Mesoporous TiO2

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Surfaces, Interfaces, and Applications

Hot Electron Transport on Three-Dimensional Pt/Mesoporous TiO2 Schottky Nanodiode Beomjoon Jeon, Hyosun Lee, Kalyan C. Goddeti, and Jeong Young Park ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 03 Apr 2019 Downloaded from http://pubs.acs.org on April 3, 2019

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ACS Applied Materials & Interfaces

Hot Electron Transport on Three-Dimensional Pt/Mesoporous TiO2 Schottky Nanodiode Beomjoon Jeon†,‡, Hyosun Lee‡, Kalyan C. Goddeti‡, and Jeong Young Park*,†,‡ †Department

of Chemistry, Korea Advanced Institute of Science and Technology (KAIST) Daejeon 305-701, Republic of Korea. ‡Center for Nanomaterials and Chemical Reactions, Institute for Basic Science, Daejeon 305701, Republic of Korea.

ABSTRACT: We present the design of a three-dimensional Pt/mesoporous TiO2 Schottky nanodiode that can capture hot electrons more effectively, compared with a typical twodimensional Schottky diode. Both chemically induced and photon-induced hot electrons were measured on the three-dimensional Pt/mesoporous TiO2 Schottky nanodiode. An increase in the number of interfacial sites between the platinum and support oxide affects the collection of hot electrons generated by both the catalytic reaction and light injection. We show that hot electron flows 2.5 times higher are detected as the current in the mesoporous system, compared with typical two-dimensional nanodiode systems that have a planar Schottky junction. Identical trends for the chemicurrent and photocurrent in the mesoporous system demonstrate that the enhanced hot electrons are attributed to the larger interface area between the metal and the mesoporous TiO2 support fabricated by the anodization process. This three-dimensional Schottky nanodiode can provide insights for hot electron generation on a practical catalytic device.

KEYWORDS: Pt/mesoporous TiO2, three-dimensional Schottky diode, hot electron, chemicurrent, catalytic activity, photocurrent

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1. INTRODUCTION A comprehensive understanding of the mechanisms in heterogeneous catalysis is particularly important for improving energy efficiency. Among the several atomic or molecular factors influencing catalytic performance, it has been suggested that the charge transport properties on mixed catalysts are crucial; therefore, a growing body of research has been performed using various catalytic systems.1-7 The electronic characteristics of catalysts have been modified by controlling the size, shape, and composition of metallic nanocatalysts, or by changing the type of the oxide support.8 In particular, metallic nanoparticles supported on a metal oxide have shown distinct catalytic activity and selectivity with the different electronic properties of the oxides, proposing that the charge flow across the metal–oxide interface provides a new pathway for activating certain reaction kinetics.9,10 Therefore, monitoring electronic behavior at the metal– oxide interface is required for understanding chemical surface dynamics on the catalyst. In general, energetic electrons (i.e., hot electrons) generated on metal surfaces by external stimuli, including heat from exothermic reactions and light incidence, play a critical role in determining chemical conversion. However, capturing and quantifying hot electrons is challenging because the hot electrons are readily thermalized within a few femtoseconds via electron–electron or electron–phonon scattering.11 A powerful technique using a metal–semiconductor Schottky nanodiode composed of a thin metal film has been introduced for direct real-time detection of hot electrons. Nienhaus and co-workers detected adsorption-induced hot electrons with a Ag/n-Si Schottky nanodiode when a hydrogen atom was being chemically adsorbed on the Ag surface.12 Since the excited electrons are energetic enough (1–3 eV) to overcome the potential barrier between the metal and the semiconductor, the generated electrons can be detected as current in the device, where the hot electron flow accompanied by a chemical energy dissipation process is

