Microscopic Electrochemical Control of Ag Nanoparticles Into


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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Microscopic Electrochemical Control of Ag Nanoparticles Into Mesoporous TiO Thin Films 2

Mercedes M. Linares Moreau, Eduardo David Martínez, María Cecilia Fuertes, Andrés Zelcer, Federico Golmar, Pablo N. Granell, Pablo E. Levy, Galo J. A. A. Soler Illia, and Leticia Paula Granja J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b11217 • Publication Date (Web): 08 Jan 2019 Downloaded from http://pubs.acs.org on January 10, 2019

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

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Microscopic Electrochemical Control of Ag Nanoparticles into Mesoporous TiO2 Thin Films M. M. Linares Moreau,a,b,c E. D. Martínez,d † M. C. Fuertes,b,c,d A. Zelcer,e,f F. Golmar,f P. N. Granell,g P. E. Levy,a,c G. J. A. A. Soler Illia,h,* L. P. Granjaa,c,** a) Departamento de Física de la Materia Condensada, Gerencia de Investigación y Aplicaciones, Centro Atómico Constituyentes, Comisión Nacional de Energía Atómica, Av. Gral. Paz 1499 (B1650KNA), San Martín, Buenos Aires, Argentina b) Instituto Sabato, Universidad Nacional de General San Martín, Centro Atómico Constituyentes, Comisión Nacional de Energía Atómica, Av. Gral. Paz 1499 (B1650KNA), San Martín, Buenos Aires, Argentina c) Instituto de Nanociencia y Nanotecnología, CONICET-CNEA, Av. Gral. Paz 1499 (B1650KNA), San Martín, Buenos Aires, Argentina d) Gerencia Química, Centro Atómico Constituyentes, Comisión Nacional de Energía Atómica, Av. Gral. Paz 1499 (B1650KNA), San Martín, Buenos Aires, Argentina e) CIBION, CONICET, Godoy Cruz 2390 (C1425FQD), CABA, Argentina f) Escuela de Ciencia yTecnología, Universidad Nacional de General San Martín, 25 de Mayo y Francia (B1650KNA), San Martín, Buenos Aires, Argentina g) Instituto Nacional de Tecnología Industrial, Centro de Micro- y Nanoelectrónica del Bicentenario, Av. Gral Paz 5445 (B1650KNA), San Martín, Buenos Aires, Argentina h) Instituto de Nanosistemas, Universidad Nacional de General San Martín, Av. 25 de Mayo y Francia (B1650KNA), San Martín, Buenos Aires, Argentina †

Present Address: “Gleb Wataghin” Institute of Physics, University of Campinas, UNICAMP

13083-859, Campinas, SP, Brazil. * [email protected] ** [email protected]

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ABSTRACT: Conductive-tip Atomic Force Microscopy (CAFM) was used to explore the effects of high electric fields on silver nanoparticles (NPs) arrays embedded inside the pores of mesoporous TiO2 thin films (MTF). A controlled local modification of the distribution of Ag NPs inside the MTF and its electronic transport properties was carried out. The process is driven by a strong electrochemically induced Ag+ migration and reduction inside the pores localized around the CAFM tip. Depending on the experimental parameters, we were able to induce drastic and violent changes, both at the surface and at the MTF/Si interface, namely through the whole scanned region. Therefore the results reported in this work contribute to the design and development of novel metal NP-based devices at the micro and nanoscale.

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INTRODUCTION Beyond the widely known applications of metallic nanoparticles (MNPs), one of the remaining challenges is their easy and efficient incorporation in electronic devices. A very promising candidate to overcome this challenge is the case of MNPs synthesized within a self-assembled mesoporous oxide thin film (MPF) matrix.1 The integration of MPF in current material processing technologies is promoted, due to their flexibility of synthesis over very diverse substrates and easy manipulation, high specific surface area and the presence of an ordered array of monodispersed pores.2 The most studied systems are SiO2 and TiO2 MPF, for which a wide variety of pore sizes and pore arrays is available. While the MPF could find application in many fields, a step forward is the combination of MPF with other nanomaterials that can complement and improve the desirable characteristics of a given MPF. In particular, the combination of MPF with MNPs to form nanocomposites present special interest, due to the catalytical and optical properties of MNPs. Their use in catalysis is related to their high specific surface area and their high surface reactivity, which drastically change when compared with bulk materials.3,4 On the other hand, MNPs optical properties are related with localized surface plasmon resonances. The optical response of MNPs depends on their size, shape and dielectric environment (refractive index of the medium, presence of neighboring MNPs, etc.) and thus, can be tuned from synthesis.5 The combined properties of MNPs and MPF give rise to possible application in different fields such as sensing, photocatalysis, solar cells, electronics, surface enhanced Raman scattering (SERS) substrates,6,7 devices that require light confinement and medical applications such as disease diagnosis and treatment, drug delivery devices, among others.8 The catalytic and optical properties of titania-silver nanocomposites have been widely demonstrated.9 A precise control of the size, shape and position of the silver NPs is fundamental for the tuning of these functionalities. There have been several studies on the possibility of creating and controlling silver nanostructures supported on different frameworks by using 3

