Half-Encapsulated Au Nanorods@CeO2 Core@Shell Nanostructures

Feb 7, 2019 - Upon 808 nm laser illumination for 7 min, the temperature of the solution containing 75 ppm half-encapsulated Au NRs@CeO2 (h-Au@CeO2) ...
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Half-Encapsulated Au Nanorods@CeO2 Core@Shell Nanostructures for Near Infrared Plasmon-Enhanced Catalysis Jing Pan, Lingling Zhang, Songtao Zhang, Zhan Shi, Xiao Wang, Shuyan Song, and Hongjie Zhang ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.9b00002 • Publication Date (Web): 07 Feb 2019 Downloaded from http://pubs.acs.org on February 10, 2019

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Half-Encapsulated Au Nanorods@CeO2 Core@Shell Nanostructures for Near Infrared Plasmon-Enhanced Catalysis Jing Pan,†,‡ Lingling Zhang,†,‡ Songtao Zhang,‡ Zhan Shi,† Xiao Wang,‡ Shuyan Song*,‡ and Hongjie Zhang‡ †



College of chemistry, Jilin University, Changchun 130012, P. R. China. State Key Laboratory of Rare Earth Resource Utilization Changchun Institute of Applied

Chemistry, Chinese Academy of Sciences, Changchun 130022, P. R. China. KEYWORDS surface plasmon resonance, anisotropy, photocatalysis, Au@CeO2, 4-nitrophenol

ABSTRACT

The technology of surface plasmon resonance (SPR) is considered to be highly attractive approach to directly harvest optical energy for photocatalytic reactions. However, photocatalytic application is limited mainly by inefficient absorption of catalysts in the visible and infrared range. In this work, an anisotropic Au@CeO2 mushroom-like core@shell nanostructure is designed by controlled hydrolysis of the cerium acetate utilizing the cetyltrimethyl ammonium bromide (CTAB) as soft template. Special aspect ratio of the Au nanorods (Au NRs) and the

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anisotropy of the nanostructures increase the plasmon absorption in the frequency near infrare range (NIR). Upon 808 nm laser illumination for 7 min, the temperature of the solution containing 75 ppm half-encapsulated Au NRs@CeO2 (h-Au@CeO2) increases up to 53.8 °C. Besides exhibiting plasmon-enhanced photothermal properties, h-Au@CeO2 also has good 4nitrophenol (4-NP) catalytic reduction activity under near infrared laser. The anisotropy of hAu@CeO2 promotes the plasmon produced hot-electron transfer and separation of electron-hole pairs resulting in better photocatalytic effect towards 4-NP reduction compared with totallyencapsulated Au NRs@CeO2(t-Au@CeO2) nanostructures.

INTRODUCTION The surface plasmon resonance (SPR) of Au, Ag and Pd nanoparticles can get strong electromagnetic fields to effectively enhance the absorption in spectrum and produce SPR hotelectron and photothermal effect.1-4 However, the photocatalytic application of SPR is limited mainly by inefficient absorption of catalysts in the visible and infrared range. Au nanorods (Au NRs) also have received the widespread attention because of their wide application, including plasmon enhanced spectrum, chemical sensor, biological imaging and treatment, light activated and optoelectronic devices, and so on. The most fascinating feature of Au NRs is that their different enhanced SPR can be obtained by adjusting the aspect ratio of nanorods.2,

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As a

result, a wide spectral absorption can be obtained by modulating aspect ratios of Au NRs to overcome the disadvantages that the plasmon frequency of noble metal itself is usually in visible and ultraviolet range.7

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Previous literature suggests that plasmon metal/semiconductor connection–a Schottky junction–can effectively output hot electron and enhance the visible-light-driven chemical processes, which can meet charge carriers both of the oxidation and reduction. 8 It is found that the hot electron escaping from the surface of the noble metal nanoparticles can be captured by the connected oxide.9 The metal nanoparticles are positively charged and generate electromagnetic fields between the noble metal and the oxide to promote energy flow.10 Therefore, the hot-electron separation can be easily expanded when Au NRs are connecting with effective electron acceptor (such as TiO211, Pd12,

