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Robust multimetallic plasmonic core-satellite nanodendrites: highly effective visible light induced colloidal CO2 photo-conversion system Dinesh Kumar, Chan Hee Park, and Cheol Sang Kim ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b00924 • Publication Date (Web): 23 May 2018 Downloaded from http://pubs.acs.org on May 24, 2018

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ACS Sustainable Chemistry & Engineering

Robust multimetallic plasmonic core-satellite nanodendrites: highly effective visible light induced colloidal CO2 photo-conversion system Dinesh Kumar, Chan Hee Park* and Cheol Sang Kim* Department of Bionanosystem Engineering, Graduate School Chonbuk National University Jeonju 561756 Republic of Korea E-mail: [email protected] (C.H. Park), [email protected] (C.S. Kim) Keywords: plasmonic nanodendrites, colloidal stability, CO2 reduction, light irradiation, HCOOH formation.

ABSTRACT: The present report demonstrated the synthesis of trimetallic Pt-nanodots (PtNDs, 3-4 nm) and TiO2 (4-5 nm) coated AuNPs (Pt@TiO2-AuNPs) plasmonic nanoparticles with spherical dendritic shape, robust structure, and high colloidal stability. Prepared plasmonic nanodendrites were tested for CO2 photoconversion to HCOOH in the aqueous medium at room temperature and showed significant conversion rate with 3.12% of chemical yield and 1.84% of quantum yield. There was a remarkable improvement in the photocatalytic efficiency of TiO2AuNPs after PtNDs decoration as 312 fold increase in the CO2 photoconversion efficiency with trimetallic Pt@TiO2-AuNPs nanodendrites in comparison to bare AuNPs in the presence of Xe

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lamp (visible light). Prepared nanodendrites also showed notable efficiency in NIR light and solar simulator illumination. The action spectra (quantum yield vs. Wavelength of monochromatic light) has shown the plasmon-enhanced reaction progress and robustness of prepared colloidal nanodendrites. The spherical nanodendrites have been tested for at least five reaction cycles and showed consistent conversion efficiency with exceptional morphological and colloidal stability.

INTRODUCTION To resolve the global climate change and demand of carbon and energy resources, the utilization of natural sunlight to reduce carbon dioxide to alternative hydrocarbon fuels and other value-added product using photocatalysts is a persuasive approach and has been tremendously interesting.1 Transition metal (Pd, Ru, Rh, Cu or Ni) catalysts are used commercially for deriving value-added chemicals, though these catalysts normally require energy-intensive conditions like high pressure and temperature.2 Light-mediated processes can be a viable alternative, in order to achieve the maximum efficiency in large-scale processes while minimizing energy consumption. Photo-catalytic semiconductors commonly TiO2, ZnO, SiC, and CdS were most frequently employed as a catalyst for CO2 photo-conversion in the ultra-violet (UV) light illumination due to numerous benefits such as plentiful abundance, high stability with less cost and toxicity.3-6 TiO2 with co-exposed facets of (001) and (101) was also tested for CO2 reduction and found that exposure of the (001) facets boosts the efficiency.7


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But the inefficient photo-response in visible light and the extensive electron-hole recombination due to large band-gap its efficiency is far less for CO2 reduction.8-9 As ∼50% of solar light irradiation consists of visible light and UV light shares only about 5%.10-11 Hence, the advancement in the preparation of photo-catalysts active in visible light is the main area from a practical application point of view.12-13 Moreover, during the CO2 photo-reduction with TiO2 hydrogen formation is a preferred process over CO2 conversion and hence upsetting the selectivity for CO2 conversion.14 Although, the combination of high selectivity of noble metals (Au, Pt, Ag, Cu, or Pd) as co-catalyst with TiO2 had a positive impact on CO2 photo-reduction efficiency.3, 14-16 Anisotropic ALa4Ti4O15 (A = Ca or Sr or Ba) along with co-catalyst (Ag) used water as a reducing reagent in CO2 reduction.17 Bimetallic Cu-Pt loaded TiO2 nanotubes reduced CO2 to hydrocarbons with 4-times improved reduction efficiency under solar irradiation.18 CuO loaded N doped TiO2 also increases the conversion efficiency of CO2 to methane (CH4) under solar illumination.19 Benzene-1,3,5-tricarboxylate combined with Cu and TiO2 based core-shell photocatalysts (Cu3(BTC)2@TiO2), where micrporous TiO2 shell transport CO2 easily which then adsorbed on Cu3(BTC)2.20 Mononuclear oxo-bridged C5H5-RuH-TiO2 complex,21 Cu2ZnSnS4/TiO2 heterostructures,22 PbS-Cu complex with TiO2 and CdSe-Pt heterostructure with TiO2,23 metal-doped NaTaO3 with co-catalyst (Ag),24 and codeposited RuO2 and Pt as oxidation and reduction co-catalysts on Zn2GeO425 are other examples of semiconductor based complexes for photoconversion of CO2. Further, in order to increase the CO2 conversion efficiency in visible light, noble metal-based plasmonic nanoscale materials, because of the LSPR (localized surface plasmon resonance) and resultant hot electron generation, have attracted significant scientific and industrial interest because of their enhanced solar energy to chemical energy abilities.26-29 However, the shorter