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denoted as ‘chemicurrent’. Park and Somorjai observed a steady-state chemicurrent during CO oxidation in a Pt/TiO2 catalytic nanodevice13,14 where the reaction-induced electron flux is proportional to the extent of the reaction. This phenomenon was confirmed by several chemicurrent studies of various catalytic systems, including H2O2 decomposition (liquid–solid phase)15,16 and H2 oxidation (solid–gas system).17-20 In most industrial catalysts, metal nanocatalysts are incorporated into porous oxide supports with high surface areas, showing a strong metal–support interaction (SMSI) closely related to charge transfer at the metal–oxide interface. One of the prominent materials is titanium dioxide, which shows representative photocatalyst properties. The oxygen vacancies available to be involved in the electronic features can be tuned using simple treatments, including thermal oxidation and doping processes.21,22 TiO2 is also a common oxide showing the SMSI effect with metal catalysts so that electrons can be easily transported from the metal to the semiconductor. The porous TiO2 can be fabricated using a HF solution, where the structure of the titanium oxide is controlled through an electrochemical oxidation process (i.e., anodization).23-26 To date, pivotal research about chemical reaction-induced current in highly defective structures has been conducted. Various catalytic reactions with the concept of a chemical–electrical transducer consisting of a metal/mesoporous oxide structure, including Pt/TiO2, have been carried out.27-29 A broad interpretation for the reaction current including hot electron flow provides scientific reinforcement of the field of hot electron chemistry.28 Meanwhile, various accounts of detecting hot electrons induced by light incidence onto a plasmonic metal have also been reported. Using a Au/TiO2 Schottky nanodiode, Lee et al. successfully observed photon-induced hot electrons as photocurrent.30-33 A nanodiode loaded with plasmonic metal allows the generation of hot electrons on the Au surface and detection of these 3



hot electrons using photocurrent. Obviously, there is a broad interest in the fabrication of new nanodevices that can simultaneously observe both phenomena (i.e., chemicurrent and photocurrent). Here, we suggest structural control of the oxide support layer of nanocatalyst devices. In this paper, we show the fabrication and performance of a three-dimensional Pt/mesoporous TiO2 Schottky nanodiode that is similar to practical catalytic systems (Figure 1a). Using this Pt/mesoporous TiO2 Schottky nanodiode, we obtain the hot electron flux generated during the hydrogen oxidation reaction and from photon absorption. We observe that the hot electron flux obtained from the Pt/mesoporous TiO2 nanodiode is amplified, compared with that from a conventional Pt/planar TiO2 system. Based on the clear relationship between hot electrons and catalytic performance, we conclude that the enhancement of hot electron flux in the Pt/mesoporous TiO2 Schottky nanodiode is attributed to an increase in the metal–oxide interface area.

2. EXPERIMENTAL SECTION 2.1. Mesoporous TiO2 Fabrication. The TiO2 oxide support layer can be considered the hot electron accepting material because it can form a Schottky barrier with the top platinum layer, as shown in the scheme (Figure 1b) and energy band diagram (Figure 1c). The mesoporous TiO2 layer can be fabricated through anodization of a planar titanium film prepared using the e-beam evaporation method.34 A 500 nm SiO2 layer loaded on a p-type silicon (100) wafer by wet oxidation was employed as the substrate for the nanodiode. The titanium dioxide layer, loaded as the oxide support, originated from a 500 nm titanium film prepared at a deposition rate of 2 Å/s. An aluminum shadow mask (4 × 10 mm2) creates a rectangular titanium layer that is optimized for

