Keywords

1 Introduction

Light of the appropriate wavelength can selectively activate a desired photocatalyst to generate reactive species that can in turn initiate processes that cannot occur under thermal conditions or that require harsh reagents [1, 2]. Undoubtedly, solar light is considered the greenest reagent for catalytic processes (free and abundant). Photocatalysis is the acceleration of a chemical reaction or transformation produced thanks to the absorption of light by a photocatalyst.

Nowadays, photochemical and photophysical applications have become fundamental for our daily life products: information technology, nanotechnology, sustainable technologies (solar energy storage, waste water cleaning, photovoltaics), lighting (LEDs), etc. [3]. Specifically, photocatalytic applications include advanced oxidation processes (e.g. water decontamination of organic debris) [4, 5], water splitting [6–11], self-cleaning surfaces [12–14], etc.

Metallic nanoparticles (MNPs) possess unique optical, electrical, mechanical, and chemical properties. Thus, their use as photocatalysts to aid in the development of green nanotechnology has attracted a great deal of interest [11, 15–17]. Size reduction increases the Fermi level of the nanoparticle leading to a lower reduction potential of the metal on its surface (see Chap. “Silver Nanoparticles: From Bulk Material to Colloidal Nanoparticles”). The total surface area in nanomaterials is considerably higher than the obtained in the bulk material at the same total concentration. This translates, for example, into higher catalytic performance [18], which is extremely important for catalytic and photocatalytic applications, since one nanoparticle can simultaneously interact with many reactants (comparatively, organic molecules or organometallic complexes only allow one to one interactions) [19].

MNPs upon interaction with an electromagnetic wave give rise to an absorption band known as Surface Plasmon Band (SPB). See Chap. “Silver Nanoparticles: From Bulk Material to Colloidal Nanoparticles” for a detailed explanation. Mahmoud et al. [20]. found that the most active MNPs for catalysis are those with sharp edges, sharp corners, or rough surfaces. The shape of the MNP influences the nature of the surface (facet) exposed to reactants. This would change how reactants and MNP interact and therefore, would influence the outcome of the catalytic processes [21].

This chapter focuses on the use of silver nanoparticles (AgNP) as catalysts. These NPs are also used for other applications such as surface enhanced Raman spectroscopy (Chap. “Surface Enhanced Raman Scattering (SERS) Using Nanoparticles ”), single-molecule spectroscopy [22–24], solar cells [25, 26] and biological uses [27] (Chap. “Biomedical Uses of Silver Nanoparticles: From Roman Wine Cups to Biomedical Devices”). For AgNP, a shift in the plasmon absorption band from 400 to 670 nm is observed as the particle shape transitions from a sphere to a cube [11, 28] AgNP’ shapes have been found to affect the outcome of the catalytic oxidation of styrene to styrene oxide [29, 30].

MNPs have recently started to be applied in photocatalysis (also called Plasmon Mediated Catalysis, PMC) [11, 31] The use of PMC in synthetic applications is fairly recent and considerable effort is being made to optimize photoreactions promoted by localized plasmons [11, 15–17]. In other words, MNPs are used to transform the energy of light into chemical energy by the formation of short-lived energetic electrons and enhanced electric fields that are generated on the surface and/or in close proximity to the NPs surface [11]. Remarkably, very high-intensity fields have been found in the regions (termed ‘hot spots’) between plasmonic nanostructures [11].

MNPs interact with the reactant molecules on their surface. Simultaneously, the electrons at their surface can be excited by light. The reaction rate therefore, will depend not only on the number and energy of those electrons, but also on the number of reactant molecules on the NP surface dictated by the affinity between the MNP surface and the reactants [15–17]. Interestingly, plasmonic nanostructures effectively couple thermal and photonic stimuli to drive chemical transformations [32].

For a detailed summary of the essential features of the preparation methods of supported AgNP, the reader is directed to a recent review written by Wang et al. and the references cited therein [17].

Similar to gold nanoparticles, AgNP have optical properties and catalytic capabilities that make them ideal for green photocatalysis (i.e. photocatalysis utilizing visible and ultraviolet light, a major part of the solar spectrum) [15–17]. An ideal green photocatalyst must absorb sunlight (since it is naturally available), must be stable under the reaction conditions (i.e. solvent, or, if there is no other choice, temperature and illumination), must be available, reusable, able to perform selective transformations, and have low toxicity [2].

