Surface plasmon-driven catalytic reactions on a patterned Co₃O₄/Au inverse catalyst

Abstract

We report hot electron- and surface plasmon-driven catalytic reactions on an inverse catalyst that is composed of a reactive cobalt oxide (Co₃O₄) for the catalyst portion and a metal as the source of localized surface plasmon resonance (LSPR). To achieve this scheme, a patterned Co₃O₄/Au nanostructure was fabricated using the nanosphere lithography technique. The enhancement of catalytic activity for CO oxidation on cobalt oxide by the transfer of plasmonic hot carriers generated from the LSPR of Au under light irradiation was observed. We found that the Co₃O₄/Au inverse catalyst exhibited 25–50% higher catalytic activity when under light irradiation. We attribute this enhancement of catalytic activity to the flow of surface plasmon-driven hot carriers that were created by the absorption of photons on the Au metal, followed by injection into the active Co₃O₄ layers.

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1 Introduction

Plasmonic metal nanoparticles have been widely used for many applications (e.g. catalytic activity, water splitting reactions, biosensors, solar-cell design, and cancer diagnostics).^1–6 Surface plasmon resonance (SPR) is a strong electromagnetic field near the surface of noble metal nanostructures that can occur in plasmonic metal nanostructures by resonant oscillation of free electrons with the same wavelength as the incident photons.^7–9 Recently, many studies have shown the generation of hot electrons with high kinetic energy (1–3 eV) by the electromagnetic decay of surface plasmons at a femtosecond timescale in plasmonic nanostructures under light irradiation and SPR excitation.^10–12 Mukherjee et al. revealed that H₂ molecules on Au nanoparticles can dissociate at room temperature because of hot electrons created from surface plasmon decay under visible light irradiation.¹³ Lee et al. found a steady-state flow of hot electrons generated by localized surface plasmon resonance (LSPR) at a metal–semiconductor junction.¹⁴ Wang et al. reported that a high concentration of hot electrons can tunnel through the insulating layer and generate photocurrent in a metal–insulator–metal device.¹⁵

To fabricate new plasmonic nanostructures with homogeneous size, shape, and interparticle distance of the structures, several nanolithography techniques have been developed (e.g. electron beam lithography, focused ion beam lithography, and nanoimprint lithography).^16–18 Furthermore, the nanosphere lithography (NSL) technique is a low-cost fabrication technique that uses 2-dimensional colloidal crystals (e.g. 2D arrays of polystyrene and silica nanospheres) as a mask. The advantage of the NSL technique for plasmonic nanostructures is that the LSPR of noble metal nanostructures can be easily controlled in the visible to near-IR region because the aspect ratio that determines the extinction maximum wavelength can be accurately adjusted by varying the size of the nanospheres used as the mask and the thickness of the deposited noble metal.¹⁹

Conventional catalysts consist of a metal as the catalyst and an oxide or semiconductor as the support for the metal catalyst. It has been demonstrated that the interaction between the metal and the oxide support at their interface (i.e. strong metal–support interactions (SMSI)) can affect the catalytic activity with a dependence on the oxidation state of the metal catalyst and the doping state or reducibility of the oxide support.^20–23 In addition, there are some studies that show the influence of hot electrons from the oxide or semiconductor support on the catalytic activity of metal catalysts when under light irradiation.^24–26

To understand the intrinsic relation between catalytic activity and hot electron flows generated across metal–oxide interfaces, the design of a fresh catalyst model is important.²⁷ In previous studies, the active site was the metal, which is influenced by hot electrons from an oxide or a semiconductor. The major shortcoming of this conventional catalyst is the short range of the electron mean free path for the metal, which can de-energize the hot electron before its arrival at the metal surface. Kim et al. reported that the change in catalytic activity for CO oxidation on Au/CeO₂ or Pt/GaN catalysts under light irradiation decreased as the size of the Au or Pt nanoparticles increased because of the length of the mean free path of the metal.^24,26 Lee et al. found that smaller Pt nanoparticles lead to higher chemicurrent yield (i.e. the number of hot electrons detected per molecule of reaction product formed on the Au/TiO₂ nanodiode surface) during hydrogen oxidation, which is associated with the shorter travel length for the hot electrons, compared with their inelastic mean free path.²⁸ Therefore, the role of the electron mean free path is critical for studying the relationship between catalytic activity and hot electron flow.

