Buffer Layer Assisted Growth of Ag Nanoparticles in Titania Thin Films

Article pubs.acs.org/JPCC

Buffer Layer Assisted Growth of Ag Nanoparticles in Titania Thin Films L. Zilberberg, S. Mitlin, H. Shankar, and M. Asscher* Institute of Chemistry, Edmund J. Safra Campus, The Hebrew University of Jerusalem, Jerusalem 91904, Israel S Supporting Information *

ABSTRACT: Developing materials with improved photocatalytic activity is important for light energy conversion and storage within chemical bonds. Here we present a new type of hybrid film of silver nanoparticles (AgNPs) embedded within TiOx (x ≤ 2) to approach this goal, introducing visible light absorption via surface-plasmon excitation of the AgNPs. Silver nanoparticles were prepared by an ultrahigh vacuum (UHV) based buffer layer assisted growth method. The titania films as a substrate and protective layers were grown by the reactive layer assisted deposition (RLAD) technique; in both cases amorphous solid water (ASW) was the buffer material. The thin titania films and the AgNPs were ex situ characterized by UV−vis, micro-Raman, XRD, XPS, SEM, and TEM techniques. The titania protective layers on top of the silver particles were found to introduce a dielectric environment for the AgNPs, leading to a significant red-shift of their plasmon resonance from 460 to 530 nm, in addition to avoiding oxidation of the small nanoparticles. Photoinduced activity of these hybrid films has been tested following the degradation of methylene blue (MB) in aqueous solution under both UV and visible pulsed laser irradiation. Preliminary results have shown photocatalytic activity of the RLAD titania film with only marginal influence due to the presence of the AgNPs. Possible reasons for this observation are discussed.

1. INTRODUCTION Diverse photocatalytic and solar energy storage applications require photoactivity in the near-UV and visible spectral range. Among the best studied materials for these applications, titanium dioxide is known to exhibit outstanding photocharacteristics.1−5 The light absorption by two main crystalline phases of TiO2, rutile and anatase, is predominantly in the near-UV range due to their wide optical gaps reported at 3.3 eV (E⊥c) and 3.4 eV (E|| c),1,6−8 where E is the electric field of the incident light and c is the crystallographic axis, with the onset of the optical absorption at ∼3.2 eV6 and the direct band gap at 3.03 eV7 for rutile and at 3.420 eV (E⊥c) and 3.460 eV (E||c)8 with the indirect optical band gap at 3.20 eV for anatase.7 Much of the interest in titanium dioxide is due to its near-UV photocatalytic activity in e.g. the water splitting reaction and also in the decomposition of organic compounds in aqueous solutions.1,3,9,10 It is commonly believed that the photocatalytic activity of the crystalline phase anatase is higher than that of rutile.1,3 This statement, however, is still in debate. Recent study has shown that as in the case of the rutile phase, also in anatase thin films, only defect sites are contributing to photocatalytic activity.11 In addition, the superhydrophilic character of TiO2 under the UV-irradiation12 enables an efficient removal of pollutants from its surface.5 The absorption range in the near-UV, accountable for the transparency of TiO2 coatings, is also one of its disadvantages because only a small fraction of the solar spectrum contributes to the photocatalytic activity.13 © XXXX American Chemical Society

Hence, an important challenge is to extend the photoactivity of titania-based compounds into the visible spectral range without affecting their transparency in a significant way by achieving an optimal balance between the photoactivity and transparency which in a first approximation depend reciprocally on the optical gaps. Insertion of metallic nanoparticles (primarily silver and gold) into titania-based photocatalysts has been demonstrated and claimed to enhance the absorption in the visible and photoactivity of hybrid systems14−17 due to the metallic particles’ surface plasmon resonance (SPR) excitation. Numerous theoretical and experimental studies have claimed that noble metal nanoparticles facilitate the photodissociation of molecules located in the vicinity of their surfaces.18−21 This phenomenon may be understood if the SPR band of the embedded metallic particles overlaps, at least partially, with the absorption of the host titania, promoting charge separation in the semiconductor due to the near field effect and/or resonant energy transfer.22−26 One of the key processes contributing to the photoactivity in the visible spectral range is the injection of hot electrons from metal nanoparticles into the conduction band of the semiReceived: October 1, 2015 Revised: November 30, 2015

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DOI: 10.1021/acs.jpcc.5b09621 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C conductor.24,27 These hot electrons are generated following the decay of the excited plasmonic oscillations, if the excitation overlaps the SPR spectral position. Surface plasmon resonance in the visible range is a well-known property of copper, silver, and gold nanoparticles.28,29 While silver exhibits the strongest SPR band among these metals, one of the problems in using silver in photocatalytic applications is associated with a high sensitivity of the Ag SPR to its exposure to ambient conditions which often leads to substantial quenching of the SPR signal.30 Several approaches to slow down the SPR degradation have been discussed in the literature involving, in particular, the encapsulation of metal nanoparticles in chemically inert dielectric matrixes, which prevents the charge transport between the system components. Another approach was the formation of Ag−Co alloys.31,32 Au nanoparticles on TiO2 have been found to undergo UV-induced photo-oxidation in aerated conditions by the reaction with photoholes yielding positively charged Au ions, Au+, that could be accommodated within the titania matrix.33 Titania films can be prepared by different techniques comprising electron-beam evaporation,33 dc and rf reactive sputtering,35 pulse laser deposition,36 chemical vapor,37 atomic layer deposition,38 and sol−gel processes.39 In this work we focus our report on the introduction of versatile UHV-based buffer/ reactive layer assisted growth/deposition (BLAG and RLAD) methods40,41 for the formation of thin hybrid films comprising a titania matrix and silver nanoparticles. The technique allows to fabricate uniform amorphous (without annealing) or crystalline (following in-vacuum or in-air annealing) titania matrixes of a desired thickness which support ensembles of silver nanoparticles of different shapes, sizes, and densities. Moreover, asdeposited AgNPs can be protected by additional layers of amorphous titania deposited on top of the nanoparticles. These protective TiOx layers were found to stabilize significantly small (4−20 nm size) AgNPs. By depositing the protective titania layers, the SPR absorption maximum can be spectrally redshifted from 460 to 530 nm. Preliminary results on the photocatalytic activity of this hybrid material are discussed.

