Plasmon and Upconversion Mediated Broadband Spectral Response

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Plasmon and Upconversion Mediated Broadband Spectral Response in TiO2 Inverse Opal Photocatalysts for Enhanced Photoelectrochemical Water Splitting Ramireddy Boppella, Filipe Marques Mota, Ju Won Lim, Saji Thomas Kochuveedu, Sunghyun Ahn, Jiseok Lee, Daisuke Kawaguchi, Keiji Tanaka, and Dong Ha Kim ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.9b00469 • Publication Date (Web): 09 Apr 2019 Downloaded from http://pubs.acs.org on April 10, 2019

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ACS Applied Energy Materials

Plasmon and Upconversion Mediated Broadband Spectral Response in TiO2 Inverse Opal Photocatalysts for Enhanced Photoelectrochemical Water Splitting Ramireddy Boppella,a,† Filipe Marques Mota,a,† Ju Won Lim,a Saji Thomas Kochuveedu,a Sunghyun Ahn,b Jiseok Lee,b Daisuke Kawaguchi,c Keiji Tanakac,* and Dong Ha Kima,* a

Department of Chemistry and Nano Science, Ewha Womans University, 52, Ewhayeodae-gil, Seodaemun-gu, Seoul 03760, Republic of Korea b

School of Energy and Chemical Engineering, Ulsan National Institute of Science and Technology (UNIST), 50 UNIST-gil, Ulsan 44919, Republic of Korea

c

Department of Applied Chemistry, Education Center for Global Leaders in Molecular Systems for Devices, and International Institute for Carbon-Neutral Energy Research (WPI-I2CNER), Kyushu University, Fukuoka 819-0395, Japan.

†

These authors contributed equally to this work.

KEYWORDS: Surface plasmon; Upconversion; Broadband absorption; TiO2 inverse opal; Water splitting

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ABSTRACT Harvesting low energy photons by strategically exploiting the photocatalytic properties of plasmonic and upconversion nanocomponents is a promising route to improve solar energy utilization. Herein, a rationally designed 3D composite photoanode integrating NIR-responsive upconversion nanocrystals (UCN) and visible-responsive plasmonic Au nanoparticles (NPs) into 3D TiO2 inverse-opal nanostructures (Au/UCN/TiO2) has been shown to extend the solar energy utilization in the UV-vis-NIR range. The NIR-responsive properties of NaYF4:Yb3+-based UCN nanocrystals doped with Er3+ or Tm3+ ions, and the effect of an alternating sequential introduction of UCN and Au have been assessed. With an extended overlap between the emission of Er-UCN and the characteristic SPR band of Au, our ternary Au/Er-UCN/TiO2 hybrid nanostructure unveiled a notable 10-fold improvement in photocurrent density under UV-visNIR illumination compared with a pristine TiO2 reference. The Au incorporation was confirmed to play a key role in enhancing the efficiency of light harvesting and to synergistically facilitate the energy transfer from UCN to TiO2. This work further dissected plausible mechanistic pathways combining collected photoelectrocatalytic results, with electrochemical impedance measurements and transient absorption spectroscopic measurements. The synthesis and catalytic performance of our Au/UCN/TiO2 and the underlying mechanism here proposed are expected to reflect extended applicability in analogous applications for efficient solar-to-energy sustainable platforms.

1. INTRODUCTION The emergence of solar radiation as a clean and sustainable energy source has unveiled promising results in a number of chemical processes including water splitting,1-3 CO2


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reutilization,4 production of fuels,5 and environmental remediation.6-8 The splitting of water molecules toward a sustainable photo-driven generation of hydrogen is of great potential to fulfill future energy demands.9-10 Photoelectrocatalytic (PEC) systems have emerged as alternative sidesteps to parallel electrocatalytic platforms requiring large amounts of electricity to overcome high-energy barriers in the applications listed above.11 In detail, the utilization of PEC materials, integrating light-responsive materials ensuring a dramatic reduction of electricity consumption and inducing higher overall efficiencies, has gained traction as an attractive alternative in water splitting technologies. Semiconductor

TiO2-based

materials

have

been

extensively

investigated

as

photoelectrodes/photocatalysts due to their high-efficiency, cost effectiveness, non-toxicity, and photostability.7,