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stable nanodiode fabrication. Through an anodization step conducted using a DC power supply (XHR 120-10, Xantrex), we fabricated the mesoporous TiO2. Using a sufficiently thick stainlesssteel film as the cathode, the titanium film can be etched as the fluoride ion is converted to titanium hexafluoride.35 Irregular formation of the mesopores results from the high water content of the solution.36 A 0.35 wt% HF solution was employed as the electrolyte for the anodization.37 The nanofilm can be oxidized to mesoporous TiO2 at 50 V for 5 min. The titanium film vanished after 5 min of anodization when the concentration of the electrolyte was higher than 0.5 wt%. The morphology of the optimized mesoporous TiO2 support is shown in Figure 2a,b. The as-synthesized mesoporous TiO2 seems to have an amorphous structure that shows high resistance. To employ TiO2 as a semiconducting material for Schottky barrier formation, an annealing process was carried out in air at 450 °C for 2 hours. There are several papers reporting resistivity and phase variation with annealing temperature.38-40 Figure 2c,d shows top and cross-sectional views of the Pt/mesoporous TiO2 layer. As depicted in the X-ray diffraction analysis (XRD) (Figure 2e), the anatase phase was clearly observed. Moreover, the crystallized mesoporous TiO2 seems to be an n-type semiconductor, thus it can be employed as the oxide support for the Pt/TiO2 Schottky nanodiode.38 2.2. Preparation of the Pt/Mesoporous TiO2 Schottky Nanodiode. The Pt/mesoporous TiO2 catalytic nanodiode was fabricated with the anatase mesoporous TiO2 support on a silica substrate (Figure 2c,d). A Au layer is widely used as the electrode for catalytic nanodiodes when measuring chemicurrent, which can effectively transport and circulate the generated hot electrons in a circuit because it is non-reactive and electrically stable. However, loading the Au layer directly on the TiO2 support results in the formation of another Schottky barrier. Thus, a 50 nm titanium film was loaded as the ohmic contacting layer prior to deposition of the 100 nm Au electrode. A stainless-

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steel mask (5 x 5 mm2) was used for depositing the titanium film. Employed as an electron-rich metal catalyst, the platinum layer was carefully deposited onto the oxide support at thicknesses of 10, 30, or 50 nm using another stainless-steel mask (2 x 6 mm2). The Schottky barrier formed between the platinum catalyst and the titanium dioxide support results in rectification of the electron flow from the metal catalyst to the semiconducting oxide support, which indicates that ntype TiO2 was formed for the mesoporous TiO2 layer. It has an inherent barrier height that depends on the metal and semiconductor materials such that electrons showing higher energy than the barrier height (i.e., hot electrons) can be transported and circulated.41 We note that we have implemented a two-dimensional electric circuit on three-dimensional mesoporous TiO2, which we refer to as a semi-three-dimensional Schottky nanodiode. To compare with our newly suggested nanodiode, a Pt/planar TiO2 nanodiode, which is considered a two-dimensional system, was also prepared. Planar TiO2 showing comparable resistivity with mesoporous TiO2 can be formed by annealing at 580 °C for 2 hours (see Figure S1 in the Supporting Information). The Pt/planar TiO2 Schottky nanodiode can be made by following the identical procedure as used to make the three-dimensional nanodevice. A system where the platinum film is deposited on planar TiO2 only offers an apparent metal–oxide interface (i.e., 0.04 cm2) that creates the active area for hot electron transfer in the diode. Therefore, we denote this sample as a two-dimensional catalytic system even if the planar TiO2 layer also has a thickness of 500 nm. 2.3. Fitting Procedure for the I–V Characteristics. Experimental results are unreliable when the Schottky barrier height changes because of the chemical reaction. To demonstrate that the electrical properties, including the Schottky barrier, remained constant, I–V characteristics were obtained before and after the reaction using a Keithley Instrumentation 2400 sourcemeter and