mass spectrometer (QMS, SRS-200) for 10 min. We assume that water condensation produces the ASW of the compact structure typical for this temperature range. Under the present conditions, therefore exposure of 1 Langmuir (1 Langmuir = 1 × 10−6 Torr· s) is considered to be equivalent to 1 ± 0.1 monolayer of D2O, assuming a coverage-independent sticking probability; namely 50 Langmuirs is equivalent to 50 ML. Directed onto the water reactive layer by e-beam evaporator (McAllister), a Ti flux was calibrated by an in situ quartz microbalance (QMB). A single RLAD step consists of an amount of Ti atoms equivalent to 5 Å thick metal film at a constant deposition rate of 1.2 Å min−1. This value of Ti deposition rate was maintained throughout the present work. By turning on and off the flux of Ti atoms with a magnetically coupled shutter, we could monitor the initial stage of titanium oxide formation by detecting D2 molecules evolution from the D2O ASW layer into the gas phase immediately upon the reaction between the impinging Ti atoms and the water molecules. After completion of a Ti deposition step, the sample was heated to 300 K, causing the remaining water molecules to react with underoxidized Ti species or to desorb. Annealing to higher temperatures (in-vacuum up to 1100 K) was carried out when we attempted to initiate a crystallization process. Multilayer titania films were obtained by performing multiple cycles of the D2O/Ti/D2O/Ti depositions while annealing the entire as-grown structure only once at the end of the deposition cycles. The thickness of the amorphous film prepared by 40 RLAD cycles was determined by ex situ ellipsometry as 18 ± 1 nm. We have also used a rutile TiO2 (110) single crystal, 1 mm thick at a surface area of 12 × 6 mm2, as a reference for the photocatalytic studies reported below. Silver nanoparticles were prepared in a similar way by using ASW as the buffer media for accommodation of impinging Ag atoms which was followed by the nucleation of multiatomic Ag clusters. Directed on the predeposited 50 ML thick layer of D2O by resistively heating a tungsten filament (0.25 mm in diameter) wrapped around a 1.0 mm in diameter, 5 mm long silver wire of 99.99% purity, the Ag flux was monitored by the QMB and adjusted to a value of 3 Å min−1 which was used consistently for each single cycle of the AgNPs deposition. Annealing of the Agin-ASW film to room temperature leads to a uniform layer of bare AgNPs adherent to the substrate surface. Deposited on both thin (3 RLAD cycles) or thick (40 RLAD cycles) titania layers, silver nanoparticles were further covered by 3−5 or more additional titania layers designed to preserve original size-density characteristics of as-grown AgNPs ensembles. The pristine titania and hybrid TiOx/AgNPs/TiOx films as well as the AgNPs on different substrates have been subsequently examined by UV−vis, micro-Raman, SEM, TEM, XPS, and XRD techniques. The chemical composition of different films comprising AgNPs embedded in titanium oxide layers on the sapphire substrates and the chemical state of AgNPs has been determined by XPS. Optical properties (both absorption and light scattering) of the hybrid films and those of bare AgNPs grown on transparent sapphire substrates were characterized by a UV−vis spectrophotometer (Cary 5000). The photocatalytic activity of the pristine RLAD-titania and hybrid TiOx/AgNPs/TiOx films has been studied by following the degradation of methylene blue dye (MB) in aqueous solutions under pulsed laser irradiation at two wavelengths. The second and third harmonics of a Nd:YAG laser at 532 and 355 nm, respectively, with pulse duration of 6 ns have been used as two separate light sources at the power of 20 mW at 10 Hz, with the lateral footprint on a sample of ∼0.5 cm2. The films were

2. EXPERIMENTAL SECTION The preparation of the family of titania-based materials comprising pristine amorphous and crystalline TiOx and hybrid TiOx /AgNPs/TiOx films was performed in an ultrahigh-vacuum (UHV) chamber pumped to a base pressure of 1 × 10−10 Torr by a Pfeiffer Turbo molecular pump. The following materials were used as substrates for the titania films: 12 × 6 × 0.5 mm3 (n-type) SiO2/Si(100) wafer for the SEM measurements, 12 × 6 × 1 mm3 aluminum oxide single crystals (synthetic sapphire, corundum phase) for optical spectroscopy and photocatalytic measurements, and ultrathin amorphous carbon (a-C) on a copper grid, a standard sample holder for transmission electron microscopy (TEM). The preparation of nanoscale amorphous titania films by the RLAD method has been described in our previous report.41 The method involves the deposition of amorphous solid water (ASW) on a precooled substrate as a reactive layer followed by the deposition of Ti atoms. Initially, the substrate mounted on a special holder was attached to the bottom of a liquid nitrogen Dewar in the UHV chamber and cooled down to about 100 K. A layer of ASW was deposited on the substrate by backfilling the UHV chamber with the vapor of D2O. 50 monolayer thick (50 ML) ASW layers were prepared by exposing the substrate to the D2O vapor at 8.3 × 10−8 Torr as measured by the quadrupole B


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The Journal of Physical Chemistry C submerged into 400 μL of 6 × 10−4 M aqueous solution of MB in a quartz optical cell and then left for 3 h prior to the exposure to a laser light to allow the equilibration between MB species in the solution and those adsorbed on the surface of the titania film and on the walls of the quartz cell. This long equilibration period was found to be necessary for achieving reproducible photodegradation measurements. The samples were then exposed to the laser beams in series of 30 min irradiation intervals after each of which the MB concentration has been determined by measuring the optical absorption of the solutions at 665 nm which corresponds to the maximum absorbance of the main molecular form of MB. The total exposure to the laser light after 3 h reached 1021 photons. Evidently, the photocatalytic performance of the given films depends not only on film properties but also on the characteristics of the light source (power density, spectral width, pulse duration, etc.), the optical absorption of organic species chosen for the test and the chemical composition and the hydrodynamics of a solution during the irradiation.22 A relevant observation regarding previous photocatalytic studies with metallic plasmon enhancement is that many are performed using wide-band and continuous wave irradiation schemes.42

formula TiOx could be considered as designating not merely the oxygen deficiency, which is more appropriate for the vacuumannealed anatase or rutile samples with oxygen vacancies, but rather the titanium excess which could be attributed to Tin+ interstitials.45 Only upon the exposure to an ambient environment a gradual oxidation proceeds with the corresponding bathochromic shift in the fundamental absorption of TiOx matrixes,46 as discussed in the Supporting Information. Grown on a sapphire substrate, our nanoscale amorphous titania films are found to be of a gray-blue color which appears to be similar to that of vacuum-annealed, reduced rutile, and anatase crystals as reported in a number of studies.47,48 Evidently, a color appearance reflects the nonstoichiometric composition of these as-grown films, which is in accord with the RLAD mechanism,41,43 and also on the presence of donor levels within the band gap.1,47,49 The blue color of reduced titania crystals seems to be related to the light absorption by quasi-free electrons45,48 which implies the presence of shallow donor levels. While the low-temperature RLAD procedure yields amorphous titania layers on a-carbon (a-C) substrates for TEM analysis, high-temperature annealing can be pursued in an attempt to alter the optical characteristics and the photoactivity of respective films. Indeed, it was found that in-vacuum annealing of the titania film grown on the a-C substrate at 850 K for 45 min or at 1100 K for 15 min leads to crystallization, as indicated by the Moiré pattern that appeared in the TEM image of the annealed sample, as shown in Figure 1A. Displayed in the inset to Figure

