significant production of ROS with a quantum efficiency of 17%, thus suggesting that the photo-induced degradation of 9ACA is promoted by the photosensitized ROS.

Therefore, silica-based nanomaterials are promising candidates for wastewater remediation, thanks to the synergism between silica and metal nanoparticles that makes the degradation of aromatic pollutants possible under white or solar irradiation.