CO2 Reduction by Plasmonic Au Nanoparticle-Decorated TiO2

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C: Surfaces, Interfaces, Porous Materials, and Catalysis 2

CO Reduction by Plasmonic Au Nanoparticles Decorated TiO Photocatalyst with an Ultrathin AlO Interlayer 2

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Huilei Zhao, Xiaoyu Zheng, Xuhui Feng, and Ying Li J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b04239 • Publication Date (Web): 02 Aug 2018 Downloaded from http://pubs.acs.org on August 4, 2018

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The Journal of Physical Chemistry

CO2 Reduction by Plasmonic Au Nanoparticles Decorated TiO2 Photocatalyst with an Ultrathin Al2O3 Interlayer Huilei Zhao†, Xiaoyu Zheng†, Xuhui Feng and Ying Li* † Authors contribute to this work equally. Department of Mechanical Engineering, Texas A&M University, College Station, TX, 77845 USA * Corresponding Author: Ying Li, Ph.D., Associate Professor, Texas A&M University Email: [email protected]; Tel.: +1 979-862-4465; Fax: +1 979-845-3081

Abstract A well-designed nanocomposite structure, Au/Al2O3/TiO2, i.e. Au decorated TiO2 with an atomic layer deposited (ALD) Al2O3 interlayer between Au and TiO2, was synthesized to study the role of plasmonic Au nanoparticles (NPs) on photocatalytic CO2 reduction as well as the influence of the interlayer. Localized surface plasmon resonance (LSPR) induced hot electron injection and near-field enhancement, together with the enhanced charge separation caused by the Au NPs were found responsible for the promoted photocatalytic CO2 reduction on Au modified TiO2. The Al2O3 interlayer inhibited electron transfer and dampened the near-field enhancement effect; however, it also served as passivation layer to suppress electron-hole recombination on the TiO2 surface. To further identify the significance of each of the mechanisms, photocatalysts with tailored thickness of Al2O3 interlayer and varied size of Au NPs were carefully synthesized, characterized and tested for CO2 photoreduction with water. A finite difference time domain (FDTD) simulation was also conducted to correlate with the catalytic activity. The results show that the hot electron injection phenomenon was observable but very weak under visible light excitation. Charge separation and near-field enhancement were more dominant mechanisms under UV-vis irradiation and were closely related to the size of Au NPs. Charge separation was more significant for smaller Au NPs, while stronger near-field enhancement was found on larger Au NPs. At an optimal thickness of Al2O3 interlayer, the 1 ACS Paragon Plus Environment

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positive effects of the interlayer outperformed the negative ones, resulting in a significant improvement in CO2 photoreduction.

Introduction The global warming effect caused by greenhouse gas especially CO2 emissions has attracted great attention, and the need of developing advanced CO2 capture and utilization technologies becomes urgent. Among different technologies, photocatalytic reduction of CO2 assisted by photocatalysts (e.g. TiO2) is potentially a promising method to reduce CO2 emissions and simultaneously convert solar energy into storable chemical energy1-5. TiO2 is widely used to photocatlytically reduce CO2 because TiO2 has high chemical and thermal stability, appropriate conduction band and valence band positions, low cost and non-toxicity6. However, as a wide band-gap semiconductor (3.2 eV for anatase and 3.0 eV for rutile), TiO2 is hardly excited under visible light irradiation.7 The rapid surface recombination of photogenerated electrons and holes also limits the photocatalytic performance of TiO2. In our past research, different approaches were performed to improve photocatalytic performance of TiO2, including metal and/or nonmetal doping8-9, defect engineering10, and enhancing surface basicity11-12. One of our recent works showed an ultrathin layer of Al2O3 coated on TiO2 synthesized by atomic layer deposition (ALD) can efficiently passivate surface recombination and promote photocatalytic activity of CO2 photoreduction.13 ALD as a surface engineering approach to improve photocatalytic activity is still an emerging area. Another widely applied approach to address the limitation of visible light activity of TiO2 is to couple it with plasmonic metals, e.g. Au14-15. The various possible effects of plasmonic nanoparticles (PNPs) on photocatalysis are shown in Figure 1. First, when Au NPs are in contact with TiO2, its suitable fermi level enables Au NPs act as good acceptors for photoexcited electrons from TiO2, promoting charge separation (I in Figure 1).16 Second, hot electron injection (II in Figure 1) from Au NPs to TiO2 conduction band via a non-radiative damping of plasmon is induced by the predominant plasmonic mode, localized surface plasmon resonance (LSPR)17 18. One of the preconditions for hot electron injection to take place is that TiO2 is in contact with Au. Third, LSPR can also cause near-field enhancement in the semiconductor by an intense localized electric field near the metal NPs’ surface, i.e. near-field enhancement19 (III in 2 ACS Paragon Plus Environment

