Recent advancements in plasmon-enhanced visible light-driven water splitting

Qingzhe Zhang; Deepak Thrithamarassery Gangadharan; Yanlong Liu; Zhenhe Xu; Mohamed Chaker; Dongling Ma

Accepted Manuscript

Recent advancements in plasmon-enhanced visible light-driven water splitting

Qingzhe Zhang, Deepak Thrithamarassery Gangadharan, Yanlong Liu, Zhenhe Xu, Mohamed Chaker, Dongling Ma

Materiomics

PII: S2352-8478(16)30098-3

DOI: 10.1016/j.jmat.2016.11.005

Reference: JMAT 80

To appear in: Journal of Materiomics

Received Date: 10 September 2016 Revised Date: 14 November 2016 Accepted Date: 21 November 2016

Please cite this article as: Zhang Q, Thrithamarassery Gangadharan D, Liu Y, Xu Z, Chaker M, Ma D, Recent advancements in plasmon-enhanced visible light-driven water splitting, Journal of Materiomics (2016), doi: 10.1016/j.jmat.2016.11.005.

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The most recently advancements in visible light-driven plasmon-enhanced water splitting were reviewed according to different enhancement mechanisms. Moreover, the influencing factors, such as shape, size and geometric position of metallic nanostructures, in plasmonic-metal/semiconductor system were also discussed in detail.

Recent advancements in plasmon-enhanced visible light-driven water splitting

Qingzhe Zhang a,15 Deepak Thrithamarassery Gangadharan a,15 Yanlong Liu a,15 Zhenhe Xu a,b,*5

Mohamed Chaker a, Dongling Ma a,**

a Institut National de la Recherche Scientifique (INRS), Centre Énergie Materiaux et Télécommunications, Université du Québec, 1650 Boulevard Lionel-Boulet, Varennes, Québec J3X1S2, Canada;

b The Key Laboratory of Inorganic Molecule-Based Chemistry of Liaoning Province, College of Applied Chemistry, Shenyang University of Chemical Technology, Shenyang, 110142, P.R. China

* Corresponding author.

** Corresponding author.

Email addresses:xuzh056@163.com (Z. Xu), ma@emt.inrs.ca (D. Ma). Phone:514-228-6920; Fax:450-929-8102. 1 These authors contributed equally to this work.

Abstract

Recently, the combination of plasmonic noble metallic nanostructures with semiconductors for plasmon-enhanced visible light-driven water splitting (WS) has attracted considerable attention. This review first presents three prime enhancement mechanisms for plasmon-enhanced photocatalytic WS, and then some state-of-the-art representative studies are introduced according to different enhancement mechanisms. Furthermore, the design parameters of plasmonic-metal/semiconductor photocatalysts are discussed in detail, focusing on the effect of shape, size and geometric position of metallic nanostructures on the photocatalytic activity of visible light-driven WS. Finally, the challenges and perspectives for plasmon-enhanced solar WS are proposed.

Key words: surface plasmon resonance; water splitting; visible light photocatalysis

1. Introduction

Solar water splitting (WS) is one of the most promising ways to store solar energy into other useful energy forms [1,2]. In WS, solar energy is converted to chemical energy mainly in the form of hydrogen. In comparison with solar-to-electrical energy conversion, solar energy stored in chemical bonds can be released on demand [3]. Even though there has been a great progress made in the research field of WS, its large scale industrial applications are still lacking due to low photocatalytic WS efficiency. To the best of our knowledge, the highest reported quantum efficiency for overall water splitting is 57 % achieved with the NiO/NaTaO3:La photocatalyst under the excitation wavelength of 270 nm [4,5]. UV light only accounts for ~5% of incident solar spectrum, so the high solar-to-hydrogen efficiency could not be obtained in this system. For the visible light-driven photocatalysts, the maximum quantum yield of 16% was achieved over a carbon nanodot-carbon nitride (C3N4) nanocomposite at 420±20 nm [6]. The efficiency is still far from the tentative goal for overall water splitting at 600 nm, with the quantum yield and solar-to-hydrogen efficiency set at 30 % and 5 %, respectively [5,7]. Inadequate visible light response of photocatalytic semiconductors is one of the major reasons causing such low efficiencies. Metal nanostructures can interact with ultraviolet (UV)-visible light through surface plasmon resonance (SPR), giving rise to improved light absorption and enhanced exciton generation in semiconductors nearby. This phenomenon can be utilized to improve the photocatalytic efficiency via appropriately coupling plasmonic metal nanostructures with semiconductors. Over the past decade, considerable efforts have been made to improve the photon absorption using the strong plasmon resonance of Ag or Au nanostructures in the visible range [8-13]. In this type of semiconductor/Au (or Ag) composite photocatalysts, the light response can be tuned and enhanced by engineering the shape and size of the metal nanostructure since the SPR property highly depends on such structural parameters [14-18].

In this review article, we start with a general introduction on photocatalytic WS and different mechanisms involved in plasmon-enhanced photocatalytic WS, followed by a summary of the important progress made in the semiconductor/plasmonic-metal catalysts in the past decade. Further, we discuss critical experimental parameters influencing plasmon-enhanced photocatalytic WS, such as the size, shape and geometric positions of the metal nanostructures.

2. Plasmon-enhanced photocatalytic WS

Semiconductors can act as photocatalysts for various chemical transformations induced by UV-visible light, including splitting of water [19,20]. Photogenerated excitons (i.e., electron-hole pairs) can dissociate and resultant charge carriers can diffuse to catalytic active sites at the semiconductor/liquid interface for respective reactions. In photocatalytic WS, holes are involved in the oxygen-evolution, whereas electrons in the hydrogen-evolution [21,22]:

H2O + 2h+ ^ 2H+ + %O2 (1)

2H+ + 2e- ^ H2 (2)

The net reaction is endothermic and converts solar energy into chemical energy.

Two typical configurations have been implemented to execute photocatalytic WS. One configuration employs a working electrode consisting of semiconductor material deposited on a conductive substrate connected to a counter electrode (usually platinum (Pt)) through an external circuit (Fig. la). Upon photoexcitation, when a n-type semiconductor is involved (p-type semiconductor can also be used), photogenerated holes will diffuse to the semiconductor/liquid interface, where they take part in the O2 evolution reaction and photogenerated electrons move to the counter electrode to be involved in the H2 evolution reaction. This is known as photoelectrochemical WS cell design. In an alternative system design, photocatalytic particles dispersed in suspension can be used to perform both half-reactions at the specific surface sites of catalyst particles (Fig. lb). The main criterion used for choosing a semiconductor for photocatalytic WS is based on valence band (VB) and conduction band (CB) potentials of semiconductor with respect to the redox potentials of the WS half-reactions (versus normal hydrogen electrode (NHE)). For the thermodynamically favorable WS reactions to occur, the CB edge should have a more negative potential than the H2-evolution potential and the VB edge should have a more positive potential than the O2-evolution potential (Fig. lc).

External circuit e'-

H, evolution

H-j h30

J A v-ounier-Semiconductor j electrode Current coflector_

(electrolyte)

°2+H* evolution <VH*

co-catalyst

oo £ 2

. CdSe

SrTi03 wo.

"J"T.....-2H+ + 2e_ -» H2

-- H20 + 2h+ -» h02 + 2H+

'2 KTaO,

Fig. l. (a) Photoelectrochemical cell design for WS (processes for an n-type semiconductor are shown). Under illumination, holes diffuse to the semiconductor surface and drive the oxygen-evolution half-reaction (2H2O + 4h+ ^ O2 + 4H+) and electrons are collected and travel to the counter electrode where they drive the hydrogen evolution half-reaction (2H+ + 2e- ^ H2). (b) Particle-based WS photocatalyst. Excited charge carriers (both electrons and holes) diffuse to the particle surface where they drive the two half-reactions, VB, valence band; CB, conduction band. c) VB and CB for a range of semiconductors on a potential scale (V) versus the normal hydrogen electrode (NHE). Reproduced with permission from Ref. [22]. Copyright © 2011 Macmillan Publishers Limited.

In particle-based WS system design, high volumetric generation of reactive species is possible compared to photoelectrochemical WS cell design. In addition, it is considered to be a simple and cheap solution for H2 generation, more relevant to its future economic, large-scale implementation. However, particulate photocatalytic system is in general less efficient because of difficulties in charge carrier separation. Separation of generated H2 and O2 represents another issue. In the case of photoelectrochemical WS cell deign, redox reactions take place at two different locations or electrodes so it clearly has an advantage in charge separation with reduced recombination. In recent years, particulate photocatalytic system has gained more and more attention because it has advantages of being much simpler and less expensive.

