High-Efficiency âWorking-in-Tandemâ Nitrogen Photofixation Achieved

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High-Efficiency “Working-in-Tandem” Nitrogen Photofixation Achieved by Assembling Plasmonic Gold Nanocrystals on Ultrathin Titania Nanosheets Jianhua Yang, Yanzhen Guo, Ruibin Jiang, Feng Qin, Han Zhang, Wenzheng Lu, Jianfang Wang, and Jimmy Chai Mei Yu J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b03537 • Publication Date (Web): 15 Jun 2018 Downloaded from http://pubs.acs.org on June 15, 2018

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Journal of the American Chemical Society

High-Efficiency “Working-in-Tandem” Nitrogen Photofixation Achieved by Assembling Plasmonic Gold Nanocrystals on Ultrathin Titania Nanosheets Jianhua Yang,† Yanzhen Guo,† Ruibin Jiang,*,‡ Feng Qin,† Han Zhang,† Wenzheng Lu,† Jianfang Wang,*,† and Jimmy C. Yu§ †

Department of Physics, The Chinese University of Hong Kong, Shatin, Hong Kong SAR, China

‡

Shaanxi Key Laboratory for Advanced Energy Devices, Shaanxi Engineering Lab for Advanced Energy Technology, School of Materials Science and Engineering, Shaanxi Normal University, Xi’an 710119, China

§

Department of Chemistry, The Chinese University of Hong Kong, Shatin, Hong Kong SAR, China

ABSTRACT: The fixation of atmospheric N2 to NH3 is one of the most essential processes for sustaining life. One grand challenge is to develop efficient catalysts to photofix N2 under ambient conditions. Herein we report on an all-inorganic catalyst, Au nanocrystals anchored on ultrathin TiO2 nanosheets with oxygen vacancies. It can accomplish photodriven N2 fixation in the “working-in-tandem” pathway at room temperature and atmospheric pressure. The oxygen vacancies on the TiO2 nanosheets chemisorb and activate N2 molecules, which are subsequently reduced to NH3 by hot electrons generated from plasmon excitation of the Au nanocrystals. The apparent quantum efficiency of 0.82% at 550 nm for the conversion of incident photons to NH3 is higher than those reported so far. Optimizing the absorption across the overall visible range with the mixture of Au nanospheres and nanorods further enhances the N2 photofixation rate by 66.2% in comparison with Au nanospheres solely. This work offers a new approach for the rational design of efficient catalysts towards sustainable N2 fixation through a less energy-demanding photochemical process compared to the industrial Haber-Bosch process.

INTRODUCTION Nitrogen is an essential element for all living organisms to build various units from amino acids and peptides to functional proteins.1 Although molecular dinitrogen is the major component of the atmosphere on the Earth (~78 vol%), it is nutritionally unavailable to most organisms because the cleavage of the N≡N bond has a very large activation barrier (941 kJ/mol).2,3 Industrially, the fixation of dinitrogen is accomplished through the wellestablished Haber-Bosch process, a dissociative reaction involving the co-activation of N2 and H2 on iron-based catalysts at high temperatures (over 300 °C) and pressures (over 100 atm). The energy required to maintain the harsh conditions for this reaction is largely derived from aerobic combustion of fossil fuels, consuming ~1.4% of the world’s annual energy supply.2 In addition, the major source of H2 used for this reaction is the steam reforming of natural gas accompanied with the emission of appreciable quantities of CO2. From the consideration of reducing the reliance on fossil fuels as the energy source and the emission of greenhouse gases, N2 fixation through an energyefficient and environmentally friendly process is highly desirable.4 More than one half of the world’s nitrogen fixation is carried out in nature by nitrogenase enzymes, which mainly consist of two metalloprotein components named

the activating MoFe-protein and the catalytic Fe-protein.5 Although the detailed mechanism of N2 fixation by the nitrogenase has still remained controversial, convincing evidences have shown that the active Fe-Mo-S cofactor at the core of the MoFe-protein first binds and activates N2 molecules through electron back-donation and that the Fe-protein hydrolyzes MgATP to provide the required energy and electrons for the reduction of activated N2 to ammonia.5,6 Such a “working-in-tandem” mechanism between the MoFe-protein and the Fe-protein rationalizes why the biological N2 fixation proceeds with a lower activation barrier (less uphill) under ambient conditions. However, the operation of the nitrogenase systems in vitro usually requires strong reducing agents, massive organic solvents and often extremely low temperatures, which makes it experimentally difficult to realize.7–9 Despite this difficulty, the concerted nature of the biological N2 fixation process does offer an inspiration that efficient N2 fixation might be achieved by mimicking the biological working mechanism, where both activating sites and catalytic centers are simultaneously equipped and well synergized. The rate-determining step of the N2 fixation reaction lies in the cleavage of the N≡N bond.10 Intriguingly, transition metal (Mo, Fe, Ru, W) complexes can weaken the N≡N bond and hence activate N2 toward further reactions

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by increasing its LUMO (lowest unoccupied molecular orbital) electron density due to the back donation of the metal dp electrons and by depleting its HOMO (highest occupied molecular orbital) electron density upon strong coordination.3,11 Trivalent titanium (Ti3+), another possible type of transition metal cations for activation, has been reported to be able to activate N2 through electron donation and to form a Ti4+–azo complex with the N=N bond,10,12 which can be cleaved by strong reducing agents such as Li and Mg to produce a Ti4+–hydrazo complex.13 Further reduction of the Ti4+–hydrazo complex produces NH3 with the regeneration of Ti3+. Ti3+ is extensively present in TiO2 that contains oxygen vacancies (OVs).10 The high density of states (d-orbital) in the conduction band endows TiO2 with an excellent electron-accepting ability. This contrasts with other common metal oxides (ZnO, In2O3, SnO2), whose conduction bands are mainly composed of the s or sp orbital of the metal atoms.14,15 In addition, OVs with their localized electronic states typically lying below the conduction band can serve as trapping sites for photogenerated electrons to promote the separation of charge carriers as well as charge transfer to adsorbates.16 However, titania containing OVs is sluggish for N2 photofixation under visible light owing to the small transition probability of electrons from the valence band to the defect states induced by OVs.10,17,18 Localized surface plasmon resonance (LSPR), collective oscillations of free electrons in metal nanocrystals under resonant excitation, has recently received much attention. It can not only endow metal nanocrystals with large absorption cross-sections but also photo-generate highenergy electrons, namely, hot electrons.14,19,20 More interestingly, when a plasmonic metal (Au, Ag, Cu) nanocrystal is in contact with an n-type semiconductor, such as TiO2 with a wide bandgap, a Schottky barrier forms at the interface through the downward bending of the conduction band of the semiconductor so that the Fermi levels in the metal and the semiconductor are aligned.14,20 The Schottky barrier works as a filter allowing for energetic electrons to pass across the interface while inhibiting their reverse movement, therefore resulting in effective electron–hole separation at the interface.21,22 Such systems made of plasmonic metals and wide-bandgap semiconductors possess the advantages of low electron–hole recombination rates, high and stable light harvesting capabilities, and tailorable response wavelengths from the visible to near-infrared region. They are superior to conventional cocatalyst–wide-bandgap-semiconductor systems that are only active under high-energy ultraviolet light.23 The incident photon-to-electron conversion efficiency has been predicted theoretically to reach 26% for Au–TiO2 nanostructures, which makes plasmonic metal– semiconductor nanostructures promising candidates for the efficient utilization of solar energy to drive chemical reactions.14,23,24 Very recently, arrays of TiO2 nanorods that are deposited with Au nanoparticles and further encapsulated with an amorphous TiO2 layer25 and (BiO)2CO3 nanodisks deposited with Au nanoparticles26 have been reported to have the ability to fix N2 under the irradiation of simulated solar light. Both of the plasmonic Au nano-