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and graphene14). Additionally, anisotropic

nanostructures of these composites usually exhibit rich morphology-dependent properties.15,

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They can promote the complete separation of the charge and exchange process inside the Au NRs and prevent the holes combining with electrons, thus the catalytic activity can be acutely enhanced.17,

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Selective molecular cladding method plays an important role in most of the

system of the anisotropic growth.19, 20 The technology has been successfully used in the growth of the anisotropic metal hybrid structures ( eg, Pt21, 22, SiO223, Fe2O324 and TiO225 on Au NRs) by covering the cetyltrimethyl ammonium bromide (CTAB). However, there have been no report of the anisotropic assembling CeO2 onto Au NRs because of the fast hydrolysis of Ce3+ and the lattice mismatch between CeO2 and Au. In recent years, CeO2 has attracted wide attention in many catalytic fields.9 CeO2 under ambient environment is generally regarded as a n-type semiconductor due to O vacancies and has excellent oxidation reduction properties mainly because of its ability to store and release oxygen.26 Furthermore, in most cases CeO2 can be a stabilizer or co-catalyst with noble metal to improve the catalytic performance of the main active phase.27,

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The synergy effect between noble metals and CeO2 can be utilized to improve

catalytic activity. Although there have been many reports about the binding of gold and cerium

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oxide to catalyze reactions,3, 29-32 the experiments on controlled synthesis of anisotropic coatings are not reported. Therefore, to produce Au nanorods/CeO2 anisotropic nanostructures is considered as an efficient method to enhance the photocatalytic activity through maximizing the SPR and synergy effect. Herein, we develop a soft templates method and control hydrolysis rate to synthesize anisotropic Au NRs@CeO2 core@shell nanostructure, as shown in Scheme 1. Firstly, the obtained uniform Au NRs are treated with CTAB. CTAB is unevenly distributed on the side of Au NRs caused by the different surface curvature of Au NRs. After adding cerium acetate (Ce(Ac)3), the solution begins to slowly heat until the specific temperature to reduce the hydrolysis rate. At the beginning, cerium oxide particles selectively deposit at one end of the Au NRs. Due to the slow hydrolysis and the lattice matching, the subsequent deposited CeO2 preferably deposit next to the former CeO2, eventually resulting in anisotropic coating. In contrast, the Au NRs without CTAB to hinder the deposition has been completely covered. The as-prepared nanostructures exhibit plasmon-enhanced photothermal properties and 4-NP catalytic reduction activity under near infrared laser. Scheme 1. Fabrication process for t-Au@CeO2 and h-Au@CeO2.

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EXPERIMENTAL METHODS Synthesis of Au nanorods (Au NRs) High quality Au NRs are prepared as reported previously.33 The details for the experiment are put in supporting information. Synthesis of half encapsulated-Au NRs@CeO2 (h-Au@CeO2)/totally-encapsulated Au NRs@CeO2 (t-Au@CeO2) nanostructures 6.66 mL Au NRs aqueous solution ( 1 mg mL-1 ) is diluted into 50 mL with DI-water in round-bottom flask, which is used in all of the preparations. 1 mL CTAB (0.1 M) (the mole ratio of surfactants and Au nanorods is 15:1) is added to the solution under stirring in room temperature for 30 min. Before rising temperature, 0.735 mL cerium acetate (0.1 M) is added into the solution dropwise and then the temperature is stayed at 80 °C for 4 h. The product is collected by centrifuge at 8000 rpm for 10 min and washed with water twice to remove the surfactant and unreacted Ce3+ precursor. The t-Au@CeO2 can be obtained by the same method just without CTAB. Structural and Optical Characterization Transmission electron microscopy (TEM), high-angle annular dark-field scanning TEM (HAADF-STEM), high-resolution TEM (HRTEM) and energy dispersive X-ray (EDX) analysis are collected a TECNAI G2 transmission electron microscope with 200 kV accelerating voltage. The morphology and microstructure of the products are characterized using a field-emission scanning electron microscope (FE-SEM, HITACHI S-4800). X-ray photoelectron spectra (XPS) measurements are conducted on an ESCALAB-MKII 250 photoelectron spectrometer (VG Co.). X-ray diffraction (XRD) patterns characterization are performed using a Bruker D8 ADVANCE with Ni filtered Cu Kα radiation (λ=1.54 Å). ICP-AES (Varian Liberty 200 spectrophotometer)