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Figure 1). The strong electric field in the vicinity of Au can further help the charge separation process (I in Figure 1) by changing it from a mode of spontaneous migration of carriers to a field guided transfer. In addition, according to Fermi’s golden rule, the enhanced electric field in TiO2 near Au NPs leads to much higher electron-hole formation rate.20-21 Recently, some researchers reported plasmon induced resonance energy transfer (PIRET) mechanism22-23, in which plasmons transfer to the semiconductor by dipole-dipole interaction without the need of direct contact (IV in Figure 1). However, for the Au-TiO2 system, PIRET effect can be neglected because the LSPR peak of Au (~550 nm) is away from the light absorption edge of TiO2 (~400 nm).24-25

Figure 1. Possible effects of plasmonic nanoparticles (PNPs) on photocatalytic CO2 reduction Au PNPs decorated TiO2 photocatalysts have been used for CO2 photocatalytic reductions, which showed significantly improved CO2 conversion, e.g. higher CO production26 or increased formation of hydrocarbon products, e.g. CH4. 27-28. The light absorption capability of Au-TiO2 materials are highly promoted due to the LSPR of Au PNPs, which extends the photoresponse to visible or even IR region.2, 29 Tahir30 found that improved CO2 conversion to CO on montmorillonite dispersed TiO2 nanocomposite after Au modification under UV light, which was attributed to a general LSPR effect. Both charge separation and near-field enhancement induced 3 ACS Paragon Plus Environment


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by LSPR of Au, as reported by Tu et. al.31, improved CO2 photoreduction, and facilitated the formation of C2H6 on Au@TiO2 yolk–shell hollow spheres. Though positive effects of attaching PNPs to TiO2 for CO2 photoreduction have been recognized, the influence of Au PNP size on different LSPR effects (as shown in Figure 1) is still not clear. It is necessary to evaluate the dependence of each possible LSPR effects on Au PNP size to design highly efficient plasmonic photocatalysts. In this work, a nanostructured catalyst consisting of Au NPs/Al2O3 atomic interlayer/TiO2 NPs was synthesized to study the possible mechanisms (e.g. charge separation, hot electron injection, and/or near-field enhancement) of plasmonic metal induced enhancement in CO2 photoreduction. The significance of each of the mechanisms was assessed by tailoring the Au NP size and the thickness of the Al2O3 interlayer. Finite difference time domain (FDTD) simulation was also conducted to correlate with the photocatalytic activity results to further elucidate the fundamental mechanism. To our best knowledge, this work is the first one to explore the effects of Au NP size and Al2O3 interlayer thickness on CO2 photoreduction by TiO2 through both experimental and simulation approaches, which provides useful insight for plasmonic photocatalyst design.

Experimental Section Material Synthesis Synthesize of gold nanoparticles Au NPs suspensions were prepared by a sodium citrate reduction method, as reported in the literature32. Briefly, a desired amount of 0.25 mM HAuCl4•3H2O solution was stirred and uniformly heated at boiling point in an oil bath. Then sodium citrate solution at a desired Na3Ct concentration was dropwise added into the Au ion solution. The mixed solution was kept at the boiling temperature until it turned a wine-red color, where a suspension of Au NPs was obtained. The size of Au NPs was tuned by varying the molar ratio of Na3Ct to HAuCl4. As reported by Ji et. al32, a molar ratio of Na3Ct:HAuCl4 at 3.5:1, 7:1 and 14:1 could result in Au NPs with 10, 20 and 30 nm, respectively, in size. The size dependent LSPR of Au is attributed to the coupling of the LSPR from d electrons to interband transitions, and the coupling disappears entirely when the size of Au is too small.33 Moreover, when the size of Au is smaller than 5 nm, quantum effects 4 ACS Paragon Plus Environment

