Plasmonic Au-Loaded Hierarchical Hollow Porous TiO2 Spheres

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Energy Conversion and Storage; Plasmonics and Optoelectronics 2

Plasmonic Au Loaded Hierarchical Hollow Porous TiO Spheres: Synergistic Catalysts for Nitroaromatic Reduction Qingzhe Zhang, Xin Jin, Zhenhe Xu, Jianming Zhang, Ulises Felix Rendon, Luca Razzari, Mohamed Chaker, and Dongling Ma J. Phys. Chem. Lett., Just Accepted Manuscript • Publication Date (Web): 28 Aug 2018 Downloaded from http://pubs.acs.org on August 29, 2018

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The Journal of Physical Chemistry Letters

Plasmonic Au Loaded Hierarchical Hollow Porous TiO2 Spheres: Synergistic Catalysts for Nitroaromatic Reduction

Qingzhe Zhang,† Xin Jin,† Zhenhe Xu,*,‡ Jianming Zhang,§ Ulises F. Rendón,† Luca Razzari,† Mohamed Chaker,† Dongling Ma*,† †

Institut National de la Recherche Scientifique (INRS), Centre Énergie Materiaux et

Télécommunications, Université du Québec, 1650 Boulevard Lionel-Boulet, Varennes, Québec J3X 1S2, Canada ‡

College of Applied Chemistry, Shenyang University of Chemical Technology, Shenyang

110142, China §

School of Chemistry and Chemical Engineering, Jiangsu University, Zhenjiang 212013,

China

* Corresponding authors. Prof. Zhenhe Xu Email address: [email protected]; Prof. Dongling Ma Email address: [email protected]; Phone: 514-228-6920; Fax: 450-929-8102.

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ABSTRACT Plasmonic Au nanoparticle (NP)-loaded hierarchical hollow porous TiO2 spheres are designed and synthesized with the purpose of enhancing the overall catalytic activity by introducing the Au plasmonic effect into the system, where Au NPs themselves are catalytically active. The constructed nanohybrid exhibits both high activity in 4-Nitrophenol reduction, compared to all the previously reported Au-based catalysts, and high selectivity. The synergy of inherent catalytic property of Au NPs and the plasmonic effect (mainly via hot electron transfer) under irradiation is confirmed by a series of control experiment. The specifically designed, porous hollow structure also greatly contributes to the good catalytic activity, because it provides large surface area, facilitates reactant adsorption and hinders charge recombination. In addition, theoretical calculations reveal that such a structure also leads to an increase in light absorption of about 21% in the range of 400-800 nm with respect to a uniform water-TiO2 background featuring the same filling factor. This work provides insight into the rational design of plasmon-enhanced catalysts that will show their versatility in various electro-/photo-catalysis. KEYWORDS: Porous hollow TiO2 spheres, Au nanoparticle, surface plasmon resonance, 4nitrophenol reduction, hot electron transfer


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1. INTRODUCTION As one of the most frequently encountered pollutants among the nitroaromatic compounds in the industrial and agricultural effluent, 4-nitrophenol (4-NP) is toxic and hazardous, provoking damages to the aquatic environment. Still worse is the biological and chemical stability of 4-NP that makes it resistant to natural degradation,1-2 which leads to many difficulties in the treatment of 4-NP-containing wastewater. The catalytic reduction of 4-NP to 4-aminophenol (4-AP) by metal catalysts, such as Au, Ag, Pd and Ru nanoparticles (NPs), in the presence of a reducing agent of NaBH4 has been widely utilized in the removal of 4NP.3-9 The reduction product, 4-AP, is considered as an important intermediate in the manufacture of dyes, fungicides, and many antipyretic and analgesic drugs.10 Hence, the catalytic reduction of 4-NP to 4-AP, making waste profitable by transforming a toxic pollutant to a useful product, is of great significance. As Au is a precious metal, achieving high reduction efficiency with the use of less amount of Au has been a “holy grail” for researchers in this area. Photocatalysis, a recognized green and clean technology, has been extensively studied for eliminating contaminants via redox reactions directly driven by solar energy.11-14 Unfortunately, up to now satisfying results meeting the industry standards have not been obtained yet for most of the photocatalytic reactions in real world operations under sunlight. A key to achieving high-efficiency photocatalysis consists in the development of an effective photocatalyst. TiO2 is considered to be the most promising photocatalyst for commercial application, owing to its low cost, nontoxicity and chemical inertness.15-17 Howbeit the intrinsic drawbacks of TiO2, i.e., the large band gap (confining its activity mainly into the ultra-violet (UV) region), and high recombination rate of photogenerated charge carriers cause low photocatalytic efficiency, and thereby severely hinder the practical applications of pure TiO2 photocatalysts whatever their size and shape.18-22 The combination of plasmonic Au NPs with TiO2 has recently emerged as a new research frontier in enhancing the photocatalytic activity by the surface plasmon resonance (SPR) of Au NPs under visible light excitation.23-32 The presence of SPR leads to light scattering enhancement and localized field amplification which can prolong the average photon path length in Au/TiO2 system and thereby enhancing the light absorption to generate charge carriers in TiO2. In addition, SPR can enhance photocatalytic activity by hot electron injection which is analogous to dye sensitization.23, 33 Essentially, the plasmonic Au NPs can act as a dye sensitizer to absorb photons and transfer SPR-induced energetic charge carriers