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is used to determine the concentration of Au and Ce. Absorption spectra is recorded with UV– vis–NIR (SHIMADZU, UV-3600). The H2-TPR studies of the different dried samlpes are performed in Chemstar TPx under a flow of 10% H2/Ar gas mixture (30 cc min-1) with a heating rate of 10°C min-1 from 50 °C to 850 °C. The samples are prepared by mixing each assynthesized active material (10 wt%) with Al2O3 (90 wt%). H2O produced during the reduction process is trapped before the TCD detector. Characterization of catalytic activity To study the catalytic activity, a freshly prepared aqueous solution of sodium borohydride (NaBH4) (3 mL, 8 mg mL-1) is added into 4-NP aqueous solution (25 μL, 0.01 M). At the stage, the nitrophenol is converted to nitrophenolate anion (aqua A). After that, the dispersive liquid of h-Au@CeO2 containing 4.5 μg Au is added in aqua A. The mixture is transferred into a quartz cuvette at once and irradiated with an 808 nm NIR laser (1.25 W cm-2). UV-Vis absorption spectra is recorded to monitor changes at 400 nm at different time intervals. In a similar way the study was carried out by the catalyst of t-Au@CeO2, Au NRs, the mixture of Au NRs and CeO2 and all with no irradiation condition. We also test the catalytic rate of h-Au@CeO2 under the hydrothermal and no irradiation condition.

RESULTS AND DISSCUSSION The Au NRs are prepared through a seed-mediated method according to the previous report with a consistent structure and morphology (Figure S1 and S2).9 The average diameter and length of the starting Au NRs is (16±2) and (72±8) nm, respectively, with an aspect ratio of 4.5. The anisotropic overgrowth of CeO2 on Au NRs is through utilizing CTAB as a soft template and controlling the rate of hydrolysis of Ce(Ac)3 to obtain the h-Au@CeO2 core@shell

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nanostructure. As displayed in Figure 1a, the product achieves the epitaxial growth on Au NR to form a one tip covered mushroom-like structure. The CeO2 shell is spherical with an average diameter of 50 nm covering about half of the Au NRs. And the structure of Au NRs remains the same. According to the HRTEM images (Figure 1c and 1d), the Au NR cores possess lattice fringes with d-spacing of 1.42 Å assigned to {220} facets and the CeO2 are crystalline with interplanar spacing of 3.15 Å which corresponds to the {111} facets. Note that the CeO2 caps and Au nanorods are crystallographic based on the results revealed in the XRD patterns (Figure S2), which matches well with the pure cubic CeO2 phase (JCPDS No.34-0394) and cubic Au phase (JCPDS No.04-0784), respectively. To confirm the anisotropic structure of h-Au@CeO2, the elemental distributions of Ce and Au in nanostructure are characterized by the HAADFSTEM analyses (Figure 1e-1g), clearly revealing the only one end covered of CeO2 shell on Au NR. No Ce signal has been found in another tip, further confirming one tip-side selectivity of the deposition. Additionally, a totally coated core-shell structure, t-Au@CeO2, is also obtained without the CTAB treatment process (Figure 1b and Figure S2). The XRD peaks of the tAu@CeO2 also match well with the CeO2 and Au. The peak of Au is weakened because Au is covered. The longitudinal plasmon (LP) peak of the Au nanorods in aqueous solutions is at 835 nm as shown in Figure 2. The LP peaks of the h-Au@CeO2 nanostructure solutions, located at 905 nm, are obviously red-shifted compared to that of the starting Au NRs sample (Figure 2). Although the UV spectrum of t-Au@CeO2 has the more obvious red shift, the absorption intensity decreases a lot, which illustrates that the cladding of CeO2 leads to red shift, but too much pack will also affect the absorption of energy. The phenomenon corresponds to a local refractive index change because of the formation of the CeO2 on the Au NRs and according to previous reports of dielectric material coating on Au NRs.

