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Understanding the superior photocatalytic activity of noble metals modified titania under UV and visible light irradiation Published on 27 January 2014. Downloaded by Aston University on 27/01/2014 16:03:38.

Ali Bumajdad*, Metwally Madkour 5

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Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX DOI: 10.1039/b000000x Although TiO2 is one of the most efficient photocatalysts, with the highest stability and the lowest cost, there are drawbacks that hinder its practical applications like its wide band gap and high recombination rate of the charge carriers. Consequently, many efforts were oriented toward enhancing the photocatalytic activity of TiO2 and extending its response to the visible region. To head off these attempts, modification of TiO2 with noble metal nanoparticles (NMNPs) received considerable attention due to its role in accelerating the transfer of photoexcited electrons from TiO2 and also due to the surface plasmon resonance which induce the photocatalytic activity of TiO2 under visible light irradiation. This insightful perspective is devoted to the vital role of TiO2 photocatalysis and its drawbacks that urged researchers to find solutions such as modification with NMNPs. In a coherent context; we discussed here the characteristics which qualify NMNPs to possess a great enhancement effect for TiO2 photocatalysis. Also we tried to stand on the reasons behind this effect by means of photoluminescence (PL), electron paramagnetic resonance (EPR) spectra, and Density Functional Theory (DFT) calculations. Then the mechanism of action of NMNPs upon deposition on TiO2 is presented. Finally we introduced a survey of the behaviour of these noble metal NPs on TiO2 based on particle size and loading amount. limited on doping with other dopants but also extended to tailoring TiO2 with high surface energy. In this regard, anatase 1. Introduction TiO2 [001] facets is known to has excellent photocatalytic TiO2 is known to exhibit photocatalytic activity due to the photo- 50 activity due to its higher surface energy (0.90 J/m2)23 and low density of defects, which could inhibit the recombination rate of generated charge carriers (negative electrons, e-, and positive photogenerated charge carriers at the grain boundaries. Therefore, holes, h+)1. Positive holes oxidize organic compounds, while the synthesis of anatase TiO2 nanocrystals with exposed [001] negative electrons mainly reduce molecular oxygen to superoxide facet is critical and challenging. radical anions 2-4. In spite of the fact that the photocatalytic activity of TiO2 is among the highest for all semiconductors, one 55 Also many attempts have been made to improve the photocatalytic activity of TiO2 by doping with noble metals of the critical drawbacks of TiO2 is its high photogenerated which acts as an electron acceptor. The capture of photogenerated electron/hole pairs recombination rate which hinders its electrons from noble metals is assumed to repress the photocatalytic efficiency5,6. If the recombination rate is quick recombination of electron-hole pairs and facilitate the transfer of (lifetime of charge carriers is about 30 ns) there is no enough time 60 holes on the TiO2 surface. Hence, the photocatalytic activity can for other chemical reactions to occur 7-9. be enhanced in terms of a longer electron-hole pair separation Another drawback for TiO2 is the fact that it cannot efficiently lifetime 24-26. utilize visible light as the band gap of TiO2 is 3.0-3.2 eV (the As deposition of noble metals on TiO2 is of a great interest, many exact value depends on the phase studied), so the generation of reviews had been published to illustrate the behaviour of noble electron-hole pairs and degradation of organic compounds only can occur under ultraviolet illumination. Therefore, many efforts 65 metals in photocatalysis for guidance purposes. For example, in a review of Kowalska et al.27 they explicated the mechanism of have been made for expanding the photoresponse of TiO2 to photocatalytic reaction on Au/TiO2 under visible light irradiation longer wavelengths because UV radiation accounts for 4% energy by examining the action spectra and the properties of of the incoming sunlight, while the visible light (wavelength photocatalysts required for a high level of activity. Linic et al. 28 above 400 nm) constitutes around 43% of solar energy 10. Hence, 70 reviewed water-splitting reactions on plasmonicone of the greatest challenges for photocatalyst study is to design metal/semiconductor and the mechanisms by which metal surface new catalysts that exhibit minimum recombination rate and high plasmon resonance, SPR, can affect the photocatalytic activity of activity when illuminated by visible light. semiconductors. Kumar et al.29 attempts to provide a review Various approaches have been utilized to enhance the visible 11-13 focused on modified TiO2 photocatalysis besides highlighting the light activity of TiO2 such as non-metal doping , incorporation of carbonaceous nanomaterials such as activated carbon, carbon 75 advancements made to improve the surface-electronic structure of titania with high efficiency. Zhou et al. 30 focused on the most nanotubes, fullerenes and graphene14-17 and coupling with other 18-22 recent advances in the synthesis and photocatalytic properties of semiconductors . Also the modification of TiO2 was not This journal is © The Royal Society of Chemistry [year]