3. RESULTS AND DISCUSSION 3.1. Preparation, Annealing, and Characterization of Pristine Titania Films. Titania nanoscale films were prepared using the reactive layer assisted deposition (RLAD) procedure as discussed briefly in the Experimental Section. At the initial stage of the titanium deposition, each incoming Ti atom reacts with two D2O molecules on the surface of a water buffer layer, forming a monomer Ti(OD)2 in the solid phase and ejecting a single D2 molecule to the gas phase. At latter stages of the deposition process, the incoming Ti atoms interact with fewer than two D2O molecules per atom, therefore forming a depleted monomer Ti(OD)z with z < 2.41 It is concluded that under the given experimental conditions at the end of a titanium deposition cycle the instantaneous oxidation state of the majority of Ti species is +2. In vacuum, the next oxidation step occurs by the interaction of Ti2+ species with the excess of the water molecules present within the condensed buffer layer at the temperature range from 100 to 200 K.43 Heating inside the UHV environment up to room temperature deepens the Ti oxidation via the reaction between the Tin+ cations with n < 4 and OD groups of other oxohydroxide species. Simultaneously, the Ti(OD) z and Ti1+yOx(OH)z monomers form a continuous TiOx (x ≤ 2) matrix by the polycondensation (olation and oxolation) in a sol− gel like process.41 A typical single RLAD cycle results in a very thin TiOx layer characterized by a relatively weak optical absorption in the nearUV range. In order to get thicker films, more appropriate for photochemical applications due to the stronger fundamental absorption in the near-UV range, we repeated the RLAD deposition procedure several times while avoiding intermittent annealing steps. The final preparation step in this case is annealing of the as-grown film up to room temperature to form a TiOx polycondensate. Ex situ ellipsometry measurements after 40 RLAD cycles reveal titania layer thickness of 18 ± 1 nm, which corresponds to 4.5 Å per layer. This result indicates that the present implementation of the RLAD technique resembles a molecular layer deposition scheme,44 but under UHV conditions. Under the present RLAD conditions, however, one gradually obtains Ti-enriched TiOx layers as the in-vacuum deposition goes on.41,43 In this regard, the subscript x ≤ 2 in the

Figure 1. (A) TEM image of 40 RLAD cycles of TiOx formation after annealing to 1100 K for 15 min on an ultrathin amorphous carbon (a-C, TEM sample holder in-vacuum). Inset: electron diffraction pattern of the annealed film. (B) TEM image of a single RLAD layer of TiOx after annealing to 1100 K for 10 min.

1A, the respective diffraction pattern indicates that the crystallization of the amorphous titania matrix under vacuum at this temperature range leads primarily to the rutile phase with no signature of the anatase phase. Furthermore, the 18 nm thick titania films retain long-range continuity after the hightemperature annealing. In contrast, Figure 1B shows that in the case of the single-layer RLAD on the a-C substrate with an estimated thickness of only 4.5 Å the high-temperature annealing leads to a local dewetting of the titania on the a-C TEM support. These observations suggest that the nanoscale titania films on the a-C support exhibit a discernible local mobility at moderately high temperatures (850 K) because of a relatively weak titania− carbon adhesion energy. We found a clear difference between the a-C and sapphire as substrates regarding the outcome of in-vacuum annealing. Unlike the a-C substrate, in the case of sapphire only amorphous titania layers were observed at annealing temperature up to 1100 K. C


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3.2. Preparation and Morphology of Silver Nanoparticles on Titania Films. AgNPs were grown by the buffer layer assisted growth method (BLAG), as mentioned in the Experimental Section. The size and density of the AgNPs can be controlled by changing the buffer layer thickness, the amount of the evaporated Ag metal, and the number of the BLAG cycles performed consecutively.40 Figure S9A demonstrates the typical dependence of the AgNPs density and size on the water layer thickness. As the water layer thickness increases, the particles size is found to increase from 4 ± 1 nm at 60 ML of ASW up to 8 ± 2 nm at 300 ML which is accompanied by the NP density decrease from (1 ± 0.5) × 1016 to (3 ± 1) × 1014 m−2. These results originate from the nucleation and growth dynamics of AgNPs within the ASW matrix during the BLAG cycle as described in details in earlier studies.40 The present as-grown ensemble of AgNPs after a single BLAG step is characterized by a narrow size distribution around 4 nm with a particle density up to 1.5 × 1016 m−2. Figure S9B reveals that if the total amount of deposited Ag is less than 5 Å, the increase in amount of deposited silver atoms in a single cycle leads to the increase in density and size of AgNPs. On the other hand, the deposition of a larger amount of Ag leads to coalescence of primary nuclei already at the deposition time, resulting in the formation of fewer but larger particles. These results indicate that when fine-tuned, the BLAG procedure enables to fabricate different ensembles of AgNPs in a controllable manner. We have previously demonstrated that the density of AgNPs can be increased by repeating several BLAG cycles.40 Aiming for larger, more stable (against SPR quenching) metal nanoparticles at a high density in order to enhance a potential Ag SPR effect, we have examined a new approach where a sandwich structure was prepared by repeating the BLAG procedure several times (designated as n(AgNPs), where n stands for the number of consecutive BLAG cycles) with the annealing step performed only once at the end of the process. A typical SEM image of the hybrid 3(TiOx) /6(AgNPs) + 6(AgNPs)/3(TiOx)/SiO2/Si (100) structure, shown in Figure 3A, demonstrates the emergence of large crystalline Ag particles with the size up to 85 nm (usually nonspherical; see ref 55 for relevant definitions) formed by the coalescence of a number of much smaller particles. The size distribution diagram in Figure 3B shows the presence of two populations among moderately elongated multiparticles with the circularity parameter ≥0.4: middle-size multiparticles with the average diameter of 8 nm and the tail up to 25 nm and large multiparticles with the average diameter of 55 nm and the tail up to 100 nm. Nearly spherical middle-size nanoparticles are formed most likely by the coalescence of primary nuclei whereas the population of large particles with a significant number of nonspherical conglomerates originates from the aggregation of the middle size and small multiparticles. The process of aggregation implies a discernible mobility of AgNPs on the titania surface in vacuum at low and room temperatures in accord with studies on the silver nanoparticle growth on crystalline titania. It is also evident in Figure 3A that middle and small size Ag NPs are well protected by additional three amorphous TiOx layers deposited on top of the nanoparticles. 3.3. Dynamics of AgNPs and the Stability of a Surface Plasmon Resonance in Different Environments. The intensity of Ag SPR could be important for the photocatalytic performance of hybrid titania films due to the near field and resonant energy transfer effects and the interfacial charge exchange, all of which have been associated with the plasmonic

Apparently, the incomplete oxidation of the as-deposited titanium-enriched TiOx and the interfacial effect of alumina50 prevent a well-ordered crystal growth. In contrast to a vacuum environment, annealing of the titania films on sapphire under ambient atmospheric conditions at 1000 K for 1 h has led to the onset of crystallization primarily into the rutile phase. Moreover, while the as-grown films are gray-blue in color due to the presence of titanium interstitials,51 the in-air high-temperature annealing leads to a light yellow coloration which is the indication for further oxidation of partially reduced titanium ions. Figure 2 displays Raman and XRD spectra of nanocrystal-

Figure 2. (A) Raman spectra of pristine sapphire, α-Al2O3, substrate (red), and nanocrystalline anatase TiO2 film (blue); see preparation procedure in the text. (B) Raman spectrum of a nanocrystalline rutile film on a sapphire substrate (green) annealed in-air to 1000 K. Selected Raman peaks corresponding to sapphire, anatase, and rutile phases are marked by the letters S, A, and R, respectively. Insets to (A) and (B) show corresponding XRD peaks: (A) the (101) peak at 25.349° for the anatase film and (B) the (110) peak at 27.53° for the rutile film.