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applying a bias of −1 to +1 V. The theoretical value of the thermionic emission current induced by the applied forward bias depends on the Schottky barrier height42 such that I = AA*T2 exp ( ― qESB / kBT) exp [q(V - IRs) / ηkBT], (1) where A is the interface area formed between the platinum and titania, A* is the Richardson constant, T is the temperature when the electrical property is measured, q is the elementary electric charge, kB is the Boltzmann constant, ESB is the Schottky barrier height, η is the ideality coefficient, and Rs is the series resistance of the nanodiode. We can easily obtain the thermionic emission current according to the applied bias calculated using eq 2 rather than eq 1. V = (ηkBT/q)ln (I/I0) +IRs (2) A reverse saturation current, which is caused by a small leakage of thermally excited electrons in the metal, is expressed as I0 = AA*T2 exp ( ― qESB / kBT). Electrical characteristics, including the Schottky barrier height, ideality coefficient, and series resistance, can be determined using the above equations. 2.4. Hydrogen Oxidation-Induced Hot Electron Characterization. Hot electrons generated on the platinum surface by the exothermic heat from the hydrogen oxidation reaction were detected with the 10 nm Pt/mesoporous TiO2 nanodiode. When the internal pressure of the batch reactor (1 L) reached 1.0 × 10−8 Torr, the limited reactant (i.e., hydrogen) was exposed to the loaded sample at 15 Torr and then the reactor was filled to atmospheric pressure with oxygen, which serves as the excess reactant. The temperature was raised from 30 to 90 °C in 10 °C increments, where the temperature increase time was set to 1 min and the holding time was set to 1 min. The process of increasing the temperature to 90 °C and then cooling it back to room temperature is 7



defined as one cycle, and a total of three cycles were repeated to obtain a reliable chemicurrent. A steady-state current was exhibited at constant temperature. The thermoelectric current measurement was employed while increasing the temperature under atmospheric oxygen conditions in which the catalytic reaction did not occur. 2.5 Reaction Rate Measurement. The reaction rate was measured on the Pt/mesoporous TiO2 on a silica substrate of sufficient size (~50 mm2) to exhibit that the measured hot electron flow was generated from the catalytic reaction. The turnover frequency (TOF), an indicator of the extent of the catalytic reaction, is denoted as the ratio between the turnover number of the H2O molecule and the reaction time at each temperature during the H2 oxidation reaction. The gas at the same partial pressure as in the chemicurrent observation was recycled through a GC line connected with the gas chromatograph (iGC 7200, DS science) at a rate of 2 L/min. Each component was quantitatively analyzed by increasing the temperature from 40 to 70 °C in 10 °C increments. Obviously, the catalytic activity follows the Arrhenius plot, which indicates that the catalytic activity exhibits an exponential relationship with temperature. Comparable activation energies for the catalytic activity and hot electron flow exhibit behavior from thermal stimuli that follow identical trends. 2.6. Light-Induced Hot Electron Characterization. The photocurrent was measured to demonstrate the feasibility of using the newly proposed nanodiode with a three-dimensional support layer as a light sensor. A 9 mW/cm2 tungsten halogen lamp was used to illuminate the sample at a distance of 10 cm between the light source and the sample. All photocurrents were measured in the batch reactor, at 30 to 90 °C in 10 °C increments, as used when measuring the chemicurrent. To ensure that the steady-state current was detected at the same conditions, the sample was illuminated for 20 s and then shaded for 20 s. A Keithley Instrumentation 2400

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sourcemeter was employed for the whole photocurrent measurement process.

3. RESULTS AND DISCUSSION 3.1. Enhancement of Chemicurrent in Mesoporous TiO2. To obtain the temperature dependence of the hot electron flows, we measured the current with the Pt/mesoporous TiO2 nanodiode at reaction conditions (i.e., 15 Torr of H2 + 745 Torr of O2) while increasing the temperature from 30 to 90 °C (Figure 3a). Because the measured signal includes contributions from the thermoelectric current as well as the chemicurrent associated with hot electron flows, we subtracted the thermoelectric current measured under pure oxygen (i.e., 760 Torr of O2) from the total current obtained during hydrogen oxidation. There is a definite difference between the currents measured with and without the catalytic reaction, which is the net chemicurrent generated on the Pt surface during H2 oxidation. Here, all the results were normalized to an apparent Pt/TiO2 interface area of 0.04 cm2 and reported as a chemicurrent density (i.e., the hot electron flux (units of A/cm2)). As shown in Figure 3a, the reaction-induced hot electron flow obtained from the Pt/mesoporous TiO2 nanodiode increased exponentially with increasing temperature, and exhibited 2.5 times higher values than those measured from the Pt/planar TiO2 systems across the entire temperature range (Figure 3a,c). By characterizing the I–V curves of both devices, we confirm that this trend is not attributed to differences in the respective electrical properties because they had comparable Schottky barrier heights of 0.75 eV (Figure S2). Therefore, it was assumed that the flow of hot electrons improved in the Pt/mesoporous TiO2 nanodiode because of the larger metal–oxide (i.e., Pt/TiO2) interface, which significantly affects hot electron transport from the metal to the