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(“hot” electrons in case of Au NPs) to neighboring TiO2. These “hot” electrons, generated by the non-radiative relaxation of excited resonant plasmons via Landau damping, populate electronic states above the Fermi level of Au NPs transiently.31, 34-35 It was verified both experimentally and theoretically that the hot electrons can overcome the potential barrier at the Au-TiO2 interface and be directly injected into the conduction band (CB) of the proximal TiO2.26, 34-35 Through this SPR-induced hot electron injection, the Au NP-TiO2 composite is endowed with visible light-photocatalytic activity. It is very attractive to investigate this system in the photocatalytic reaction of 4-NP as it can potentially lead to superior catalytic activity and can help answer stimulating scientific questions. In most of the studies in the area of plasmon-enhanced catalysis, Au itself is not an active catalyst; mainly its SPR effect plays a critical role in the overall photocatalytic activity. In the 4-NP reaction, Au is one of the most commonly used, active catalysts in the dark. If the plasmons of Au nanostructures are excited, can they directly participate in the reduction reaction and how? Will the plasmon excitation affect the reaction rate and even reaction products and if yes, how? Is it possible to harvest both plasmonic and catalytic effects to gain higher catalytic activity? The work can help fill these knowledge gaps. It is worth mentioning that to date the direct photocatalysis on different plasmonic metal surfaces has been studied for many reactions,23,

36-41

but the

detailed mechanism is still elusive. Moreover, as for the reduction of 4-NP to 4-AP, it has so far mainly relied on catalysts such as Au, Pt, Pd, Ag and AuPt alloy NPs,4, 42-50 with the presence of the reduction agent of NaBH4, while without involving lights. Exploration of this reaction under light opens an alternative door for enhancing its reaction rate. Herein, for the first time, we designed and synthesized one type of hybrid photocatalysts, Au NP loaded hierarchical hollow porous TiO2 sphere (Au-hollow TiO2) for the synergistic catalytic reduction of 4-NP under combined plasmonic effects (mainly via the hot electron transfer) and inherent catalytic property of Au NPs. The hollow porous TiO2 spheres are composed of a great number of very thin nanosheets, which shorten the diffusion distance of charge carriers and make the photo-generated charges largely and immediately accessible to the reactants, greatly suppressing charge carrier recombination. The porous TiO2 network provides sufficient area for the Au NP loading and a great number of active sites for redox reactions. The “nanojet effect” (a concentration of the local electric field on the surface of a dielectric sphere opposite to the illumination side

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) of the hollow TiO2 sphere results in

enhanced optical absorption arising from the SPR of Au NPs, which was confirmed by our numerical simulations. In addition to the photocatalytic power arising from UV-excited TiO2 or visible-excited Au NPs, the inherent catalytic activity of Au NPs plays an important ACS Paragon Plus Environment

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synergistic role in the reduction of 4-NP. As a result, the novel Au-hollow TiO2 composite, with low Au NP loading (down to 0.1 ppm), exhibits high activity in the reduction of 4-NP to 4-AP, surpassing all the reported efficiency in this reaction system using Au-based catalyst. The catalyst is also very selective. By conducting a series of experiments, we offer new physical insights on the mechanisms responsible for achieved high catalytic activity of the porous Au-hollow TiO2 structure, which can contribute to not only the rational development of the high-performance catalysts for the 4-NP reduction, but also the broad field of plasmon enhanced photocatalysis. 2. EXPERIMENTAL SECTION 2.1. Materials. Au target (purity > 99.99%, diameter × thickness: 8 mm × 1.5 mm), sodium hydroxyl (NaOH), nitric acid (HNO3), Iron(III) chloride hexahydrate (FeCl3·6H2O), trisodium citrate (Na3Ct), sodium acetate (NaAc), ethylene glycol (EG, ≥ 99%), anhydrous isopropanol (IPA, ≥ 99.5%), diethylenetriamine (DETA, 99%), titanium isopropoxide (TIP, 97%), hydrochloride acid (HCl, 38% in water), dimethylformamide (DMF), 4-nitrophenol (4NP), sodium borohydride (NaBH4), deuteroxide (D2O), 1,4-benzoquinone (BQ), methanol, and tert-butyl alcohol (t-BuOH) were purchased from Sigma-Aldrich and all used as received. Water was purified by a Millipore Ultrapure water system with the resistivity no less than 18.2 MΩ cm, and used in all the experiments. 2.2. Fabrication of Au Nanoparticle (NP) Colloids. The Au NP colloids were prepared by using a “pulsed laser ablation in liquid” (PLAL) method.4, 12, 52 The laser ablation was performed with a KrF excimer pulsed laser (GSI Lumonics PM-846) that provides pulse width of 25 ns at a wavelength of 248 nm (repetition rate: 20 Hz; pulses: 6500). The Au target was placed at the bottom of one glass beaker, which was filled with 5 mL of NaOH solution (pH ≈ 9.5) with a depth of about 10 mm. The laser was focused by a lens having the focal length of 7.5 cm onto the target. The Au NPs with good quality and an average diameter of ~10 nm were obtained at the fixed laser fluence of ~40.0 J cm−2. In the present study, the concentration of Au atoms in the form of NPs was ~18.0 ppm, which was measured by the neutron activation analysis (NAA) method. It means that 16 µg of Au NPs were generated per laser pulse. 2.3. Synthesis of Fe3O4 Microspheres. The Fe3O4 microspheres were prepared according to a modified solvothermal method.53 In brief, FeCl3·6H2O (2.16 g), Na3Ct (0.25 g) and NaAc (2.0 g) were successively dissolved in EG (40 mL) under vigorously magnetic stirring for 1 h. The obtained yellow-brown solution was transferred into a Teflon-lined stainless steel