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lifespan of hot electrons because of the intrinsic ultrafast relaxation restricts the photocatalytic ability of monometallic plasmonic nanomaterials. Moreover, the photocatalytic properties of such nanomaterials can be enhanced to a great extent by the synthesis of different morphologies of bimetal or trimetal or multimetal nanomaterials.30-31 TiO2 coated gold nanomaterials have been reported with increased efficiency for CO2 conversion in visible light.32-34 The easily controlled physical properties enabled the core-shell complex formation and the combined nanostructures exhibited strong surface plasmon resonance which enhanced the photocatalytic efficiency.32-33 Hence, a thin layer of TiO2 on gold nanoparticles (AuNPs) corresponds to a stronger electric field persuaded by LSPR on or close to the TiO2-AuNPs surface, and thus there is more enhancement in the light energy absorption surrounding the core-shell TiO2-AuNPs nanoparticles as compared to non-coated or bare AuNPs.35 Although, the large band gap and short lifetime of hot electrons need further improvement in the TiO2 coated gold nanocomplex, in order to enhance photocatalytic activity in visible and NIR light. Few reports rerated to Pt-TiAu trimetallic system are available in the literature but they had neither colloidal dispersibility/stability nor have well-defined morphology and also they have used a high amount of prepared nanoparticles for the photocatalytic applications.36-38 In comparison, colloidal nanoparticles system required much less amount of particles for photocatalytic reactions.39 The synthesis of TiO2 coated gold nanoparticles based three-component system with colloidal stability and dispersibility could be an attractive and qualitative improvement. However, there are very few reports available in literature related to three component or trimetallic particles with well-defined morphology, colloidal dispersibility, and structural stability. As during the multimetallic nanoparticles preparation, different reduction potentials of different metals precursors causing significant complexities while the nucleation and growth rate


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control of nanoparticles.40-41 Therefore, preparation of such multimetallic nanomaterials is still a challenging task and relatively unexplored area of research, and the advancement of an easy and efficient method to synthesize colloidal multimetallic plasmonic nanoparticles with a stable structure and possible multiple applications is significantly demanding.40 Moreover, for real-life application, CO2 photoconversion should be carried out in an aqueous medium to utilize water as an electron source, because it is easily available and an abundant resource. Though, in literature, only a few reports have been found which can be employed for CO2 photoreduction in an aqueous medium, even while using a sacrificial agent.1 In present work, we have synthesized Au, Pt and TiO2 based core-satellite colloidal plasmonic nanoparticles (Pt@TiO2-AuNPs) with precise spherical dendritic morphology composed of AuNPs coated with TiO2 (4-5 nm) and platinum nanoparticles (3-4 nm). The Au-Pt bimetallic nanodendrites (Pt@AuNPs) were also prepared for the comparative study and to analyze the influence of TiO2 layer on possible applicative studies. The prepared Pt@TiO2-AuNPs nanodendrites were studied as a reusable plasmonic photo-catalytic process for CO2 conversion to HCOOH with high colloidal and structural stability.

RESULTS AND DISCUSSION Characterization of prepared plasmonic nanomaterials: The TEM (transmission electron microscopic) image (Figure 1c), and EDX (energy dispersive X-ray) mapping images (Figure 1d) of Pt@TiO2-AuNPs show a deposition of Pt-nanodots (3-4 nm) and uniform TiO2 layer (4-5 nm) around the AuNPs core. The coating of thin layer (4-5 nm) of TiO2 is clearly visible in TEM (Figure S2b) and HRTEM image (Figure 1a). The lattice fringes in HRTEM image with 0.20 nm


