Plasmonic-TiO₂ Nanohybrid for Environmental and Energy Applications

Abstract

Engineering the plasmonic nanohybrid structures to provide the advancement in their optical and photocatalytic profiles is one of the important aspects for the development of several environmental and energy applications. Plasmonic nanohybrids, integration of semiconductors and noble nanoparticles provide efficient charge separation due to Schottky junction and plasmon nanoparticle induced electromagnetic field. Effective charge separation and electromagnetic features make plasmonic nanohybrids a promising candidate for SERS-based detection environmental detoxification and energy harvesting applications. In the present chapter, we will summarize and elaborate the different strategies and modification techniques to enhance photocatalytic-driven environmental and energy applications. Moreover, the current chapter also includes the detection of various harmful pollutant molecules and their decomposition under sunlight using several plasmonic nanohybrids. This chapter also reveals the origins of morphological, optical, and plasmonic variations on TiO2 nanostructures for enhanced photocatalytic efficiency. We have also highlighted the probable mechanism due to the plasmonic nanoparticles’ aspects over TiO2 nanostructures and their future perspectives of advanced photocatalysis. This chapter provides the fundamental synthesis aspects of plasmonic nanohybrid and their possible usage in energy and environmental applications significantly. This chapter will provide a basic understanding for the readers to develop several plasmonic nanostructures for environmental applications.

1. Introduction

Nanostructured semiconductor-based photocatalysts have proven tremendous candidates after the outstanding water-splitting discovery by using TiO₂-based electrodes in 1972 [1]. Owing to the band structure properties, semiconductors have the ability to perform and enhance the redox reaction with different light exposure [2]. The nontoxicity, ease of availability, and cost-effectiveness properties of semiconductors make them fascinating for environmental and energy applications [3, 4, 5]. Several nanostructured semiconductors such as ZnO, CuO and TiO₂, WO₃, and V₂O₅ have been employed for different energy and environmental applications [6, 7, 8, 9, 10]. In comparison to other semiconductors, TiO₂ has been found more beneficial for photocatalytic and energy generation devices due to its high photostability, unique band gap, lower cost, and non-toxic nature [11]. TiO₂ is known to be an n-type semiconductor and is the most explored towards energy harvesting and energy generation applications owing to its fascinating electrical and optical behavior [12]. TiO₂ contains three crystalline phases with wide bandgap; anatase rutile (3 eV), brookite (3.1 eV), and (3.2 eV) [13]. Among these phases, the rutile phase is more stable as compared to the brookite and anatase phases. Brookite and anatase phases are stable only at low temperatures and can be transformed into rutile phases by using high-temperature thermal annealing. The anatase phase is found to be more efficient as a photocatalyst as compared to both other phases of TiO₂. Rutile TiO₂ is the most stable phase, while. As compared to the rutile and brookite phases, anatase TiO₂ is more suitable for energy-harvesting reaction processes [13]. The formation of the biphasic TiO₂ is also one of the effective approaches to improving the charge separation in TiO₂ without any external modification. Several research groups have demonstrated the improved photodecomposition ability of mixed-phase TiO₂ in comparison to single-phase TiO₂ [14, 15, 16, 17]. Singh et al. [13] used the hydrothermal method and synthesized mixed-phasic TiO₂ nanoflowers and used them as photocatalysts for the water remediation applicators. They have shown that mixed-phase TiO₂ nanoflowers exhibited high photocatalytic activity owing to the creation of the heterojunction interfaces among the rutile and anatase phases of TiO₂.

As compared to the bulk, nanostructured TiO₂ contains superior photocatalytic efficiency due to their effective active sites and high surface-to-volume ratio, which provides a strong tendency for molecular interactions. TiO₂ with a wide band gap (3.2 eV) absorbs ultra-violet light, following the charge separation, yielding the photo-induced electrons in the conduction band and the complementary holes in the valance band. These photo-generated carriers are short-lived, so they quickly recombine and result in diminishing photocatalytic efficiency. In order to resolve these issues, several methods have been adopted by various research groups, such as doping with metals [18], non-metals [19], and, more recently, through surface modification by noble metal nanoparticles [20, 21, 22]. Metal nanoparticles functionalizing TiO₂ duplicate as an electron sink capturing electrons from TiO₂ and also help to furnish more charged carriers using its localized electric field or Surface Plasmon Resonance (SPR) [20, 21]. With the attachment of plasmonic metal nanoparticles (MNP), the reduction in the recombination rate takes place by the migration of electrons from the conduction band of TiO₂ to MNP. In addition, the electromagnetic field generated by the plasmonic nanoparticles attached over the TiO₂ surface under electromagnetic radiation also helps to reduce the recombination rate. Owing to the SPR effect of plasmonic nanoparticles, the plasmonic nanoparticles modified TiO₂ enable visible light adsorption. Thus plasmonic nanoparticles functionalized TiO₂ nanostructures are expected to exhibit superior energy harvesting efficiency as compared to bare nanostructured TiO₂ [23]. Figure 1 reveals the working mechanism of the plasmonic nanoparticles functionalized TiO₂ nanostructures. Bare TiO₂ nanostructures contain a high probability of charge recombination. With the modification with plasmonic nanoparticles, efficient charge transfer occurs; consequently, a reduction in the recombination rate takes place. A high density of electrons in the conduction band produces superoxide radicals by reacting with the surface oxygen, while holes transform the water molecule into hydroxyl radicals [23]. These two unsaturated radicals effectively control the different energy generation and environmental applications, such as H₂ production and water purification, respectively. Plasmonic nanoparticles functionalized TiO₂ nanostructures significantly improve its efficiency in various applications such as sensors [24], solar cells [25], photocatalytic activity [26], energy storage [27], and energy production [28]. Various physical and chemical techniques such as sol-gel [29], impregnation method [30], sputtering [31] pulse laser deposition method [32], and photo-deposition method [33] have been embraced by different research groups for the fabrication of plasmonic nanocomposites such as Ag-TiO₂ and Au-TiO₂.