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In addition, after the cladding of

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CeO2, there are a new absorption peak locating in the ultraviolet region which is the typical charge transfer absorption peak of CeO2 from O 2p to Ce 4f.34-36

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Figure 1. TEM images of (a) h-Au@CeO2 and (b) t-Au@CeO2. High-resolution TEM (HRTEM) images of (c) CeO2 and (d) Au NRs. (e), (f) and (g) HAADF-STEM image of h-Au@CeO2 and the elemental maps of Au and Ce.

Figure 2. UV-vis-NIR absorption spectra of t-Au@CeO2, h-Au@CeO2 and Au NRs with 75 ppm.

Figure S3 shows the X-ray photoelectron spectra (XPS) of h-Au@CeO2 and t-Au@CeO2, which is performed to examine the surface elements and their valence states. The XPS element survey scans of the h-Au@CeO2 and t-Au@CeO2 display that gold, cerium, oxygen, silver and carbon are present in the h-Au@CeO2, as shown in Figure S3a. The silver is used to prepare Au nanorods. However, the gold in t-Au@CeO2 corresponding to the cores, is not detected in the full spectrum. This is due to that the detectable depth of XPS technique is no more than 10 nm,

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which is much less than the CeO2 shell thichness. This is a strong proof that the Au nanorods are well encapsulated by the CeO2 shells. The peaks centers of Au 4f are at 83.3 and 87.0 eV belong to metallic Au0. The position shift may be due to the electron transferring from Au to CeO2. The Ce 3d spectra is fitted with 10 peaks, as described in previous studies.37 The peaks labeled “u“are indexed to Ce 3d3/2 and the peaks labeled “v“ are indexed to Ce 3d5/2. The peaks centered at u2, v2, u0 and v0 are assigned to Ce3+, and others are assigned to Ce4+. For h-Au@CeO2, the Ce3+ ratios relative to the total amounts of Ce are estimated to be 25.93%, which is higher than 22.8% of tAu@CeO2. Previous literatures show a high Ce3+ concentration can give ceria nanostructures an admirable catalytic capability to reversibly release/store oxygen from/into them through the increase of O vacancies.9 The presence of Ce3+ in the samples is on account of the small size of the CeO2 particles, which have abundant defects. The Raman spectra is mainly the F2g of the cubic CeO2 structure around 445 cm-1, which is attributed to a symmetrical stretching mode of the Ce-O8 vibrational unit. The peak of h-Au@CeO2 has a obvious migration and is wider than tAu@CeO2 in the Raman spectra (Figure S4), which further proves that there are more oxygen defects. For O 1s XPS spectrum, the primary peak located at 530.0 eV and 531.9 eV are indexed to the chemisorbed oxygen and weakly bonded oxygen species in the sample. And the peak at 528.8 eV is denoted as lattice oxygen, which represents the O 1s ionization for oxygen mainly associated with the ceria. Subsequently, the mechanism of anisotropic CeO2 growth on the Au NRs is explored. First, we study the effect of different surfactants, including cationic surfactants (sodium dodecyl sulphate (SDS)), short carbon chain of surfactant (dodecyltrimethylammonium Bromide(C12TAB)), different halogen ions surfactant (cetyl trimethyl ammonium chloride(CTAC)), simply polymer surfactant (polyvinylpyrrolidone(PVP)) and KBr. The morphology of different experimental