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and 0.23 nm of d-spacing corresponds to Au (200) and Au (111) fcc planes, respectively. Also, the lattice d-spacing of 0.19 nm and 0.23 nm representing Pt (200) and Pt (111) fcc planes, respectively (Figure 1b). The AuNPs showed surface plasmon resonance band (λmax) at 525 nm and there was a redshift observed from 525 to 545 nm after TiO2 coating in the UV-visible spectrum (Figure 1e). A further redshift from 545 to 550 nm with broadening in peak was observed after PtNDs coating on TiO2-AuNPs (Figure 1e). The redshift in the UV-visible spectrum was because of an overall increment in the refractive index of the gold nanoparticles dielectric environment after coating of TiO2.42 Also, the XRD (X-ray diffraction) spectral patterns exhibited the peaks at 2θ ~ 37.8°, 44.1°, 64.1° and 77.1° which be in agreement with Au (111), Au (200), Au (220) and Au (311) fcc planes,43 respectively (Figure 1f). The presence of TiO2 in anatase form was indicated by the peaks at 48.08° (200) and 25.3° (101) (Figure 1f). Moreover, the peaks at 2θ ~ 40.6°, 46.7°, 67.6°, 81.2° and 87.6° corresponds to (111), (200), (220), (311) and (222) fcc planes of the Pt crystal lattice,44 respectively (Figure 1f). The TEM image of the Pt@AuNPs shows deposition of Pt nanoparticles (size = 3-4 nm) on the AuNPs (Figure S3b). Also, the redshift has been observed from 525 nm to 545 nm in the UV spectrum as AuNPs were coated with PtNDs (Figure S3a).


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Figure 1. Characterization of Pt@TiO2-AuNPs nanoparticles. HRTEM images of (a) TiO2AuNPs, and (b) Pt@TiO2-AuNPs. (c) TEM image of Pt@TiO2-AuNPs. (d) EDX (energy dispersive X-ray) mapping image of Pt@TiO2-AuNPs. (e) UV-Visible spectra and (f) XRD spectral patterns of AuNPs, TiO2-AuNPs and Pt@TiO2-AuNPs (□ = Au, ⃰ = Ti, and ♦ = Pt). Photocatalytic carbon dioxide conversion: In a typical CO2 photoreduction process, the CO2 saturated (0.24 mg/mL) nanoparticles solution (10 mL, OD at 545 nm = 1.0) was placed in a chemical reactor (Pyrex glass, capacity = 10 mL) equipped with an outer jacket for water circulation to maintain temperature. Then, the reaction vessel was illuminated with visible light (Xe lamp, power density = 5.71 W/cm2) whereas 808 nm near-infrared laser (2.94 W/ cm2) has been used for NIR irradiation. After the 5 h of reaction time (completion of reaction) the reaction mixture was centrifuged (15,000 rpm/15 min) to remove nanoparticles (Pt@TiO2-AuNPs) and to obtain the supernatant containing final products. The supernatant was analyzed for obtained reaction products using GC-MS (gas chromatography-mass spectrometry). For nuclear magnetic


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resonance (1H-NMR and

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C-NMR, 600 MHz, CDCl3), Raman and FTIR (Fourier Transform

Infrared Spectrometer) studies, the supernatant pH was adjusted to 12 with dilute NaOH solution additions to convert HCOOH to HCOO−Na+ and rotary evaporated. The QY (quantum yield) and CY (chemical yield) of HCOOH formation were computed by Gas chromatographic and proton-NMR spectroscopic data. In typical 1H-NMR spectrum, the chemical shift at 8.02 ppm corresponds to the aldehyde proton (H−C=O) which indicated the formation of formic acid. Further, the chemical shift at 166.1 ppm in the

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C-NMR spectra also confirmed the formation of formic acid (Figure 2a).45

The presence of two major peaks at the retention time of 104 and 201 s in the Gas chromatogram indicated the presence of methanol (CH3OH, area % = 30%) and formic acid (HCOOH, area % = 70%) from the Pt@TiO2-AuNPs assisted reaction mixture, and likewise CH3OH (area % = 35%) and HCOOH (area % = 65%) from the Pt@AuNPs assisted reaction mixture,46 respectively (Figure 2b). Further, the HCOONa+ obtained after the pH adjustment to 12.0 and removal of the aqueous medium was also analyzed by FT-IR. The O=C−O symmetric stretching (1369 cm−1), O=C−O asymmetric stretching (1617 cm−1), C=O vibrational stretching (1703 cm−1), and C−H stretching 2855 cm−1 after 1 h, 3 h and 5 h of reaction time supported the HCOOH formation.47 For Raman analysis, 10 µL of CO2 reduction reaction mixture was allowed to dry on the quartz substrate and analyzed with 532 nm laser excitation (50 mW). The spectral range used was 400– 1800 cm−1 with 10 sec of integration time. The final Raman spectra of reaction product after 5 h showed characteristic shifts corresponding to OCO bending (624 cm−1), HOC stretching (919 cm−1), HCO stretching (1048 cm−1), CO stretching (1220 cm−1), and HOC bending (1420 cm−1) and confirmed HCOOH formation (Figure 2d).48


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Figure 2. (a) 1H-NMR and 13C-NMR spectra of CO2 photo-reduction reaction products. (b) Gas Chromatogram of the reaction products after CO2 photo-reduction reaction with visible light irradiation in the presence of Pt@AuNPs and Pt@TiO2-AuNPs, respectively. (c) FT-IR spectra and (d) Raman spectra of the CO2 photo-reduction reaction products obtained after 0 h, 1.0 h, 3.0 h, and 5.0 h.