Some oxide materials (e.g. cobalt oxide (Co₃O₄), cerium oxide (CeO₂) and cuprous oxide (Cu₂O)) are catalytically reactive because these oxide materials can be easily and reversibly oxidized and reduced.^29–31 Among these reactive oxide materials, Co₃O₄ is specifically known as the most catalytically active oxide material for CO oxidation. Haruta and co-workers demonstrated that some base metal oxides (e.g. Co₃O₄, MnO₂, and NiO) are intrinsically reactive for CO oxidation without a metal catalyst and that the catalytic activity of base metal oxides decreases in the following order: Co₃O₄ > MnO > NiO > CuO > Fe₂O₃ > Mn₂O₃ > Fe₃O₄ > CeO₂ > TiO₂.³² Xie et al. found that Co₃O₄ nanorods with predominantly exposed [110] planes are very reactive and stable for CO oxidation at temperatures as low as −77 °C, giving 100% CO conversion because of the presence of active Co³⁺ species at the surface.³³ Song et al. reported that mesoporous cobalt oxide nanoparticle aggregates can achieve 100% CO conversion to CO₂ at −60 °C under normal conditions.³⁴ It was found that a small Co₃O₄–SiO₂ nanocomposite catalyst prepared using an activated carbon template exhibits very high activity for CO oxidation at −76 °C.³⁵

To overcome the short electron mean free path problem in the metal, we used an inverse catalyst in this study that is composed of a reactive oxide for the catalytic portion and a noble metal for the LSPR source. Triangular noble metal nanostructures show much higher surface-enhanced Raman scattering (SERS) activities than flat noble metals because of the large number of confined hot spots between adjacent triangles.³⁶ Subsequently, we grew Co₃O₄ on a patterned triangular Au nanostructure to make the patterned Co₃O₄/Au inverse catalyst using NSL (Fig. 1a). We studied the catalytic activity for CO oxidation on the patterned Co₃O₄/Au inverse catalyst with and without light irradiation to identify the role of hot electrons from LSPR on the catalytic reaction. The energy band diagram of the Co₃O₄/Au junction (Fig. 1b) shows the transfer of hot electrons and their effect on catalytic activity. The Schottky barrier height for Au/Co₃O₄ is 1.6 eV, which was determined by the difference between the work function of the Au (5 eV) and the electron affinity for Co₃O₄ (3.4 eV).³⁷ The main merit of our inverse catalyst is the long mean free path in the oxide created by using a reactive oxide material on a plasmonic noble metal. Energetic hot electrons having high kinetic energy (2.0–2.1 eV) formed via LSPR upon the absorption of photons on the plasmonic gold. The hot electrons then travelled over the Schottky barrier and arrived at the Co₃O₄ surface because of their long mean free path. Thus, these plasmonic hot carriers can affect the catalytic activity of CO oxidation on Co₃O₄.

Fig. 1 (a) Schematic drawing of the patterned Co₃O₄–Au nanostructure and the effect of hot electrons on catalytic activity under light irradiation. (b) Energy band diagram for the Au–Co₃O₄ inverse catalyst representing the flow of hot electrons generated from the LSPR of the patterned Au nanostructures and their effect on the CO oxidation reaction when under light irradiation.