Figure 1.

Scheme reveals the efficient charge transfer mechanism in TiO₂ using plasmonic noble metal nanoparticles.

2. Environmental applications of plasmonic-TiO₂ nanohybrid

In this section, plasmonic-TiO₂ nanostructure-driven environmental applications have been explained in detail. Environmental applications include the SERS-based detection of pollutant molecules solution and the breakdown of pollutant molecules in water under light illumination. This section also explains the underline mechanism responsible for the outstanding performance of plasmonic-TiO₂ nanohybrid for photocatalytic water purification and SERS-based detection process. This section will provide an in-depth understanding for the readers and encourage them to develop several plasmonic nanostructures for environmental applications.

2.1 Photodecomposition of organic pollutants in water

Plasmonic-TiO₂ nanohybrids have been proven the outstanding photocatalyst which can effectively decompose industrial pollutants [20, 21, 22]. Plasmonic-TiO₂ nanohybrids photocatalyst with efficient charge separation properties reveals the light-harvesting ability from UV to visible regions, which make them fascinating for photodecomposition reactions. Various research groups studied the modification of plasmonic nanoparticles over different morphologies of TiO₂ for photocatalytic applications [34, 35, 36]. The attachment of plasmonic nanoparticles (Ag, Au) with TiO₂ nanostructures is an effective way to improve their photodecomposition behavior. The existence of plasmonic nanoparticles on nanostructured TiO₂ forms the Schottky junctions interface and captures the photo-generated electrons, and decreases the rate of recombination in TiO₂. Moreover, due to the presence of plasmonic nanoparticles over TiO_2, their light-harvesting ability significantly extended [20, 21, 37]. Previous reports indicate the remarkable improvement in TiO₂ nanostructures with the modification of plasmonic nanoparticles such as Ag and Au [20, 21, 22, 23, 34, 35, 36, 37].

Uniformly interspersed Ag nanoparticles on 1-D TiO₂ nanostructures have been a favorite choice for their efficient SPR excitation ability, feasibility, electron mobility, and cost-effectiveness. Ag-dispersed TiO₂ nanorods [38], nanobelts [39], and nanowires [34] have been synthesized using different synthetic procedures for effective decomposition of a range of dyes such as Rhodamine B, Malachite Green, MB, R6G, etc. A contemporary study conducted by Singh et al. [40] postulates electron scavenging and LSPR of Ag as predominant factors responsible for the exceedingly high efficiency of their synthesized photocatalyst nanocomposites towards toxin degradation. Three different Ag-modified TiO₂ nanorods with increasing Ag loading, respectively, were synthesized using one pot wet chemical process. Their optical, structural, and morphological profiles were well characterized. Using sunlight (950 W/cm²) as their illumination source, they degraded model organic dyes methyl orange and methylene blue. In another report, Bian et al. [41] informed the synthesis of Au-attached TiO₂ nanohybrids using the impregnation reaction and demonstrated photodegradation of chlorophenol, MB, and RhB 4 pollutant solution under Xe lamp exposure. In their photocatalysis studies, 10 μM of chlorophenol, MB, and RhB were decomposed in 4 hours, 20 minutes, and 2 minutes, respectively. Li et al. [42] reported the formation of Au-functioned TiO₂ nanostructures by a wet chemical process. Pyrolysis method-driven TiO₂ nanostructures were attached with plasmonic Au nanoparticles with various concentrations (2, 5, and 10 wt%). The prepared Au-TiO₂ nanohybrids (100 mg) exhibited photodegradation by diminished RhB (20 mg/L) molecules solution in 180 minutes under visible light exposure. Singh et al. [43] reported the Ag-TiO₂ thin films formation by combining the sol-gel method with the spin-coating method. Figure 2(a-d) show the morphological changes in modified Ag-TiO₂ thin films in comparison to the bare Ag-TiO₂ and. They tuned the photocatalytic profiles of Ag-TiO₂ thin films by using the ion-beam irradiation technique. They modulate the optical properties effectively, and consequently, the light-harvesting behavior of the Ag-TiO₂ was largely affected. They have observed a significant shift in Ag SPR peak from 456 to 446 nm under different ion fluence values (Figure 2(e)). In their photocatalytic activity, they have demonstrated the breakdown of 5 μM MB pollutant molecule solution. They have shown that the most efficient Ag-TiO₂ thin film decomposed the MB solution in 30 minutes of sunlight exposure (Figure 2(f)).

Figure 2.

SEM images of (a) bare Ag-TiO₂ thin films and (b-d) ion beam modified Ag-TiO₂ thin films, (e) UV-Visible spectra of bare and modified Ag-TiO₂ thin films, (f) photocatalytic activity of bare and modified Ag-TiO₂ thin films under sunlight. Reprinted from the reference [43].

