For energy harvesting with plasmonic photocatalysis, it is important to optimize geometrical arrangements of plasmonic nanomaterials, electron (or hole) acceptors, and co-catalysts so as to improve the charge separation efficiency and suppress charge recombination. Here, we employ a photocatalytic system with Au nanocubes on TiO2 and introduce MnO2 as an oxidation co-catalyst onto the nanocubes via site-selective oxidation based on plasmon-induced charge separation (PICS). However, it has been known that PbO2 is the only material that can be deposited onto Au nanomaterials through PICS with sufficient site-selectivity. Here we addressed this issue by introducing an indirect approach for MnO2 deposition via site-selective PbO2 deposition and subsequent galvanic replacement of PbO2 with MnO2. The indirect approach gave nanostructures with MnO2 introduced at around the top part, bottom part, or entire surface of the Au nanocubes on a TiO2 electrode. The activity of those plasmonic photocatalysts was strongly dependent on the location of MnO2. The key to improving the activity is to separate MnO2 from TiO2 to prevent recombination of the positive charges in MnO2 with the negative ones in TiO2.
When a nanoparticle exhibiting localized surface plasmon resonance (LSPR) is in contact with a semiconductor having an appropriate band structure, plasmon-induced charge separation (PICS) takes place at the interface.1,2 PICS has been widely studied since it has a wide variety of applications, including photovoltaics and photocatalytic energy harvesting.2 The charge separation mechanism is often explained in terms of a photoelectric effect, and it has been suggested that energetic electron–hole pairs generated through a plasmon decay process3 play an essential role in PICS. Electrons from the pairs are injected into the semiconductor conduction band and then transported to the semiconductor surface or a counter electrode to drive photocatalytic reduction reactions while the remaining holes are relaxed and the potential corresponding to the particle Fermi level shifts positively, resulting in photocatalytic oxidation reactions at the entire nanoparticle surface based on the charge accumulation mechanism.4,5 Some oxidation reactions are driven by holes or trapped holes before relaxation, preferentially around the resonance sites of the metal nanoparticle. In the case of the latter, namely the hole ejection mechanism, the oxidation reaction site can thus be controlled by selecting the LSPR mode to be excited.4
Because LSPR confines light to a nanoscale area, which is smaller than the wavelength of light, PICS enables photo-induced nanofabrication beyond the diffraction limit.6–15 We have reported a variety of plasmonic nanostructures fabricated via site-selective oxidation reactions, including Ag dissolution and PbO2 deposition.16–19 In the case of a plasmonic nanocube (NC) on a highly refractive substrate such as TiO2, LSPR splits into two modes at different wavelengths, namely distal and proximal modes,20 in which electrons oscillate at the top and bottom face of the NC, respectively. Selective excitation of one of those modes of an AgNC on TiO2 results in the partial oxidation and dissolution of the AgNC preferentially at the corresponding resonance site through PICS.18 Such site-selectivity was also observed in the case of photo-oxidation of Pb2+ to PbO2 on AuNCs.19 This leads to the fabrication of chiral plasmonic nanostructures by circularly polarized light in an enantioselective manner.21,22 Thus, by taking advantage of PICS, the morphology of nanoparticles can be manipulated at the nanoscale, and the nanofabrication allows optical, electronic, and chemical properties of the particles to be modified. If a catalytic material is introduced to a plasmonic photocatalyst as a co-catalyst, its ability for plasmonic energy harvesting would be enhanced. Photocatalytic activity and specificity could be improved if the location of the co-catalyst is optimized in terms of mass transfer, efficient hole ejection and electron injection, and suppressed charge recombination. However, whether the hole ejection mechanism or the charge accumulation mechanism is chosen depends on the reaction involved in PICS.4,5 Oxidative polymerization of pyrrole and coordinative dissolution of Au nanoparticles do not necessarily proceed site-selectively.5 Thus far, oxidative PbO2 deposition is known as the only deposition reaction in which sufficient site-selectivity is achieved.5,19,21,22
