Effect of Modification of TiO2 with Metal Nanoparticles

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The effect of metal cluster deposition route on structure and photocatalytic activity of mono- and bimetallic .... scavenging by metal are responsible for this ratio.
Effect of Modification of TiO2 with Metal Nanoparticles on its Photocatalytic Properties Studied by Time Resolved Microwave Conductivity Hynd Remita,* María Guadalupe Méndez Medrano and Christophe Colbeau-Justin

Laboratoire de Chimie Physique, CNRS UMR 8000, Univ Paris-Sud - Université Paris-Saclay, 91405 Orsay, France E-mail : [email protected]

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INTRODUCTION

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1.

DEPOSITION OF METAL NANOPARTICLES BY RADIOLYSIS AND BY PHOTODEPOSITION METHOD 5

2.

ELECTRONIC PROPERTIES STUDIED TIME RESOLVED MICROWAVE CONDUCTIVITY

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3.

SURFACE MODIFICATION OF TITANIA WITH MONOMETALLIC NANOPARTICLES

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3.1.

Surface modification of titania with Pt clusters

3.2.

Surface modification of TiO2 with Pd nanoparticles

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3.3.

Modification of TiO2 with Ag nanoparticles

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3.4.

Modification of TiO2 with Au nanoparticles

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3.5.

Modification of TiO2 with Bi clusters

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4.

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SURFACE MODIFICATION OF TIO2 WITH BIMETALLIC NANOPARTICLES

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4.1.

Surface modification with Au-Cu nanoparticles

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4.2.

Surface modification with Ag and CuO nanoparticles

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4.3.

Co-modification of TiO2 with Ni and Au nanoparticles for hydrogen production

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4.4.

TiO2 modified with NiPd nanoalloys for hydrogen evolution

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4.5. The effect of metal cluster deposition route on structure and photocatalytic activity of mono- and bimetallic nanoparticles supported on TiO2 29

5.

SUMMARY

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REFERENCES

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Introduction TiO2 is a very efficient photocatalyst due to its strong oxidation capacity, high photochemical and biological stability and low cost. Since the discovery of photoinduced decomposition of water on a TiO2 electrode, TiO2-based photocatalysts have attracted wide attention [1]. The limitation in TiO2 application, results from low quantum yield due to fast charge carriers (electron/hole e-/h+) recombination and its activation only under UV irradiation because of the value of its band gap (3.2 eV for anatase and 3.0 eV for rutile) [2]. UV light constitutes only about 3-4 % of the solar spectrum impinging on the Earth’s surface, therefore modification of titania to extent its absorption to visible domain and to enhance its activity is a very active area of research. Doping TiO2 with N, C or S has been used to extent its activity towards the visible [3,4]. Doping TiO2 with ions such as Rh3+ or Bi3+ was also used to enhance its photocatalytic activity under visible light [5–7]. Surface modification with noble metal (platinum, palladium, silver, gold) nanoparticles (NPs) can result in enhancement of the photo-conversion quantum yield and may allow the extension of the light absorption of wide band-gap semiconductors to the visible light [8–10]. In particular, plasmonic photocatalysts have appeared as a very promising way to induce a photocatalytic activity of TiO 2 in the visible [11,12]. Coupling titania with another semiconductor of a smaller band gap (such as CdS or Bi2S3) is also a way to enhance the photocatalytic efficiency by decreasing the recombination rate and inducing a photocatalytic activity in the visible range [13]. The applications of photocatalysts concern mainly: self-cleaning surfaces, water and air treatment, and solar fuel production. Artificial photosynthesis to recycle CO 2 is also a topic of increasing interest. The photocatalytic activity of TiO2 compounds is related to the creation and the evolution of charge-carriers in the photocatalyst [14]. Thus, the knowledge of the relation existing between charge-carrier lifetimes and material structural parameters can help to understand the mechanisms leading to the photoactivity. To follow the charge-carrier dynamics in TiO 2, the variation of the sample conductivity after illumination must be determined. Time Resolved Microwave Conductivity (TRMC) is a contactless method, based on the measurement of the change of the microwave power reflected by a sample induced by laser pulsed illumination [15,16]. The TRMC signal allows following directly the decay of the number of electrons and holes after the laser pulse by recombination or trapping of the charge carriers. We summerize in this chapter studies on surface modification of TiO2 with mono- and bimetallic nanoparticles (NPs) (mainly synthesized by radiolysis) for photocatalytic applications (water depollution and hydrogen generation). We show here that TRMC is a very powerful method to study charge-carrier dynamics and to understand the effect of semiconductor modification on its photocatalytic activity. In this chapter, we present different examples of modification of TiO2 with metal nanoparticles for photocatalytic applications: water depollution (mainly phenol photodegradation) and hydrogen production. The relation between titania modification, chargecarrier dynamics (electronic properties) and photocatalytic activity has been investigated. It has been evidenced that one can correlate the modification with metal NPs with the change of the photoconductivity signal and the photocatalytic activity. A strong influence of structural parameters on the photoconductivity is observed, and a relation between the photoconductivity and the photoactivity may be evidenced. 4

1. Deposition of metal nanoparticles by radiolysis and by photodeposition method Noble metal nanoparticles (and in particular plasmonic NPs) have been the subject of strong interest, because of their catalytic properties [17], and their ability to confine high electromagnetic energy within their small particle size owing to the localized surface plasmon oscillations of the conduction band electrons. Gold and silver nanoparticles (AuNPs and AgNPs) have attracted increasing attention because of their optical, catalytic and electrocatalytic properties. Metallization of TiO2 surface with noble metals such as Pt, Ag, and Au has been investigated from the early times of photocatalysis to increase the photocatalytic activity [1,11,18–21]. Different studies have shown that metal doped semiconductor composites exhibit shifts in the Fermi level to more negative potentials. One important factor that can influence the electronic properties of the TiO 2-metal composite is the size of the metal nanoparticles and the shift in the Fermi level is size-dependent. This shift enhances the efficiency in the interfacial charge transfer process and improves the energetics of the composite system. Different methods have been developed for the synthesis of metal nanoparticles on inorganic semiconductors such as TiO 2. Among them, the photochemical and radiolytic methods are versatile and powerful methods to synthesize metal nanoparticles of controlled size, shape and to induce bimetallic nanoparticles and composite materials [22,23]. These methods present the advantage of simple physico-chemical conditions (room temperature, absence of contaminants) and lead to homogeneous reduction and nucleation. In the case of radiolysis, solvated electrons and reducing radicals are generated by solvent excitation. These reducing species reduce the metal precursors (present in solution), which undergo nucleation and growth. In the photochemical approach, the metal precursors (salts or complexes) can be, either directly excited by light and then reduced, or photochemically generated intermediates, such as excited molecules and radicals, can be used for their reduction. [23] Radiolysis is a powerful method to synthesize nanoparticles of controlled size and shape in solution and in heterogeneous media [22]. Solvent radiolysis induces formation of solvated electrons and radicals, which reduce metal ions homogeneously in the medium leading to a homogeneous nucleation. Small and relatively monodisperse nanoparticles can therefore be obtained. Radiolysis presents the advantage of inducing a homogeneous nucleation and growth in the whole volume of the sample and has been used successfully in order to synthesize various noble (such as silver, gold and platinum) and non-noble (such as nickel, iron and colbalt) metal nanoparticles in solution or on supports [24]. The primary effects of the interaction of high-energy radiation such as electron or ion beams, X-rays or gamma photons with a solution of metal ions are the excitation and th e ionization of the solvent. For example, in aqueous solutions according to equation (1): 𝛾−𝑟𝑎𝑦

𝐻2 𝑂 →

− 𝑒𝑎𝑞 , 𝐻3 𝑂+ , 𝐻 . , 𝐻𝑂. , 𝐻2 , 𝐻2 𝑂2

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(1)

Solvated electrons e -aq (E0 (H2O/e-aq) = - 2.87 VNHE) [25] and alcohol radicals are strong reducing agents able to reduce metal ions to lower valences and finally to metal atoms. During . the irradiation of deoxygenated water, hydroxyl radicals (HO ), which are very strong oxidative .