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are involved.34-35 Therefore, to study the size dependent LSPR of Au, particle sizes of Au 10 nm, 20nm and 30 nm were chosen, to minimize interferences from the quantum effects. Synthesis of Al2O3/TiO2 A commercial ALD system (Savannah S200, Ultratech) was used to coat TiO2 with a thin layer of Al2O3 through a similar procedure reported in our previous work13. Pre-washed commercial TiO2 P25 powder was used as the substrate, while H2O and Trimethylaluminum (TMA) were used as O2 and Al precursor, respectively. For each ALD procedure, the reaction chamber was set at 200 °C, and 0.1 s pulse time was used for both H2O and TMA. The “expo” mode was chosen to ensure a uniform coating layer of Al2O3 over TiO2 nanoparticles. The thickness of Al2O3 was controlled by varying the cycle number of ALD. Theoretically, under “expo” mode with 0.1s pulse time, the growth rate of Al2O3 would be ~1 Å per cycle.13 Therefore, to obtain a 1, 5, 20 or 50 Å thick Al2O3 layer, 1, 5, 20 or 50 cycles of ALD operation were conducted, respectively. The as-prepared Al2O3/TiO2 composites were denoted as xAl/Ti, where x was the number of ALD cycles. Synthesis of Au/TiO2 and Au/Al2O3/TiO2 heterostructure To prepare Au/Al2O3/TiO2 heterostructure, a desired amount of Au nanospheres suspension was mixed with xAl/Ti powder under sustained stirring for 12 h. The obtained solids were then washed and centrifuged, and subsequently dried at 60 °C overnight. The Au content was set to be 0.5 wt.% for all the catalysts in this work since 0.5 wt.% Au modified TiO2 exhibited the best photoactivity among all samples in the range of 0.25 to 2.0 wt.%, as illustrated in Figure S1. Catalysts composed of only Au NPs or very high concentration of Au (i.e. little TiO2 with many Au NPs) were not tested because as shown in Figure S1, a higher ratio of Au/TiO2 led to decreased CO2 conversion. When there are too many Au NPs, they may become charge recombination centers and thus the catalytic activity is reduced. This optimum Au concentration was also consistent with findings reported in the literature.36-37 The synthesized Au/Al2O3/TiO2 heterostructured materials were denoted as Auy/xAl/Ti, where y is the size of the Au NPs. For example, Au30/20Al/Ti denotes 30 nm Au NPs decorated TiO2 with 20 Å Al2O3 interlayer. Au/TiO2 without the Al2O3 interlayer were also synthesized as a comparison, which were denoted as Auy/Ti.

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Material characterization X-ray diffraction (XRD, BRUKER D8) with Cu Kα irradiation at 45 kV and 40 mA diffracted beam monochromator was used to characterize the crystal structures of photocatalysts. A scanning electron microscopy (SEM, JEOL JSM7500F) equipped with a cold cathode UHV field emission conical was applied to obtain the surface morphology. UV-vis diffuse reflectance spectra were obtained via a Hitachi U4100 UV-vis-NIR Spectrophotometer with Praying Mantis accessory. Photocatalytic activity test The activity testing procedure and apparatus were similar to those used in our previous work13. A schematic of the photoreaction system is shown in Figure 2. For each test, 10 mg catalyst was mixed with 1 mL de-ionized water to obtain a uniform slurry. Then the slurry was dispersed onto a piece of 25×50 mm rectangular glass fiber filter, which was pre-calcined at 450°C for 2 h to remove any potential organic contaminants on the fiber filter. After drying at 60°C for 2 h, the catalyst-loaded glass fiber filter was placed in a cylindrical quartz reactor. CO2 (99.999%, Airgas) continuously passed through a water bubbler and brought the mixed CO2 and H2O vapor (2.3 vol% of H2O) into the reactor. The reactor was first purged by 0.3 L/min CO2 for 1 h to eliminate air, and then maintained at 3 mL/min during the 4 h photoreaction period. The light irradiation was provided by a 450W Xe lamp (Newport). A long-pass filter (λ > 435 nm) was applied when only visible light was needed. Another infrared lamp was used to maintain the chamber temperature at 90°C instead of room temperature, as a relatively higher reaction temperature can promote the desorption of the products and improve the activity of photoactivated TiO2,11,38 so that the competition of possible photocatalytic mechanisms (e.g. charge separation, hot electron injection, and near-field enhancement) would be more distinguishable. Experiments at lower temperatures (e.g. room temperature) were not conducted because the study of temperature effect was not the objective of this work. The gas products were analyzed by a gas chromatography (GC, Fuel Cell GC-2014ATF, Shimadzu) equipped with a thermal conductivity detector (TCD) and a methanizer assisted flame ionization detector (FID).