For the photoelectrochemical water splitting, the photocurrent and/or energy conversion efficiency are always used to evaluate the performance. For photocatalytic water splitting, being carried out in particle systems, the "gas production vs time" is more suitable for the assessment of photocatalytic activity. For the performance evaluation of the photocatalysts and photoelectrodes, the solar hydrogen production efficiency and diagnostic efficiencies (including, for example, quantum efficiency at a specific wavelength), highly depending on the configuration used for the water splitting, were reviewed in detail recently [23,24].

For the development of affordable and widespread commercial application of this technology, with no doubt highly efficient photocatalysts should be developed. One of the major disadvantages of semiconductor photocatalysts (such as the most commonly used TiO2 and Fe2O3) is inefficient absorption of photons across the entire UV-visible region of the solar spectrum and subsequently inefficient separation of excitons into useful electrons and holes. Wide bandgap materials like TiO2 only can mainly absorb photons in the UV region of the solar spectrum, which merely represents ~5% of the solar radiation (Fig. 2) [25]. In the case of lower bandgap Fe2O3, although its absorption can be extended into the visible range, it still suffers from limited photon absorption in the visible regime, along with another disadvantage of very low charge carrier mobility.

300 400 500 600 700 800

Wavelength (nm)

Solar Spectrum

CD O £= 03 _Q

To overcome the inherent problems of semiconductor photocatalysts, a composite plasmonic-metal/semiconductor photocatalyst can be used. During the past decade, a significant number of articles reporting on the improved photocatalytic activity by plasmons have emerged [26-29]. Plasmonic metal nanostructures can interact with light through localized surface plasmon resonance (LSPR), which arises from the collective oscillation of electrons confined to the surface of the metal nanostructure. In a composite plasmonic-metal/semiconductor photocatalyst, LSPR can play a central role in boosting the rate of O2- and H2-evolution half-reactions by transferring energy or hot electrons from the metal to the semiconductor [30-32]. In addition to LSPR, plasmonic metal nanostructures may also act as electron reservoirs for photo-induced charge carriers in the semiconductor and thereby extend the lifetime of photogenerated charge carriers.

There are mainly three plasmon-related enhancement mechanisms, namely light scattering mechanism, near-field mechanism, and hot electron transfer mechanism. A brief presentation and discussion on these three mechanisms are essential to understand plasmon-enhanced photocatalytic WS.

3. Mechanisms involved in plasmon-enhanced photocatalytic water-splitting

3.1 Light scattering mechanism

Enhanced light scattering due to the presence of plasmonic metal nanoparticles (NPs) can be associated with a radiative energy transfer process from the plasmonic to semiconductor materials, which can be used to enhance the efficiency of WS. Basically, the enhanced light scattering increases the average photon path length in plasmonic-metal/semiconductor composites and thereby enhancing the formation of charge carriers in the semiconductor that can take part in WS reaction (Fig. 3a). Contribution from this aspect highly depends on the light scattering efficiency of plasmonic nanostructures, which can be estimated by calculated light scattering-to-absorption ratios. A high scattering-to-absorption ratio is a very important condition to be satisfied for utilizing photons effectively to enhance the efficiency of plasmon-enhanced WS.

The scattering cross section (csca) and absorption cross section (Cabs) are expressed as [33],

Cabs = 47tka3Im[^N (4)

where k = 2-kIX, X represents wavelength, a and s are the diameter and dielectric constant of a NP respectively, £m is the dielectric constant of surrounding medium and Im denotes the imaginary component of the dielectric constant. At the resonance frequency (where s becomes equal to —2£m) Csca is largely enhanced. From Equations 3 and 4, it can be seen that both Cscaand Cabs largely and directly depend on the size of metal NPs. Nonetheless, since the former scales as a6

and the latter a3, it can be straightforwardly anticipated that the ratio of light scattering to absorption strongly relies on the size. Larger particles will yield higher scattering-to-absorption ratios. As an example, the calculated Csca /Cabs of differently sized spherical NPs is shown in Fig. 3b. A ~80 nm sized NP predominately scatters incident electromagnetic field whereas a 20 nm sized NP mainly absorbs light and localizes the field [34].

Photon scattering

Optically . excited metal particles

0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0 -0.1

20 30 40 50 60 70 80

Nanosphcrc diameter D (nm)

Fig. 3 (a) Schematic illustrating the scattering mechanism. The addition of optically excited plasmonic NPs increases the average path length of photons in the composite structure. Reprinted by permission from Macmillan Publishers Ltd: Nature Materials [22], copyright 2011. (b) Variation of the calculated ratio Csca/Cabs as a function of the Au nanospheres (NSs) size. Reprinted with permission from Ref. [34]. Copyright © 2006 American Chemical Society.

In addition to the size of metallic nanostructures, shape and loading amount also determine the enhancement of WS efficiency by scattering. For instance, it has been observed that photoreaction rate was highest for Ag nanocubes compared to Ag NSs and nanowires mainly due to difference in their scattering efficiency [35]. Theoretical simulations further confirmed that the Ag nanocubes had the highest scattering efficiency.

3.2 Near-field mechanism

Enhanced near-field electromagnetic field can also be involved in the energy transfer process, by which photocatalystic WS can be enhanced and it is often known as plasmon resonance energy transfer (PRET). In a composite photocatalyst system, radiative energy from the SPR of the metal can be transferred to the semiconductor, lead to the generation of more electron-hole pairs in the semiconductor [36,37]. Photo-excited metal NPs can induce a significantly enhanced field extending to ~ 20-30 nm or even longer with the intensity falling off as 1/s4 at short distance and as 1/d6 at long distance, where s and d are the distances from the surface and center of the metallic NP respectively [37]. The much stronger electric field produced by photo-excited plasmonic nanostructures, with respect to the incident field, can increase the rate of electron-hole pair formation in the semiconductor nearby since the rate of electron-hole formation is proportional to | E | 2 [38,39]. A strong near-field electromagnetic resonance would also be beneficial to reduce the charge carrier recombination in a semiconductor by allowing the use of a thinner layer of the semiconductor with metal NPs

situated at the semiconductor/liquid interface (Fig. 4). Such energy transfer can occur even in the presence of a thin non-conductive layer between the plasmonic metal and semiconductor [40].

Fig. 4. (a) Illustration of the phenomenon where electron-hole pairs generated deep under the surface of the semiconductor recombine easily, (b) Illustration of the situation when plasmonic metals are loaded at the surface of the semiconductor. The light-absorption layer becomes thinner where electron-hole pairs are generated at a high rate as a result of near-field electromagnetic resonant energy transfer. The generated electron-hole pairs take part in the reactions easily owning to their shorter migration length. Reprinted with permission from Ref. [37]. Copyright C> 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Wemheim.

Quantum efficiency of PRET is described as below [3,41]:

1l.PRET

-I* — 1

where the T{~jiRET is rate of PRET, T^r is non-radiative decay, Tjy is radiative decay and T is elastic dephasing rate. The three terms in denominator of the above equation are competing processes to the rate of PRET. The sum of these three competing processes is termed as time constant T2. For quantum efficiency to be unity, T^pRET should be shorter than competing processes. It should be noted that T2 is longer for the smaller metal NP; apart from the size of the metal NP, shape and composition also influence the ??pret-

Most of the experimental results suggest that energy of the near-field should be above the bandgap of the semiconductor for the enhanced generation of electron-hole pairs through PRET. However, Wu and co-authors observed unprecedented plasmonic enhancement with an energy below the bandgap [40]. They well explained their observation by proposing that plasmon-induced resonance energy transfer (PIRET) is indeed a non-radiative process and relies on the dipole-dipole interaction between the semiconductor and metallic NP [42]. For utilizing PIRET in plasmon- enhanced WS, it should meet two important requirements: 1) the distance between plasmonic metal and semiconductor needs to be short, 2) the SPR band of a plasmonic material needs to overlap the intrinsic absorption region of the semiconductor. Very recently, such a process was also found to play a dominant role in Au nanostar-enhanced quantum dot based solar cells [43].