2 Experimental

2.1 Fabrication of the two-dimensional patterned Co₃O₄/Au nanostructures by nanosphere lithography

Chemicals. Tetraethyl orthosilicate (TEOS, Aldrich, >99.0%), sodium dodecyl sulfate (SDS, Aldrich, >99.0%), ammonium hydroxide (NH₄OH, Dae Jung Chemicals, 25.0–28.0%), chloroform (CHCl₃, Junsei, 99.0%), ethanol (C₂H₅OH, Merck), and methanol (CH₃OH, Merck) were used. 300 nm SiO₂ nanospheres were synthesized using the Stöber method, as follows. 0.55 mL of NH₄OH in 3.5 mL of deionized water was added to 10 mL of ethanol and stirred. 2.3 mL of TEOS in 10 mL of ethanol was injected dropwise into the previous solution. After stirring the reaction solution at room temperature for 12 h, the mixture was centrifuged at 3000 rpm for 10 min and washed with ethanol. After repeating this washing process three times, the 300 nm SiO₂ nanospheres were dispersed in methanol. Self-assembled monolayer arrays of silica nanospheres were prepared on a clean SiO₂/Si substrate by utilizing the Langmuir–Blodgett (LB) technique using a Nima Technology 311D. To spread the SiO₂ nanospheres onto the deionized water subphase in a trough, 3 mg of SDS was mixed with 2 mL of the previous silica solution dispersed in methanol. After ultra-sonication of the mixed solution for 40 min with heating, 3 mL of chloroform was added to float the silica nanospheres on the water and sonicated again for 30 min. This prepared solution was spread and floated on the water surface; the surface pressure was monitored using a Wilhelmy plate. While maintaining the target surface pressure at 15 mN m⁻¹, the self-assembled packed silica nanospheres were transferred onto the SiO₂/Si substrate through the monolayer by slowly lifting the substrate at a constant speed of 1 mm min⁻¹. For fabrication of the Co₃O₄/Au nanostructures, thin films consisting of 5 nm of titanium (Ti), 30 nm of gold (Au), and 40 nm of cobalt (Co) were deposited sequentially using electron beam evaporation on the previously closely packed SiO₂ nanospheres on SiO₂/Si substrate. At room temperature, all the thin films were deposited using 10 kV DC voltage in a high-vacuum chamber with a base pressure of 5 × 10⁻⁷ Torr, evacuated using rotary pumps and turbo molecular pumps. The metal deposition rate was 0.1 Å s⁻¹ until 2 nm was deposited for adhesion. The deposition rate was then increased to 0.5 Å s⁻¹ until reaching the final metal thickness. The closely packed SiO₂ nanosphere arrays act as a mask, thus all the metal was deposited with the SiO₂ nanospheres creating the patterning. After finishing the metal deposition, the cobalt was oxidized to Co₃O₄ by heat treatment at 553 K for 2 h 30 min. Finally, the 300 nm SiO₂ nanospheres were removed by sonication for 1 minute with deionized water. The patterned Co₃O₄–Au islands were thus obtained using NSL. Based on our experience fabricating Au/TiO₂ catalytic nanodiodes, in the case of the Au/Ti interface, the electrical contact is an ohmic contact and is very unlikely to have a Schottky barrier. To form a Schottky barrier, we would need to heat the Ti layer under oxygen to form TiO₂ before depositing the Au layer.

2.2 Characterization, chemical modification, and structures of the patterned Co₃O₄/Au nanostructures

The crystalline phases of the cobalt oxide and the Au nanostructure before and after heat treatment were revealed using X-ray diffraction patterns (XRD, Rigaku D/MAX-2500) taken at a 2θ scan range of 10–70°, scan speed of 4° min⁻¹, and step size of 0.01 Å using Cu Kα radiation. The oxidation state of the cobalt oxide (Co 2p) before and after CO oxidation was identified using X-ray photoelectron spectroscopy (XPS, Thermo VG Scientific Sigma Probe system with an Al Kα X-ray source (1486.3 eV)) with an energy resolution of 0.47 eV full width at half maximum (FWHM) under ultrahigh vacuum (10⁻¹⁰ Torr). The thickness and morphology of the patterned Co₃O₄/Au nanostructures were characterized using cross-section transmission electron microscopy (TEM, FEI Titan G2 Cubed 60-300), scanning transmission electron microscopy and high-angle annular dark field (STEM-HAADF, FEI Titan G2 Cubed 60-300), corresponding energy-dispersive X-ray spectroscopy (EDS, FEI Super X (4 SDD EDS)) elemental maps for 300 s, energy-dispersive X-ray spectroscopy (EDS) line profiles, and scanning electron microscopy (SEM, Magellan 400). The TEM specimen was prepared using the focused ion beam (FIB, Helios Nanolab 450 F1) preparation technique for cutting cross sections of the Co₃O₄/Au nanostructures. Two sizes of triangular Au nanostructures were created by changing the size of the SiO₂ nanosphere mask. The optical properties of these Au nanostructures were characterized using a UV-vis spectrophotometer (Lambda 1050). Triangular Au nanostructures (30 nm high) were prepared on a quartz window (1 mm thick) using the same method described previously. The spectral range was 400–800 nm, and a clean quartz window was used as a reference.