species (E0(HO /H2O) = +2.34 V NHE at pH 7) [26], are also formed. To avoid competitive oxidation reactions, which may limit or even prevent metal reduction, hydroxyl radical scavengers are added in solution prior to irradiation. Among these scavengers, primary or secondary alcohols (such as propan-2-ol) molecules or formate ions, which also react with hydrogen atoms, are generally used [27]: 𝐶𝐻3 𝐶𝐻𝑂𝐻𝐶𝐻3 + 𝑂𝐻 • → 𝐶𝐻3 𝐶 • 𝑂𝐻𝐶𝐻3 + 𝐻2 𝑂 𝑘1 = 1.9𝑥109 𝐿𝑚𝑜𝑙 −1 𝑠 −1 𝐶𝐻3 𝐶𝐻𝑂𝐻𝐶𝐻3 + 𝐻 • → 𝐶𝐻3 𝐶 • 𝑂𝐻𝐶𝐻3 + 𝐻2

𝑘3 = 7.4𝑥107 𝐿𝑚𝑜𝑙 −1 𝑠 −1

𝐻𝐶𝑂𝑂− + 𝐻𝑂• 𝑜𝑟 𝐻 • → 𝐶𝑂𝑂•− + 𝐻2 𝑂 𝑜𝑟 𝐻2

(2) (3) (4)

.

Due to their redox potentials (E 0((CH3)2CO/(CH3)2 C OH) = - 1.8 VNHE at pH 7 [28] and E0 .

(CO2/ COO -) = -1.9 VNHE) [29], the radicals formed by reactions (3) and (4) are almost as .

.

powerful reducing agents as H atoms (E0 (H+/H ) = - 2.31 VNHE). When a solution containing metal ions is in contact with a solid support, the ions can diffuse in the pores and can be adsorbed on the surface. Therefore, the penetration of the ionizing radiation enables in situ reduction of metal ions and then the further coalescence of metal atoms inside the confined volumes of polymeric membranes, mesophases, porous materials, such as zeolites, alumino-silica-gels or colloidal oxides such as TiO 2. Metal nanoparticles can be induced in porous matrixes (including mesoporous oxides such as SiO 2 or TiO2). A preparation process of a composite material (metal NPs/porous oxide) consists of impregnating a microporous or mesoporous solid material with metal precursors, then of reducing the impregnated material by radiolysis. Gold nanoparticles were directly synthesized by radiolysis on TiO2 for photocatalytic applications [30]. Photocatalytic reduction of metal complexes on semiconductors is also an efficient way to design metal-semiconductor composites [8]. Generally, the photoirradiation is carried out for solutions containing metal ions, a semiconductor support, and hole scavengers. The photo absorption of semiconductor supports generates electrons and holes. The metal precursors (ions or complexes) adsorbed on the semiconductor surface can be reduced on the surface by the photogenerated electrons. This approach has been used to deposit metal nanoparticles on TiO 2 to enhance its photocatalytic activity [31,32] and to extend its absorption from the ultraviolet (UV) to visible range. Gold nanoparticles were photodeposited on different TiO 2 surfaces: during the photodeposition process under UV light, methanol was used as a sacrificial hole scavenger resulting in Au/TiO2 powders of different colors (violet, pink and grey) with broad absorption bands in the wavelength range of ca. 400-700 nm and with a peak maximum at ca. 530-610 nm. [19,33]. Silver nanoparticles can also be easily deposited on TiO 2 by photoreduction (using for example benzophenone as photosensitizer) or by photocatalytic deposition (by direct formation on illuminated TiO2 surface) [34]. 6

2. Electronic properties studied Time Resolved Microwave Conductivity Charge-carrier lifetimes in bare and modified TiO 2 after UV illumination were studied by Time Resolved Microwave Conductivity method (TRMC) [15,16]. The TRMC technique is based on the measurement of the relative change of the microwave power reflected by a sample (semiconductor), ΔP(t)/P, during its simultaneous irradiation by a laser pulse. Such relative variation can be correlated to small perturbation of the sample conductivity, Δσ, as shown in the following equation: ∆𝑃(𝑡) = 𝐴∆𝜎(𝑡) 𝑃

(5)

where A is a time independent proportionality factor. Because the electron mobility, μe, in TiO2 is much larger than the hole mobility, Δσ(t) can be attributed to excess electrons: 𝐴∆𝜎(𝑡) ≈ ∆𝑛(𝑡)𝑒𝜇𝑒

(6)

The signal obtained by this technique displays the evolution of the sample conductivity, I(t), (denominated photoconductivity) as a function of time (ns). The main data provided by TRMC are given by the maximum value of the signal (Imax), which reflects the number of the excess charge-carriers created by the laser pulse, and the decay is due to the decrease of the excess electrons (free electrons) [16]. To analyze the decay, the signal is divided in two sections: shortand long-range decays. The short-range decay, arbitrarily fixed up to 40 ns after the maximum of the pulse, is represented by the I 40ns/Imax ratio, which reflects the fast processes active during and just after the pulse. Most probably electron-hole recombination and possibly electron scavenging by metal are responsible for this ratio. The long-range decay, here fixed from 200 until 1000 ns, is related to slow processes involving trapped species, i.e., interfacial charge transfer reactions and decay of excess electrons controlled by the relaxation time of trapped holes. In this range, the decay of TRMC signal can be fitted to a power decay according to: 𝐼 = 𝐼𝐷 𝑡 𝑘𝐷

(7)

where ID is the intensity of the signal due to charge-carriers that recombine after 200 ns, and kD is an adimensional parameter related to their lifetime: higher k D values correspond to faster decays of the TRMC signal. The numbers of incident photons in the sample, expressed as nanomole of photons also called nanoEinstein (nano-ein), were calculated by the following equation:

𝑛ℎ𝜐 =

𝐸∙𝜆 ℎ ∙ 𝑐 ∙ 𝑁𝐴

7

(8)

where E is the excitation energy (J),  is the wavelength (nm), h is the Planck constant (J.s), c the speed of light (m/s) and N A the Avogadro constant (mol -1). Imax/𝑛ℎ𝜐 values for each wavelength were plotted. The obtained graphic is called “TRMC action spectra”.

3. Surface modification of titania with monometallic nanoparticles 3.1. Surface modification of titania with Pt clusters Several studies report on visible light photoactivity of small metal clusters. These clusters formed by a few metal atoms exhibit molecular-like excited state properties with well defined absorption and emission features [35–38]. [Pt3(CO)6]n2- (n=3-10) clusters (called Chini clusters) absorb strongly in the visible domain and their optical and electronic properties can be tuned with size [39]. These clusters can be easily synthesized by radiolysis by reduction of Pt complexes in alcohol solution under CO atmosphere. Doping silver halides with [Pt 3(CO)6]n2- (n = 3-10) clusters induces enhancement of the photoconversion yield by inhibition of the electron-hole recombination [40]. Pt clusters/TiO2 composites absorb in the visible range due to high absorption of platinum Chini clusters in this region [21]. [Pt3(CO)6]6 2- are green and present two specific narrow absorption bands at 430 and 802 nm. TiO2-P25 (P25 is a commercial TiO2 with high activity under visible light, it exhibit a surface area of ca. 50 m 2 g-1 and consists of a mixture of the crystalline phases anatase (73-85%), rutile (14-17%) and amorphous titania (0-13%)) [41] and synthesized by sol-gel method) modified with platinum complexes (Pt(II) and Pt(IV) complexes) or [Pt3(CO)6]62- clusters exhibit higher photocatalytic activity compared to bare titania. The photocatalytic properties of Pt-modified TiO2 were studied for phenol and rhodamine B (RB) degradation (taken as model pollutants). Phenol is one of the most employed test molecule. It has been proposed by Serpone et al. [42] as standard test molecule, and presents some advantages: - It does not undergo degradation by photolysis or catalysis. - It presents an absorption band at 269 nm detectable by UV-Visible spectroscopy. - Its degradation mechanism is quite identified; the principal intermediates are benzoquinone, hydroquinone, and catechol [43]. - It follows a complete mineralization to CO 2 and H2O. - It adsorbs very weakly at the surface of TiO 2. - It is a real pollutant of water. Guo et al. suggested that, in photocatalytic degradation of phenol by TiO 2, the attack of •OH radicals on phenyl ring is the first stage of photocatalytic process which leads to the formation di- and trihydroxybenzenes and, subsequently, to opening of the phenyl ring and forming maleic acid among other intermediate products [44]. Faster degradation of phenol and RB was obtained with Pt-modified TiO2 both under UV and visible light (Figure 1). In this work, platinization lowers the signal, but the influence on the decay is variable and can be related to the activity in the case of phenol degradation with UV light. Surface modification by Pt clusters slows the I decay, showing a slower charge-carrier recombination, which is beneficial to the photoactivity. TRMC signals show that under UV irradiation platinum clusters act as charge scavengers hindering charge carrier recombination ( 8

