Light-Induced Assembly of Metal Nanoparticles on ZnO Enhances the

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Light-Induced Assembly of Metal Nanoparticles on ZnO Enhances the Generation of Charge Carriers, Reactive Oxygen Species and Antibacterial Activity Xiumei Jiang, Weiwei He, Xiaowei Zhang, Yong Wu, Qian Zhang, Gaojuan Cao, Hui Zhang, Jiwen Zheng, Timothy R Croley, and Jun-Jie Yin J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b10578 • Publication Date (Web): 07 Dec 2018 Downloaded from http://pubs.acs.org on December 9, 2018

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The Journal of Physical Chemistry

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Light-Induced Assembly of Metal Nanoparticles on ZnO Enhances the

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Generation of Charge Carriers, Reactive Oxygen Species and

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Antibacterial Activity

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Xiumei Jiang,a Weiwei He,b* Xiaowei Zhang,a Yong Wu,c Qian Zhang,d Gaojuan Cao,a Hui

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Zhang,a Jiwen Zheng,c Timothy R. Croley,a Jun-Jie Yin a*

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a Division

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Applied Nutrition, U.S. Food and Drug Administration, College Park, Maryland 20740, United

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States

of Analytical Chemistry, Office of Regulatory Science, Center for Food Safety and

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b Key

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Institute of Surface Micro and Nanomaterials; Henan Joint International Research Laboratory of

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Nanomaterials for Energy and Catalysis, Xuchang University, Xuchang, Henan 461000, China

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c Division

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Laboratories, Center for Devices and Radiological Health, U.S. Food and Drug Administration,

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Silver Spring, Maryland 20993, United States

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d Department

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20742, United States

Laboratory for Micro-Nano Energy Storage and Conversion Materials of Henan Province,

of Biology, Chemistry and Materials Sciences, Office of Science and Engineering

of Chemistry and Biochemistry, University of Maryland, College Park, Maryland

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*

Corresponding author: [email protected], [email protected]

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ACS Paragon Plus Environment

The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Abstract

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Increasing the photocatalytic activity of semiconductors by forming heterojunctions with metal

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improves their energy transfer efficiency and environmental remediation capabilities. However,

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our knowledge regarding the structure-activity relationship of semiconductor/metal hybrid

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nanostructures is lacking due to poor control over their physicochemical properties. Here, we

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report a facile way to make ZnO/metal heteronanoparticles by mixing/irradiation process of ZnO

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and metal nanoparticles. The resultant products provide an expedient model to explore the effects

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of various metal NPs on light-induced electron/hole separation, reactive oxygen species (ROS)

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production and antibacterial activities of ZnO NPs. Electron spin resonance spectroscopy was used

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to compare the effect of mixing Pt, Au, or Ag NPs at different size and concentration with ZnO

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NPs on light-induced electron-hole separation and ROS production. The enhancing effect of metal

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NPs depends on particle size, composition, and mass ratio of the metal NPs to the ZnO NPs.

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Smaller-sized Pt NPs are more efficient in promoting charge carrier generation and ROS

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production. At 5 nm, Ag NPs promoted charge carrier generation more efficiently than Pt and Au

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NPs, but Pt NPs promoted ROS generation more efficiently than Au and Ag NPs. These results

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provide valuable strategy to design the synthesis of semiconductor/metal hybrid nanostructures

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and easy tailoring of ROS and charge carrier production.

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1. Introduction

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Semiconductor/metal hybrid nanostructures have attracted great interest because of their potential

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for environmental remediation (such as wastewater treatment and air filtration),1-2 solar energy

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conversion,3-4 and antibacterial applications.5-6 Various combinations of semiconductors (metal

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oxides and metal sulfides) and metals (Au, Ag, Pt, Pd, and binary metals) have been synthesized

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in the past to optimize catalytic efficiency.7-12 Specific combinations of semiconductor and metal 2


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has been well-demonstrated as an effective way to enhance the photocatalytic performance of

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semiconductors, but their catalytic efficiency is affected not only by particle size, shape, and metal

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composition, but also by the metal/semiconductor composition ratio and connection type.13-17 For

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example, TiO2 decorated by smaller-sized Pt NPs exhibited superior photocatalytic hydrogen

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production,18 and the combination of TiO2 with Au nanorods or nanocages exhibited improved

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photocatalytic activity in visible and near-infrared regions.19

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Various methods for preparation of semiconductor/metal hybrid nanostructures have been

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reported, including photo-deposition,16 chemical reduction,9 impregnation,20 electrodeposition,8

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and atomic layer deposition.21 Unfortunately, among these methods, applied potential, temperature,

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instruments, and/or difficult operations are involved, and even with these conditions, it is yet

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insufficient to control the size, shape, and composition of the metal components.