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noble metal-based plasmonic composites under visible light. Pelaez et al. 31 presented the development of different strategies to modify TiO2 for the utilization of visible light, including nonmetal and/or metal doping, dye sensitization and coupling semiconductors. Hou et al. 32 focused on recent work demonstrating plasmon-enhanced photocatalytic water splitting, reduction of CO2 with H2O to form hydrocarbon fuels, and degradation of organic molecules. Xuming et al. 33 aimed primarily to stand on the major mechanisms in plasmonic photocatalysis and to provide physical explanations accordingly. Also due to the increased Accretion of the plasmonic photocatalysts, Wang et al. 34 reviewed the recent synthetic routes and photocatalytic reaction tests performed using plasmonic photocatalysts. They dealt broadly with all types of plasmonics photocatalysis (e.g. NMNPs on semiconductors, insulators, carbon, halides, and graphene) and finally they provided important guidelines to be considered in constructing new plasmonic photocatalysts. This perspective focus on the reasons behind the superior photocatalytic activity of TiO2 upon deposition with NMNPs. For better understanding how the noble metals affect the electronic properties of TiO2, experimental analytical studies (Photoluminescence, PL, and electron paramagnetic resonance , EPR, spectra) and theoretical investigations (Density Functional Theory, DFT) are reviewed. Also the proposed mechanisms for this superior activity under both ultraviolet and visible light irradiations are summarized. From the discussed studies, one can reached to a conclusion that simplifying the parameters that govern the NMNPs role (e.g. referring it just to particle size and loading amount effect) is inadequate and more details and focused studies are necessary to understand the full picture. 2. General overview

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To the best of our knowledge, the first attempt for doping TiO2 with noble metals was reported by Tauster et al. 35. Since then, many studies focused on the modification of TiO2 by noble metal nanoparticles, NMNPs, like: Au 36-41, Ag 42-49, and Pt 50-58. Long back, researchers have directed their focus on loading noble metal nanoparticles on the common TiO2. However, the work on the visible-light-responsive anatase TiO2 [001] facets using the SPR effect of noble metals has been rarely investigated. Yu et al.59 demonstrated the photocatalytic hydrogen production via novel visible-light driven plasmonic Pt/TiO2 nanosheets with a high percentage of exposed [001] facets. Also, Ag/TiO2 singlecrystal nanosheets dominated by [001] facets was disclosed by Jiang et al.60 and assessed for the reduction of p-nitrophenol, Rhodamine B (RB) and ciprofloxacin (CIP). Their results revealed that the the photocatalytic activity of Ag/TiO2 is much higher than of TiO2. Therefore this hot topic is expected to grab the attention of many researchers for designing highly efficient visible light active photocatalyst. In order to stand on the most reactive noble metal upon deposition on TiO2, one needs to compare such noble metals. But few studies have been oriented to this point. In a study of Rupa et al. 61, a series of different noble metals loaded TiO2 was prepared via the same method (photodeposition) and at the same loading percentage (1%). Their photoreactivity was assessed toward the photocatalytic decolourisation of tartazine dye under identical 2 | Journal Name, [year], [vol], 00–00

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operational parameters, and the results revealed the order; Au/TiO2> Ag/TiO2 ≈ Pt/TiO2 even all of them possess the same surface area (≈88 m2g-1). Also in analogue study the order obtained was: Au/TiO2> Pd/TiO2 ≈ Pt/TiO2> Ag/TiO2 under the same conditions and at the same loading (1%) and approximately possessing similar surface area as well (≈47 m2g-1)62. Based on that, one can conclude that Au could be regarded as the most reactive among the others. But as shown in Fig.1, one cannot confirm which metal induces the best performance among noble metals, as the obtained results were subjected to different parameters (e.g. loading amount) which is expected to affect the reactivity. So, to date, there is no report disclosed clearly the most reactive noble metal on TiO2. We believe that the Fermi level energy for each noble metal will be a major entrance to solve this puzzle. According to the electrochemical reduction for the following noble metals (Eqs. 1-4): Au3+ + 3e− → Au E°reduction = 1.5 V (1) Pt2+ + 2e− → Pt E°reduction = 1.2 V (2) Pd2+ + 2e− → Pd E°reduction = 0.83 V (3) Ag+ + e− → Ag E°reduction = 0.8 V (4) The closer values of the reduction potential to the valence band of TiO2, the more reactive noble metal will be. So, the expected order will be Au/TiO2> Pt/TiO2 > Pd/TiO2 ≈ Ag/TiO2 which is matched, somehow, with the disclosed results. In an attempt to reveal to what extent the researchers discussed the noble metal deposition behavior, presented herewith a summary of many studies in which, a focus is given only on how they emphasize on the superior effect. For example, Subramanian et al. 63 demonstrated that, in the case of Au/TiO2 NPs, the Fermi

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Fig.1 Photocatalytic activity of the NMNPs/TiO2 composites after irradiation with UV light for 1 h plotted against initial metal content. Taken from ref. 62 2

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level is shifted to more negative potentials than that of pure TiO2. This is referred to an increase in the charge storage and an improve of charge separation. Christopher et al. 64 referred the enhanced visible activity of Ag/TiO2 to radiative transfer of energy mediated by surface plasmons from Ag NPs to TiO2, which lead to higher concentrations of the photogenerated charge carriers (e-/h+ pairs) in the semiconductor, and hence leading to higher photocatalytic activity. Seh et al. 65 showed enhanced visible light activity for anatase Au/TiO2 NPs and referred this enhancement to the strong localization of plasmonic near-fields This journal is © The Royal Society of Chemistry [year]

Physical Chemistry Chemical Physics Accepted Manuscript

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Table 1 tabulated a brief literature survey of NMNPs/TiO2 preparation method and the promotion effect and how greatly the deposition of NMNPs affects the photodegradation rate. Table 1. Overview of preparation method and the effect of deposition of noble metals on TiO2 upon the photocatalytic degradation of some pollutants. Pollutant Chloroform

The results The efficiency of Pt/TiO2 was almost twice as that of TiO2.