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autoclave (50 mL in capacity) and then heated in an oven at 200 oC for 10 h. After the autoclave cooled down to room temperature naturally, the black precipitates were separated by magnetic decantation and washed with water and ethanol in sequence, and then dried in a vacuum oven at 60 oC for 6h. 2.4. Preparation of Fe3O4@TiO2 Core @ Porous-Shell Microspheres. Typically, the assynthesized Fe3O4 microspheres (0.02 g) were dispersed in IPA (20.74 mL), followed by the addition of 15 µL of DETA. After gentle stirring for 10 min, 1 mL of TIP was dropwise added to the solution before it was transferred into a Teflon-lined stainless steel autoclave (50 mL in capacity), and kept at 200 oC for 24 h. After cooling down to room temperature naturally, the product was separated by magnetic decantation and washed with ethanol, and then dried in a vacuum oven at 80 oC overnight. The powder was calcined in N2 atmosphere with a heating rate of 1 oC min−1 up to 400 oC and maintained at this temperature for 2h to get the highly crystalline anatase TiO2. 2.5. Synthesis of Porous Hollow TiO2 Spheres. The as-prepared Fe3O4@TiO2 core @ porous-shell microspheres (0.02 g) were dispersed in concentrated HCl solution (7.5 M, 30 mL). The mixture was stirred vigorously at room temperature for 5 h. After that, the product was recovered by centrifugation, and then washed with distilled water for 3-5 times. Finally, the product was dried in air at 60 oC for 6 h. 2.6. Deposition of Au NPs on Porous Hollow TiO2 Spheres. The freshly prepared Au NP colloid (200 µL) was added to porous hollow TiO2 spheres (10 mg). After being shaken for 5 min, the thoroughly mixed solution was incubated at room temperature for 30 min. The pink precipitate was collected by centrifugation, and was washed with water and ethanol in sequence. The final product was dried in air at 60 oC for 6 h and labeled as Au-hollow TiO2. For comparison, the Au NP loaded commercial Degussa P25 TiO2 (Au-P25) was also prepared under the exactly same conditions. The weight content of Au in Au-TiO2 samples was determined by NAA method to be 0.036 wt%. 2.7. Characterization. The morphology and the microstructure of the prepared catalysts were observed by means of a field emission scanning electron microscopy (FESEM, JSM7401F, JEOL, operated at 5.0 kV) and a transmission electron microscope (TEM, 2100F, JEOL, operated at 200 kV), equipped with an energy-dispersive X-ray (EDX) spectroscopy. The x-ray diffraction (XRD) patterns were obtained by a PANalytical X’Pert MRD instrument with a Cu Kα radiation source (λ = 1.5406 Å). A Varian Cary 5000 scan spectrometer was used to measure the UV-vis absorption spectra of samples, and the absorbance of 4-NP and 4-AP in aqueous solutions. NAA technique was performed using a ACS Paragon Plus Environment

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SLOWPOKE nuclear reactor to determine the Au content in Au NP colloids, as well as in Au-hollow TiO2 and Au-P25 samples. The 1H Nuclear Magnetic Resonance (NMR) spectra were recorded on a Bruker 600 MHz NMR spectrometer (Avance III HD). 2.8. 4-NP Reduction Reaction. The Au-hollow TiO2 spheres were used for the catalytic reduction of 4-NP to 4-AP in the presence of NaBH4 at room temperature in the dark, or under different light irradiations. Typically, 10 mg of the catalysts were added to 30 mL of 4NP aqueous solution (0.12 mM) in a 100 mL quartz flask, followed by magnetic stirring for 30 min in the dark to establish the adsorption-desorption equilibrium between 4-NP and the surface of the catalysts. Subsequently, 30 mL of freshly prepared NaBH4 aqueous solution (5 mM) was injected to trigger the reduction reaction under continues stirring. Finally, the reaction mixture contains 10 mg of 0.036 wt% Au-TiO2 sample and the mole ratio of 4-NP to NaBH4 is 1:42. For the photocatalytic reduction, the photo-irradiation was also turned on at the same time with the addition of NaBH4 solution. The UV-light photoreduction reaction was carried out in a commercial UV reactor (LUZ-4V, Luzchem), which was equipped with fourteen 8 W ultraviolet lamps (1.799 mW cm-2, Luzchem LZC-UVA). The visible light irradiation was supplied by a 300 W Xenon lamp with a UV cut-off optical filter (1800 mW cm-2, λ > 420 nm). The simulated solar light photocatalysis reaction was performed using a 150 W solar simulator (100 mW cm-2). The light intensities at the position of reactor with different light irradiation were measured by one Power Meter (Newport 843-R). At varied time intervals, 0.6 mL of the reaction solution was taken out and immediately filtered by a pressure syringe filter (0.22 µm), which can terminate the reaction via separating the catalysts from 4-NP. The concentrations of 4-NP and 4-AP in the filtrate were measured by a UV–vis absorption spectrometer at a constant pH value of 10 at the wavelengths of 400 nm and 300 nm, respectively. The reactants and products of the reaction were analyzed quantitatively by a 1

