Plasmonic CoO-Decorated Au Nanorods for Photoelectrocatalytic

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Plasmonic CoO-Decorated Au Nanorods for Photoelectrocatalytic Water Oxidation Piue Ghosh, Ashish Kar, Shikha Khandelwal, Divya Vyas, Ab Qayoom Mir, Arup Lal Chakraborty, Ravi S Hegde, Sudhanshu Sharma, Arnab Dutta, and Saumyakanti Khatua ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.9b01258 • Publication Date (Web): 19 Aug 2019 Downloaded from pubs.acs.org on August 19, 2019

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ACS Applied Nano Materials

Plasmonic CoO-Decorated Au Nanorods for Photoelectrocatalytic Water Oxidation Piue Ghosh⁋, Ashish Kar†, Shikha Khandelwal†, Divya Vyas†, Ab Qayoom Mir†, Arup Lal Chakrabortya⁋, Ravi S Hegde⁋, Sudhanshu Sharma†, Arnab Dutta†*, and Saumyakanti Khatua†* † ⁋

Discipline of Chemistry, Indian Institute of Technology Gandhinagar, Gujarat, India

Discipline of Electrical Engineering, Indian Institute of Technology Gandhinagar, Gujarat, India

KEYWORDS. Photocatalysis, plasmonic catalysis, gold nanorod, plasmonic antenna-reactor, photocatalytic water oxidation

Abstract

Harvesting the full bandwidth of the solar spectrum, especially the near infrared portion, remains a challenge for solar-to-fuel conversion technology. Plasmonic nanostructures have recently attracted attention in this connection due to their enhanced yet tunable broadband absorption and photochemical stability. Here we report a nanoplasmonic photocatalytic construct by decorating plasmonic Au Nanorods with CoO for harvesting visible and NIR light via photo-electrochemical water oxidation reaction (WOR). In contrast to previous reports of plasmonic photocatalyst constructs, our structure does not require complicated fabrication or rely on rare-earth heavy-atom elements and exhibits excellent photostability without leaching of either cobalt or gold into the

ACS Paragon Plus Environment

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reaction solution under photoelectrochemical conditions. This catalytic construct triggered photoelectrochemical WOR with generation of significant photocurrent (~ 100 µAcm-2) while producing photogenerated oxygen at 18.1 mmoles h-1 of and hydrogen at 40.2 mmoles h-1 (on counter electrode) per milligram of cobalt under broadband excitation of 410-1700 nm with photon to oxygen conversion efficiency of ~0.05% in neutral aqueous conditions. The broadband photocatalytic activity of CoO-decorated Au Nanorod was attributed to the hot holes generated by the photoexcitation of plasmonic gold nanorods.

Introduction Efficient transformation of solar radiation to sustainable chemical feedstocks is considered to be one of the most viable approaches towards the development of a successful global renewable energy infrastructure. Artificial photosynthetic systems mimicking the architectural blueprint of natural photosystem leads the way for the appropriate implementation of this strategy 1–5. WOR is a key step in the solar light harvest to chemical fuels, however the thermodynamic and kinetic barriers pose serious questions on its practical implementaion due to the multiple electron/proton exchange involved in the reaction (Equation 1). 𝟐𝑯𝟐 𝑶 → 𝟒𝑯+ + 𝟒𝒆− + 𝑶𝟐 , ∆𝑮 = 𝟏𝟏𝟑 𝒌𝒄𝒂𝒍/ 𝒎𝒐𝒍.6 Here, the presence of an interactive chromophore-catalytic assembly is vital as the water oxidation executed by the photo-induced charge-separated holes initiates the light to chemical energy conversion process7. Both organic dyes and semiconductor based materials have been widely employed for the role of photosensitizer albeit with limited success due to the narrow solar energy spectrum harvest and low energy efficiency8–12. Plasmonic nano-materials have emerged as potential alternative light-harvesting materials owing to their the strongly enhanced yet tunable aborbance profile spanning the entire visible and NIR wavelengths13,14. Such improved and widely spread absorbance properties stem from the resonances of the collective oscillation modes of their


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conduction electrons, known as surface plasmon resonance(SPR)

