Application of Plasmonic Metal Nanoparticles in TiO2-SiO2 Composite as an Efficient Solar-Activated Photocatalyst: A Review Paper

Over the last decade, interest in the utilization of solar energy for photocatalysis treatment processes has taken centre-stage. Researchers had focused on doping TiO₂ with SiO₂ to obtain an efficient degradation rate of various types of target pollutants both under UV and visible-light irradiation. In order to further improve this degradation effect, some researchers resorted to incorporate plasmonic metal nanoparticles such as silver and gold into the combined TiO₂-SiO₂ to fully optimize the TiO₂-SiO₂’s potential in the visible-light region. This article focuses on the challenges in utilizing TiO₂ in the visible-light region, the contribution of SiO₂ in enhancing photocatalytic activities of the TiO₂-SiO₂ photocatalyst, and the ability of plasmonic metal nanoparticles (Ag and Au) to edge the TiO₂-SiO₂ photocatalyst toward an efficient solar photocatalyst.

Introduction

The first breakthrough in producing the photocatalytic effect of TiO₂ was reported by Fujishima-Honda in 1972, in which, photosplitting of water in the presence of TiO₂ was achieved. The team documented the generation of oxygen gas bubbles at the electrode containing TiO₂ placed under an electrical contact with a piece of platinum metal. While both were immersed in water and exposed to light, hydrogen gas was detected at the platinum electrode. The explanation for this effect was that TiO₂, under ultraviolet irradiation with a wavelength lower than 380 nm, produces an electron-hole pairs according to the following equation:

${\text{TiO}}_2+\mathit{hv}\to {\text{TiO}}_2\left({\mathit{e}}_{\mathit{C}{\mathit{B}}^{-}}+{\mathit{h}}_{\mathit{V}{\mathit{B}}^{+}}\right)(1)$

The electron-hole pairs diffuse evenly on the surface of the TiO₂ particle, to react and decompose oxygen and water present in the atmosphere in order to produce HO^•, hydroxyl radicals, and O⁻₂, superoxide ions, according to the following equations:

${\text{TiO}}_2\left({\text{h}}_{{\text{VB}}^{+}}\right){+\text{H}}_2\text{O}\to {\text{TiO}}_2{+\text{H}}^{+}+{\text{OH}}^{-}(2)$

${\text{TiO}}_2\left({\mathit{h}}_{\mathit{V}{\mathit{B}}^{+}}\right)+{\text{OH}}^{-}\to {\text{TiO}}_2+\text{OH}(3)$

${\text{TiO}}_2\left({\mathit{e}}_{\mathit{C}{\mathit{B}}^{-}}\right)+{\text{O}}_2\to {\text{TiO}}_2+{\text{O}}_2^{-}(4)$

These oxidants will disintegrate and restructure the pollutants through redox reactions taking place on the surface of catalyst, into H₂O, CO₂, and mineral acids (Zangeneh et al., 2015). The mineral acids are generated due to presence of heteroatoms, i.e., S, N, and Cl in the organic compounds (Gardin et al., 2010). TiO₂, is a commonly used photocatalyst due to its availability, its efficient photoactivity, highest chemical stability, harmless nature, lowest cost, and ability in mineralizing various organic contaminants including dyes, insecticides, aromatics, alkanes, haloalkanes, alcohols, and surfactants (Gardin et al., 2010; Abdullah et al., 2016).

TiO₂-based photocatalysis has long been utilized as one of the methods to remediate wastewater. The main obstruction for the application of TiO₂ under solar energy is its high recombination rate of photoexcited electron-hole pairs and wide band gap which is 3.2 eV (Liu et al., 2013). Due to its wide band gap, it requires photon energy equals or higher than 3.2 eV, in order to induce the photoexcitation of electrons and holes. Unfortunately, majority of photons in visible region have photon energy less than 3.0 eV (based on Figure 1). In order to overcome these restrictions, TiO₂ is prepared as a composite catalyst in which the newly added material enables the utilization of TiO₂ in the visible-light region. For instance, doping CuBiS₂, a p-type semiconductor (with a band-gap value of 2.19–2.62 eV) onto TiO₂, enabled the direct utilization of TiO₂ photocatalytic activity in the visible-light region (Abdullah & Kuo, 2015).

FIGURE 1

FIGURE 1. The solar energy spectrum based on the number of photons received per second per unit area of 1 m² vs. the photon energy detected on a clear sunny day with the sun at 60° above the horizon. I: IR region, II: visible region, and III: UV region cited by Choi, 2007 which was originally adapted from Oriel-Instruments (Book of Photon Tools, Oriel-Instruments, 1999).

Another hindrance in utilizing TiO₂ photocatalysis on an industrial scale is the requirement of a large amount of energy to power the UV lamps, resulting in its high operating costs (Chan et al., 2012). In order to overcome these drawbacks, employing sunlight energy to power the photoexcitation of the electrons and holes in the TiO₂ semiconductor would be most ideal. The tapping of solar energy to drive the remediation of wastewater by photocatalysis processes will enable the classification of conventional photocatalysis methods as renewable energy with green technology characteristics.

Due to these benefits, many investigators are currently working on extending the application of TiO₂ photocatalysis into visible-light region. In order to achieve this, one of the methods focused on is the synthesizing of SiO₂-modified TiO₂ photocatalysts which have shown remarkable efficiency under visible-light region.