3.3 Hot electron transfer mechanism

Combination of plasmonic metallic NPs showing the SPR in the visible range with wide bandgap semiconductor photocatalysts can extent semiconductors' light absorption range towards visible wavelengths by the so-called hot electron mechanism. In a metallic NP/semiconductor composite, metal NPs can behave as sensitizers, absorbing incident photons and transferring the excited, higher-energy electron (i.e., hot electrons) to the nearby semiconductor [44]. In metallic NPs, there is no HOMO-LUMO or analogous VB-CB separation and plasmonic charges reside at the Fermi energy level. A continuous upshift of electronic states (surface plasmon states) was observed in metal NPs upon photoexcitation, resulting in hot electron injection into the CB of semiconductor (Fig. 5). These hot electrons can then be used for photocatalysis. It should be noted that, hot electron injection only becomes possible when electrons can gain sufficient energy to overcome the Schottky barrier formed between the metal NP and semiconductor through their direct contact. Hot electron injection stops as soon as surface plasmon energy dissipates and restores excited electronic states back to the Fermi level. Hot electron injection efficiency depends on multiple factors, including the size, shape, and composition of the metallic NPs, the physical and electrochemical properties of the semiconductor, as well as the boundary conditions between them [45]. For instance, it was found that the photogeneration of charge carriers becomes more energetic when metal NPs are quite small with the radius in the range of 1-4 nm [46]. Regarding the shape, NPs with sharp tips or corners can be beneficial for hot electron injection [47]. For example, a metal nanocube is found to be more e Orient for plasmonic hot electron generation than a NS. Plasmonic field enhancement and the inhomogeneity of electric fields inside a metal nanostructure, both of which strongly depend on the shape, can amplify the photogeneration of charge carriers. Geometry of the metal nanostructure can also influence hot electron generation via geometry-induced intraband transitions that result in the generation of "hotter" electrons via nanoconfinement effects. In this case, anisotropic momentum-direction distributions of hot carriers are affected by crystal orientation and plasmon polarization of metal [37].

Plasmon-induced charge transfer

Fig. 5. Mechanism of SPR-induced charge transfer with approximate energy levels on the NHE scale. Dashed red lines refer to the water-splitting redox potentials. (i) Electrons near the metal Fermi level, Ef are excited to surface plasmon (SP) states; (ii) the electrons transfer to a nearby semiconductor particle; (iii) this activates electron-driven processes such as the hydrogen-evolution half-reaction. Reprinted by permission from Macmillan Publishers Ltd: Nature Materials [22], copyright 2011.

The exact mechanism of the generation hot charge carriers and their kinetics are not fully understood and require more investigations. Further, controlling the interface of the metal NP and semiconductor is very important to reduce the recombination of charge carriers and to more efficiently use plasmon generated energetic carriers.

4. Advancements in plasmon-enhanced WS under visible light

Since Liu et al. [10] reported the use of a plasmonic photocatalyst and resultant enhanced photocurrent generation for WS, considerable attention has been paid on hydrogen generation via plasmon-enhanced WS under visible light illumination [48]. In this section, we will review the recent advancements in plasmon-enhanced WS according to the abovementioned major enhancement mechanisms. In particular, since the light scattering and near-field mechanisms are often present simultaneously and are not differentiated in many published articles, we categorize our examples according to i) light scattering and near-field enhancement and ii) hot electron transfer. The focus is put on the actively studied structural parameters in plasmonic-metal/semiconductor photocatalysts. Specifically, the effect of shape, size and geometric position of plasmonic metals on plasmon-enhanced WS under visible light will be discussed in detail.

4.1 Recent studies on plasmon-enhanced WS with different enhancement mechanisms

4.1.1 Light scattering and near-field enhancement

Sun and co-workers [49] coated ZnO nanorod (NR) using Ag film with various thickness on a polyethylene terephthalate flexible substrate, which was then used as a photoanode for photoelectrochemical WS. When the thickness of the film was less than 30 nm, the film was composed of many small Ag islands that were distributed on the surface of ZnO NRs. Under the visible light (400-800 nm) irradiation, compared with bare ZnO NRs, the ZnO NRs coated with the Ag film exhibited distinctly enhanced light absorption. It was mainly the LSPR effect of Ag islands and the dipole-dipole scattering that contributed to the enhancement in light absorption. The enhanced light scattering with the presence of the Ag islands lengthened the path of light propagation, improving the light trapping and absorption of ZnO NRs. Augustynski et al. [50] fabricated a Ag NP/WO3 film photoanode by depositing Ag NPs onto WO3 films coated on fluorine-doped tin oxide (FTO) glass. Under the illumination of simulated AM1.5, the Ag NP/WO3 photoanode showed a plateau photocurrent of 2.1 mA cm-2, which was about 1.6 times that for pristine WO3 photoanode without Ag NPs. The increase in photocurrent of the Ag NP/WO3 film photoanode associated with WS was attributed to the combination of light scattering and local electromagnetic-field effect mediated by the LSPR of Ag NPs.

The well dispersed triplex Ag@SiO2@TiO2 core-shell photocatalysts with different thickness of SiO2 interlayer and TiO2 shell were designed and fabricated by Zhu et al. [51], which exhibited significant photocatalytic activity under visible light. The photocatalysts with a 20 nm TiO2 shell and 2 nm SiO2 interlayer showed the highest photocurrent density, which was ca. 38 times higher than P25 under visible light irradiation (A > 400 nm). The enhanced electric field intensity induced by the LSPR of Ag gave rise to the efficient formation of electron-hole pairs in TiO2. In addition, when the characteristic length of Ag nanostructures was more than 40 nm, the overall extinction spectrum of Ag@SiO2@TiO2 was contributed by the pure TiO2 absorption (directly associated with the formation of useful electron-hole pairs) and the scattering from Ag. The scattering effect of Ag nanostructures prolonged the transport path of photons and further enhanced the light absorption of TiO2, leading to concentration increase in electron-hole pairs.

Liu et al. [10] reported for the first time in 2011 in utilizing plasmonic photoelectrodes, which were fabricated by depositing Au NPs onto TiO2 films, as photoanodes for photoelectrochemical WS [48]. The photoelectrochemical WS was performed in 1 M KOH solution using a three-electrode system, with the Au NPs/TiO2 films, graphite electrode and Ag/AgCl electrode as the working, counter and reference electrodes, respectively (Fig. 6a). Compared to bare TiO2 films with no Au NPs, the photocurrent value of Au NPs/TiO2 films, which can be used to quantitatively determine H2 production, was enhanced by the factor of 66 under visible light (A = 633 nm) irradiation (Fig. 6b). While the energy of the irradiation (1.96 eV) was significantly below the bandgap of TiO2 (3.2 eV), the photocurrent was still greatly enhanced, indicative of the LSPR effect of Au NPs on photoelectrochemical WS reaction. The authors performed the electromagnetic response simulations of Au NPs/TiO2 films by using the finite-difference time-domain (FDTD) method, confirming that it was the local electromagnetic field enhancement near the TiO2 surface, rather than the direct hot electron transfer from the Au NPs to TiO2 films, that contributed to the large improvement of photoelectrochemical WS activity under visible light illumination. The near-field optical enhancement effectively facilitated the generation of electron-hole pairs at the surface of TiO2. The research provided a possible route to achieve more efficient conversion from solar energy to fuel. Chen et al. [52] synthesized Au NPs/TiO2 particles by using the photodeposition method for visible light-driven photocatalytic WS. The increased H2 production suggested that Au NPs not only acted as electron traps [11] and active sites for producing H2, but also made great contribution to the SPR enhancement to intensify the electromagnetic field at the interface between TiO2 and Au NPs. The intensified electric field effectively promoted the production of H2 in photocatalytic WS under the irradiation of visible light with appropriate wavelengths.

Time (sec)

Fig. 6. (a) The three-electrode configuration for photoelectrochemical WS under UV/visible light illumination; (b) Photocurrent of anodic TiO2 with and without Au NPs irradiated with X = 633 nm light for 22 s. Reproduced with permission from Ref. [10]. Copyright © 2011 American Chemical Society.