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Scanning Probe Microscopy techniques.10,11,12 Some examples of this are dip-pen nanolithography13 and solid-state electrochemistry reactions generated by a polarized Scanning Tunneling Microscopy tip.14 Most of the works usually take place over non-mesoporous oxide substrates. However, the growth and control of silver nanostructures inside and on the surface of a mesoporous silica film has also been reported.10,15 These experiments were carried out in SiO2 mesoporous films impregnated with a silver nitrate solution, which is a typical precursor for Ag nanoparticles (AgNPs) synthesis,1 and the nanostructures were created inside the pores using a Conductive-tip Atomic Force Microscope. Martinez et al.16 developed a method to photoreduce Ag NPs inside the pores of ordered mesoporous TiO2 thin films (MTF). The main advantage of this method is the possibility of incorporating a mask to perform a Photocatalysis Assisted Mesopore Patterned Array (PAMPA) lithography to create patterns of AgNPs included within mesoporous TiO2 thin films ([email protected]). In a previous work we characterized the dependence of the electrical transport properties of this [email protected] system with different structural parameters Using Conductive-tip Atomic Force Microscopy (CAFM).17 There, we showed that the PAMPA method generates well defined conducting (with AgNPs inside the pores) and non-conducting (without AgNPs inside the pores) regions at the microscale. Moreover, in the conducting regions the electrical conduction develops in the three dimensions of the nanoparticles array, improving with the amount of AgNPs, the section of the current flow and the accessible porous volume. In this work, we have focused on the local control of the electrical and morphological properties inside the [email protected] patterns. These experiments resulted in a direct local modification of the distribution of Ag nanoparticles embedded in the mesoporous titania films, with a consequent change in the electrical conductivity. In previous reports, CAFM has been used to synthesize the NPs inside mesoporous SiO2 thin films impregnated with the Ag precursor solution, which could not be repeated once the precursor solution was rinsed.15 In contrast, our

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method is a ‘dry’ procedure without any precursor mediation. Furthermore the electric field can be used to control and localize the Ag NPs in the TiO2 porous matrix, generating buried conduction patterns that could be modified a posteriori as many times as required. This inspires a wide scenario for NPs potential applications in metal-oxides interfaces and conducting nanofilements for memristors devices.18,19

METHODS Mesoporous TiO2 thin films were synthesized following the sol-gel route in combination with the Evaporation Induced Self Assembly strategy (EISA).20 A templating agent, Pluronic® F127, was added to an ethanolic solution of TiCl4 (Merck) and then water was incorporated to promote hydrolysis of the Ti precursor. The molar ratio TiCl4:F127:H2O:EtOH used was 1:0.005:10:40. The solution was heated up to 32 °C and was dip coated at a withdrawal speed of 1.0 mm·s-1 in a humidity controlled environment of RH  35%. The templating agent is used to produce the micelles that will become pores after the thermal treatment at 350 ºC. The pore diameter is about 10 nm for Pluronic® F127. The substrate used was silicon (N-doped P (100), 3-7 Ω cm). Patterned nanocomposites formed by TiO2 mesoporous thin films loaded with Ag nanoparticles were produced by the PAMPA lithography method.16 This method consists in an Ag+ impregnation step, followed by UV illumination through a lithography mask (UV source: Phillips, 15W, λmax = 355nm). UV-excited electrons in the anatase TiO2 assist Ag+ ions photoreduction in the presence of ethanol. Ag loading is controlled by UV-light irradiation time. Finally, the samples are rinsed with water and EtOH and cleaned with optical tissue cleaning paper to remove any excess of Ag particles formed on the surface. The results shown in this work were obtained for samples with a thickness of 80 nm and irradiation time of 90 min. In this way, a set of TiO2-Ag

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composite thin films was produced, with conductive (Ag-infiltrated) and non-conductive regions resolved on a micrometer scale. The [email protected] samples were studied by CAFM in a Solver Pro NT-MDT Atomic Force Microscope. Electric current and topography were simultaneously measured in contact mode, using Pt-coated conductive tips. The voltage (V) was applied between the nanoscale tip and the sample substrate within ±10 V voltage range (Figure 1a). The silicon substrate was contacted with silver paint to a conductive sample holder electrically grounded. Electrical characterization included constant voltage scans and current-voltage (I-V) curves. The surface of the samples was also studied by Scanning electron microscopy (SEM) and optical dark field microscopy. A cross section of the samples prepared by Focused-Ion Beam (FIB) was also studied by SEM.