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Finally, the Mass spectra demonstrated the molecular ion and fragmentation peaks at m/z value 46, 45, and 44 corresponded to HCOOH+, HCOO+, and COO+, respectively (Figure 3a) and at 32, 31, and 29 corresponded to CH3OH+, CH2OH, and COH+, respectively (Figure 3b). Both the products have important respective aspects as HCOOH has termed as a promising H2 carrier with 53 g H2/L of high volumetric ability with lesser toxicity and low flammability,49 whereas, CH3OH has very low pollutant emission and elevated operating power density and is being used in fuel cells.50

Figure 3. Mass spectroscopic analysis of (a) HCOOH and (b) CH3OH obtained after CO2 reduction. Reaction mechanism of CO2 photo-reduction: In a two-step mechanism of CO2 photoreduction, first of all, light illumination generated H2 from water (reaction medium) through water splitting and in the second step generated H2 and CO2 dissolved in reaction mixture were reacted to produce HCOOH or CH3OH (as shown in Scheme 1).45, 51 The other possibility is the direct injection of hot electrons to CO2 molecule and formation of CO2•- which in turn going to react with H+ and resulting in the HCOOH formation (Scheme 1).52-54


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To support the mechanism two experiments were conducted for the confirmation of formation of H2 and reaction of H2 with CO2 to form HCOOH. In the first experimentation, Pt@TiO2AuNPs in the aqueous solution (without CO2) were irradiated under the visible light (Xe lamp). The reaction vessel was sealed from the top and connected to an aqueous copper sulfate solution which was bright blue in color and acts as a hydrogen gas indicator. The hydrogen gas generation from the aqueous solution of the Pt@TiO2-AuNPs was detected as there were a color change and precipitate formation in the copper sulfate solution because first H2 reduced Cu2+ to Cu+ and then to Cu. In the second experiment, H2 gas was reacted for 5 h with CO2 dissolved in distilled water at room temperature without light irradiation (the detailed procedure in Section 2.2, supporting information) and the CY found to be 0.94% for formic acid formation (Figure 5d). The lower yield in comparison to Pt@TiO2-AuNPs assisted reaction was because of the absence of addition formic acid formation pathways like the oxidation of methanol to formic acid, direct injection of electrons to CO2 molecule which then combines with H+ to form HCOOH, and absence of plasmonic photoconversion pathways.55-56

Scheme 1. Schematic representation of Pt@TiO2-AuNPs mediated CO2 reduction into HCOOH and CH3OH under light irradiation.


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Photo-catalytic performance of plasmonic nanodendrites for CO2 reduction: The hot electron generation is crucial for photo-activity of plasmonic nanoparticles which depends on the LSPR of AuNPs. Moreover, the wavelength maxima (wavelength at which plasmon can be excited) is responsible for strong LSPR, which in turn depends on the morphology of plasmonic nanostructure (size and shape). Therefore, the visible (Xe lamp) and NIR (808 nm laser) light irradiation were used for the photo-conversion of CO2. The short lifespan of hot electrons was responsible for the non-noticeable conversion rate of non-coated AuNPs in the presence of visible and NIR light illumination even though their wavelength maxima (520 nm) lies in the visible range.57-58 However, after the coating of a TiO2 layer (4-5 nm) on AuNPs, there is significant conversion efficiency found with 0.84% of QY and 1.45% of CY for the formation of formic acid in the presence of visible light (Figure 4a). The increment in efficiency could be due to the enlargement of surface area from 2947 nm2 (AuNPs) to 4184 nm2 (TiO2-AuNPs) as shown in Table S1, red-shift with broadening of surface plasmon band of AuNPs after TiO2 coating as shown in the UV-visible spectrum of TiO2-AuNPs (Figure 1f), and the selectivity of TiO2 (24%) for CO2 photo-reduction. Also, there was further increase in the absorption of light energy absorption surrounding the core-shell TiO2-AuNPs nanoparticles after the thin layer coating of