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results have shown in Figure S5, with same dosage (the mole ratio of surfactants and Au nanorods is 15:1). It can be seen that fully CeO2 coating can be realized by using CTAC and PVP, which suggests that bromide ions play an important role during the anisotropic growth. It has been reported that bromine ions interact strongly with noble metals, and they prefer to be attached to surfaces with high surface free energy, which can affect the growth of surface nanostructures.38 We also synthesize Au NRs@CeO2 under the condition of adding only KBr. We can find from the results that only bromine ions cannot form the desired semi-clad structure, indicating that the hindering effect of long carbon chains is also critical. In addition, the uniform distribution on the Au NRs of PVP cannot help CeO2 selectively clad. Interestingly, when the surfactant is SDS, we get the CeO2 nanowires growing on the tips of Au nanorods. Under the condition of C12TAB, we also obtain half cladding structure, which shows that the length of carbon chain has almost no obvious effect on selective coating. The effects of surfactant dosage are also examined by adjusting the mole ratio of CTAB: Au is 5:1, 10:1, 15:1, 20:1 and 50:1 respectively (as showed in Figure S6 and Figure 1a). It is found that superfluous CTAB can result in CeO2 independently nucleating and cannot achieve coating on the surface of gold rods. Once the CTAB is too few to hinder the CeO2 depositing, which cannot achieve coating selectively. Therefore, a suitable amount of CTAB is very important for the uneven distribution of CeO2 on the Au NRs. The detail growth process can be summarized as follows. CTAB is mainly distributed on the side of the Au NRs and little distributed on the tips because of the different surface curvature of Au NRs. At the beginning, selective deposition of cerium oxide is firstly at one tip of the Au NRs where there is hardly any CTAB. Due to the slow hydrolysis and lattice matching, the following formed CeO2 preferably deposits around the former CeO2 rather than other location on the Au NRs, eventually resulting in uneven coating.

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The influence of temperature is also discussed in details (Figure S7). The different temperatures of 60 °C, 80 °C and 100 °C lead different hydrolysis rate of Ce3+. As a result, the CeO2 crystals with different size gather on the Au NRs. In the 100 °C, the hydrolysis rate is too fast to get small crystals. At high temperatures, fast deposition of CeO2 is likely to disturb the distribution of CTAB, which influences the deposition of growth CeO2.39, 40 Under the condition of 60 °C, the hydrolysis rate slows down because of the low hydrolysis temperature. As a resulting, the size of CeO2 particles on the Au NRs is relatively large, which have no enough defect to catalyse. Therefore, 80 °C is the best reaction temperature in this work. The reaction in the 80 °C has suitable hydrolysis speed to realize the selective coating and can minimize the size of CeO2 particles to increase the oxygen defects to improve catalytic ability. Finally, we study the sample with different reaction time with the CTAB (mole ratio =15:1) in 80 °C and find out that CeO2 starts growing on the tip (Figure S8a). With the gradual growth process of CeO2, the subsequent deposited CeO2 preferably deposits next to the former CeO2 due to the slow hydrolysis and the lattice matching, which eventually results in anisotropic coating. (Figure S8b and S8c). The reaction is over after proceeding 4h by completely consuming Ce3+ (Figure S8d). To survey the photothermal performance, different aqueous solutions of the samples with same concentrations are exposed to an 808 nm NIR laser at 1.25 W cm-2 for 7 min. Under the given laser power, the temperature of the different aqueous solution increases with the passage of irradiation time. Figure 3a presents photographs of aqueous solutions of different samples captured by infrared camera, when they are irradiated by laser. The photothermal performances of different samples are shown in Figure 3b. The pure water as controlled experiment gives a temperature increase of less than 2 °C, while upon laser illumination for 7 min, the temperature

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of the solution containing 75 ppm h-Au@CeO2, t-Au@CeO2 and Au NRs increases up to 53.8 °C, 46.4°C and 53.6 °C, respectively. Highly increased temperature of Au NRs is observed, which is ascribed to the strong absorption at 808nm of Au NRs in the same concentration. According to the UV absorption spectrum, Au NRs, h-Au@CeO2 and h-Au@CeO2 have different absorption at 808nm. The absorption value of Au NRs is almost twice that of hAu@CeO2. Therefore, when the light at 808 nm is used for the same time, the temperature of the Au NRs and h-Au@CeO2 is almost the same, indicating a better photothermal conversion efficiency with the CeO2. We also find out that the absorption of t-Au@CeO2 at 808 nm is much lower than that of h-Au@CeO2. According to previous literature41, 42, due to the long narrow gaps between Au NRs and CeO2 induce a strong plasmonic coupling effect, t-Au@CeO2 still has a photothermal conversion effect. The heating effects come from the collision electrons with the phonon-phonon relaxation coupled with the metal lattice and the semiconductor lattice.43 Moreover, h-Au@CeO2 also exhibited good stability, and there is no change after laser irradiation (808 nm, 1.25 W cm-2) for 30 min (Figure 3c).