12

The wide band gap of TiO2 can, however, only absorb UV-light photons,

which account for about 5% of the total solar spectrum and therefore exclude the non-absorbed 45% and 50% ranges for visible and near-infrared (NIR) light photons, respectively. With limiting efficiency for a large-scale application, significant efforts have been attempted to extend the optical absorption edge of TiO2-based photocatalysts within the vis-NIR region.13 Works of interest have included metal/non-metal doping,14-16 self-doping by Ti3+ and oxygen vacancies,1718

coupling with other semiconductors,19 and incorporation of carbon-based materials.20

However, alongside a limited enhancement of visible light absorption, poor stability and increased carrier-recombination centers remain commonly pointed shortcomings in relevant reports 8. In recent years, the use of plasmonic Au nanoparticles (NPs) exhibiting surface plasmon resonance (SPR) properties in the visible region has become a widely-applied approach for tackling light absorption limitations in photocatalysis.11, 21-25 In detail, plasmon-induced resonant


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energy transfer (PIRET) and/or direct electron transfer (DET) mechanisms have been proposed in Au-incorporated wide band gap semiconductor-based systems.23,

26-29

The resulting energy

transfer has been claimed to improve light absorption of TiO2, to induce charge separation in the Au/TiO2 hybrid photoanode, to extend the lifetime of electron-hole pairs, and is enhanced through a direct contact of both components free of organic ligands.30-32 Alternatively, a non-linear optical process known as upconversion (UC) has emerged as a valuable pathway in photocatalysis and solar cells.33-39 Lanthanum-doped upconversion nanocrystals (UCN) have unique ability to convert low-energy NIR radiation to high energy visible or UV light, via an anti-Stokes emission process. 21-25 Integration with semiconductors is thus a feasible solution to utilize the NIR light in photocatalysis.40-42 Among the UCN, Tm3+ or Er3+-doped NaYF4:Yb3+ have shown unusual optical properties, e.g. tunable emission color and high photochemical stability, and light emission within the UV-vis region.40,

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With both

Lanthanum-doped UCN however, the enhancement in photocatalytic performances remains severely limited due to the large mismatch between the emission energy of latter and the bandgap energy of TiO2. For the rational design of UCN-incorporated photocatalytic systems, spectral overlap between the emitted upconversion luminescence and absorption wavelength of the selected adjacent nanocomponents is required for an effective utilization light in the extended UV-vis-NIR range.30, 44 Although the effect of upconversion nanocrystals and the incorporation of plasmonic Au NPs have separately disclosed remarkable advances to eclipse light absorption limitations in photocatalysis, attempts to disclose eventual synergetic effects arising from both strategies remain relatively unexplored.30 Herein, we demonstrate the rational design of a 3D composite photoanode integrating both UCN (NaYF4:Yb3+, either doped with Er3+ or Tm3+) and plasmonic


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Au NPs into TiO2 to improve solar energy utilization in the UV-vis-NIR range. Periodically ordered macroporous inverse opal structures with interconnected pores and extended surface area were designed to enhance electron transport throughout the electrode, and work as light trapping architectures readily interacting with light in the UV-vis-NIR region.11 The incorporation of visible-responsive Au NPs and NIR-responsive UCN into a UV-responsive TiO2 nanostructure was confirmed to provide a notable improvement in harvesting low energy photons while achieving higher solar energy conversion efficiency in the photoelectrochemical water splitting reaction

2. EXPERIMENTAL 2.1. Fabrication of TiO2 IO structures The TiO2 IO were prepared following our previously reported methodology.11 Polystyrene (PS) spheres with a 600 nm-size were obtained by emulsion polymerization method (synthesis details provided as ESI). PS spheres (0.1 mL) were drop-casted on the FTO substrate and carefully evaporated overnight under room temperature. The resulting self-assembled opal film was heated at 70 °C for 1 h, to tighten the distance between neighboring PS spheres and therefore enhance the structural stability of the film by. The TiO2 precursor solution was prepared by mixing TTIP (2.6 mL), HCl (0.1 mL) and ethanol (20 mL). The PS opal was vertically dipped into the precursor solution for 1 min and dried at room temperature. The TiO2 inverse opal (IO) nanostructure was obtained at 550 °C for 3 h at a ramping rate of 2 °C min−1. 2.2. Synthesis of NaYF4:Yb3+,Er3+/Tm3+ UCN Hexagonal-phased NaYF4:Yb3+,Er3+/Tm3+ (80:18:2) UCN were synthesized using a modified method from a previous report.45 Briefly, YCl3 (240 mg, 0.8 mmol), YbCl3 (70 mg, 0.18 mmol),