Wang et al. [44] successfully prepared Au-TiO₂ nanohybrids with the joint effort of the sol-gel method with the solvothermal method. In their photocatalytic studies, 500 mg/L of the fabricated photocatalyst was employed for the decomposition of 50 mg/L MB solution under the exposure of a Hg lamp (500 W). In their studies, they have highlighted that Au-modified porous TiO₂ nanostructures exhibited superior photodegradation nature as compared to bare porous TiO₂. The superior photodecomposition nature of the porous Au-TiO₂ nanostructures could be assigned to the effective charge separation and improved light harvesting properties owing to the SPR effect in plasmonic nanoparticles. Recently, Singh et al. [20] reported that Au nanoparticles encapsulated TiO₂ nanospheres by using the chemical precipitation method. They have engineered the size of Au nanoparticles by varying the concentration of Au precursors during TiO₂ growth. Figure 3(a-e) shows the morphology and elemental mapping of Au-encapsulated TiO₂ spheres. With the variations in the size of Au nanoparticles, their optical properties improved significantly. Modified Au-TiO₂ nanohybrids absorb the entire visible region (500–800 nm) and majorly contributed to increased photocatalytic activity (Figure 3(f)). The quenching in the rate of recombination with the variations in the size of Au nanoparticles is further confirmed by the reduction in the intensity of photoluminescence spectroscopy.

Figure 3.

FESEM images of (a) Au-TiO₂ nanohybrids revealing the Au encapsulated TiO₂ spheres, (b) combined mapped FESEM image of Au-TiO₂ nanohybrids, elemental mapping showing the individual distribution of (c) Au, (d) Ti and (e) O, (f) UV-DRS curves for synthesized Au-TiO₂ nanohybrids with tuned optical properties, (g) Scheme for enhanced photodecomposition of Au-TiO₂ nanohybrids. Reprinted from the reference [20].

They have highlighted that the outstanding photocatalytic behavior of the Au-TiO₂ nanohybrids can be assigned to the formation of the Schottky junction interface among the Au nanoparticles and TiO₂. Owing to the creation of the Schottky junction, the density of electrons in the conduction band significantly improves the photodecomposition reaction and enhances the degradation rate of the pollutants solution. In their photocatalytic studies, 10 μM of solution three industrial well-known pollutant molecules solution (MB, MG, and MO) were decomposed in 30, 20, and 30 minutes of sunlight exposure, respectively, by using 5 mg of the photocatalyst. Singh et al. [45] prepared the Au-TiO₂ thin films by combining the spin coating method with the thermal evaporation method and employed them for the decomposition of a 5 μM solution of MB under natural sunlight exposure. They have demonstrated that 30 minutes of sunlight exposure can decompose a significant amount of pollutants.

Enhanced photocatalytic activity of Au-TiO₂ nanohybrids thin film can chiefly correspond to different electronic transfers occurring at the composite surface (Figure 4). When Au-TiO₂ photocatalyst with adsorbed MB molecules is illuminated by a light source of appropriate wavelength, two major electron pathways get activated. Primarily, upon excitation, electrons from the valence band of TiO₂ gain energy and subsequently get transferred to its conduction band, leaving a hole behind and thus generating a pair of photo-induced charge carriers. The presence of Au nanoparticles in the vicinity helps the composite system by taking up the photo-induced electrons from the conduction band of TiO_2, thereby suppressing recombination and felicitating catalytic reactions at the surface by acting as an electron sink. Secondly, Au nanoparticles upon light interaction, exhibit surface plasmon resonance (SPR), driving the generation of increased electron-hole pairs in TiO₂ on account of intense electric fields localized around these plasmonic nanostructures close to the TiO₂ surface. This phenomenon is referred to as Plasmon Resonance Energy Transfer (PRET) [45]. These plasmonic nanoparticles have been reported to serve as “hot spots” allowing increased electron-hole generation and improved photocatalytic reactions [45]. Though Au-TiO₂ electron exchange is an important step towards improved catalytic behavior of the nanocomposite system, it is not the key rate-determining step for the reaction. As a consequence of Schottky junction formation, electronic flow is maintained from TiO₂ to Au until equilibrium is attained. [45] Apart from Au, the adsorbed pollutant molecules also contribute to increased photocatalytic activity by absorbing light leading to electron transfer into the TiO₂ conduction band. The photo-generated electrons produced through different pathways react with oxygen molecules at the surface, converting them into superoxide radicals (•O₂⁻), while the holes in the valance band react with water molecules generating hydroxyl radicals (•OH). These unsaturated highly active radicals transform the pollutant molecules into their degradation products [20, 21].

Figure 4.

Schematic picture revealing the efficient charge transfer among Au and TiO₂ for enhanced photodecomposition. Reprinted from the reference [45].

From the above discussion and excellent studies, it can be concluded that the tremendous properties of plasmonic-TiO₂ nanohybrids have significantly improved the photodecomposition process by providing additional light adsorption and reduction in the recombination rate.

2.2 Detection of pollutants in water

The contamination of pollutant molecules in the form of organic dyes, pesticides, pharmaceuticals, and biomolecules requires proper monitoring due to the serious impacts on human health, aquatic life, and the terrestrial environment [46]. As a fingerprint recognition technique, Raman spectroscopy reveals deep chemical and structural insights such as chemical bonding and molecular interactions and thus enables the identification of unknown molecular species [47]. The Raman spectral features consist of bands corresponding to vibrational or rotational transitions; however, its direct applicability is hindered due to inherently weak Raman scattering. Surface-enhanced Raman spectroscopy (SERS) has proven its uniqueness as a reliable spectroscopic technique for the direct identification of bio-molecules and chemicals owing to its selectivity and sensitivity for the sensing of ultralow concentrations of analyte molecules [48]. Nowadays, As a sensitive, specific, non-destructive, and label-free identification technique, SERS has shown enormous potential in several fields inducing medical sciences, life sciences, and analytical chemistry for the targeted identification of chemical and biological hazardous trace species. Primarily, the overall signal enhancement through SERS is governed by electromagnetic as well as chemical enhancement mechanisms. The modified SERS spectral features indicate the adsorption or chemical interaction of analytes with suitable SERS substrates and, thus, imply the involvement of charge transfer-initiated chemical enhancement along with the predominated electromagnetic enhancement. The formation of the charge-transfer complex is critically dependent on the atomic-scale substrate’s surface properties along with the surface density of the adsorbates; therefore, the chemical state and topography of the surface strongly determine the overall SERS enhancement [49, 50].