15,16

. These resonances lead to

enhanced light absorption in an area much larger than the actual physical cross-section of the nanoparticle17. Photoexcitation of plasmonic nanomaterials leads to the formation of energetic charged carriers, often referred as hot electrons and holes18,19. These energetic carriers can induce and/or accelerate chemical reactions at nanoparticle’s surface albeit rather low efficiencies due to the occurence of rapid electron-hole recombination18,20–25. In an alternate approach, these hot carriers can be coupled with a nearby catalytic unit in an antenna-reactor construct to boost the photocatalytic activity of the catalyst. Several such antenna- reactors have been developed to drive variety of chemical reactions. Halas and co-workers have combined various plasmonic nanoparticles (Al and Ag) with non-plasmonic catalytic (Pt and Pd) nanoparticles for light-driven hydrogen dissociation and CO oxidation26,27. Recently, they also combined plasmonic Alnanoparticles with a semiconductor Cu2O catalyst for photocatalytic CO2 conversion with high specificity28. Linic et al. have used Pt-coated Ag-nanobars for photocatalytic CO oxidation reaction29. The research group led by Li have studied Au-nanoparticle -TiO2 construct for photocatalytic water oxidation and demonstrated that hot holes at the Au-TiO2 inteface drives the chemical reaction30. Gold nanoparticles on TiO2 nanowires were used by Wei and co-workers for visible light-driven hydrogen production31. Liu and co-workers have used combinations of gold nanoparticles and CdS nanospheres or ZnO/CdS nanorube arrays or Fe2O3 nanotubes arrays for photocatalytic hudrogen evolution, water splitting and methanol oxidation reactions 32–34. Wang et al. have combined plasmonic nanospheres with 2D materials in a Au/XS2/Au (X=Mo, Re) construct for hydrogen evolution reaction in the presence of hole quenchers for better utilization of the hot electrons35. Amirav and co-workers have shown that Au nanoprism antenna can enhance bi-exciton generation in a neighboring semiconductor nanoparticle to promote photocatalyzed


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two-electron reactions36. Song et al. have developed Au-CdS double shell hollow nanocubes for photocatalytic hydrogen generation reactions37. Very recently, Chen and co-workers have shown photocatalytic methanol oxidation with a Pd-Ag alloy nanotube hybrid nanostructures38. A hierarchical plasmonic Pt-TiO2 nanomaterial was designed by Tan and coworkers to induce visible light-driven water splitting39. The above mentioned plasmonic antenna-reactors present significant success in utilizing only the UV and visible light that constitutes only 50% of the solar irradiation. However, limited success has been achieved in the proper exploitation of the near-infrared (NIR) part of sunlight, which shares the major portion of the solar energy reaching earth’s surface. Moskovits et al. used a dense array of vertically oriented Au-nanorods containing Pt-nanoparticles on top (separated by a thin layer of TiO2) and Co-OEC catalyst on sides40. Significant photocurrent beyond 750 nm demonstated its NIR activity. However, the requirement of expensive lithographic fabrication method restricts its adaptation to large-scale upgradation. Feldman and his group recently developed unique nanopeapod (NPP) structure by assembling Au-Niobium(Nb) core-shell nanoparticle inside a HxK1−xNbO3 nanoscrolls41. This NPP assembly exhibited plasmon induced water splitting under broadband solar stimulation, including the NIR range. The uniform distribution of the Au-Nb nanoparticles inside the nano-scrolls is crucial for the water photoelectrolysis. However, the requirement of maintaining this intricate NPP structure can pose a barrier for long-term industrial scale usage. Analogous NIR light activated water splitting via Au-layer based local surface plasmon resonance (LSPR) excitation was also observed for the multilayered AuNR-TiO2-cobalt phosphate heterostructure developed by Nam and co-workers.42 However, this heterostructure required multiple layers (more than 15) of AuNR and TiO2 nanoparticles for exhibiting optimized photocatalysis, where the presence of the toxic TiO2 was