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TiO2 on AuNPs because of the stronger electric field stimulated by the LSPR. Though, there was a remarkable increment in the quantum (1.84%) and chemical yield (3.12%) when PtNDs (3-4 nm) were deposited to TiO2-AuNPs while using visible light illumination (Figure 4a). The reasons for such increase in yield were the combination of PtNDs and TiO2 with AuNPs which caused significant increase in the surface area (Pt@TiO2-AuNPs = 5445 nm2), and broaden the surface plasmon band with redshift to 545 nm along with their high selectivity (Pt = 83%) and TiO2 = 24%) for CO2 reduction. However, TiO2-AuNPs have shown 0.12% of QY and 0.2% of CY with NIR light, the lesser efficiency was due to the absence of wavelength maxima in the NIR region and lower power density (2.94 W/cm2) of NIR laser as compared with Xe lamp (5.71 W/cm2). While Pt@TiO2AuNPs showed reasonable conversion rate for HCOOH with 0.42% of QY and 0.71% of CY in the presence of NIR light irradiation (Figure 4b). The first factor for high efficiency was the coating of PtNDs, which is highly efficient co-catalyst, accelerated the reduction of CO2 due to high selectivity (83%), and TiO2 layer, which in spite of having large band gap has 24% selectivity for CO2 reduction, are deciding factors along with the hot electron generation due to plasmonic effect as elaborated schematically in Figure 2a.59 The second factor associated with the TiO2 and PtNDs coating which increases the surface area from 2947 nm2 (AuNPs) to 5445 nm2 (Pt@TiO2-AuNPs), and makes it more effective in visible and NIR light. The third reason was the higher photo-generated electrons (hot electrons) consumption rate of PtNDs59-60 which helped to avoid ultrafast relaxation of hot electrons due to non-radiative decay and high band gap of TiO2, and in turn prevent electron-hole recombination which leads to the significant increment in the photo-conversion efficiency of CO2 to HCOOH (Figure 4a-b).


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In addition, the photo-induced electron and holes separation efficiency have been examined using photoluminescence (PL), transient photocurrent response, and electrochemical impedance spectroscopic analysis of AuNPs, TiO2-AuNPs, and Pt@TiO2-AuNPs (Figure S4a). The PL spectra of Pt@TiO2-AuNPs showed comparatively lower PL intensities than TiO2-AuNPs and AuNPs (Figure S4a) which was associated to the strong internal electric field in the close proximity of AuNPs and efficient photo-induced electron consumption by Pt nanoparticles, which foster effective charge separation of photo-induced electrons. Alternatively, TiO2-AuNPs showed higher PL intensities in comparison to Pt@TiO2-AuNPs and AuNPs and hence is less effective for photo-induced charge separation could be because TiO2 has a large band gap.61 Moreover, in order to interpret the interface charge transport behavior and to evaluate the photoinduced electron and holes separation efficiency, electrochemical impedance spectroscopic (EIP) analysis of AuNPs, TiO2-AuNPs, and Pt@TiO2-AuNPs has been performed. As shown in the Figure S4b, the semicircle at high-frequency region in Nyquist plot is negligible and diffusion line is almost perpendicular towards the imaginary axis, highly suggested that the charge transfer phenomena between electrode and electrolyte in Pt@TiO2-AuNPs is higher than that of AuNPs and TiO2-AuNPs (Figure S4b). The TiO2-AuNPs nanoparticles exhibited least charge transfer which might be because TiO2 has a large band gap. The semicircle radius shows the charge transfer resistance, highly depends on electrode materials and its surface, and concentration nature of the electrolyte.62-63 IPCE (Incident photon to charge carrier efficiency) of Pt@TiO2AuNPs was also studied and the IPCE value increase was observed in the visible and near infrared region (450 nm to 800 nm) of a spectrum (Figure S4c). The IPCE enhancement is in a large region which may be due to the attachment of three components (Au, Ti, and Pt) together.


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On the basis of PL, EIS, and IPCE analysis, we can conclude that Pt@TiO2-AuNPs allowed more transfer of electrons and hence there are less chances of electron-hole recombination. Therefore, during the photocatalytic CO2 reduction Pt@TiO2-AuNPs found to be highly efficient due to the effectual separation of the photo-induced hot-carriers (hot electrons and holes). The desirable and ultimate goal of energy source to carry out photochemical reactions is solar light. To analyze the Pt@TiO2-AuNPs efficiency for CO2 conversion in sunlight, a solar simulator (AM 1.5) with a power density of 0.23 W/cm2 was used as a source of light. The aqueous solution of prepared nanoparticles viz., TiO2-AuNPs, Pt@AuNPs, and Pt@TiO2-AuNPs were saturated with CO2 and placed in solar simulator light irradiation for 5 h, and the quantum yields and chemical yield were calculated to be 0.11%, 0.21%, 0.28%, and 0.19%, 0.35%, 0.47%, respectively (Figure 4c). The low efficiency for the CO2 reduction in solar simulator light was because of its low power density when compared with Xe lamp. On the contrary, bimetallic Pt@AuNPs nanoparticles also showed significant conversion efficiency for the CO2 reduction to HCOOH with 1.44% of QY and 2.45% of CY under Xe lamp irradiation (Figure 4a). While using NIR light, the QY and CY for CO2 reduction were found to be 0.36% and 0.61%, respectively (Figure 4b). The efficiency towards CO2 reduction could be attributed to the proficient selectivity and electron consumption capability of PtNDs along with plasmonic effect (plasmon resonance band expansion) of AuNPs. The absence of TiO2 layer caused the lesser conversion efficiency in case of Pt@AuNPs assisted reaction as compared to Pt@TiO2-AuNPs. Attachment of TiO2 layer increased the surface area from 3948 nm2 (Pt@AuNPs) to 5445 nm2 (Pt@TiO2-AuNPs) and played a crucial role in higher photocatalytic efficiency (Table S1 and Figure 1a-c). In order to rule out any contamination from photocatalyst, the isotopic 13CO2 experiment has been performed and the yield was found to be almost similar