Also in 2011, Ingram et al. [36] reported the design and use of composite photoanodes, by depositing Au NPs or Ag NPs onto nitrogen-doped TiO2 (N-TiO2) electrodes, for photoelectrochemical WS. For these composite photoanodes, the plasmonic metal and N-TiO2 were separated by coating the plasmonic metal with a thin insulator layer of organic stabilizer molecules, which could prevent the direct electron transfer from metal to TiO2. As shown in Fig. 7a, the SPR band of Ag NPs appeared at around 400 nm, overlapping the absorption spectrum of N-TiO2, while that of Au NPs existed near 600 nm and there was no overlap with N-TiO2 absorption. Therefore, compared to the Au/N-TiO2 photoanode and bare N-TiO2 photoanode, the Ag/N-TiO2 photoanode exhibited much greater H2 and O2 generation (Fig. 7b) and photocurrent responses (Fig. 7c) under the visible light irradiation. The FDTD simulations showed the enhancement in the electromagnetic field of a 120 nm Ag cube in water (Fig. 7d). The intense electric field significantly facilitated the generation of electron-hole pairs on the surface of the N-TiO2 electrode in the vicinity of Ag NPs, accounting for the enhancement in photoelectrochemical WS of the Ag/N-TiO2 photoanode.

b j—i

300 400 500 600 700 800

Ag/N-Ti02 Composite / Au/N-Ti02 Composite

J 0.6 ^ 0.5

«0.1 o

■ Ag/N-Ti02

■ •

■ • * 8 f ■ 1 |N-Ti02

300 400 500 600 700

Wavelength [nm]

12 3 4

Illumination Time [hours]

□ I I I I I I I I

Ag/N-TiO, /

Au/N-TiO.

Ag only

(J 1000

50 100 150

Time [seconds]

c/[nm]

Fig. 7. (a) UV-visible extinction spectra of TiO2, N-TiO2, Ag/N-TiO2 and Au/N-TiO2. The inset shows difference spectra for Ag and Au (i.e., Ag/N-TiO2 or Au/N-TiO2 spectrum minus the N-TiO2 spectrum). (b) H2 and O2 (•) production upon visible illumination of N-TiO2 (black symbols) and Ag/N-TiO2 (blue symbols) photocatalysts, as measured by mass spectrometry. (c) Photocurrent responses (per macroscopic electrode area) upon illumination with a broadband visible light source (400-900 nm). (d) Average electric field enhancement around a Ag cube with an edge length of 120 nm as a function of the distance d from the cube, as calculated using FDTD simulations. Inset: Local enhancement of the electric field calculated from an FDTD simulation of a 120 nm Ag cube in water. Reproduced with permission from Ref. [36]. Copyright © 2011 American Chemical Society.

The same near-field effect was also reported by Wang et al. [53]. In that scenario, the Au/TiO2/Au nanosheets on Ti foil enhanced the photocurrent by 3 times compared to that of TiO2 electrodes in the absence of Au NPs in the wavelength range from 400 to 650 nm. Wu's group [42] fabricated a novel photoanode by incorporating a hematite NR array into a Au nanohole array pattern on FTO glass, as shown in Fig. 8a. The incorporated photoanode greatly improved the photoelectrochemical WS performance, enhancing the incident photon-to-electron conversion efficiency (IPCE) at 650 nm by 18 times as compared to the hematite NR array on bare FTO without Au (Fig. 8b). The dramatic enhancement was attributed to both enhanced light absorption due to the PIRET effect and reduced charge carrier recombination in the hematite NR photoanode thanks to the presence of the intensified local field. Most recently, they further reported that the incorporation of nitrogen-doped La2Ti2O7 (NLTO) with Au NPs and reduced graphene oxide nanosheets led to a much higher H2 generation rate in photocatalytic WS (Fig. 8c) [54]. The doping of nitrogen extended the light-absorption range of LTO to 550 nm, which overlapped with the LSPR band of Au NPs and thus allowed the occurring of the PIRET process that significantly augmented the photocatalytic WS activity.

400 500 600 700 800 Wavelength (nm)

0 2 4 6 8 10 Time (hr)

Fig. 8. (a) Scheme for the growth of the hematite NR array on the Au nanohole array; (b) IPCE spectra,the insert is the IPCE in 600-700 nm range. Reproduced with permission from Ref. [42]. Copyright © 2013 Macmillan Publishers Limited. (c) Hydrogen generated by the photocatalysts. Reproduced with permission from Ref. [54]. Copyright © 2015 American Chemical Society.

Lin et al. [55] designed a new nanosystem for WS by the assembly of Ag@Ag3(PO4)i-x core-nanoshell NPs on the support of ZnO NRs and it was the first time demonstration for photosensitizing ZnO with plasmonic Ag@Ag3(PO4)i-x in a nonsacrificial electrolyte. The Ag@Ag3(PO4)i-x/ZnO photoelectrodes exhibited excellent photoelectrochemical WS activity with a maximum photocurrent of 3.1 mA cm-2 at 0.6 V vs. Pt counter electrode under simulated AM 1.5 solar illumination. The photocurrent density of Ag@Ag3(PO4)i-x/ZnO photoelectrodes was about 6 times larger than that of bare ZnO NRs. The augment in photoelectrochemical WS activity displayed by Ag@Ag3(PO4)1-x/ZnO in the region of visible light from 400 to 590 nm was ascribed to the enhanced near-field amplitudes induced by the LSPR of the Ag core and the extended absorption edge at 550 nm contributed by the Ag3(PO4)1-x nanoshell.

4.1.2 Hot electron transfer enhancement

In some plasmon-enhanced photocatalytic WS systems, the enhancement in the activity of H2 production was attributed to hot electron injection from plasmonic metals to semiconductors. García and co-workers [27] in 2011 reported that Au NPs supported on P25 TiO2 (Au/TiO2) exhibited photocatalytic WS activity under visible light (laser at 532 nm or polychromatic light at X > 400 nm) irradiation, which was ascribed to the energetic electron injection from the Au

NPs to the CB of TiO2 photocatalyst. In this photocatalytic WS process, it was proposed that the holes remained in the Au NPs and the electrons in the CB of TiO2 gave rise to the production of O2 and H2, respectively. The study reported by Wei et al. [56] indicated that the lifetime of hot electrons induced by LSPR and transferred from Au NPs to TiO2 was 1-2 orders of magnitude longer than that of the electrons generated via UV excitation within TiO2. These long-lived electrons were involved in WS to evolve H2 under visible light (k > 515 nm) illumination, heralding their potential for solar energy conversion. Majima et al. [57] fabricated Pt-tipped Au NRs by selectively growing Pt on two ends of Au NRs, exhibiting enhanced H2 production activity under visible light illumination due to the plasmon-induced hot electron injection from Au NRs to Pt.

Different plasmonic-metal/semiconductor configurations were also realized for WS by other groups. Moskovits's group [8] developed a composite photoanode by depositing TiO2 on the top of Au NR arrays followed by loading an oxygen evolution catalyst (OEC). The Au NR/TiO2-OEC photoanode significantly improved the photocurrent under visible light as compared to that under UV light irradiation. When the LSPR of the Au NRs were excited by visible light illumination, hot electrons were generated and injected from the Au NRs to TiO2 and then further transported to the Pt electrode to be involved in the H2 evolution. Simultaneously, the holes left behind in the Au NRs were accumulated in OEC to oxidize H2O to produce O2 (Fig. 9a). The latter process is much slower than the former one, leading to a steady state of the system when the electrons trapped by deposited TiO2 were saturated (Fig. 9b). Following this work, the same group further developed a novel efficient, autonomous plasmon-enhanced whole WS system based on a Au NR array (Fig. 10) [58]. The upper portion of Au NRs were covered by a thin TiO2 layer for charge separation. Subsequently, Pt NPs, which act as a co-catalyst for H2 evolution, were deposited onto the TiO2 layer. A cobalt-based OEC was loaded onto the exposed portions of the Au NRs, as shown in Fig. 10a and 10b. The main processes involved in WS are illustrated in Fig. 10c, clearly revealing that all the charge carries participating in the H2 and O2 evolution reactions are exclusively derived from the SPR of the Au NRs. Each Au NR without

external wiring exhibited a H2 production rate of 5*1013 H2 molecules cm-2 s-1 under 1 sun

irradiation (AM 1.5G and 100 mW cm ) with long-term stability (Fig. 10d). In addition, a stoichiometric ratio (2:1) for the quantity of generated H2 and O2 was observed (Fig. 10e).

w CD C LU

Au NRs

'H, ~ 2 ■o

Measured photocurrent density

H, produced as measured ,9 from GC

0.2 § 0.1 ~

10 20 30 40 50 60 Time (min)

Fig. 9. (a) Energy band diagram of composite plasmonic photoanode unit. The electron-hole pairs created in Au NR upon excitation in visible light are separated as electron hole pairs with energetic electrons injected into TiO2. The energetic holes are efficiently extracted by Co-OEC and used for water oxidation. (b) The quantity of evolved hydrogen (blue trace) measured (gas-chromatographically) as a function of time. (Black curve) The photocurrent simultaneously recorded at 1 V vs RHE with visible light illumination. (Red trace) The photocurrent calculated from the evolved H2. Reproduced with permission from Ref. [8]. Copyright © 2012 American Chemical Society.