Figure 2). Modification of TiO2 with metal nanoparticles or clusters leads to more efficient electronhole separation and to enhanced HO and O2.- radicals formation. O2•−radicals can be subsequently transformed, via H 2O2, to •OH radicals which are thought to be the most responsible for photocatalytic degradation of phenol. HO• radicals can be simultaneously generated via direct oxidation of water molecules by photogenerated holes [45]. Thus, higher efficiency of HO• radical generation can lead to higher photoactivity.

Figure 1. Degradation of phenol (2 x 10-4 M phenol initial concentration) with pure or modified titania (1 g/L photocatalyst) synthesized by sol-gel method (surface modified and non-modified with Pt salt (II), PtCl4 2-; Pt salt (IV), PtCl6 2-; Pt cluster, Pt3(CO)6]6 2-): (a) Under UV/vis light;(b) Under visible light (>450 nm) [21]. (Copyright 2008 American Chemical Society).

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Figure 2. TRMC signals after excitation at 355 nm of pure or modified titania with Pt salt (II), PtCl 42-; Pt salt (IV), PtCl62-; Pt cluster, [Pt3-(CO)6]62-: (a) P25; (b) TiO2 synthesized by sol-gel technique [21]. (Copyright 2008 American Chemical Society).

3.2. Surface modification of TiO 2 with Pd nanoparticles The surface of four commercial TiO 2 compounds (Cristal Global PC-series) has been modified with 3 nm-Pd nanoparticles induced by gamma radiolysis [46]. Their photocatalytic properties have been studied for phenol and rhodamine B photocatalytic degradation in aqueous suspensions under UV and visible light. Their electronic properties have been studied by Time Resolved Microwave Conductivity (TRMC) method to follow the charge-carrier dynamics. The experiments evidence a complex behavior of the surface-Pd. Its influence depends on the pollutant and on the irradiation. The modification may be strongly favorable to the photocatalytic activity. The results have been interpreted in terms of modification of charge carrier dynamics with TRMC measurements. For phenol photocatalytic degradation, under UV irradiation, the surface-Pd increases the photocatalytic activity of PC50 and PC10. Those results can be directly related to the slowdown of the TRMC decay proving that the surface-Pd can help to avoid charge-carrier recombination. For RB photocatalytic degradation, the surface-Pd always promotes the photocatalytic activity under UV irradiation, especially for PC50, whereas it is without effect under visible irradiation. The surface-Pd can play a role in charge-carrier separations, leading to an improved photocatalytic activity under UV-light. However, the Pd surface modification does not lead to an important change in the absorption properties and to a significant formation of charge-carriers at 532 nm to create a real photocatalytic activity under visible light. Pd NPs play a role in charge-carrier separations, increasing the photocatalytic activity under UV-light, but show no effect on the absorption properties, preventing the creation of an activity under visible light. In another study, Litter et al. reported heterogeneous photocatalytic reduction of nitrate (which is a pollutant related to human activities, with particular impact in groundwaters and drinking water). (2 mM) in the presence of formic acid (10 mM) using bare and modified TiO2 samples under UV–vis irradiation (at pH 3) [47]. Commercial samples (Evonik P25 and Cristal Global PC500 and PC10) were modified with two noble metal (Ag and Pd) nanoparticles (induced by radiolysis). P25 was modified with Ag (0.5 and 2% w/w), while PC10 and PC500 were modified with Pd (1% w/w). The order of the photocatalytic activity of the materials for 10

NO3− transformation was Ag2%-P25 > PC500 > Ag0.5%-P25 ≈ P25 >> Pd1%-PC500 > PC10 > Pd1%-PC10. Nitrite formation was observed in all cases, but at low amounts, and its concentration was negligible after complete NO 3− reduction. Ammonium was found as final product and remained in considerable amounts at the end of the irradiation. The nitrogen balance accounted for a large amount of non-identified nitrogen products formed during the photocatalytic reaction, probably N 2 or NO; this amount was higher for the P25 and PC500 bare samples. The efficiency on the use of formic acid as donor was evaluated and PC500 was found to be the most efficient sample in this sense. Radiolytic modification of TiO 2 with noble metal nanoparticles such as Ag or Pd does not always increase the photocatalytic efficiency of NO 3− reduction. The reasons for this differential behavior are related to the inherent mechanism of nitrate degradation and the possible side reactions that can be involved, such as H2 generation. The modification of PC samples with Pd NPs deteriorates NO3− transformation and decreases significantly the efficiency in the use of the donor, probably because of the competence of H2 evolution.

3.3. Modification of TiO 2 with Ag nanoparticles Silver nanoparticles attract a lot of interest because of localized surface plasmon resonance (LSPR) [48–50], their size- and shape-dependent optical properties [51,52] their catalytic activity, [53–55] and their potential applications in chemical and biological sensing based on surface-enhanced Raman scattering (SERS) [56,57], and metal-enhanced fluorescence (MEF) [58,59]. TiO2 modified with silver nanoparticles exhibit enhanced photocatalytic activity under UV and visible light and improved anti-bacterial properties [60–62]. Ag NPs show a very intense localized surface plasmon (LSP) absorption band in the near-UV region [63] and this is associated with a considerable enhancement of the electric near-field in the vicinity of the Ag NPs. This enhanced near-field can boost the excitation of electron−hole pairs in TiO 2 and therefore increase the photocatalytic activity. Surface of commercial TiO 2 compounds (P25 and ST-01) has been modified with Ag nanoparticles induced by radiolysis [64]: An alcohol suspension of TiO 2 conaining Ag+ ions was irradiated (by gamma rays or electron beams) under N 2 atmosphere. Silver ions are reduced by solvated electrons: 𝐴𝑔+ + 𝑒𝑠− → 𝐴𝑔0

(9)

Reduction of free silver ions by alcohol radicals proceeds via the formation of a complex involving metal ions and the alcohol radicals, which act as ligands. It has been observed in case of 2-propanol [65]: 𝐴𝑔+ + (𝐶𝐻3 )2 𝐶 • 𝑂𝐻 → [𝐴𝑔(𝐶𝐻3 )2 𝐶 • 𝑂𝐻]+

(10)

[𝐴𝑔(𝐶𝐻3 )2 𝐶 • 𝑂𝐻]+ + 𝐴𝑔+ → 𝐴𝑔2+ + (𝐶𝐻3 )2 𝐶𝑂 • + 𝐻 +

(11)

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The same reactions probably occur also with methanol radical. However, direct reduction by .