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essential for it reactivity.43,44 Thus, there is an immediate requirement for developing non-toxic, NIR-active photoanodic materials with appreciable water oxidation capabilities. Here, we demonstrate a robust plasmonic antenna-reactor construct, CoO-decorated Au nanorod by combining an earth-abundant 1st row transition metal oxide with a plasmonic gold nanorod antenna with its surface plasmon resonance tuned to the near-infrared wavelengths (Scheme 1) for driving photoelectrocatalytic water oxidation reaction (WOR) under broadband radiation. We observe that this plasmon-transition metal-oxide dyad performs photogenerated hot-hole driven WOR even at neutral aqueous conditions (pH 7), showcased by the generation of notable photocurrent densities of 33 µA cm-2 for 850-1700 nm excitations, 65 µA cm-2 for 610-850 nm, and 110 µA cm-2 for 410-650 nm excitation bands, highlighting its broadband light-harvesting capability. This catalyst construct produced 18.1 𝑚𝑚𝑜𝑙𝑒𝑠 ℎ−1 photogenerated oxygen and 40.2 𝑚𝑚𝑜𝑙𝑒𝑠 ℎ−1 hydrogen per miligram of cobalt under broadband excitation of 410-1700 nm with photon to oxygen conversion efficiency of 0.05%. Moreover, it displays appreciable stability under the photocatalytic conditions as supported by the consistent photocurrent generation under irradiation over 3000 s with minimal leaching of either cobalt or gold in the reaction solution. Hence, a straightforward wet-chemical preparatory method, use of earth-abundant metal-based catalyst, excellent photochemical stability, and appreciable photocatalytic activity over visible and NIR wavelengths highlights the potential of CoO-decorated Au Nanorods for large-scale application in the renewable energy sector.


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Results and discussions CoO-decorated Au nanorods were prepared by depositing cobalt oxide on already synthesized gold nanorods (AuNRs). AuNRs were synthesized by following a seed-growth method reported elsewhere

45,46

. The average dimensions of nanorods were found to be 82 nm × 13 nm (Fig S1)

with longitudinal plasmon resonance (LSPR) at 1015 nm in water. The cobalt oxide layer was deposited on gold nanorods via in situ reduction of a cobalt precursor (H2CoCl4) with a reducing agent (Hydroquinone). Cobalt deposition was monitored via the extinction coefficient of gold nanorods, which showed a gradual time-dependent blue shift till 56 nm. No significant broadening of the spectral shape was observed (Fig S2), which indicated that the nanorods preserved their shapes. Indeed, the transmission electron microscope (TEM) image (Fig S1) confirmed that the overall nanorod geometry remains unchanged during cobalt deposition. Note that the LSPR did not exhibit any significant shift when either the cobalt precursor or the reducing agent was absent (Fig S2).

Scheme 1: Schematic description of different steps involved in preparation of photocatalytic substrate.

The presence of cobalt on gold nanorods was confirmed from elemental mapping with a scanning transmission electron microscope-coupled with energy dispersive X-ray analysis (STEM-EDS) as well as XPS studies. Figs. 1(a,b) show STEM-EDS mapping of Cobalt and gold on a nanorod (shown in the inset of fig. 1a) confirming that Cobalt is present on the gold nanorod. The XPS spectrum displayed two major signals at 779.7 eV and 795.7 eV that can be attributed to the 2p3/2


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and 2p1/2 main transitions originated from Co (II) center present in CoO (Figure 1(c), right panel). 47,48

Two distinctive satellite peaks at 784.6 eV (2p3/2) and 801.4 eV (2p1/2), also corroborated the

assignment of the oxidation state of the Cobalt. The presence of the gold in the catalytic construct was also supported by the characteristics Au 4f5/2 and 4f7/2 signals observed at 85.6 eV and 81.9 eV, respectively.49,50 Further studies with energy-dispersive X-ray spectroscopy (EDS) revealed that the amount of cobalt varies from 4-12% (w/w, Fig S3). Crystalline nature of gold nanorods were also confirmed from X-ray diffraction studies (Figure S4).

Figure 1: (a,b) Elemental mapping of Co, and Au on a CoO-decorated Au nanorod. TEM image of a nanorod is shown in the inset of (a). (c) XPS spectra of CoO-decorated Au Nanorod confirming presence of Au and CoO. (d) SEM image of CoO-decorated Au Nanorod coated Indium Tin Oxide (ITO) substrate.