Pt nanoparticle reduction catalyst

"00, electron filter

Au nanorod photovoltaic unit

0 6 12 18 24 30 36 42 48 54 60 Time (h)

Time (h)

Fig. 10. (a) Schematic of the cross-section of an individual unit showing the inner Au NR, the TiO2 cap decorated with Pt NPs, which functions as the hydrogen evolution catalyst, and the Co-OEC material deposited on the lower portion of the Au NR. (b) Corresponding transmission electron microscopy (TEM) image (left) and magnified views of the Pt/TiO2 cap (top right) and the Co-OEC (bottom right). (c) Energy level diagram superimposed on a schematic of an individual unit of the plasmonic solar water splitter, showing the proposed processes occurring in its various parts and in energy space. EF, Fermi energy. (d) Hydrogen evolution under visible light illumination (X>410 nm) as a function of time, measured by gas chromatograph. Experiments were conducted in a 6-h cycle. No significant decrease in activity is noted over the 66 h of solar irradiation time. (e) Measured O2 and H2 photoproducts as a function of time for a second device illuminated by 300 mW cm-2 of white light (AM 1.5). The hydrogen/oxygen ratio is ~2, trending downward as the experiment progresses, undoubtedly due to contamination by atmospheric oxygen during the extraction of the gaseous products by syringe through a septum. Reproduced with permission from Ref. [58]. Copyright © 2013 Macmillan Publishers Limited.

In addition to serving as the hot electron source, plasmonic Au NPs have been proved to facilitate charge transfer between semiconductors. Due to the dual role of Au NPs, a sandwich-structured CdS/Au NPs/TiO2 NRs photoanode for WS exhibited a maximum solar-to-chemical energy conversion efficiency of 2.8% [59]. The similar effect of Au NPs was also demonstrated in a CdS@Au/SrTiO3 WS system, which enhanced the activity for H2 production under visible light (A>400 nm) irradiation [60]. Wang et al. [61] designed a Au-Pt-CdS hetero-nanostructure

with all the three components contacting with each other using a well-controlled synthesis strategy. As shown in Fig. 11a and 11b, Pt NPs and CdS shells were deposited on the tips and surface of Au nanotriangles (AuNT), respectively, allowing the Pt NPs fully exposed as the co-catalyst. The Au@CdS/Pt hybrids were also synthesized by growing Pt NPs on the surface of Au@CdS core-shell structures. The Au-Pt-CdS hybrids exhibited the highest photocatalytic activity in H2 generation (778 jimol h-1 g-1) under visible light (A>420 nm) irradiation by using the Na2SO3-Na2S aqueous solution as sacrificial reagents, which was about 8 times and 2.5 times higher than that of the Au@CdS and CdS/Pt nanostructures, respectively (Fig. 11c). It was also considerably higher than that of the Au@CdS/Pt (570 jimol h-1 g-1) structure. As well illustrated in Fig. 11d, the direct contact facilitated the transfer of plasmon-induced hot electrons from Au to Pt in the Au-Pt-CdS nanostructure, while in the case of Au@CdS/Pt, the direct pathway of electron transfer was blocked by the CdS shell. The enhanced local electric field around the three tips of the Au NT and the plasmon-induced hot electron transfer significantly improved the separation of charge carries, leading to a remarkable enhancement of H2 evolution in the photocatalytic WS reaction.

Fig. 11. (a) Schematic illustrating the Au-Pt-CdS hetero-nanostructures. (b) TEM image of and Au-Pt-CdS hetero-nanostructures. (c) Schematic illustration of the multipathway electron transfer in Au-Pt-CdS hetero-nanostructures. (d) Photocatalytic H2 production of CdS, Au@CdS, CdS/Pt, Au@CdS/Pt, and Au-Pt-CdS under visible light (k > 420 nm) irradiation. Reproduced with permission from Ref. [61]. Copyright © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Gong's group employed Au/TiO2 branched nanorod arrays (Au/TiO2 BNRs) as a photoanode for photoelectrochemical WS under visible light irradiation (Fig. 12a) [62]. As shown in Fig.

12b, the Au/TiO2 BNRs showed stronger visible light absorption at 534 nm than TiO2 NRs and TiO2 BNRs because of the SPR of Au NPs. A photocurrent of 0.125 mA cm-2 under visible light (X > 420 nm) was achieved by Au/TiO2 BNRs electrode (Fig. 12c), which was comparable to the highest value of photocurrent in papers ever published. The excellent performance in photoelectrochemical WS of Au/TiO2 BNRs was mainly ascribed to the enhanced visible light absorption and hot electron injection from Au to the CB of TiO2 BNRs resulted from the SPR effect of Au NPs

wCS^i - ■

- |H|| »*

Wavelength /nm

Time I s

Fig. 12. (a) Schematic diagram of TiO2 BNRs and charge transfer mechanism between Au NPs and TiO2 in visible and UV light. (b) UV-visible absorption spectra of TiO2 NRs, TiO2 BNRs, and Au/TiO2 BNRs. (c) Amperometric I-t curves of TiO2 BNRs and Au/TiO2 BNRs at an applied potential of 0.5 V vs. Ag/AgCl under the illumination of visible light with wavelength > 420 nm with on/off cycles. Reproduced with permission from Ref. [62]. Copyright © The Royal Society of Chemistry 2013.

4.2 Design parameters for plasmonic photocatalysts in WS

In plasmon-enhanced WS system, the photocatalytic activity of plasmonic photocatalysts in WS can be controlled, because the SPR effect and related enhancement mechanisms described above are highly associated with the size, shape and the intrinsic properties of plasmonic metal nanostructures, and their geometric arrangements in composite systems [22,37,57,63-67]. In this section, some parameters in designing plasmonic nanostructures towards more efficient WS will be summarized, focusing on the size, shape and geometric position of plasmonic metals in the system, which are known to play significant roles in the plasmon-enhanced WS.

4.2.1 Effect of the morphology of plasmonic metal nanostructures

The morphologies, mainly referring to the shape and size, of plasmonic metal nanostructures greatly affect the SPR intensity and wavelength, as well as the hot-electron generation and transfer process. For example, as shown in Fig. 13 a, the Ag wires, spheres and cubes possessed different LSPR peak wavelengths from about 400 nm to 500 nm. Moreover, by tuning the size of Ag nanocubes from 56 ± 8 nm to 129 ± 7 nm, their LSPR peak was red-shifted from about 450 nm to 550 nm (Fig. 13b) [22,68]. Such tunability offers great latitude for adjusting the degree of spectral overlap between plasmonic metal nanostructures and semiconductors, highly relevant to plasmon-enhanced photocatalysis.

Fig. 13. (a) Normalized extinction spectra for Ag wire, cube and sphere NPs. Wire-shaped particles are 90+12 nm diameter and >30 aspect ratio, cubic particles are 79+12 nm edge length and spherical particles are 38+12nm diameter. (b) Normalized extinction spectra for Ag nanocubes as a function of size (56 + 8 nm, 79 +13 nm and 129 + 7 nm edge lengths correspond to orange, red and blue spectra respectively). The inset shows a photograph of the three nanocube samples suspended in ethanol. Reproduced with permission from Ref. [22]. Copyright © 2011 Macmillan Publishers Limited.

Shape of plasmonic metal nanostructures

Li et al. [64] demonstrated that the effective enhancement in the photoactivity for photoelectrochemical water oxidation of Au decorated TiO2 electrodes under both UV and visible light (300 - 800 nm) irradiation can be achieved by tuning the shape of decorating Au nanostructures. The Au NPs and Au NRs were separately deposited onto TiO2 nanowire arrays on FTO glass to fabricate the electrodes (Fig. 14a and 14b). As shown in Fig. 14c and 14d, both Au NP-TiO2 and Au NR-TiO2 exhibited enhancement in photoactivity under visible light illumination, while only Au NP-TiO2 enhanced the photocurrent in the UV region compared with bare TiO2 nanowires. The IPCE peaks of Au NP-TiO2 and Au NR-TiO2 electrodes were also observed at different light wavelengths (Fig. 14e), well-matching with their respective SPR peaks in the visible light region, which indicated that the improved visible light activity of Au-TiO2 was ascribed to the SPR excitation of Au nanostructures. The FDTD simulation suggested that the electric field intensities upon SPR excitation of Au NP-TiO2 and Au NR-TiO2 were amplified in different levels (Fig. 14f). The amplified electric field in the UV region of the NP-

3 Ag spheres fc*

Ag wires Q Ag cubes

300 400 500 600 700 800 Wavelength (nm)

TiO2 overlaps with the TiO2 absorption (Fig. 14f), which was partially responsible for its exclusively enhanced photoactivity in the UV region. In addition, the hot electron generation and amplified electrical field upon SPR excitation mainly contributed to the enhancement in photoactivity for water oxidation under visible light irradiation.