H NMR spectrometer at 25 oC in D2O. The reduction reaction was also performed with D2O

as solvent for product analysis. In addition, in order to identify the involved active species in the photocatalytic reduction of 4-NP, methanol (1 mM), BQ (1 mM) and t-BuOH (1 mM) were employed as hole, superoxide radical (•O2−) and hydroxyl radical (•OH) scavengers, respectively. The trapping experiment was carried out under otherwise the same conditions as photocatalytic tests with the only exception being the addition of scavengers. 2.9. Numerical Simulation Details. We performed three-dimensional electromagnetic simulations for the mixtures of shell-like porous TiO2 and Au NPs by using a finite element method commercial software (COMSOL Multiphysics, optics module). We simulated an ACS Paragon Plus Environment

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infinitely large water volume by using perfectly matched layer boundary conditions, thus avoiding inward reflections. In this domain, the refractive index of water was set to be dispersive following the reported work,54 and the complex refractive index for Au NPs was taken from the literature.55 For appropriately describing the optical property of the porous TiO2 structure, we implemented an effective medium approximation (EMA). In particular, by considering a “flake” morphology as the basic constituent of the TiO2 porous shells, an EMA based on the Bruggeman mixing formula can properly depict this specific layout:56-57

(1 − f )

ε H O − ε eff ε TiO − ε eff  (1 − f ) (ε H O − ε eff ) + f (ε TiO − ε eff )  +f + 2  = 0, εH O ε TiO ε eff   2

2

2

2

(1)

2

2

in which the effective permittivity of the shell ( ε eff ) can be extracted starting from the permittivities of water ( ε H2O ) and bulk TiO2 ( εTiO2 , taken from the work of Kischkat et al.58), as well as considering a filling factor ( f ) that represents the TiO2 volume fraction in the porous shell. From the optical extinction measurements, we know that the 5 nm Au NPs incorporated in the shell structure have a surface plasmon resonance (SPR) red-shifted by 14 nm when compared to the same particles in water, due to the increase of ambient refractive index. From this, we extracted an average filling factor ( fav ) of 0.12 for the shell. Following our TEM characterizations, we considered a 65 nm thick shell with an outer diameter of 355 nm in the simulation model. The 5 nm Au NPs in our experiments are much smaller than the radiation wavelength, so that their scattering is negligible when compared to absorption.59-60 The number of Au NPs per hollow TiO2 sphere was estimated to be 140 by calculating the weight of single Au NP and hollow TiO2 sphere, and the total weight of them in the system. 3. RESULTS AND DISCUSSION 3.1. Synthesis and Characterization of Au-Hollow TiO2 Spheres. The designed Auhollow TiO2 catalysts were prepared by a multi-step method (Figure 1a). Uniform and welldispersed Fe3O4 microspheres (~225 nm in diameter) were firstly synthesized via a solvothermal reaction (Figure S1, Supporting Information). The as-synthesized Fe3O4 microspheres were then used as template for the preparation of Fe3O4@pourous-TiO2 core@shell spheres with the shell thickness of ~65 nm by a wet chemical approach (Figure S2). To obtain the hollow porous TiO2 spheres, the Fe3O4 core in the annealed core@shell structure was removed by an optimized acid etching strategy. It is worth noting that the crystallinity of TiO2 shell and the acid etching conditions are crucial to the formation of welldefined hollow porous structure. If the crystallinity is not sufficiently high, the hollow


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structure can easily collapse during the etching treatment. In the last step, Au NPs fabricated by using our reported “pulsed laser ablation in liquid (PLAL)” method,12, 52, 61 were loaded onto the TiO2 nanosheets of the hollow porous structure via simple incubation at room temperature. Figure 1b clearly shows the hollow porous TiO2 spheres with narrow size distribution and uniform morphology, which are composed of numerous TiO2 nanosheets. The average size of these hollow TiO2 spheres is ~355 nm and ~225 nm in outer and inner diameters, respectively. Compared with the initial core@shell structure (Figure S2a-S2e), the optimized acid etching approach used in our case to remove the Fe3O4 core did not affect the porous structure of the TiO2 shell. The survey (Figure S2f) and Fe 2p (Figure S2g) XPS spectra of hollow TiO2 show no peaks for Fe, indicating that Fe3O4 core was completely removed after etching. The hollow porous TiO2 structure favors easy loading of Au NPs and redox reactions by providing significantly more surface sites. Different from the Au NPs prepared by a general chemical method, the PLAL-generated Au NPs possess “bare and clean” surface free of any organic ligands (Figure S3), which is beneficial for the efficient charge transfer between the anchored Au NPs and TiO2.4, 12 The transmission electron microscopy (TEM) images of Auhollow TiO2 microspheres are displayed in Figure 1c and 1d. The successful incorporation of Au NPs onto the porous TiO2 network was clearly observed in Figure 1d. The darker spots are expected to be Au NPs considering the much higher atomic mass of Au with respect to TiO2, which was further corroborated by energy-dispersive X-ray (EDX) measurements (Figure 1e). It was observed that the loaded Au NPs are uniformly distributed over the hollow TiO2 shell without significant aggregation. Furthermore, the absence of Fe element in the EDX spectrum confirms the complete removal of Fe3O4 core from the core@shell structure once again. The high resolution TEM images of Au-hollow TiO2 (Figure 1f-1g) clearly show the lattice fringes of Au and TiO2, suggesting the high crystallinity of Au and TiO2. The lattice fringes with d-spacing of 0.25 nm and 0.35 nm are attributed to the (111) lattice plane of Au and (101) lattice plane of TiO2, respectively. It also shows good attachment of Au NPs onto TiO2 that highlights the sharp boundaries created at the int,erface between these two components. The intimate contact and strong metal-support interaction are highly likely to lead to efficient charge carrier transfer between them and are favorable for photocatalysis.62