2.3 Measurement of the catalytic activity of CO oxidation on patterned Co₃O₄/Au inverse catalysts with and without light irradiation

Reaction studies were performed in an ultrahigh vacuum chamber (1 L) with a base pressure of 1 × 10⁻⁸ Torr, evacuated using rotary pumps and turbo molecular pumps. After isolating with a gate valve, the CO oxidation reaction was carried out in the batch reactor system under 40 Torr CO and 100 Torr O₂ as the reactant gases and 620 Torr He as the balancing gas. All the gases were circulated continuously through the reaction line by a Metal Bellows recirculation pump at a rate of 2 L min⁻¹. After 30 min of recirculation at room temperature for gas equilibration, a DS 6200 gas chromatograph (GC) equipped with a thermal conductivity detector and a 6 in. × 1/8 in. SS molecular sieve 5A was used to separate the products for analysis. The CO conversion was monitored at various temperatures (523–553 K) and the measured reaction rates were reported as turnover frequencies (TOF), which were measured in units of product molecules of CO₂ produced per active Co₃O₄ surface site per second of reaction time. All the reaction data were obtained by measuring the rate of CO oxidation at low conversion (i.e., conversion below ∼15%) to check the initial reaction rate within a kinetically controlled regime, assuming different reaction conditions. The number of active sites was calculated from the geometry in the SEM measurements of the surface area of the nanoparticle arrays. A halogen lamp (Dolhan-Janner D150) was used as the light source to measure the enhancement of catalytic activity while under light irradiation. The distance between the sample and halogen lamp was about 8 cm through a sapphire window and the light intensity was measured to be 49.4 mW cm⁻² at this distance by an optical power meter (ADCMT 8230).

2.4 Finite-difference time domain (FDTD) simulations

The distributions of the electric field by localized surface plasmon resonance (LSPR) on the patterned Co₃O₄–Au nanostructure and the patterned Au nanostructure were calculated using a two-dimensional FDTD simulation with a commercial FDTD simulation package (FDTD-Lumerical™). The optical properties of the gold were obtained from the materials database in the software using the Drude model. The refractive index of Co₃O₄ was obtained experimentally using a spectroscopic ellipsometer (Woollam M2000D). Variable angle spectroscopic ellipsometric (VASE) data (i.e. 65.00, 70.00, 75.00°) were acquired from a Co₃O₄ thin film on a SiO₂ wafer (173 nm cobalt oxide/1000 nm SiO₂) and modelled in B-spline mode to calculate the refractive index for cobalt oxide (2.8 at 622 nm). The electric field distribution was acquired with a mesh size of 0.5 nm at an incident light of 622 nm, which is the extinction maximum wavelength (λ_ex) of the nanostructures derived from the UV-vis absorbance spectra. The geometric model used in these calculations for the patterned Co₃O₄/Au nanostructure and the patterned Au nanostructure was built from the SEM and TEM images.

3 Results and discussion

3.1 Characterization of the patterned Co₃O₄/Au inverse catalysts

We fabricated reactive oxide/plasmonic noble metal hybrid structures using the NSL technique. The Co₃O₄ on triangular Au nanostructures were thus fabricated: first, 300 nm SiO₂ nanospheres were synthesized using the well-known Stöber method, and self-assembled monolayer arrays of silica nanospheres were prepared on a clean SiO₂/Si substrate using the LB technique. Next, 30 nm of gold as the LSPR source and 40 nm of cobalt as the catalyst were deposited sequentially by electron beam evaporation on the previously fabricated closely packed SiO₂ nanospheres on SiO₂/Si substrate. The closely packed SiO₂ nanosphere arrays act as a mask so that all the metal was deposited using the SiO₂ nanospheres for patterning. Finally, the cobalt was oxidized to Co₃O₄ by heat treatment at 553 K for 2 h 30 min and the 300 nm SiO₂ nanospheres were removed by sonication for 1 minute with deionized water, thus obtaining the patterned Co₃O₄–Au islands. All the fabrication procedures and SEM images are shown in Fig. 2a–d and e–h, respectively.

Fig. 2 Fabrication procedure for the patterned Co₃O₄–Au nanostructures and corresponding SEM images. (a and e) Deposition of 300 nm SiO₂ nanospheres using the LB technique. (b and f) E-Beam deposition of 30 nm of Au. (c and g) E-Beam deposition of 40 nm of Co. (d and h) Heat treatment for oxidation of the cobalt to cobalt oxide and removal of the SiO₂ nanospheres.