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semiconductor. To further clarify the origin of the enhanced hot electron flows in mesoporous TiO2, we performed several additional experiments. One method for revealing an origin of chemicurrent is comparing the activation energy (Ea) from the chemicurrent with that obtained from the turnover frequency (TOF) of the catalytic reaction. For that, we measured the catalytic activity (TOF) on both the Pt/mesoporous TiO2 and Pt/planar TiO2 systems. The oxidation states of the platinum catalyst were checked by measuring the X-ray photoelectron spectra (XPS) before and after the reactions (Figure S3), where comparable metallic platinum phases (74% and 76% before and after the reaction, respectively) were maintained. From the Arrhenius plot of chemicurrent and TOF, we found that the catalytic activity (TOF) exhibited an activation energy of 12.98 kcal/mol for the Pt/mesoporous TiO2 Schottky nanodiode (Figure 3b). In the case of H2 oxidation on the Pt surface, a rate determining step is the formation of H2O as an adsorbed phase on the catalyst (i.e., H(a) + OH(a) → H2O(a)) that requires an activation energy of 12–16 kcal/mol.43-45 This indicates that the value of the activation energy obtained on the Pt/mesoporous TiO2 catalytic nanodiode is well matched with the values reported in previous studies.19,20 We also estimated the activation energy from the chemicurrent results using identical procedures, where an activation energy of 12.01 kcal/mol was obtained (Figure 3b). These comparable activation energies obtained from the TOF and the chemicurrent indicate that the chemicurrent measured in the Pt/mesoporous TiO2 catalytic nanodiode consist mostly of hot electrons that are excited by the platinum-catalyzed exothermic reaction (Figure 3b). In a typical 10 nm Pt/planar TiO2 catalytic device, both activation energies gained from chemicurrent and TOF are consistent not only with each other, but also with those of the mesoporous system (Figure 3d), implying that the reaction pathways do not change with structural changes in the oxide layer at these reaction conditions.

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We also show that the hot electron current is negligible when we use the Au (10 nm)/mesoporous TiO2 catalytic nanodiode (Figure 4a), which is consistent with the chemical inertness of the Au layers. This result reinforces that the hot electron flow detected from the Pt/mesoporous TiO2 catalytic nanodiode could indeed be generated by the catalytic reaction. We also confirmed a decrease in the hot electron current as the thickness of the Pt layer increased (Figure 4b). Because electrons excited on the metal surface lose their energy while going through the metal film, the transferred hot electron flux decreases exponentially depending on the metal thickness with the relation -d

i = i0e

λ

(3)

where i is the chemicurrent, i0 is the initial hot electron flow, d is the thickness of the metal film, and λ indicates the inelastic mean free path of electrons in the metal film.46,47 In the case of Pt, the inelastic mean free path is ~13 nm.48 To quantify the decrease in chemicurrent, we used Pt/mesoporous TiO2 nanodiodes with platinum films 30 and 50 nm thick. According to this relationship, we found that the nanodiode with the 30 nm Pt film transported 21.5% of the reactioninduced hot electrons compared with the 10 nm Pt system, and the current drastically dropped to zero when the thickness of the Pt films is much thicker than the mean free path of the electrons (i.e., 50 nm). The comparable Schottky barrier height of the three Pt/mesoporous TiO2 nanodiodes with different thicknesses of Pt film were confirmed by the characteristics of the I–V curves during the hydrogen oxidation reaction (Figure S2). We demonstrate here that proton spillover, which could significantly affect the chemicurrent measurement, was negligible at our reaction conditions. Proton spillover is the lateral migration of dissociated protons from the metal surface to an oxide in direct contact with the metal catalyst. It