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Figure 1. a) Synthesis of Au-hollow TiO2 microspheres. Typical TEM images of b) hollow TiO2 microspheres, c, d) Au-hollow TiO2 microspheres and e) the EDX spectrum of the circled area in d). f, g) HR-TEM images of Au-hollow TiO2 microspheres. Inset of b) is the magnified hollow TiO2 sphere.

The X-ray diffraction (XRD) patterns reveal the formation of anatase TiO2 (JCPDS card no. 21-1272) in the Fe3O4@TiO2, hollow TiO2 and Au-hollow TiO2 samples (Figure 2a). With respect to the core@shell structure, the disappearance of diffraction peaks, corresponding to the cubic inverse spinel Fe3O4 (JCPDS card no. 19-0629), in the XRD pattern of the Auhollow TiO2 sample, confirms that the Fe3O4 core was completely etched away, which is consistent with the EDX analysis. The optical properties of the as-prepared samples were investigated and are shown in Figure 2b. Both the hollow TiO2 and Au-hollow TiO2 samples show strong absorption in the UV region with an absorption edge at ~400 nm, which is attributed to the band gap excitation of anatase TiO2. In the case of Au-hollow TiO2 sphere, an additional, obvious visible light absorption band centered at around 540 nm was observed, suggesting the SPR excitation of Au NPs.27


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Figure 2. a) XRD patterns of Fe3O4, Fe3O4@TiO2, hollow TiO2 and Au-hollow TiO2, b) UV-visible absorption spectra of the hollow TiO2 before and after Au loading. The inset in b) shows the absorption spectrum of colloidal Au NPs.

3.2. Catalytic Reduction of 4-NP by Au-Hollow TiO2 Catalysts with and without Irradiation. The as-synthesized Au-hollow TiO2 catalysts were utilized for the reduction of 4-NP under visible light irradiation in the presence of excessive NaBH4 (Figure 3a-3b). The detailed procedures for photocatalytic testing are described in the experimental section in Supporting Information. Figure 3a displays the time-dependent UV-visible (UV-vis) absorption spectra of the reaction mixture during the photocatalysis process. The absorption peak at 400 nm is ascribed to the n→π* transition of the 4-NP anion,5 decreases steadily with time and eventually disappears after 22 min under irradiation. For simplicity, 4-NP is also used to represent the 4-NP anion (4-nitrophenolate). Simultaneously, the peak at 300 nm, attributed to 4-AP, emerges and increases with the decoloring of the reaction solution from light yellow to colorless (Figure S4). The kinetic study of 4-NP reduction under visible light irradiation was carried out by fitting the absorption data at 400 nm, reflecting the concentration of 4-NP, with the pseudo-first order reaction model (Figure 3b). The analysis of the reaction kinetics gave the apparent rate constant kapp of 1.83×10−3 s-1 at the Au NP concentration (cAu) of 0.3033 µmol L-1. This rate constant was further normalized with cAu, yielding the k value of 6.034×10−3 s-1 µmol-1 L, which is about 1.7 times higher than the previously reported highest value (3.631×10−3 s-1 µmol-1 L),63 and several orders of magnitude higher than that of most of the reported catalytic 4-NP reaction systems using Aubased catalyst (Table S1). To explore the mechanism for the greatly enhanced catalytic activity for 4-NP reduction over the Au-hollow TiO2 catalyst, a series of control experiment was carried out (Figure S5S7). In the dark, the reaction cannot proceed without adding Au (Figure S5), or in the