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particles and the semiconductors are excited under the simulated solar light, which makes it difficult to uncover the plasmonic effect from the cocatalyst effect. In addition, there has still been much room to improve the efficiencies of the existent plasmonic N2 photofixation systems. In this work, using Au nanocrystal-decorated ultrathin TiO2 nanosheets containing abundant OVs (Au/TiO2OV), we demonstrate that hot electrons generated from plasmonic Au nanocrystals under visible light can efficiently reduce activated N2 to NH3 at ambient pressure and temperature. The TiO2-OV nanosheets not only work as a structural motif to support plasmonic Au nanocrystals and provide a large number of OVs on the surface, but also bring a large specific contact area, which increases substantially the probability of excited charge carriers to interact with the reaction solution.27,28 Hot electrons excited from the plasmonic Au nanocrystals overcome the Schottky barrier and inject into the conduction band of TiO2-OV. The injected hot electrons diffuse and get trapped at the defect states of the OVs. On the surface, the trapped hot electrons reduce OV-activated N2 into NH3. The OVs and Au nanocrystals work in tandem for the N2 photofixation, which is similar to the “working-intandem” mechanism in nitrogenase enzymes. The apparent quantum efficiency (AQE) of 0.82% achieved at 550 nm for the photo-driven conversion of N2 to NH3 is higher than those reported for other N2 photofixation systems at visible wavelengths. We further show an improvement of 66.2% in the N2 fixation rate by extending the light absorption with the mixture of Au nanospheres and Au nanorods across the entire visible range in comparison with the sole use of Au nanospheres. Although our N2 photofixation route is unlikely to replace the industrial HaberBosch process at present, this study represents a new paradigm for achieving efficient photo-driven N2 fixation into NH3 through a less energy-consuming photochemical process. EXPERIMENTAL SECTION Catalyst Preparation. Au nanospheres were prepared in aqueous solutions according to our previously reported seed-mediated growth method with slight modifications.29 Briefly, for the preparation of the seed solution, a HAuCl4 solution (0.01 M, 0.25 mL) was added into a cetyltrimethylammonium bromide (CTAB) solution (0.1 M, 9.75 mL), followed by the rapid injection of a freshlyprepared, ice-cold NaBH4 solution (0.01 M, 0.6 mL) under vigorous stirring. The resultant solution was placed in an oven at 30 °C and kept undisturbed for 4 h. Afterwards, 0.25 mL of the seed solution was rapidly injected into a growth solution made of CTAB (0.1 M, 9.75 mL), water (190 mL), HAuCl4 (0.01 M, 4 mL) and ascorbic acid (0.1 M, 15 mL). The reaction mixture was gently shaken for 30 s and then left undisturbed overnight at 30 °C. The resultant Au nanospheres were centrifuged and washed by water twice and finally redispersed into water for further use. Au nanorod samples were also synthesized using the seed-mediated method30,31 in a similar manner to the


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growth of the Au nanospheres. Two aliquots of the growth solution were both made of CTAB (0.1 M, 40 mL), HAuCl4 (0.01 M, 2 mL), ascorbic acid (0.1 M, 0.32 mL), AgNO3 (0.01 M, 0.4 mL) and HCl (1.0 M, 0.8 mL). Two aliquots (6 μL, 15 μL) of the same seed solution as used in the growth of the Au nanospheres were added in the growth solutions to prepare Au nanorod samples with longitudinal plasmon wavelengths of 615 nm and 697 nm, respectively. To prepare the ultrathin TiO2–OV nanosheets, titanium isopropoxide (0.853 g) was added slowly into a concentrated HCl solution (5 M, 0.780 g) under vigorous stirring. The light yellow solution was then drop-added into a solution prepared in advance by dissolving triblock copolymer (ethylene oxide)20-(propylene oxide)70-(ethylene oxide)20 (0.24 g) in absolute ethanol (3.2 g) with stirring. After being stirred for 45 min, 3.5 mL of the mixture solution together with ethylene glycol (30 mL) was transferred into an 80-mL Teflon-lined stainless autoclave and reacted at 145 °C for 20 h under the autogenous pressure. The resultant product was collected and washed with absolute ethanol four times to remove residual species. The final product was dried at room temperature in a vacuum oven and then calcined at 350 °C in Ar atmosphere for 6 h to produce ultrathin TiO2-OV nanosheets. The OV-free TiO2 nanosheets were prepared by calcining the TiO2-OV nanosheets in air at 350 °C in a muffle furnace for 12 h. To assemble the Au nanospheres onto the ultrathin TiO2-OV nanosheets, the TiO2-OV nanosheets (0.15 g) were added into an appropriate volume (typically 8.0 mL) of the Au nanosphere solution (particle concentration: 4.0 × 1012 particles·mL–1) under vigorous stirring. The mixture solution was then transferred into a beaker on a portable heating magnetic whisk (45 °C) under a continuous bubbling of Ar in order to evaporate water. The resultant red powder was collected and then redispersed into 10 mL water under vigorous stirring before sonication for 10 min. The well-dispersed mixture was centrifuged and washed twice with water to remove unattached Au nanospheres. The final precipitate was then transferred into a flask containing 20 mL of acetone to remove residual CTAB through refluxing at 75 °C in an oil bath for 10 h. The product was centrifuged, collected and washed with acetone. The refluxing treatment was repeated three times. The red product was collected and dried at room temperature in a vacuum oven. The obtained powder was calcinated in a tube furnace in Ar atmosphere at 350 °C for 6 h. The Au/TiO2 sample was synthesized in a similar way to the Au/TiO2-OV sample, except that the OV-free TiO2 nanosheets were used. The Au loading amounts on the TiO2-OV and OV-free TiO2 nanosheets were varied by changing the volume of the Au nanosphere solution. The loading of the Au nanocrystal mixture followed the same procedure, with the weight percentages of the Au nanospheres, 615-nm Au nanorods and 697-nm Au nanorods adjusted to be 5, 55 and 40 wt%, respectively. In the experiment to examine the H2 evolution, the asprepared Au/TiO2-OV sample (0.3 g, dispersed in 10.0 mL of water) was further loaded with Pt nanoparticles. H2PtCl6 solution (0.01 M, 0.77 mL) was injected into the