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and ErCl3 (7.6 mg, 0.02 mmol) or TmCl3 (7.6 mg, 0.02 mmol) were mixed with oleic acid (6 mL) and octadecene (12 mL) in a 100 mL three-neck flask. The mixture was heated to 160 °C for 30 min to form a homogeneous clear solution, and then cooled down to room temperature. A methanolic solution (10 mL) containing NaOH (40 mg, 2.5 mmol) and NH4F (148.5 mg, 4 mmol) was slowly added into the flask under stirring for 30 min. The solution mixture was then slowly heated to remove methanol at 100 °C for 30 min. The solution was subsequently heated up to 300 °C for 1.5 h under nitrogen flow. UCN were precipitated with the addition of ethanol (20 mL), and collected after extensive washing (three times) with ethanol/hexane (1:1 v/v). 2.3. Fabrication of Au/TiO2 and UCN/TiO2 structures For the fabrication of UCN/TiO2, TiO2 IO substrate was vertically dipped in a solution of UCN (NaYF4:Yb3+,Er3+/Tm3+) dispersed in cyclohexane (0.3 mg/ml) and placed in an oven at 40 °C overnight. With the slow evaporation of the solvent the UCN were self-organized into the void space of TiO2 IO, driven by the capillary force of the liquid in the evaporation process. For the deposition of Au NPs through a facile ionic layer adsorption and chemical reduction approach, the TiO2 IO substrate was immersed in a HAuCl4.H2O solution (5 mM) for 12 h at room temperature. Then NaBH4 (1 mL, 0.01 M) was added to reduce the trivalent Au ions to metallic Au NPs, with a color change from white to light wine-red of the TiO2 IO substrate being observed. The Au/TiO2 substrate was rinsed with water and dried at 80 °C overnight. The size and density of the Au NPs was adjusted by changing the HAuCl4.H2O concentration. The ternary hybrid UCN/Au/TiO2 and Au/UCN/TiO2 nanostructures were obtained by subsequently incorporating both UCN and Au NPs, following the methodologies above outlined. 2.4. Photoelectrochemical Measurements (PEC)


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All PEC measurements were assessed in Na2SO4 aqueous solution (0.5 M, pH ≈ 6.5) anteriorly bubbled with N2 gas for 30 min. Measurements were conducted in a three-electrode system using a potentiostat (CH Instrument, CHI 660) with the photoanode as the working electrode, Ag/AgCl (3 M KCl) as the reference, and Pt foil as the counter electrode. Measured potentials were converted to the reversible hydrogen electrode (RHE) scale using the Nernst equation. Conductive silver paste was painted on the top of the electrode. The working electrodes were illuminated from the front side with simulated sunlight with a 300 W Xe lamp equipped with an AM 1.5 G filter. Electrochemical impedance spectroscopy (EIS) measurements (SI 1287, Solartron, Leicester, UK) were carried out in the frequency range of 10000 kHz–0.1 Hz with an AC amplitude of 10 mV. The evolved H2 and O2 gases were systematically monitored by gas chromatography (Shimadzu GC-2014, molecular sieve 5A column) equipped with a thermal conductivity detector (TCD). 2.5. Femto-second transient absorption spectroscopy (TAS) The photo-carrier formation process in the samples was examined by TAS measurements, for which a transient absorption spectrometer (Ultrafast Systems, Helios) and a regenerative amplified Ti:sapphire laser (Spectra-Physics, Solstice) were used. The amplified Ti:sapphire laser provided fundamental pulses with   800 nm and width of 100 fs (FWHM) at a repetition rate of 1 kHz, which were split into two beams with a beam splitter to generate pump and probe pulses. One of them was converted into pump pulses, which mechanically modulated with a repetition rate of 500 Hz, with   400 nm with a second harmonic generator. Another was converted into probe pulses with the  region from 400 to 1600 nm. The samples were set in a cryostat (Oxford Instruments, Optistat CF-V) and excited by pump pulses with a laser power of


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200 μW. The transient absorption spectra, and thereby decays, were followed over the time range from -5 ps to 1 s at 298 K under vacuum.