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presence of the hollow TiO2 solely, as expected since TiO2 is not a good catalyst for this reaction (Figure S6a). The reduction reaction is triggered with the addition of the Au NP or Au-hollow TiO2 catalysts. With the support of the hollow TiO2, the agglomeration of the tiny Au NPs during the catalysis was greatly suppressed, which contributes to the slightly enhanced reduction rate of Au-hollow TiO2 sphere (Figure S6c) with respect to that of Au NPs dispersed in the reaction solution (Figure S6b). This advantage is anticipated to become more significant for long-term or multiple-cycle catalytic tests. As shown in Figure S6d, the k value (1.969×10−3 s-1 µmol-1 L) for the reaction in the dark is more than 3 times lower than that under visible light irradiation (6.034×10−3 s-1 µmol-1 L) in the presence of the same catalyst of Au-hollow TiO2 sphere. It indicates that the excitation of plasmons in the Au catalyst, which is also an active catalyst in the dark, further significantly enhanced its catalytic activity. In clear contrast, under visible light irradiation and only in the presence of hollow TiO2, the unaltered peak intensity at 400 nm with time, suggests that the reduction reaction does not occur (Figure S7a). Furthermore, only in the presence of Au NPs, no significant enhancement of the reduction rate was observed for the reaction under visible illumination compared to that in the dark (Figure S6b, FigureS7b-S7c), implying that the SPR induced hot carriers in the Au NPs preferentially and rapidly recombine, instead of directly participating in the reduction reaction. In other words, without the TiO2 support, the hot electrons generated inside the Au NPs seem “useless” for this reaction and do not contribute to the catalysis. The observation that the Au NPs demonstrated the same reaction rate in the dark and under visible light also rules out the possible heating effect on the reduction reaction with the exposure of visible light at the intensity we applied. The loading of Au NPs onto the hollow TiO2 spheres also remarkably enhances the catalytic reduction rate of 4-NP (irradiation time: 11 min; conversion: 90%) with respect to that can be achieved in the presence of single components, Au NPs (irradiation time: 60 min; conversion: 67%) or hollow TiO2 (irradiation time: 60 min; conversion: 77%), under UV light illumination (Figure S8 and Figure 3c-3d). Such observations imply that the energy level of TiO2 is appropriate for the reduction of 4-NP; the different activities of Au and hollow TiO2 could be ascribed to the different kinetic barriers or pathways involved on their surfaces. Moreover, the obtained k value for Au-hollow TiO2 under UV exposure (12.859×10−3 s-1 µmol-1 L) is about 6.5 times higher than that in the dark (1.969×10−3 s-1 µmol-1 L), suggesting that the excited electrons in TiO2 can contribute to the catalysis either directly or by injecting into Au to participate in the reduction reaction, with the latter being more likely. According to the literature, the negative charge transfer from Au NPs to H atom ACS Paragon Plus Environment

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is necessary for the formation of highly reactive Au-H species.64 It was also reported that the charge transfer from TiO2 support to Au NPs can generate negatively charged Au NPs and form a reactive Au-TiO2 interface,65-66 which has been confirmed to facilitate H2 dissociation and show higher activity in hydrogenation reaction than pure TiO2 or Au NPs.67-69 In our case, Au-H species can be more easily formed on the Au-TiO2 sample and thus accelerate the reduction rate of 4-NP to 4-AP. These results strongly indicate that the existence of synergistic effect in Au-hollow TiO2 catalysts, under both visible and UV light irradiation, which contributes to the superior performance in the catalytic reduction of 4-NP. However, few efforts have been previously made to utilize the synergistic photocatalysis in the reduction of 4-NP.

Figure 3. Time-dependent absorption spectra of the reaction mixture, and c/c0 and ln(c0/c) as a function of reaction time for the reduction of 4-NP over Au-hollow TiO2 microspheres under a, b) visible light and c, d) UV light irradiation.

For the purpose of facilitating practical application, the reduction of 4-NP in the presence of Au-hollow TiO2 sphere was also conducted under simulated solar light irradiation and compared with the behavior of Au-P25 (Figure S9 and Figure 4a-4b). A k value of 8.935×10−3 s-1 µmol-1 L was acquired, representing a 2.7-fold enhancement over that of AuP25 under otherwise identical experimental conditions. P25 has served as a benchmark for photocatalytic reactions. Herein the saliently improved k value with the use of the hollow,


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porous TiO2 spheres underlines the advantages of such a structure. As compared to solid P25, the large surface area of hollow, porous TiO2 provides significantly more active sites on the internal and external surface of the hollow structure as well as on the surface of numerous constituent nanosheets, highly favorable for the catalytic reaction. 3.3. Wavelength-Dependent 4-NP Reduction by Au-Hollow TiO2 Catalysts. To directly analyze the contribution of SPR effect of Au NPs on the 4-NP reduction, the wavelengthdependent reaction rates of Au-hollow TiO2 catalysts were measured by using a series of long-pass filters and a 300W Xeon lamp. The k value at a specific wavelength range, such as 420-475 nm, was estimated by subtracting k1 value measured with a 420 nm long-pass filter from k2 value measured with a 475 nm long-pass filter after 15 min irradiation. The k values at other wavelength ranges were all measured and calculated by this way with all other experimental conditions being identical. As shown in Figure 2b and Figure 4c, the k values achieved by the Au-hollow TiO2 catalyst generally follow its light absorption: the greater the light absorption, the higher the k value. Under λ>420 nm irradiation, the strong contribution from TiO2 excitation can be ruled out. The Au-hollow TiO2 sample shows the highest k value in the wavelength range of 515-570 nm, which is well consistent with the SPR absorption of Au NPs. These results confirm the positive effect of the SPR excitation in the Au-hollow TiO2 sample on the 4-NP reduction. 3.4. Detection of Active Species and Nuclear Magnetic Resonance Analysis. To better understand the pathways involved in the catalytic reaction of 4-NP under simulated solar irradiation covering UV and visible wavelengths, the expected, major active species were detected by performing trapping experiment using three different types of scavengers (Figure 4d and S10). UV-excited TiO2 is known to possess a high redox potential. The photoinduced electrons and holes can react with O2 and OH-/H2O to generate superoxide radicals (•O2−) and hydroxyl radicals (•OH), respectively. In principle, these active species are capable of oxidizing 4-NP into CO2 and H2O. However, since in our reaction system excessive, strong reductant NaBH4 is used, the photooxidation is not expected to occur at any meaningful level. To confirm it, methanol, 1,4-benzoquinone (BQ) and tert-butyl alcohol (t-BuOH), acting as the scavengers of hole, •O2− and •OH, respectively, were applied in the photocatalytic tests. Only slight difference in the k value, within the range of experimental errors, was observed with the addition of these scavengers. These results supported our hypothesis that 4-NP photo-oxidation did not take place in our investigated system at a measurable level,70 and electrons from photocatalyst are therefore crucial to the 4-NP reduction to 4-AP under light illumination. The conclusion was also testified by quantifying the products using the Nuclear ACS Paragon Plus Environment