Initially, a wide range of isotropic and structurally modified anisotropic noble metallic nanostructures such as nanorods, nanotriangles, nanowires, pyramids, nanostars (NSs), nanocubes, nanodendrites (NDs), nanoflowers (NFs), and core-shell NPs in the form of nano-crystalline aggregates and roughened surfaces were utilized for the ultra-sensitive sensing of different analytes. Along with efficient signal enhancement capabilities, the stability, reproducibility, cost-effectiveness, biocompatibility, and spectral background of SERS substrates are the vital parameters restricting the reliable and long-term practical applicability of plasmonic nanostructures-based SERS platforms for the identification of analyte molecules [51]. Owing to their high surface area, excellent physical properties, and flexible and low-cost fabrication peculiarities, semiconductor materials have shown the potential to address the issues with noble plasmonic SERS substrates and recently emerged as promising platforms for SERS-based molecular detection applications utilizing the chemical enhancement and the associated charge-transfer between the analyte and semiconductor’s surface. The signal enhancement produced by semiconductor SERS (SC-SERS) substrates relies on the chemical enhancement mechanism and depicts lesser enhancement than traditional plasmonic materials [52]. However, semiconductor SERS substrates recently gained significant research attention as the prospective potential candidates with improved SERS performance. The high sensitivity and resolution of the SERS technique enable the study of the chemical and physical nature of the interaction among molecule and semiconductor nanostructures, as well as the properties of semiconductor nanomaterials. Direct proximity among the adsorbate molecule and SERS substrate initiates the charge transfer (CT) between them and contributes to some distinct spectral features [53, 54]. The varied adsorption direction of the adsorbate molecules on the SERS substrate could produce distinctly resolved spectral features, which in turn provide significant insights into charge-transfer-induced chemical enhancement mechanisms.

Interestingly, the synthesis simplicity, high stability, chemical inertness, non-toxicity, and corresponding bio-compatibility of nano-structures titanium (IV) oxide (TiO₂) make him a potential candidate for stable SERS signal requirements along with the excellent SERS response. A rich variety of novel TiO₂ NPs-based SERS substrates have been fabricated through varying the nanostructure’s morphology, including spherical nanoparticles (NPs), nanowires [NWs], and nanorods [NRs] employing the various fabrication strategies such as sol-gel, hydrothermal, and solvothermal methods [55]. Especially, the three-dimensional (3D) architectures such as nanoflowers [NFs] and porous nanomaterials provide the supply larger surface area for the adsorption of analyte molecules and further support the chemical enhancement in SERS. The limited Raman intensity enhancements from semiconductor TiO₂ substrates can be improved by optimizing the micro/nano-structured morphology or more efficiently through incorporating plasmonic nanomaterials. Musumeci and coworkers reported the strong SERS signal enhancements of adsorbed enediol molecules over the pure TiO₂ NPs surface through the essential formation of the charge-transfer (CT) complex of TiO₂ with the molecule. The enhancement was attributed to the increased surface atoms and the consecutive adsorbed analytes molecules as well as borrowing the intensity from the allowed continuum state transitions (Figure 5) [56].

Figure 5.

(a) Absorption, and (b) normal Raman (0.1 M ligands) and SERS spectra of 5 nm pure TiO₂ NPs (1 × 10⁻⁴ M NP colloidal solution) after the modification with different (4 × 10⁻² M) ligands. In figure λ_exc represents the laser excitation wavelength. Reprinted from the reference [56].

Anatase and rutile phases of TiO₂ NPs-based SERS substrates were prepared by Yang and coworkers utilizing the sol-hydrothermal method and explored for the TiO₂ phase-dependent study towards the sensitive identification of 4-mercaptobenzoic acid probe molecules significantly contributed by the TiO₂-to-molecule charge transfer mechanism [57].

As shown in Figure 6, the charge-transfer initiated SERS activity of analyte molecule adsorbed TiO₂ NPs is dominated by the three different kinds of transition (i) the excitation of electrons from the valence band (VB) to the conduction band (CB) of TiO₂ and their subsequent transition to LUMO of the adsorbed molecules with excitation larger than the TiO₂ band gap energy (~3.2ev), (ii) the excitation of electrons from HOMO to LUMO of adsorbed molecules followed by the injection into TiO₂ CB, and (iii) the excitation of electrons from TiO₂ VB to surface state energy levels and their transfer to the LUMO of the adsorbed molecules [58]. The abundance availability of TiO₂ surface defects or surface oxygen vacancy defects can bind the electrons and result in the formation of surface state energy levels. Lin et al. reported the ultrahigh SERS activity with EF ~10⁵ for the 4-NBT probe molecule utilizing photo-induced charge transfer employing crystal-amorphous core-shell black TiO₂ NPs by a controllable solid-state synthesis method [59]. Electrochemical anodization of titanium substrates was employed for the fabrication of TiO₂ Nanotube Arrays for the improved detection of cytochrome C with EFs ~10⁴ and LOD down to the ~10⁻⁷ M concentration [59]. Notably, the crystalline femtosecond laser generated TiO₂ nanofibers in the form of three-dimensional chained nanoparticles provided the resultant EFs of ~1.3 × 10⁶ of the CV dye molecules, which is comparable enhancement produced by noble metal-based substrates. This significant contribution to SERS is assigned to the fabrication of 3D nanonetworks, including the combined contribution of nanogap, nanocluster, and plasmonic hybridization [60].