TiO₂-SiO₂ Photocatalysis

Effect of SiO₂ on Specific Surface Area of TiO₂-SiO₂ Composite

SiO₂ is typically used as a catalyst support or dopant dispersed within the TiO₂ lattice (Liga et al., 2013). This doping affects the TiO₂’s fundamental properties and thus influences the photocatalytic activities too. The structure modification of TiO₂ by SiO₂ drastically increases the specific surface area of the TiO₂-based photocatalyst. The high surface area of TiO₂-SiO₂ is due to its high porosity. Structures with high porosity have large internal surface area per weight, and it is this property which provides high accessibility and diffusivity in order to allow molecules to penetrate through pores, resulting in higher degradation of pollutants on the catalyst’s surface (Jeon et al., 2015).

The morphological improvement by SiO₂ on the TiO₂’s surface is evident from the literature. Balachandran et al. (2014) reported that the BET specific surface area increased from 65 m² g⁻¹ for TiO₂ to 75 m² g⁻¹ for TiO₂-SiO₂ while the mean pore size calculated from BET isotherms was 10 nm for TiO₂ and 15 nm for TiO₂-SiO₂. Bellardita et al. (2010) reported specific surface area of TiO₂-Cabot SiO₂, TiO₂-Axim SiO₂, and TiO₂-Fly Ash SiO₂ as 177, 49, and 29 m² g⁻¹, respectively. Fatimah et al. (2015) reported TiO₂-SiO₂ produced by using rice husk ash as the precursor for SiO₂ with a specific surface area of 91.91 m²/g with a pore volume of 15.18 cc/g compared to that of rice husk ash with a specific surface area of 25.09 m²g⁻¹ and pore volume of 11.60 cc g⁻¹. These results were reportedly higher than that of TiO₂-SiO₂ synthesized by using tetraethyl orthosilicate (TEOS) as the SiO₂ precursor, resulting in catalyst material with a specific surface area of 59.22 m²/g and pore volume of 14.92 cc/g. Table 1 shows the BET surface area and crystallite size values of TiO₂-SiO₂ photocatalyst reported by Ramamoorthy et al., 2016.

TABLE 1

TABLE 1. BET surface area and crystallite size values of TiO₂-SiO₂ catalysts.

Data in Table 1 clearly indicate that the high surface area of TiO₂-SiO₂ is attributed to the high surface area of SiO₂ itself. SiO₂ can be synthesized easily via sol gel with a large surface area and pore volume (Ren et al., 2010; Bahadur et al., 2012) and later can be added into TiO₂ sol, generating composite photocatalyst with increased surface area and pore volume. The enhanced surface area of TiO₂-SiO₂ definitely facilitates in achieving higher photocatalytic activity under solar irradiation.

Effect of SiO₂ on Crystalline Size of TiO₂-SiO₂ Composite

Deposition of SiO₂ onto TiO₂ reduces the overall particle size of TiO₂-SiO₂ composite particles. Based on the spectra from the UV-Vis spectrophotometer, Balachandran et al. (2014) reported absorption peak of maximum absorbance of TiO₂ and TiO₂-SiO₂ at 372 and 352 nm, respectively. The blue shift indicates decrease in particle size due to quantum confinement effect. As shown in Table 1, Ramamoorthy et al. (2016) reported the reduction in crystallite size of TiO₂-SiO₂ (9–12 nm) compared to TiO₂ (18 nm). In addition, using data from the SEM and TEM analysis, Balachandran et al. (2014) reported that pure TiO₂ showed irregular morphological structure due to the agglomeration of its particles and has an average diameter of 15–20 nm. Meanwhile, TiO₂-SiO₂ showed regular morphology with an average particle size of 7–10 nm. This proves that the SiO₂-modified TiO₂ photocatalyst consists of smaller particles but with larger surface area.

The reduced size of TiO₂-SiO₂ particles reduces the pathway in which the photoinduced electrons and holes are used to migrate to the active sites on the TiO₂ surface. This increases the efficiency of the redox reactions by electrons and holes while reducing the recombination rate of photoinduced electrons and holes, thus making TiO₂-SiO₂ a better photocatalyst as compared to TiO₂.

Effect of SiO₂ on Surface Acidity of TiO₂-SiO₂ Composite

Many previous researchers have reported high surface acidity of TiO₂-SiO₂ composite which aids in enhancing the photo decomposition of pollutant molecules. According to Wei et al. (2014), TiO₂ exhibits Lewis acidity and SiO₂ does not exhibit any acidity while TiO₂-SiO₂ exhibits both Bronsted and Lewis acidity. For TiO₂-SiO₂ with TiO₂ as the main component, the Lewis acid sites are dominant while the Bronsted acid sites are dominant for TiO₂-SiO₂ with SiO₂ as the major component (Fateh et al., 2013). It should be noted that the Ti/Si atomic ratio can be manipulated to control the acidity of the TiO₂-SiO₂. TiO₂-SiO₂ with atomic ratio of four (Ti:Si = 4) exhibits highest Lewis acidity while TiO₂-SiO₂ with atomic ratio of one (Ti:Si = 1) exhibited highest Bronsted acidity (Wei et al., 2014).