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Magnetic Resonance (NMR) spectra (Figure S11). The 1H NMR spectrum (in D2O) of 4-NP exhibits peaks around the chemical shift (δ) of 6.9 ppm and 8.1 ppm, corresponding to the four aromatic protons in 4-NP.3, 71 After the addition of NaBH4, all the peaks show shifts toward the lower δ possibly due to the increased pH value and the change of -OH group to O-. In the presence of Au-hollow TiO2 catalysts, the 1H NMR spectra of the products obtained in the dark or under irradiation both show peaks at δ ~6.56 ppm and δ ~6.65 ppm, confirming the formation of 4-AP.3, 71 Furthermore, the almost same integrated 1H peak areas (~4) of initial reactants and final products, and the absence of other detectable products strongly suggest that 4-NP has been completely reduced to 4-AP. The study rules out not only the photo-oxidation reaction, but also the hydrogenation of the benzene ring. It shows the high selectivity of the catalysts developed herein for the reduction reaction.

Figure 4. a) Time-dependent UV-vis absorption spectra of the reaction mixture and b) c/c0 and ln(c0/c) as a function of irradiation time for the 4-NP reduction in the presence of NaBH4 and Au-hollow TiO2 sample under simulated solar light irradiation. c) Wavelength-dependent rate constants (k) for the 4-NP reduction by Auhollow TiO2 photocatalysts. The purple and red bars represent the wavelength ranges of 610 nm, respectively. The inset shows enlarged wavelength-dependent H2 production rate at wavelengths longer than TiO2 absorption. d) ln(c0/c) vs reaction time plot in the absence and presence of three types of scavengers.

3.5. Theoretical Calculations on the Optical Property. It is now worthwhile to examine the plasmonic property of Au NPs in hollow, porous spheres, which were reported to increase light-harvesting.72-73 Plasmonic nanostructures may be involved in an optical phenomenon, called the “nanojet effect.” When it takes place light can be concentrated and reach a very


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high intensity at the shadow-side surface when it transports through dielectric microcylinders or microspheres, which have a diameter much larger than the irradiation wavelength.74-75 In our case, the expected largely increased light intensity at one side of hollow TiO2 spheres may be favorable for the SPR excitation of the loaded Au NPs situated within the “nanojets” in this side, resulting in overall enhanced light absorption of Au-hollow TiO2 spheres. The numerical modelling was thus conducted (refer to Experimental Section for calculation details). As shown in Figure 5a, the porous TiO2 structure with an inner cavity can produce a “nanojet” effect, resulting in an increase of light absorption by 21% (integrated from 400-800 nm) with respect to a uniform water-TiO2 background featuring the same filling factor (Figure 5d). Furthermore, the SPR hot spots of the Au NP in the shell structure (Figure 5b) appear to be “brighter” (i.e., featuring higher local electric field values) than those in a uniform background case (Figure 5c). In the simulations, it is also feasible to separately extract the absorption spectra within the gold and the TiO2 matrix. We can then independently multiply the gold absorption spectrum (averaged over all the possible illumination directions) by the number of Au NPs embedded in the shell (estimated as 140 per TiO2 sphere, see Supporting Information) to get a full description of the Au-hollow TiO2 hybrid structure. As an example, the red curve in Figure 5e presents the total absorption spectrum of the hollow, porous TiO2 shell comprising 140 Au NPs. This simulation result is in fair agreement with the absorption measurement shown in Figure 2b.


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Figure 5. a) Local electric field distribution around the Au-hollow-TiO2 structure. The cross section is taken in the E-k (E: Electric field of incident wave; k: wave vector) plane, and it passes through the center point of the sphere. b) The zoomed-in field enhancement map for the 5 nm Au NP in the Au-hollow-TiO2 shell. c) The field enhancement map for the same Au NP in a homogenous water-TiO2 hybrid mixture (f = 0.12). d) The visible absorption cross sections for these two cases. In the shell case, the absorption is calculated averaging over all the possible illumination directions. e) The absorption spectra comparison between the hollow-TiO2 (blue) and Auhollow-TiO2 (red) structures, the latter calculated averaging the absorption for all the possible illumination directions and considering 140 Au NPs in the shell. All the 2D electric field maps presented in a), b) and c) are calculated at the SPR peak wavelength (540 nm).