Catalyst synthesis

Rhodamine B (RB)

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Methyl tertbutyl ether (MTBE ) Methyl Orange (MO)

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Sol-Gel

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Electron beam evaporation Chemical reduction

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The photodegradation using the Ag/TiO2 was 44% compared to the 35% obtained using pure oxide. 74 Au nanoparticles significantly enhance the catalytic activity of P-25 titania toward the photodegradation of 4chlorophenol. 75

Ag/TiO2 and Pt /TiO2 enhanced the photocatalytic activity compared to pure TiO2. 76 Deposition of gold NPs increased the photoactivity degradation of TiO2 in 80%.77 The photocatalytic mineralization of 4-CP :Au/TiO2 > Pt/TiO2 > Ag/TiO2 > TiO2.78 The conversion enhanced from 23% (Meso-TiO2) to 60% (Au/Meso-TiO2) and 100% (Pt/Meso-TiO2) after only 100 s of UV irradiation The degradation rate by Ag/TiO2 was 2.52 times higher than that by TiO2 nanoparticles. 80 Ag/TiO2 showed a 30% increase in the RB photodegradation compared to TiO2. 81 Ag/TiO2 showed higher photocatalytic efficiency (81%) than TiO2 (60%). 82 The MB degradation efficiency was 80% for Au/TiO2 and 44% for TiO2.

H2O Splitting

TiO2 and Au–TiO2 nanoparticles shows catalytic activity with about 37% and 42% MB degraded. 84 MB photodegradation was 63% and 82% using pure and Pt–TiO2 TiO2 respectively. 85 Au/TiO2 exhibited a threefold rate enhancement compared to unmodified TiO2. 86 TiO2 showed 35% of methyl orange degradation whereas Au/ TiO2 possessed 82%.87 The photocatalytic activity of 2.0 wt% Ag/TiO2 is about 2.3 times as that of TiO2. 88 The activity of water splitting was improved by the loading of Au NPs on TiO2. 89 The Au/TiO2 and Au-Pd/TiO2 exhibit enhanced photoactivity and stability for photocatalytic water splitting under UV and sunlight rather than TiO2. 90

Photoreduction

Photoreduction

Depositionprecipitation

Borohydrate reduction method Depositionprecipitation Photodeposition

Frens method

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close to the Au/TiO2 interface and the coupling of plasmonic near-fields to optical transitions involving localized electronic states in TiO2. Subramanian et al. 63 also attributed an enhancement in the photocurrent under UV illumination after modification of TiO2 with NMNPs (Au or Ag) to the metal nanoparticles’ ability to capture and store the photogenerated electrons 66, 67. It was evidenced also by the work of Koci et al. 68 that the yield of methane and methanol from the photocatalytic reduction of CO2 increases upon modifying the TiO2 by Ag incorporation under UV illumination and they interpreted this phenomenon to the formation of a Schottky barrier in the Ag/TiO2 contact region, which causes charge separation, thereby decreasing the electron–hole recombination rate, and increasing their lifetime 69. The work of Yogi et al.70 showed the enhancement effect upon deposition of Ag, Pt, and Au NPs on TiO2, for photodegradation of methylene blue dye and they attributed this to the trap of the photogenerated electrons by these metals, leading to high efficiency of charge separation. In one of our previous studies, the effect of deposition of Ag NPs on TiO2 under ultraviolet irradiation was discussed. The rise of photocatalytic activity was attributed to electronic interaction occurring at the contact region between the metal deposits and the semiconductor surface which cause the removal of electrons from TiO2 into the vicinity of the metal particle leading to the formation of the Schottky barriers and in turn promotes the charge separation 71. Also, in a recent study by our group72, deposition of Au NPs on TiO2 under both ultraviolet and solar irradiation was investigated and the enhancement effect was referred to Schottky barriers under ultraviolet irradiation and to the localized surface plasmon resonance (LSPR) that induce a collective coherent oscillation of the conducting band electrons of the metal nanoparticles under solar irradiation72. As a result to the SPR effect, Au NPs absorb resonant photons and transfer the electron which originates from the intraband excitation of its 6sp electrons to the titania and this reduces molecular oxygen adsorbed to superoxide, O2•-, on the surface of titania thereby facilitating dye degradation.

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Photoreduction

Photodeposition

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Fig. 3 SPR effect of Au NPs on the TiO2. Taken from ref.98

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Electron beam evaporation

The surface plasmon band (SPB) is known to be a strong and a broad band in the UV-visible spectrum for metallic NPs bigger than 2 nm [Fig. 4]. The position, the shape and the intensity of the SPB strongly depends on various factors: (i) the dielectric constant of the surrounding medium, (ii) the electronic interactions between the stabilizing ligands and the nanoparticle, which alter the electron density inside the nanoparticle, and (iii) the size, shape and monodispersity of the NPs 99.

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Herein the main features behind the enhancement effect of noble metals are summarized in the context of LSPR effect and Schottky junction. Firstly, an important characteristic of the NMNPs is that they have strong absorption of visible light due to surface plasmon resonance (SPR) effect 97. Surface plasmon is the oscillations that can propagate at the interface of metal and semiconductor. As shown in Fig. 2, the incoming irradiation, which is an oscillating electromagnetic field, induces surface plasmon oscillation of the metal electrons. As the light wave passes through NMNPs, the metal electron density is polarized to one side and oscillates in resonance with the light frequency.