Photocatalysis is currently used in two particularly beneficial albeit contrasting purposes; (i) pollutant reduction and the destruction of harmful organic pollutants in contaminated water and air using techniques collectively known as advanced oxidation processes (AOPs) [5], and (ii) the synthesis of chemical compounds, “positive photocatalysis”, whose aim is to generate new products [33–37].

Several organic compounds, organometallic complexes and semiconductors have been used as photocatalysts in both homogeneous and heterogeneous systems [1, 2]. Colloidal heterogeneous nanocatalysis in particular, refers to those reactions that occur on the surface of nanoparticles [20]. The active site in heterogeneous catalytic reactions is a collection of surface atoms that adsorb reactants and facilitate chemical bond transformation [21]. The loading of nanoparticles on solid supports is a brilliant strategy that is broadening the way of thinking about heterogeneous thermal catalysis and PMC. This chapter will focus on this strategy and its use in PMC.

There are several advantages in using heterogeneous PMC;

  1. (i)

    MNPs have much better affinity than solid supports to many reactants, especially organic molecules [15].

  2. (ii)

    MNPs are more resistant to degradation than organic photocatalysts.

  3. (iii)

    Heterogeneous photocatalysts are easier to remove from the reaction mixture (filtration, centrifugation) than heterogeneous catalysts. This heterogeneity can favor the reaction of one kind of reactant in the presence of others by selective adsorption [1].

  4. (iv)

    The MNPs get energy from the irradiation source, resulting in high-energy active surface atoms, which is desirable for activating molecules for chemical reactions.

  5. (v)

    The predictable aggregation of the NPs is prevented by the interaction with the solid support [15].

There are several issues that need to be resolved for the successful application of PMC;

  1. (i)

    Reshaping of the nanocatalyst during catalysis reaction [21].

  2. (ii)

    Reaction products deposited on the NP surface could decrease NP stability and limit their recycling.

  3. (iii)

    The reaction rate may be adversely affected due to the different states of matter in the mixture, or inefficient reactant adsorption/desorption processes, on the solid photocatalysts.

  4. (iv)

    Reduced availability of active surface area due to NP interaction with the solid support.

  5. (v.)

    The synthesis of MNPs usually involves reducing agents, organic cappings, and/or growth-directing molecules. Some of those compounds could end up attached to the particle, and thus affect the catalytic process itself [21].

  6. (vi)

    Particularly in the case of AgNP, it is difficult to prevent the oxidation of AgNP to silver oxide [38].

Despite the above unresolved issues, recent work has demonstrated a plasmonically active and self-regenerative Ag/AgOx nanostructure to be useful for catalytic reactions and this may open up a new perspective in the field [39]. The support-free catalyst shows plentiful surface-adsorbed oxygen species along with excellent localized surface plasmon resonance (LSPR) and measurable photoluminescence [39].

2 Heterogeneous Photocatalysts Based on AgNP

Semiconductors and insulator supports with surface-embedded metallic nanoparticles for improved stability are being developed and tested. Interestingly, the support can also help in some cases to perform the photocatalytic process, thanks to a synergistic effect generated by the association between the support and the nanoparticle. Accordingly, a thorough understanding of the properties of the new material is required in order to prepare the most efficient photocatalyst possible. Based on the above, research has been carried out on the photocatalytic activity of silver nanoparticles supported on titanium (AgNP@TiO2) [38, 40–44], and other metal oxides (e.g. ZrO2, SiO2 and graphene oxide), as well as zeolites [45]. Other metal–semiconductor composite photocatalysts based on the Ag@AgX (X = Cl, Br, I) system have also been developed [17].

It is reasonable to expect different mechanisms for insulator supported AgNP and semiconductor-supported AgNP when applied to photocatalytic processes [15–17]. In this section, we describe these mechanisms in detail to afford the reader with a more in-depth understanding of the photophysical processes required to determine the appropriate strategy for their particular needs. While reviewing these mechanisms, the use of AgNP-based photocatalysis for advances oxidation processes will be briefly summarized and following that, a section focused on positive photocatalysis is presented.