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Figure 2. Schematic of the experimental setup for CO2 photoreduction

Results and Discussion Structural, textural and optical properties The crystal structure of photocatalyst samples was characterized by XRD. Figure 3a shows the XRD patterns of three selected samples, Au30/Ti, Au10/Ti and Au30/50Al/Ti. From the XRD patterns, it is clear that all three samples had TiO2 (anatase/rutile) characteristic peaks with similar intensity to that of TiO2 P25.39 No peaks of Au and Al2O3 were discernable, which can be explained by the very small amount of Au loading and the amorphous nature of Al2O3. The morphology of Au30/Ti was observed by SEM and shown in Figure 3b. The catalyst was an assembly of ~20 nm nanoparticles, the size of which coincides with the primary particle size of TiO2 P25.40 Au NPs were not obvious on the SEM image as their sizes were close to those of TiO2 and the number of Au NPs was much smaller than that of TiO2 due to the very low concentration of Au.



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Figure 3. XRD patterns of Au30/Ti, Au10/Ti and Au30/50Al/Ti (a), and SEM image of Au30/Ti (b) The presence of Au on the catalyst was confirmed by UV-vis diffuse reflectance spectra as shown in Figure 4. Pristine TiO2 barely absorbs visible light with wavelengths larger than 400 nm, while a red shift of light absorption edge to visible range was noticed for all three Au NPs decorated TiO2. Moreover, an absorption peak at 555, 550, and 543 nm was observed for Au10/Ti, Au20/Ti and Au30/Ti, respectively, which were attributed to the plasmonic absorption peaks of Au nanoparticles at different sizes. The blue shift in peak position is correlated with the increase of Au particle size. As shown in Figure S2a, the Al2O3 coating at 5, 20 or 50 layers did not make significant changes on the optical property of TiO2. However, for Au10/Ti samples showing a plasmonic absorption peak at around 550 nm, a decrease of the height of plasmonic absorption peaks was observed with an increase of the Al2O3 interlayer thickness, as shown in Figure S2b. This observation suggests that the LSPR effect of Au nanoparticles decreased with the Al2O3 interlayer thickness.


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Figure 4. UV-vis diffuse reflectance spectra of TiO2, Au10/Ti, Au20/Ti and Au30/Ti.

Photocatalytic activity for CO2 photoreduction with water Control experiments were first conducted by illuminating the catalyst-loaded glass fiber filter in an Ar flow and by illuminating a blank glass fiber filter without catalyst in a CO2 flow. Neither case gave rise to discernable peaks of CO, CH4, or other carbon-containing products from the GC, indicating CO2 was the only possible carbon source of CO and CH4 observed in later experiments when the catalyst was illuminated in a CO2 flow. The comparison of photocatalytic activities of pristine TiO2 and Au/TiO2 with different Au NP sizes is demonstrated in Figure 5. Under UV-vis illumination (Figure 5a), only CO production was detected for pristine TiO2, while for Au10/Ti, Au20/Ti and Au30/Ti, the yield of CO was increased by about 7, 5 and 4 times, respectively. CH4 was also detected as the secondary product for CO2 photoreduction on Au modified TiO2. Because the sizes of Au and 9 ACS Paragon Plus Environment


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TiO2 are comparable, it is possible that one Au NP may be in close vicinity with more than one TiO2 particles, thus augmenting the LSPR effect even when the number of Au NPs is not large. To further confirm the effect of LSPR, TiO2 and Au modified TiO2 were also tested under visible irradiation (>435 nm), and the results are shown in Figure 5b. For pristine TiO2 that does not absorb visible light no CO production was observed. Whereas, significantly enhanced CO production was noticed on Au modified TiO2, with Au10/Ti performing the best. The smaller sized Au nanoparticles and the larger contact area can lead to a higher efficiency of hot electron injection, because the generated hot electrons in smaller Au particles can have shorter travel paths and lower possibility to loose energy.41 From the data in Figure 5, one can see positive effects of Au NPs on CO2 photoreduction under both UV-vis and visible light irradiation, which could be attributed to the three possible effects (I, II and III) outlined in Figure 1. However, the visible light activity in general was two orders of magnitude lower than that under UV-vis, indicating that UV light is the most effective excitation source. In addition, the production of CH4 was not observable under visible light. Because it takes 8 electrons to produce CH4 and 2 electrons to produce CO, the change in product selectivity indicates fewer electrons generated under visible light, which agrees with the result that smaller amount of CO produced. Since visible light activity of Au/TiO2 is primarily derived from hot electron generation on Au NPs and subsequent transfer to TiO2, the experimental result from this work suggests that the hot electron injection mechanism does exist but is not the dominating mechanism for the enhanced CO2 reduction induced by Au NPs under UV-vis irradiation.