The CoO-decorated AuNR were deposited on an ITO-coated glass slide (SEM images shown in Fig. 1d and Fig. S5) and utilized as the working electrode in a standard three electrode system for further investigation on its electrochemical properties. A Pt wire and Ag/AgCl (in 3M KCl) were employed alongside as the counter and the reference electrode, respectively. A linear sweep


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voltammetry (LSV) experiment was performed with this CoO-decorated AuNR on ITO working electrode under neutral aqueous condition (buffered at pH 7.0). The current response observed for CoO-decorated AuNR in the LSV curve shows a steep rise at 1.1 V following the stoichiometric signal (Fig. 2 (a)). This sharp change in current possibly signifies the onset of catalytic process on the working electrode. Previous studies have demonstrated that at sufficiently anodic potential Co(III) species can be oxidized to Co(IV) and this species in turn reacts with water molecule to produce O2.42 A chronocoulometry (also known as bulk electrolysis) experiment was performed with the same CoO-decorated AuNR on ITO working electrode at 1.36 V to probe the catalytic process. In this experiment evolution of gas bubbles was observed on the working electrode over time along with the accumulation of substantial amount of positive charge signifying the oxidizing nature of this process (Fig. 2 (b)). The evolved gas in this experiment was collected from the head space of a closely packed electrochemical cell and analyzed as O2 via gas chromatography (GC) ( subset of Fig. 2 (d)) Thus, the catalytic process observed during LSV is assigned as the water oxidation process. The catalytic wave commenced at 1.1 V and this potential is considered as the onset potential for the water oxidation process by the sample at pH 7.0. The overpotential requirement (η) (i.e. the energy required to initiate certain chemical reaction beyond its thermodynamic potential) for this process was calculated as the difference between the onset potential and the thermodynamic potential for O2/H2O reaction at pH 7.0 (Equation 2, Fig. 1 (a)). 𝜼 (𝑶𝒗𝒆𝒓𝒑𝒐𝒕𝒆𝒏𝒕𝒊𝒂𝒍) = 𝒐𝒏𝒔𝒆𝒕 𝒑𝒐𝒕𝒆𝒏𝒕𝒊𝒂𝒍 (𝑽) − 𝑬𝑯𝟐𝑶/𝑶𝟐 (𝑽) (𝟐) Only 283 mV of overpotential requirement was measured for water oxidation electrocatalysis by the CoO-decorated AuNR hybrid system in neutral aqueous solution. The LSV data recorded


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on five different working electrode showed similar behaviour with an averaged onset overpotential of 295 ± 16 mV (a few examples are shown in Fig S6). This data clearly establishes this catalytic assembly to be one of the most energy efficient heterogeneous electrocatalytic systems to date, as the other analogous contemporary catalytic constructs require > 400 mV overpotential for water oxidation41,42.

Figure 2: (a) Linear sweep voltammograms (LSV) recorded on ITO substrate (dotted magenta line), AuNRs deposited on ITO (dotted green line), and CoO-decorated Au nanorods deposited on ITO (solid red line) at 10 mV/s of scan rate in pH 7.0 aqueous solution (containing 0.1 M Na2SO4 as electrolyte, reference electrode: Ag/AgCl (in saturated KCl), counter electrode: Pt wire). (b) LSV on CoO-decorated Au nanorods deposited on ITO in dark (black line) and under light. Inset shows a zoom-in version of LSV curve between 0.65 V and 1.15 V. Excitation light intensity was 400 mWcm-2. (c) Charge accumulation during chronoamperometry at 1.36 V at pH 7 under dark condition (black line) and under illumination with visible light (red line). Excitation intensity was 400 mWcm-2 (d) Time course of H2 and O2 production from water


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at 1.36 V in dark (black, Oxygen: circles, Hydrogen: square) and in the presence of 400 mW/cm2 excitation (red, Oxygen: circles, Hydrogen: square). Inset shows a typical gas chromatogram for the sample gas collected at the head space showing the presence of both oxygen and hydrogen.