2.1 Heterogeneous Photocatalysts Composed of AgNP and Semiconductors

Semiconductor photocatalysts based on TiO2 are widely studied for applications in environmental remediation and solar energy conversion [46–49]. Unfortunately, TiO2 can only be excited by ultraviolet (UV) light due to its relative wide band gap (3.2 eV). Various strategies have however been developed to make it sensitive to visible light [46–48, 50–57].

Upon illumination of a semiconductor with light, one electron from the conduction band (CB) is promoted to the valence band (VB), leaving behind a vacancy, commonly known as a hole, in the CB [6]. The energy carried by photons from the illumination source has to be greater than the bandgap energy, Eg, (the ΔE between the CB and the VB) for this process to occur. These electron-hole pairs can recombine or be trapped and, most importantly, the photogenerated charge carriers can migrate to the photocatalyst surface and a redox process can take place, giving rise to the corresponding reduced and oxidized products. Some semiconductors respond to visible light (e.g. CdS), allowing scientists to use sunlight or indoor lighting to excite them [2, 58]. Admittedly, the use of photocatalysts based on heavy metals is not the ideal scenario for water and air purification. Thus, in order to overcome this impropriety, several metal-free quantum dots are being developed as potential photocatalysts [59].

A particularly effective strategy for narrowing the band gap of TiO2 is called defect engineering [55, 60–64]. Introducing oxygen vacancies into the lattice of TiO2, the light absorption of TiO2 can be extended to the visible or even infrared region [61, 65]. It is equally important to optimize the photogenerated electron/hole separation characteristics over the TiO2 surface. Notably, the addition of noble metals to TiO2 could reduce the photogenerated electron/hole recombination [50, 51, 66].

AgNP supported on TiO2 (AgNP@TiO2) have shown photocatalytic activity under UV and visible light and also antibacterial properties [38, 40–43, 67–73]. The ability of AgNP@TiO2 nanoparticles to degrade phenol [43, 74], methyl red [40], methyl orange (MO) [67] and methylene blue (MB) [38] has been evaluated. An electron transfer reaction takes place upon UV irradiation (Scheme 1) and, where spherical AgNP coated with TiO2 are concerned, a spectral shift of the plasmon resonance band to a shorter wavelength was detected and elucidated to be due to electron trapping on the AgNP induced by the charge separation after the electron transfer process [40–42]. The interfacial charge transfer between TiO2 and silver could be enhanced by a negative shift in the Fermi level of the AgNP@TiO2 composite resulting from the accumulation of electrons produced by the surface plasmon resonance [16].

Scheme 1
scheme 1

Proposed mechanism for UV excitation of the AgNP@TiO2 photocatalyts: a semiconductor excitation and electron transfer to AgNP [42, 43] and b photoinduced charge separation and charging of the AgNP [41]

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If an electron acceptor such as thionine dye or oxygen is present, the photogenerated electrons are scavenged and electrons fail to accumulate in the AgNP core [42]. This is also the case for Ag@ZnO [75–79]. It is in fact possible to carry out a redox titration and obtain quantitative information about the stored electrons in the Ag@TiO2 colloids [42]. The capacity of electron storage is determined by the size of the metal core and its ability to undergo charge equilibration with the TiO2 shell. Once this maximum storage limit is attained, electron-hole recombination in the TiO2 shell dominates [42].

Electrons accumulated in AgNP can lead to generation of •O2 − when reacting with O2, and finally to •OH. On the other hand, photogenerated holes in TiO2 can be scavenged by OH− or H2O to generate •OH directly [4, 80]. Moreover, the pH value also dictates the amount of •OH generated [40] and the acidity of the hydroxyl groups on the TiO2 surface could be enhanced in the presence of AgNP (as described for gold nanoparticles [81]). Thus, AgNP reduce electron-hole pairs recombination and enhance the efficiency of photoreduction owing to the Fermi level equilibration, ultimately leading to a higher yield of •OH [40]. Indeed, due to the electronegativity of AgNP, their Fermi level can shift to negative potentials, resulting in charging effects or original chemical reactivity [82].