3. RESULTS AND DISCUSSION In the following subsections the preparation, characterization, and PEC evaluation of the Au/TiO2, UCN/TiO2, Au/UCN/TiO2 and UCN/Au/TiO2 inverse opal electrodes is presented. The fabrication procedure illustrated in Figure 1 further reflects our focus in assessing the effect of an alternating sequential introduction of Au and UCN (either doped with Er3+ or Tm3+ ions). Further corroborated by electrochemical impedance spectroscopy and transient absorption spectroscopic measurements, we finally investigated possible mechanistic pathways reflecting the results herein introduced. 3.1. Characterization of the Synthesized Materials Prior to the fabrication of the binary and ternary systems, NaYF4:Yb3+,Tm3+ (Tm-UCN) and NaYF4:Yb3+,Er3+ (Er-UCN) nanocrystals with an average 40 nm-size were synthesized following the methodology outlined in the experimental section. To confirm the suitability of the approach, Au NPs were prepared following facile chemical reduction using NaBH4 (0.01 M) (Figure S1). The X-ray diffraction (XRD) patterns of all materials were found to be in agreement with those reported in the literature (Figure S2). With similar morphology features suggesting a minor effect of the selected Tm3+ and Er3+ dopant ions (Figure S3a-d), the discussion below was selectively focused on representative top-view SEM images of Tm-UCN incorporated hybrids (Figure 2). Large-scale self-assembly of the PS opal template with a face-centered cubic arrangement is depicted in Figure 2a. The successful infiltration of the TiO2 precursor was reflected in the long-range ordered hexagonal arrangement with a center-to-center distance of


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~500 nm of the resulting inverse opal (IO) structure (Figure 2b). The void size of the IO decreased by nearly 20% due to shrinkage during the removal of the PS template. The resulting seemingly well-interconnected empty voids enabled subsequent introduction of both Au NPs and UCN. Incorporation of uniformly distributed Au NPs anchored onto the walls of TiO2 was achieved through the adsorption and subsequent chemical reduction of introduced Au ions (Figure 2c), whereas UCN were randomly dispersed inside the TiO2 IO structures following solvent evaporation (Figure 2d). Ternary systems were fabricated by sequential deposition of Au and UCN on the UCN/TiO2 and Au/TiO2 hybrids, respectively (Figure 2e,f). In line with the observations in Figure 2c and d, the simultaneous incorporation of both ~40 nm-size UCN and ~10 nm-size Au NPs can be easily distinguished in the fabrication of these ternary systems as depicted in representative SEM images (Figure S4). It is presumed that when ensuing the insertion of UCN, direct Au deposition may additionally occur over the surface of the formerly incorporated UC nanocrystals, in line with the established electrostatic interaction between both components prior to chemical reduction (Figure S4). In the evaluation of the fabricated binary and ternary electrodes, XRD patterns of all samples agreed with those reported for the TiO2 anatase-crystalline phase (JCPDS 21-1272) and the hexagonal NaYF4 crystal phase of the incorporated UCN (JCPDS 16-0334) (Figure 3a).13, 46 Indiscernible characteristic Au diffraction peaks were ascribed to low Au content and a presumed overlap with other components in the structures.30 High-resolution X-ray photoelectron spectroscopy (XPS) measurements were carried out to assess the chemical interaction between the introduced nanocomponents. In each case, the C 1s line position was used as a reference in the calibration. Comparison of the prepared TiO2, UCN/TiO2, Au/TiO2,