The Ti-O-Si bond, a strong acidic bond, in the TiO₂-SiO₂ composite photocatalyst, results in a charge imbalance due to the different coordination numbers of the Ti and Si metal center. To offset the negative imbalanced charges over Ti-O, a great deal of protons is extracted from H₂O molecules generating HO⁻ groups, which in turn enhances the surface acidity. The decomposition rate of pollutant molecules is enhanced due to higher amount of hydroxyl groups being on the surface of TiO₂-SiO₂, representing a better photocatalytic performance in visible region compared to TiO₂ (Kibombo et al., 2012; Xu et al., 2015).

The increased surface acidity attracts and adsorbs more hydroxyl groups which later act as hole-scavengers and readily oxidize the adsorbed H₂O molecules. These surface hole-scavenger active sites effectively increase the charge separation and reduce the recombination of photoinduced electrons and holes (Kibombo et al., 2012).

Effect of SiO₂ on Band Gap of TiO₂-SiO₂ Composite

One of the main drawbacks of TiO₂ is the fast recombination of photoinduced electrons and holes which decreases the efficiency of its photocatalytic activity. Deposition of SiO₂ onto TiO₂ caused an increase in the band-gap value of the TiO₂-SiO₂ composite, which is due to the quantum-size effect resulting in an increase in the band-gap value and the interface interaction between the oxide phases, either an SiO₂ matrix or SiO₂ support effect. The interface interaction leads to a formation of Ti-O-Si bonds strongly modifying the electronic structure of the Ti atoms (Panayotov and Yates, 2003).

As an example, the band-gap value of TiO₂ and TiO₂-SiO₂ as reported by Balachandran et al. (2014) was 3.3 and 3.54 eV, respectively. Bellardita et al. (2010) documented the band-gap value of pure TiO₂, TiO₂-Cabot SiO₂, and TiO₂/Axim SiO₂ as 3.00, 3.02, and 2.98 eV, respectively. The increased band gap in TiO₂-SiO₂ indicates that the electrons and holes possess stronger reduction (Kibombo et al., 2012) and oxidation abilities and these abilities enhance the photocatalytic activity of the TiO₂-SiO₂ photocatalyst in visible region.

Effect of SiO₂ on Thermal Stability of TiO₂-SiO₂ Composite

Addition of SiO₂ onto TiO₂ increases the overall thermal stability of TiO₂-SiO₂, thus preventing the conversion from anatase TiO₂ into rutile TiO₂ crystalline structure (Kibombo et al., 2012). Table 2 shows the specific surface and pore volume of CTS-1 (Ti:Si = 1), CTS-4 (Ti:Si = 4), and TiO₂ as a function of the calcination temperature reported by Wei et al. (2014).

TABLE 2

TABLE 2. Specific surface area and pore volume of CTS-1, CTS-4, and TiO₂ in relation to calcination temperature (source: Wei et al. (2014)).

When the calcination temperature increased from 500 to 950°C, the specific surface area decreased from 437.4 to 176.3 m² g⁻¹ for CTS-1 and from 309.5 to 121.2 m² g⁻¹ for CTS-4. When the calcination temperature was increased from 500 to 950°C, the pore volume of CTS-1 changed from 1.39 to 0.52 ml g⁻¹ and the pore volume of CTS-4 changed from 0.55 to 0.30 ml g⁻¹. Meanwhile, for TiO₂, both specific surface area and pore volume were undetectable after calcination at 650°C or higher due to sintering of particles and collapse of interstitial pores. However, both CTS-1 and CTS-4 are less affected by the increase in calcination temperature due to enhanced thermal stability of TiO₂ with the addition of SiO₂ (Wei et al., 2014).

Photocatalytic Activity of TiO₂-SiO₂ Composite

Binary mixed oxide, TiO₂-SiO₂, has been widely documented as a better photocatalyst compared to TiO₂ alone due to simultaneous roles played by the TiO₂ and SiO₂ as photocatalyst and adsorbent, respectively, according to the reaction mechanism shown in Figure 2. The doping of SiO₂ onto TiO₂ enhances the adsorption of pollutant molecules to be near the photoactive center of TiO₂, resulting in more pollutant molecules being broken down, thus producing high degradation rate of pollutant molecules under visible-light region. Due to the presence of more effective adsorption sites in the TiO₂-SiO₂ composite system, the photogenerated holes can reach the sites before recombination with electrons thus further enhancing the photocatalytic activity of TiO₂.

FIGURE 2

FIGURE 2. Photocatalytic degradation of phenol over TiO₂-SiO₂ photocatalyst (source: Kibombo et al. (2012)).

Ramamoorthy et al. (2016) reported that 30% of the TiO₂ incorporated in TiO₂-SiO₂ (200 mg) exhibited 51% degradation of acid orange dye (300 ppm) as compared to only 19% by TiO₂ (200 mg) after 10 h of visible-light irradiation (150 W tungsten filament lamp). Mungondori et al. (2015) reported an absorption band of 366 nm for TiO₂ and 397 nm for TiO₂-SiO₂ analyzed through shift in absorption band edge. This proves that incorporation of SiO₂ into TiO₂ shifts the absorption band of TiO₂ toward the visible-light region.