Fig. 3a shows the crystalline phase of the cobalt oxide according to annealing temperature, as revealed by XRD patterns. After annealing at 553 K, diffraction peaks corresponding to a cubic spinel structure were confirmed, indicating that the cobalt was fully oxidized to Co₃O₄. Co₃O₄ has a cubic spinel structure and the Au nanostructure was polycrystalline after annealing at 553 K, as shown in Fig. 3a and S1 in ESI.† Fig. 3b illustrates the UV-vis extinction spectra of the 30 nm triangular Au nanostructures fabricated by NSL using 200 and 300 nm SiO₂ nanospheres as masks. All the Au cones remained and there were no SiO₂ nanospheres after sonication, as shown in Fig. S2 in ESI.† It is well known that the optical properties (e.g. the intensity and extinction maximum wavelength) of the LSPR for noble metal nanoparticles are influenced by shape, size, and dielectric environment. Previous studies show that the extinction maximum wavelength of triangular noble metal nanostructures fabricated by NSL can be tuned by changing the aspect ratio (i.e. the ratio of the length and the height) of the triangular nanostructures.^38,39 In this work, the aspect ratio was controlled by changing the size of the SiO₂ nanospheres used as the mask. The extinction maximum wavelength peak of the triangular Au nanostructures from the 300 nm SiO₂ nanospheres was red-shifted from the wavelength peak of the triangular Au nanostructures from the 200 nm SiO₂ nanospheres because of the increased aspect ratio from the large SiO₂ nanospheres, which reveals that the extinction spectrum represents absorption by the LSPR of the triangular Au nanostructures. In addition, there was an ultraweak absorption band about 750 nm away from the extinction maximum wavelength peak. This broad LSPR band came from some defects in the triangular Au nanostructures that were caused by misalignment of the SiO₂ nanospheres or by the size distribution of the SiO₂ nanospheres, as shown in Fig. S2 in ESI.†³⁹

Fig. 3 (a) XRD patterns of the cobalt oxide at different annealing temperatures. (b) UV-visible absorbance spectra of the patterned Au nanostructures fabricated using 200 or 300 nm SiO₂ spheres.

To identify the thickness and morphology of the patterned Co₃O₄/Au nanostructures, a cross-sectional TEM image was taken (Fig. 4a). During fabrication of the Co₃O₄/Au nanostructures, vacancies between three closely packed SiO₂ nanosphere arrays created cobalt oxide/Au nanostructures with nanopyramidic shape. Fig. 4d–h shows a STEM-HAADF image and EDS elemental maps for 300 s of the patterned Co₃O₄/Au nanostructure. It clearly shows that the top layer is cobalt oxide (i.e. the catalyst portion) and that there is a Au triangle under the cobalt oxide (i.e. the LSPR source). An EDS line profile of the patterned Co₃O₄/Au nanostructure is shown in Fig. 4b and c. The heights of the Au triangle and cobalt oxide are 30 and 40 nm, respectively.

Fig. 4 (a) TEM image of the Co₃O₄–Au nanostructure. (b and c) EDS line profile of the Co₃O₄–Au nanostructure. (d) STEM-HADDF image of the Co₃O₄–Au nanostructure and (e–h) EDS elemental maps of each component.

3.2 Surface-plasmon driven CO oxidation on patterned Co₃O₄/Au inverse catalysts

Here, we carried out CO oxidation on the patterned Co₃O₄/Au inverse catalyst with and without light to show the intrinsic relation between catalytic activity and hot electron flow from the LSPR on the triangular Au nanostructure during light irradiation. Reaction studies were performed in an ultrahigh vacuum chamber (1 L) with a base pressure of 1 × 10⁻⁸ Torr, evacuated using rotary and turbo molecular pumps. After isolation using a gate valve, the CO oxidation reaction was carried out in the batch reactor system under 40 Torr CO and 100 Torr O₂ as the reactant gases and 620 Torr He as the balancing gas. CO conversion was monitored at various temperatures (523–553 K) and the measured reaction rates were reported as TOF, which are measured in units of product molecules of CO₂ produced per active Co₃O₄ surface site per second of reaction time.