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Figure 3. (a) Photos and (b) Temperature rising curves of t-Au@CeO2, h-Au@CeO2, Au NRs and H2O upon 1.25 W cm-2 808 nm laser irradiation. (c) Photos of h-Au@CeO2 before and after the laser irradiation 30 min. The results of the hydrogen temperature programmed reduction (H2-TPR) of the different samples are shown in Figure 4. The reduction of the CeO2 surface oxygen is responsible for that the H2-TPR spectrum of CeO2 has one reduction peak at 471 °C, which is in agreement with result for conventional CeO2.44 And the broad reduction peak between 700 °C to 800 °C ascribes to the reduction of lattice oxygen.45 In the H2-TPR spectrum of Au NRs, two very low intensity and broad reduction peaks are registered between 345 °C to 551 °C and 600 °C to 735 °C. The peaks are related to gold particles reduction process. Two peaks are observed in the H2-TPR spectrum of h-Au@CeO2 and t-Au@CeO2. According to the literature32, the low-temperature peak can be assigned to the co-reduction of Au and surface oxygen of the CeO2, and the high-

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temperature peak represents to the reduction of lattice oxygen. The temperature of the lowtemperature peak for h-Au@CeO2 and t-Au@CeO2 is lower than that for Au NRs and CeO2, which is due to the interaction between Au and CeO2 in the samples.29 As gold is reduced, hydrogen atoms from H2 dissociation by gold spill over to the ceria and reduce its surface. And nearly all surface oxygen species of ceria can be reduced at low temperature in the presence of gold.43 So after growth of CeO2 on Au NRs, CeO2 have strongly synergistic effect with the Au NRs, thus changing the reducibility.

Figure 4. H2-TPR curves of different samples. The catalytic activities of the h-Au@CeO2 is investigated by the reduction of 4-NP in the existence of NaBH4, which is usually used as a model system to quantitatively judge the activity of catalysts with the advantage of the possibility of monitoring by UV-vis spectroscopy.46 The results of conversion of 4-NP to 4-aminophenol (4-AP) in water at room temperature of different catalysts are shown in Figure 5. In the Figure 5a, 4-NP in water has a maximum absorption at 317 nm (Figure 5a1). The color of the solution changes from the light yellow to intense yellow after adding NaBH4. Meanwhile, the maximum absorption peak changes to 400 nm because of the formation of 4-nitrophenolate ions under alkaline conditions (Figure 5a2). 4-nitrophenolate

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ions solution is placed in 20 min without the catalyst, whose absorption spectrum has few change (Figure 5a3). After placing two days, the absorbance of the 4-NP solutions in 400nm has only a little decline (Figure 5a4). After adding the catalysts into the solution containing 4-NP (accurately, 4-nitrophenolate) and NaBH4, the peak at 400 nm decreases and a new peak appears at about 300 nm which corresponds to the formation of 4-AP, as shown in Figure S9. The reaction kinetics could be monitored easily from the time-dependent absorption data (Figure 5b). After adding the h-Au@CeO2, t-Au@CeO2, the mixture of Au NRs and CeO2 and Au NRs, when the reaction continues 9 min, 11 min, 15min and 20 min respectively, the peak originated from the 4-NP is no longer observed, meaning that the catalytic reduction of 4-NP has totally completed. In order to detect the physical adsorption property of the catalysts, we also test the absorption spectra of 4-NP by only adding catalyst and without reducing agent. As shown in Figure S10, we find that the absorption spectrum dose not change significantly within 20 min. Therefore, it is proved that the reduction of 4-NP is due to hydrogenation reduction after adding catalyst and reducing agent, and there is no other physical adsorption process.