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is known that dissociated protons on the Pt surface quickly move into the TiO2 where platinum makes contact with the anatase TiO2 substrate. The activation energy barrier for spillover is almost zero;49 thus, additional current can be induced by proton spillover even at low temperatures. This phenomenon does not vary with the thickness of the platinum layer. If proton spillover is dominant, a considerable current would be detected in the 10, 30, and 50 nm Pt/mesoporous TiO2 nanodiodes. However, the exponential chemicurrent decay we observed as the metal catalyst gets thicker shows that the spillover phenomenon did not occur. Therefore, it is reasonable that the spillover effect was suppressed in the oxygen-rich condition containing hydrogen at only 2%. We have also checked that spillover can be mitigated in this oxygen-rich condition via further observations (Figure S4). Without oxygen exposure, in pure hydrogen (15 and 30 Torr of H2), our nanodiode showed ohmic properties when sweeping across voltages from –1 to +1 V. As oxygen is introduced into the chamber, however, a rectifying feature was observed because a Schottky barrier was formed. We show that the Schottky barrier height can be maintained at the reaction conditions with partial pressures of 15 Torr of hydrogen and 760 Torr of oxygen. The Schottky barrier decreases as the hydrogen fraction becomes larger, which is accompanied by higher conductivity and electrical properties of the nanodevices. Thus, injecting only 15 Torr of H2 for the chemicurrent measurement does not affect the critical Schottky barrier loss, and we can rule out proton spillover affecting the magnitude of the measured chemicurrent. The fact that there is no spillover phenomenon in the excess oxygen environment can reinforce our assumption that the titanium dioxide layer is not involved in the catalytic reaction and is only employed as a material to accept hot electrons. Earlier studies show the existence of a spilloverinduced electromotive force, whose magnitude is identical to an open-circuit voltage, when the catalytic reaction occurs for a long time (i.e., slow reaction mode).27-29 For several Pt/metal oxide

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structural catalysts including Pt/TiO2 and Pt/ZrO2, at hydrogen-dominant conditions, protons migrating to the oxide surface can be converted to H2O with oxygen and plenty of electrons provided by the upper conduction band of the n-TiO2.27 This reaction also accompanies electron transfer; however, spillover current was not detected because the amount of oxygen is much greater than hydrogen, which does not allow H+ adsorption onto the TiO2 surface. In our experiment, the open-circuit voltage measured under reaction conditions is quite low (less than 10 mV). Because our chemicurrent measurement was conducted within several minutes (i.e., fast reaction mode), no electromotive force was produced by the TiO2-involved reaction. Previous reports have discussed the hydrogen concentration cell effect induced by electromotive force, which may allow significant electron flow.28,50 Because of the very low open-circuit voltage, we can also exclude the hydrogen concentration cell effect as a major mechanism in our experiment. We also confirmed that the mesoporous TiO2 in the Schottky nanodiode exhibited no significant catalytic activity during hydrogen oxidation (Figure S5), which also supports that the measured chemicurrent was only from the platinum catalyst surface, and not from other species such as TiO2. Previous reports observed an additional electron flow with reverse polarity to the hot electron flows in the Pt/mesoporous TiO2 catalytic system, which can form through additional mechanisms (e.g., charge transfer or TiO2-involved slow reaction mode).27-29, 50 The polarity of the observed electron flow in our study is consistent with the hot electron effect; therefore, hydrogen-induced charge flow (e.g., spillover or the concentration fuel cell effect) can also be ignored. The noncatalytic activity of the mesoporous TiO2 indicates that the reactivity of the TiO2 pores is also negligible. Based on all the experimental evidence we obtained, it was confirmed that the chemicurrent detected on the Pt/mesoporous TiO2 nanodiodes indeed originated from the reaction performed on