Fig. 5a and b shows the catalytic activity for CO oxidation measured on the patterned Co₃O₄/Au inverse catalyst at four different temperatures (523–553 K) with and without light. The patterned Co₃O₄/Au inverse catalyst shows a higher catalytic activity under light irradiation than in the dark. Previous studies show that the enhancement of the activity of the Pt NPs/p-doped GaN wafer under light irradiation was 11–33% which was measured using the same batch reactor system and halogen lamp as used in this study;²⁶ thus, the enhancement under light irradiation (25–50%) observed on the Co₃O₄/Au inverse catalyst is significant. It is associated with hot electrons generated from LSPR excitation of the triangular Au nanostructure by light absorption that can affect the catalytic activity for CO oxidation on the reactive Co₃O₄. However, the thermal effect of light irradiation can also result in the enhancement of catalytic activity. We measured sample temperature data with a temperature controller with and without light at 523 K under 40 Torr CO, 100 Torr O₂, and 620 Torr He (Fig. S3 in ESI†). There was no change in the sample temperature when light was irradiated on the sample, demonstrating that any heating effect by light irradiation can be ignored in our experiments. Also some previous studies show a decrease in catalytic activity for CO oxidation on Pt/n-Si and Pt nanoparticle/n-type GaN resulting from a reduced net charge on the Pt nanoparticles when under light irradiation.^25,26,40 CO oxidation was also carried out on the patterned Co₃O₄ nanostructure with and without light under the same conditions to compare the catalytic activity of the patterned Co₃O₄/Au inverse catalyst (Fig. 5c). Under light irradiation, the catalytic activity of the Co₃O₄ exhibited an insignificant difference (i.e. within the error of measurement). Even if a hot electron is generated on the Co₃O₄ from electron–hole pair excitation when under light irradiation, it will eventually turn into a low-energy electron through inelastic scattering within a time scale of femtoseconds. Hot electrons from LSPR excitation on Au can be transferred when a metal–oxide interface is formed, revealing that a role for Au nanostructures is as an LSPR excitation source, leading to a change in catalytic activity for CO oxidation. The TOF values of the patterned Co₃O₄/Au inverse catalyst were similar to the TOF values of the patterned Co₃O₄ nanostructure in the same reaction temperature range, demonstrating that there was no synergetic effect between the Au and the Co₃O₄. We also carried out CO oxidation on the patterned Au nanostructure at 573 K without Co₃O₄, but there was no CO₂ product although the triangular Au nanostructure was polycrystalline. Thus, the CO oxidation reaction was occurring on the Co₃O₄, and not on the Au nanostructure. While the triangular Au nanostructure is not reactive in this reaction temperature range, it does perform a role as the LSPR source when under light irradiation. Hence, we can confirm that the enhancement of catalytic activity only originated from hot carriers generated by LSPR excitation of the Au that were then transferred to the reactive Co₃O₄.

Fig. 5 (a) Catalytic activity for CO oxidation on the patterned Co₃O₄–Au inverse catalyst with and without light at four different temperatures. (b) Arrhenius plots of the patterned Co₃O₄–Au inverse catalyst with and without light at four different temperatures. (c) Catalytic activity for CO oxidation on the patterned Co₃O₄ nanostructure with and without light at four different temperatures. (d) TOF for CO oxidation with sequential light ON/OFF on the patterned Co₃O₄/Au at 543 K. The TOF value was calculated from the slope of the turnover number with time.

To confirm whether the plasmonic hot carrier effect on catalytic activity was reversible, CO oxidation was carried out continuously on the patterned Co₃O₄/Au inverse catalyst as the light was turned on and off at 543 K (Fig. 5d). The TOF value was similar to prior experiments (Fig. 5a): the catalytic activity was enhanced and then returned to the original catalytic activity as the halogen lamp was turned on and off, respectively, showing that the plasmonic hot carrier effect from the triangular Au nanostructure to the Co₃O₄ was reversible. The small increase in the TOF value in the second cycle was the result of calcination of the sample during the reaction. After the reaction, the morphology of the patterned Co₃O₄/Au nanostructure had minor changes, but there were no significant morphology changes, as verified by SEM images taken before and after the CO oxidation reaction (Fig. S4 in ESI†). XPS measurements showed that the oxidation state of the cobalt oxide was a mixture of Co²⁺ and Co³⁺, representing Co₃O₄. After the reaction, a higher percentage of Co³⁺ was observed (Fig. S5 in ESI†).

To understand the electric field distribution from the LSPR around the patterned Co₃O₄–Au nanostructure, we carried out two-dimensional FDTD simulation analysis. The electric field distribution was obtained with a mesh size of 0.5 nm and incident light of 622 nm, which corresponds to the extinction maximum wavelength of the nanostructures from the UV-vis absorbance spectra. We set the refractive index of the Co₃O₄ at 622 nm to 2.8, which was acquired experimentally, and the geometric model for the patterned Co₃O₄/Au nanostructure was built on experimentally observed structural parameters from SEM and TEM images. The calculated simulation results show an electric field distribution that indicates an enhancement of light absorption for the top and cross-section views of the nanostructure. Fig. 6a shows that the electric field was formed around the vertex points of the patterned Au nanostructure, which was oriented along the direction of polarization. In Fig. 6b, the electric field was confined prominently along the gold edge between two adjacent Au triangles. In particular, a strong electric field was formed at the interface between the Au and Co₃O₄ due to the higher refractive index of the Co₃O₄.⁴¹ For the patterned Au nanostructure without Co₃O₄, there was no electric field on the top surface of the Au nanostructure and the electric field was formed only along the side of the Au nanostructure (Fig. S6 in ESI†). This interesting LSPR confined at the Au–Co₃O₄ interface can support the plausible hot electron transfer mechanism, indicating that the generated hot electrons at the Au edge or the Au–Co₃O₄ interface were transferred to the Co₃O₄ over the Schottky barrier and can easily transfer to the Co₃O₄ surface owing to their long electron mean free path affecting the catalytic activity on the cobalt oxide.