Figure 6.

Different modes representing the charge transfer between the TiO₂ and adsorbed analyte molecule. Reprinted from the reference [58].

Nowadays, the noble metal-semiconductor nanocomposites have attracted significant research attention as noble SERS substrates utilizing the synergistic interaction between the noble metal and semiconductor surfaces. The relatively poor SERS activity of pure TiO₂ can be significantly improved by incorporating plasmonic materials with TiO₂ substrates through the composite formation. Notably, noble metal NPs can be simply incorporated on the TiO₂ surface through the photocatalytic process and therefore offers the ease of fabrication of noble metal-TiO₂ hybrid nanocomposites [61]. The SERS activity of semiconductor substrates is dominantly assigned to metal-molecule charge-transfer contribution and the correspondingly involved chemical enhancement mechanism results in the weak SERS activity of pure TiO₂ NPs-based substrates. The considerably increased photophysical characteristics can be achieved through the attachment of metal nanoparticles with TiO₂, through moving the titania’s Fermi level to more negative potentials and inhibiting the electron-hole pair recombination [62].

Initially, some studies reported the fabrication and SERS performance of hybrid Ag/TiO₂ and Au/TiO₂ composites [55]. The excitation of SPR of AgNPs produces enhanced electromagnetic fields at the AgNPs surface and, in turn, the generation of hot electrons at the metal-semiconductor interface. The correspondingly formed Schottky junction promotes the transfer of generated hot-electrons towards the conduction band of TiO₂ from the metal-semiconductor interface. Thus, the SERS activity of Ag/TiO₂ hybrid substrates is mainly governed by the three contributions, namely, the plasmonic NPs, the transition of electrons between the highest occupied molecular orbital (HOMO) energy level, and the lowest unoccupied molecular orbital (LUMO) energy level of analyte molecule and initiation of charge transfer between the Fermi level of Ag nanoparticle and HOMO level of the analyte molecule, as explained by Zhao et al. for adsorbed R6G molecules over surface of Ag/TiO₂ nanowire substrates [63].

Fu at el. employed the modified photocatalytic method for the AgNPs deposited TiO₂ films for the detection of rhodamine 6G dye molecules [64]. However, the fast photocatalytic process results in the inhomogeneous synthesis of AgNPs under UV exposure and so restricts the applicability of the photocatalytic method for reproducibly uniform deposition of AgNPs over TiO₂ surface. Especially, Song and coworkers demonstrated the applicability of metal composite nanofibers by detecting 4-MPy molecules adsorbed over the AgNPs-coated TiO₂ nanofibers by an enhancement factor of ~10⁵ [65]. In another report, Wang and coworkers fabricated the in-situ AgNPs-deposited well-ordered TiO₂ nanosheets utilizing hydrothermal synthesis exhibiting excellently uniform and renewable SERs activity with EF ~10⁸ for 4-mercaptobenzoic acid [66]. Yang et al. utilized the wet chemical approach for the preparation of Ag/TiO₂ core-shell NPs and reported higher sensitivity as compared to Ag cores through synergistic SERS enhancement. The prepared ultra-thin coated Ag/TiO2 core-shell NPs were employed for the 4-MBA and tetramethyl thiuram disulfide (TMTD) insecticide with significantly lowered detection limits of 10⁻¹¹ M and 10⁻¹⁰ M concentrations, respectively [67]. Notably, Lu et al. synthesized the hierarchical micro/nanostructured TiO₂/Ag architectures utilizing a combined approach of femtosecond laser structuring and hydrothermal treatment. The prepared SERS substrates demonstrated four-fold enhancement compared to the hydrothermally fabricated bare Ti surface along with LODs down to ~10⁻¹⁴ M R6G concentration [68]. In addition, the applicability of Ag/TiO₂ hybrid SERS substrates was also explored towards the detection of various biomolecules such as antibiotics, circulating tumor cells (CTC), uracil DNA glycolase (UDG), and bacteria E. coli and S. aureus [69, 70, 71, 72]. Recently, photo-catalytically synthesized TiO₂@Ag nanostructured SERS bioprobe substrates were developed by Xu et al. targeted towards the ultralow (~10⁻¹⁴ M) concentration detection of R6G molecules and further utilized to specifically recognize CTC with LOD down to 1 cell per mL (Figure 7) [73].

Figure 7.

(a) SERS signal of different concentration of R6G molecules adsorbed over TiO₂@Ag nanostructure. The inset depicts the SERS spectra at low R6G concentrations (10⁻¹², 10⁻¹³, and 10⁻¹⁴ M), and (b) SERS intensity at ~613 cm⁻¹ R6G peak adsorbed over TiO₂@Ag–R6G SERS system at different R6G concentrations. Reprinted from the reference [73].

Therefore, the recent SERS studies unveiled the amplified SERS performance of hybrid Ag/TiO₂ substrates comparable to noble plasmonic NPs based substrates with detectability down to picomolar and femtomolar concentration range.