Au/TiO2-OV suspension. The resultant solution was irradiated under a Xe lamp (300 W) with Ar bubbling for 1 h. The resultant sample was collected by centrifugation and washed three times with water. The nominal weight ratio between Pt and Au/TiO2-OV is calculated to be 0.5% assuming that the Pt precursor is totally reduced. To prepare the Au@SiO2/TiO2-OV sample, (Au core)@(SiO2 shell) nanostructures were first prepared following a reported procedure with slight modifications.32 Briefly, 160 µL of (3-aminopropyl)trimethoxysilane solution (1 mM, 7.3 µL of the molecule dissolved in 40 mL of deionized water) and 1.28 mL of sodium silicate solution (diluted to 0.54 wt% with deionized water and acidified to pH = ~10.2 with HCl) were successively added into 4 mL of the Au nanosphere solution under vigorous stirring. The mixture was kept under stirring for further reaction for 20 h at room temperature. The product was centrifuged and washed with water twice for further use. The Au@SiO2/TiO2-OV sample was prepared according to the same procedure of Au/TiO2-OV, except the use of the Au@SiO2 nanostructures. Nitrogen Photofixation Reaction. The N2 photofixation reaction was performed in a home-made threenecked reactor with two ends opened and one end sealed by a rubber plug. The catalyst (100 mg), water (72 mL) and methanol (8 mL) were added into the reactor under vigorous stirring. The mixture was then sonicated for 15 min, followed by bubbling with a constant high-purity N2 flow (50 mL·min–1) at a pressure of 1 atm for 30 min to remove dissolved air in the solution. The mixture was irradiated afterwards with a Xe lamp (300 W). A 420-nm cut-off filter was placed in front of the lamp when needed to provide a visible light source. The N2 flow was carefully monitored with a gas flow meter. An aliquot of the reaction solution (3.0 mL) was taken out and transferred into a tube every 15 min. It was centrifuged to precipitate the photocatalyst. The concentration of ammonia in the supernatant was determined using the indophenol-blue method on an ultraviolet/visible/near-infrared spectrophotometer. Each photocatalytic N2 fixation experiment, including the control experiments, was repeated three times under the same conditions. The average amount of produced NH3 and the standard deviation were calculated. The action spectrum was obtained by performing N2 fixation under monochromatic light using appropriate bandpass filters at different wavelengths (420, 475, 520, 550, 600, 650, 700 nm). The power intensity of the monochromatic light was measured using an optical power meter (Molectron POWER MAX 5200). The full widths at half maximum of the monochromatic lights are all 20 nm. Detection of Ammonia Concentration. The indophenol-blue method was employed to determine the produced amount of ammonia. In the presence of sodium nitroprusside (Na2[Fe(CN)5NO]), NH4+ can react with salicylic acid (C6H4(OH)COOH) and hypochlorite ions (ClO–) to produce a blue complex (indophenol), which has an absorption peak at 697 nm. For the measurements, to eliminate the influence of possible organics, the supernatant was diluted to 10 mL using deionized water, fol-



lowed by the addition of H2SO4 (1.0 M, 0.4 mL). The acidic mixture was gently heated to evaporate the solvent until the remaining solution became 1.0 mL. NaOH solution (2.8 M) was then added to the remaining solution until the Congo red indicator paper was changed from blue to red. Subsequently, the remaining solution (0.6 mL), salicylic acid (1.0 M in 1.17 M NaOH solution, 0.4 mL), sodium nitroprusside (0.1 g in 10 mL water, 0.025 mL) and NaClO (available chlorine content 0.35 wt% in 0.7 M NaOH, 0.025 mL) were sequentially added into deionized water (1.9 mL) in a cuvette (capacity: 4.5 mL, optical path: 1 cm). The solutions were mixed uniformly and then left undisturbed at room temperature for 1.5 h to allow for the complete color development. The amount of the formed indophenol blue was determined by measuring the peak absorbance at 697 nm. To obtain the calibration curve for this colorimetric method, a standard NH4Cl solution (0.1 mM) was prepared in advance. Eight batches of the standard NH4Cl solution with volumes of 0, 0.1, 0.2, 0.4, 0.6, 0.8, 1.6 and 2.4 mL were added into eight cuvettes labelled as 0, 1, 2, 3, 4, 5, 6 and 7 with 2.5, 2.4, 2.3, 2.1, 1.9, 1.7, 0.9 and 0.1 mL of deionized water, respectively, while all of the other agents were kept the same in the measurements. The absorbance values at 697 nm were measured and plotted as a function of the ammonia concentration. Note that if the ammonia concentration in the reaction solution is too high, the solution can be diluted by certain times before the indophenol-blue test. Electrochemical Measurements. The Mott-Schottky plots and photocurrent measurements were conducted in a three-electrode, single-compartment quartz cell using an electrochemical workstation (CHI 760E). The working electrode was constructed by depositing the catalyst sample on a transparent fluorine-doped tin oxide glass slide (1 cm × 2 cm). Specifically, the catalyst sample was dispersed in ethanol to form a 10 mg mL–1 solution. 0.15 mL of the suspension was then dip-coated on the conductive surface of the glass slide with a deposition area of ~1 cm × 1 cm at 1.5 mg·cm–2. The prepared working electrode was allowed to dry in vacuum overnight at room temperature. A Pt plate and a standard Ag/AgCl electrode served as the counter and reference electrodes, respectively. All measurements were carried out at room temperature in Na2SO4 (0.1 M, 80 mL) that had been deoxygenated by bubbling high-purity Ar for 30 min. The Mott-Schottky measurements were conducted in the potential range from -1.5 to + 0.2 V (vs Ag/AgCl) at a potential step of 0.05 V and a frequency of 1.0 kHz under light. The light source used in the electrochemical measurements was a Xe lamp