Unfortunately, the photo-oxidation of AgNP by O2 takes place under visible light irradiation of a Ag@TiO2 film [83] and the photogenerated holes residing on the electron-rich AgNP, drive dissolution of Ag atoms from the AgNP via Ag+ ejection [84–86]. This can be prevented by either the deposition of TiO2 made on AgNP, covered with a silica (SiO2) shell [38], or the preparation of Ag@TiO2 photocatalyst using Montmorillonite as a support [87]. Either approach effectively prevents the loss of the metal particles during the photocatalytic reaction.

Recently, metal-core@semiconductor-shell nanoparticles consisting of Ag@Cu2O core-shell were shown to have photocatalytic activity where degradation of MO was concerned. This activity is due to the presence of localized surface plasmon resonance (LSPR) in the AgNP core [88]. This strategy avoids corrosion and dissolution of the metal particles and maximizes the metal—support interaction, thereby facilitating the plasmonic energy transfer processes. Lastly, the local electromagnetic field of the LSPR penetrates the shell, which can be used to tune the center wavelength of the LSPR by changing the shell thickness [88].

2.2 Heterogeneous Photocatalysts Composed of AgNP and Insulator Supports

In 2008, Awazu et al. demonstrated that the photolysis of methylene blue (organic dye) was promoted by a plasmonic effect when visible light was irradiated onto AgNP with a SiO2 layer placed inside a TiO2 layer whose plasmon resonance band was near the band edge of TiO2 [38].

Very recently, AgNP were loaded onto inert supports, which are photocatalytically inactive supports. In this scenario, the charge density is partially localized on the AgNP surface, and this localization is increased by charge separation derived from the LSPR effect [89].

Insulators have different properties than semiconductors since the forbidden band gap between the valence band and conduction band is larger in an insulator, and as a result to this, electrons cannot be promoted from the valence band to the conduction band, even by near UV light [15].

Consequently, when AgNP are deposited onto an insulating oxide, there is no electron transfer between the photo-excited AgNP and the insulator support, and hence, only the nanoparticles participate in the photocatalysis directly [17]. In this case, the photocatalytic activity of noble metal NPs is determined by two factors: the enhanced electromagnetic field of neighboring NPs and the direct interaction of the LSPR excited energetic electrons with reactants. A distinct feature of these systems is that both light absorption and activation of the reactants take place on the AgNP. Charge transfer between the AgNP and support is not required. The particles can be dispersed on an insulating solid (or a very wide band gap semiconductor, such as Al2O3 (band gap, 8.0 eV) [90], ZrO2 [45], SiO2 (band gap, 9.0 eV) [45, 91], SBA-15 [92], zeolite [45], etc.), which has large specific surface areas and porosity.

The energy of incoming photons is concentrated into small volumes surrounding the AgNP. In fact, a build-up of intense, spatially non-homogeneous oscillating electric fields takes place in the neighborhood of the nanoparticle in the AgNP@insulator nanostructure due to the localize plasmon resonance of AgNP [23, 25, 93]. Hot spots also display very high-intensity fields [94] and these enhanced electric fields play a key role in photocatalysis. Indeed, for nanoparticles smaller than 30 nm, surface plasmons lead to the formation of energetic charge carriers [93, 95], which can be transferred to the surroundings [96] or relaxed by locally heating the nanoparticle [26]. This mechanism is thus quite different from that which occurs in the presence of a semiconductor, where electron transfer between the AgNP and the semiconductor takes place.

AgNP supported on zirconia, silica and Zeolite Y (Ag@ZrO2, Ag@SiO2 and Ag@Zeolite Y) are active photocatalysts for sulforhodamine-B (SRB) and phenol degradation [45]. The degradation of SRB by these photocatalysts increases proportionally with silver content.

Also, Ag@SiO2 exhibited superior photocatalytic performance toward MB photodegradation [91]. Once AgNP are excited, the generation of photo-excited electrons and electron vacancies takes place and are responsible for MB degradation (Scheme 2).

Scheme 2
scheme 2

Schematic diagram illustrating a AgNP@SiO2 nanohybrid that display photocatalytic activity upon surface plasmon resonance (SPR) excitation and degrades methylene blue (MB). Reprinted with permission from Ref. [91]. Copyright 2012 American Chemical Society

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Another interesting aspect of photocatalysts containing AgNP is the possible enhancement of the photocatalytic effect of an anchored molecule. For example, core–shell Ag@SiO2 nanoparticles with an anchored [Ru(bpy)3]2+ dye, showed an enhancement of the photo induced oxidation activity of the ruthenium(II) complex that was attributed to the surface plasmon resonance [97].