Figure 5. Photocatalytic CO2 reduction to CO and CH4 on TiO2, Au10/Ti, Au20/Ti and Au30/Ti under 4 h UV-vis light (a) and visible light (>435 nm) irradiation (b). 10 ACS Paragon Plus Environment

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Theoretical analysis in the literature indicates that electrons excited by photoirradiation that overcome the Schottky barrier height (SBH) at the interface of Au and TiO2 are able to transfer through the interface to TiO2. Based on Schottky-Mott rule, the SBH at the interface of Au and TiO2 can be estimated by = − , where is the SBH, is work funtion of Au (5.4 eV42), and is the electron affinity of TiO2 (4.8 eV for rutile and 5.1 eV for anatase43). TiO2 P25 is composed by about 80% of anatase and 20% rutile. Therefore, the SBH of the interface of Au and TiO2 can be estimated to be 0.3-0.6 eV. 10 nm Au NPs theoretically have a slightly larger work function42, about 0.03 eV larger than 30 nm NPs, leads to a higher SBH and lower possibility for hot electron injection. However, from Figure 5b, it is obvious that the photocatalytic activity of Au decorated TiO2 was in the order of Au10/Ti > Au20/Ti > Au30/Ti. TiO2 modified by smaller Au NPs exhibited higher activity under visible irradiation could be explained by more uniform distribution of smaller Au particles and a larger contact area between Au and TiO2, at a fixed weight ratio of Au:TiO2. The larger contact area between Au and TiO2 could lead to more efficient damping of plasmon to hot electron and hot electron injection to CB of TiO2. This result was in agreement with a theoretical prediction in the literature, which argued that a thinner film of gold has a higher probability of yielding hot electron through geometryassisted intraband transition.43



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Figure 6. CO2 photocatalytic reduction to CO and CH4 on prepared catalysts under 4 h UV-vis irradiation To further unravel the role of Au NPs in CO2 photoreduction, ALD was applied to add an interlayer of Al2O3 between Au nanoparticles and TiO2. The thickness of Al2O3 layer was varied from 0 to 50 Å by controlling the ALD cycle number, with a growth rate about 1 Å/cycle.13 As a comparison, pristine TiO2 was also coated with Al2O3. The photocatalytic activity for all samples with an Al2O3 layer were tested for CO2 photoreduction with H2O under UV-vis irradiation, and the results are shown in Figure 6. Without Au decoration, an ultrathin layer of Al2O3 on TiO2 in the range of 1 to 50 Å coating improved CO production in general, with 5 Å being the optimal thickness. This enhancement can be attributed to the passivation effect of the Al2O3 overlayer on inhibiting surface charge recombination,13,

44-45

thus extending the lifetime of photo-excited

electrons. Notably, 5 layers of coating also resulted in the highest CH4 yield. The formation of one CH4 molecule requires 8 electrons or 6 more electrons than that of CO. Surface passivation by Al2O3 layer extended the lifetime of surface accumulated photoelectrons, resulting in a higher density of photoelectrons, which could explain the higher CH4 production on Al2O3 coated TiO2. 12 ACS Paragon Plus Environment

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However, the product yield, especially CO yield started to decrease with further increased thickness of Al2O3 layer, highly possible due to the inhibited charge transfer from TiO2 to surface adsorbed gas molecules, and/or the shielding effect for near-field enhancement. Table 1. The normalized effect of Al2O3 layer on catalytic activity Influence Factor (IF) at different thickness of Al2O3 interlayer Catalyst 1Å

5Å

20 Å

50 Å

TiO2

1.21

2.80

2.11

1.81

Au10/Ti

0.91

2.97

1.63

0.80

Au20/Ti

0.98

1.99

1.21

0.88

Au30/Ti

1.10

2.02

1.35

1.31

( ) =

# !"#$ %"#"&'(# )*#+ ,&- ./ * #!&"'! # !"#$ %"#"&'(# )*#+0# ,&- ./ * #!&"'!

(eq. 1)