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(QY = 1.79% and CY = 3.02%) which ruled out any contamination from an organic residue contaminating catalyst materials.

Figure 4. Impact of various plasmonic hybrids nanostructure on CO2 photoreduction with (a) visible light (Xe-lamp), (b) NIR (808 nm) laser and (c) sunlight (solar simulator). Effect of (d) nanoparticles concentration (OD at 545 nm) for CO2 photo-conversion using Pt@TiO2-AuNPs in Xe lamp irradiation. (*N/P = No Product). Impact of nanoparticles amount and reaction duration: The impact of nanoparticles concentration on the reduction of CO2 to HCOOH formation was analyzed by changing O.D.


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(optical density) of nanoparticles (Pt@TiO2-AuNPs) in the range of 0.5 to 3.0. All the reactions were performed for 5 h of reaction duration in the presence of Xe lamp. A linear increase in the QY and CY was observed as the O.D. was raised from 0.5 (QY = 0.81%, CY = 1.37%) to 1.0 (QY = 1.84%, CY = 3.12%) and modest increase was found while O.D. was raised from 1.0 to 2.0 (QY = 1.97%, CY = 3.34%). Although there was a minor decrease in yield with O.D. = 3.0 (QY = 1.86%, CY = 3.16%) because of the formation of nanoparticles aggregate due to high concentrated (Figure 4d). Pt@TiO2-AuNPs (O.D. = 1.0) mediated CO2 photo-conversion reaction progress has been studied by varying the reaction time in the range of 1h to 5h and then analyzing them with GC and 1H-NMR for quantum and chemical yield calculations. After 1 h of reaction time, the QY and CY have been observed as 0.58% and 0.98%, respectively, and increased linearly to 1.84% (QY) and 3.12% (CY) after reaction duration of 5 h (Figure 5a). After studying different reaction parameters for CO2 photo-conversion it has been observed that PtNDs coating on TiO2-AuNPs shows much higher photoconversion proficiency in comparison to TiO2-AuNPs. Also, there was 71 to 312 fold increment found for conversion efficacy in NIR and Xe lamp irradiation when gold nanoparticles combined with TiO2 and PtNDs (Figure 5b and 5c). These results supported that the combination of PtNDs with TiO2AuNPs was essential to increase the efficiency (up to 70%) and selectivity for HCOOH formation in high yield.


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Figure 5. (a) Impact of reaction time on the progress of Pt@TiO2-AuNPs mediated CO2 reduction in visible light irradiation. Comparative fold change for the quantum yield and chemical yield of TiO2-AuNPs, Pt@AuNPs, and Pt@TiO2-AuNPs versus AuNPs CO2 reduction in (b) visible light (Xe lamp), and (c) NIR (808 nm) light. (d) Effect of H2 gas purging on CO2 photoconversion.

Morphological robustness of prepared nanodendrites: To demonstrate the robustness of the prepared nanodendrites (Pt@TiO2-AuNPs), the CO2 photoconversion reactions were performed by varying monochromatic wavelengths such as 450, 550, and 650 nm. The QY and CY at 450, 550, and 650 nm were found to be 0.21% and 0.39%, 0.40% and 0.69%, and 0.24% and 0.40%,


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respectively (Figure 6a). The highest efficiency was found at 550 nm which is the plasmon resonance band (550 nm) of Pt@TiO2-AuNPs and hence correlated well (Figure 6a). The lesser efficiency of Pt@TiO2-AuNPs with 550 nm filter in comparison to complete visible light region (390-770 nm) illumination with Xe lamp was because of the decrease in the power density of Xe lamp when wavelength filters were used. The impact of variable power densities of NIR laser (5 W, 7 W, and 10 W) have been monitored with Pt@TiO2-AuNPs nanodendrites. As the power density of NIR laser was increased from 5 W to 7 W the QY was found to increase from 0.42% to 0.54% for HCOOH formation and with further increase of power density from 10 W the quantum yield was found to be 0.75% (Figure 6b). The increase in the yield varied linearly with an increment in the power density of NIR laser. The lesser efficiency in NIR (808 nm) light as compared to Xe lamp illumination could be because of the absence of plasmon resonance band of Pt@TiO2-AuNPs (550 nm) in NIR region.