Continuous formation of hydrogen gas was also observed at the Pt-counter electrode during the bulk electrolysis (inset of Fig. 2d). This observation completes the full picture of electrocatalytic water splitting by CoO-decorated AuNR under neutral pH. The LSV data was also recorded for blank ITO and for AuNR-deposited ITO as control samples under analogous conditions (pH 7.0). Two distinct changes were noticed for these control samples compared to the CoO-decorated AuNR on ITO system (Fig. 2a). First, the Co-based stoichiometric signals were distinctly missing in both the samples. Second, the water oxidation catalytic signal was significantly diminished for the control samples whereas it is observed only at very high anodic potentials (> 1.4 V). This data clearly indicates that the CoO is playing the most crucial catalytic role during the water oxidation by the CoO-decorated AuNR hybrid. The water oxidation property of CoO-decorated AuNR system was also probed by monitoring the advent of the photocurrent in a photo-electrochemical set up containing an identical threeelectrode system. The LSV experiment was performed with this arrangement to monitor the response current while the CoO-decorated AuNR on ITO working electrode was illuminated with a broadband light source. An increase in the oxidation current response (i.e. photocurrent) was noticed in presence of light showcasing the photoelectrocatalytic activity of the CoO-decorated AuNR assembly (Fig. 2b and inset). The generation of photocurrent is also consistent with the increase in overall charge accumulation in chronocoulometry experiment (Fig. 2c). The evolution of photocurrent correlates well with the increased production of hydrogen and oxygen. Fig. 2d shows the amount of hydrogen produced as function time in dark (black circles)


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and under light excitation (red circles). In both the scenarios, amounts of hydrogen and oxygen increase linearly with time while maintaining a stoichiometric ratio between hydrogen and oxygen. The slopes, however, are significantly steeper in the presence of light yielding ~ 90 µL (~ 4.02 µmoles) more hydrogen and 40.5 µL (~ 1.81 µmoles) oxygen in 1-hour illumination (given that headspace volume is 15 mL in the experimental set up). This leads to an estimated oxygen and hydrogen production rate of 18.1 mmoles and 40.2 mmoles h-1 mg-1 of cobalt under broadband light illumination with incident photon to hydrogen conversion efficiency of ~0.12% at the given bias of 1.36 V (overpotential, η = 0.53 V) (see supporting information for detailed calculation). The

photocurrent

response

of

CoO-decorated

AuNR

was

further

studied

via

chronoamperometric experiments at different applied overpotentials (ηOP), which was calculated from the difference between applied potential (ηap) and the thermodynamic potential requirement (ηth) according to equation (3). 𝜼𝑶𝑷 = 𝜼𝒂𝒑 − 𝜼𝒕𝒉 , 𝜼𝒕𝒉 = [𝟏. 𝟐𝟑 − (𝒑𝑯 × 𝟎. 𝟎𝟓𝟗)]𝑽 … … … (𝟑). The CoO-decorated AuNR working electrode was kept in the dark at a particular applied potential for 60 s to attain a stable current response. The electrode was then illuminated by light that was periodically switched on and off with an on-time of 10s followed by an off-time of 60s. The magnitude and response patterns of the photocurrent over time varied significantly with the applied potential. Initially, with an applied overpotential of 0.13 V, only stoichiometric cobalt oxidation was noticed (Fig. 3a). As shown in Fig. 3(a), a sharp rise and fall in the photocurrent, of magnitude of approximately 4 μAcm-2, was observed in the illumination period at this potential. We note that characteristics overshooting of photocurrent was observable during both the initiation and termination of the illumination, as reported earlier40,51. The photocurrent response improved by ~10 times when the applied potential (ηOP = 0.33V) crosses the thermodynamic barrier (Fig. 3a). The magnitude of photocurrent continued to increase as the applied overpotential was moved


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towards more anodic direction and it reached to 1.5 mA/cm2 when the ηOP = 0.63V. The appearance of the photocurrent traces over time were similar when appreciable overpotential (ηOP>0.13 V) was applied (Fig. 3b). At this condition, the photocurrent attained a plateau rather slowly compared to the response observed for the stoichiometric condition (ηOP = 0.13 V). Analogously, the decay of the photocurrent response to the background (dark) level occurred over ~50 s at higher applied overpotential values (ηOP>0.13 V) compared to the rapid equilibration of the current response at ηOP = 0.13 V (Fig. 3b).


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Figure 3: (a) Chronoamperometry data recorded on CoO-decorated AuNR on ITO working electrode at different applied potential (ηOP) under repeated light on-off cycles. Light-on time was 10 seconds and light-off time was 60 seconds. Excitation power was 400 mW/cm2. (b) Normalized photocurrent response for 1 light-on-off cycle. (c) Chronoamperometry data recorded on CoO-decorated AuNR on ITO working electrode at 0.53 V under light on-off cycle at different excitation wavelength bands. Light-on time was 10 seconds and light-off time was 60 seconds. Excitation power was kept at 150 mW/cm2 in all excitation bands. (d) Incident photon to current conversion efficiency (IPCE) for different excitation bands (blue: 410-650 nm, red: 610-850 nm, yellow: 850-1700 nm) at 0.53V applied overpotential. Extinction spectra (black line) of the CoO-decorated gold nanorods on ITO. (e) A schematic diagram describing the mechanism of photoelectrocatalytic water oxidation.