line anatase and rutile films obtained by annealing of amorphous 18 nm thick titania matrixes. The nanocrystalline anatase film was prepared by initially depositing 40 RLAD cycles on the sapphire substrate, exposed to ambient oxidizing condition in air, and subsequently submerged in deionized water for 70 days and finally in-vacuum annealed at 875 K. This film is characterized by a distinct Raman spectrum which comprises a complete set of fundamental Raman bands pertaining to the anatase phase52 with the following assignments (in cm−1 units): 142 (Eg), 195 (Eg), 395 (B1g), 515 (A1g, B1g), 638 (Eg). The inset in Figure 2A shows the distinct XRD peak at 25.349° (in 2θ scale) attributed to diffraction from the (101) plane in the anatase phase.53 Figure 2B displays the Raman and XRD spectra of the nanocrystalline rutile film on the sapphire obtained after the in-air annealing to 1000 K for 1 h. The present Raman spectrum comprises the following bands pertaining to the rutile phase vibrational modes54 (in cm−1 units): 238 (the multiphoton band associated with the fundamental B1g mode reported at 143), 450 (Eg), 610 (A1g), 830 (B2g). The broad XRD peak at 27.53° is attributed to the (110) diffraction of the rutile phase.53 The XRD data demonstrate that the anatase film (Figure 2A) is better crystallized compared to the rutile film (Figure 2B) in our experimental conditions. D


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widths are very different than those observed in colloidal nanoparticles whose SPRs are typically significantly narrower ́ and hypsochromically shifted, e.g., the work of Gutierrez and Henglein63 where 25 nm wide and sharp SPR peaks appear at 385 nm. Our studies on the plasmon dynamics demonstrate that the SPR band of bare AgNPs obtained by a single BLAG cycle on the amorphous-TiOx/α-Al2O3 substrate (up to 4 nm diameter) exhibits a rapid decay following its exposure to an ambient atmosphere as evident in Figure 4A. Moreover, the plasmonic resonance in Figure 4A shows the characteristic bathochromic shift from the initial value at 460 to 510 nm and the concomitant broadening from 153 to 277 nm during 40 min exposure in air. These observations imply rapid electron transfer from AgNPs to the titania host or to oxygen molecules in the atmosphere. In contrast, the fast SPR decay observed for bare AgNPs deposited directly on the sapphire substrate is not accompanied by such a pronounced spectral shift which can be attributed to the dielectric character of α-Al2O3 in comparison with the semiconductor (or even semimetallic) nature of TiOx. In other words, for AgNPs on α-Al2O3, the electron transport from silver nanoparticles to the alumina is insignificant. Peculiarities of the plasmonic dynamics of AgNPs/TiOx in air evident in Figure 4A could be interpreted in the following three terms: (1) the charge transfer from AgNPs to the TiOx host57,58 or to molecular oxygen (2) the discernible decrease in the remaining “resonant volume” that is the plasmon-active fraction of AgNPs and (3) possibly the increase in electron scattering within individual resonant volumes in AgNPs.59 With respect to the charge distribution between the metallic and semiconductor components of the hybrid system in vacuum, it is plausible that the low affinity of the tinanium-enriched TiOx host in comparison to the work function of Ag (see schematic in Figures 5A,B) leads to the accumulation of negative charges on silver nanoparticles and a positive charge on the titania. The pronounced plasmonic absorption in AgNPs/TiOx samples immediately after the removal from vacuum is consistent with the excess of electron density on the silver nanoparticles. Upon the exposure to an ambient atmosphere, the oxidation of the reduced titania host occurs most likely by the reaction of molecular oxygen with Tim+ (m < 4) interstitials leading to the generation of holes in accord with the following reaction written in the Kröger−Vink notations:45

Figure 3. (A) SEM image of AgNPs within the hybrid 3(TiOx)/ 6(AgNPs) + 6(AgNPs)/3(TiOx)/Si structure. The silver nanoparticles were prepared by two deposition-annealing steps each included six standard BLAG cycles. The as-grown AgNPs were protected by three additional layers of amorphous titania deposited on top of the nanoparticles. The hybrid film was prepared on a native oxide Si wafer. (B) Size distribution for particles possessing the criterion of roundness between 0.4 and 1. The particle analysis has been performed within the ImageJ program (see ref 55 for further explanation).

activity as scrutinized in numerous studies.14−27 However, the SPR of noble metal nanoparticles is sensitive to the environment.30−32 One of the goals of the present study, therefore, has been to determine how sensitive our unique hybrid films are regarding the intensity of the SPR signal vs time under ambient air conditions and how the SPR signal can be stabilized. At the initial moment at atmospheric conditions, we observed a wide range of peak plasmon positions of bare, small AgNPs on the amorphous titania matrixes ranging from 420 to 465 nm with the average at 440 ± 20 nm and the width at 150 nm (full width at half-maximum). The spectral position of Ag SPR evidently depends on the dielectric response of its immediate environment56 and the electrostatic charge on AgNPs57,58 which is expected as the consequence of establishing an electrochemical equilibrium between sliver nanoparticles, the TiOx host, and the environment. The SPR width reflects electron scattering process, and therefore, it depends on the level of crystallinity and dimensions of the resonant volume and charge within the particles.57−59 In addition, the inhomogeneous broadening is highly plausible in the present case considering the level of randomness in the AgNPs distribution over various sizes and shapes and, even more importantly, variations in the local dielectric environment due to the metal−support interaction. These latter inferences are in accord with those of Tatsuma and co-workers60 and Dahmen and co-workers,61,62 who have observed a very broad, 150−250 nm wide, Ag SPR bands with maxima between 550 and 600 nm in titania. These spectral

X Ti im • + O2,gas → Ti Ti + 2OOX + m h+

(1) 27,33,62

These energetic holes, h+, can oxidize silver nanoparticles (see the band bending at the Ag/TiOx interface in Figure 5C) which will gain a positive electric charge as expressed in eq 2, and as a consequence the respective SPR band will exhibit a bathochromic shift and broadening as evident in Figure 4A. X k(Ti im • + O2,gas ) + AgNP → k(Ti Ti + 2OOX) + AgNP(km) +

(2)

Furthermore, as depicted in Figure 5D, following the positive charging, relatively large AgNPs could eject positively charged Ag clusters, as appears in eq 3, which could be accommodated within the amorphous titania host: AgNP NM + → AgNP(NM−−nm) + + Ag mn +

(3)

The process expressed in eq 3 is thought to contribute to the SPR decay in ambient atmospheric conditions because positively charged small Ag clusters most likely do not exhibit discernible E


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Figure 4. (A) UV−vis spectra of bare AgNPs with 4 nm in diameter on a TiOx matrix measured at time intervals of 10 min during the exposure to ambient atmosphere. (B) UV−vis spectra of the hybrid sample, comprising the AgNPs embedded into a TiOx matrix and covered by additional three RLAD layers of titania, measured every 10 min under ambient atmospheric conditions. (C) UV−vis spectra of the hybrid TiOx/AgNPs/TiOx sample identical to (B) in a water-submerged state revealing the additional stabilization of the plasmonic band. (D) Decays of the Ag SPR intensity vs time for the three samples shown in (A)−(C). A linear decay is found in the case (A) and exponential decays in the cases (B) and (C). Shown in Figure S11, we demonstrate that larger bare silver particles (80 nm on average) practically do not decay even after 13 h.