Figure 6. (a) Effect of different incident monochromatic wavelengths on the yield estimated for the CO2 photo-reduction to HCOOH using prepared Pt@TiO2-AuNPs nanoparticles. (b) Effect


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of different power densities of NIR (808 nm) laser on the yield of CO2 photoconversion to HCOOH.

Recyclability and morphological stability studies: The recyclability of colloidal nanoparticles depends on the colloidal stability mainly and is a major factor for practical application of any photo-catalytic system. The Pt@TiO2-AuNPs were recovered after 5.0 h of reaction duration and used again for another five reaction recycles of CO2 photo-conversion reaction under Xe lamp irradiation. The conversion rate for each cycle was constant as confirmed by gas chromatographic analysis of the reaction product of five recycles and no observable change was found in the final HCOOH yield (Figure 7a and 7b). Moreover, there was no distortion in morphology and change in particle distribution of Pt@TiO2-AuNPs after 5 recycles as shown by UV-visible spectrum (Figure 7c) as well as TEM and XRD analysis (Figure S5). The colloidal stability of TiO2-AuNPs, Pt@AuNPs, and Pt@TiO2-AuNPs during CO2 conversion reaction under visible light irradiation has been checked by analyzing particles after every one hour with UV-visible spectroscopy and all the three kinds of plasmonic nanoparticles have shown excellent stability (Figure 7d). The excellent colloidal and morphological stability of Pt@TiO2-AuNPs was the major reason for its consistent performance during reusability processes of CO2 photoconversion.


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Figure 7. (a) Reusability studies of Pt@TiO2-AuNPs for CO2 reduction using Xe lamp. (b) Comparative Gas chromatogram for Pt@TiO2-AuNPs mediated reaction product after 5.0 h of visible light irradiation for five recycles. (c) The UV-Visible spectrum of Pt@TiO2-AuNPs before reaction and after 5 recycles. (d) Stability of Pt@AuNPs, TiO2-AuNPs, and Pt@TiO2AuNPs under visible light illumination for five hours.

CONCLUSIONS In conclusion, the plasmonic trimetallic spherical nanodendrites composed of AuNPs, TiO2, and PtNDs have been prepared in the colloidal form using simple solution based methods and tested


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for CO2 photoreduction. The attachment of PtNDs on TiO2-AuNPs boosted the CO2 photoconversion efficiency significantly under visible (1.84% = QY and 3.12% = CY), NIR light (0.42% = QY and 0.71% = CY) and sunlight (0.28% = QY and 0.47% = CY) illumination and turned out to be a promising plasmonic nanomaterial for energy conversion. There was 71 and 312 fold increase with Pt@TiO2-AuNPs in the yield for HCOOH formation in comparison to non-coated AuNPs in NIR and Xe lamp illumination, respectively. Pt@TiO2-AuNPs showed excellent morphological and colloidal stability during the CO2 reduction reaction as well as after multiple reaction cycles. The quantum yield vs. Wavelength of monochromatic light based action spectra supported the robustness of colloidal photo-system and the plasmon-enhanced reaction progress as maximum conversion efficiency was found at the wavelength (550 nm) where plasmon resonance band of Pt@TiO2-AuNPs existed. The results suggested that a higher rate of hot (photo-generated) electrons consumption by PtNDs, increase in the surface area, and broadening of plasmon resonance band of AuNPs after TiO2 and PtNDs coating excels the efficiency of Pt@TiO2-AuNPs enormously for CO2 reduction.

EXPERIMENTAL SECTION Preparation of positively charged AuNPs: Positively surface charged AuNPs (λmax = 525 nm) were synthesized by using the cysteamine-modified method.64 To an aqueous mixture of HAuCl4 (20 mL, 1.42 mM) and cysteamine solution (200 µL, 213 µM), fresh sodium borohydride (5 µL, 10 mM, NaBH4) was mixed with gentle stirring; then, the final reaction mixture was placed on a shaker for 12 h at 25 °C. The red-wine colored reaction product was centrifuged twice at 2,000 rpm (15 min) to obtain the purified AuNP solution (30-35 nm, Figure S2a). Preparation of Pt@AuNPs: The coating of Pt nanoparticles (3-4 nm) on AuNPs core was performed by mixing ascorbic acid (0.6 mL) to the 10 mL of AuNPs solution having optical