We, therefore, developed an indirect approach for the site-selective introduction of some other metal oxides that are difficult to introduce directly by the hole ejection mechanism. Because the redox potential of the Pb2+/PbO2 couple is relatively positive (E◦Pb2+/PbO2 = +1.46 V vs NHE), PbO2 can be substituted with a metal oxide (MOx) spontaneously via galvanic replacement by exposure to Mn+ (2x > n), whose redox potential is more negative than E◦Pb2+/PbO2. Actually, bulky PbO2 film (12 µm thick) was replaced with MnO2, Co3O4, and SnO2 by galvanic replacement, namely simultaneous reductive PbO2 dissolution and oxidative MOx deposition.23–25 However, for PbO2 in a nanometer scale, it has not yet been reported whether the deposition site is retained or not during the replacement process. We, therefore, examine galvanic replacement of site-selectively deposited PbO2 with MnO2. MnO2 is known as one of the co-catalysts for water oxidation,26–29 and has been used as that for semiconductor photocatalysts30–32 and plasmonic photocatalysts based on PICS.33,34 Manganese oxide is also applicable to multi-electron oxidation reactions because of its positive charge accumulation ability.35 For plasmonic photocatalysts, the optimal position of co-catalysts should be dependent on the locations of the semiconductor and the resonance sites. Therefore, the selective introduction of MnO2 to one of the different resonance sites of plasmonic nanoparticles would lead to PICS systems with better efficiency and specificity as well as wider applications.
In the present study, we propose the indirect approach based on the site-selective deposition of PbO2 onto Au nanocubes via PICS and its galvanic replacement with an MnO2 co-catalyst and fabricate efficient plasmonic photocatalysts (Fig. 1). The relationship between the MnO2 deposition site and photocatalytic activity is investigated for the novel structural design of PICS photocatalysts. This approach would be applied to the fabrication of plasmonic nanostructures with many different materials and morphologies.
AuNCs with the size of 80 nm [Fig. S1(a)] were deposited onto a TiO2 thin layer (thickness ∼40 nm) formed on an indium tin oxide (ITO) electrode.19 The distal and proximal modes in which electrons oscillate around the top and bottom face of the NC were observed at ∼550 and ∼660 nm, respectively, in the extinction (= absorption + scattering) spectrum [Fig. S1(b)]. The extinction of ITO was subtracted from the spectrum. The AuNC-modified TiO2 electrode as the working electrode (electrode area ∼2.2 cm2) was immersed in 0.05 M Pb(NO3)2 and a bare ITO electrode as the counter electrode was dipped in 0.05 M Ag(NO3)2. The electrolytes were connected through a salt bridge. The two electrodes were short-circuited and the working electrode was irradiated with 560 nm light (FWHM = 20 nm, ∼10.2 mW cm−2, front incidence) or 625–800 nm light (∼136 mW cm−2, back incidence) for 48 h to introduce PbO2 onto the distal or proximal site, respectively [Figs. 2(a) and 2(b)].19 We also prepared a sample with AuNCs entirely coated with PbO2 by exciting both the distal and proximal modes with 480–800 nm light (∼30.1 mW cm−2, back incidence) [Fig. 2(c)].
The PbO2-deposited electrode was immersed in an aqueous solution containing 0.02 M Mn(CH3COO)2 and 0.05 M CH3COOH (pH 5.3) for 24 h at room temperature. Although the morphology of the deposits was changed, the deposition site was almost maintained [Figs. 2(d)–2(f)]. We analyzed chemical composition changes after the galvanic replacement treatment of PbO2 at the distal sites by X-ray photoelectron spectroscopy (XPS). The spectrum obtained before the replacement showed Pb 4f7/2 and Pb 4f5/2 peaks [Fig. S2 and Fig. 3(a)]. The former can be deconvoluted into two peaks at 137.4 and 138.5 eV and the latter into those at 142.3 and 143.4 eV. Regarding the Pb 4f7/2 peaks, it has been reported that PbO2 and PbO show peaks at 137.2–137.6 eV37–39 and 137.7–137.9 eV,36,37,40 respectively, while that for metallic Pb is observed at lower binding energies.36 It is also known that electroconducting PbO2 shows a plasmon loss satellite at ∼1 eV higher energy than the pristine peak.41,42 We, therefore, assigned the deconvoluted Pb 4f7/2 peaks at 137.4 and 138.5 eV to the pristine and satellite peaks, respectively, of PbO2. After galvanic replacement, we observed a 10-fold lower Pb 4f7/2 signal at 137.9 eV [Fig. 3(a)], which is assigned to PbO. Also, Mn2p signals appeared at 642.0 and 653.5 eV [Fig. 3(b)]. These peaks can be assigned to MnO2.43 We, therefore, conclude that most of the introduced PbO2 was replaced with MnO2, and the residual PbO2 was reduced by Mn2+ to PbO.