Figure 5. (A, B) Schematics of the band structure, charge distribution and AgNP configurations related to the hybrid RLAD titania plus silver matrix in vacuum. (C, D) Schematics of the band structure, the redistribution of charges, and the disintegration of AgNPs in ambient atmospheric conditions. Silver cations could be captured by the titania host forming [Ag−TiOx] complexes. In addition, oxygen can adsorb on the AgNPs forming distinct adsorbed species as reported by Schlögl and co-workers39 which decrease noticeably the surface energy of silver.

SPR bands. To support the hypothesis regarding the formation of positively charged Ag clusters, we demonstrate in Figure S7 several difference UV−vis spectra for AgNPs/TiOx samples exposed to ambient atmospheric conditions. The emergence of two small positive bands (an increasing absorbance) at 312 nm and below 265 nm is evident in Figure S7. These bands have been attributed in the literature to the absorption of positively charged silver clusters, Agnm+, and single silver ions, Ag+, respectively.56,57

These positively charged silver species could react with the TiOx matrix forming a [Ag−(TiOx)] complex in spite of the large difference in sizes of silver and titanium ions. The similar reaction path has been proposed in the literature for the evolution of AgNPs in different environments.65−68 In particular, the works of Lai and Goodman,64 Antad et al.,65 and Kazuma and Tatsuma66 are relevant to our observations. Particles size redistributions of AgNPs on titania as the result of exposure to air has been F


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Figure 6. (A) Low-resolution TEM image of bare AgNPs on a single layer of amorphous TiOx. Large size AgNPs formed in the course of diffusion, aggregation, and coalescence processes occurring during the postgrowth evolution, room temperature, under the UHV conditions. (B) Low-resolution TEM image of AgNPs protected by three RLAD TiOx layers. The AgNPs size is around 4 nm, as shown in the magnified inset. (C) Influence of the number of RLAD TiOx layers on the surface plasmon resonance intensity (black line) and the SPR band position (red line).

the conduction band, and the top of the valence band in titania, respectively. φAg, χTiOx, and φSB are the work function of AgNPs, the electron affinity of the titania, and the respective Schottky barrier. In this stage the AgNPs are prone to the attack of holes once the latter will be generated. The dynamics in ensembles of small AgNP on different substrates in ambient atmosphere could be attributed in part to oxygen-induced surface effects including the significant reduction in the surface energy of AgNPs and the titania host and the enhancement in the mobility of silver atoms and clusters upon the oxygen and water adsorption.65,69,70 In order to explore the role of RLAD titania films as a protective layer in preserving size density and Ag SPR characteristics of as-grown particles, UV−vis spectra of samples with AgNPs precovered by a thin amorphous TiOx overlayer were recorded. Figure 4B reveals how the coating of AgNPs by the titania film results in a significant bathochromic shift of the SPR band from 460 to 530 nm due to the dielectric effect of the overlayer as discussed in section 3.4. In contrast to the bare, unprotected AgNPs, the SPR band drops initially relatively fast to ∼80% of its initial value, but subsequently it decays slowly, remaining at about 50% of its original value for more than 24 h. It is evident that both the bathochromic shift in the SPR position from 520 to 550 cm−1 and the corresponding broadening during the first 40 min reflect a much slower process compared to the case of the bare AgNP ensemble. This is consistent with a significant barrier for the charge transfer reaction from AgNPs to the titania host. In addition, we found that submerging of an identical hybrid sample, TiOx/AgNPs/TiOx /sapphire, in deionized water further slows down the plasmon decay as evident in Figure 4C. In this case, the surface plasmon intensity remains at 75% of its original value even after 3 days of submerging in water. Figure 4C shows also that the plasmon band remains on its original spectral position at 530 nm, clearly indicating that there is no recharging of AgNPs in the submerged case, unlike the exposure to ambient atmospheric conditions depicted in Figure 5A−D. In other

attributed to the Ostwald ripening mechanism promoted by O2,65 while only the interaction of charged oxygen species from plasma sources and not neutral oxygen were shown to lead to SPR quenching of AgNPs.66 Furthermore, the illumination of AgNPs results in either disintegration or reassembling of AgNPs depending on the light energy.67,68 In the course of TEM measurements (not shown), we have found that the postgrowth exposure to air of various ensembles of primarily small bare AgNPs on the titanium-enriched amorphous titania leads to the preferential disintegration of relatively large, above 6 nm, and middle size particles and the concomitant strong rise (up to a factor of 5) in the total density due to the increase in amount of small nanoparticles below 2 nm. All AgNPs examined by TEM were found in the crystalline metallic state with no signature of silver oxides. With respect to the potential influence of atmospheric corrosion, in spite of the close similarity in the present spectral features of Ag SPR in Figure 4A and those observed by McMahon et al.67 during the much slower SPR decay in a system of Ag nanodisks under ambient atmospheric conditions, we assume that it is insignificant at the given time scale and that the formation of silver oxide layers on the AgNPs could play only a limited role in the present case.69 It is concluded that the SPR dynamics in Figure 4A appears to be consistent with an oxygen-driven electrochemical process resulting in significant morphological changes in the initial ensemble of AgNPs on titania as schematically depicted in Figure 5A−D and presented in eqs 1−3. It is proposed that upon the electrochemical equilibration the AgNPs and the titania carry negative and positive charges, respectively, as the result of the high Fermi level in the titanium-enriched host with respect to AgNPs. The nanoparticles are locked by the partial encapsulation with the amorphous host promoted by the migration of interstitial {Tiin•} defects. The majority of negatively charged AgNPs displays relatively strong SPR bands having inhomogeneous broadening due to imperfections in AgNPs. In Figure 5 EF, ECB, and EVB stands for a Fermi level, the energy of the bottom of G