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(300 W) with a 420-nm cut-off filter. The photocurrent was recorded as a function of time by chopping the light on and off at an interval of 40 s at the open-circuit potential. Density Functional Theory Calculations. The density functional theory (DFT) calculations were performed using the Vienna Ab initio Simulation Package (VASP) software. The generalized gradient approximation by Perdew, Burke and Ernzerhor33 was employed for the exchange-correlation energy. The ionic cores were described by projector-augmented wave potentials. The (010) facet was used to model the surface of the TiO2 nanosheets because the experiments showed that the main exposed surface of the TiO2 nanosheets is the {010} facets. The (010) facet was constructed using a (1 × 2) supercell with five atomic layers. An oxygen vacancy was created on the surface of the (1 × 2) supercell. The vertical separation between the successive slabs was set at 1.5 nm. All structural optimizations were done using a cut-off at 400 eV for the plane-wave basis set. The Brillouin zones were sampled with (3 × 5 × 1) Monkhorst-Pack grids.34 RESULTS AND DISCUSSION Catalyst Characterization. In nature, the nitrogenase is composed of two functional proteins, i.e., the MoFeprotein and the Fe-protein, which work in tandem to fix N2 under ambient conditions (Figure 1a). Enlightened by the working mechanism of the natural nitrogenase, we prepared our catalysts from Au nanospheres and ultrathin TiO2 nanosheets, with the latter possessing abundant OVs. The TiO2-OV nanosheets are similar to the MoFeprotein to provide active sites for N2 adsorption and activation. The Au nanospheres function like the Fe-protein to harvest visible light and create hot electrons for N2 reduction. Their sizes are relatively uniform with an average diameter of 20.0 ± 1.4 nm (Figure 1b). They exhibit a strong plasmon peak at 520 nm when dispersed in aqueous solutions (Figure 1c). Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) imaging shows that the as-prepared TiO2-OV nanosheets possess a sheet-like morphology (Figure 1d). The nearly transparent nature of the sheets under TEM imaging suggests that they are ultrathin. From atomic force microscopy (AFM) characterization (Figure S1), the average thickness of the nanosheets was found to be 4.3 ± 0.3 nm, which is approximately eleven times the lattice constant (0.38 nm) of anatase TiO2 along the [010] direction.


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Figure 1. Synthesis of the Au/TiO2-OV catalyst. (a) Schematic illustrating the “working-in-tandem” mechanism of the nitrogenase with the Fe-protein and MoFe-protein in biological N2 fixation. (b) TEM image of the Au nanospheres. (c) Extinction spectrum of the Au nanospheres in water. (d) SEM (left) and TEM (right) images of the TiO2-OV nanosheets. (e) SEM (left) and TEM (right) images of the Au/TiO2-OV sample. (f) Absorption spectra of the Au/TiO2-OV, Au/TiO2, TiO2-OV and TiO2 samples. (g) EPR spectra of the Au/TiO2-OV, Au/TiO2, TiO2-OV and TiO2 samples.

Because the Au nanospheres and the TiO2-OV nanosheets are oppositely charged, with their respective zeta potentials being +33.2 and -32.2 mV (Figure S2), they can spontaneously assemble together through electrostatic attraction by refluxing them in acetone.35,36 The assembly results in an intimate contact between Au and TiO2OV, producing the Au/TiO2-OV nanostructures. Except being specifically mentioned, the actually measured content of Au in Au/TiO2-OV in our study is mostly 1.5 wt% relative to 100 wt% of TiO2-OV. The Au nanospheres are distributed on the TiO2-OV nanosheets relatively uniformly without aggregation, as revealed by SEM/TEM imaging (Figure 1e), high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) imaging and elemental mapping (Figure S3a). Highresolution TEM (HRTEM) imaging shows the clear (111) lattice fringes of Au, the (103) and (004) lattice fringes of anatase TiO2 (Figure S3b). The observed lattice fringes of anatase TiO2 indicate that the largely exposed surfaces of the TiO2-OV nanosheets are the (010) facets. For comparison, we also synthesized the Au/TiO2, TiO2-OV and TiO2 samples from the Au nanospheres and the TiO2 nanosheets. X-ray diffraction (XRD) patterns indicate that

the Au/TiO2-OV sample contains the cubic Au phase (JCPDS #: 01-1172) and the anatase TiO2 phase (JCPDS #: 04-0477) (Figure S4), which is consistent with the HRTEM result. The synthesized samples possess large specific surface areas arising from the ultrathin nature of the nanosheets, with that of the Au/TiO2-OV sample being 288 m2 g–1 (Figure S5 and Table S1). The successful assembly of the Au nanospheres on the TiO2-OV nanosheets was confirmed by the absorption spectra (Figure 1f). The OV-free TiO2 sample exhibits a sharp absorption edge at 375 nm in the ultraviolet region. Upon the introduction of the Au nanospheres, Au-TiO2 shows a broad LSPR band at 550 nm with a clear redshift in comparison with the LSPR band of the Au nanospheres in aqueous solutions. The redshift is caused by the larger refractive index of TiO2 compared with water. Intriguingly, the TiO2-OV sample displays a decaying tail over the visible region, corresponding to the absorption of OVinduced defect states within the bandgap. The Au/TiO2OV sample combines the absorption characteristics of both of the Au/TiO2 and TiO2-OV samples, as reflected by the coexistence of a broad LSPR band and a long absorption tail. The determined bandgaps of the four catalyst



samples are nearly identical (Figure S6 and Table S1). The introduction of OVs creates donor states in the bandgap of TiO2, but does not change the bandgap.37 The loading of the Au nanospheres on the TiO2 or TiO2-OV nanosheets does not change their bandgaps either. The intimate contact between the Au nanospheres and the TiO2-OV nanosheets was further confirmed by the X-ray photoelectron spectroscopy (XPS) analysis of Au 4f. The peak corresponding to Au 4f7/2 in the Au/TiO2-OV sample displays a clear negative shift of ~0.5 eV relative to 84.0 eV for bulk Au (Figure S7a), suggesting electron transfer from the TiO2-OV nanosheets to the Au nanospheres due to the equilibration of the Fermi levels.38 In addition, a small positive shift of ~0.15 eV for Ti 2p3/2 after Au loading also indicates electron transfer from the TiO2-OV nanosheets to the Au nanospheres (Figure S7b). The electron transfer clearly suggests the intimate contact between the TiO2-OV nanosheets to the Au nanospheres. OVs play an important role in catalysis. The presence of OVs in the Au/TiO2-OV sample is evidenced by the highresolution XPS spectra of Ti 2p (Figure S7b). The Ti 2p spectra of the TiO2 and Au/TiO2 samples exhibit two peaks that arise from the 2p1/2 at 463.6 eV and 2p3/2 at 457.8 eV, respectively, indicating the characteristic +4 oxidation state. In comparison to the TiO2 and Au/TiO2 samples, two additional peaks with lower binding energies at 461.7 and 456.1 eV appear in the Ti 2p spectra of the TiO2-OV and Au/TiO2-OV samples, which are originated from the partial reduction of Ti4+ by localized electrons at OV sites.39 Low-temperature electron paramagnetic resonance (EPR) analysis was performed to examine the existence of OVs (Figure 1g). No EPR signals were observed on the TiO2 and Au/TiO2 samples. In contrast, a characteristic OV signal with a g factor of 1.998 was observed on the TiO2-OV and Au/TiO2-OV samples, suggesting the presence of OVs.40 The large specific surface area of the TiO2-OV nanosheets is advantageous for increasing the probability of OVs being located on the surface and therefore the accessibility of OVs by reactant molecules.27 N2 Photofixation. The successful design of the Au/TiO2-OV catalyst featuring enhanced light absorption allows for the exploration of its N2 photofixation activity under visible light. Methanol was employed as the hole scavenger in our study. The produced ammonia amount was determined using the well-established indophenolblue method, for which the calibration relationship between the peak absorbance and the NH4+ concentration was first obtained (Figure S8). A Xe lamp was employed for irradiation, and a 420-nm cut-off filter was used to