Photocatalytic water splitting can be explained qualitatively with the scheme illustrated in Fig. 3d. Photoexcited gold nanorod cores create energetic carriers, often referred to hot electron and holes21,52. The hot electrons transiently occupy the empty states above the Fermi level before decaying via radiative or thermal processes52. A fraction of such electrons can enter the ITO and eventually reach the counter Pt electrode via the external circuit. This process leaves the hot holes in gold nanorods, making them transiently electron deficient. Electrons from the CoO layer enter the gold nanorods and quench the photogenerated hot holes. This process facilitates the formation of higher oxidation states of cobalt, which drives the water oxidation process (Fig. 3d). Hot-hole facilitated CoII to COIII formation is supported by the increased current and the noticeable cathodic shift of CoII/CoIII onset potential (Fig. 2c, inset) under illumination. Such observations were also reported in a recent report by Kim et al

42

. With further increase in applied overpotential the

magnitude of the photocurrent increases significantly as more hot-holes participate in the water oxidation process. We note that the rate of charge carrier generation and their transfer is much faster than their consumption in water oxidation process. This leads to a non-equilibrium condition between charge carrier generation and utilization, which is reflected in the slow rise of photocurrent 41. The involvement of hot carriers is also consistent with the linear dependence of photocurrent on the excitation light intensity (Fig.S8). The linear dependence of photocurrent with excitation laser power also indicates that photo-thermal heating is not a significant component


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during this energy transduction process. The photocurrent increases at higher applied potential as larger number of hot holes can participate in the catalysis process. At a given excitation frequency (𝜔), the hot holes energies are distributed between Fermi Level and ℏ𝜔 and this distribution can be modelled as a triangular function with highest population near the Fermi Level and gradually decreases to zero away from the Fermi Level (Figure S9)18,24. The efficiency of hot-hole induced oxidation (observed photocurrent) depends on the overlap between the HOMO of Co(III) and the triangular hot-hole energy distribution function. With the increase of positive potential, the Fermi Level goes down causing a larger overlap between hot hole energy distribution function and the enhancing the efficiency of hot hole-induced oxidation process resulting in larger photocurrent density (Fig. S9).

The photoelectrocatalytic activity of the CoO-decorated AuNR catalyst was also monitored at different excitation wavelength bands (410< λ 98%) was purchased from TCI America. Hydrogen tetrachloroaurate(III) trihydrate (HAuCl4.H2O, ≥99.995%), Silver nitrate (AgNO3, >99%), sodium borohydride (NaBH4, >98%) were purchased from Sigma Aldrich. Hydroquinone (99%) was procured from Alfa Aesar. Hydrochloric acid (HCl, 35%) and nitric acid (HNO3, 63%) were procured from Rankem. Cobalt (II) chloride hexahydrate (CoCl 2.6H2O, 98.0 %), were procured from SD-fine chemicals. All the chemicals were used without any further purification. Milli-Q (resistivity 18.1 MΩcm) water at 25oC was used in all experiments. All glassware was thoroughly cleaned with aqua regia before their use. [CAUTION: aqua regia is a hazardous material and it has to be handle with appropriate safety precautions]. B. Synthesis of gold nanorods: (i) Synthesis of seeds: 5 mL of 1 mM of HAuCl4 was mixed thoroughly with 5 mL of 0.2 M CTAB solution at 28°C. Then 450 µl of a freshly prepared 0.1 M NaBH4 was added to this solution under continuous stirring. The solution became dark brown. This color change indicates the formation of the seed particles. The seed solution was kept undisturbed at 28°C for 30 minutes before performing growth reaction. (ii) Synthesis of gold nanorods: Growth solution was prepared by adding 25μl of 100 mM AgNO3 in 2.5 ml of 0.2 M