CH2OH of silver cations adsorbed on silver clusters or on TiO2 is also possible (the reduction potentials of Agn+ and Ag+/TiO2 are more positive than the one of Ag +): •

𝐶𝐻2 𝑂𝐻 + 𝐴𝑔+ ⁄𝑇𝑖𝑂2 → 𝐻𝐶𝐻𝑂 + 𝐴𝑔0 ⁄𝑇𝑖𝑂2 + 𝐻 + •

𝐶𝐻2 𝑂𝐻 + 𝐴𝑔𝑛+ → 𝐻𝐶𝐻𝑂 + 𝐴𝑔𝑛 + 𝐻 +

(12) (13)

Very small Ag nanoparticles of 1-2 nm were synthesized by radiolysis on TiO2 P25 (by irradiation of a suspension of TiO 2 in an alcohol solution containing Ag + ions), while on TiO2 TS-01, two populations of Ag nanoparticles were obtained, small nanoparticles (1-2 nm) and larger ones (mean diameter 7 to 12 nm depending on the silver loading from 0.5 to 2% in mass) [64]. Modification of TiO 2-P25 with Ag clusters induced by radiolysis leads to a wide absorption of the photocatalysts in the visible with two maxima at 410 and 540-560 nm (Figure 3a). In the case of Ag-modified ST01, a wide absorption is also observed with a maximum at 410 nm (Figure 3b). Silver nanoparticles exhibit a plasmon band with a maximum at around 400 nm in water and this plasmon band is sensitive to the environment and can be shifted depending on the stabilizer or on the substrate. This plasmon band is blue-shifted when these NPs are supported on titania because of the coupling between the Ag NPs and TiO2 having a high reflective index (the absorption coefficient and refractive index are for anatase phase 90 cm-1 and 2.19 at a wavelength of 380 nm, respectively) [66,67]. Surface modification with silver nanoparticles induced a modification of the absorption properties of the photocatalysts inducing an activity under visible light. The modified TiO2 samples absorb in the 540-560 nm region, and this absorption has been attributed to small (1-2 nm) Ag clusters [68]. The diffusion reflectance spectra of the modified samples show a slight shift in the band-gap transition to longer wavelengths. The red shift in the band gap transition revealed by diffuse reflectance spectra can be related to the electronic interaction between metal NPs and TiO 2.

b

a

Figure 3. Diffuse reflectance spectra of pure and modified TiO2: (a) P25 and (b) ST-01 with different silver loading (0.5 to 2% in mass) and recorded respectively using BaSO4 as reference [64]. (Copyright 2013 American Chemical Society).

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Figure 4. TEM image showing Ag clusters induced by radiolysis on P25 and TRMC signals of bare and modified with Ag clusters P25 obtained with excitation at 355 nm. Inset: A scheme showing electron scavenging by Ag clusters decreasing the charge carrier recombination [64]. (Copyright 2013 American Chemical Society).

The photocatalytic activity of Ag-modified TiO2 (P25 and ST01) is enhanced both under UV and visible light for phenol degradation. Faster decay of TRMC signals were obtained with AgTiO2 compared to bare titania (See Figure 4). TRMC measurements have shown that TiO2 modification with Ag nanoparticles play a role in charge-carrier separations increasing the activity under UV-light and that Ag NPs act as electron scavengers. Titania modification with Ag accelerates the overall decay of the TRMC signals. The modification of TiO2 with Ag nanoparticles causes an increase in the photocatalytic activity. The TRMC signal is mainly related to the electron mobility. The decrease of the TRMC signals is due to efficient electron scavenging by silver nanoparticles deposited on TiO2. It implies a decrease of the charge-carrier recombination, which is beneficial to the photoactivity. It has been also shown in another study that deposition of Ag nanoparticles on titania increases the affinity of the surface to oxygen [69], and this affinity to oxygen can have an influence on the photocatalytic activity. Silver modification of TiO 2 results also in enhancement of the bactericidal activity of TiO 2 under UV light because of the improved microorganism adsorption to the particle surface and lower electron-hole recombination [70,71]. Kowalska et al. reported in another study on Ag/TiO2 system, action spectra (AS) analysis proving that the photocatalytic activity under visible light irradiation is due to localized surface plasmon resonance (LSPR) of silver NPs [72].This composite Ag/TiO2 system showed also antimicrobial properties under visible light irradiation indicating that not only intrinsic properties of silver in the dark, but also plasmonic properties of Ag/TiO2 were responsible for bacteria killing. The evolution of carbon dioxide indicated mineralization of bacteria cells, and therefore possible application of silver-modified titania for decomposition of chemical and biological pollutants. It has also to be mentioned that modification of titania with silver clusters of a few atoms such as Ag8 and Au25 induces a photocatalytic activity under visible light. These clusters are photochemically reactive and can act as electron donnors under visible excitation due their molecular-like properties [21,39,40].

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3.4. Modification of TiO 2 with Au nanoparticles In the plasmonic photocatalytic Au/TiO 2 system, the presence of Au is essential, due to their localized surface plasmon resonance (LSPR), i.e., the oscillation of metal free electrons in constructive interference with the electric field of the incident light. These plasmonic properties can be used to induce photocatalytic activity of the semiconductor material under visible light [19,73–79]. However, the support presents also important effects on the photocatalytic processes, i.e., (i) the electron-hole recombination for anatase and rutile particles has been studied and was found to be higher for rutile than anatase [16,41,80,81]; (ii) the higher Fermi level of anatase results in stronger electronic interaction with Au-NPs, which can inhibit the growth and aggregation of metal nanoparticle [82] ; (iii) the difference in energy band gap of anatase (3.2 eV) and rutile (3.0 eV) [2] could result in activation of TiO2 at longer wavelengths for rutile [83] and finally, (iv) the dielectric constant of the support could shift the LSPR of metal nanoparticles toward the red spectral region [11,84]. For example, it has been shown that crystalline composition and surface properties of Au/TiO 2 photocatalysts, prepared by photodeposition of gold on fifteen commercial titania, influenced significantly the resultant photocatalytic activities in a different manner under UV and visible light irradiation [19]. Under UV irradiation, the gold presence resulted in a high enhancement of the photocatalytic decomposition of acetic acid (1-3-fold) for all the modified samples. On the other hand, under visible light irradiation, mainly Au/TiO 2 samples possessing large crystallites of rutile exhibited the highest level of photocatalytic activity. It was proposed that polydispersity of gold deposits, i.e., various sizes and shapes (nanoparticles, nanorods) resulted in broad LSPR, and therefore in higher overall photocatalytic activity than that of Au/TiO2 of fine gold NPs with narrow LSPR. Small gold nanoparticles (Au-NPs) around 2-3 nm on the surface of titanium dioxide work as visible-light absorbers and thermal redox active centers. Au-NPs were synthesized on commercial TiO2 (P25) by reduction with tetrakis(hydroxymethyl)-phosphonium chloride [79]. The optical properties of the modified surface of TiO 2-P25 were studied by DRS. The spectrum of TiO2-P25 shows an absorption edge at around 400 nm due to the presence of rutile [83]. The photo-absorption properties of Au/P25 materials are higher than that of pure TiO 2-P25, since the Au-NPs induce a shift of the absorbance toward the visible light attributed to the interaction between the metal and the semiconductor, i.e., the so-called Schottky barrier [75] and because of the plasmon of gold. The position of the LSPR peak of the Au-NPs at 520 nm for 13-nm gold NPs, in aqueous solution is sensitive to the dielectric constant of the surrounding medium [11,84] This shift in the plasmon is due to the interaction between the Au-NPs and the semiconductor TiO2-P25. Indeed, this plasmon band is sensitive to the size, and environment, and can be shifted depending on the stabilizer or the substrate. Because of the coupling between Au nanoparticles and TiO 2 support, the plasmon band in case of modified titania is usually redshifted [19,33,85,86]. A weak LSPR band from 500 to 650 nm with a maximum values at 548, 554 and 560 nm for Au 0.5 wt%, Au 1 wt% and Au 2 wt% respectively, are observed in the specta (Figure 5), due to the plasmon of small nanoclusters [85,87]. These absorptions result in a pink-purple color of the modified TiO 2-P25 samples. The modification of TiO 2-P25, with very small Au nanoparticles (< 5 nm) localized on the anatase phase and with a low metal loading induced increase of the photocatalytic activity under UV and visible-light [79]. 14

Figure 5. TEM images of a) Au0.5%/P25; b) Au1%/P25, with the corresponding histogram of the size distribution of Au-NPs. c) DRS spectra of Au/P25 samples, where the LSPR of Au-NPs is observed between marked lines [79].