3.6. Mechanism for the Catalytic Reaction. On the basis of the above experimental and calculation results, we are able to propose the possible catalysis mechanisms under visible and UV light irradiation (Figure 6). Under visible light, only the SPR of Au NPs can be excited (Figure 6a). The resonant photons are absorbed by Au NPs to generate SPR-induced energetic electrons which can be injected into the CB of neighboring hollow TiO2, given that their energy relative to the Au Fermi level (EF) is higher than the Schottky barrier height.23-25, 34, 76-77

The “nanojet” effect of the hollow, porous TiO2 further enhances the Au SPR. The

TiO2 network also serves as a conduit to transport hot electrons that can be further transferred to the lowest un-occupied molecular orbital (LUMO) of 4-NP.11 In the presence of excess hydrogen atom donor BH4- or H2O, the photocatalytic hydrogenation of 4-NP proceeded on the surface of TiO2. These energetic electrons, may directly participate in the catalytic reduction reaction on the surface of Au NPs, considering the inherent catalytic activity of Au


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NPs in this specific reduction reaction. The reaction can even be facilitated considering that hot electron injection into TiO2 reduces the electron density of Au NPs, promoting the adsorption of both BH4- and 4-nitrophenolate onto the surface of Au NPs. In this case, the electrons transfer from the adsorbed BH4- to Au NPs and activate the produced H2 to form highly reactive Au-H species,78-80 reducing the adsorbed 4-nitrophenolate into 4-AP. In addition, the excited electrons reflected by the Schottky barrier can transfer their excess energy to BH4- to promote the breaking of B-H bond, thus releasing the excessive reactive hydride ions which in turn can facilitate the 4-NP reduction on Au atoms. Under UV light irradiation, although Au SPR is not excited, the band gap-excitation of TiO2 occurs (Figure 6b). The electrons in the valence band (VB) of TiO2 are excited to the CB, and then transferred to Au NPs, which can be further injected into the LUMO of 4-NP for reduction. On the other hand, the richening of electrons can largely promote the formation of H2 from water splitting or Au-H species on the surface of Au NPs for more efficient reduction of 4-NP. Furthermore, the excited electrons in the CB of TiO2 can be directly injected to the LUMO of adsorbed 4-NP for reduction, while the holes in the VB of TiO2 can react with BH4- ions to generate H2 for hydrogenation, altogether promoting the photocatalytic reduction of 4-NP on the surface of TiO2. From the above discussion, it can be concluded that the strong, synergistic interactions between the Au NPs and hollow TiO2 significantly enhance the catalytic activity in the 4-NP reduction under both UV and visible light irradiations with respect to the reaction undertaken in the dark.

Figure 6. Schematic diagram showing the energy band structure and electron transfer in the synergistic catalytic reduction of 4-NP over Au-hollow TiO2 microspheres under a) visible and b) UV light irradiation.

4. CONCLUSIONS In summary, the synthesis of novel plasmonic Au loaded hollow porous TiO2 spheres have been demonstrated herein. The porous hollow TiO2 sphere composed of thin nanosheets serves as the scaffold of PLAL-generated Au NPs, favouring the adsorption of reactants and


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suppressing the recombination of charge carriers by decreasing their diffusion length. Regarding the plasmonic effects, the structure presents an increased absorption and promotes the hot electron transfer under visible light illumination. The synergy of inherent catalytic property of Au NPs, and their plasmonic activity under light illumination was confirmed by a series of control experiments. As a result, the designed Au-hollow TiO2 catalyst exhibits high activity in the reduction of 4-NP to 4-AP in the presence of excessive NaBH4 under light irradiation compared to the best reported catalysts. We believe that the current work paves a new way for designing novel plasmonic functional materials that will find extensive applications in photoelectrocatalysis and photocatalysis. ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website (http://pubs.acs.org). SEM and TEM images; Control experiments for 4-NP reduction; 1H NMR spectra; Reaction rates table. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] *E-mail: [email protected]; Phone: 514-228-6920; Fax: 450-929-8102. Notes The authors declare no competing financial interests. ACKNOWLEDGEMENTS Financial support from the Natural Sciences and Engineering Research Council of Canada (NSERC) in the context of NSERC-Discovery Grant and NSERC-Strategic Grant (with the support of Canadian Solar Inc.), and le Fonds de recherche du Quebec-Nature et technologies (FRQNT) is greatly appreciated. M.C. is also grateful to the Canada Research Chairs Program. Q.Z. acknowledges the Q.Z. acknowledges the scholarships from the China Scholarship Council (CSC, No. 201506220152) and FRQNT (258513), and J.Z. is grateful for the funding support from Jiangsu University (17JDG008). REFERENCES


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Table of Contents 4.77 cm × 5.25 cm (300 × 300 DPI)


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Plasmonic Au-Loaded Hierarchical Hollow Porous TiO2 Spheres

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