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obtain visible light (Figure S9). Control experiments revealed that ammonia cannot be produced in the absence of any one of the following: light irradiation, N2 or the catalyst. Under the visible light, because of the limited visible light absorption, the TiO2-OV sample exhibits a negligible N2 photofixation activity of 2.2 μmol h–1 g–1, which is 4.8% relative to the activity under the white light (Figure S10). For the Au/TiO2-OV sample under the visible light, the color of the testing solution for ammonia becomes greener with the irradiation time (Figure 2a). The light absorption peak of the testing solution at 697 nm becomes stronger, clearly suggesting that the NH4+ concentration increases gradually with the irradiation time (Figure 2b). In comparison with the TiO2, TiO2-OV and Au/TiO2 samples, the N2 photofixation activity of the Au/TiO2-OV sample under the visible light is greatly boosted, showing a linear increase of the ammonia amount with the irradiation time (Figure 2c). The N2 photofixation rate normalized against the irradiation time and the amount of the catalyst is 78.6 μmol h–1 g–1 for Au/TiO2-OV (1.5 wt% Au) under the visible light, which is respectively 98 and 35 times those of Au/TiO2 and TiO2OV. The N2 photofixation activity remains nearly unchanged after five runs of the photocatalytic test, showing that Au/TiO2-OV has a good photostability for N2 photofixation under visible light (Figure 2d). In addition, we found that the amount of ammonia produced in the control experiments using aprotic solvents of dimethylformamide and acetonitrile is very small in comparison with the case of using water as the solvent, demonstrating the importance of H2O as the proton source for N2 photofixation (Figure S11). We note that H2 is produced as a coproduct in the reported biological hybrid of CdS nanocrystals and the nitrogenase MoFe protein for N2 photofixation.41 To identify if the H2 co-product was produced on the Au/TiO2-OV sample, the reaction solution was bubbled with N2 for one hour, followed by the irradiation of the visible light with all three neck openings sealed. The produced H2 amount was found to be 27 mol% with respect to the harvested ammonia (Figure S12). The amount of H2O in the reaction solutions is overwhelmingly larger than that of N2. Therefore, the competition from H2 evolution reaction constitutes one of the bottlenecks in N2 photofixation, which also occurs in CO2 photoreduction. Nevertheless, H2 evolution in N2 photofixation would not be an issue considering the fact that formed NH3 is highly soluble in water and can be readily separated from gaseous H2. In this study, we focus on the ammonia evolution and leave the selectivity between ammonia and H2 in future works.


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Figure 2. N2 photofixation under the visible light. (a) Photograph of the aliquots obtained during N2 photofixation with the Au/TiO2-OV sample and subjected to the indophenol-blue test. The colors of the solutions change gradually with the irradiation time. (b) Absorption spectra of the testing solutions showing the intensification of the absorption peak of the indophenol at 697 nm. (c) Ammonia concentrations as functions of time for the four catalysts. (d) Cycling test for N2 photofixation with Au/TiO2OV. (e) Shift of the Raman peak of the anatase Eg mode with the time of calcination in air for Au/TiO2-OV. (f) Ammonia production rate as a function of the Raman peak shift. The Raman peak shifts are the absolute wavenumber difference values relative to the peak wavenumber before calcination. (g) Effect of the Au loading amount on the light absorption at 550 nm (left axis) and on the ammonia evolution rate (right axis) for Au/TiO2-OV. (h) Absorption (left axis) and ammonia evolution action (right axis) spectra of Au/TiO2-OV (1.5 wt% Au).

To elucidate the dependence of the N2 photofixation rate on the OV concentration, Au/TiO2-OV samples with different concentrations of OVs were prepared by calcinating the Au/TiO2-OV sample in air for different periods of time. Oxidation of oxygen-deficient TiO2 reduces the concentration of OVs and simultaneously results in increases in the lattice spacing, which induces redshifts of the anatase Eg mode in the Raman spectra (Figure 2e). The Raman peak has been found to redshift linearly with the concentration of OVs.42 We observed a nearly linearly dependence of the N2 photofixation activity on the Raman peak shift (Figure 2f), suggesting that the N2 photofixation rate decays nearly linearly with the reduction of the OV concentration. To identify the effect of the Au loading amount on the rate of N2 photofixation, the Au nanospheres were loaded on the TiO2-OV nanosheets at different amounts. Light absorption measurements revealed no broadening or redshift for the plasmon peak, suggesting that no aggregation occurs for the loaded Au nano-

spheres. The N2 photofixation rate under the visible light shows a nearly volcano-shaped dependence on the Au loading amount, with the maximum located at 2.53 wt% (Figure 2g). The reduction of the N2 photofixation rate when the Au loading amount is above 2.53 wt% is attributed to the weakening or inhibition of N2 adsorption on OVs by the excessive Au nanospheres. The dependence of the N2 photofixation rate on the LSPR of the Au nanospheres was further investigated by acquiring the action spectrum (Figure 2h). To obtain the action spectrum, N2 photofixation was performed on the Au/TiO2-OV sample under the irradiation of monochromatic visible light. The AQE of the NH3 production at each wavelength of light was calculated from the ratio between the number of electrons involved in the N2 fixation reaction and the number of the incident photons. The former is three times the number of the produced NH3 molecules. The trend of the determined AQEs is in good agreement with the absorption spectrum of