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Figure 5. (a) UV-vis spectra of 4-NP (1) before and (2) after the addition of NaBH4 solution and the time-dependent for (3) 20 min and (4) 2 day without any catalyst. (b) Plots of I/I0 against time for the reduction of 4-NP over h-Au@CeO2, t-Au@CeO2, Au NRs and the mixture of Au

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NRs and CeO2. (c) Plots of I/I0 against time for the reduction of 4-NP over h-Au@CeO2, tAu@CeO2, Au NRs and the mixture of Au NRs and CeO2 under the illumination with 808 nm laser in 1.25 W. (d) Plots of I/I0 against time for the reduction of 4-NP over h-Au@CeO2 in the condition with illumination, water bath and no illumination and no water bath. (e) Plots of – ln(I/I0) against time for h-Au@CeO2, t-Au@CeO2, Au NRs and the mixture of Au NRs and CeO2 under the illumination with 808 nm laser in 1.25 W. (f) Cycling test of h-Au@CeO2 for the reduction of 4-NP under laser irradiation. Under the irradiation of 808 nm laser in 1.25 W, the same experiments are carried out. Under the irradiation, the reactions based on different catalysts take 4 min, 8 min, 13 min and 15 min to complete, respectively (Figure 5c). It is found that the catalytic properties have obvious improvement under the light irradiation and the reactions are finished more thoroughly. The temperature of the solution contained h-Au@CeO2 as catalyst increases from room temperature (23 °C) to 27 °C after reduction reaction due to laser irradiation. To eliminate the influence of temperature, the contrast catalytic test used h-Au@CeO2 as catalyst without light but in water bath heating to keep the same temperature at 27 °C is carried out. The plots of intensity of the peak at 400 nm changed over time is shown in Figure 5d. From the results, it is concluded that the improvement of catalytic activity of the h-Au@CeO2 is contributed by plasmon-excitationinduced hot electrons more than plasmonic photothermal conversion. The Au nanorod cores are responsible for light absorption and get energy to generate hot electrons under the NIR illumination. Then the hot electrons overcome the Schottky barrier to flow from Au NRs to CeO2 (Scheme 2) and are available for photo reduction with CeO2 which serves as the electron-transfer medium. The BH4- are adsorbed onto the surface of Au NRs with holes left, which have positive potential. Then the BH4- give the electrons to the Au NRs and active hydrogen species (H) are

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produced to convert 4-NP to 4-AP. CeO2 on the other tip obtains electrons with a negative potential, which absorbs hydrogen ions from water to form H on the surface.17 4-NP molecules are reduced to 4-AP. Au NRs cores and half coated CeO2 shells generate an electromagnetic field that forms continuous electrons flux, and thus enhances hot-electron generation from the excitation of the SPR. As for t-Au@CeO2, the fully coated structure not only affects the absorption of excitation light, but also is not conducive to the separation of hot electrons and holes, thus affecting the progress of the catalytic reaction. Therefore, the anisotropic core@shell nanostructures play an important role to determine the photocatalytic activity by promoting separation and exchange of charge to improve catalytic capability. The presence of CeO2 also enhances its stability and synergistic effect to improve the catalytic properties of the noble metal. Scheme 2. Proposed mechanism for the reduction of 4-NP over the h-Au@CeO2 catalyst under the near infrared light illumination.

Figure 5e displays the linear relationship between -ln(I/I0) and reaction time (t), indicating that all these plots also accord with the pseudo-first-order reaction kinetics. The reducing reaction rate constants (K) are calculated to be 0.7039 min-1, 0.39414 min-1, 0.2631min-1 and 0.0864 min1

for h-Au@CeO2, t-Au@CeO2, the mixture of Au NRs and CeO2 and Au NRs, respectively. So

as to compare with reports in the literature, we calculate the ratio of rate constants (k) over the total weight of the catalysts, k=K/m.47 The activity factor k of the h-Au@CeO2, t-Au@CeO2, the ACS Paragon Plus Environment