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Fig. 4 UV-vis absorbance spectra of various noble metal nanoparticles having the same geometry and the corresponding SPB band thereof. Taken from ref.100 Secondly, an important feature of the noble metal NPs is the Schottky junction which results from the contact of the NMNPs and the semiconductor. It builds up an internal electric field close to the metal/semiconductor interface [Fig. 5]. Once they are created inside or near the Schottky junction, this would force the electrons and holes to move in different directions 101.

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Fig. 2 surface plasmon resonance in which oscillations of electrons due to electromagnetic wave. For further clarification, Fig. 3 reveals how the SPR effect enhances the photoactivity of titania under visible light irradiation upon deposited with Au NPs. Owing to the photogenerated electrons which have negative potential higher than that of the conduction band (CB) of TiO2, the photogenerated electrons transfer from an excited Au NPs to a TiO2 NPs 98.

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Fig. 5 Schottky barrier between a metal and a semiconductor. Eg is the band gap of the semiconductor, EF is the Fermi level of the metal and ФoB,n is the Schottky barrier height. Taken from ref.102

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CO2 Reduction

Hydrogen obtained in the presence of Pt was >100 times higher than in its absence. 91 Deposition of Pt nanoparticles lead to activity improvement up to 13 times compared with TiO2-P25. 92 Ag/TiO2 exhibited Photocatalytic Reduction of CO2 9.4 times higher than that of pure TiO2. 93 Pt/TiO2 nanofilms exhibited high efficiencies for photocatalytic reduction of CO2 compared to TiO2. 94 Photoreduction of CO2 showed a 24-fold enhancement in the presence of Au NPs.95 Ag/TiO2 showed more than three times higher activity than TiO2 (P25). 96

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Fig. 6 Photoluminescence spectra of as-synthesized TiO2, Ag/TiO2, and Au/TiO2. Taken from ref.111

4. Detection of noble metal role Herein, the experimental and theoretical approaches to understand the role of noble metal surface plasmon resonance (SPR) are discussed.

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4.1. Experimental approaches Photoluminescence (PL) spectroscopy technique has been used to investigate the efficiency of charge carrier trapping, migration, and transfer with the aim of better understanding the fate of electron–hole pairs in semiconductor particles 103-105. Several reports studied the effect of deposition of noble metals on TiO2 in terms of PL intensity and its impact on the photocatalytic activity of the semiconductor. When Au NPs are deposited on TiO2, the PL intensity was found to decrease as a function of Au NPs loading. This is because of the decrease in the recombination which is due to the interfacial electron transfer from TiO2 to Au NPs 106. Likewise, for the Ag/TiO2, the PL intensity was found to decrease with increasing Ag content, indicating that the Ag NPs could effectively hinder the recombination of the photoinduced electrons and holes 107. In a recent study, 108 PL emission spectra had been used for interpreting the superior photocatalytic activity of TiO2 upon deposition with Pt NPs. Compared with the pure TiO2, deposition of Pt NPs on TiO2 leads to a decrease of both the UV and visible emissions. The decrease of the UV emission for the Pt/TiO2 can be ascribed to the electron-scavenging effect of Pt, which acts as an electron acceptor due to transfer of electrons in the conduction band of the semiconductor toward the electron acceptors in the solution, thus inhibiting the recombination of charge carriers on TiO2. Also, the considerable decrease of the visible emission suggests that Pt/TiO2 may inhibit the radiative recombination process of photogenerated electrons and holes in TiO2. The slower recombination of the photogenerated charges is advantageous for photocatalysis. In a study of Yang et al., 109 PL intensities of Ag/TiO2 and Au/TiO2 were found to decrease which indicate the efficient separation of electron–hole pairs for optimal doping of noble metals on the TiO2 matrix. The same finding was disclosed in a study of Kwak et al. 110, in which PL intensity of Pd/TiO2 was lower than that of pure TiO2. They attributed this behavior due to palladium atoms which play the role of electron capturers, and hence suppress the recombination process. The same finding was also discovered by Chen et al., 111 in which, the deposition of Ag and Au nanoparticles on TiO2 leads to extinction of the PL intensity Photoluminescence as shown in [Fig. 6].

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In addition to experimental techniques, methods that based on density functional theory, DFT, calculations have become a robust research tool for looking into the geometric and electronic properties of metal/oxide interfaces. DFT calculations were found to be as a useful approaches to investigate the effectiveness of chemical bonds at the interface of Au/Pt clusters on the TiO2 [110] surface 112. Hybrid PBE (Perdew, Burke and Ernzerhof) calculations predict that the oxygen vacancy formation energy decreases by about 1.2–2.2 eV in the presence of Pt clusters and by about 0.2–0.7 eV in the presence of Au clusters. In contrast, GGA-PBE (Generalized Gradient Approximations of PBE) calculations predict that the oxygen vacancy formation energy decreases only by 0.9–1.5 eV in the presence of Pt clusters and by 0.2–0.4 eV in the presence of Au clusters. The effect is more pronounced for Pt clusters since Pt makes a strong covalent type interaction with the TiO2 surface, while Au makes only a weak filled-filled type interaction with TiO2. The formation of oxygen vacancies appears to lead to an enhancement of the photocatalytic activities of NMNPs-TiO2 under visible-light irradiation.