When the molar ratio of TiO₂ in TiO₂-SiO₂ was increased, the total surface area of TiO₂-SiO₂ decreased causing low photocatalytic activity of the composite catalyst (Ramamoorthy et al., 2016). In a study reported by Jeon et al. (2015), at high molar ratio of TiO₂ (molar ratio of TiO₂ precursor to SiO₂ precursor of 0.05) in the TiO₂-SiO₂, dense thin film with mostly blocked pores was observed. This indicated that at higher concentration of TiO₂ precursor relative to SiO₂ precursor, the size of the pores became smaller if not completely blocked. In contrast, at high SiO₂ precursor molar ratio relative to TiO₂ precursor, the amount of vertically perforated pores increased and formed large micron-sized cracks due to the inability in enduring thermal shrinkage stress as the film thickness increased. Thickness of the film has been reported to decrease with increase in molar ratio of SiO₂ in TiO₂/SiO₂ film (Fateh et al., 2013).

When too small amount of SiO₂ (<2%) or too large (>5%) is used to modify TiO₂, it causes lower photocatalytic degradation of TiO₂-SiO₂ as compared to commercial P25 TiO₂. In addition, from the XRD analysis, it was reported that doping a small amount of SiO₂ on TiO₂ will not effectively prevent the rutile phase transformation, which also contributes to a lower photocatalytic activity while a high amount of SiO₂ doping will influence the optical absorption of TiO₂ which is unfavourable in photocatalytic reactions (Kang et al., 2009). Table 3 presents the gist of the some of the previous studies on TiO₂-SiO₂ photocatalyst.

TABLE 3

TABLE 3. Some of the previous studies on TiO₂-SiO₂ photocatalyst.

Challenges in TiO₂-SiO₂ in Solar Photocatalysis

Modification of TiO₂ with SiO₂ alone is insufficient to maximize the utilization of the TiO₂ semiconductor in a solar-energy-powered wastewater remediation process. In order to fully utilize the photocatalytic activity of the TiO₂-SiO₂ in the visible region, there is a need for third material to be added to the TiO₂-SiO₂ in which the third material donates the electrons excited via absorption of photon from sunlight irradiation. Thus, the third material needs to have a low band-gap energy (Eg < 3) in order to utilize the energy directly from the sunlight. Xu et al. (2015) utilized Fe₂O₃, which has a band-gap energy of 2.2 eV, to extend the application of TiO₂ in the visible-light region by coupling with Fe₂O₃.

Wang et al. (2016) reported on utilizing Fe₂O₃ coupled TiO₂-SiO₂ photocatalyst irradiated under room light to degrade the methyl orange dye. Based on the diffuse reflectance spectra analysis, the absorption edge of SiO₂-TiO₂ was approximately 400 nm. However, the absorption edge of SiO₂-TiO₂-Fe₂O₃ and SiO₂-Fe₂O₃-TiO₂ is in the visible-light region as shown in Figures 3, 4. This shift directly enables and enhances the photocatalytic activity of TiO₂-based catalyst in the visible-light region due to the narrowing of the band-gap value. Compared to SiO₂-TiO₂-Fe₂O₂, SiO₂-Fe₂O₃-TiO₂ exhibited a higher photocatalytic activity in the visible region due to strong interaction between Fe₂O₃ and TiO₂ and due to the formation of shallow trapping sites for photoinduced electrons and holes.

FIGURE 3

FIGURE 3. UV-Vis diffuse reflectance spectra of various SiO₂-TiO₂-Fe2O3 combinations. Inset: the plots of (αhv)1/2 vs. photon energy of different catalysts. Ratio percentage of Fe/Ti amount: a, 0%; b, 1%; c, 3%; d, 5%; e, 8% (source: Wang et al. (2016)).

FIGURE 4

FIGURE 4. UV-Vis diffuse reflectance spectra of various SiO₂-Fe₂O₃-TiO₂ combinations. Inset: the plots of (αhv)1/2 vs. the photon energy of different catalysts. Ratio percentage of Fe/Ti amount: a, 0%; b, 1%; c, 3%; d, 5%; e, 8% (source: Wang et al. (2016)).

The codoped SiO₂-Fe₂O₃(5%)-TiO₂ by N and B exhibited high photocatalytic activity of methyl orange under weak room light irradiation (4 W fluorescent lamp with rare earth phosphor) due to synergistic effect of both N doping and Fe₂O₃ coupling which enhanced the visible-light absorbance and reduced the band-gap energy of TiO₂-based catalyst and at the same time, and both N and B doping on TiO₂ caused high separation efficiency for photoinduced electrons and holes (Wang et al., 2016).

Deposition of Plasmonic Metal Nanoparticles

Another option to maximize the application of TiO₂-SiO₂ photocatalyst in the visible-light region is by incorporating plasmonic metals such as Ag and Au into TiO₂ in order to utilize their ability in absorbing strongly in visible region. Their ability in absorbing of solar photons is attributed to the Localized Surface Plasmon Resonance (LSPR) (Hao et al., 2016). LSPR is defined as collective motions of conduction electrons induced by light irradiation (Chen et al., 2012). Some of the previous attempts to deposit plasmonic metal onto the TiO₂-SiO₂ composite are listed in Table 4.

TABLE 4

TABLE 4. Some of the previous studies on plasmonic metal deposited TiO₂-SiO₂.