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the SPR signal obtained from larger bare particles (diameter of 80 ± 20 nm) is practically not sensitive at all to ambient atmospheric conditions (see Figure S11). 3.4. Dielectric Response of the AgNPs Optical Plasmon Resonance. The titania thin film on top of the silver nanoparticles strongly modifies the optical plasmon resonance, as demonstrated in Figure 6C. We found that for as-grown AgNPs located on an amorphous TiOx/α-Al2O3 substrate and submerged in water the maximum absorbance is at 460 nm, which is substantially bathochromically shifted with respect to the SPR of small colloidal AgNPs in water reported at 380 nm. This very significant red-shift of 80 nm can be primarily attributed to the dielectric effect of the substrate which also implies that AgNPs are in a firm contact with the substrate surface. Furthermore, the titania-coated AgNPs reveal SPR absorption at 520−530 nm (Figures 4A and 4B, respectively); i.e., the SPR is bathochromically shifted by additional 60−70 nm with respect to the uncoated AgNPs, an effect that can primarily be attributed to the dielectric effect of TiOx, as also discussed in the literature.55,58,72 The protecting TiOx layers impose the gradual bathochromic shift and increased intensity of Ag-SPR with a number of layers, as demonstrated in Figure 6C. Above three titania layers the redshift and the increase of intensity are saturated which indicates that the dielectric environment of AgNPs becomes stable beyond this point. In order to confirm that the optical surface plasmon resonance responds to the dielectric function of the environment of AgNPs, simple simulations of Ag SPR have been performed. For nanoparticles that are much smaller than the wavelength of the exciting light (λ) (2r ≪ λ), where r is the spherical particle radius, the main contribution to the excitation cross section arises from the collective electrons dipolar oscillations. The quasielectrostatic approximation simplifies the Mie theory and establishes the relations between the extinction cross section (σext) and the dielectric function of the surrounding medium (εd).59

words, this difference in the plasmonic dynamics implies that under atmospheric conditions the oxidized silver species, Ag+ and Agnm+, are more stable than the same species in aqueous media in accord with studies of Gallardo et al.71 Overall, we may conclude that the Ag SPR appears to be the sensitive indicator of the partial pressure of oxygen gas and the moisture level in its surroundings. In Figure 5 C,D the scheme describes oxygen molecules oxidizing the titanium-enriched titania host causing the decrease in the Fermi level position of TiOx which could become lower than that of AgNPs. As a consequence, the AgNPs and the titania host acquire the positive and negative charges, respectively. The anodic reaction takes place on the surface of silver nanoparticles whereas the oxygen reduction occurs on the titania interface which is covered by a few monolayer thick layer of adsorbed water. Formed in the course of the anodic process at the surface of AgNPs, positively charged Agnm+ clusters diffuse across the titania interface and could precipitate at titaniumenriched sites. Figure 4D summarizes the intensities of surface plasmon features of AgNPs vs time for the three samples discussed above. It provides the solid base for suggesting that the coating of AgNPs with protective titania layers and water-submerging lead to the significant stabilization of metastable ensembles of small silver particles and slows down possible morphological and chemical changes in this ensemble. The exponential law of the plasmon decay shown in Figure 4D (curve B) is found to be the characteristic feature of protected AgNPs on TiOx when exposed to atmosphere. Another very significant observation is that the titania protective layer leads also to the pronounced and controllable bathochromic spectral shift of the plasmon feature from 440−460 to 520−530 nm as discussed in section 3.4. With respect to the protective nature of top TiOx layers on the dynamics of AgNPs, one aspect of this protection is found to be associated with the prevention of diffusion, aggregation, coalescence, and, perhaps, also some disintegration of AgNPs. In particular, Figure 6A shows the TEM image of bare AgNPs deposited via a single BLAG cycle on an amorphous TiOx layer prepared by a single RLAD cycle on the a-C TEM sample holder. Large Ag nanoparticles up to 30 nm in diameter are formed due to the aggregation and coalescence of as-grown, much smaller Ag particles of 1−2 nm in diameter. The small particles are present on this image but remain invisible at the given magnification. The dynamic processes leading to the coalescence of AgNPs take place to some extent during a postgrowth history of the sample in the UHV chamber but most likely after exposure to the ambient environment.64 Figure 6B shows the TEM image of AgNPs, 5 Å of Ag/50 ML of D2O grown on a single layer of amorphous TiOx, and subsequently covered by three RLAD layers of TiOx. In this case, the ensemble of AgNPs is found to retain original sizedensity characteristics with an average diameter of 4 nm. The aggregation and coalescence taking place in the ensemble of bare as-grown AgNPs leads to a 2 orders of magnitude reduction in the particle density from 1.4 × 1016 m−2 in the as-grown ensemble to 2.9 × 1014 m−2 in the postgrowth ensemble. Therefore, we conclude that the protective TiOx layers serve as an effective barrier which prevents the diffusion of AgNPs, Ag atoms, ions, or positively charged clusters, thus avoiding the Ostwald ripening that could lead to the aggregation.64 To conclude this part, the fast quenching of small (4 nm) bare AgNPs (Figure 4A) is attributed to a complex relaxation dynamics in the metastable ensemble of as-grown silver nanoparticles driven primarily by the interaction with the support and the environment, primarily oxygen gas. In contrast,

σext = 9

ε2 ω 3/2 εd V c [ε1 + 2εd]2 + ε2 2

(4)

where V is the volume of a spherical particle of radius r, ω is the angular frequency of the exciting light, c is the speed of light, εd is the dielectric function of the surrounding medium, εm(ω) = ε1(ω) + iε2(ω) is the dielectric function of the metal, and n(ω) is the refractive index of the surrounding material defined as n(ω)2 = εd. The parameters of the dielectric function of the metal, ε1 and ε2, found in the literature are usually those of the bulk materials. In order to adjust the dielectric function value for the case of small NPs, we use the following correction that takes into account the electron scattering from the NP boundaries: εm(ω) = εinter(ω) + ε D(ω , ωp , X(r ))

(5)

where size-independent εinter(ω) describes the interband and the core electrons contribution to the dielectric function whereas the Drude free-electron contribution εD(ω, ωp) is size-dependent through a parameter X; ωp stands for the plasmon frequency. The parameter X is a size-dependent adjustable parameter that is system dependent and takes the density of particles into account.59,72 The best fit value of the surrounding dielectric constant, εd, for bare silver nanoparticles on TiO2/Al2O3 that were submerged in water is 3.85, while the value for the TiOx protected AgNPs is 6.5. H