2.3 Heterogeneous Photocatalysts Composed of AgNP Ag@AgX (X = Cl, Br)

Silver halides such as AgCl and AgBr are another class of semiconductor supports, which have been extensively used in photographic films due to their photosensitivity. The band gaps, Eg, of AgCl and AgBr are 3.25 and 2.69 eV, respectively [26, 38, 98–100]. Silver halides have been mainly used to host AgNP. Although, the synthesis of silver halide nanostructures has not been as well studied as that of AgNP [101], the synthesis of heterogeneous photocatalysts composed of AgNP Ag@AgX (X = Cl, Br, I) has been extensively investigated over the last decade.

The photocatalytic activity of Ag@AgCl nanoparticles fabricated with different morphological structures was recently shown to extend to visible light due to the SPB of AgNP [102–113]. Various morphotypes of the same NPs have been explored including cubes [106, 109, 112, 113], cubic cages [114], wires [103] films [110] and other shapes [115, 116]. Cubic Ag@AgCl have higher specific surface area, more active sites and facets as compared with spherical counterparts of similar size, all of which are favorable to the enhancement of photocatalytic activity [108]. However, enhanced photocatalytic activities have been reported when using red Ag@AgCl photocatalysts due to their even larger surface area [111].

AgNP supported on AgCl (Ag@AgCl) with high stability and high photocatalytic efficiency under illumination have been used to disinfect water and decompose organic pollutants such as phenol, and the dyes; 2,4-dichlorophenol, methyl orange (MO) [103, 107, 109–114], orange 7 [109], rhodamine B and methylene Blue [106]. These nanocomposites also show reductive activities such as, conversion of CO2 into liquid carbon fuels (methanol and ethanol) [111, 115] and reduction of CrVI to CrIII [111, 117].

Upon UV light excitation, a silver halide particle generates an electron-hole pair. The electrons migrate to the surface of the nanoparticle, while the holes migrate to the surface of the AgCl. Subsequently, the photogenerated electron combines with an Ag+ ion to form an Ag0 atom [107]. Meanwhile, the photogenerated holes within the silver halide photocatalysts are able to oxidize halide ions to halide atoms (i.e. Cl− ions to Cl0 atoms (radicals)), which are strongly oxidative for organic molecules, finally being reduced back to Cl− ions [107, 109].

The electrons would be trapped by O2 in solution to form superoxide ions and other reactive oxygen species (ROS) that could also promote the decomposition of dyes [4, 109, 118].

Another similar photocatalyst, AgNP supported on AgBr (Ag@AgBr), catalyzed the decomposition of MO more efficiently than Ag@AgCl under visible light irradiation [116, 119, 120]. Also, reduction of CO2 to methanol has been observed by plasmon mediated catalysis with Ag@AgBr hexagonal nanoplates [115]. Note AgBr nanoparticles that have shown face-dependent photocatalytic properties for example in the degradation of MO [121]. In this chapter only those semiconductor halides containing AgNP are reviewed. We refer to other papers and the references cited therein for further details on AgX and AgX@semiconductor nanocomposites photocatalytic activity [111, 121].

The photocatalytic activity of Ag@AgI spherical nanoparticles has been recently studied and effectively show antibacterial activity against both E. coli and S. aureus [122]. In all three cases, Ag@AgCl, Ag@AgBr and Ag@AgI plasmon mediated catalysis, the surface of AgX nanoparticles is terminated by X− ions and therefore, is supposed to be negatively charged [107, 109, 119]. Consequently, AgNP on their surface should polarize the electron distribution such that the regions of its negative charges are far from the Ag/AgX interface, whereas the positive ones are close to it [109]. Then, under visible light illumination, where the SPB of AgNP absorbs, AgNP produce excited electrons and holes [107, 109, 119], thus promoting the photoinduced electron ejection from AgNP into the conduction band of AgX [111, 119]. This is possible because the Fermi energy level of AgX is lower than that of AgNP. Therefore, electrons are transferred from AgNP to AgX until the two systems attain equilibrium and form the new Fermi level [119].