Figure 1: (a) Scheme of the electrical configuration used for the CAFM measurements. (b) Topography and (c) current map, during a -10 V bias scan at the edge of the patterning that defines the Ag-infiltrated and non-infiltrated regions of the composite films (scan size 60 µm). The bright area of the current map is conductive while the dark one is insulating (I = 20 nA), corresponding to Ag-infiltrated and non-infiltrated regions respectively.

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RESULTS AND DISCUSSION Figure 1a illustrates the electrical configuration used in the CAFM measurements performed in the [email protected] samples. Figures 1b and c show respectively a CAFM topography and current (I) map at the edge of the Ag-patterning, measured simultaneously with a -10 V bias voltage. There is a clear contrast in the electrical conduction between the highly conductive Ag-infiltrated regions and the non-conductive TiO2 mesoporous matrix (ΔI = 20 nA). However little differences can be appreciated in the topography between these regions, where the rugosity (Δr) is about 6 nm for the whole scanned area. A sequence of CAFM scans with different polarity was performed on the Ag-infiltrated region of the sample. Figures 2a and b illustrate the procedure followed for this experiment. First, two separate squares of 5 µm × 5 µm were scanned with +10 V and -10 V (top and bottom squares of Figure 2a respectively). This procedure, named as writing scan (WS) was followed by a -5 V reading scan (RS) in a larger region of 55 µm × 55 µm, which included the smaller scanned squares (Figure 2b). Figures 2c and d show the topography and the current map obtained, measured simultaneously with an applied RS voltage (VRS) of -5 V, including the regions previously scanned with -10 V (bottom square) and +10 V (top square). The results indicate that the electrical properties are modified after applying a WS voltage (VWS) with the AFM tip. The area scanned with -10 V becomes less conductive, while the area scanned with +10 V becomes more conductive than the surrounding ‘virgin’ state. Moreover, a singular conduction pattern can be appreciated surrounding the original written area.

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Figure 2: (a,b) CAFM scanning sequence: two 5 μm squares were scanned applying +10 V (top) and -10 V (bottom); subsequently a 55 μm region was scanned at -5 V including both previous scans. (c) Topography and (d) current map of the 55 μm square scanned at -5V. The scan velocity during WS was 7 µm·s-1. The characteristics of the electrical conduction patterns can be observed more in detail in Figure 3 for negative and positive VWS cases. In Figure 3a, for negative VWS, the current map shows a pattern with an enhanced conductivity ring surrounding the insulating scanned square, immediately followed by a low conductivity area, after which the conductivity is the normal offset for low bias (-4 V) in the virgin regions. In Figure 3b, for positive VWS, a conduction drop develops around the conductive scanned center, in turn followed by a more conductive ring, until the normal offset current of the ‘virgin’ state is reached. Note that in Figure 3b an imperfection in the mask, which avoided the UV exposure during the formation of nanoparticles, gave rise to some black 8

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(insulating) regions inside the conductive square. Thus it was possible to evidence at the microscale that these dark spots, without Ag content inside the pores, do not present any modification of their conductivity after the scans. (The topographies corresponding to the Figures 3a and b are displayed at the Supporting Information (SI). It can be observed there that no particular feature associated to the black spot of Figure 3b appears). In both cases of Figure 3, the conduction patterns display an affected surrounding area 5 times bigger than the original scanned area. A dendritic-like radial pattern for the electrical conduction can be appreciated. The current values for the inner and the surroundings WS effect are clearly visualized in the current profiles of Figures 3c,d. Therefore, these results strongly suggest a migration and redistribution process of Ag inside the porous titania matrix, induced by the application of VWS.