100 nm

illll 4110 50(1 60« 7011 8110 Wavelength {nm}

Fig. 14. SEM and TEM (inset) images of (a) Au NP-TiO2 nanowires and (b) Au NR-TiO2 nanowires. Chronoamperomertic I-t curves collected at 0 V versus Ag/AgCl for Au NP-TiO2 and (c) Au NP-TiO2 and (d) Au NR-TiO2 electrodes under white-light (AM 1.5G, 100 mW/cm2) and visible light (with a 430 nm long pass filter, 73.3 mW/cm2) illumination. (1)-(5) in (c) represent the Au NP-TiO2 electrodes fabricated by using different time for coating Au precursor. (e) Magnified IPCE plots of Au NP-TiO2 and Au NR-TiO2 electrodes in the incident wavelength between 450 and 800 nm, highlighted by the dashed box in panel a. The corresponding absorption spectra of Au NP-TiO2 and Au NR-TiO2 are also included for comparison. (f) Simulated electric-field intensity plot for bare TiO2, Au NP-TiO2 and Au NR-TiO2 nanowires as a function of incident light wavelength. Inset shows schematic model of Au NP-TiO2 and Au NR-TiO2 nanowires for FDTD simulation. Reproduced with permission from Ref. [64]. Copyright © 2013 American Chemical Society.

Majima et al. [57] demonstrated that Pt-tipped Au NRs exhibited a significant increase in H2 evolution rate than both Pt-covered Au NRs and Au NSs under visible light (460 < X < 820 nm) irradiation (Fig. 15a). Moreover, compared with Pt-covered Au NSs, Pt-covered Au NRs showed improved H2 evolution rate. As shown in Fig. 15b-d, the three samples displayed different action spectra of apparent quantum efficiency (AQE), which were consistent with their respective absorption spectra, indicating that the H2 generation was indeed induced by the SPR excitation of the Au NRs or Au NSs. The AQE of Pt-covered Au NRs reached a much higher value in the light region (> 600 nm) than that of Pt-covered Au NSs, resulting from the much stronger light capturing capability of Au NRs. In addition, the unique anisotropic heterostructure of Pt-tipped Au NRs, facilitating the charge carrier separation and transportation, was partially responsible for their highest H2 evolution rate among the three samples.

Pt-tipped Au NRs • Pt-covered Au NRs •

-*-1-■-r

Pt-tipped Au NRs

«00»

0.5 0.4 H

x 0.2-j LU

Time (h) ' i— Pt-covered Au NRs

400 600 800 1000 Wavelength (nm)

-j—■—i—1—i—'—i—'— Pt-covered Au NSs

Mi •

^ c0.3

0.2& .9

' o LU c o.2

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400 600 800 1000

Wavelength (nm)

400 500 600 700

Wavelength (nm)

0-4 o <

0.12 h0.10 0.08

0.06 LU

0.04 <

0.00 800

Fig. 15. (a) Time course of H2 evolution from water-methanol (20 vol %) suspensions of Pt-modified Au NPs (0.188 mg) under visible light irradiation (460 < X < 820 nm). Extinction and action spectra of AQE obtained for (b) Pt-tipped Au NRs, (c) Pt-covered Au NRs and (d) Pt-covered Au NSs. Reproduced with permission from Ref. [57]. Copyright © 2014 American Chemical Society.

Size of plasmonic metal nanostructures

In general, the size increase of plasmonic metal nanostructures will result in the red-shift of LSPR extinction peaks and the ratio change of scattering-to-absorption [57,69]. Moreover, the size of plasmonic metal nanostructures can also affect the intensity and area of LSPR-induced electromagnetic fields, all of which can have effects on plasmon-enhanced photocatalysis [3,69,70].

As shown in Fig. 16a-c, Seh et al. [71] synthesized Janus Aud-TiO2 nanostructures using Au NPs with different sizes (30, 50 and 70 nm in diameter) to study the effect of Au NP size on the photocatalytic activity in H2 production. With the size increase of Au NPs from 30 nm to 70 nm, the LSPR extinction peak shifted from about 550 nm to 600 nm (Fig. 16d). The intensities of LSPR extinction and the visible light H2 generation rate followed the order of Au70 nm-TiO2 > Au50 nm-TiO2 > Au30 nm-TiO2 (Fig. 16d and 16e). The stronger SPR effect of larger Au NPs, resulting in more intensified near-fields and enhanced light absorption, was shown in the optical-absorption maps of Aud-TiO2 nanostructures (Fig. 16f-h). Therefore, the higher H2 generation rate of Aud-TiO2 with larger Au NPs was ascribed to the intensified near-field and enhanced light absorption.

However, Zhang et al. [72] reported that smaller Au NPs (20 nm) incorporated with TiO2-based photonic crystal substrate showed higher visible light absorption intensity and

photocurrent density than larger size Au NPs (40 nm). It is simply because that the SPR wavelength of smaller Au NPs matched better with the photonic bandgap of photonic crystals. Kim et al. [9] fabricated size-controllable Au nanodot arrays (dot size: 50, 63 and 83 nm) with TiO2 overcoats on ITO substrates. It was found that with the decrease of Au dot size, the induced plasmonic enhancement for the photoelectrochemical WS reaction increased greatly. For light on/off experiments, the plasmonic enhancements achieved were 25 times and 10 times for electrodes with the Au nanodots size of 50 nm and 83 nm, respectively. According to the UV-vis measurement, with the decrease of Au nanodot size, the LSPR peak energy (Elspr) increased and LSPR line width (r) decreased. It means that the quality factor (defined as Q = Elspr/T) value of the electrodes with smaller Au nanodot size is larger, which was proportional to the activity of the catalyst. The finding that electrodes with smaller Au nanodot size exhibited better WS performance was possibly because that the enhanced local field upon LSPR excitation was stronger due to the larger quality factor for the array made of smaller dots.

Wavelength (nm)

Fig. 16. TEM images of Janus (a) Au70 nm-TiO2, (b) Au5q nm-TiO2 and (c) Au30 nm-TiO2 nanostructures. (d) Optical extinction spectra of Janus Au^TiO2 nanostructures with various Au NPs sizes. (e) Volume of hydrogen generated (equation image) under visible light irradiation using Janus Aud-TiO2 photocatalysts with various Au NP sizes d (in nm). Optical-absorption maps (cross-section view at z = 0) of Janus (f) Au70 nm-TiO2, (g) Au50 nm-TiO2 and (h) Au30 nm-TiO2 photocatalysts, simulated using the optical properties of amorphous TiO2. The maps show the power absorbed by the nanostructures at their LSPR wavelengths, with incident polarization along the x-axis and an incident intensity of 0.5 W cm-2. Reproduced with permission from Ref. [71]. Copyright © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Silva et al. [27] investigated the photocatalytic activity of Au NPs with different sizes loaded onto P25-TiO2 (Au/TiO2) for visible light WS. The H2 generation performance of Au/TiO2 improved with the decrease of Au NP size, which was attributed to the enhanced light absorption of smaller Au NPs. Wei et al. [70] also fabricated similar, Au-loaded TiO2 (Au-TiO2) heterostructures by depositing Au NPs with different sizes onto P25 (Degussa) TiO2 (insets in Fig. 17a). As shown in Fig. 17b, under the excitation with X > 400 nm for 2 h, the small Au-P25 (with Au NP size of 4.4 ± 1.7 nm) exhibited significant improvement in the amount of H2 evolution, which was about 20 times higher than larger Au-P25 (with Au NP size of 67 ± 17 nm).