Plasmonic nanoparticles (such as Ag and Au) exhibit a property identified as surface plasmon resonance (SPR) especially in the visible region of the electromagnetic spectrum (Cho and Krishnan, 2013; Abdullah and Kuo, 2015). Many previous research studies deposited Ag/Au nanoparticles onto TiO₂-SiO₂ composite photocatalyst due to their ability to enhance photocatalytic activities by trapping the generated photoelectron and reducing the recombination rate of generated electrons and holes by acting as an electron trap (Chen et al., 2012; Abdullah & Kuo, 2015). The Ag/Au nanoparticles enhance the efficiency of TiO₂-SiO₂ photocatalyst by absorbing the solar photons and transferring the energetic electron, formed via SPR excitation, into the TiO₂ (Hamal and Klabunde, 2010; Linic et al., 2011). This mechanism is demonstrated in Figure 5, 6.

FIGURE 5

FIGURE 5. Schematic representations of excited electron generated in AuNS and transfer to the TiO₂ CB, where ECB, Ef, and EVB represent the energies of the conduction band, Fermi level, and valence band, respectively (source: Shi et al. (2016)).

FIGURE 6

FIGURE 6. Reaction mechanism of the Ag@AgBr/mp-TiO₂ photocatalyst under visible-light irradiation (source: Hayashido et al., 2016).

Evidence of higher efficiency achieved by plasmonic metal deposited TiO₂-SiO₂ was reported by Liu et al. (2013). Liu showed that under 4 h of visible irradiation, the TiO₂-SiO₂-Ag photocatalyst exhibited an almost complete degradation of rhodamine B (10ppm) as compared to 80% by TiO₂-Ag and only 30% by neat TiO₂. In comparison, Hayashido et al. (2016) documented that Ag@AgBr/mp-TiO₂ is an efficient photocatalyst in the visible-light region (λ > 400 nm) due to the visible-light activity of Ag@AgBr.

Addition of Ag into TiO₂ modifies the lattice parameters of TiO₂ by generating oxygen vacancies which act as active sites for photocatalysis process, reduces recombination rates of photoexcited electrons and holes due to the formation of Schottky barrier between Ag and TiO₂, reduces band-gap value, and generates defect site Ti³⁺ (Gupta et al., 2006; Chen et al., 2007; Hamal and Klabunde, 2010). Hamal and Klabunde (2010) documented that 5% AgCl-SiO₂, 5% AgBr-SiO₂, and 5% AgI-SiO₂ exhibited band-gap absorption in visible region as shown in Figure 7 and were able to degrade rhodamine B dye (concentration of 2 × 10^–5 M) in the liquid phase and acetaldehyde in the gas phase under visible region (λ > 420 nm). Among these three, the 5% AgI-SiO₂ was found to be the best photocatalyst due to its low band gap and high surface area as well as high stability.

FIGURE 7

FIGURE 7. UV-Vis spectrophotometer absorption spectra of 5% silver halide supported on silica (Hamal and Klabunde (2010)).

When molar ratio of TiO₂ in the TiO₂-SiO₂ photocatalyst structure is increased, the total surface area of TiO₂-SiO₂ decreases causing low photocatalytic activity of the composite catalyst (Ramamoorthy et al., 2016). In a study reported by Jeon et al. (2015), at high molar ratio of TiO₂ (molar ratio of TiO₂ precursor to SiO₂ precursor of 0.05) in the TiO₂-SiO₂ photocatalyst, a dense thin film with mostly blocked pores was observed. This indicates that at higher concentration of the TiO₂ precursor relative to the SiO₂ precursor, the size of the pores becomes smaller and may be completely blocked. In contrast at high SiO₂ precursor molar ratio relative to TiO₂ precursor, the amount of vertically perforated pores increases and forms large micron-sized cracks due to the inability in enduring thermal shrinkage stress due to the high film thickness. Thickness of the film has been reported to decrease with the increase in molar ratio of SiO₂ in TiO₂/SiO₂ film (Fateh et al., 2013).

When the amount of SiO₂ used to modify the TiO₂ is too small (<2%) or too large (>5%), it causes lower photocatalytic degradation of TiO₂-SiO₂ as compared to commercial P25 TiO₂. In addition, from the XRD analysis, it was reported that doping small amount of SiO₂ on TiO₂ will not effectively prevent rutile phase transformation, which also produces lower photocatalytic activity while high mount of SiO₂ doping will influence the optical absorption of TiO₂ which is not favourable in photocatalytic reactions (Kang et al., 2009).

Lee et al. (2017a) employed the citrate reduction method to prepare Au and polyol method to prepare Ag nanoparticles, respectively. After that, modified Stober process was employed to coat Au and Ag surfaces with SiO₂. The prepared composites were later deposited onto TiO₂ (Degussa P25) by employing maleic acid as an anchoring agent. Lee et al. (2017b) reported that Ag@SiO₂-doped TiO₂ nanoparticles are significantly more effective in photocatalytic activity than Ag-doped TiO₂. Ag25@SiO₂/TiO₂ exhibited the highest activity in decomposing aqueous salicylic acid and aniline under UV-visible light irradiation; which was 3.8 and 2.5 times, respectively, that of the bare TiO₂. The authors attributed the high photocatalytic efficiency of Ag25@SiO₂/TiO₂ to strong LSPR effect of Ag.