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Optimizing the photocatalytic efficiency of semiconductor/metal hybrid nanostructures is also

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limited by our lack of knowledge of the underlying mechanisms. Metal nanocomponents have

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been proposed to improve the photoactivity of semiconductors by (1) storing electrons from photo-

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irradiated semiconductors, which increases electron/hole separation efficiency, (2) providing more

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active sites for chemical reactions, and (3) introducing surface plasmonic fields to improve light

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absorption and create hot-charge carriers.22-25 However, the fundamental photochemistry

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underlying the photocatalytic reactions is still unclear. For example, how do specific metal

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components and their connection types affect the generation of charge carriers and reactive oxygen

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species (ROS) and their photocatalytic activities? Answering these questions requires a model

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structure with changeable metal components (size, shape, coating, and composition). Therefore, it

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would be very desirable to develop an easy way to fabricate semiconductor/metal hybrid objects

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with tunable structures, so their structure-activity relationships and applications could be

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determined.

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During light irradiation of semiconductor nanoparticles, the nanoparticles absorb energy that

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results in charge separation and the creation of electron/hole pairs. The electron/hole pairs have

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reductive/oxidative power that can form ROS, including superoxide radicals [O2 ], singlet oxygen

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[O21], and hydroxyl radicals [•OH]. The ROS determine the chemical reactivity of the photoexcited

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nanoparticles, and this mechanism has been proposed as the mainstay of semiconductor

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photocatalytic activity.26-28 Detecting active intermediates, such as photo-induced electron/hole

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pairs and ROS, can be difficult because they are very active and short-lived entities. In our previous

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studies, we have established a powerful tool that can identify charge carriers and ROS formulations

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in nanomaterials. Specific spectra associated with the paramagnetic characteristics of each ROS

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spin adduct can be obtained using electron spin resonance (ESR) spectroscopy coupled with spin

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labelling and spin trapping agents,29 enabling us to both identify and quantify the charge carriers

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and ROS induced by nanomaterials.

•–

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In a suspension containing nanoparticles, Brownian motions and the large specific surface

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area of NPs can lead to nanoparticle collision and coagulation if the nanoparticles are between 0.1-

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10 µm in size, even without the assistance of additional chemical/physical forces. We hypothesized

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that particle collision or coagulation of mixed solution of semiconductor and metal NPs may

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facilitate photoinduced electron transfer and enhance the photocatalytic activity of the

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semiconductor. In this study, we used zinc oxide (ZnO) NPs and noble metal NPs (Au, Ag, and

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Pt) to prove our concept because they are commercially available and widely used in photocatalysis.

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Physicochemical properties of ZnO NPs and metal NPs were characterized before and after

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sunlight irradiation. ESR coupled with spin trapping and spin labelling was applied to identify and 4


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quantify the ROS generation during sunlight irradiation of mixed solution of ZnO and different

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metal NPs. By examining the effect of metal composition, particle size, crystal facets, and particle

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concentrations on photocatalytic activity of ZnO NPs, we found a connection between

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photocatalytic activity and ROS/charge carrier production.

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2. Material and Methods

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2.1. Chemicals and Materials

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An aqueous dispersion of zinc oxide NPs (20 wt%, 30-40 nm) was purchased from US Research

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Nanomaterials, Inc. (Houston, TX). Aqueous solutions of Pt NPs (5 nm, 30 nm, 50 nm, 1 mg/ml,

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Citrate), Au NPs (5 nm, 50 nm, 1 mg/ml, Citrate), Ag NPs (5 nm, 50 nm, 1 mg/ml, Citrate), silica-

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shelled Au NPs (Au-Si NPs, 50 nm Au core diameter, 1 mg/ml, Silanol), and silica-shelled Ag

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NPs (Ag-Si NPs, 50 nm Ag core diameter, 1 mg/ml, Silanol), were purchased from nanoComposix,

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Inc. (San Diego, CA). Pd nanocubes and Pd octahedrons were kindly provided by Dr. Cuicui Ge

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from Soochow University. The spin-trap 5-tert-butoxycarbonyl 5-methyl-1-pyrroline N-oxide

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(BMPO) was purchased from Dojindo Molecular Technologies, Inc. (Rockville, MD); the 4-Oxo-

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2,2,6,6-tetramethylpiperidine (4-Oxo-TEMP) was purchased from Wako Chemicals USA, Inc.

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(Richmond, VA); the 4-Oxo-2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPONE) and 1-Hydroxy-

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2,2,6,6-tetramethyl-4-oxo-piperidine•HCl (TEMPONE-H. hydrochloride) were purchased from

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Enzo Life Sciences, Inc. (Farmingdale, NY); and the 3-Carbamoyl-2,2,5,5-tetramethyl-3-pyrrolin-

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1-oxyl (CTPO), sodium chloride (NaCl), rhodamine B (RhB) and methylene blue (MB) were

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purchased from Sigma-Aldrich (St. Louis, MO). Milli-Q water (18 MΩ·cm) was used to prepare

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all the solutions.