The conclusion that smaller Au-TiO2 heterostructures showed better activity under above conditions was in agreement with the reports of Priebe et al. [73] and Seh et al. [71]. However, under X > 435 nm irradiation, there was no H2 production in the smaller Au-TiO2 system, while significant amount of H2 was still detected over large Au-P25 under identical conditions (Fig. 17a). These interesting results suggested that the weaker SPR effect of Au in smaller Au-TiO2 was not responsible for its enhanced activity under X > 400 nm irradiation. Rather, the slight "tail" beyond 400 nm in the absorption spectrum of P25 made the electrons excitation possible. The incorporation of Au NPs with P25 significantly suppressed the recombination of charge carriers excited by X > 400 nm, contributing to the higher H2 production over small Au-P25. Under X > 435 nm irradiation, the extended tail in the excitation spectrum of small Au-P25 was due to light scattering rather than direct absorption of P25. The authors believe that the weaker SPR intensity of Au NPs in small Au-P25 made the energy level of transferred, accumulated electrons insufficient for H2O reduction (Fig. 17c, right). While the more intense SPR of Au NPs in larger Au-P25 facilitated the injection of hot electrons from Au to the CB of TiO2 that further participated in H2O reduction (Fig. 17c, left).

V Small Au-P25 O Large Au-P25

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Time (min)

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Reduction Potentials

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[PtCI6]2 + 4e = Pt+ 6CI-

1 2. 3~

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Fig. 17. H2O reduction activities of Au-P25 photocatalysts under (a) X > 435 nm and (b) X > 400 nm irradiation. Insets are HR-STEM images of small (left) and large (right) Au-P25. (c) Proposed mechanism for manipulating SPR-mediated electron transfer for photocatalysis by controlling Au NP size. Reproduced with permission from Ref. [70]. Copyright © 2014 American Chemical Society.

4.2.2 Geometric position of plasmonic metal nanostructures

Same as the shape and size, the geometric position of plasmonic metal nanostructures with respect to the semiconductor also plays an important role in the magnitude of SPR-induced

activity enhancement in WS. For the composite catalysts, where metallic NPs and semiconductors are separated from each other by non-conductive layers, both the light scattering and near-field mechanisms could play a role [22,29,36,74]. If the metallic NPs are in direct contact with the semiconductor, either on the surface of or embedded inside the semiconductor, all the aforementioned mechanisms for SPR-induced enhancement can take effect. The intensity, distribution and area of the increased electromagnetic field, which directly affects the photon absorption and charge carrier separation of the semiconductor in the vicinity of the plasmonic NPs, are all strongly dependent on the geometric arrangement between the plasmonic metal nanostructure and semiconductor [22].

Thomann et al. [75] used SiO2-coated Au NPs (50 nm) located at the bottom or on the top of an a-Fe2O3 photoelectrode layer as two kinds of photoanodes for WS (insets in Fig. 18a and 18b, respectively). The thin SiO2 shell acted as a barrier to prevent the hot electron transfer between the Au and a-Fe2O3 and avoid the catalytic effect of Au NPs. As shown in Fig. 18a and 18b, the photocurrent enhancement spectra of both the two photoelectrodes exhibited excellent qualitative agreement with their respective simulated absorption enhancement spectra, indicating that the SPR effect contributed to the enhanced photocatalytic activity in WS. Nevertheless, the maximum of photocurrent enhancement was achieved at a much higher level in the Au-top-configuration (Fig. 18b) than in the Au-bottom-configuration (Fig. 18a). The more effective enhancement in the former configuration was attributed to the increased charge carrier separation nearby the Au NPs due to the near-field effect, which allowed significant light concentration close to the electrode/liquid interface. Because of that, most of the generated charge carriers had much shorter diffusion length to the surface reaction sites with respect to those in the second configuration. Recently, they designed one Schottky junction-free device for water splitting, which could generate hot electrons efficiently and directly inject them from Au NPs to adsorbed water molecules [76]. The NiOx was used as the selective transport layer (blocking electron transport, while allowing for hole transport) to facilitate the separation of carriers. Even without the expensive Pt as a cocatalyst, the architecture exhibited comparable photocurrent with the best-published results.

500 550 600 650 wavelength (nm)

Figure 18. Photocurrent enhancement spectra for Au NPs with a silica shell. Measured photocurrent (red symbols) and simulated (solid blue lines) absorption enhancement spectra that show the beneficial effects of placing silica-

Zhan et al. [77] fabricated three kinds of photoanodes for WS under visible light irradiation, which were Au NPs sitting on TiO2 (Au-on-TiO2) (Fig. 19a), Au NPs embedded in TiO2 (Au-in-TiO2) (Fig. 19b), and 3D Au-embedded TiO2 (two layers of Au-in-TiO2) (Fig. 19c). As displayed in Fig. 19d, the Au-on-TiO2 electrode showed much enhanced absorption in the visible region (400 nm-800 nm) compared with the bare TiO2 electrode, with a red-shift of absorption peak to ~625 nm, which was ascribed to the dielectric interaction between the Au NPs and TiO2. The absorption was further improved by the Au-in-TiO2 electrode and the improvement was even more remarkable in the 3D Au embedded TiO2 electrode. It was due to the increased absorption resulted from a larger volume of TiO2 with the presence of more plasmonic Au NPs in the TiO2 matrix. The enhanced light absorption in these cases resulted in photocurrent increase by 3 times

in Au-in-TiO2 (1.1 |iA cm-2) and 5 times in 3D Au-embedded TiO2 (1.8 |iA cm 2) compared with

that of the Au-on-TiO2 electrode (0.35 |iA cm ) under visible light (k > 420 nm) irradiation (Fig. 19e-g).

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— Au-on-TiOz

— Au-in-Ti02

— 3D Au-embedded Ti02

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500 600 700 Wavelength(nm)

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20 40 60 80 100 120 140 Time(s)

20 40 60 80 100 120 140 160 Time(s)

100 120

Fig. 19. Schematic illustrations of (a) Au-on-TiO2 electrode (Au particles sitting on the surface of TiO2 layer), (b) Au-in-TiO2 electrode (Au particles embedded in TiO2 matrix), and (c) 3D Au-embedded TiO2 electrode (two layers of Au particles embedded in TiO2 matrix). The total thickness of TiO2 is 300 nm for all three electrodes. (d) UV-vis absorption of TiO2, Au-on-TiO2, Au-in-TiO2, and 3D Au-embedded TiO2 electrodes. Current-time curve of (e) Au-on-TiO2 electrode, (f) Au-in-TiO2 electrode and (g) 3D Au-embedded TiO2 electrode with visible light irradiation on and off at a bias of 0.2 V. Reproduced with permission from Ref. [77], Copyright © 2014 American Chemical Society.

Seh et al. [71] investigated the photocatalytic performance of as-prepared non-centrosymmetric Janus (Figure 16b) and symmetric Au@TiO2 core-shell nanostructures based

on 50 nm Au NPs for WS under visible light irradiation. A smaller red-shift in the LSPR spectrum was observed for the Janus Au-TiO2 compared with the core-shell structure, due to the different dielectric environments surrounding the two sides of Au NPs in the Janus structure. Stronger plasmonic near-fields, formed close to the Au-TiO2 interface in the Janus Au-TiO2, enhanced the light absorption and the generation of charge carriers that significantly increased the rate of H2 production under visible light irradiation.

Stucky and co-workers [78] reported a bottom-up synthesis of Au NR/TiO2 nanodumbbells (Au NR/TiO2 NDs) by the anisotropic overgrowth of TiO2 onto the two tips of Au NRs with the sides exposed (Fig. 20a). Using a similar process, the fully coated Au NR@TiO2 core-shell structure (Au NR@TiO2) was prepared by the pre-modification of Au NRs (Fig. 20b). As shown in Fig. 20c, the red-shifts of the SPR spectrum were observed in the Au NR/TiO2 NDs and Au NR@TiO2 core-shell samples with respect to the Au NRs, reflecting the local refractive index change with TiO2 coating. The results were consistent with previous studies of incorporating dielectric materials onto Au NPs [79,80]. Under visible light irradiation, the as-prepared Au NR/TiO2 NDs exhibited higher photoactivity in H2 evolution than the mechanical mixture of Au NRs and amorphous TiO2 (Fig. 20d). It was indicated that the highly intimate contact and strong plasmonic coupling between the Au NR and TiO2 in the ND structure played an important role in facilitating the separation and transfer of charge carries. In addition, there was no H2 production in the Au NR@TiO2 core-shell NPs under the same conditions (Fig. 20d), suggesting that it was the hot electron transfer upon LSPR excitation, not the near-field effect that mainly contributed to the plasmon-enhanced photocatalytic water reduction. For the NRs, the SPR-induced hot electrons could be transferred to TiO2 to participate in the water reduction, leaving the positively charged holes in the Au NRs for the oxidation reaction. As shown in Fig. 20e, the oxidation reaction could take place on the bare lateral side of the Au NR to restore the charge balance when the hot electrons were transferred to TiO2 for water reduction. While for the Au NR@TiO2 NPs, there was no exposed regions on the Au NRs for the oxidation reaction to consume the generated holes, blocking the continuous generation of hot electrons upon SPR excitation (Fig. 20f).

c 1-81

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* 0 0.8 -Ï0.6-

S0-4: 0.2-

l-1-1-1-r

400 600 800 1000 Wavelenqth (nm)

Visible light.