Au/TiO2-OV. We can therefore reason that N2 photofixation is induced by the light absorption caused by the LSPR of the Au nanospheres that are attached on the TiO2-OV nanosheets. The AQE reaches 0.82% at 550 nm, which is higher than those reported so far (Tables S2 and S3). Because the N2 photofixation rate peaks at 2.53 wt% for the Au loading amount, we believe that an even higher AQE can be obtained with the Au/TiO2-OV catalyst. Understanding of the “Working-in-Tandem” Mechanism. Chemisorption of N2 molecules is an essential step in the N2 fixation reaction. We employed N2 temperature-programmed desorption (TPD) to ascertain the N2 adsorption ability of the catalysts (Figure 3a). For both of the TiO2 and Au/TiO2 samples, only one adsorption peak was observed, which is originated from N2 physisorption. In contrast, in addition to the physisorption peak, both of the TiO2-OV and Au/TiO2-OV samples show a peak at a higher temperature, which is ascribed to N2 chemisorption. This finding suggests that N2 chemisorption takes place at the OV sites on the TiO2 nanosheets, which is believed to be mainly induced by electron transfer between OV-induced Ti3+ and N2. Notably, besides the main desorption peak at 277 °C, a desorption shoulder was observed at 246 °C on the TPD profile of Au/TiO2-OV. Since chemisorption peaks were not observed on Au/TiO2, the chemisorption of N2 molecules cannot be ascribed to the loading of the Au nanospheres. The desorption shoulder is therefore reasoned to be still associated with the adsorption of N2 on OVs. When a gold nanosphere is located close to an OV, local electron transfer occurs from TiO2 to the Au nanosphere due to the thermal equilibration of the Fermi levels. Such electron transfer reduces the local electron density at the OV and therefore weakens its ability of N2 adsorption, giving rise to the desorption shoulder at 246 °C. The weak adsorption means that N2 molecules are less activated, which is disadvantageous for N2 photofixation. As a result, there exists an optimal loading amount of the Au nanospheres (Figure 2g). Each OV on the TiO2 surface introduces two localized electrons,16 leading to the partial reduction of Ti4+ to Ti3+. Alt-

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hough there have been experimental evidences for that OV-induced Ti3+ can cause N2 chemisorption,10,12 no theoretical calculations have been conducted to understand the phenomenon. We performed DFT calculations to examine N2 chemisorption on the OV sites of TiO2. Upon the adsorption of a N2 molecule on the OV-free TiO2, the charge density difference cannot be observed even at the isovalue of 0.01 e Å–3 (Figure S13). A negligible charge density difference implies that the electron transfer between N2 molecules and OV-free TiO2 is very small and that N2 molecules are mainly adsorbed on OV-free TiO2 through physisorption. In contrast, when a N2 molecule is adsorbed at an OV site (Figure 3b), there is a clear charge density difference at the isovalue of 0.05 e Å–3, indicating that electron transfer between N2 and OV is significant and that N2 chemisorbs. The electron-enriched isosurface on the adsorbed N2 molecule displays a π-orbital shape, suggesting that electrons transfer from the OV to the lowest unoccupied π-orbital of the N2 molecule (back donation).3 In addition, the electron density between the two N atoms is clearly reduced, indicating electrons are donated from the highest occupied σ-orbital of the N2 molecule to the OV (donation). Since the lowest unoccupied π-orbital of N2 is an antibonding orbital and the highest occupied σ-orbital is a bonding orbital, both electron donation and back donation can weaken the N≡N bond, as reflected by the elongated N≡N bond (1.162 Å versus 1.109 Å in a free N2 molecule). The electron back donation phenomenon has been found in many transition metal–N2 complexes, where the metals (Mo, Fe, Ru) donate their available d-orbital electrons into the π antibonding system to activate the N2 molecule, leading to the effective coordination of N2 to transition-metal complexes.3,43 More interestingly, it has been reported that a strongly reducing transition metal with available d-orbital electrons can activate N2 through the electron back donation effect, leading to the formation of strongly reduced dinitrogen ligands such as N=N (N22–) or even N–N (N24–) moieties, which can largely facilitate full N2 cleavage.44


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Figure 3. Chemisorption of N2 molecules on the OVs. (a) N2 TPD profiles of the four catalysts. (b) Adsorption configuration (left) and charge density difference (right) of a N2 molecule adsorbed at an OV site on the TiO2 surface. The light blue and yel–3 low isosurfaces represent charge accumulation and depletion in space, respectively. The isovalue is 0.05 e Å . (c) Schematic illustration of the in situ DRIFTS setup. (d) In situ DRIFTS spectra recorded as a function of time during the N2 photofixation reaction on the Au/TiO2-OV catalyst. The numbers indicate the wavenumbers of the different vibrational modes.

The effective N2 chemisorption at the OV sites was further verified by the N2 TPD behavior of the Au/TiO2-OV samples with different OV concentrations. The Au/TiO2OV samples were treated at 350 Å in air for different periods of time. The TPD peak decreases in intensity with increasing calcination time, suggesting that the amount of N2 chemisorbed on the Au/TiO2-OV samples decreases (Figure 14Sa, b). Because the chemisorbed N2 amount is proportional to the OV concentration and the Raman peak linearly shifts with the OV concentration,42 the integrated TPD peak areas and therefore the chemisorbed N2 amounts display a linear dependence on the Raman peak shift, as verified by the plot in Figure S14c. This finding also explains the linear relationship between the N2 photofixation rate and the Raman peak shift (Figure 2f) as well as the OV concentration. We performed in situ diffuse-reflectance infrared Fourier transform spectroscopy (DRIFTS) in a home-made reactor to directly visualize the N2 activation and further reduction on the Au/TiO2-OV catalyst. Such a highly sensitive spectroscopy technique allows for studying the time-dependent transformation of the functional groups on the catalyst surface in a moist stream of N2 under visible light. In the control experiment performed in Ar atmosphere, two bands at 3305 cm–1 and 1443 cm–1 were observed. They increase in strength with the reaction time (Figure S15). The two bands are respectively originated from the ν(O–H) stretching mode and σ(O–H) bending mode of adsorbed water molecules or surface hydroxyl

groups produced under the light irradiation. No bands that are related to N-containing species were observed. In N2 atmosphere, the observed absorption band at the wavenumber of 1988 cm–1 is believed to arise from the Ti– N=N complex formed through N2 binding to the Ti3+ sites.10,45,46 The wavenumber is smaller than the stretching wavenumber of free N2 molecules (v(gaseous N2) = 2331 cm–1) and larger than that of hydrazine (H2N–NH2) (1111 cm–1), indicating that the N–N bond order is reduced and the N–N bond length is increased.3,44,47 The DRIFTS result is consistent with our DFT calculations described above. The significant redshift of the stretching vibration of the adsorbed N2 molecules relative to free N2 ones, which depends on the amount of electron donation and back donation between the OVs and N2, firmly suggests the great extent of N2 activation at the OV site.3 In addition, a predominant overlapped absorption band ranging from 3600 to 2980 cm–1 appears and becomes stronger with the irradiation time. The bands in this range can be assigned to the v(N–H) stretching mode of NH3 and the v(O–H) stretching mode of surface hydroxyl groups or adsorbed water, respectively. The bands at 1726 and 1477 cm–1 are originated from the σ(N–H) bending mode. The bands at 2800 and 1405 cm–1 can be attributed to the characteristic absorption of NH4+. The direct observation of these functional groups in situ strongly confirms the N2 activation and further reduction to NH3 on the Au/TiO2-OV catalyst. In the working mechanism of the nitrogenase, in addition to the N2 activation sites, a steady stream of electrons