(Copyright 2016 American Chemical Society).

Charge-carrier dynamics in Au-TiO2 system has been studied by Time-Resolved Microwave Conductivity. Figure 6 showed the electrons are trapped by Au NPs under UV and the injection of electrons from Au-NPs into the conduction band of TiO2 under visible-light excitation, as a result of the activation of the localized surface plasmon resonance (LSPR) of the Au-NPs [79].

Figure 6. TRMC signals of pristine and modified TiO2-P25 at different excitation wavelengths: a) 365 nm and 400 nm UV irradiation, b) 450 nm and 470 nm visible irradiation, and c) 500 nm and 560 nm (Plasmon excitation) [79]. (Copyright 2016 American Chemical Society).

The photocatalytic activity of Au/TiO2 was evaluated for degradations of phenol, 2-propanol, acetic acid and for H2 production from aqueous methanol solution [79] This photocatalytic activity is much higher for the plasmonic TiO 2 compared to the activity of bare titania. The 15

stability of these plasmonic photocatalysts was also investigated, and the results showed that t he Au/TiO2 system is a stable photocatalysts and can be reused several times, without appreciable change in structure, activity nor composition [79]. For this system, the apparent quantum efficiency was obtained using action spectra (AS) calculated as the rate of CO 2 evolution from the decomposition of acetic acid versus the flux of incident photons, assuming that four photons were required. Action spectra (quantum yield per unit of incident photons as a function of the wavelength) correlate with the absorption spectra (DRS spectra), Figure 7, for pure TiO2-P25 and modified TiO2-P25 with Au 0.5 wt%, and an appreciable response is obtained under UV and visible light, confirming that the decomposition of acetic acid occurs by a photocatalytic mechanism and much lower quantum yield under visible light than under UV irradiation for plasmonic photocatalysts has been usually observed and reported. Under visible range excitation, the samples modified with Au-NPs show a higher apparent quantum efficiency (Φapp) in the range between 450–650 nm, which is related to the LSPR [79].

Figure 7. a) Comparison between DRS spectra and the action spectra of TiO2-P25 and modified TiO2-P25 with Au 0.5 wt%, b) superposition of action spectrum (in blue) and absorption spectrum (in red) in the Au plasmon range [79]. (Copyright 2016 American Chemical Society).

The modified TiO2-P25 was also found to provide promising results for hydrogen generation under UV and visible-light [79]. In the photocatalytic water splitting (PWS) process by photocatalysis, electrons in the conduction band reduce protons into H 2 and holes in the valence band oxidize water to O 2, but the main drawback of the PWS process by photocatalysis is the low H2 production due to the fast recombination of the charge carriers [88–90]. For H2 production tests under λ=400 nm and 470 nm, the addition of Au-NPs activate the titania support [79]. Figure 8 shows that a considerable higher amount of H2 is produced with Au/P25 16

samples, compared to the amount produced with bare-P25. Under excitation at 400 nm (See Figure 8a), the activity for H 2 production decreases with the Au-loading. The excess of Au-NPs on TiO2-P25 can act as recombination centers of photo-generated charges reducing the photocatalytic activity in H2 production. Under visible irradiation at 470 nm (Figure 8b), only a small amount of H2 is obtained (0.23 µmol g -1 s-1) with the Au/P25 samples. Similar results have been reported by Joo et al. in the range of 300-600 nm using methanol as holes scavenger (~4 µmol g-1 s-1 using Au/P25 and around 0.2 µmol g -1 s-1 using Au@SiO2(Thin)/P25) [90]. This activity does not depend on the Au loadings (Au 0.5, 1 and 2 wt%) [79].

Figure 8. Production of H2 under visible light at: a) λ = 400 nm and b) λ = 470 nm for pure TiO 2-P25 and TiO2P25 modified with Au-NPs at different loadings The relative uncertainties at 400 and 470 nm are 5% and 7% respectively [79]. (Copyright 2016 American Chemical Society).

Small gold nanoparticles can induce a much higher change of the work function compared to larger particles, and this change is an indication of a larger charge separation and improved reduction potential for the photocatalyst and metal/TiO 2 interface. The action spectra showed that the photocatalytic activity under visible light is directly correlated to the LSPR, and TRMC signals showed that Au-NPs inject electrons in the conduction band of the semiconductor due to their surface plasmon resonance inducing an activity in the visible [79]. When the samples were irradiated at 470 nm, 500 nm and 550 nm a small TRMC signal is observed for the Au-P25 samples, attesting the generation of free electrons in the conduction band of the Au modified TiO 2-P25. This suggests that excess electrons are injected in the conduction band of TiO 2-P25 after excitation of the Au nanoparticles at a wavelength very close (or equal) to their LSPR. This electron injection from excited AuNPs to the CB of TiO 2 was demonstrated for the first time by TRMC. Direct proof for electron transfer by EPR technique has been reported by Caretti et al. and Priebe et al. [91,92]. Existence of both trapped electrons (Ti +3) and holes (O -) were observed under UV excitation, but only trapped electrons under visible excitation were observed confirming that the electron transfer from the metal nanoparticles to the semiconductor was the main mechanism of plasmonic titania activation [91]. Priebe et al. reported EPR results proving electron transfer from Au-NPs and their trapping by lattice Ti +4 or surface oxygen vacancies [92]. The electrons are probably injected from the metal NPs to the CB of TiO 2 by electrons transfer or by energy transfer on account of the LSPR of Au-NPs, and this corresponds to a higher photocatalytic activity see Figure 9 [79]. 17

Figure 9. Mechanism purposed for modified TiO2 modified by Au-NPs a) UV and b) visible irradiation by electron and c) energy transfer [79]. (Copyright 2016 American Chemical Society).

In another study, TiO2 was surface modified with silver, gold or platinum clusters to improve its photocatalytic activity. The effect of metal content, the kind of dopant and titanium dioxide source (commercial - P25 and ST-01) used during preparation procedure on photoactivity were investigated. The photocatalytic activity was estimated by measuring the decomposition rate of 0.21 mM phenol aqueous solution under UV-Vis and visible (>400 nm). The highest photoactivity was observed for TiO2 loaded with silver (2%Ag on P25), gold (1%Au on P25) and platinum (0.5% Pt on ST-01) clusters. After 60 min. of irradiation under UV light phenol solution was degraded in 91%, 49% and 91%, respectively [30].

3.5. Modification of TiO 2 with Bi clusters Development of cheap and efficient TiO 2-based photocatalysts without noble metals is a challenge. In this context, small (subnanometer) Bi zero-valent clusters were synthesized on TiO2-P25 by radiolysis for application in photocatalysis [93]. Surface modification of TiO 2 with zero-valent Bi nanoclusters induced high photocatalytic activity under visible light for rhodamine B and phenol degradation. Very small amounts of Bi (0.5 wt%) can activate titania for photocatalytic applications under visible light, but the photocatalytic activity under UV light with bare and modified TiO 2 were very similar. It has been shown that these photocatalysts are very stable with cycling. The intensity of the TRMC signals of Bi-modified TiO2 under UV illumination was higher compared to the one obtained with bare TiO 2 (Figure 10) proving that more electrons are induced in the conduction band of Bi-modified TiO2. This indicates that Bi clusters inject electrons in the conduction band of TiO 2. Bi clusters do not have any influence on the TRMC signal decay. More importantly, Time Resolved Microwave Conductivity measurements indicated that under visible irradiation Bi nanoclusters inject electrons into the conduction band of TiO2 (see Figure 10). These electrons react with O 2 forming oxidizing O 2.-. These oxidizing

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radicals are responsible for photocatalytic degradation of phenol (taken as model pollutant) under visible light irradiation.