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and Au/UCN/TiO2 revealed similar Ti 2p signals at 459.1 and 464.8 eV corresponding to Ti 2p3/2 and Ti 2p1/2, and attributed to the Ti4+ state of anatase TiO2 (Figure S5).17, 47 After incorporation of Au and UCN, no reduced Ti3+ could be detected. Conversely, the Au 4f core-level XPS spectra showed two sharp peaks centered at 84.0 and 87.7 eV ascribed to Au 4f7/2 and Au 4f5/2, respectively, and confirming the metallic state of the incorporated Au.48-49 In detail, the presence of oxidized Au species (Au3+ and Auδ+) in both binary and ternary hybrid systems could be herein excluded upon deconvolution, in good agreement with the use of NaBH4 as a strong reducing agent. 3.2. Optical properties Optical properties of these hybrid materials were then investigated to assess the broadband photoresponse in the UV-vis-NIR range of these hybrid nanostructures in a single rationally designed architecture. Diffuse reflectance UV-vis spectra (Figure 3b) of all samples disclosed a sharp decrease at a wavelength range shorter than 360 nm due to the strong intrinsic absorption of TiO2 (≈380 nm, 3.2 eV).46 The reflectance intensity of the UCN/TiO2 is comparatively reduced to that of TiO2, indicating the extension of the light responsive range.13 The SPR properties of the incorporated Au NPs are reflected across the entire spectral range, but particularly at a characteristic 520 nm wavelength in the prepared Au/TiO2.50-51 Luminescence spectra of the as-synthesized hybrid nanostructures were examined under an excitation wavelength of 980 nm (Figure 4a). Upon irradiation Tm-UCN yielded two peaks in the blue region at 451 nm and 476 nm, and three peaks within the red to NIR region at 646 nm, 697 nm and 743 nm (Figure 4a). The listed emissions were ascribed to the electronic transitions from 1D2 to 3F4 and 1G4 to 3H6, and 1G4 to 3F4, 3F2 to 3H6 and 3F3 to 3H6, respectively (Figure 4b). Reported peaks yielded in the UV region at 345 nm and 361 nm could not be observed as a result


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of our equipment limitation.30 Er-UCN exhibited three prominent peaks at 525 nm, 541 nm, and 655 nm in accordance with transitions from 4S3/2, 2H11/2 and 4F9/2 to the 4I15/2 level, respectively (Figure 4a,b). Being consistent with prior reports, the emerging emissions serve as critical fingerprints of the successfully doped Tm3+ or Er3+ species into these UCN nanostructures Upon Tm-UCN incorporation into the TiO2 IO nanostructure, an intensity enhancement ascribed to an increase of light scattering was witnessed (Figure 4c).52 Upon Au introduction, prior reports have substantiated the generation of an electric field by plasmonic resonance and a subsequent enhancement of the upconversion emission intensity.53 In this work however, the consecutive incorporation of UCN and Au led to a dramatic decrease of the PL intensity, particularly over high-energy ranges. The observation is attributed to the overlap of UCN emissions with the Au SPR absorption peak at 520 nm, and a resulting energy migration from the UCN to Au by radiation–reabsorption processes of upconverted emissions in the same region.44 When the incorporation of Au ensued that of UCN (Au/Tm-UCN/TiO2), a greater PL intensity decrease compared to that of Tm-UCN/Au/TiO2 was observed. Assuming similar dispersion of both nanocomponents, a varying extent of related energy and charge transfer in the interfaces of the ternary systems is here suggested. In detail, a greater interaction between Au and UCN is presumed to occur with the Au/Tm-UCN/TiO2, leading to an increase in the absorption of the upconverted radiation. Similar conclusions could be drawn in corresponding Er-UCN-based ternary counterparts, with the resulting striking decrease in the PL intensity being here clearly assigned to the extended overlap between the UCN emissions at 525 and 541 nm, and the Au SPR peak at 520 nm (Figure 4d).44 Whereas the present results shed clear light in the interaction between UCN and Au, an analogous visualization of this effect in the UV region reflecting the