Zhang et al. (2018) found that SiO₂-Ag/TiO₂ (SAT) showed enhanced visible-light activity and UV light activity compared to SiO₂-TiO₂/Ag (STA) for degrading tetracycline and traditional dyes. According to them, SiO₂ serves as an efficient support for the Ag nanoparticle immobilization; meanwhile, TiO₂ retains the hierarchical structure and prevents agglomeration of Ag nanoparticles during photocatalytic reaction. They attributed the excellent photocatalytic activity of SAT to improve the transport path of photogenerated electrons, diminished recombination probability of electron-hole pairs, and reduced threat of oxidation and corrosion. It should be noted that the SAT retained its photocatalytic efficiency even after five consecutive runs.

Conclusion

Plasmonic metal particle-incorporated TiO₂-SiO₂ composite plays an essential role as a solar photocatalyst which can transform solar energy into chemical energy for application in photocatalysis. Various methods and strategies were presented in this work that highlighted this incorporation, yielding different results. By taking advantage of three synergistic effects that can influence the photocatalytic activity of TiO₂, ability to absorb solar photons by plasmonic metal nanoparticles (Ag or Au), and high adsorption activity by SiO₂, it is possible to utilize the renewable solar energy for water and wastewater remediation effectively. With greater focus on this composite photocatalyst, the next few years will bring major advancement in utilizing the Ag/Au-incorporated TiO₂-SiO₂ in water and wastewater treatment plants at an industrial scale.

Author Contributions

CJ supervised the student, visualised, wrote, and edited the manuscript. YH wrote and edited the manuscript. BM made funding acquisition and edited the manuscript. MS made funding acquisition and edited the manuscript. LE conducted the research and investigation process and wrote the original draft.

Funding

This research was supported by the Research and Innovation Management Center of University Malaysia Sabah (grant no. SBK0354-2017), and it is gratefully acknowledged.

Conflict of Interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

References

Abdullah, H., Kuo, D. H., Lee, J. Y., and Wu, C. M. (2016). Recyclability of thin nylon film-supported p-CuBiS₂/n-TiO₂ heterojunction-based nanocomposites for visible light photocatalytic degradation of organic dye. Appl. Phys. A. 122 (8), 750. doi:10.1007/s00339-016-0276-4

CrossRef Full Text | Google Scholar

Abdullah, H., and Kuo, D. H. (2015). Photocatalytic performance of Ag and CuBiS₂ nanoparticle-coated SiO₂@ TiO₂ composite sphere under visible and ultraviolet light irradiation for azo dye degradation with the assistance of numerous nano p–n diodes. J. Phys. Chem. C. 119 (24), 13632–13641. doi:10.1021/acs.jpcc.5b01970

CrossRef Full Text | Google Scholar

Bahadur, J., Sen, D., Mazumder, S., Sastry, P. U., Paul, B., Bhatt, H., et al. (2012). One-step fabrication of thermally stable TiO₂/SiO₂ nanocomposite microspheres by evaporation-induced self-assembly. Langmuir 28 (31), 11343–11353. doi:10.1021/la3022886

PubMed Abstract | CrossRef Full Text | Google Scholar

Balachandran, K., Venckatesh, R., Sivaraj, R., and Rajiv, P. (2014). TiO₂ nanoparticles versus TiO₂–SiO₂ nanocomposites: a comparative study of photo catalysis on acid red 88. Spectrochim. Acta Mol. Biomol. Spectrosc. 128, 468–474. doi:10.1016/j.saa.2014.02.127

CrossRef Full Text | Google Scholar

Bellardita, M., Addamo, M., Di Paola, A., Marcì, G., Palmisano, L., Cassar, L., et al. (2010). Photocatalytic activity of TiO₂/SiO₂ systems. J. Hazard Mater. 174 (1–3), 707–713. doi:10.1016/j.jhazmat.2009.09.108

PubMed Abstract | CrossRef Full Text | Google Scholar

Chan, P. Y., El-Din, M. G., and Bolton, J. R. (2012). A solar-driven UV/Chlorine advanced oxidation process. Water Res. 46 (17), 5672–5682. doi:10.1016/j.watres.2012.07.047

PubMed Abstract | CrossRef Full Text | Google Scholar

Chen, H. W., Ku, Y., and Kuo, Y. L. (2007). Photodegradation of o‐cresol with Ag deposited on TiO₂ under visible and UV light irradiation. Chem. Eng. Technol. 30 (9), 1242–1247. doi:10.1002/ceat.200700196

CrossRef Full Text | Google Scholar

Chen, J. J., Wu, J. C., Wu, P. C., and Tsai, D. P. (2012). Improved photocatalytic activity of shell-isolated plasmonic photocatalyst Au@ SiO₂/TiO₂ by promoted LSPR. J. Phys. Chem. C. 116 (50), 26535–26542. doi:10.1021/jp309901y

CrossRef Full Text | Google Scholar

S. H. Cho, and S. Krishnan (Editiors) (2013). Cancer nanotechnology: principles and applications in radiation oncology. (Boca Raton, FL: CRC Press), 296.

CrossRef Full Text | Google Scholar

Choi, K. S. (2007). Band gap tuning of zinc oxide films for solar energy conversionA CASPiE Module. West Lafayette, Ind: Purdue University, 38.