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the platinum catalyst surface. Finally, this conclusion can be drawn: The magnitude of the hot electron flux increased because of the larger metal–oxide interface area provided by the mesoporous TiO2. 3.2. Amplification of Photocurrent in Mesoporous TiO2. Typically, hot electrons can also be detected when light is irradiated on a Schottky nanodiode. We measured the light-induced hot electrons as an elecrical current (i.e., photocurrent) from the Pt/TiO2 nanodiode with the mesoporous oxide. No leakage current flowed when the nanodiode system was in the dark (Figure S6). Once exposed to light, photon-induced hot electrons were generated that can overcome the Schottky barrier formed between the platinum and the TiO2 support. The photocurrent increased 2.58 fold when the oxide layer had a mesoporous structure compared with the thin film oxide (Figure S6). The photocurrent was also measured as the temperature increased from 30 to 90 °C, confirming that the enhanced photocurrent ratio of the mesoporous to planar TiO2 remained constant across the entire temperature range (Figure 5b). The same trend was observed even when the top Pt layer was replaced with a Au layer (Figure S7). Although the photocurrent exhibited insignificant temperature dependence compared with the chemicurrent, the ratio observed for the mesoporous system remained at 2.5 in both the photocurrent and chemicurrent (Figure 5a,b), reinforcing the fact that the enhanced hot electron flows are attributed to the increased active metal–oxide interface area in the mesoporous TiO2 support. These results also demonstrate that the mesoporous layer is indeed an effective structure for hot electron sensing. 3.3. Active Area Enlargement Effect on Hot Electron Capture. Because it is challenging to estimate the real active area in the mesoporous TiO2 system, we

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used the apparent Pt/TiO2 interface area (i.e., 0.04 cm2) instead of the genuine active area when normalizing the current signal. Furthermore, we ensured that the enhanced chemicurrent density in the mesoporous systems is only from the increased active sites and not from other factors, such as different electrical properties of the oxide or a change in the reaction mechanism. To further verify this, we analyzed the chemicurrent yield in both the planar and mesoporous Pt/TiO2 catalytic nanodiodes. The relationship between the chemicurrent density and catalytic performance for the planar and mesoporous TiO2 is depicted in Figure 6a,b. In both cases, the measured chemicurrent was linearly proportional to the turnover frequency; moreover, the slopes were also comparable to each other. Chemicurrent yield (α) can be expressed as α = 𝑗 qNPtTOF

(4)

where 𝑗 is the net chemicurrent density, q is the elementary electron charge, and NPt is the number of Pt catalyst active sites per square centimeter. This chemicurrent yield represents the number of hot electrons captured during a unit reaction forming one H2O molecule. Based on eq 4, we obtained chemicurrent yields of 1.32 ± 0.1 × 10−5 and 1.27 ± 0.1 × 10−5 electrons/H2O molecule for the planar and mesoporous systems, respectively (Figure 6b). Our results are comparable to the chemicurrent yield reported in a recent study using Pt/TiO2 composites.34 These identical chemicurrent yields in both systems clearly indicate that the improved hot electron flows were mainly attributed to the larger area of the Pt/TiO2 interface. To our knowledge, this is the first study simultaneously observing two types of hot electrons generated by photon absorption and chemical reaction on a mesoporous Schottky nanodiode. Controlled mesoporous Schottky nanodiodes can make it possible for universal hot electron collection including reaction- and photon-induced hot electrons. The identical chemicurrent yield of both catalytic systems and the maintenance of an enhanced hot electron ratio throughout the entire

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temperature range scientifically support the potential of mesoporous TiO2 as both a chemo- and photosensor.