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mixture of Au NRs and CeO2 and Au NRs is 156.4 min-1 mg-1, 87.6 min-1 mg-1, 58.5 min-1 mg-1 and 19.2 min-1 mg-1. As shown in Figure S11, the activity factor k of the h-Au@CeO2, tAu@CeO2, the mixture of Au NRs and CeO2 and Au NRs without illumination is 73.3 min-1 mg1,

62.2 min-1 mg-1, 40 min-1 mg-1 and 28.9 min-1 mg-1. As summarized in Table S1, our catalyst,

Au NRs@CeO2, have a catalytic activity comparable to and even highest in those of some reported Au/CeO2 catalysts for the conversion of 4-NP to 4-AP. Catalytic reusability is one of the most important factors in practical catalytic applications. Thus, the reusability of the catalyst of h-Au@CeO2 under laser irradiation is also investigated. After the catalytic reaction, the 4-NP and NaBH4 are directly added into the solution, which are used for the second cycle. The catalyst still has great activity during the reduction of 4-NP. The relevant UV-Vis absorption spectra of the three runs of the catalytic reduction over h-Au@CeO2 is presented in Figure 5f. The data shows that the used catalyst is not as same as the fresh one but it still works efficiently. There is slightly loss of catalytic activity observed after the catalytic system is re-used three times and more than 95% of the 4-NP can be reduced. CONCLUSIONS In summary, we have prepared the Au NRs@CeO2 anisotropic core@shell mushroom-like nanostructure via controlled hydrolysis of the cerium acetate utilizing CTAB as soft template. And the whole experimental process is also discussed in detail to modulate the half coated structure. H-Au@CeO2 utilizes the anisotropy to promote the surface plasmon resonance produced hot-electron separation and transfer resulting in better photocatalytic effect compared with t-Au NR@CeO2 nanostructures. The construction of anisotropic nanostructures in our work is therefore promising candidate for applications in plasmon regulation, biomedical photothermal therapy and solar energy harvesting.

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ASSOCIATED CONTENT Supporting Information. The following files are available free of charge. Further details of the experiment step, additional TEM, XRD, XPS, Raman spectra and the UV spectra of catalytic process are available. AUTHOR INFORMATION Corresponding Author * E-mail: [email protected]; Phone: +86-8526-2037. Web page: http://reru.sklonline.cn/Talent.asp. 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 National Natural Science Foundation of China; Youth Innovation Promotion Association of Chinese Academy of Sciences; Chinese Academy of Sciences-Commonwealth Scientific and Industrial Research Organization. ACKNOWLEDGMENT The authors are grateful for the financial aid from the National Natural Science Foundation of China (21590794, 21771173 and 21521092), Youth Innovation Promotion Association of Chinese Academy of Sciences (2011176), the project development plan of science and

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technology of Jilin Province (20180101179JC and 20160520126JH) and CAS-CSIRO project (GJHZ1730).

ABBREVIATIONS SPR, surface plasmon resonance; CTAB, cetyl trimethyl ammonium bromide; NIR, near infrare range; h-Au@CeO2, half-encapsulated AuNR@CeO2; t-Au@CeO2, totally-encapsulated AuNR@CeO2; Au NRs, Au nanorods; Ce(Ac)3, cerium acetate; NaBH4, sodium borohydride; 4NP, 4-nitrophenol; PVP, polyvinylpyrrolidone; CTAC, cetyl trimethyl ammonium chloride; C12TAB, dodecyltrimethylammonium Bromide; SDS, sodium dodecyl sulphate; LP, longitudinal plasmon; H2-TPR, hydrogen temperature programmed reduction; TEM, Transmission electron microscopy; HAADF-STEM, high-angle annular dark-field scanning TEM; HRTEM, highresolution TEM; EDX, energy dispersive X-ray; FE-SEM, field-emission scanning electron microscope; XRD, X-ray diffraction; XPS, X-ray photoelectron spectra; SDS, sodium dodecyl sulphate ;, short carbon chain of surfactant. REFERENCES (1)

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