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Fig. 7. Optimized structures, selected bond distances (in Å), and adsorption energies (in eV) of the Mn /TiO2 surface calculated using the stoichiometric and reduced rutile TiO2 (110) cluster model Ti46O92 /Ti46O91 with PBE0 functional (only a part of the structure is given here for clarity). Taken from ref.112

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Also, DFT calculations were employed to investigate the effect of off-stoichiometric defects (oxygen vacancies or extra oxygen atoms at the Au-oxide interface) on the chemistry of oxidesupported Au nanostructures 113. The enhanced activity of supported Au is related to the existence of strong covalent bonds between the off-stoichiometric defect on the oxide support and Journal Name, [year], [vol], 00–00 | 5

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4.2. Theoretical approaches

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Under UV irradiation, the dominant proposed mechanism is the charge separation mechanism 118 in which the photogenerated electrons in the semiconductors can transfer from the conduction band to the noble metal as shown in Fig. 9. As the presence of noble metals inhibits the electron-hole pair recombination in semiconductors and therefore enhances the photocatalytic activity.

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5. Mechanism of superior activity of NMNPs/TiO2 Understanding the enhancement mechanism after deposition of noble metal upon titania is useful in the design of photocatalysts with high photocatalytic efficiencies. In this section, several mechanisms will be discussed in order to stand on the optimum mechanism. There are two forms of electronic transitions due to the interaction with electromagnetic light interband and intraband transitions 116, 117. Interband transitions occur from the occupied to the empty bulk bands. The energy required for this transition usually falls in the UV region of the electromagnetic spectrum. For Intraband transitions, it occurs due to an electronic transition at the Fermi level in incompletely filled bands, or when a filled band overlaps in energy with an empty band. The energy required for this transition usually falls in the visible-NIR region of the electromagnetic spectrum. A question must be raised here, upon irradiation with UV and visible light, whether noble metals and titania excited either separately or simultaneously? To answer this question four possibilities have been postulated as shown in Fig. 8, according to the following 33:

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Fig. 9. Schematic diagram illustrating the charge separation mechanism.

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Also another mechanism is postulated and based on the interband transitions in the NMNPs from fully occupied d-bands below the Fermi energy to the half filled sp band 119. NMNPs exhibit ultraviolet light absorption, causing the interband transition of 5d electrons to the 6sp band, 4d electrons to the 5sp band and 5d to the 6sp band for Au, Ag and Pt NPs respectively 120-122. For more clarify, as shown in Fig. 10, in a study of the photodegradation of phenol, due to absorption of ultraviolet irradiation, excitation of electrons from the 4d band to the 5sp band, yields high energy photogenerated electrons. Also the holes left in the inner d band have a greater tendency for capturing electrons from phenol than those in the outermost sp band 121.

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Fig. 8 Different irradiation states of the plasmonic photocatalytic systems. (a) Bandgap-excitation state, (b) LSPR state, (c) Dualexcitation state; (d) Separate-excitation state. SC stands for semiconductor. Taken from ref.33 The four states (i) Bandgap excitation state, only the semiconductor is excited; (ii) LSPR state, only the metal nanoparticle is excited due to localized surface plasmon resonance (LSPR); (iii) Dual-excitation state, both the metal nanoparticle and the semiconductor are excited simultaneously by the same light intensity; (iv) Separate-excitation state, the semiconductor is excited by UV and the metal nanoparticle is excited by visible light due to LSPR. Since the enhancement mechanisms under UV and visible illumination are different, they will be discussed on an individual basis.

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Fig. 10. The band structures of the supported Ag NPs. Taken from ref.121 As the energy of the UV irradiation is much higher than the SPR, therefore, the photocatalytic enhancement under ultraviolet is not related to the surface plasmon resonance of the NMNPs. So one can state that the two main characteristics of the NMNPs (namely, Schottky barrier and SPR) are the key factors for its function under ultraviolet and visible light illumination respectively 123-125.

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the Au nanostructure. Defects on oxide surfaces play a critical role not only in anchoring metal particles, as had been proposed previously, but also in imparting chemical activity on the particles 113. Other study of Andrea et al. 114 disclosed that DFT calculations predict a considerable increase of TiO2 reactivity upon deposition of titania films on Ag NPs, and they attributed this to the charge transfer from Ag NPs to the 3d Ti empty states. The same findings were published by Mete et al. 115, in which by DFT calculations they proved that Pt and Au noble metals promote the photocatalytic activity of TiO2 [110] by narrowing of the band gap towards the visible region which results from impurity driven defect states of Pt and by acquiring metallic character due to an unpaired 6s electron.