Scheme 3
scheme 3

Schematic diagram for the charge separation in a visible-light irradiated Ag/AgBr/TiO2 system. Adapted from Ref. [124]

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These photogenerated electrons in the conduction band, together with the injected SPR electrons from AgNP, could then initiate the catalytic reaction. Obviously, the interfacial junction between AgNP and AgX could facilitate the charge separation in SPR-excited AgNP and produce long lived charge carriers from the AgNP [119].

As explained above, the holes transferred to the AgX will combine with halide ions X− ions to form X0, which will oxidize organic molecules and then reduce back to X−. In the absence of organic pollutants, X0 can react with OH− to form •OH [109, 123]. The electrons would generate superoxide ions and other reactive oxygen species after reacting with O2 [111].

2.3.1 Heterogeneous Photocatalysts Composed of AgNP Ag/AgX (X = Cl, Br, I) Loaded on Other Supports

Relevant studies based on this type of photocatalysis mechanism have been widely reported; examples include loaded Ag@AgX (X = halide) hybrid structures on TiO2 [98, 124–126], Al2O3 [100, 127], ZnO [128], BiOBr [129], WO3 [116], Ag3VO4 [130], and graphene (also sulphonated graphene [131] and graphene oxide (GO)) [101, 108, 132].

These nanohybrids have proven useful to degrade organic pollutants such as chlorophenols [100, 124, 127], MO [98, 124, 128, 129], rhodamine B [128, 130], acid orange 7 [124], azo-dyes [125], MB [130], volatile organic compounds (VOCs) in the gas phase (benzene and acetone) [126] and also bacteria [116, 125, 133].

The mechanism that Ag@AgX nanoparticles loaded onto semiconductors undergo upon visible light irradiation is depicted in Scheme 3 (Br and TiO2 have been chosen as example) [98, 124]. The excited electron was transferred to the conduction band of TiO2, and a hole was subsequently recovered when the bromide ion and bromide radical were formed.

As discussed above, the oxygen molecule can trap the electron injected into the conduction band of the TiO2, and form superoxide ions, which promoted the photooxidation reaction. Simultaneously, the highly active bromide radical decomposed the organic molecules near the particles [124].

In another recent contribution, AgNP were used to induce visible light photocatalytic reduction of GO leading to reduced graphene oxide (rGO) in the presence of an electron donor [134]. Indeed, AgNP have also been used as a bridge to connect activated GO and Ag2Mo3O10 or Ag3PO4 [101, 135]. This strategy was also described using Ag@AgX nanoparticles [108, 136–138].

Both, GO or rGO can act as an electron acceptors to effectively suppress the charge recombination, resulting in more reactive species [139]. The advantages of their hybridization can be found in most silver halide and phosphate cases [135–138, 140, 141]. When combined with semiconductors, the photoactivity enhancement of semiconductors after hybridization with rGO or GO has been attributed to a 2D network with an electron sink to accept and shuttle electrons photogenerated in the semiconductors [142–144].

Thus, for Ag2Mo3O10, a reinforced charge transfer and a suppressed recombination of electron hole pairs result from hybridization of Ag@AgX with GO nanosheets, and contribute to the enhanced photocatalytic activity in the degradation of methyl orange [108].

Silver molibdates such as Ag2Mo3O10 Ws can also generate an electron/hole pair and to reduce Ag+ ions to Ag0 particles under illumination, leading to a cluster of silver atoms on an Ag2Mo3O10 Ws backbone [101]. However, although they show lower photocatalytic activity than silver salt-based photocatalysts, Ag@Ag2Mo3O10@GO nanohybrids are able to photodegrade rhodamine B an 4-chlorophenol [101].

The proposed mechanism for the activity of Ag0 is as a solid-state electron mediator that accepts electrons from activated GO and Ag2Mo3O10 Ws (Scheme 4) [101].

Scheme 4
scheme 4

Photocatalytic process of Ag@Ag2Mo3O10/activated graphene oxide composite through assistance of solar light. Adapted from reference [101]

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Interestingly, Wang et al. [145] have just reported that Ag@rGO@TiO2 nanocomposites can act as photocatalysts to generate hydrogen and are also able to photodegrade RhB. The authors claim that in this system the photogenerated electron separation is improved. Different sizes of AgNP were tested. Small sized AgNP (2–5 nm) could store a photoexcited electron that was generated from TiO2 leading to photocatalysis improvement; whereas, large sized AgNP were able to absorb visible light. Scheme 5 illustrates the two possible pathways for the photocatalytic process in these Ag@rGO@TiO2 composites.