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CTAB under slow stirring. The solution was left undisturbed at 28°C for 15 minutes. Subsequently, 2.5 ml of 1 mM HAuCl4 and 250 μl of 0.1M hydroquinone was added under continuous stirring. A small amount of HCl was added to adjust the pH of the growth solution to be 2.5. Finally, 50 μl seed solution was added to the growth solution and was left overnight. After overnight growth, the nanorods were centrifuged and dispersed again in 200 μM CTAB solution. The nanorods were then characterized via UV-VIS spectrometer (Analytik Jena, specord 210 plus) and scanning electron microscopy (SEM, JEOL JSM7600F). C. Synthesis of CoO-decorated Au Nanorods: Cobalt oxide was deposited on already synthesized gold nanorods via in situ reduction of a cobalt precursor with a reducing agent. H2CoCl4 was used as a metal precursor. The cobalt precursor, H2CoCl4 of 500 mM was prepared in 1 M HCl using chloride hexahydrate salt. A volume of 60 µL of 500 mM H2CoCl4 and 80 µL of 500 mM of hydroquinone were added in 1.5 mL of 0.2 M CTAB. An appropriate amount of AuNRs (optical density was kept at 0.5) were added to this solution. The final concentration of CTAB and cobalt ion was kept at 0.1 M and 10 mM respectively by adding an adequate amount of Milli-Q water. The final solution was undisturbed at 28oC. CoO-decorated AuNRs for 24 hours, were then characterized by a UV-VIS spectrometer (Analytik Jena, specord 210 plus), SEM, XPS, and STEM-EDS. D. Preparation of CoO-decorated AuNR on ITO electrodes: CoO-decorated AuNRs were deposited on an ITO coated glass substrate via drop casting from a dilute solution in water. The sample was then washed with water to remove additional CTAB. SEM and UV visible spectroscopy were used to confirm CoO-decorated AuNR deposition on ITO. E. Electrochemistry set up: Electrochemical studies were performed using conventional three electrode system at room temperature. A platinum wire was used as a counter electrode, Ag/AgCl


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with 3 M KCl (aqueous) was used as a reference electrode, and CoO-decorated AuNRs ITO substrate is used as a working electrode. All potentials are converted to the standard hydrogen electrode (SHE) by calibrating with K4Fe(CN)6. Phosphate buffer was used to maintain pH 7. F. Photoelectrochemistry set up: A white light supercontinuum laser was used as light source. Appropriate filters were used to select excitation wavelength bands. Laser output was chopped using a mechanical chopper. G. ICP-OES: For the inductively coupled plasma optical emission spectrometry (ICP-OES) experiment, the ITO-bound CoO-decorated Au nanorod was dipped into a freshly prepared 5 mL aqua-regia and kept at 50 ºC for 30 minutes with continuous stirring to allow complete digestion of the deposited cobalt and gold. Later this sample was further diluted 50 times with milli-Q water. Finally, 1 ml of that diluted sample was added to 9 ml of 2% HNO3 solution for the ICP-OES measurement (Model Nexion 2000B ICP-MS). H. XPS: The AXIS ULTRA instrument with multi-technique X-ray Photoelectron Spectroscopy with XPS-mapping capability, was utilized for the XPS data collection of ITO-bound CoOdecorated Au nanorod samples (before and after electrocatalysis).

ASSOCIATED CONTENT Supporting Information. Figures S1 to S15 contain additional experimental details, sample characterization, excitation power dependent photocurrent measurement, and a schematic description of hot hole generations at different excitation energies. AUTHOR INFORMATION Corresponding Author


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*[email protected], [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Funding Sources DST Nanomission grant (SR/NM/NS-65/2016), SERB ExtraMural grant (EMR/2015/001013), Ramanujan Fellowship (SB/S2/RJN-112/2015) and Extramural research grant from DST-SERB (EMR/2015/002462). Notes Any additional relevant notes should be placed here. ACKNOWLEDGMENT RH, AD, and SK acknowledge financial support from DST Nanomission (SR/NM/NS-65/2016). SK acknowledges ExtraMural research support from SERB (EMR/2015/001013). AD acknowledges the Ramanujan Fellowship (SB/S2/RJN-112/2015) and Extramural research grant from DST-SERB (EMR/2015/002462). AD, RH, and SK also thank IIT Gandhinagar for the seed grant and excellence in research fellowship from IIT Gandhinagar. REFERENCES (1)

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Plasmonic CoO-Decorated Au Nanorods for Photoelectrocatalytic

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