Figure 3: Current maps under VRS = -4V of (a) a region where a 5 μm square was previously written with VWS = -10 V at 13 µm·/s-1and (b) a 25 μm square was written with VWS = +10 V at 5 µm·s-1. The dark (non-conducting) spots inside the written square in (b) are due to an imperfection in the lithography mask used for the Ag impregnation method and are not related to

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the CAFM writing/reading process. (c) and (d) show typical current profiles including WS of (a) and (b) respectively.

Figure 4: Effect of scan velocity on the resulting pattern. All current maps measured with VRS= 4V. (a) Topography and (b) current map of a 5 μm square written with VWS = +10 V and vs = 2.50 µm·s-1. (c) Topography and (d) current map of a 10 μm square with VWS = +10 V and vs = 16 µm·s-1. (e) Topography and (f) current map of a 5 μm square with VWS = +6V and vs = 9 µm·s-1. Additionally, the dependence of the resulting conductivity patterns with the scan parameters involved in CAFM experiments was studied. Figure 4 displays the effect of the scan velocity (vs) during the writing process. For the same VWS (+10 V), a slower scan (2.5 µm·s-1) produces 10

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enlarged, circular and very conductive (ΔI = 17 nA) patterns (Figure 4b), while a faster scan (16 µm·s-1) produces a pattern where only the scanned area is clearly modified, attaining ΔI = 5 nA (Figure 4d). Thus, it would be possible to compensate the voltage effect by performing a slower scan. This can be observed comparing Figures 4d and 4f, where the last was performed with VWS = +6 V but vs = 9 µm·s-1 and achieved ΔI = 9 nA. Figure 5 shows the typical current-voltage (I-V) curves obtained for the [email protected] samples. They were performed in three different voltage ranges: -10 V to 0 V, 0 to +10 V and -10 V to +10 V, displayed in Figures 5a, b and c. respectively. Each set of curves was measured on a virgin sample spot, connecting the electric circuit between the CAFM tip as sample surface top electrode and the grounded substrate bottom electrode. For all cases, the I-V curves are not linear and initially they all behave irreversibly. The electric response becomes reversible after about 20 cycles, suggesting that the processes involved have reached an equilibrium state for the corresponding experimental conditions. Moreover, this irreversibility is an evidence of the evolution of the electric transport behavior with the successive I-V curves that clearly depends on the polarity of the voltage range. The electrical conduction decreases for the successive negative I-V cycles (-10 V – 0 V, Figure 5a) and it increases with the positive cycles (0 – +10 V, Figure 5b). This behavior is consistent with the results described formerly, where the regions became less (more) conductive after being scanned with negative (positive) VWS. A barrier is observed in all the I-V curves of Figure 5. From the evolution of the successive voltage cycling of the I-V curves, it can be concluded that the barrier will depend on the “electric story” of the region under test, setting conditions on the value of VWS and VRS for the scan experiments. Furthermore, when the voltage polarity is alternated within the (-10 V – +10 V) range, the appearance of a rectified response is observed while increasing in cycling (Figure 5c), that favors the current flow for positive bias and almost inhibits current for negative bias above the tenth cycle. This rectifier characteristic was previously observed for the mesoporous titania

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system (without MNPs filling) by CAFM,21 suggesting that at this point the electric transport is mostly controlled by a Schottky barrier at the interface between the Pt AFM tip and the titania film. We had already estimated (SI of Ref.21) a barrier of 1.95 eV for a Pt-TiO2 junction, based on a 5.65 eV vacuum Pt workfunction22 and a 3.7 eV electron affinity for sol-gel titania,23 originating the rectifier behavior that favors the electron flow from the TiO2 into the tip.

Figure 5: CAFM Current-voltage (I-V) curves sets applied on three different virgin spots. First and last I-V curves of 20 cycles performed without alternating polarity (a) (-10 V – 0 V) and (b) (0 V – 10 V). (c) Cycles alternating bias from -10 V to 10 V, showing the first, second and last curves of the 20 cycles performed on the same spot.

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For negative VWS, the decrease in the electrical conductivity is accompanied by an increase in the local roughness of the scanned area, which can be detected over the initial roughness (20 – 40 nm RMS) originated by the Ag infiltration method. These effects on the topography after the voltage scans were confirmed by SEM (Figure 6) and dark field (DF) optical microscopy (Figure 7). Particularly in Figure 7, the edge of the Ag lithography is shown, evidencing that this effect only appears within the Ag infiltrated region. The negatively polarized tip would induce the migration of Ag

+

ions toward the surface forming new Ag nanoparticles, what would result in a

notable increment of the scanned area’s roughness. Conversely, for positive VWS no change on the surface is observed either in SEM or in optical microscopy.