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of 80 ± 20 nm diameter (AgNPs/TiOx/sapphire) (see section 3.2), presenting 10 times stronger absorption compared to the small particles in Figure 7A. The black curve represents the UV− vis spectrum corresponding to large AgNPs protected by a thin titania film. Comparison between the graphs shows that the dielectric effect of the surrounding medium due to the protecting titania film is strong also in the case of large particles. Interestingly, a similar spectral red-shift effect has been reported for self-assembled monolayer (SAMs) on silver particles of comparable dimensions. The important fraction of the measured absorptivity of large particles is due to light scattering as shown in Figure S12. In addition, it is important to note that the SPR of the larger AgNPs is significantly more stable upon exposure to an ambient atmosphere on the time scale of few weeks, as discussed in section 3.3. 3.5. Photocatalytic Activity. Photocatalytic activity of pristine TiOx/sapphire, AgNPs/sapphire and hybrid TiOx/ AgNPs/TiOx/sapphire films prepared by the buffer layer assisted growth methods were investigated by following the photocatalytic degradation of methylene blue (MB), a water-soluble dye of a phenothiazine family strongly absorbing at 550−700 and 230−350 nm. This molecule is often used as a convenient standard in photocatalytic studies.42,75,76 MB photocatalytic oxidation kinetics was shown to be consistent with the Langmuir or Langmuir−Hinshelwood mechanisms,75 indicating the significance of the MB adsorption on the catalyst’s surface (titania or silver in the present case) prior to its decomposition process. Depending on the spectral range of irradiation, the photodegradation of MB is understood as the process based on oxidation/decomposition driven either by photoelectrons and/ or holes generated in the photocatalyst upon the UV absorption. Under the UV irradiation of both the pristine titania and the hybrid TiOx/AgNPs/TiOx films, photohole-driven anodic reactions comprise the oxidation of adsorbed water/surface hydroxyl species, (H2O)ad /(OH−)ad, on the TiOx surface yielding highly reactive hydroxyl radical species, (OH•)ad76 and the direct oxidation of MB species adsorbed on the surface. Under aerated conditions for the hybrid films containing AgNPs embedded in the titania matrix, photoholes can also oxidize Ag metal yielding positively charged ions, Ag+, and clusters, Agnm+, which become incorporated in the TiOx matrix as depicted in Figure 5D.27,66 In the latter case, AgNPs act as photohole scavengers which can result in degradation of the photoactivity in comparison with the pristine TiOx system. The photochemical process of the silver oxidative dissolution within the titania matrix can be attested by the SPR decay.27,66 In the present study, samples comprising the active hybrid films deposited on a transparent sapphire substrate were submerged in diluted MB solutions. After establishing the equilibrium between the MB molecules in the solution and those adsorbed on the cuvette walls and on the titania film for 3 h, the samples were exposed to pulsed laser irradiation at 355 or 532 nm. The photon energy of these light beams excludes the possibility of the direct photodecomposition of MB molecules. This implies that under the 532 nm irradiation of pristine titania and hybrid TiOx/AgNPs/TiOx films, a key anodic process involving the charge transfer from photoexcited MB species to the conduction band of the photocatalysts75 does not take place. In the case of hybrid films, however, the light absorption by the Ag SPR results in the formation of hot electrons and holes which, under certain conditions,13−15,22−27 could drive the anodic and cathodic processes, i.e., the oxidation of adsorbed species of MB and the oxygen reduction, respectively.76

The change in εd is responsible for the observed red-shift from 460 nm for the bare AgNPs to 530−540 nm for the TiOx protected AgNPs, as shown in Figures 4A and 4B. We note that our values are qualitatively consistent with reports in the literature of a value of about 2 for εd of water at 450 nm,72 3.2 for sapphire, and a value of about 7 for the dielectric constant of TiO2 at 540 nm.74 The relatively large deviation of the value of εd for pure water is thought to arise from the fact that our particles are in direct contact with solid substrate which comprises a single titania layer on sapphire. Figure 7A shows that a reasonably good agreement is obtained between the experimental UV−vis spectra of two samples: Figure

Figure 7. (A) UV−vis spectrum of bare, small AgNPs deposited on TiOx (black) in water-submerged conditions and the corresponding simulated SPR peak at 460 nm (dashed red) and the UV−vis spectrum of titania-covered AgNPs (blue) submerged in water and the corresponding simulated SPR peak at 530 nm (dashed red). A bathochromic shift and the increase in the Ag SPR intensity are evident due to the dielectric effect of the TiOx protective coverage. (B) UV−vis spectrum of large bare (80 nm size particles) AgNPs on TiOx (red) and of titania-covered large AgNPs (black). Again, the protective layer of TiOx imposes significant and characteristic changes in the Ag SPR.

7A1, AgNPs/TiOx/sapphire (black); Figure 7A2, TiOx/AgNPs/ TiOx/sapphire (blue), both submerged in water and the simulated spectra (dashed red line in Figure 7A) that were calculated on the basis of eqs 4 and 5. The important outcome of these simulations is that the distinct dielectric influence of the protective TiOx layers develops at the frequency of SPR in AgNPs. The similar red-shift effect of the protecting TiO2 layers is observed also for much larger AgNPs, as shown in Figure 7B. The red curve represents the UV−vis spectrum of bare silver particles I


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arise from the surface density of specific defect sites,11 our results appear to suggest that in the present RLAD preparation scheme the surface density of photoactivity relevant defects is similar in all our samples. Another plausible inference concerns the density of defects which could act as charge recombination centers in these titania films causing the noticeable reduction in photoactivity.14 The sample containing bare AgNPs directly deposited on the sapphire substrate (Figure 8, purple stars) displays a negligible activity upon irradiation at 355 nm. This observation indicates that AgNPs alone do not lead to the decomposition of MB at the given photon energy. The lack of photoinduced and general photoactivity of AgNPs26,27 is attributed to the combination of two unfavorable factors: the inefficiency of the plasmon and interband excitations in the present AgNPs/α-Al2O3 system at this wavelength (see the absorption spectrum in Figure 7B, curve 2) and/or to the inefficiency of the direct decomposition of MB molecules, adsorbed on the silver surface, by plasmonic holes or by oxygen. The hybrid sample comprising AgNPs (see Figure 3 for the SEM image and the size distribution profile) deposited on an amorphous titania film and protected by additional three TiOx overlayers exhibits a discernible photoactivity at 355 nm as shown in Figure 8 (purple-star spectrum). This AgNPsdecorated amorphous sample is found to exhibit nearly the same activity as the pristine anatase, rutile, and amorphous TiOx films as evident in Figure 8. From the one hand, the lack of photocatalytic enhancement in this hybrid film in comparison with the pristine titania allows to suggest that the near field plasmonic effect22 of the large AgNPs is negligible at this energy which is in good accord with the very weak, out of resonance, absorption at 355 nm as evident in Figure 7B,1 for a similar hybrid sample. On the other hand, taking into account the measured photoactivity of the pristine amorphous titania films and the possibility of the exciton dissociation at the AgNP/TiOx interface due to an interfacial electric field14 as depicted in Figure 5A,C, it can be proposed in accord with the works of Kamat and co-workers27,33 that the AgNPs and the titania host will be sites for respective cathodic or anodic processes depending on the illumination and the relative values of the Ag work function and the electron affinity and the Fermi level of the titania. The 355 nm illumination generates excitons in the titania host which can dissociate at the AgNP/TiOx interface in accord with Figure 5A such that the holes and electrons will be attracted to the AgNPs and the titania host, respectively, suggesting that AgNPs will support anodic whereas the titania host cathodic reactions. Therefore, according to this band configuration (Figure 5A), the protective titania layer around AgNPs is expected to act as reductive media which apparently cannot oxidize the MB species. The opposite circumstances take place for the AgNP/oxidized titania system as depicted in Figure 5C. The mechanism involving the interfacial charge transfer implies the direct participation of AgNPs in the photoelectrochemical reaction chain and requires the access of reactive species to the surface of AgNPs which could be hindered by the protective titania layers. In the latter case, the isolation of AgNPs from external chemical species blocks this important photochemical mechanism from operating which seems to agree with the nearly identical kinetics on pristine and hybrid films with the amorphous titaniumenriched host at 355 nm as shown in Figure 8. No detectable photoactivity has been observed for the titaniaprotected hybrid sample upon irradiated at 532 nm as appears in Figure 8 (green rhombus). Both pulsed and CW excitations at