CrossRef Full Text | Google Scholar

Fateh, R., Dillert, R., and Bahnemann, D. (2013). Preparation and characterization of transparent hydrophilic photocatalytic TiO₂/SiO₂ thin films on polycarbonate. Langmuir 29 (11), 3730–3739. doi:10.1021/la400191x

PubMed Abstract | CrossRef Full Text | Google Scholar

Fatimah, I., Said, A., and Hasanah, U. A. (2015). Preparation of TiO₂-SiO₂ using rice husk ash as silica source and the kinetics study as photocatalyst in methyl violet decolorization. Bull. Chem. React. Eng. Catal. 10 (1), 43–49. doi:10.9767/bcrec.10.1.7218.43-49

CrossRef Full Text | Google Scholar

Fu, N., Ren, X.-C., and Wan, J.-X. (2019). Preparation of Ag-coated SiO₂@TiO₂ core-shell nanocomposites and their photocatalytic applications towards phenol and methylene blue degradation. J. Nanomater. 2019, 8. doi:10.1155/2019/8175803

CrossRef Full Text | Google Scholar

Gardin, S., Signorini, R., Pistore, A., Giustina, G. D., Brusatin, G., Guglielmi, M., et al. (2010). Photocatalytic performance of hybrid SiO₂−TiO₂ films. J. Phys. Chem. C. 114 (17), 7646–7652. doi:10.1021/jp911495h

CrossRef Full Text | Google Scholar

Gupta, A. K., Pal, A., and Sahoo, C. (2006). Photocatalytic degradation of a mixture of Crystal Violet (Basic Violet 3) and Methyl Red dye in aqueous suspensions using Ag⁺ doped TiO₂. Dyes Pigments 69 (3), 224–232. doi:10.1016/j.dyepig.2005.04.001

CrossRef Full Text | Google Scholar

Hamal, D. B., and Klabunde, K. J. (2010). “Heterogeneous photocatalysis over high-surface-area silica-supported silver halide photocatalysts for environmental remediation,”in Nanoscale materials in chemistry: environmental applications. (Washington: American Chemical Society), 191–205.

CrossRef Full Text | Google Scholar

Hao, C. H., Guo, X. N., Pan, Y. T., Chen, S., Jiao, Z. F., Yang, H., et al. (2016). Visible-light-driven selective photocatalytic hydrogenation of cinnamaldehyde over Au/SiC catalysts. J. Am. Chem. Soc. 138 (30), 9361–9364. doi:10.1021/jacs.6b04175

PubMed Abstract | CrossRef Full Text | Google Scholar

Hayashido, Y., Naya, S. I., and Tada, H. (2016). Local electric field-enhanced plasmonic photocatalyst: formation of Ag cluster-incorporated AgBr nanoparticles on TiO₂. J. Phys. Chem. C. 120 (35), 19663–19669. doi:10.1021/acs.jpcc.6b04894

CrossRef Full Text | Google Scholar

Hu, C., Tang, Y., Jiang, Z., Hao, Z., Tang, H., and Wong, P. K. (2003). Characterization and photocatalytic activity of noble-metal-supported surface TiO₂/SiO₂. Appl. Catal. Gen. 253 (2), 389–396. doi:10.1016/S0926-860X(03)00545-3

CrossRef Full Text | Google Scholar

Jeon, H., Lee, C. S., Patel, R., and Kim, J. H. (2015). Well-organized meso-macroporous TiO₂/SiO₂ film derived from amphiphilic rubbery comb copolymer. ACS Appl. Mater. Interfaces. 7 (14), 7767–7775. doi:10.1021/acsami.5b01010

PubMed Abstract | CrossRef Full Text | Google Scholar

Kang, C., Jing, L., Guo, T., Cui, H., Zhou, J., and Fu, H. (2009). Mesoporous SiO₂-modified nanocrystalline TiO₂ with high anatase thermal stability and large surface area as efficient photocatalyst. J. Phys. Chem. C. 113 (3), 1006–1013. doi:10.1021/jp807552u

CrossRef Full Text | Google Scholar

Kibombo, H. S., Peng, R., Rasalingam, S., and Koodali, R. T. (2012). Versatility of heterogeneous photocatalysis: synthetic methodologies epitomizing the role of silica support in TiO₂ based mixed oxides. Catal. Sci. Technol. 2 (9), 1737–1766. doi:10.1039/C2CY20247F

CrossRef Full Text | Google Scholar

Lee, J. E., Bera, S., Choi, Y. S., and Lee, W. I. (2017a). Size-dependent plasmonic effects of M and M@ SiO₂ (M= Au or Ag) deposited on TiO₂ in photocatalytic oxidation reactions. Appl. Catal. B Environ. 214, 15–22. doi:10.1016/j.apcatb.2017.05.025

CrossRef Full Text | Google Scholar

Lee, R., Kumaresan, Y., Yoon, S. Y., Um, S. H., Kwon, I. K., and Jung, G. Y. (2017b). Design of gold nanoparticles-decorated SiO₂@TiO₂ core/shell nanostructures for visible light-activated photocatalysis. RSC Adv. 7 (13), 7469–7475. doi:10.1039/C6RA27591E

PubMed Abstract | CrossRef Full Text | Google Scholar

Liga, M. V., Maguire-Boyle, S. J., Jafry, H. R., Barron, A. R., and Li, Q. (2013). Silica decorated TiO₂ for virus inactivation in drinking water–simple synthesis method and mechanisms of enhanced inactivation kinetics. Environ. Sci. Technol. 47 (12), 6463–6470. doi:10.1021/es400196p

PubMed Abstract | CrossRef Full Text | Google Scholar

Linic, S., Christopher, P., and Ingram, D. B. (2011). Plasmonic-metal nanostructures for efficient conversion of solar to chemical energy. Nat. Mater. 10 (12), 911–921. doi:10.1038/nmat3151