AuNR /TiO.

AuNR and TiO.

photocathode

dumbbell structure

core-shell structure

Fig. 20. TEM images of (a) Au NR/TiO2 NDs and (b) Au NR@TiO2. (c) UV-Vis spectra: Aqueous solution of Au NR-TiO2 dumbbells and Au NR@TiO2 after cleaning and their corresponding seeds (Au NR: 32 nm in diameter) (d) H2 evolution rate by various catalysts under visible illumination and in the presence of methanol and water. Structure and mechanism of operation under visible light of (e) an individual Au NR/TiO2 dumbbell and (f) Au NR@TiO2 core-shell. In (e) hot electrons generated from plasmonic Au NRs are filtered out by the Au/TiO2 Schottky barrier for photoreduction and regenerated from the electron donor (methanol here). Reproduced with permission from Ref. [78]. Copyright © 2016 American Chemical Society.

5. Conclusions and outlook

Plasmon-enhanced WS has been attracting enormous attention and promising results have been consistently reported by many research groups in recent years. In this review, we briefly introduce the fundamentals of WS and widely accepted three major enhancement mechanisms in plasmon-enhanced WS, i.e., light scattering, near-field and hot electron transfer mechanisms. We also summarize recent advancements in plasmon-enhanced WS under visible light irradiation by including some representative systems reported by different groups. Moreover, the important parameters, including the shape, size and geometric position of metallic structures in plasmonic-metal/semiconductor photocatalysts, which need to be taken into consideration for designing efficient systems, are discussed in detail.

Although large numbers of encouraging results have been obtained by introducing the concept of plasmon-enhanced photocatalysis to the field of visible light-driven WS, the study of plasmon-enhanced WS is still in its infancy, and there remain many challenges, including the mechanistic understanding and the design and large scale realization of highly efficient plasmonic photocatalysts.

Firstly, the exact mechanisms of plasmon-enhanced WS are not fully understood. For example, the SPR-induced hot electron transfer pathway and kinetics (including the rate of back transfer), which play an important role in improving the photoactivity, are still difficult to be determined. More explorations have to be conducted in this research direction. Unified predictive models are highly desired to give guidance in the effective design of plasmonic-metal/semiconductor composite photocatalysts. In addition, state-of-art characterization techniques are in urgent demand. In particular, the ultrafast in situ characterizations of the behavior and fate of charge carriers at the surface and interface during the WS process will be able to provide valuable information and largely help with better understanding of the processes.

Secondly, the experimental design and realization of effective and wideband plasmonic photocatalysts are of great importance in solar energy conversion to fuel. Most reports to date mainly focused on plasmon-enhanced WS in the UV and partial visible light regions, which at most account for ~ 50% of the solar irradiation. The SPR wavelength and intensity and the plasmonic effects on photocatalysis are strongly dependent on the size, shape and geometric positon of plasmonic nanostructures, hence, in principle it is possible to utilize the entire solar spectrum by manipulating the influencing factors in plasmonic photocatalysts. The rational design of plasmonic nanostructures themselves and their arrangement with semiconductors are thus important, which may allow to take full advantage of the LSPR effect in WS. Moreover, the design of plasmonic photocatalysts with higher efficiency can reduce the amount of photocatalysts to be involved in WS system, which allows the investigation and use of very efficient yet expensive materials in WS. There is still a long way to go to implement the widespread practical application of WS. With in-depth understanding and progress on rational synthesis, breakthroughs in plasmon-enhanced WS in the not very distant future can be expected.

Acknowledgement

Financial support from the Natural Sciences and Engineering Research Council of Canada, Canada Research Chairs Program, and le Fonds de recherche du Quebec-Nature et technologies (FRQNT) is greatly appreciated. In addition, Q. Zhang acknowledges the support under State Scholarship Fund from the China Scholarship Council (CSC, NO. 201506220152), D. Thrithamarassery Gangadharan acknowledges the support from FRQNT Merit scholarship program for foreign students and Dr. Z. Xu acknowledges the National Natural Science Foundation of China (NSFC 51402198) and Natural Science Foundation of Liaoning Province (201602592) for financial support.

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Qingzhe Zhang is currently a Ph.D. candidate under the supervision of Prof. Dongling Ma and Prof. Mohamed Chaker at INRS, Canada. He earned his Master's degree from Shandong University (China) in 2015 and was awarded a 4-year pursuing Ph.D. degree scholarship from the China Scholarship Council (CSC) under the State Scholarship Fund. His current research interests focus on the synthesis of multifunctional nanomaterials for applications in catalysis, including photocatalytic degradation of pollutants and solar water splitting.

Deepak Thrithamarassery Gangadharan completed his MSc degree in physics with a specialization in Non-Conventional Energy from Mahatma Gandhi University, Kottayam, India. Currently, he is a PhD candidate at the Institut national de la recherche scientifique (INRS), Canada. He works on plasmonic-enhanced hybrid inorganic-organic perovskite solar cells under the guidance of Prof. Dongling Ma. His other areas of interest include quantum physics, nanotechnology for energy applications.

Yanlong Liu received BSc degree from Huazhong University of Science and Technology (2010), China; MSc degree from Wuhan University of Technology (2013), China. He started his doctoral research as a member of Dongling Ma's group in 2014. His main research interests focus on the preparation and application of plasmonic metals nanomaterials.

Zhenhe Xu is currently an associate professor at Shenyang University of Chemical Technology. He received his Ph.D. degree (inorganic chemistry) from the Changchun Institute of Applied Chemistry (CIAC), Chinese Academy of Sciences in 2011. He was awarded the "ZhuLiYueHua Scholarship" by Chinese Academy of Sciences. He became a postdoctoral fellow in Prof. Ma's group at the center of Énergie, Matériaux et Télécommunications, Institut national de la recherche scientifique, University of Quebec (Canada) in 2013. His current research interests include the development of nanostructured materials and multifunctional composite materials mainly for catalytical applications.

Mohamed Chaker has been professor at INRS since 1989. As a holder of Tier-I Canada Research Chair on "Plasma applied to micro- and nano-manufacturing technologies", he is leading a research program in plasma-based synthesis and etching of innovative materials at the nanoscale for the fabrication of RF and photonic devices. In career, his research work resulted in

245 articles and over 300 conference presentations (including 50 invited presentations). From 1999 to 2005, he was the director of the Center Energy, Materials and Telecommunications of INRS.

Dongling Ma is currently a professor at INRS. Her main research interest consists in the development of various nanomaterials (e.g., quantum dots, catalytic nanoparticles, plasmonic nanostructures, and different types of nanohybrids) for applications in energy, catalysis and biomedical sectors. Before joining INRS in July 2006, she was awarded Natural Sciences and Engineering Research Council Visiting Fellowships and worked at National Research Council of Canada from 2004 to 2006. She received her Ph.D. degree from Rensselaer Polytechnic Institute (USA) in 2004.

Qingzhe Zhang

Deepak Thrithamarassery Gangadharan

Yanlong Liu

Zhenhe Xu

Mohamed Chaker

Dongling Ma

Recent advancements in plasmon-enhanced visible light-driven water splitting Academic research paper on "Nano-technology"

Abstract of research paper on Nano-technology, author of scientific article — Qingzhe Zhang, Deepak Thrithamarassery Gangadharan, Yanlong Liu, Zhenhe Xu, Mohamed Chaker, et al.

Similar topics of scientific paper in Nano-technology , author of scholarly article — Qingzhe Zhang, Deepak Thrithamarassery Gangadharan, Yanlong Liu, Zhenhe Xu, Mohamed Chaker, et al.

Academic research paper on topic "Recent advancements in plasmon-enhanced visible light-driven water splitting"