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Tian et al. 126 proposed a charge transfer mechanism to explain the enhancement effect upon loading Au or Ag NPs into TiO2 films under visible light illumination. In their proposed mechanism, the plasmon resonance excites electrons in Au or Ag, which are then transferred to the conduction band of TiO2. This charge transfer mechanism is similar to that of a dye-sensitized solar cell 127. Subsequently, other reports disclosed this mechanism to explain the enhanced photocatalytic water splitting 128 , methyl orange decomposition 129, and photooxidation of formaldehyde 130 observed under visible illumination. Mubeen et al. 131 further elucidated this charge transfer mechanism, in which the surface plasmon decay generates electron-hole pairs in the gold, and the hot electrons produced in the decay of localized surface-plasmon polaritons which excited from Au NPs are directly injected into TiO2 by quantum tunneling. Another mechanism proposed for plasmon enhanced photocatalysis is based on the local electric field enhancement associated with plasmonic nanoparticles 132. In this mechanism, the plasmon enhancement was attributed to the strong electric fields produced by the surface plasmon resonance of Au or Ag NPs. The collective oscillations of conduction electrons in metal nanoparticles resonate with the electromagnetic field of the incident light, which results in a considerable enhancement of the local electromagnetic fields at the metal-semiconductor interface. The schematic diagrams in Fig. 11 illustrate the difference between the local electric field mechanism and the charge transfer mechanism.

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Fig. 12. Photoinduced electron transfer and photodegradation of malonic acid at Au/TiO2. Taken from ref.139 80

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Fig. 11. Schematic diagrams of the charge transfer mechanism (A) and the local electric field enhancement mechanism (B). In a study of Liu et al. 133 electromagnetic simulations indicate that the enhanced photocatalytic activity in the visible range is due to the local electric field enhancement near the TiO2 surface, rather than the direct transfer of charge between the two materials. Recently Zhang et al. 134 suggested three mechanisms that could explain the enhanced photochemical reactivity of the noble metal modified TiO2 compared to pure TiO2 under visible light irradiation. The first mechanism includes photoexcited metal due to plasmon resonance can inject electrons into the conduction band of TiO2 upon excitation. Nevertheless, the band gap energies for e-/h+ are very different from the metal– semiconductor interface and there are no specific models for this process 135, 136. Another mechanism is based on the interaction of TiO2 with localized surface plasmon resonance (LSPR) induced electromagnetic fields (near field effect), which causes plasmon

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Also, for another evidence of electron transfer direction under both visible and UV irradiations, EPR spectroscopy was utilized to monitor the electron migration through noble metal-oxide interface. Vijayan et al. 140 disclosed the EPR spectra of TiO2–NT (nanotubes) and Pt/TiO2–NT under UV and visible light illumination. They indicated that photogenerated electrons localize both in the bulk and at the surface of anatase TiO2-NT under UV light illumination. While upon illuminated at λ > 400 nm, the photon energy is inadequate to excite electrons from the valence to conduction band of anatase; thus the EPR spectrum exhibits the absence of any signals for oxidized nanotubes [Fig. 13d]. Therefore, based on the disclosed results we can comfortably affirm the conclusion about the electron transfer through the noble metal/TiO2 interface under different illuminations (i.e. from Au NPs to TiO2 under visible and from TiO2 to Au NPs under UV).

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resonance energy transfer (PRET) from metal NPs to TiO2 27,137. The effect of PRET is to enhance the electric field intensity in a small, well-defined location of the semiconductor, thereby increasing the power absorbed in that region. This procedure results in the rapid formation of e-/h+ pairs in the semiconductor. The third mechanism is based on efficient scattering mediated by LSPR (far field effect), which leads to longer optical path lengths for photons in TiO2 that raise the excitation of e-/h+ pairs 138. In order to differentiate between the mechanism under ultraviolet and visible irradiations a study of Hu et al. 139 disclosed that Attenuated total reflectance (ATR) spectra revealed that the interface electron transfer from Au NPs to TiO2 conduction band under visible light irradiation and from TiO2 to Au NPs under UV illumination [ Fig. 12].

Fig. 13. EPR spectra of anatase nanotubes under UV and visible light illumination. (a, d) nanotubes calcined at 400 C in a flow of oxygen; (b, e) nanotubes calcined at 400 °C in a flow of Journal Name, [year], [vol], 00–00 | 7

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hydrogen;(c,f) nanotubes with 0.5% Pt-doped nanotubes calcined at 400 °C in a flow of hydrogen. Taken from ref. 140

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For the sake of ease understanding the electron migration direction, herein we present the photocatalytic process reactions of Au/TiO2 for photodegradation of organic pollutants upon illumination with UV (Eq. 5-11) and visible light (12-13) irradiations 141: TiO2+ hυ → TiO2(ecb− + hvb+) hvb+ + H2Oads→ OH• + H+ hvb+ + OH−→ OH• TiO2(ecb−) + Au → TiO2–Au(e−) TiO2–Au(e−) + O2→ TiO2–Au + O2•− • OH + pollutant → H2O + CO2 O2•− + pollutant → H2O + CO2 Au + h(visible) → Au+(e−) TiO2+ Au+(e−) → TiO2(e−) + Au

(5) (6) (7) (8) (9) (10) (11) (12) (13)

Under UV irradiation, Au NPs trap the photogenerated electrons from TiO2 thereby inhibits the recombination (Eqs. 8, 9). However, under visible light irradiation, Au NPs act as light harvester therefore absorbing photons and then injecting electrons into TiO2 conduction band (Eqs. 12, 13) 141.