Furthermore, TiO₂ substrates coated with Au nanohybrid structures were widely utilized to obtain higher sensitivity, stability, and superior compatibility for detecting biological molecules. The incorporation of Au with TiO₂ substrates improves the biocompatibility of the substrates along with the promoted stability of the SERS signal due to the non-oxidizing properties of Au. For instance, Jiang et al. proposed Au-deposited TiO₂ (Au-TiO₂) nanocomposites through photocatalytic reduction of HAuCl₄ on TiO₂ NPs [74]. The prepared Au-TiO₂ nanocomposites produced recyclable and sensitive SERS detection of 4-MBA molecules with the lowest detectable concentration of ~10⁻⁸ M, which benefitted from the synergistic contribution of metals and semiconductors. The employment of nanometer-sized inter-particle gaps with densely packed AuNPs deposited on vertically aligned large-area TiO₂ nanosheets (NSs) reported superior sensitivity along with the ultralow detectability of ~10⁻¹⁴ M crystal violet concentration (Figure 8) [75].

Figure 8.

(a) SEM image depicts the TiO₂ NS/Au NPs@SiO₂/Au NPs sample, and the inset shows a typical SEM image, and (b) SERS spectra of CV molecules with TiO₂ NS/Au NPs@SiO₂/Au NPs-based substrates. Reprinted from the reference [75].

Interestingly, Singh et al. employed the combined thermal deposition and spin coating procedure with the improved optical response and targeted towards efficiently sensitive and simultaneous identification of two organic pollutants rhodamine 6G (R6G) and methylene blue (MB), exhibiting the Raman intensity enhancement factors of the order of ~10⁷ along with excellent multiplexed detection capabilities [45]. Photo-reduced growth of AuNPs over TiO₂ (TiO₂/AuNWAs) nanowire arrays used as recyclable and sensitive SERS platform for detecting ~10⁻⁹ M concentration of R6G molecules. In addition, the comparative SERS study of bare TiO₂, AuNPs, and TiO₂/Au NWAs reveal remarkably improved SERS performance of TiO₂/Au NWAs substrates [76]. Recently, AuNPs decorated ordered porous TiO₂ thin films based SERS sensors were optimized by Liang and coworkers for detection of rhodamine 6G probe molecules [77]. The improved bio-compatibility of Au/TiO₂ nanostructures offers an additional degree of freedom for detecting various biomolecules with high sensitivity. The highly sensitive detection of adenine biomolecules was performed by Jiang and coworkers utilizing Au/TiO₂/Au nanosheet substrates through sputtered deposition of AuNPs with a detection limit of ~10⁻⁷ M concentration [78].

Apart from monometallic NPs deposited TiO₂ SERS substrates, bimetallic Au/Ag nanoparticles (NPs) decorated well-aligned TiO₂ nanorod arrays (NRAs) with strong absorption from 400 to 1300 nm spectral region generated the superior NIR-SERS activity for antibiotic chloramphenicol and ciprofloxacin in real-world water samples with nano-molar detection limits [79]. Additionally, Borges et al. demonstrated the incorporation of bimetallic Ag-Au NPs with TiO₂ and the strong LSPR band and correspondingly improved SERS activity [80]. Along with noble (Ag and Au) metals, some recent SERS studies also focused on the implementation of poor plasmonic materials for the development of hybrid TiO₂ substrates. To prove the applicability of non-plasmonic materials, the uniformly deposited PtNPs onto the vertically aligned TiO₂ nanorod arrays (Pt@TiO₂ NTAs) for the detection of R6G through effective prevention of electron-hole pairs recombination [81]. In another study by Jiang et al., the SERS activity of Fe₂O₃@TiO₂ hybrid substrates was investigated employing exosomal miRNAs via integrated miRNA-triggered hot-spot SERS and Fe₃O₄@TiO₂-based exosome accumulation [82].

In summary, the excellent physical, chemical, and biological properties of metal-oxide TiO₂ semiconductor enables broad SERS detection applications through facile incorporation of plasmonic materials utilizing synergistic interaction between TiO₂ and metallic NPs. The SERS substrate design, material composition, and synthesis strategy strongly govern the stability, reproducibility, and compatibility with biomolecules and subsequently offers a new perspective of SERS substrate development benefitting from the charge-transfer induced chemical enhancement SERS mechanism.

3. Energy applications of plasmonic-TiO₂ nanohybrid

In this section plasmonic-TiO₂ nanostructure-driven energy production applications have been elucidated in detail. This section includes the applicability of plasmonic-TiO₂ nanohybrids for photovoltaics applications. This section explains the basic mechanism responsible for the tremendous usage of plasmonic-TiO₂ nanohybrid for photovoltaic applications. This section will expect to enhance knowledge and understanding of plasmonic nanostructures-based solar cells.

As we have discussed in the previous sections, the plasmonic nanoparticles can significantly control the charge kinetics in TiO₂ nanostructures and can additionally provide visible light absorption ability. By using these outstanding properties, different research groups reported the improved performance of plasmonic nanohybrid-based dye-sensitized solar cells (DSSC) [83, 84, 85]. After the discovery of TiO₂-based DSSC in 1991 by O’Regan and Grätzel the development of DSSC has been made using other semiconductors [86]. DSSC known to be the third generation of photovoltaics has similar working as we observed in the photosynthesis process. Photoanode in plasmonic nanohybrids based DSSC formed by the layer of plasmonic-TiO₂ nanohybrid which is further covered by the light-sensitive commercially available dye molecules such as black dye, which help to harvest solar light. Firstly, a light-sensitive dye molecule starts the conversion of light energy into electric energy by using a plasmonic-TiO₂ photoanode, electrolyte, and counter electrode. The basic process in Plasmonic-TiO₂ based DSSCs has been depicted in Figure 9 below.