Figure 6: (a) SEM image of the surface of the film over a region where a 30 μm square had been scanned while applying -10 V with the AFM tip. On the right: images with higher magnification (b) inside and (c) outside the scanned square, showing the difference in surface roughness and particle size.

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Figure 7: (a) Topography and (b) current map at the edge of the Ag patterning, applying VWS = -10 V. (c) Bright field and (d) dark field optical images of the surface of the film including the 60 μm × 60 μm scanned region shown in (a) and (b). The yellow region (bright field) corresponds to Ag-infiltrated part of the composite, and the blue region to the titania matrix without Ag nanoparticles.

We had previously reported CAFM studies of MTF in relative humidity (RH) controlled environments,21 and we found that the electrochemical behavior of the water-titania interface strongly depends on RH. The reduction of the MTF surface combined with titania-assisted water electrolysis is reflected on the local electric transport and topography modifications, enhanced by the high accesibility of the pores (spectroscopic ellipsometry measurements in controlled 14

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humidity conditions for MTF are shown at the SI). Although in the present work the porous volume of the MTF in [email protected] samples is mostly filled with AgNPs, it is known that there is a remaining accessible porosity after the AgNPs filling procedure.24 It was also demonstrated that the environmental humidity is relevant in general for the growing of Ag nanostructures by CAFM.11,15 Considering these previous findings, we studied the effect of the environmental humidity on the CAFM experiments performed on these [email protected] samples. However, the results described here for the [email protected] do not seem to be substantially affected by RH in the range explored. (CAFM results for [email protected] in different humidity conditions are shown at the SI.)

Figure 8: SEM image of a FIB cross-section of the film in two different regions of the sample. As part of the FIB process, a layer of Pt was deposited on the surface of the film. Image (a) was taken from a virgin region, while (b) corresponds to an area that was previously scanned applying VWS = -10 V with CAFM. The film in the modified regions appears to be lifted from the substrate by a series of bubbles, the shape of the surface is distorted and displays a higher roughness. The virgin zone, in contrast, appears straight and flat. Inset: SEM image of a surface damaged due to a very slow VWS = -10 V scan.

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In order to further investigate the processes within the TiO2 film and at the TiO2-Si interface, SEM images of the FIB cross-sections performed at the virgin and after VWS = -10 V regions (Figures 8a and 8b respectively) were analyzed. Both the surface and the TiO2-Si interface, appear straight and flat at the virgin zone (Figure 8a). However, once -10 V was applied, the WS regions of the film presented a huge increase of the surface roughness and bubble-like holes at the interface between the film and the substrate, producing a distortion that propagates through the thickness of the film (Figure 8b). These structural changes are clear consequences of the high electric field applied during WS. At the Inset of Figure 8 it can be observed that holes besides AgNP appear frequently on the film surface at extreme scan conditions. Proposed Mechanism. From the set of results displayed above, it can be concluded that the electric field applied by the CAFM tip acts electrochemically on the system. An interpretation in terms of the oxidation-reduction processes in the Ag-TiO2 system and the migration and redistribution of Ag inside the film is proposed. The process is sketched in Figures 9a for cathodic and 9b for anodic polarizations of the tip.

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Figure 9: Schematic illustration of the possible scenarios of Ag+ ion migration induced by the (a) negatively and (b) positively polarized CAFM tip. Water oxidation and the generation of oxidized Ag nanoparticles (ox-AgNP) and TiO2 oxygen vacancies (Vo2+) close to the anode is complemented by water reduction and Ag0 NPs accumulation at the cathode. Cathodic Polarization: The main effect of the negative VWS is to increase the electric resistance and the surface roughness in the scanned area (Figures 6 and 7), which is surrounded by a highly conductive ring followed by a depleted edge (Figures 2d and 3a). The proposed mechanism is, as a consequence of the cathodic polarization (Figure 9a), the Ag+ ions, mainly remnant from the Ag NPs infiltration process,7 are electrostatically attracted by the AFM tip and reduced to form metallic NPs (Ag+ + e- → Ag0). The Ag+ ions in the film are redistributed, emerging from the titania matrix and the surroundings of the scanned area. They are reduced into Ag nanoparticles (Figure 6) accumulating below and around this region (Figure 3a), and producing the observed rough profile. The Ag+ migration could be favored by the adsorbed water layered over the pore walls25 at the remaining accessible porosity.24 Although the relative environmental humidity does not seem to be critical for our experiments, this minimum amount of adsorbed water inside the pores would be enough to assist the ion migration and redox process,26 playing the role of the electrolyte. Moreover, considering the reactions at the TiO2-Si interface (the counter-electrode), the generation of oxygen vacancies (VO2+) at the titania pore surface could contribute to the oxidation of Ag nanoparticles (ox-AgNP) near the anode (Ag0 → Ag+ + e-). Thus, titania would be a necessary participant in this process. This argument was confirmed by similar experiments performed in mesoporous SiO2 filled with Ag nanoparticles, where no change in electric conduction nor topography was found (these results are displayed at the SI). The water oxidation next to the anode (4 OH- → O2 (g) + 2 H2O + 4e-) results in O2 released through the MTF matrix. The effect of the negative VWS seems to be so strong on the diffusion of Ag+ ions and O2 toward