To investigate the effect of Al2O3 layer on charge transfer, the Influence Factor (IF) of Al2O3 layers on catalytic activity is calculated based on eq. 1, and the results are summarized in Table 1. The number of e- in eq. 1 is the total electrons generated on the catalyst to form CO (2 e- per CO formation) and CH4 (8 e- per CH4 formation). The IF reflects the overall positive (if greater than 1) or negative (if less than 1) effect the Al2O3 interlayer has on the catalytic activity of Au/TiO2 catalysts. The positive effect is resultant from surface passivation to inhibit surface charge recombination; while the negative effect is resultant from retarded charge transfer to the surface and the mitigation of near-field enhancement due to the interlayer. The IF reaches the highest value for each catalyst with 5 Å Al2O3 layer, due to the balance of positive and negative effects of the ALD layer. For an extremely thin layer (1 Å), the IF is close to one, i.e. there is a minimal effect. As the thickness of Al2O3 increases above the optimal value (5 Å), the IF decreases. With a 50 Å of Al2O3 interlayer, the IF for pristine TiO2 is 1.81, indicating the positive effect of surface passivation by Al2O3 is still dominating. Whereas, on Au10/Ti, the IF is less than 1, indicating the negative effect of retarded charge transfer and obstruction of near-field enhancement outweighs the positive effect of surface passivation by the 50 Å Al2O3 interlayer. In comparison, the IF of Au30/Ti is larger than that of Au10/Ti and larger than 1. This result 13 ACS Paragon Plus Environment


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suggests the more intense LSPR effects induced by larger Au NPs still exhibit near-field enhancement despite the “thick” Al2O3 interlayer. The results in Table 1 show the Au NP size and the thickness of Al2O3 interlayer are two important factors that influence the overall catalytic activity of Au/TiO2 catalysts. FDTD simulation Finite-difference time-domain (FDTD, Yee’s method46) simulation, a time-domain finite difference method solving Maxwell’s equation for modeling computational electrodynamics, was applied to investigate the significance of near-field enhancement. A commercial software Lumerical FDTD Solution was used to conduct FDTD simulation. The resonant wavelength of Au NPs was evaluated by a reduced grid density simulation for all the configurations over 400~800 nm incident lights. The optimum wavelength was found to be 540~560 nm for 10-30 nm Au NPs in contact with 20 nm TiO2 NPs. This wavelength range is consistent with the measured UV-vis absorbance results (Figure 4) and some literature reports.47 Therefore, a unitary wavelength λ = 550 nm was set as the light source, reducing the simulation complexity. Total field/scatted field (TF/SF) method and incident light propagating along y+ direction (shown in Figure 7) were used for all simulations, where E0 is the unit field strength induced by incident light, and E is the strength of the localized electrical field generated by Au NPs. Simulated |E|2 results of different configurations are plotted as the centerline x-z plane view of attached Au and TiO2 nanospheres, and the color bar is scaled by the log10 value of |E|2, as shown in Figure 7. Au NPs showed a strong response to the incident light and formed a strong electric field in TiO2 when the Al2O3 layer was absent. The maximum |E|2 value was more than 10000 times higher than the incident light’s |E0|2. According to Fermi’s Golden rule, the intense electric field induced by Au is able to boost the photoexcitation in TiO2, i.e. resulting in near-field enhancement. In contrast, in the presence of Al2O3 coating, the near-field enhancement was shielded and is confined in the vicinity of Au NPs. The enhanced field inside the TiO2 particle was much smaller with 20 or 50 layers of Al2O3 coating. One can also see that a larger size Au NP induced stronger field enhancement because of its stronger plasmonic response, which was consistent with the results shown in Table. 1.


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Figure 7. FDTD result of |E|2: the x-z plane of (a) 10 nm, (b) 20 nm and (c) 30 nm diameter Au sphere (bottom) attached to 20 nm diameter TiO2 sphere (top) with different thickness of Al2O3 coating on TiO2. Incident light wavelength = 550 nm. To quantitatively understand the effect of near-field enhancement, the overall field enhancement (1223 45 ) with the incident light at 550 nm was calculated using FDTD data by the following equation: 1223 45 =

∭ 7(8,:,;)|=|- >? ∭ 7(8,:,;)|=3 |- >?