Scheme 5
scheme 5

Proposed mechanism for the photodegradation of RhB and photocatalytic hydrogen production (in brackets) by Ag@rGO@TiO2 under a UV part and b visible part of the simulated sunlight irradiation. Adapted from from reference [145]

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3 Positive Plasmon Mediated Catalysis

Catalysis is helping to improve chemical conversion, energy production and pollution mitigation. However, most commercial heterogeneous catalytic reactions are being run at relatively high temperatures [90].

Many interesting and useful PMC reactions have been reported using AgNP combined with supports and lower temperatures than their conventional reactions that use only thermal activation [90]. Up to now, water splitting [11], photopolymerization [146–149], isomerization [150], and synthesis of organic compounds [31] have been carried out. Regarding positive PMC, e.g. PMC to make or transform chemicals without destroying them, only CO [90], NH3 [90] and alcohol oxidations [45, 90, 151], epoxidations [90, 152] and coupling of p-aminobenzenethiol to 4,4'-dimercaptobenzene [112, 153–162] have been explored. Previous examples along this chapter illustrate that most efforts are being devoted to AOPs.

Similar mechanisms for PMC either with AuNPs or AgNP have been described under UV and/or visible light illumination. For AgNP there is an enhanced activity under UV irradiation as compared with visible irradiation, which is sometimes difficult, as explained before, due to the oxidation and/or dissolution of silver. In this chapter, only those examples that use AgNP are mentioned. It is important to remind that: (i) a linear dependence between the temperature and photocatalytic reaction rate at constant light intensity has been observed; (ii) photocatalytic reaction rates on excited MNPs exhibit an intensity dependent transition from the linear to super-linear regime, which indicates an electron-driven chemical reaction [163]. Moreover, this super-linear power law dependence on light intensity takes place at significantly lower intensity than required for super-linear behavior on extended metal surfaces and; (iii) the photocatalytic reaction rate at a given temperature can be obtained by subtracting the rate of the pure thermal process [32]. Readers are strongly recommended to get more insight about linear and super-linear dependences in references [32] and [163] and references cited therein.

After AgNP absorb light, heat is generated and an electron transfer to available adsorbate (reactant) states can occur, forming a negative ion species or a radical. This negative ion can, can either undergo a rapid reaction on the metal surface [90] often referred to as the transient negative ion (TNI) [11, 164–169]; or diffuse to the solution where it could react [45].

This process is very similar to the above-discussed electron injection from an excited AgNP with a nearby semiconductor (i.e. TiO2, AgX), except that the electron is injected in “adsorbate” states rather than in a semiconductor conduction band [11]. Note that, if the incident light is in the UV range, the formation of charged ion adsorbates could also be initiated by photon-induced interband transition in silver [45].

The interaction of excited plasmons with reactants will be affected by the spatially non-homogeneous distribution of plasmons on the surface of nanoparticles. Therefore, hot spots might play a critical role [11].

Thus, if AgNP are illuminated in the presence of an adsorbate, for example oxygen, the O–O bond is activated at lower temperatures due to a photothermal (electron–phonon driven) elementary step [90]. Scheme 6 contains more detailed information of the formation and subsequent relaxation of the O2–TNI on the AgNP surface, leading to the dissociation of the adsorbed molecular O2 [11, 21, 32, 90].

Scheme 6
scheme 6

Proposed molecular mechanism in the linear regime. A single electron excitation deposits vibrational energy into the adsorbate by accelerating the molecule along the TNI potential energy surface (PES) for the lifetime, τe. If vibrational energy is not sufficient to overcome the activation barrier, Ea, the adsorbate returns to the thermally equilibrated state. Reprinted with permission from reference [32]. Copyright 2011 Macmillan Publishers Limited

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Another mechanism, where a AgNP can transfer electron density to O2 either directly through chemical interface damping or indirectly through the decay of plasmons into energetic electron/hole pairs and subsequent transfer of the e− to O2 was proposed by Brus [23]. Irrespective of the mechanism, electrons can transiently populate unoccupied states (orbitals) of adsorbed O2 and induce electron-mediated O2 dissociation [23]. Thus, plasmon mediated electron transfer from AgNP to the O2 2π* orbital forms a transient negative O2 − species [32, 170, 171].