Figure 9.

Schematic diagram revealing the basic process of plasmonic-TiO₂ based photoanodes of DSSC.

Dissanayake et al. [87] used Ag and Au-decorated TiO₂ nanostructures via chemical reduction methods. The fabricated Ag-TiO₂ and Au-TiO₂ nanohybrids were employed to design the photoanodes of DSSCs. In their study, they highlighted that the obtained PCE values for pure TiO₂, Ag-TiO_2, and Au-TiO₂-based photoanodes were 5.12%, 6.51%, and 6.23%, respectively. In addition, they have concluded that Ag-functionalized TiO₂ plasmonic nanohybrids exhibited higher PCE values compared to pure TiO₂ and Au-functionalized TiO₂ nanohybrids.

Vaghasiya et al. [88] fabricated Ag-TiO₂ hybrid thin films and used them as photoanodes in DSSCs. The Ag-functioned TiO₂ nanohybrids contained thin films were found to result in superior PCE than bare TiO₂-based photoanodes. The computed values of J_sc, V_oc, FF, and PCE for Ag-TiO₂ thin films were found to be 5.7 mA/cm², 0.621 V, 54%, and 1.9%, respectively. The estimated J_sc, V_oc, FF, and PCE for pure TiO₂ thin films were 4 mA/cm², 0.507 V, 46%, and 1.1%, respectively.

Guo et al. [89] prepared core-shell of Ag@TiO₂ nanostructures and employed them to design DSSCs. They have modulated the Ag content for enhanced photocurrent density. In their photovoltaic studies, they have highlighted that the optimum concentration of Ag (0.15 wt %) provides the maximum photo conversion efficiency (PCE = 5.33%) in comparison to bare TiO₂₋based DSSC (PCE = 3.96%).

Lim et al. [90] successfully prepared Ag-functionalized N-TiO₂ nanohybrids by using thermal annealing combined with the chemical method. The obtained nanohybrids were embraced to fabricate the photoanode of DSSC. They varied the Ag loading by varying the concentration from 2.5 to 20 wt% over the surface of N-TiO₂ and obtained a significant value of PCE of 8.15%. Figure 10(a-b) showing the surface morphology of the Ag-TiO₂ nanohybrids while their elemental mapping is presented in Figure 10(c-d). The improvement in the charge separation due to the attachment of Ag nanoparticles over TiO₂ is assured by PL spectroscopy (Figure 10(e)). They have highlighted that the PCE value is higher for the N-TiO₂-based photoanodes as compared to the PCE of photoanodes based upon pure TiO₂ (2.19%), N-TiO₂ (2.93%) and Ag-TiO₂ (4.86%) (Figure 10(f)). In their study, they showed that increments in the Ag content significantly improve the PCE of DSSC.

Figure 10.

(a-b) SEM images of Ag functionalized N-TiO₂ nanostructures, (c-d) Elemental mapping of Ag functionalized N-TiO₂ nanostructures showing the existence of Ti, O, N, and Ag atoms, (e) Photoluminescence spectra of N-TiO₂ and Ag decorated N-TiO₂, (f) Current density characteristic of fabricated DSSC using TiO₂, N-TiO_2, and Ag decorated N-TiO₂ nanostructures. Reprinted from the reference [90].

In another study, Wang et al. [91] prepared Ag nanoparticles functionalized TiO₂ nanostructures via a hydrothermal process. They have used Ag nanoparticles to functionalize feather-like TiO₂ nanostructures. They have varied the Ag content over the surface of TiO₂ varying from 1 to 7.5 wt %. They concluded that the optimum amount of Ag content on TiO₂ can remarkably enhance the PCE value of Ag-TiO₂-based photoanode. With the modulation of Ag loading on the TiO₂ nanostructures, the obtained photo conversion efficiency increased from 6.19% to 6.74%. Bhullar et al. [92] studied the effect of Ag implantation in TiO₂ thin films for DSSCs application. They have doped the Ag ions by varying the fluence value from 10¹³ to 10¹⁶ per cm². They have highlighted that due to the incorporation of Ag ion, the optical absorption is significantly improved and also controls the recombination rate, which significantly contributed to enhanced the PCE of DSSCs. They demonstrated that at a fluence value of 10¹⁴, the Ag-doped photoanode significantly showed 21.82% better efficiency as compared to the bare TiO₂-based photoanode of DSSCs.

Muduli et al. [93] informed the formation of Au-TiO₂ nanohybrid by using the hydrothermal process. The prepared Au nanoparticles functionalized TiO₂ nanostructures were embraced for the fabrication of photoanodes of DSSCs. They observed the superior PCE for Au-TiO₂-based photoanodes as compared to the pristine TiO₂-based photoanode. They have concluded that due to the Schottky junction creation among Au and TiO₂ nanostructures, the quenching in the recombination rate takes place, and consequently, the enhancement in the PCE of Au-TiO₂-based photoanodes takes place as compared to bare TiO₂. Alamu et al. [94] prepared Ag-decorated TiO₂ nanohybrids and applied them for the fabrication of photoanodes of DSSCs. They have used natural plant extract of Azadirachta indica and Lawsonia inermis with the commercially available dye N719. They have revealed that the modification of Ag nanoparticles over TiO₂ particles enables effective band gap narrowing and improvement in the optical absorption in the visible region. In their study, they concluded that Ag-TiO₂-based photoanode with the usage of natural dyes exhibited tremendous photoconversion efficiency. The combined contribution of plasmonic nanoparticles with natural dye sensitizer effectively enhances the PCE for bare TiO₂-based photoanode as compared to bare TiO₂ nanoparticles based photoanodes.