Figure 10. TRMC signal of bare and Bi-modified samples obtained by irradiation at 355 nm (a) and 450 nm (b). Inset: A scheme showing electron injection from Bi nanoclusters in the conduction band of TiO 2 under visible light excitation [93]. (Copyright 2015 RSC).

4. Surface modification of TiO 2 with bimetallic nanoparticles 4.1. Surface modification with Au-Cu nanoparticles Bimetallic nanomaterials exhibit unique catalytic, electrocatalytic, electronic and magnetic properties, which differ from their monometallic counterparts [94,95]. In particular, bimetallic nanoparticles often show enhanced catalytic performances in terms of activity, selectivity and stability, compared to the separate components [96]. Au-Cu nanoalloys homogeneous in size were synthesized on TiO2 (P25) (0.5 wt. %) via deposition precipitation method with urea (DPU) followed by radiolytic reduction [97]. This deposition procedure ensured a complete adsorption of Au and Cu ions on TiO 2. The alloyed structure of Au-Cu NPs was confirmed by high angle annular dark field scanning transmission electron microscopy (HAADF−STEM), Energy dispersive X-ray (EDX) mapping, X-ray photoelectron spectroscopy (XPS) and Diffuse reflectance spectroscopy (DRS). Because of the plasmon of gold and copper, the modified titania absorb in the visible spectral range. Modification with Au-Cu bimetallic nanoparticles induced an enhancement in the photocatalytic activity under UV irradiation for photodegradation of methyl orange (MO). The highest 19

photocatalytic activity was obtained with Au-Cu/TiO2 (atomic ratio Au/Cu = 1:3). Modification of TiO2 with Cu and Au-Cu bimetallic NPs resulted in a decrease in the photoluminescence (PL) emission intensity indicating less electron-hole recombination rates. Modification of TiO2 with Au-Cu nanoparticles induced a better charge carrier separation because the NPs act as a sink for electrons, and consequently leads to an enhancement of the photocatalytic activity under UV light. In another study, Au, Cu and bimetallic Au–Cu nanoalloys were synthesized on surface of commercial TiO2 compounds (P25) by reduction of metal precursors with tetrakis(hydroxymethyl) phosphonium chloride (THPC) (0.5% in weight) [94]. The Au-Cu nanoalloys on TiO2 were characterized by HAADF-STEM, EDX mapping, (Figure 11) HRTEM and XPS techniques. Modification with Au-Cu nanoparticles induced an increase in the photocatalytic activity for phenol and RhB photodegradation in aqueous suspensions under UVvisible irradiation. The highest photocatalytic activity was obtained with Au–Cu/TiO2 (with the atomic ratio Au/Cu 1:3) (see Figure 12).

Figure 11. Top: energy dispersive X-ray spectroscopy line scans across a nanoparticle of AuCu1:1/P25 (the profile was taken along the green line, the blue line corresponds to CuK and the red one to the AuL signal) and corresponding STEM images for the samples. Bottom: mapping EDS analysis performed on a nanoparticle of AuCu1:1/P25 (left) [94]. (Copyright 2013 RSC).

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Figure 12. Rate constants of the first-order kinetics of phenol photodegradation by pure and modified TiO2 photocatalysts under UV-visible illumination [94]. (Copyright 2013 RSC).

TRMC measurements showed that Au, Cu and Au–Cu nanoparticles act as a sink for electrons, decreasing the charge carrier recombination: Indeed, the overall decay of the TRMC signal was accelerated for Au-Cu-modifed TiO2 compounds (see Figure 13). Importanly, TRMC measurements also showed that the bimetallic Au–Cu nanoparticles were more efficient in electron scavenging than the monometallic Au and Cu ones. This influence on charge carried recombination can be related to the photocatalytic activity under UV light. This acceleration in the decay has also been observed for the modification of TiO2 with Ag and Au clusters. It is different from the previous observations made with Pt [21] and Pd [98] modified TiO2, where a slowdown of the overall decay was observed. Indeed, contrary to metals such as Pt and Pd which provide an ohmic contact, metals such as Ag, Au and Cu exhibit capacitive properties. Furthermore, the modification with copper nanoparticles increased the initial TRMC signal intensity in the case of Cu/P25 (Figure 13) [94]. This indicated that more electrons were produced under UV-illumination in the conduction band of Cu-modified P25. These excess electrons are due to the injection of electrons in the conduction band of TiO2 after excitation of copper nanoparticles, which are more easily oxidized.

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Figure 13. Time resolved microwave conductivity signals of modified P25 photocatalysts prepared by the chemical method with THPC. Inset: scheme depicting the electron scavenging and transfer on the Au–Cu modified TiO2 surface after the absorption of UV photons [94]. (Copyright 2013 RSC).

4.2. Surface modification with Ag and CuO nanoparticles Ag and CuO nanoparticles (NPs) were synthesized on the surface of commercial TiO 2 P25 by radiolytic reduction (the loading of Cu or Ag metal was 0.5 wt%) [48]. These nanoparticles were characterized by HAADF-EDS, HRTEM, XPS and X-ray Absorption Near Edge Spectroscopy (XANES). In the case of modification with silver and copper, Ag@CuO nanoparticles (large silver cores decorated with small clusters of CuO) were obtained on TiO2−P25 (Figure 14).

Figure 14. a) Representative aberration corrected STEM-HAADF image for Ag@CuO1:1/P25 sample, and b) A schematic morphology of the modified TiO2 –P25 with Ag-CuO nanoparticles [48]. (Copyright 2016 American Chemical Society).

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The photocatalytic properties of bare and modified TiO2 were studied for phenol degradation and for acetic acid oxidation under UV and visible irradiation. Modification with Ag nanoparticles or CuO nanoclusters induced an increase in the photocatalytic activity under both UV and visible light. The photocatalytic activity of Ag@CuO/TiO2 was higher under UV light, but lower under visible light compared to the activity of CuO/TiO2 and Ag/TiO2 (Figure 15). TRMC measurements showed that surface modification of TiO 2 with Ag, CuO, and Ag@CuO nanoparticles played a role in charge-carrier separation, increasing the activity under UV-light, and that Ag@CuO NPs were more efficient electron scavengers than Ag NPs and CuO nanoclusters. The localized surface plasmon resonance of Ag NPs, and the narrow band gap of CuO induced an activity under visible light. The TRMC signal showed also responses under visible-light irradiation at different fixed wavelengths indicating that electrons are injected from Ag NPs in the conduction band (CB) of TiO 2. Under visible light, the photocatalytic activity of CuO/P25 was higher than that of plasmonic Ag/P25. The study showed that CuO clusters can activate TiO2 in a wider range of wavelengths under visible-light irradiation, compared to the activation obtained with silver modification. Action spectra correlated with the absorption spectra for irradiation wavelengths in the range of 350−470 nm proving that decomposition of acetic acid was carried out by a photocatalytic mechanism [48].

Figure 15. Degradation curves of phenol under a) UV and b) visible light (λ > 450 nm) of pure system TiO2-P25 and modified systems with, Ag, Ag@CuO1:1and CuO [48]. (Copyright 2016 American Chemical Society). Another study reports on modification of TiO2 nanotubes with Cu, Ag core/Cushell and Bi nanoparticles induced radiolysis (Figure 16) [99]. Here again, surface modification with metal nanoparticles leads to enhanced photocatalytic activity under UV–vis irradiation because of the electron trapping by the NPs, and this effect depends on the amount of deposited metal. Modification of TiO2 with metal nanoparticles leads to more efficient electron-hole separation 23

.

and to enhanced HO and O2 - radicals formation. The mechanism of phenol degradation on titania nanotubes is shown in Figure 17. The photocatalytic activity (for phenol degradation) of TiO2 nanotubes modified with AgCu nanoparticles was higher compared to monometallic samples modified with the same amount of Cu. The photoelectrochemical experiments performed under the influence of simulated solar light irradiation also confirmed the enhanced photoactivity of metal-modified nanotubes. The saturated photocurrent for the most active Biand AgCu-modified samples, was over two times higher than for bare TiO2 nanotubes. The modified TiO2 nanotubes were resistant towards photocorrosion, and this enables their application for long term photoinduced processes.