Factors that influence the capacity of AgNP to drive photocatalytic transformations have been postulated to include:

  • (i) temperature-dependent distribution of excited vibrational states, which significantly affects the probability for the plasmon-mediated formation of TNI, and the probability that the TNI will gain sufficient energy to overcome the reaction barrier; and

  • (ii) also, much lower activation barriers for chemical transformations on metals as opposed to semiconductors. This allows for lower adsorbate energy required to overcome the activation barrier. Remarkably, Linic et al. suggested that this is the main reason why fairly short-lived TNI on metals (on the order of a few femtoseconds) can induce chemical transformations [11].

AgNP supported on insulators such ZrO2, Al2O3 and SiO2 or Zeolite Y were found to be highly efficient photocatalysts under visible light for the selective oxidation of benzyl alcohol (Scheme 7, Table 1) [45]. Supports cannot absorb visible light, due to their wide band gaps and there is no electron transfer from the irradiated AgNP to the insulator supports.

Ag@TiO2 with oxygen vacancies gave rise to benzaldehyde under visible light irradiation although better results were reported when using palladium nanoparticles instead of AgNP [51]. The SPB of Ag@TiO2 cannot be excited with UV. Therefore, upon UV illumination, AgNP on TiO2 trap the photoexcited electrons from the surface of TiO2, prolonging the lifetimes of holes and electrons [151]. The holes remaining on the surface of TiO2 can oxidize absorbed methoxy coming from the thermal dissociation of methanol on TiO2 to HCHO and HCO. The residual methoxy couples with HCO to form methyl formate [151, 172].

Scheme 7
scheme 7

Oxidation of benzyl alcohol to benzaldehyde

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Table 1 Oxidation of benzyl alcohol to benzaldehyde
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Notably, AgX NPs are also promising materials to reduce CO2 under visible light irradiation due to the more negative conduction band (CB) edges than φΘ(CO2/CH3OH) and φΘ(CO2/C2H5OH) (0.38 and 0.085 eV vs. NHE) [111, 112, 115].

Ag@Al2O3 nanocomposite has also proven to be an active photocatalyst under visible light irradiation for the selective oxidation (epoxidation) of ethylene to form ethylene oxide (EO), which is a commercially important reaction [32, 90]. The dependence of the photocatalytic rate on the light intensity confirmed that the energetic electrons excited by the LSPR effect of AgNP, directly participate in the photocatalytic process of this oxidation.

Plasmonic excitation of AgNP influences protonation on photoreactions of p-aminobenzenethiol (PATP) leading to its coupling reaction product, 4,4'-dimercaptoazobenzene (DMAB) [153]. In the absence of O2 or H2, the plasmon-driven photocatalysis mechanism (hot electron—hole reactions) is the major reaction channel. In the presence of O2 or H2, the plasmon-assisted surface catalysis mechanism (activated oxygen/hydrogen reactions) is the major one [154]. Since experimental and theoretical evidence on this surface catalyzed reaction of DMAB produced from PATP, assisted by AgNP in 2010 [173], a series of investigations have been reported to support, or further confirm the conclusions [112, 155–162].

Last but not least, plasmonic mediated water splitting caused by visible light and improvement of photovoltaic devices have attracted attention among scientists [10, 11]. The reader can consult references [10, 11, 38, 174–179] to find more detailed information regarding this topic.

4 Concluding Remarks

MNPs such as AgNP are currently contributing to the development of greener technologies that are based on light driven applications. In this chapter we reviewed several examples of AgNP-support composites that are being used for photocatalysis and plasmon mediated catalytic reactions. Many of them are reusable without losing activity and remain stable under the reaction conditions. Although much effort is being placed in advanced oxidation processes, it is reasonable to assume that significant development will take place in the area of synthetic chemistry promoted by plasmon mediated catalysis. This will allow scientists to utilize the full range of solar energy to activate this new class of photocatalysts for both synthetic and AOPs processes in the near future. This green approach would help to save energy and design more environmentally friendly photocatalytic processes.