The samples were also subjected to elemental analysis by energy dispersive X-ray spectroscopy (EDS). The atomic ratio of Pb to Au at the electrode surface before the replacement was determined to be 0.35, and that of Mn after the replacement was 0.15. The ratios of Mn before replacement and Pb after replacement were lower than the detection limit (∼0.04). About half of the deposited oxides were lost during the galvanic replacement process, likely due to detachment induced by structural changes. Taking the detachment into account, the initial amount of PbO2 should be about twice as much as the targeted amount of MnO2.
The TiO2 electrode with AuNCs was subjected to a photocurrent measurement, followed by PbO2 deposition and galvanic replacement with MnO2, with a photocurrent measurement after each step. Photocurrents based on the oxidation of ethanol (0.5 M) were measured in an aqueous 0.1 M KNO3 electrolyte using the working electrode irradiated with monochromatic light (FWHM = 20 nm, ∼3.3 × 1015 photons cm−2 s−1) for 10 s at each wavelength from 790 to 430 nm with a 30 s interval. A Pt counter electrode was short-circuited with the working electrode. Action spectra for the external quantum efficiency (EQE) of the measured photocurrents are shown in Fig. S3. All the electrodes exhibited anodic photocurrent responses in the wavelength range examined. Those in the >500 and <500 nm ranges are derived from plasmonic absorption and interband transition, respectively. The former photocurrents are explained in terms of PICS: Charge separation occurs by electron injection from AuNCs into the TiO2 conduction band, resulting in oxidation of ethanol at the NCs and reduction of dissolved oxygen at the counter electrode, whose potential is negatively shifted due to the injected electrons. Photocurrents at around 800 nm or longer were relatively small even though AuNCs absorb light. This phenomenon, which is often observed for PICS, is attributed to a decreased population of sufficiently energetic electrons to overcome the Au–TiO2 Schottky barrier (energy height ∼0.9 eV44) in the long wavelength range.
Here we define the EQE enhancement factor (EFEQE) as the ratio of EQE after the MnO2 introduction to that before PbO2 deposition (AuNC only). The EFEQE values were plotted as a function of the excitation wavelength in Fig. 4(a). After PbO2 deposition, EQE decreased in almost all of the wavelength ranges, regardless of the deposition site (Fig. S3). Although PbO2 is electroconducting, the conductivity is lower than Au, and it has not been known as a good co-catalyst. After replacement of PbO2 with MnO2, however, EQE in the 500–700 nm range increased up to two times when MnO2 was introduced, chiefly to the top part of the AuNC, reflecting that the ethanol oxidation via PICS was enhanced by the co-catalytic effect of MnO2. In contrast, when MnO2 was introduced to the bottom part of the AuNC or the entire AuNC surface, the EQE values were lowered, indicating that the co-catalytic effect in PICS is susceptible to the location of co-catalysts.