is also necessary for the further reduction of activated N2 to NH3. The experiments described above have indicated that the OVs on our catalyst act as N2 activation sites. The function of the Au nanospheres should be to provide a steady electron stream for the reduction of activated N2. To verify our reasoning, several control experiments were performed. First, the addition of an electron scavenger, Cr2O72–, considerably suppressed the N2 photofixation rate of the Au/TiO2-OV sample under the visible light, suggesting that electrons contribute to N2 photofixation (Figure 4a). To ascertain the electron stream originated from the Au nanospheres, Au@CTAB/TiO2-OV was prepared by preserving CTAB molecules on the surface of the Au nanospheres, which was realized by skipping the refluxing treatment in acetone. Both thermogravimetric analysis (TGA) (Figure S16a) and Fourier transform infrared (FTIR) analyses (Figure S16b) reveal the existence of CTAB molecules on the surface of the Au nanospheres without the refluxing treatment.48 The N2 photofixation activity is largely inhibited in the case of the Au@CTAB/TiO2-OV sample (Figure 4a). Furthermore, the CTAB molecules were replaced with a thin SiO2 shell of 2.1 nm in thickness on the Au surface (Figure S17). For Au@SiO2/TiO2-OV, a similar suppression of the N2 photofixation rate to that of Au@CTAB/TiO2-OV was observed. In these two control experiments, both of the CTAB layer and the thin SiO2 shell serve as an insulating barrier pre-

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venting hot electrons from transferring from the Au nanospheres to the TiO2-OV nanosheets, which leads to the great reduction in the N2 photofixation rate. We therefore reason that the N2 photofixation with the Au/TiO2-OV sample is driven by hot electrons generated on the plasmonic Au nanospheres under the irradiation of visible light. This reasoning was further supported by photocurrent measurements (Figure S18). In comparison with the TiO2-OV sample, the photocurrent of the Au/TiO2-OV sample is greatly enhanced, indicating that the plasmon excitation on the Au nanospheres generates a large number of electrons under the visible light. Interestingly, an exponential decay was observed on the photocurrent of the Au/TiO2-OV sample, suggesting a long lifetime of the generated electrons due to the presence of the OV trap states.49 In contrast, the photocurrents of both of the Au@CTAB/TiO2-OV and Au@SiO2/TiO2-OV samples decrease rapidly back to the values in dark, suggesting the rapid recombination of electrons and holes. In addition, both of the magnitude and shape of the photocurrent profiles are nearly the same with those of the TiO2-OV sample, implying again that hot electrons are prohibited to transfer from the Au nanospheres to the TiO2-OV nanosheets by the CTAB layer and the SiO2 shell. Taken together, our control experiments evidently highlight that plasmonic Au nanocrystals provide hot electrons to the reduction of N2 molecules.


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Figure 4. Mechanism for plasmonic N2 fixation. (a) N2 photofixation rates with different catalyst samples under the visible light. 2– Cr2O7 ions function as an electron capture agent. The CTAB molecules and SiO2 shell on the surface of the Au nanospheres act as the inhibitors for electron transfer. (b) Schematic of the SPV setup. (c) SPV spectra of the different catalysts. (d) MottSchottky plots measured under the visible light for the different catalysts. (e) Transient absorption spectra of the Au/TiO2-OV catalyst measured in Ar and N2 atmosphere, respectively. Ar and N2 were used to simulate the inert and reactive environment. (f) Photocurrent responses of the different catalysts recorded in Ar and N2 atmosphere, respectively, with the visible light irradiation switched on and off repeatedly. (g) Schematic illustrating the plasmonic hot electron generation, injection, N2 reduction processes in the N2 photofixation with the Au/TiO2-OV catalyst under visible light. (h) Artistic illustration of the efficient plasmonic N2 photofixation.

Surface photovoltage (SPV) spectroscopy is a powerful tool for characterizing charge separation at nanoscale.50 It can directly and rapidly probe charge separation within a photoresponsive material by recording the surface voltage change under light irradiation (Figure 4b). No steadystate SPV signal was detected on the OV-free TiO2 nanosheets under the visible light (Figure 4c). For the TiO2-OV sample, a weak decaying SPV signal was observed in the range of 400–550 nm, which is caused by the excitation of the OV states (Figure 1f). With the attachment of the Au nanospheres on the OV-free TiO2 nanosheets, a weak SPV signal at ~550 nm was detected, which is originated from the plasmon resonance of the Au nanospheres. In stark contrast with the small SPV signal of the TiO2-OV and Au/TiO2 samples, the SPV response of the Au/TiO2-OV sample is dramatically enhanced under the visible light, implying the efficient separation of elec-

trons and holes due to the concerted action of the plasmon resonance and the OVs. The apparent charge carrier concentrations in the catalysts were determined from the Mott-Schottky plots (Figure 4d). All of the four catalyst samples show positive slopes, indicating the n-type character of their electronic band structures.51 The MottSchottky plots can also provide the flat-band potentials and apparent carrier concentrations of the catalysts (Table S4). The flat-band potentials are nearly the same, suggesting that the major characteristics of the electronic band structure of the TiO2 nanosheets are maintained. From the reported flat-band potential position relative to the conduction band edge of an n-type semiconductor52 and the bandgap of TiO2, we can draw the band edges of TiO2 along the energy and electrochemical potential axes and compared them with the reported LUMO and HOMO energy levels of free N2 molecules.53 The LUMO



and HOMO energy levels straddle the conduction and valance band edges of TiO2 (Figure S19), signifying again the importance of active sites for N2 activation during N2 photofixation. Because of the concerted action between the plasmon resonance and the OVs, the Au/TiO2-OV sample under the visible light possesses the highest apparent electron carrier concentration (3.63 × 1021 cm–3). The apparent carrier concentrations of the other catalysts decrease in the order of TiO2-OV (1.71 × 1021 cm–3), Au/TiO2 (5.64 × 1020 cm–3) and TiO2 (2.89 × 1020 cm–3). The determined apparent electron carrier concentrations reveal that the plasmon excitation can generate hot electrons, which inject into the conduction band of the TiO2 nanosheets, and that the OVs can facilitate the injection of plasmonic hot electrons. As a result, more electrons in the Au/TiO2-OV sample can be used to reduce activated N2 molecules, leading to the enhanced N2 photofixation activity. As has been widely reported, upon the excitation of Au nanoparticles under visible light, hot electrons generated from plasmonic Au nanocrystals can inject in the conduction band of TiO2, leading to the temporary accumulation of electrons in the conduction band of TiO2 and holes in the Au nanoparticles.24,54 The fate of hot electrons in the conduction band of TiO2 is critical for subsequent reduction reactions. Transient absorption spectroscopy (TAS) was therefore employed to characterize the electron dynamics among the various electronic states in the TiO2 nanosheets. To trace the fate of hot electrons in the TiO2 nanosheets, TAS was performed for the catalyst samples under reactive (N2) and inert atmosphere (Ar), respectively. The OV-free TiO2 nanosheets show very small responses in both Ar and N2 atmosphere under the visible light, with the peak ratio between the former and latter being 1.15 (Figure S20). With the introduction of OVs, a weak signal was detected in Ar on the TiO2-OV nanosheets. The signal in N2 is weaker than that in Ar. The peak ratio in Ar and N2 is 1.35. The weakening of the TAS signal in N2 atmosphere is attributed to the consumption of electrons by N2 molecules that are adsorbed at the OV sites. With the loading of the Au nanospheres on the TiO2 nanosheets, a strong signal was detected on the Au/TiO2 sample under the visible light in Ar, indicating the transfer of hot electrons from the Au nanospheres to the conduction band of the TiO2 nanosheets. Because of the absence of OVs, the TAS signal is only decreased slightly when Ar atmosphere was changed to N2 atmosphere for the Au/TiO2 sample, with the peak ratio being 1.10. As shown in Figure 4e, the TAS signal of the Au/TiO2-OV sample in Ar displays three regimes. The first regime corresponds to an initial fast decay being complete in 400 ns, which accounts for 51.5% of the initial ΔJ/J0 intensity. Intriguingly, the fast decay is followed with a slow growth of the signal intensity from 400 to 900 ns, giving rise to an increase of 14.8%. The growth of the transient signal, reflecting a delayed excitation of electronic states, results from the migration of electrons and relocation of electrons in traps that have a longer lifetime in the electronic band structure of titania.55 The OVs in the Au/TiO2-OV samples are the main traps located be-