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direct interaction of UCN (in particular, Tm-UCN) and TiO2 has been limited by the equipment utilized in this work. With a focus on the electron transport dynamics and interfacial electron transfer between the plasmonic Au metal and neighboring TiO2 centers, TAS measurements were conducted.54-55 The selected pulse laser with λ = 400 nm preferably matched the absorption band of TiO2. The collected spectra were recorded at different time delays following excitation. All evaluated materials revealed a broad absorption band in the assessed 800 to 1600 nm range, which could be distinctively assigned to trapped and free electrons in the 800–1000 nm and 1000–1600 nm regions, respectively (Figure S6).55-57 The concentration of electrons was shown to decrease within a time period of 0–100 ps in a multi-exponential fashion owing to the charge recombination between electrons and holes.57 Time profiles of absorption probed at 1200 nm have been depicted in Figure 5. The lifetimes of the transient species were extracted by a curve-fit as further detailed in the ESI, with fitted results being conveniently summarized in Table S1. TAS results for pristine TiO2 were fitted by a double exponential function with time constants within 1 ps 1) and ca. 5 ps 2). The two indicated time scales have been ascribed to two different electron decay processes, namely the trapping of free electrons and the non-geminate recombination between holes and electrons, respectively. Minor contribution on the carrier dynamics of TiO2 is expected to arise from the incorporation of UCN in these nanostructures under 400 nm. In the case of Tm-UCN/TiO2 however, the non-geminate recombination process was slightly accelerated at the Tm-UCN/TiO2 interface. This may be tentatively ascribed to the large surface defects and ligands with highenergy vibrational modes on the surface of the incorporated small 40 nm-size UCN nanocrystals, which have been reported to induce notable quenching effect, acting as trapping centers for


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photoinduced electrons and leading to reduced photoactivity.58 The drawback here underlined has paved the way for works of interest in the preparation of large NaYF4 UC particles as scattering and upconversion centers in photoanodes.59 Prompting the excitation of TiO2 under λ = 400 nm, TAS analysis demonstrates a facilitated injection of photo-generated carriers into adjacent Au centers reflecting the more negative work function of the incorporated Au in the TiO2 3D nanostructure. The lifetimes for a transient species at 1200 nm were extracted from the time dependence of optical density OD). Three decay lifetimes for Au/TiO2 were determined, i.e. Au/Tm-UCN/TiO2 (10.9 μmol/cm2) > TiO2 (6.2 μmol/cm2). The collected gas evolution quantities represent a maximal 2.2-fold enhancement compared with the reference material, further underlying the promising fingerprints of the architecture here reported. Analogous trends in H2 evolution were observed under visible light (≥ 420 nm) irradiation (Figure S8).


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Figure S9 shows the calculated applied bias photon-to-current efficiency (ABPE, %) as a function of the external potential versus RHE. Calculation details have been conveniently summarized in the Supporting Information. The pristine TiO2 sample exhibited an optimal ABPE efficiency of 0.19% at 0.65 V. A remarkable boost in the ABPE efficiency was witnessed for the ternary architectures. Au/Er-UCN/TiO2 and Au/Tm-UCN/TiO2 samples attained 0.44 and 0.34% at 0.65 VRHE, representing a striking 2.1 and 1.9-fold enhancement compared to the TiO2 reference, respectively. The stability of the evaluated photoelectrodes was further assessed over 10,000 s through chronoamperomertic i−t curves at 1.23 VRHE (Figure S10). Obtained values were found to be consistent with the I-V characteristics of UCN and Au incorporated TiO2 under solar light illumination in Figure 7. Most importantly, with seemingly constant photocurrent density during the evaluated time range, we believe the materials here reported evidence excellent stability for practical implementation.

3.4. Reaction mechanism: Concluding remarks On the basis of collected PEC performances, EIS, and TAS, plausible mechanistic pathways reflecting possible charge transfer processes have been schematized in Figure 9. Drawn conclusions reflect the structural and optical features of these nanostructures, the established interfaces between the individual moieties, selected operating conditions and the light shed during catalytic evaluation. Under light irradiation, the electrons in the VB of the individually evaluated UV-responsive TiO2 are excited to the CB, which are then subsequently transferred to the corresponding electrodes. As discussed in previous reports by some of us, the rational design of the TiO2 IO structure induces photon trapping toward an effective optical path length, triggered by scattering effects.11