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It is known that the function of noble metals upon deposition on TiO2 NPs is dependent on some different operational parameters which presides its efficiency even under identical photodegradation conditions. The morphology and the particle size are important parameters for understanding the optical properties of the noble metals 142. Also the deposition method such as photo-deposition (PD), deposition-precipitation (DP), impregnation (IMP) and colloidal deposition (CD) has a great effect on the characteristics of the obtained nanoparticles in terms of shape, dispersion and particle size which in turn affects the photocatalytic efficiency. That way, it was disclosed that the photoproduction of H2 using Pt/TiO2 and Au/TiO2 NPs is decreased in the order photodeposition> deposition-precipitation > impregnation method 143. However there is a quasi-consensus that the PD method gave the most reactive catalyst among the others 144, there are no rigid interpretation to this finding. In this perspective, our focus is exclusively oriented toward the two major factors: dosage and particle size.

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Wongwisate et al. 147 attributed the enhanced photodegradation of 4-ChloroPhenol upon decreasing the Au and Ag loading amount on TiO2 to: (1) Au NPs on the TiO2 surface could cause a strong increase of the rate of oxygen reduction, (2) the accelerated formation of superoxide anion radical, O2 - , in case of small amount of Ag NPs loading resulting in decreasing the charge recombination probability. In this study the reported optimum value was (> 0.1%) for both Ag and Au NPs. Tada et al. 148 disclosed the same behavior for the proportional relation between the loading amount and enhancement effect of Ag NPs loaded TiO2 for the photodegradation of Bis(2-dipyridyl)disulfide and referred the detrimental effect upon exceeding the optimal dosage (0.24 %) to the shielding effect and recombination of photogenerated charge carriers. Also Smith et al. 149 obtained the same finding and attributed the detrimental effect upon exceeding the optimal dosage due to the reduced activity in the TiO2 surface area. And they reported the value of 0.25 % for loading Ag NPs on TiO2 as an optimal dosage for the photodegradation of MB dye 149. In a recent study of Wen et al. 150, the optimal dosage for Au NPs deposited on TiO2 nanotubes for photodegradation of MB was found to be 5% and they interpreted the detrimental effect in terms of blocking TiO2 pores and in turn reducing the surface area. Liu et al. 151 studied the photocatalytic activity of Ag/TiO2 NPs for phenol oxidation. They attempted to interpret the correlation between the Ag NPs loading amount and the photocatalytic efficiency in terms of EPR spectra. One can see from Fig. 14 that the intensities of signals C and D which are assigned to O2•- produced on the loaded Ag NPs and surface Ti3+ ,respectively, are first enhanced with increase of silver loading up to 1.026 wt.%, and then decreased with further increasing of silver loading. Such EPR behavior of the NPs was found to be corresponding well with phenol photocatalytic oxidation. Therefore based on these results one could somehow refer the proportional relation between the in photocatalytic activity and the EPR signals intensity to the amount of photogenerated electrons. This could be clarified in terms of electron-hole separation as upon increasing the loading amount of Ag NPs up to the optimum content, better electron hole separation was achieved. Upon further increase of Ag NPs beyond the optimum content, Ag NPs became a recombination center of photogenerated electrons and holes. Another supportive explanation according to the authors was based on the possibility of decreasing the amount of received photons when high (more that the optimal) loading of Ag NPs is used.

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The dosage of NMNPs is a critical component for determining the effect of noble metal deposition on TiO2 NPs to verify whether it will be a separation or recombination centers 145. Therefore, many reports studied such effect and almost all the results are consistent in the same trend that upon increasing the loading amount up to a specified limit, the photoreactivity increases. Beyond this value, the photoreactivity gradually decreases due to the so called “screening effect” which obscures a significant portion of the semiconductor surface from photons for band gap excitation and decrease in O2 - formation 146. 8 | Journal Name, [year], [vol], 00–00

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Fig. 14. EPR spectra of Ag/TiO2 with different silver loading after UV light irradiation TiO2 (0), 0.366 wt.% loading (1), 0.763 wt.% loading (2), 1.026 wt.% loading (3), 1.474 wt.% loading (4), 1.867 wt.% loading (5), 2.320 wt.% loading (6). Taken from ref. 151 Another explanation for decreasing the photocatalytic activity beyond the optimum loading is the work of Jovic et al.152, where they found that for Au/TiO2-P25 photocatalysts, the activity continuously decreasing with Au loading above 2 % (optimal dosage). The reason was referred to the fact that at higher Au loadings, electron transfer between rutile, anatase and Au at the ‘hot spot’ sites is adversely affected which in turn affects the H2 production rate. This is because Au NPs deposits at the interface sites between TiO2 crystallites (which creates hotspots and favors H2 production) and also deposits on isolated anatase or rutile crystallites (which creates less active sites for H2 production) as clearly shown in Fig. 15 152.

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The size of NMNPs is regarded as one of the main factor among the other in controlling its function. As a general rule, upon decreasing the particle size, the photocatalytic activity decreases. In spite of that, there are different interfering parameters which esteem the particle size effect like surface area, surface coverage, dispersion and the screening effect. Many studies showed a great interest in studying the opposite correlation between the size of noble metals specially Au NPs deposited on TiO2.This relation could arise from several parameters, including the surface plasmon band position and intensity, Fermi levels of NMNPs, efficiency of electron ejection and interfacial contact between noble metals and titania 153. Recently in a relevant study by our group 72, we showed results revealing that the variation of Au NPs sizes deposited on TiO2 affects the Safranin-O photodegradation as the photodegradation rate constant (k) of Safranin-O dye clearly decreases with increasing the size of Au NPs as shown in Fig. 16. We proposed some reasons for the superior effects of Au deposits on photoactivity including: (1) the greater number of Au NPs at the surface of the catalyst, as the smaller sizes of the deposited Au NPs, the more the Au atoms present at the surface; (2) the quantum size effect of the Au NPs (3nm < dAu < 8 nm) 154, and (3) Also we considered the so-called “screening effect” of the large Au NPs, as the deposited large Au makes part of the photocatalyst surface less accessible for photons which results in less photoreaction rate.