Here we explain the mechanisms for the susceptibility using the electronic structure and schematic illustrations in Figs. 4(b)–4(e). In order to determine the potential of MnO2 for hole acceptance, MnO2 was deposited on an ITO electrode by the electrodeposition of PbO2 at +1.50 V vs NHE and the galvanic replacement under the same conditions. Its potential for the Mn(IV/V) redox reaction45 was determined by cyclic voltammetry to be ∼+1.1 V vs NHE (Fig. S4), and the potential is shown in Fig. 4(b). In the case where MnO2 is introduced to the top part, electrons excited in the AuNC are injected into the TiO2 conduction band, while holes or positive charges accumulated via hole relaxation are used for oxidation of Mn(IV) in MnO2 to Mn(V) [Fig. 4(b)]. Then, Mn(V) oxidizes an electron donor in the solution, such as ethanol or water, and Mn(V) is reduced back to Mn(IV). Namely, MnO2 serves as a co-catalyst through the accumulation and transfer of positive charges to the donor. Incidentally, the distance between the top part of the nanocube and TiO2 is comparable to the mean free path length of ballistic electrons in Au that can cross the Schottky barrier at the Au–TiO2 interface (∼42 nm).46
Since the MnO2 moiety is spatially separated from TiO2, recombination of the positive charges in MnO2 with the negative ones in TiO2 is suppressed and the photocatalytic oxidation at MnO2 is enhanced, resulting in improved PICS efficiency [Fig. 4(d)]. Although Fig. 4(b) illustrates the separation of one electron–hole pair, it is also possible that two electron–hole pairs lead to a single charge separation process (Fig. S5).47 Even when MnO2 is introduced to the bottom part of each AuNC or the entire NC surface, electrons and holes should be injected into TiO2 and MnO2, respectively. However, recombination of the separated charges is likely to take place, since MnO2 is in direct contact with TiO2 [Fig. 4(e)]. Since the TiO2 layer thickness (∼40 nm) is much smaller than the space charge layer thickness for bulk TiO2 (typically ∼200 nm) and electrons accumulate in TiO2, the energy barrier at the TiO2–MnO2 interface, if any, should be low. The recombination could be the reason why the PICS efficiency decreased at >500 nm for the case where MnO2 is introduced to the bottom part or the entire surface of AuNCs.
Incidentally, EQE was enhanced in the <500 nm range, in particular, in the case where MnO2 was in contact with TiO2. This is probably because a partially reduced state of MnO2, namely Mn2O3,48 on the AuNC works as a photosensitizer of TiO2 [Fig. 4(c)]. MnO2 itself does not play a role as a sensitizer because the conduction band edge potential of MnO2 is more positive than that of TiO2,50–51 whereas that of Mn2O3 (−0.35 to 1.1 V vs NHE52,53) is more negative, so that photoexcited electrons in the Mn2O3 conduction band can be injected into the TiO2 conduction band. Although Mn(III) constituting Mn2O3 was not detected by XPS after the galvanic replacement, trace amounts of Mn2O3 less than the detection limit or Mn2O3 reduced from MnO2 by photoexcited electrons in the AuNC may contribute to the sensitization. Thus, it was indicated that controlling the co-catalyst site is of critical importance for the efficiency improvement of PICS-based photocatalysis.
In conclusion, MnO2 co-catalyst was introduced site-selectively to AuNCs on TiO2 via site-selective PbO2 deposition based on PICS and subsequent galvanic replacement. Photoelectrochemical oxidation of ethanol through PICS was performed using the electrode with AuNC–MnO2 nanostructures thus obtained. The PICS efficiency was improved only when MnO2 was deposited on the top part of AuNCs. In contrast, the efficiency decreased in the case of MnO2 in contact with TiO2, for example, MnO2 introduced to the bottom part of NCs. Thus, we demonstrated that the PICS system is susceptible to the location of co-catalysts and that it is important to control the location. The present approach for controlling the deposition sites is expected to be applicable to plasmonic nanoparticles with different materials and morphologies, leading to the development of various plasmonic nanomaterials and nanodevices.
See the supplementary material for an SEM image, an extinction spectrum, XPS spectra, photocurrent EQE spectra, a cyclic voltammogram, and a schematic band diagram of the samples.
This work was supported, in part, by a Grant-in-Aid for Scientific Research (A) (Grant No. JP20H00325) and a Grant-in-Aid for Scientific Research (B) (Grant No. JP20H02706) from the Japan Society for the Promotion of Science (JSPS).
Conflict of Interest
The authors have no conflicts to disclose.
Kangseok Kim: Data curation (equal); Visualization (equal); Writing – original draft (equal). Hiroyasu Nishi: Conceptualization (equal); Data curation (equal); Funding acquisition (supporting); Visualization (equal); Writing – original draft (equal). Tetsu Tatsuma: Conceptualization (equal); Data curation (equal); Funding acquisition (lead); Supervision (lead); Visualization (equal); Writing – review & editing (lead).
The data that support the findings of this study are available from the corresponding author upon reasonable request.