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As demonstrated above by TAS measurements, a facilitated injection of photo-generated carriers into adjacent Au centers is observed in accordance with the more negative work function of the incorporated Au in the TiO2 3D nanostructure. When visible light irradiation in a wavelength range above 420 nm is additionally shed, conclusions on the proposed mechanism over Au-incorporated hybrid are, however, distinct from those under UV light (Figure 9). Herein, the Au SPR excitation with higher energy is proposed to be the major source of hot charge carriers, which are then injected to the CB of TiO2, due to the close contact between both components, by crossing the Schottky barrier. The SPR-induced injected hot electrons are then readily transferred to the electrode and generate current density under longer wavelength ranges. Reflecting prior literature reports, aside from the above discussed direct hot electron transfer (DET), the SPR-enhanced performance may further involve plasmon-induced resonant energy transfer (PIRET) and light scattering enhancement. Herein, however, the above contributions may be excluded, as PIRET requires the spectral overlap between SPR and TiO2 absorption and scattering preferentially observed when the SPR energy is larger or at least equal to the bandgap of the incorporated semiconductor. When UC nanocrystals are incorporated, the NIR-responsive doped Yb3+ ion possessing only one excited state acts as a sensitizer under 980 nm light (Figure 4b). The excitation of Yb3+ ions is followed by a successive energy transfer to adjacent Er3+/Tm3+, which operate as activators. Owing to a close proximity of incorporated UCN sites and neighboring TiO2 and Au centers, generated emissions of Tm3+ ions can be readily transferred and reabsorbed by adjacent centers of both components. Conversely Er3+ ions primarily yield green emissions at 521 and 540 nm well-matched with the SPR band of Au NPs (Figure 4). The observed high current density for Er ion-doped UCN-incorporated ternary systems can be explained by the emission induced hot


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electron generation in Au and subsequent injection to TiO2. The energy transferred to both Au and TiO2 accelerates the relaxation of the excited states of the incorporated UCN cation activators, enhancing the overall transition rates. Most importantly, the rational design of these structures establishes an indirect utilization of NIR light by both Au and TiO2 toward enhanced photocatalytic performances (Figure 7e,f). To be equally denoted is the underlying distinction in the mechanism between prepared ternary systems. Over Au/UCN/TiO2 a direct deposition of Au is presumed to occur over the surface of the formerly incorporated UC nanocrystals, in line with an electrostatic interaction between both components prior to chemical reduction of Au. The direct deposition of Au over upconversion centers could hence facilitate the reabsorption of generated UCN emissions compared to UCN/Au/TiO2 counterparts.65-66 The assumption, corroborated by provided representative SEM images (Figure S4), is reflected on a superior PEC performance and enhanced gas evolution quantities, and underlines the conceptual design assessed in this work in the construction of hybrid architectures.

4. CONCLUSION This contribution shares a facile approach to architect broadband photocatalysts based on a synergetic plasmonic and upconversion-enhanced approach. In detail, UV, visible and NIRresponsive nanocomponents – respectively, TiO2, Au nanoparticles and UC nanocrystals – were incorporated in a high surface rationally designed architecture believed to favor an efficient electron or energy transfer. While assessing the possibility of emerging synergetic effects established between the introduced nanocomponents in our ternary photoanodes, the established conceptual design further highlighted the importance of the incorporation methods for both Au


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and UCN. PL measurements confirmed the unique ability of Lanthanum-doped UCN to convert low-energy NIR light illumination to high-energy radiation. Conversely the Au nanoparticles incorporated in the UCN/TiO2 interface were shown to play a unique role to improve the solar utilization efficiency by effectively interacting with the embedded UCN. The ternary system able to capture photons over a wide wavelength range from UV, visible, to NIR, and show excellent photocatalytic activity, significantly superior to the pristine TiO2. Based on PL and photocurrent studies visible to NIR excitations different involved mechanisms and relevant factors are discussed in detail. In particular, the results highlighted Au/Er-UCN/TiO2 as a highly efficient photocatalyst. Our new material evidenced a 10-fold increase in photocurrent when exposed to vis-NIR irradiation compared with a pristine TiO2 reference. The mechanistic analysis of these rationally designed hybrid photocatalysts corroborated by additional electrochemical impedance spectroscopy and transient absorption spectroscopic measurements has further reflected the advantage of both plasmon and upconversion effects in harvesting low energy photons. In addition to an injection of SPR induced hot electrons from Au NPs to TiO2, NIR-light irradiation was shown to pave the way for an indirect excitation of both TiO2 and Au centers, driving the photocatalytic water splitting by producing the required charge carriers. This work is believed to serve as a valuable addition to current literature reports unveiling strategies to efficiently harvest low energy photons toward a sustainable future.

ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website at DOI:


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Figures S1-S7 provide characterization of the prepared hybrid materials and individual nanocomponents, including TEM photographs, high-resolution SEM, XRD, XPS, and transient absorption spectra, in addition to supplementary catalytic data. Table S1 summarize the kinetic parameters of decays observed for the prepared samples.

AUTHOR INFORMATION Corresponding Author *Email: [email protected] Author Contributions †These

authors contributed equally. All authors have given approval to the final version of the

manuscript. ORCID Ramireddy Boppella: 0000-0002-0370-5490 Filipe Marques Mota: 0000-0002-0928-3583 Ju Won Lim: 0000-0001-7021-3173 Saji Thomas Kochuveedu: 0000-0003-2699-6830 Jiseok lee: 0000-0002-5762-6085 Daisuke Kawaguchi: 0000-0001-8930-039X Keiji Tanaka: 0000-0003-0314-3843 Dong Ha Kim: 0000-0003-0444-0479 Notes The authors declare no competing financial interest.


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ACKNOWLEDGMENT This work was supported by the National Research Foundation of Korea Grant funded by the Korean Government (2017R1A2A1A05022387; 2015M1A2A2058365; 2011-0030255).


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Figure 1. Schematic illustration of the rationally designed fabrication processes of TiO2 inverse opal nanostructures loaded with Au NPs and UCN.


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Figure 2. SEM images of (a) PS opal, (b) TiO2 inverse opal (c) Au/TiO2, (d) UCN/TiO2, (e) Au/UCN/TiO2 and (f) UCN/Au/TiO2. Scale bar indicated in all images is 500 nm.


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Figure 3. (a) XRD spectra of Au and UCN incorporated TiO2 inverse opal structures and (b) corresponding reflectance spectra.


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Figure 4. (a) PL spectrum of NaYF4:Yb3+,Er3+ and NaYF4:Yb3+,Tm3+ arrays on FTO substrates under the irradiation of 980 nm laser source with a power output of 1 W. The secondary axis indicates the absorption spectrum of the Au NPs (red). (b) UCN energy transfer between Yb3+ and Tm3+ (left) and Yb3+ and Er3+ (right). Photoluminescence spectra of (c) Tm-UCN and (d) Er-UCN incorporated hybrid materials


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Figure 5. Normalized transient absorption profiles observed at 1200 nm for both (a) Tm-UCN and (b) Er-UCN incorporated samples excited under 400 nm light irradiation.


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Figure 6. EIS Nyquist plots of (a) TiO2 and UCN/TiO2, and (b) Au/TiO2, UCN/Au/TiO2, and Au/UCN/TiO2 in the dark and under illumination. Er-UCN-based materials have been selected as representative examples.


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Figure 7. Photoelectrochemical properties of (a), (c) NaYF4:Yb3+,Tm3+ and (b), (d) NaYF4:Yb3+,Tm3+-incorporated TiO2 hybrids. (a) and (b) display the I-V characteristics of UCN and Au incorporated TiO2 under solar light illumination (AM 1.5; 100 mW/cm2), (c) and (d) show ampherometric I−t curves of different photoelectrodes measured at 1.23 VRHE with repeated on−off cycles under Vis-NIR irradiation (≥ 420 nm). (e) and (f) show the ampherometric I−t curves of hybrids photoelectrodes measured at 1.23 VRHE with repeated on−off cycles under NIR irradiation.


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Figure 8. H2 and O2 evolution quantities under simulated sunlight illumination at 1.23 VRHE with a pristine TiO2 reference, Au/Tm-UCN/TiO2, and Au/Er-UCN/TiO2.


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Figure 9. Proposed photocatalytic mechanism over the prepared Er-UCN and Tm-UCN-based ternary photocatalysts under UV-vis-NIR extended light range.


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TOC Broadband-responsive hybrid TiO2 inverse opal nanostructure incorporating Au nanoparticles and upconversion nanocrystals for enhanced photoelectrochemical water-splitting.


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Plasmon and Upconversion Mediated Broadband Spectral Response

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