Fig. 6 FDTD calculations of the electric field intensity from LSPR on the patterned Co₃O₄–Au nanostructure: (a) top view and (b) cross-section view.

CO molecules are known to be well adsorbed on Co₃O₄, especially on the Co³⁺ site, and the mechanism for CO oxidation over Co₃O₄ is believed to the Mars–van Krevelen type in which CO molecules adsorb on the Co₃O₄ surface and react with lattice oxygen at Co₃O₄ followed by O₂ filling in the oxygen vacancy of the Co₃O₄.^32,42 We can explain that the enhancement of catalytic activity when under light irradiation is attributed to the Co₃O₄ surface becoming negatively charged by hot electrons from LSPR excitation of the triangular Au nanostructure. Previous studies used ambient-pressure X-ray photoelectron spectroscopy (APXPS) and near edge X-ray adsorption fine structure spectroscopy (NEXAFS) to show that the main reaction intermediate for CO oxidation on Cu-based catalysts is adsorbed CO₂^δ−.⁴³ Maximoff et al. shows that partially charged oxygen (O^δ−) can react with CO and produce CO₂^δ− (i.e. the partially negatively charged reaction intermediate), and finally produce CO₂ and generate chemicurrent.⁴⁴ Calaza et al. reported that electrons are shuttled from the ultrathin MgO(001) film grown on Ag(001) to Au islands, thus inducing the formation of a negative CO₂ radical that reacts to form an oxalate.⁴⁵ There are also earlier studies showing that the catalytic activity of CO oxidation on Pt catalysts supported on F-doped TiO₂ was enhanced by O activation because of electron spillover from the highly n-type TiO₂.²² The mechanism for electron spillover is still not clear; it was postulated that electrons in the highly doped TiO₂ can activate lattice oxygen in the TiO₂, which is then followed by CO oxidation with adsorbed CO at the Pt/TiO₂ interface in a Mars–van Krevelen type mechanism.⁴⁶ In our studies, hot electrons at negatively charged Co₃O₄ can transfer to the lattice oxygen at Co₃O₄ and produce CO₂ through the reaction intermediate (CO₂^δ−), O (Co₃O₄) + δe⁻ → O^δ− (Co₃O₄), CO(g) + O^δ− (Co₃O₄) → CO₂^δ−(ad) → CO₂(g); or transfer to the reaction intermediate (CO₂^δ−) and produce gas-phase CO₂, CO(g) + O (Co₃O₄) → CO₂(ad), CO₂(ad) + δe⁻ → CO₂^δ−(ad) → CO₂(g). Thus, hot electrons from LSPR excitation of the triangular Au nanostructure can easily transfer to the Co₃O₄ surface because of their long electron mean free path, and the negatively charged species can transfer to lattice oxygen at Co₃O₄ or adsorbed CO₂ to generate negatively charged oxygen (O^δ−) or adsorbed CO₂ (CO₂^δ−), which play important roles for CO₂ production from CO and oxygen via the Mars–van Krevelen type mechanism.