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Fig. 15. Effect of Au particle size on the photodegradation rate constant (k) of SO dye. Taken from ref. 72 However, the majority of researcher’s interpretations to this phenomenon were directed mainly to the shift of Fermi level of NMNPs. Cojocaru et al. 141 stated that as the Fermi energy of the NMNPs increases with the decrease of the particle size, due to a quantum size effect, NMNPs with a definite size can possess an energy level in between that of the conduction band of TiO2 and the adsorbed oxygen to be able to capture the photoelectrons and subsequently transferred to the adsorbed oxygen thus leading to Journal Name, [year], [vol], 00–00 | 9

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Fig. 15. Transmission Electron Microscopy,TEM, images for (a) P25 TiO2; (b) 1 wt.% Au/P25 TiO2; (c) 3 wt.% Au/P25 TiO2; (d) 5 wt.% Au/P25 TiO2; (e) 8 wt.% Au/P25 TiO2 and (f) 10 wt.% Au/P25 TiO2. Average Au nanoparticle sizes were similar at all Au loadings. Taken from ref.152

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Based on the above parameters, we can comfortably conclude that the photodegradation process is a complicated process as it depends on a variety of parameters in non-systematic regime. The optimal dosage and size of NMNPs required for achieving the best photocatalytic performance of TiO2 could be different in various systems due to different morphologies, surface coverage, deposition method, and the operational parameters.

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Acknowledgements The authors gratefully acknowledge the support of Kuwait University Research administration, Project No. (SC 05/12). 65

Notes and references

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Conclusion

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It is well known that TiO2 photocatalysis is attracting many researchers’ interests. This is due to its distinctive physicochemical and electronic properties which enable it to be used in a variety of applications like photoelectrochemical conversion, memory devices, air and water purification and water splitting to produce hydrogen, etc. The fact that TiO2 is able to absorb only UV irradiation which regarded as a small portion of sunlight represents a great limitation to its use as a photocatalyst. So the main point in this perspective was oriented toward understanding the enhancement effect upon modification of TiO2 with noble metals in order to minimize the major drawbacks of TiO2 as a photocatalytic material, i.e. its high band gap energy and recombination of charge carriers. In order to clarify the role of noble metal deposition effect in a better way, experimental and theoretical explanations were provided in this perspective. The mechanism of noble metal effect had been issued in many reports. So herein, a detailed description of most of the suggested mechanisms was provided. Also, to a certain extent, the controversy of electron migration direction through noble metal/TiO2 interface under UV and visible irradiation was resolved by discussing EPR results. Finally we presented many researchers’ investigations about the effect of loading amount and particle size of NMNPs on the photocatalytic activity of TiO2. Their findings in this regards were discussed with the help of Transmission Electron Microscopy, TEM, and the innovative EPR technique. We believe that loading amount and particle size parameters have complex impacts. So, one should not only refer the effect of such parameters to the screening and the quantum size effects. Breaking in these complex impacts will open the floodgates for rational design of ideal noble metal modified titania 10 | Journal Name, [year], [vol], 00–00

photocatalyst. So, in spite of the great efforts that have been made to study NMNPs/TiO2, it is obvious from this perspective that further progress is still needed to sheds further light on the superior photocatalytic behaviors of NMNPs-modified titania.

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an effective separation of charges, and an increase of the photocatalytic activity of titanium. For further clarification, the apparent Fermi levels for Au/TiO2 were determined to be -250, -270, and -290 mV vs. NHE (Normal Hydrogen Electrode) for 8, 5, and 3 nm size gold NPs respectively. Since the energy levels in the gold NPs are discrete, it is expected that a greater shift in the energy level for smaller size Au NPs than for larger ones. Thus, the composite with smaller Au NPs is expected to be more reductive than the one with larger Au NPs 155-157. In a recent study, 8.6 nm of gold nanoparticles in Au/TiO2 was found to be the optimal size in improving the photocatalytic activity of TiO277. In another study, for the photocatalytic production of H2 over Au/TiO2, the reaction rates increased with Au particle size up to 12 nm 158. Based on the disclosed results in this trend, standing on the exact appropriate size of noble metal deposited on TiO2 so as to achieve the maximum photoefficiency is elusive so far.

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Electron trapping

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eVis

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Surface Plasmon Resonance effect

Dual role of NMNPs for enhancing TiO2 photocatalytic activity under both UV and visible light irradiations

ysical Chemistry Chemical Physics Accepted Manuscri

Published on 27 January 2014. Downloaded by Aston University on

Physical Chemistry Chemical Physics

Understanding the superior photocatalytic activity of noble metals modified titania under UV and visible light irradiation.

Although TiO2 is one of the most efficient photocatalysts, with the highest stability and the lowest cost, there are drawbacks that hinder its practic...
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