4. CONCLUSIONS In summary, a Pt/mesoporous TiO2 Schottky nanodevice has been fabricated to establish electron behavior during catalytic reactions in a real stereoscopic environment. The shapemodified support allowed an increase in the number of adjacent spots located at the interface between the platinum nanofilm and the electron-accepting material to effectively capture the electrons induced by chemical energy dissipation. Compared with a conventional Pt/planar TiO2 catalytic nanodiode, the hot electron flux improved by a factor of 2.5. The identical chemicurrent yield was obtained in both the two- and three-dimensional systems, ensuring that the large chemicurrent observed in the three-dimensional system actually resulted from the numerous sites for hot electron transportation. We also confirmed that the mesoporous support efficiently collected the photoelectrons formed on the surface of the plasmonic metal; moreover, the amplification rate was also consistent with that of the chemicurrent. Although both chemicurrent and photocurrent exhibited different tendencies to increase with increasing temperature, the 2.5fold hot electron enhancement from the mesoporous support remained constant over the entire temperature range in both cases. The simultaneous detection of both chemical-reaction-induced and photon-induced hot electrons with this nanodiode confirms the universal nature of hot electron generation. Based on various analyses, including I–V characteristics and XPS, we confirmed the stability of the mesoporous TiO2 Schottky diode. This work can be useful for providing a fundamental understanding of hot carrier excitation and transfer in practical metal–oxide interfacial systems with potential applications of Schottky nanodiodes as both chemo- and 16


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photosensors.

ASSOCIATED CONTENT Supporting Information. Supporting characterization data (scanning electron microscopy, I–V characteristics, X-ray photoelectron spectroscopy, hydrogen partial pressure dependence on I–V characteristics, catalytic performance, and mesoporous support effect on photocurrent) AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] (J.Y.P.) Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS This work was supported by the Institute for Basic Science (IBS) [IBS-R004].

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Figure 1. Schematic illustrations of (a) the Pt/mesoporous TiO2 Schottky nanodiode and (b) effective hot electron generation during the catalytic reaction by the larger Pt/TiO2 interface area. (c) Energy band diagram of the Pt/TiO2. A Schottky barrier, whose height is ESB, is formed where the platinum and TiO2 come into contact. The mesoporous TiO2 creates a larger effective area for the Schottky contact.

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Figure 2. Scanning electron microscopy (SEM) images: (a) top and (b) cross-sectional views of the mesoporous TiO2, and (c) top and (d) cross-sectional views of the 10 nm Pt/mesoporous TiO2. The inset of (d) is taken in back-scatter electron mode. (e) X-ray diffraction spectra of the mesoporous TiO2 before and after annealing at 450 ºC for 2 hours. After the thermal annealing process, the anatase phase of TiO2 was obtained.

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Figure 3. (a) Chemicurrent density and (b) Arrhenius plots for the chemicurrent density and TOF for the 10 nm Pt/mesoporous TiO2 nanodiode. Activation energies of the sample are shown. (c) Chemicurrent density and (d) Arrhenius plots for the chemicurrent density and TOF for the 10 nm Pt/planar TiO2 nanodiode. Activation energies of the sample are shown.

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Figure 4. Chemicurrent density comparison between (a) the 10 nm Pt/mesoporous TiO2 and the 10 nm Au/mesoporous TiO2 and (b) the 10, 30, and 50 nm Pt/mesoporous TiO2.

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Figure 5. (top) Enhanced hot electron ratios of the mesoporous TiO2 to planar TiO2 from 30 to 90 °C for (a) chemicurrent and (b) photocurrent. (bottom) Comparison of hot electron flows obtained in both systems.

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Figure 6. Relationship between the turnover frequency (TOF) as a catalytic performance indicator and the chemicurrent density obtained for (a) 10 nm Pt/planar TiO2 and (b) 10 nm Pt/mesoporous TiO2. The apparent TOF in (b) was calculated by applying the apparent Pt/TiO2 interface area (i.e., 0.04 cm2) instead of the real active area for an intuitive comparison of the catalytic activity between the planar and mesoporous systems. The turnover frequency was measured from 40 to 70 °C. (c) Chemicurrent yield of the 10 nm Pt/mesoporous TiO2 and 10 nm Pt/planar TiO2.

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Table of Contents Graphic

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Mesoporous TiO2

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