Another possible mechanism for explaining the enhancement of catalytic activity by LSPR of the triangular Au nanostructure under light irradiation is the partial reduction of Co³⁺ to Co²⁺. Marimuthu et al. reported that selectivity for propylene oxide in propylene epoxidation increases on the Cu₂O shell–metallic Cu core structure when under light irradiation. They found that Cu₂O was reduced to metallic Cu, which is attributed to LSPR of the metallic copper under visible light irradiation using an in situ UV-vis extinction experiment.⁴⁷ Kim et al. demonstrated the reduction of Fe³⁺ ions bound to a cyanide/Ag nanoparticle system induced by plasmonically generated hot electrons from the Ag nanoparticles.⁴⁸ Also, Li et al. reported the selectivity of benzyl alcohol oxidation to benzaldehyde on a gold core@ceria shell catalyst under visible light irradiation that resulted from reducing Ce⁴⁺ to Ce³⁺ by hot electron transfer from the LSPR of the gold core to the CeO₂ shell.³⁰ Shen and co-workers found that Co₃O₄ nanorods with predominantly exposed [110] planes are very catalytically active for CO oxidation at low temperature, giving 100% CO conversion because of the presence of active Co³⁺ species at the surface. This unexpected catalytic reactivity of Co₃O₄ nanorods for CO oxidation is explained in the following mechanism: Co₃O₄ has a spinel structure containing Co³⁺ in an octahedral coordination and Co²⁺ in a tetrahedral coordination. In these surface atomic configurations, there are only Co²⁺ cations on the [001] and [111] planes, which are almost inactive, while the [110] plane is mainly composed of Co³⁺ cations, which are regarded as the active site for CO oxidation. Thus, CO only adsorbs on the surface Co³⁺ site and oxidation of the adsorbed CO then occurs by abstracting the surface oxygen that was coordinated with the Co³⁺ cations. The Co³⁺ cations are partially reduced to Co²⁺ and, finally, the partially reduced cobalt oxide can be re-oxidized by a gas-phase oxygen molecule to become active Co³⁺ cations again.³³ The reaction between the adsorbed CO on Co³⁺ and the lattice oxygen is the overall rate-determining step in CO oxidation.⁴² Broqvist et al. performed a first-principles density functional theory (DFT) study regarding the CO oxidation mechanism at the Co₃O₄(110) surface and found that the reduction of Co³⁺ to Co²⁺ (Co³⁺ + e⁻ → Co²⁺) in Co₃O₄(110) is crucial for the O abstraction step for CO₂ formation.⁴⁹ Here, in our patterned Co₃O₄–Au inverse catalysts, if a negatively charged hot electron from LSPR excitation on Au is transferred to the cobalt oxide, reduction of the cobalt oxide can occur, which is a decisive step for CO oxidation.

To find the exact mechanism for the enhancement of catalytic activity at the cobalt oxide by hot electron transfer from the Au, theoretical DFT studies or in situ spectroscopic and microscopic experiments (e.g. ambient-pressure techniques) are needed for these catalysts under light irradiation. But our studies here on these reactive oxide/plasmonic noble metal inverse catalysts can connect the enhancement of catalytic activity to LSPR excitation from the metal, having possible applications for surface plasmon-based oxide/metal hybrid nanocatalysts or catalytic nanodiodes because of the long electron mean free path of the oxide materials.

4 Conclusions

In summary, Co₃O₄ is known as a reactive oxide material for CO oxidation. Triangular noble metal nanostructures fabricated using the NSL technique exhibit higher SERS activities than flat noble metals. We used cobalt oxide as the catalytic portion and a triangular Au nanostructure as the SPR source for the patterned Co₃O₄/Au nanostructure. The patterned Co₃O₄/Au inverse catalyst nanostructures were prepared using the NSL technique with SiO₂ nanospheres as the mask for metal deposition by electron beam evaporation. FDTD simulations showed that a particularly strong electric field was formed at the Au–Co₃O₄ interface because of the higher refractive index of the Co₃O₄. CO oxidation on the patterned Co₃O₄/Au nanostructure was carried out with and without light to find the relationship between hot electron transfer to the reactive oxide from LSPR excitation on the triangular Au nanostructure and the change in catalytic activity for CO oxidation. The patterned Co₃O₄–Au inverse catalysts showed 25–50% higher catalytic activity for CO oxidation when under light irradiation than in the dark. The reversibility of the plasmonic hot carrier effect on catalytic activity was confirmed by observation of two catalytic activities with the light turned on and off, respectively. The change in catalytic activity implies that hot carriers generated from LSPR excitation of the Au nanostructure by absorption of photons on the Au metal can transfer to the Co₃O₄, and thus reversibly enhance the catalytic activity for CO oxidation.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the Institute for Basic Science (IBS) [IBS-R004-A2-2017-a00].

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Footnote

† Electronic supplementary information (ESI) available: XRD patterns of the Au nanostructure (Fig. S1). SEM image of triangular Au nanostructure after sonication (Fig. S2). Sample temperature measured by temperature controller with and without light at 523 K (Fig. S3). SEM images (Fig. S4) and XPS spectra (Fig. S5) of the patterned Co₃O₄/Au inverse catalyst before and after CO oxidation. FDTD simulations on the patterned Au nanostructure (Fig. S6). See DOI: 10.1039/c7ra10450b

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