Ran et al. [95] decorated the TiO₂ nanostructures with Ag, Au nanoparticles and Ag, Au nanowires for application in DSSCs. They have highlighted that the improved PCE could correspond to the high mobility of plasmonic nanostructures and the SPR effect of Ag and Au nanostructures. Plasmonic nanoparticles functionalized TiO₂-based photoanodes attained a higher PCE value as compared to the pristine TiO₂-based photoanodes. Under sun simulator exposure, a PCE of 5.74% was found in the Ag nanowires decorated TiO₂. The obtained efficiency of Ag-TiO₂-based photoanode attained a 25.3% improvement in comparison to photoanodes prepared by using pure TiO₂ film (4.58%). Improved electron mobility properties of Ag nanowires with enhanced optical absorption majorly contributed and enhanced the PCE of photoanodes. In addition, Ag nanowire also provides an enhancement in the light scattering, which is also favorable to improve the PCE of DSSCs.

Nbelayim et al. [96] reported the preparation of Ag@TiO₂ core-shell with variations in the Ag content from 0.1 to 1% by wet chemical synthesis process. The synthesized Ag@TiO₂ with varied Ag loading was used to fabricate the photoanode of DSSCs. In their study, they optimized the Ag doping concentration for the enhanced PCE of the DSSCs and concluded that 1% of Ag doping showed the maximum PCE. They have concluded that 1% Ag doping provides the optimum band alignment for injection from the Ag nanoparticles to TiO₂. In addition, the optimum concentration of Ag nanoparticles can effectively control the recombination rate in TiO_2, which majorly contributed to enhancing the PCE of the fabricated DSSCs. The PCE value of the Ag@TiO₂ and Ag-doped TiO₂ was found to be significantly enhanced as compared to the pristine sample. Song et al. [97] informed the fabrication of different morphology of Ag nanostructures (spherical and multi-shaped) and Au nanoparticles and explored their effect on the PCE of the DSSCs. In order to fabricate photo anodes of DSSCs, individual Ag, Au nanoparticles, multi-shaped Ag, Au, and their mixture were employed. Each plasmonic nanostructure and its combination were mixed with the mesoporous TiO₂ and were employed to fabricate photo anodes of DSSCs. Among all samples, a photoanode containing a multishape of Ag and Au with TiO₂ was found to be the most efficient for the photovoltaics performance. The enhanced PCE performance of multishaped Ag and Au nanostructures can be assigned to the wider optical absorption as compared to the spherical Ag and Au nanoparticles.

Villanueva-Cab et al. [98] prepared Au-decorated TiO₂-based DSSCs and adjusted the concentration of Au nanoparticles on TiO₂ for efficiently enhanced PCE. In their study, they have highlighted several parameters, such as charge collection efficiency, light absorption efficiency, and charge injection efficiency of dye majorly influenced the PCE of DSSC. Moreover, they showed that the existence of various Au content on the TiO₂ surface majorly influences the collection efficiency and consequently, PCE of DSSC varied.

We have successfully elaborated the importance of various parameters which majorly affect the photo-conversion efficiency of plasmonic nanostructures-based DSSCs. Plasmonic nanohybrids with their fascinating optical and electronic properties largely increase the PCE of the DSSCs. Plasmonic nanoparticles with TiO₂ tune the band alignment and significantly control the charge injection properties. Apart from this, plasmonic nanostructures over the TiO₂ surface also quench the rate of recombination and increase the lifetime of the charge carriers. Moreover, plasmonic nanoparticles also enhance the charge conduction properties of photoanodes which is also beneficial for the improvement in the PCE value of DSSCs. Thus this chapter explains the preparation and characterization of plasmonic-TiO₂ nanohybrids and their usage for water purification, SERS-based detection, and photovoltaic applications. This chapter provides a wide overview related to the preparation and employment of plasmonic nanostructures for different environmental and energy applications. This chapter also includes the importance of specific parameters of plasmonic-TiO₂ nanohybrids, which largely influence their performance for different energy and environmental applications and thus provide a better understanding to the readers.

4. Conclusion

In brief, plasmonic nanohybrids have the tremendous ability to show remarkable performance for environmental sensing and energy harvesting applications. Noble metal nanoparticles functionalized TiO₂ nanostructures-based studies provided significant evidence for their capability to address the environmental detoxification and energy crises problems. This chapter provides the recent trends, development, and applicability of plasmonic-TiO₂ nanohybrids for energy and environmental applications. This chapter includes fundamental aspects of the plasmonic-TiO₂ nanohybrid designing and brief details for the mechanism for SERS-based detection, environmental remediation, and solar cell applications. The fascinating properties of plasmonic nanohybrids include tunable enhanced optical absorption and efficient charge separation properties which open up several new opportunities for the researcher to engineer them for targeted new applications. In this chapter, we have presented the roadmap of each application by using the plasmonic-TiO₂ nanohybrids which can provide a better understanding for the readers to develop the plasmonic nanohybrids for particular applications. Certainly, the research field dealing with plasmonic nanohybrids is growing rapidly and in the future, properties, and applications will be explored for the plasmonic-TiO₂ nanohybrids.