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density 1.0, and then 90 µL of Pt precursor (H2PtCl6·6H2O, 0.01 M) was added followed by the 90 µL of dilute hydrochloric acid (0.01 M) solution. The final reaction mixture then was left undisturbed at 28 °C in water bath for overnight, and then the reaction product was washed twice (8000 rpm for 15 min) and re-dispersed in deionized water (Figure S3b, supporting information). Preparation of TiO2-AuNPs: First, 1.4 mL of ammonia solution (4%) was added to the mPEG modified 5 mL of AuNPs (O.D. = 2.0) followed by the addition of 100 µL of PAA solution. The resulted mixture placed on the magnetic stirrer for 4 h and then sonicated for 0.5 h. To the PAAAuNPs, 100 µL of TPO solution (20 µL of titanium isopropoxide mixed with 1000 µL ethanol) was mixed and the final solution was placed on a magnetic stirrer for overnight in dark at 25 °C. Then, the resultant reaction mixture has been washed with distilled water and ethanol and redispersed in 10 mL of distilled water. Preparation

of

Pt@TiO2-AuNPs:

Prepared

TiO2-AuNPs

first

modified

with

APS

(aminopropyltrimethoxysilane) and then with 1% sodium citrate solution. Pt nanoparticles (3-4 nm) were decorated uniformly on TiO2-AuNPs by the same process used for Pt@AuNPs synthesis. Photo-conversion of CO2: The CO2 photoreduction has been carried out in a Pyrex glass reactor (capacity = 10 mL) contains outside water circulation system to maintain temperature (Figure S1). 10 mL of prepared nanoparticles solution (OD = 1.0) was added in the reactor and CO2 gas purged until saturation (0.24 mg/mL). Then the reaction mixture was irradiated for 5 h with visible light (Xe lamp, power density: 5.71 W/cm2) or NIR (808 nm laser, 2.94 W/cm2) light. HCOOH formation under H2 gas reaction with CO2: Formic acid formation through CO2 reduction without light illumination was performed in a continuous flow of H2 for 5 h. H2 gas was produced by the addition of an aluminum foil to NaOH solution (200 mL, 2.0 M).65 The as-


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generated H2 gas has flowed into CO2 saturated distilled water constantly for 5 h. Then, after the completion of a reaction, the pH was maintained to 12.0 by adding dilute NaOH and the mixture was rotary evaporated to dryness. CO2 photo-reduction reaction product characterization: After the completion of reaction (5 h) the nanoparticles were separated from the reaction products through centrifugation of reaction mixture at 8,000 rpm for 15 min. Then the reaction products were characterized using gas chromatographic analysis (GC). For GC characterization, the oven temperature was varied from 35 °C to 100 °C using helium gas as the carrier gas with 200 °C of an injector temperature and 20 min of sampling time for GC-MS analysis. The equation obtained from the standard deviation curve was used to calculate the number of moles of formic acid formed. Also, after the adjustment of pH of the resulted reaction mixture to 12, the rotary evaporated product was characterized using NMR (1H-NMR and

13

C-NMR), FTIR (Fourier-transform infrared) and

Raman analytical studies. The small aliquot (10 µL) of CO2 reduction reaction mixtures was dried on a quartz substrate and then analyzed for Raman spectroscopy (the samples were analyzed with 532 nm laser excitation (50 mW)). Spectral data were collected over the range 400–1800 cm−1 with 10 sec of integration time.

ASSOCIATED CONTENT Supporting Information. The pdf file contains experimental details, additional TEM, UV, BET, PL, IPCE, electrochemical impedance spectra, and XRD data.The Supporting Information is available free of charge on the ACS Publications website. AUTHOR INFORMATION


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Corresponding Author *E-mail: [email protected] (C.S. Kim), [email protected] (C.H. Park). ORCID Dinesh Kumar: 0000-0001-6769-827X Chan Hee Park: 0000-0001-6894-521X Cheol Sang Kim: 0000-0002-7321-8954 Author Contributions D.K. was involved in the discovery and initial development of the photocatalyst platform. D.K., C.H.P., and C.S.K. designed and synthesized the catalysts. The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no conﬂict of interest.

ACKNOWLEDGMENT The work was supported by the Basic Science Research Program through National Research Foundation of Korea (NRF) by Ministry of Education, Science and Technology (2016R1D1A1B03934226 and 2016R1A2A2A07005160). We are thankful to CURF, Chonbuk National University, South Korea for EDX mapping, TEM, NMR, and Raman spectroscopic analysis.


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Page 36 of 36

For Table of Contents Use Only

Trimetallic

colloidal

monodisperse

nanodendrites were synthesized and used as plasmon enhanced light induced highly efficient

system

for

carbon

dioxide

reduction.


36

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