Figure 16. SEM micrographs of pure TiO2 nanotubes (a–c) and AgCu-NT III sample (d); STEM image of AgCuNT III sample (e) [99]. (With permission from Elsevier, Copyright 2016).

Figure 17. Proposed mechanism of phenol decomposition in the presence of TiO 2 nanotubes decorated with metal nanoparticles under UV–vis irradiation [99]. (With permission from Elsevier, Copyright 2016).

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4.3. Co-modification of TiO 2 with Ni and Au nanoparticles for hydrogen production Au and/or Ni nanoparticles were synthesized by radiolysis on TiO 2 (commercial P25) at various composition (metal content) [100]. The modified photocatalysts were characterized by High Resolution TransmissionMicroscopy (HRTEM), Energy-Dispersive X-ray Spectroscopy (EDS), UV–vis Diffuse Reflectance Spectroscopy (DRS) and X-ray Photoelectron Spectroscopy (XPS). The charge-carrier mobility was studied by Time Resolved Microwave Conductivity (TRMC). The photocatalytic activities were tested for H2 production under UV–vis irradiation using polychromatic and monochromatic light (action spectrum analysis of apparent quantum efficiency). According to the characterization results, a segregation of the two metals was observed. Large Au NPs and Ni nanoclusters (partially oxidized) were obtained on TiO 2. The surface modification of TiO2 with Ni and Au NPs resulted in an increase of the photocatalytic activity for hydrogen production using a methanol-water solution under UV light. The highest production of hydrogen was obtained with the NiAu/TiO 2 catalysts, which was explained in terms of a synergetic effect by the presence on Au NPs and Ni(O) clusters on TiO 2, acting as recombination sites for atomic hydrogen conversion to molecular hydrogen. It was found that a very small amount of gold associated to nickel (atomic ratio Ni:Au 5:1 and total metal 0.5 to 1at%) can induce a significant increase in H 2 formation, thus the costs of photocatalyst preparation are relatively low.

Figure 18. Absorption spectra (photon absorption), TRMC spectra (charge-carrier creation) and action spectra (apparent quantum efficiency) of modified samples, metal loading of 0.5 at% [100]. (Copyright 2016 American Chemical Society).

Figure 18 displays the AQE determined by the action spectra for each sample. The absorbance obtained by DRS, and the Imax/photons obtained by TRMC have also been plotted to 25

follow the evolution with the wavelength of the three steps of the photocatalytic mechanism: photon absorption, charge-carrier creation, and chemical surface reaction. It can be observed that the AQE of bare TiO 2 is very weak. The action spectrum shows that the maximum amount of hydrogen is obtained at a wavelength of 350±5 nm. It suggests that the highest density of electrons in the conduction band is obtained at this energy. This agrees with the TRMC results, where the highest photoconductivity was obtained under irradiation at 355±5 nm. Detailed analysis of AQE spectra of the three modified compounds suggests appreciable differences among them; the action spectrum of Au/TiO2 shows a low level and a maximum at 380 nm, while the action spectra of compounds containing Ni present higher levels and fol low the absorption spectra. Ni/TiO2 and NiAu/TiO2 samples show similar profiles, but an enhancement of the AQE is clearly shown for NiAu/TiO2. Considering the small amount of gold, the enhancement in H 2 production cannot be explained only by an additional effect of gold, but by a synergistic effect of gold with nickel. P25 is a mixture of anatase (main componant) and rutile with absorption edge at 380 and 410 nm, respectively. Thereby a shoulder of its absorption spectrum at ca. 400 nm is assigned to rutile phase. The observed action spectra of modified compounds suggest that gold and nickel particles were loaded predominantly on rutile, and anatase particle, respectively. It has been reported that platinum particles were photodeposited preferentially on rutile in P25 if the number of platinum particles was small and the corresponding action spectrum showed a dip in the wavelength region at around 350 nm [41]. This was explained by the disturbance of rutile photoabsorption by inactive anatase crystallites in the relatively short wavelength region. Thus, anatase and rutile crystallites mainly work in Ni/TiO2 and Au/TiO2, respectively, even though both crystallites absorb light, and NiAu/TiO2 might show activity higher than the sum of activities of singly modified samples due to both crystallites work effectively. For bare TiO2, the low AQE values are associated to high TRMC signal. In comparison, the modified compounds present higher AQE values corresponding to slightly lower TRMC signal. This point confirms the assumption that the positive effect of the NPs is more effective on the H2 overpotential, i.e. its ability to act as a recombination center of atomic hydrogen, than on the separation of charge-carriers. The NiAu/TiO2 samples are much more efficient in photocatalytic hydrogen generation than the monometallic samples. Clearly, the improvement of the photocatalytic performance was due to a synergetic effect between Au and Ni(O) since it was not a simple additive effect. A reaction scheme is proposed for hydrogen photoproduction on NiAu/TiO2 samples (Figure 19). The generation of the electron-hole pair takes place on the TiO 2 and NiO surfaces. The holes, coming from TiO2, oxidize water and/or the methanol mixture generating protons, which are then reduced at the surface of both TiO 2 and NiO forming atomic hydrogen. Finally, H ● recombination occurs on the surface of metal NPs forming H 2. The improvement of hydrogen generation compared with that of the monometallic samples is attributed to a synergetic effect between both Ni(O) and Au acting as a better atomic hydrogen recombination site than the monometallic samples.

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Figure 19. A proposed mechanism for H2 production on NiAu/TiO2 samples. [100]. (Copyright

2016 American

Chemical Society).

4.4. TiO 2 modified with NiPd nanoalloys for hydrogen evolution A systematic study of surface modification of commercial TiO 2 (P25) with mono- and bimetallic (Ni, Pd and Ni-Pd) NPs synthetized by radiolysis has been realized [101]. The photocatalysts were characterized by High Resolution Transmission Microscopy (HRTEM), Scanning Transmission Electron Microscope (STEM), X-Ray Diffraction (XRD), EnergyDispersive X-ray Spectroscopy (EDS), X-ray Photoelectron Spectroscopy (XPS), and UV–vis Diffuse Reflectance Spectroscopy (DRS). The charge-carrier dynamics was studied by Time Resolved Microwave Conductivity (TRMC). The photocatalytic activity was evaluated for hydrogen generation under UV–vis irradiation using polychromatic and monochromatic lights (action spectra analysis of apparent quantum efficiency). TiO2 modified with Pd-Ni bimetallic NPs exhibits a high activity for H 2 generation, and a synergetic effect of the two metals was obtained. The study of light absorption, charge-carrier dynamics and photocatalytic activity revealed that the main role of the metal NPs is to act as catalytic sites for recombination of atomic hydrogen. The characterization of the Ni-Pd NPs showed that the NPs size was sensitive to the Ni:Pd atomic ratio. Large aggregates (30 nm) were observed in the Pd rich sample Ni1Pd10/TiO2, while the metal NPs on the Ni rich sample Ni10Pd1/TiO2 exhibit a small size (3 nm). In Ni10Pd1/TiO2 samples, once of Pd NPs were formed some Ni ions (remaining in solution) were reduced on their surface, leading to Pd core-Nishell nanoparticles. For Ni1Pd10/TiO2 samples, the amount of Ni ions was small facilitating their complete adsorption on the support, where Ni and Pd ions were reduced independently: mono-metallic nanoparticles of Pd and Ni without interaction were observed on TiO2. In both cases, small amounts of NiO, PdO and PdO x have been observed.

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Figure 20. Light absorption, charge carrier separation and H2 evolution of (a) bare TiO2 (b) 0.5-Ni10Pd1/TiO2 and (c) 1-Ni1Pd10/TiO2 measured by DRS, TRMC and action spectra respectively [101]. (Copyright 2017 American Chemical Society).