CrossRef Full Text | Google Scholar

Liu, C., Yang, D., Jiao, Y., Tian, Y., Wang, Y., and Jiang, Z. (2013). Biomimetic synthesis of TiO₂–SiO₂–Ag nanocomposites with enhanced visible-light photocatalytic activity. ACS Appl. Mater. Interfaces 5 (9), 3824–3832. doi:10.1021/am4004733

CrossRef Full Text | Google Scholar

Matos, J., Llano, B., Montaña, R., Poon, P. S., and Hidalgo, M. C. (2018). Design of Ag/and Pt/TiO₂-SiO₂ nanomaterials for the photocatalytic degradation of phenol under solar irradiation. Environ. Sci. Pollut. Control Ser. 25 (19), 18894–18913. doi:10.1007/s11356-018-2102-3

CrossRef Full Text | Google Scholar

Mungondori, H. H., Tichagwa, L., and Green, E. (2015). Synthesis and glass immobilization of carbon and nitrogen doped TiO₂-SiO₂ and its effect on E. coli ATCC 25922 bacteria. Br. J. Appl. Sci. Technol. 5 (5), 447. doi:10.9734/bjast/2015/11049

CrossRef Full Text | Google Scholar

Oriel-Instruments (1999). The book of photon tools. Stratford, CT: Oriel Instrument, 1–3.

PubMed Abstract | CrossRef Full Text | Google Scholar

Panayotov, D., and Yates, J. T. (2003). Bifunctional hydrogen bonding of 2-chloroethyl ethyl sulfide on TiO₂−SiO₂ powders. J. Phys. Chem. B. 107 (38), 10560–10564. doi:10.1021/jp0304273

PubMed Abstract | CrossRef Full Text | Google Scholar

Ramamoorthy, V., Kannan, K., Joseph, J., Infant, A., Kanagaraj, T., and Thiripuranthagan, S. (2016). Photocatalytic degradation of acid orange dye using silver impregnated TiO₂/SiO₂ composite catalysts. J. Nanosci. Nanotechnol. 16 (9), 9980–9986. doi:10.1166/jnn.2016.12071

CrossRef Full Text | Google Scholar

Ren, Y., Chen, M., Zhang, Y., and Wu, L. (2010). Fabrication of rattle-type TiO₂/SiO₂ core/shell particles with both high photoactivity and UV-shielding property. Langmuir 26 (13), 11391–11396. doi:10.1021/la1008413

CrossRef Full Text | Google Scholar

Shi, X., Lou, Z., Zhang, P., Fujitsuka, M., and Majima, T. (2016). 3D-Array of Au–TiO₂ yolk–shell as plasmonic photocatalyst boosting multi-scattering with enhanced hydrogen evolution. ACS Appl. Mater. Interfaces 8 (46), 31738–31745. doi:10.1021/acsami.6b12940

CrossRef Full Text | Google Scholar

Singh, R., Bapat, R., Qin, L., Feng, H., and Polshettiwar, V. (2016). Atomic layer deposited (ALD) TiO₂ on fibrous nano-silica (KCC-1) for photocatalysis: nanoparticle formation and size quantization effect. ACS Catal. 6 (5), 2770–2784. doi:10.1021/acscatal.6b00418

CrossRef Full Text | Google Scholar

Wang, T., Zhu, Y. C., Xu, Z. Y., Wu, L. G., Wei, Y. Y., Chen, T. N., et al. (2016). Fabrication of weak-room-light-driven TiO₂-based catalysts through adsorbed-layer nanoreactor synthesis: enhancing catalytic performance by regulating catalyst structure. J. Phys. Chem. C. 120 (22), 12293–12304. doi:10.1021/acs.jpcc.6b03721

PubMed Abstract | CrossRef Full Text | Google Scholar

Wei, Q., Li, Y., Zhang, T., Tao, X., Zhou, Y., and Chung, K., et al. (2014). TiO₂–SiO₂-composite-supported catalysts for residue fluid catalytic cracking diesel hydrotreating. Energy Fuels 28 (12), 7343–7351. doi:10.1021/ef500799t

Google Scholar

Xu, Z., Huang, C., Wang, L., Pan, X., Qin, L., Guo, X., et al. (2015). Sulfate functionalized Fe₂O₃ nanoparticles on TiO₂ nanotube as efficient visible light-active photo-Fenton catalyst. Ind. Eng. Chem. Res. 54 (16), 4593–4602. doi:10.1021/acs.iecr.5b00335

Google Scholar

Zangeneh, H., Zinatizadeh, A. A. L., Habibi, M., Akia, M., and Isa, M. H. (2015). Photocatalytic oxidation of organic dyes and pollutants in wastewater using different modified titanium dioxides: a comparative review. J. Ind. Eng. Chem. 26, 1–36. doi:10.1016/j.jiec.2014.10.043

Google Scholar

Zhang, Y., Chen, J., Tang, H., Xiao, Y., Qiu, S., Li, S., et al. (2018). Hierarchically-structured SiO₂-Ag@ TiO₂ hollow spheres with excellent photocatalytic activity and recyclability. J. Hazard Mater. 354, 17–26. doi:10.1016/j.jhazmat.2018.04.047

Google Scholar