An extensive volume of studies in the literature has been devoted to the potentially different role of the two main crystallographic phases of titania, namely rutile and anatase, on overall photocatalysis reactivity.1,11 In this work we have developed a special procedure to obtain both pristine rutile and anatase nanoscale films (18 nm thick) employing our RLAD technique in combination with different annealing steps in order to study their photocatalytic reactivity and compare it with those of the hybrid TiOx/AgNPs/TiOx films with the amorphous titania host. A summary of all relevant samples we have studied is presented in Figure 8. The time evolution of the integrated, normalized

Figure 8. Absorbance of MB (arbitrary units, normalized) as the function of the number of irradiated photons for different samples. The samples irradiated by the 355 nm pulsed laser are (1) anatase (annealed RLAD-TiO2, 40 cycles) on sapphire (red circles), (2) rutile (annealed RLAD-TiO2, 40 cycles) on sapphire (blue triangles), (3) large bare AgNPs on sapphire (purple stars), (4) large AgNPs deposited on the amorphous layer (RLAD-TiO2, 40 cycles, 18 nm thick) and further protected by three additional amorphous TiOx layers (pink triangles), and (5) TiO2(110) single crystal (black square). The sample irradiated by the 532 nm pulsed laser comprises large AgNPs deposited on the amorphous layer (RLAD-TiO2, 40 cycles, 18 nm thick) and further protected by three additional amorphous TiOx layers (green rhombus).

absorbance of MB observed in the course of sequential 30 min intervals between irradiations of five different films is displayed. It was found that under the 355 nm laser irradiation (3.49 eV photon energy) the pristine anatase (red circle), rutile (blue triangle), and amorphous TiOx (not shown for clarity) films on the sapphire substrate exhibit practically identical photoactivity. A rutile TiO2 (110) single crystal (1 mm thick) (black squares) reveals slightly higher activity at the 355 nm irradiation in comparison with nanoscale amorphous and crystalline RLAD samples. It is most likely that the main reason for this observation is the significantly stronger optical absorption of this sample at the near-UV region due to its thickness. It is noteworthy that in spite of the distinctly different fundamental absorption and significant differences in the adsorption edge, all three pristine RLAD titania films exhibit similar and relatively low level absorption at 355 nm (see Figure S5 and Table S1) which is just at the edge of the absorption band gap of titania. More precisely, the difference in the absorption at 355 nm among these nanoscale films could be insufficient in order to translate into the different photoactivity at the given experimental conditions. Furthermore, based on recent claims that most of the activity in both the rutile and the anatase phases J


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similarity to close levels of the optical absorption at 355 nm and to the similar surface density of photocatalytic-relevant defect sites on these surfaces. Moreover, no further enhancement of activity by the presence of titania-protected AgNPs due to potential plasmonic and interfacial charge transfer effects could be observed. This lack of improved activity is consistent with a recent study that examined the direct role of plasmon excitation of AgNPs photoreactivity under UHV conditions.77

that wavelength were examined; only the results from the pulsed excitation are shown. This lack of activity is in spite of the fact that the irradiation at this wavelength was aimed to excite the Ag surface plasmon resonance absorption in accord with the respective spectrum in Figure 7B,1. In order to be active at 532 nm (2.33 eV), this hybrid system should enable hot electrons, generated via the plasmon relaxation, to overcome the Schottky barrier, φSB, at the Ag/TiOx interface as shown in Figure 5A and migrate across the thin protecting titania film in order to reduce oxygen/water species at the TiOx interface. The isolation of AgNPs from a direct contact with the MB species in the solution could affect to some extent the absence of the photocatalytic activity. Another important question with respect to the plasmonic photoactivity is related to the correspondence between the energy levels of hot carriers in AgNPs and respective LUMO and HOMO levels of a photoprobe, i.e., a MB molecule in the present case. It cannot be ruled out that the energy of hot carriers generated by the irradiation at 532 nm is insufficient to decompose MB species in spite of the fact that the fundamental absorption of the latter is at 665 nm (1.87 eV). In contrast to several reports in the literature22−26 that discussed the potential plasmon effect on photocatalysis, the present results imply that the plasmonic excitation in AgNPs alone could be energetically insufficient to drive the photoactivity. A recent work by Toker et al.77 under well-defined UHV conditions has demonstrated that the overlap between the plasmon-induced carriers energy and the adsorbent’s unoccupied affinity levels is mandatory for plasmon enhancement of photoreactivity on surfaces.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b09621. Detailed spectral analysis of the absorption edge of various titania films grown via the RLAD process (Figures S1−S8) to obtain their relevant band gap estimate and the presence of spectral evidence for ionic silver atoms and clusters (Figure S7); determination of silver clusters density and size distribution (Figure S9) and presentation of XPS data showing plasmon absorption (Figure S10); the stability and actual absorption of large silver particles (Figures S11 and S12) (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected] (M.A.). Notes

The authors declare no competing financial interest.

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4. CONCLUSIONS With the goal to extend the photoactivity of titania-based photocatalysts into the visible spectral range, we have developed a clean, UHV-based method for the growth of hybrid films composed of silver nanoparticles embedded within a thin layer of titania matrix. Employing amorphous solid water (ASW) as a reactive layer, the growth of titania thin films by the reactive layer assisted deposition (RLAD) and of silver nanoparticles by the buffer layer assisted growth (BLAG) has been demonstrated. 40 cycles of the Ti/ASW deposition reaction with a single annealing step at the end resulted in a uniform film of amorphous titanium oxide becoming amorphous or crystalline structures upon invacuum or in-air annealing procedures. Growth of silver nanoparticles of 4−80 nm in diameter with additional three protective titania layers has led to stable hybrid films that exhibit a noticeable optical absorption in the visible due to the surface plasmon excitation of AgNPs. The titanium oxide protective layers cause a significant redshift of the plasmon resonant peak from 460 to 530 nm while maintaining fixed particles shape and size. A simple Mie theory analysis has attributed a major contribution to this large spectral shift to the surrounding dielectric constant (εd), dominated by the three protective titania layers. The investigation of Ag SPR dynamics affected by various environmental conditions reveals two different effects of the TiOx thin films: (1) the dielectric response of the titania layers and its effect on the surface plasmon resonance of silver nanoparticles and (2) the electrochemical/ galvanic effect comprising of charge transfer between AgNPs and titania and the chemical interaction of positively charged Ag species with the TiOx matrix. Photocatalytic activity studies of three types of titania films amorphous, rutile, and anatase phasesreveal significant and identical degradation rates of MB molecules. We attribute this

ACKNOWLEDGMENTS The support of the stuff of The Harvey M. Krueger Family Center for Nanoscience and Nanotechnology of the Hebrew University of Jerusalem is acknowledged. Specifically we thank Drs. Inna Popov, Vitaly Gutkin, Anna Radko, and Vladimir Uvarov for their help in conducting the sample characterization. This work was partially supported by The Israel Science Foundation (ISF) and by a dedicated grant provided by the Israel National Nanotechnology Initiative (INNI) via its FTA program, on which L.Z., H.S., and S.M. were partially funded.

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REFERENCES

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