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the surface that the interface with the substrate is affected and the film could be severely damaged as shown in Figure 8b. This bubble-like deformation has already been reported for TiO2 under strong electric fields.27 Thus, the system evolves until the source of Ag+ ions mostly depletes. The conductive paths between the tip and the substrate disappear, and the electric transport becomes mainly governed by the MTF, as can be seen in the IV curves of Figure 5. Anodic Polarization: for the positive polarization of the tip (Figure 9b), the Ag nanoparticles closer to the film surface are oxidized (Ag0 → Ag+ + e-) increasing the effective concentration of Ag+ ions. The trapped water in the pores would be the medium for the Ag+ diffusion toward the substrate and away from the tip, generating a redistribution of the AgNPs. Thus, silver would be reduced (Ag+ + e- → Ag0) next to the cathode (substrate), promoting the AgNPs homogeneous filling of the pores below and around the WS area. This effect is evidenced by an increase of the electrical conduction inside this region, being especially noticeable in Figure 4b, where the WS was performed with a very slow scan velocity (vs = 2.50 µm·s-1). Nevertheless, as the new AgNPs accumulate next to the substrate, no trace of the positive VWS appears at the topography by AFM or any other microscopies (see for instance Figure 4a).

CONCLUSIONS Embedding Ag nanoparticles inside mesoporous TiO2 thin films generates well-defined conductive and non-conductive regions.17 We demonstrated here that it is possible to modify the conductivity inside the Ag-loaded regions of the film by applying an electric field with a CAFM tip. The process achieves a redistribution of the Ag nanoparticles inside the pores, induced electrochemically and localized around the CAFM tip. From a detailed analysis of the SEM-FIB images, we conclude that the effect of Ag+ migration produces important changes both at the MTF/Si interface and inside the film. Unlike previous metal nanoparticles reports,11,15 our three-

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dimensional and ‘dry’ procedure is favored by the catalytic properties of titania and develops inside the mesoporous structure without any AgNPs precursor solution mediation. Summarizing, this work combines the photoreduction PAMPA silver infiltration method16 with the possibility of modifying electrically these already synthesized structures with microscale resolution by CAFM. This research is a an alternative starting point for nanoparticles manipulation in the design of tridimensional NPs arrays, whose improvements and implementation now depend on tuning the proper external control settings (i.e. the writing voltage, scan time, etc.) and exploring new structural parameters. Particularly, it was formerly reported that environmental humidity influences on ions migration and filaments formation in MIM and MIS memory cells, affecting the current and voltage operation of the cell. Hence, new studies of the [email protected] system playing with the combination of Ag content and RH could help to understand and improve the performance of these devices. Moreover, this work could contribute to the development and miniaturization of memory cells and other electrical devices and sensors. Particularly, it could inspire more complex designs based on metal-oxide interfaces and conducting nanofilaments for memristors nanodevices18,28,29 and the most recent applications of memristors on neuromorphic computing.19,30

Supporting Information The supporting information provided free of charge online includes: (I) Water accesibility characterization of MTF, (II) Complementary

characterization of CAFM experiments on

[email protected], (III) Effect of the environmental humidity for CAFM experiments on [email protected] and (IV) CAFM experiments on SiO2 mesoporous thin films loaded with Ag nanoparticles. Acknowledgments The authors acknowledge financial support received from ANPCyT (PICT 2008-1345, PICT 2012-1506) and UNSAM (SJ10/20). We thank A. Wolosiuk for fruitful discussions and S.J. 19

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Ludueña and M.C. Marchi for the SEM images performed at CMA belonging to the Sistema Nacional de Microscopía (MINCYT). M.C.F., A.Z., F.G., P.L., G.J.A.A.S. and L.P.G. are CONICET staff members.

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