(eq. 2)

in which w(x, y, z) is the weight function caused by recombination in order to depict a higher possibility for electron near the surface to reach TiO2 surface. Electrons with a closer distance from the center of a nanosphere need to travel a longer distance and corresponding time to reach the surface. However, as the characteristic diffusion lengths of electrons and holes in both 15 ACS Paragon Plus Environment


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anatase and rutile TiO2 exceed the dimension of P25 NPs,48 the influence of recombination effect can be neglected and set w(x, y, z) to be 1. From a statistical point of view, the non-uniformity of semiconductor lattice was neglected. From the calculated results shown in Figure 8, the effective field enhancement 1223 45 of a single 10 nm, 20 nm or 30 nm Au NP to a single TiO2 was about 2.5, 7.5 or 15, respectively. In consistent with the visualization in Figure 7 and results in Figure 6, a larger size Au induced a larger field enhancement P value in the TiO2 region, while a thicker Al2O3 interlayer shielded the near-field enhancement and caused P value to decrease.

Figure 8. Calculated effective field enhancement of a single Au NP to attached TiO2 nanosphere Since only 0.5 wt. % of Au was applied on TiO2, the volume ratio of Au to TiO2 NPs was only ~0.001. Therefore, even for 10 nm Au, the theoretical count ratio of Au NPs to TiO2 NPs was less than 0.01. To estimate the field enhancement by Au NPs under UV-vis light irradiation, considering the real structure of Au/TiO2, an average P value DEF was calculated by the following equation: 16 ACS Paragon Plus Environment

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DEF =

GK D⋅I(J) >J GK I(J) >J

(eq. 3)

in which (M) is the intensity of light source corresponding to wavelength M. Since DEF is much smaller than 1223 45 , direct near-field enhancement induced photo-excitation is not the sole reason for the significantly increased photoactivity of Au/Ti samples under UV-vis light. Another possible reason for the higher activity of Au/Ti than bare TiO2 is through charge separation effect, because of the small intrinsic Schottky barrier between Au and P25. LSPR induced strong electric field in the vicinity of Au can help the charge separation process by changing it from a spontaneous migration of charge carriers to a field guided transfer. Au NPs can trap electrons and retard the fast surface recombination, thereby extending the lifetime of photo-excited charge carriers. Furthermore, because the Al2O3 interlayer is in angstrom length scale, the LSPR induced electric field can probably facilitate electrons to tunnel through the ultrathin interlayer.49

Conclusion Au/Al2O3/TiO2 heterostructure with different Au NP sizes and Al2O3 layer thicknesses were synthesized and tested. The addition of Au NPs on TiO2 significantly enhanced the photolytic CO2 reduction under both UV-vis and visible light irradiation. Based on the results of CO2 reduction experiments and FDTD simulations, the promoted photocatalytic performance of Au decorated TiO2 was attributed to three reasons: (I) charge separation, (II) hot electron injection, and (III) near-field enhancement. Hot electron injection was the major effect that led to enhanced activity under visible light irradiation; however, under UV-Vis irradiation, its effect was shadowed by the two other mechanisms that made significantly more contributions to the enhanced activity through charge separation and near-field enhancement. The Al2O3 interlayer exhibited a shielding effect on near-field enhancement and inhibited charge transfer to the surface; on the other hand, the ultrathin coating could suppress surface charge recombination and improve photocatalytic activity. By tailoring the thickness of the Al2O3 interlayer between Au NPs and TiO2, it was found that charge separation and near-field enhancement were closely related to the size of Au NPs. Charge separation was more significant for smaller Au NPs due to more uniform Au distribution, while stronger near-field enhancement was found on larger Au NPs. By applying the Al2O3 interlayer, we were able to identify that charge separation and near17 ACS Paragon Plus Environment


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field enhancement are the major enhancement effects induced by LSPR of Au NPs for CO2 photoreduction under UV-vis irradiation.

Supporting Information Available: Comparison of CO2 reduction activity of pristine TiO2 and TiO2 with different Au loading (Figure S1); UV-vis diffuse reflectance spectra of TiO2 and Au10/Ti coated with 0, 5, 20 and 50 ALD layers of Al2O3 (Figure S2). This material is available free of charge via the Internet at http://pubs.acs.org.

Acknowledgement This work is supported by U.S. National Science Foundation CAREER Award (CBET 1538404).

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CO2 Reduction by Plasmonic Au Nanoparticle-Decorated TiO2

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