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low the conduction band of TiO2. Clearly, the injected hot electrons in the conduction band of TiO2 are further trapped by the OV states, leading to the growth of the transient signal from 400 to 900 ns in the second regime. After 900 ns, the transient signal undergoes a final decay in the third regime. Upon exposure to N2 atmosphere, the three regimes of the transient signal are still observable while the signal intensity is largely quenched. The peak ratio in Ar and N2 is 1.26. In addition, each regime for the Au/TiO2-OV sample in N2 is shortened. The first regime is from 0 to 300 ns, which is followed with the second regime from 300 to 700 ns and the third regime after 700 ns. The quenching and shortening of the TAS signal suggest the consumption of electrons by N2 at the OV sites. These TAS results firmly prove that the plasmonic Au nanospheres serve as the sources of hot electrons and that the OVs act as the mediating electron acceptors to facilitate the transfer of hot electrons to activated N2 molecules. This “working-in-tandem” mechanism between plasmonic Au nanocrystals and OVs is notably significant considering that both of the inorganic components resembling the Fe- and MoFe-proteins are supported on the ultrathin TiO2 nanosheets and perform their own roles for efficient N2 fixation under ambient conditions. To further understand the “working-in-tandem” mechanism, photocurrent measurements were conducted on the four catalyst samples under the visible light in both Ar and N2 atmosphere (Figure 4f). For the OV-free TiO2 nanosheets, no photocurrent signals were detected in Ar or N2 atmosphere. A weak photocurrent response was observed for the TiO2-OV sample under Ar atmosphere, while the photocurrent was strongly reduced in N2. In comparison with the TiO2 and TiO2-OV samples, the photocurrent of the Au/TiO2 sample was greatly enhanced in Ar. There was no significant change in photocurrent when Ar atmosphere was changed to N2, implying negligible electron transfer from TiO2 to N2 in the absence of OVs. Interestingly, with the concerted action between the Au nanospheres and the OVs, the photocurrent of the Au/TiO2-OV sample in N2 is only one fourth that in Ar. The other three fourths of the photocurrent can be ascribed to the electron consumption by activated N2 molecules at the OV sites. Based on the TAS and photocurrent measurements, the plasmonic N2 photofixation process can be divided into four steps: activation of N2 molecules at the OV sites, injection of hot electrons from the Au nanospheres into the conduction band of the TiO2 nanosheets, trapping of the injected hot electrons by the OV-induced defect states, and reduction of the activated N2 molecules by the hot electrons at the OV sites (Figure 4g). Hot holes are mainly generated and consumed by the hole scavenger, methanol, on the plasmonic Au nanospheres. Figure 4h shows an overall artistic view of the efficient plasmonic N2 photofixation with the Au/TiO2OV catalyst under ambient conditions. Understanding the “working-in-tandem” mechanism is critical for future thorough investigations to further improve the N2 photofixation activity. We mixed together the Au nanospheres with two Au nanorod samples and assembled them onto the TiO2-OV nanosheets in order to increase the visible


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light absorption (Figure S21). The three Au nanocrystal samples have different plasmon wavelengths. The extinction of the mixture covers nearly the entire visible region. The N2 photofixation rate of the Au mixture/TiO2-OV sample (1.5 wt% Au) under the visible light was measured to be 130.5 μmol/h/g (Figure S21g), which is 66.2% larger than that of the Au/TiO2-OV sample (Figure 2c). Moreover, the N2 photofixation rate of the Au mixture/TiO2-OV sample reaches 187.1 μmol/h/g under the irradiation of the white light. CONCLUSIONS In summary, enlightened by the “working-in-tandem” mechanism of the natural nitrogenase, we have designed and constructed an efficient N2 photofixation system, inorganic Au/TiO2-OV catalyst, which employs the OVs as the activation sites for N2 molecules and plasmonic hot electrons as the reducing agents for the final fixation of the activated N2 molecules to NH3. The OVs and plasmon resonance work in tandem to endow the Au/TiO2-OV catalyst with an AQE of 0.82% at 550 nm, which is the highest among those reported so far at visible wavelengths. The N2 photofixation rate can be further enhanced by optimizing the absorption of visible light. Such an “working-in-tandem” strategy effectively tackles the two critical steps of the activation and reduction of N2 molecules and allows for future thorough optimization for high-performance and sustainable N2 photofixation through the less energy-demanding photochemical process. Our strategy could also be potentially extended to other photochemical systems involving two or more functional components in successive steps.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/jacs.xxxxxxx. AFM, TEM, XRD, XPS characterizations, Zeta potential measurements, N2 adsorption-desorption isotherms, Tauc plots, calibration lines, light sources, control experiments, photocurrent measurements, TAS analysis, TEM images and extinction spectra of the Au nanorodinvolved samples (Figures S1–S21), measured specific surface areas, AQEs, N2 photofixation performances in other works, flat-band potentials, electron carrier concentrations (Tables S1‒S4) and additional references (PDF)

AUTHOR INFORMATION Corresponding Authors *[email protected] *[email protected]

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was supported by Hong Kong Research Grants Council (J.F.W. and J.C.Y., Theme-based Research Scheme, T23-407-13N; J.F.W., NSFC/RGC Joint Research Scheme,

N_CUHK440/14) and Hong Kong PhD Fellowship Scheme (J.H.Y.)

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High-Efficiency âWorking-in-Tandemâ Nitrogen Photofixation Achieved

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