A systematic study of the three major steps involved in the photocatalytic H 2 generation has been realized: 1) light absorption by the sample, 2) the charge-carrier dynamics, and 3) surface reactions for H2 generation. These steps were studied by DRS, TRMC and action spectra, respectively. In Figure 20, the measurements of the three mentioned steps for bare TiO 2, 0.5Ni10Pd1/TiO2 and 1-Ni1Pd10/TiO2 samples are presented. As shown in Figure 20a, bare TiO2 exhibits a strong light absorption below 410 nm. The TRMC spectrum reveals that such absorption is coherent with the charge (electron and hole) separation because no significant TRMC signal was detected with an excitation wavelength higher than 410 nm. Despite the good efficiency of bare TiO2 to absorb light and to generate electron-hole pairs, it is inefficient to produce H2. This suggests that bare TiO 2 does not efficiently perform the third step involved in the photocatalytic H2 evolution, though our TRMC study reveals that it has a large amount of excited electrons to reduce the protons. On the other hand, a better performance of the third step is observed when titania surface is modified with metal NPs. The three involved steps in the H 2 evolution using modified titania are shown on Figure 20b and 20c. The samples exhibited an UV and visible absorption attributed to the semiconductor and the metal NPs, respectively. Even though the samples absorb in the visible range, a charge separation was obtained using only excitation wavelengths shorter than 380 nm, as shown its TRMC profile. The latter implies that TiO2 is the only one involved in the electron-hole pair generation. The amount of H 2 generated by modified sample is much higher than that obtained using bare TiO 2 as their respective AQE profile shows. The metal NPs are the only difference between bare - and modified TiO2, suggesting that H2 generation (third step) is controlled by the metal NPs.

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Figure 21. Schematic representation of: (a) H2 evolution using metal-NPs/TiO2 as photocatalysts where the proton reduction occurs on the surface of TiO2 while the molecular H2 is generated on the surface of metal NPs, (b) H2 evolution using metal-Ni10Pd1/TiO2 as photocatalysts, and (c) H2 evolution using metal-Ni1Pd10/TiO2 as photocatalysts [101]. (Copyright 2016 American Chemical Society).

Therefore, the enhancement in H 2 evolution is achieved because the surface metal NPs catalyze H2 generation reaction. So that, the metal NPs act as catalytic sites, and this is their main role in H2 generation. Figure 21a depicts the proposed photocatalytic mechanism to generate H2. In the Ni10Pd1/TiO2 samples two groups of NPs were found: a) small Ni NPs, and b) large NPs of Pd covered by Ni NPs forming a kind of core-shell structure. The interaction between Ni and Pd NPs might be the main reason that metal NPs act a better catalytic site than the monometallic samples (Figure 21b). In the sample Ni1Pd10/TiO2 large Pd NPs and small Ni NPs (not in contact) were observed. This is very interesting, because even if there is not contact between Ni and Pd NPs, their concomitant presence on TiO 2 leads to increase the H 2 rate compared to monometallic samples. The distance between supported metal NPs can also influence in the reactivity of the co-catalyst in the same way as their size and shape do it [102]. In the Ni1Pd10/TiO2 sample, the proximity between Pd and Ni NPs seems to be enough to enhance photocatalytic activity for H 2 production compared to the monometallic samples. Furthermore, the highest amount of hydrogen evolution was observed with the Ni 1Pd10/TiO2 sample, and can also be due to the electron trapping by the Pd-based nanoparticles, avoiding their recombination and consequently favoring the proton reduction reaction (Figure 21c).

4.5.

The effect of metal cluster deposition route on structure and photocatalytic activity of mono - and bimetallic nanoparticles supported on TiO 2 Zaleska et al. reported the influence of metal deposition method on metal nanocluster morphology and structure and its impact on TiO 2 photocatalytic activity under Vis and UV–vis irradiation [103]. TiO2 (P25) was modified with small and relatively monodisperse mono- and bimetallic clusters (Ag, Pd, Pt, Ag/Pd, Ag/Pt and Pd/Pt) induced by radiolysis to improve its photocatalytic activity. The photocatalysts were characterized by X-ray fluorescence 29

spectrometry (XRF), photoluminescence spectrometry (PL), diffuse reflectance spectroscopy (DRS), X-ray powder diffractometry (XRD), scanning transition electron microscopy (STEM) and BET surface area analysis. Both, simultaneous and subsequent deposition of Ag/Pd, Ag/Pt and Pd/Pt metal pairs resulted in formation of alloy-like structures. The effect of metal type (mono- and bimetallic modification) as well as deposition method (simultaneous or subsequent deposition of two metals) on the photocatalytic activity in toluene removal in gas phase und er UV–Vis irradiation (light-emitting diodes- LEDs) and phenol degradation in liquid phase under visible light irradiation (> 420 nm) were investigated. The highest photoactivity under Vis light was observed for TiO 2 co-loaded with platinum (0.1%) and palladium (0.1%) clusters. Simultaneous addition of metal precursors resulted in formation of larger metal nanoparticles (15–30 nm) on TiO2 surface and enhancement of the photocatalytic activity of Ag/PdTiO2 in the visible range up to four times, while the subsequent metal ions addition resulted in formation of metal particle size ranging from 4 to 20 nm. Photocatalysts, where the metals were introduced sequentially, exhibited higher photocatalytic activity in the toluene degradation in the gas phase under the UV–vis irradiation and the photocatalytic activity was stable after four cycles. Direct electron transfer from the bimetallic metal nanoparticles to the conduction band of the semiconductor is responsible for visible light photoactivity, whereas superoxide radicals (such as O2• − and •OOH) are responsible for pollutants degradation over metal-TiO2 composites.

5. Summary Development of efficient photocatalysts under solar light for water and air treatment and solar fuel production is a main challenge to solve energy and environment issues. Chargecarrier dynamics is a main key in photocatalysis. Therefore, to develop efficient photocatalysts, it is of crucial importance to understand the effect of semiconductor modification on chargecarrier dynamics and to correlate it with their photocatalytic activity. Time Resolved Microwave Conductivity is a very powerful technique to study these charge-carrier mobility and dynamics. Surface modification of TiO2 with metal nanoparticles is a very efficient way to enhance its photocatalytic activity under UV and visible light. Radiolysis is a very powerful method to synthesize metal nanoparticles of controlled size and composition on semiconductors. Enhancement of the photoactivity of modified semiconductors with metal nanoparticles under UV irradiation originates from prolongation of lifetime of charge-carriers (photogenerated electrons and holes): indeed noble metal nanoparticles act as an electron sink as proved by TRMC studies on different systems, and thus accelerating the transfer of electrons from the semiconductor to substrates. Under visible excitation, surface modification with metal nanoparticles can lead to activation of the semiconductor and induce a photocatalytic activity in this spectral domain. Increasing attention was paid to plasmonic photocatalysis: modification of wide bandgap semiconductors (in particular TiO 2) with plasmonic nanoparticles (mainly Ag, Au and Cu) leads to visible light absorption due to the plasmon resonance and to the photocatalytic activity under visible light. The LSPR and the Schottky junction properties are characteristic of the plasmonic photocatalysts and are the main properties responsible of the enhancement of the photoactivity of the composite system (metal nanoparticle/TiO 2). An electric field is created by the LSPR, which may power the generation of more electrons and holes, and heat up the surrounding environment, inducing an increase of the redox reaction rates and the mass transfer, and also polarization of the nonpolar molecules for better adsorption. In case of Au-TiO2, TRMC studies have shown that hot electrons can be ejected form the excited nanoparticles at their plasmon band to the conduction band of the SC inducing an activity under visible light. 30

Modification with bimetallic nanoparticles can lead to enhancement of the photocatalytic activity (water and air depollution and hydrogen generation) compared to modification with monometallic NPs. A systematic study of light absorption, charge-carrier dynamics, and reaction efficiency (the three main steps involved in H 2 evolution) demonstrated that the main role of the metal NPs for hydrogen generation is to act as catalytic sites. TRMC studies also showed that the electron transfer from TiO 2 to metal NPs is favorable, but not an indispensable factor for photocatalytic H 2 generation.

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