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2.2. Simulated sunlight irradiation

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A Universal Arc Lamp Power Supply (model 69920, Newport, Irvine, CA) coupled with an Xenon

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Lamp Solar Simulator (model 91190, Newport) was used to generate simulated sunlight. The light

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was filtered through a solar simulation bandpass to provide the simulated sunlight irradiation. This

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light source was used to irradiate the ZnO and ZnO/metal mixture to generate electron/hole and

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produce ROS. The power of the simulated sunlight was 380 mW/cm2 measured by a Newport

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optical meter.

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2.3. ZnO NPs and metal NPs characterization

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The metal and metal oxide NPs used in our study include Pt NPs of different sizes (5, 30, and 50

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nm), Au NPs (5 nm, 50 nm), Ag NPs (5 nm, 50 nm), silica-shelled Au NPs (50 nm Au core), silica-

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shelled Ag NPs (50 nm Ag core) and ZnO NPs. Transmission electron microscopy (JEM-1400

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TEM [JEOL, Tokyo, Japan]) was used to characterize the size and shape of all the metal NPs and

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ZnO NPs. UV-Vis absorption of all the metal NPs and ZnO NPs were characterized by Varian

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Cary 300 (Agilent, Santa Clara, CA, USA). Zeta potential and the hydrodynamic diameter of the

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nanoparticles dispersed in ddH2O were measured with Zetasizer Nano ZS90 (Malvern,

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Worcestershire, UK).

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2.4. ZnO/metal heterojunction characterization

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For TEM imaging: ZnO NPs (250 µg/ml) and metal NPs (5 nm, 10 µg/ml) are mixed in a 1.5 ml

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Eppendorf tube and irradiated with simulated sunlight for 15 min. After irradiation, the mixture

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was centrifuged 8000 rpm/min for 10 min at room temperature. Discard the supernatant and re-

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disperse the pellet with ddH2O and ultrasonic for 3 min, pipet 10 µl particle suspension on a TEM

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grid and air-dry. The TEM images were taken by JEM-1400 TEM.

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For UV-Vis absorption spectra: the mixture of ZnO NPs (250 µg/ml) and metal NPs (5 nm, 10

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µg/ml) was divided in two groups, one group was irradiated with simulated sunlight and the other 6


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one does not. Then the samples were transferred to a quartz cuvette and record the UV-Vis

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absorption spectra.

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2.5 Antibacterial activity of ZnO/metal NP mixture under light irradiation

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The photo-enhanced antibacterial activity of ZnO/metal NP mixture was determined by the

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survival rate of Gram-positive Staphylococcus aureus (S. aureus) and Gram-negative Escherichia

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coli (E. coli) after sunlight irradiation in the presence of ZnO/metal NP mixtures. Briefly, ZnO NP

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(250 µg/ml) and each of the three metal NPs (Pt, Au, Ag) (10 µg/ml) were mixed in solution and

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irradiated with sunlight for 15 min to form ZnO/metal hybrid NPs. S. aureus (ATCC 29213TM)

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and E. coli (ATCC 25922TM) grown in lysogeny broth (LB) agar were incubated overnight at 37

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ºC and a colony was selected and inoculated in 3 ml of liquid LB and shake for 18 h at 37 ºC.

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The resulting liquid culture was diluted with liquid LB to an optical density of ca. 0.2 at 600 nm.

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Then bacteria solutions (50 µl) were mixed with 5 nm metal NPs (25 µl, 10 µg/ml), or ZnO NPs

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(25 µl, 250 µg/ml) or ZnO/metal hybrid NPs (25 µl) and add 0.9 % NaCl solution to make a

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solution of 5 ml, and irradiated with sunlight for 10 min while stirring, after turned off the light

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the bacteria and nanoparticle solution continually stirred for 1 h. After that, the bacteria were

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diluted 200 times for S. aureus and 50 times for E. coli with 0.9 % NaCl and 50 µl diluted bacteria

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solution was added into the LB agar plates and spread with L shaped spreader to evenly grow the

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bacterial on the plate. Finally, the plates were imaged and the number of the colony forming units

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(CFU) was counted with Scan 1200 (interscience Laboratories Inc, Woburn, MA) to calculate the

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survival cell percentage.

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2.6. Photocatalytic activity of ZnO/metal NP mixtures towards organic pollutant dyes

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The photocatalytic activity of ZnO NPs and ZnO/metal NP mixtures was determined by measuring

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the degradation of two organic dyes: Rhodamine B (RhB) and methylene blue (MB). ZnO NPs or 7



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ZnO/metal NP mixtures were added to 10 ml of aqueous solution containing 2 µg/ml of MB or 2

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µg/ml of RhB. Before irradiation, the mixed solution was stirred in the dark for 30 min to

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equilibrate the suspension, then the solution was exposed to simulated sunlight using an 800 W

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Xenon lamp. During irradiation, the suspension was continuously stirred to avoid precipitation.

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Aliquots of each suspension were collected at selected time intervals and centrifuged to eliminate

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the nanoparticles. Dye absorption was measured at 554 nm and 650 nm to monitor the residual

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concentrations of RhB and MB, respectively.

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2.7. Photoinduced charge separation and free radical generation of ZnO/metal NP mixtures

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The number of electrons, holes, and free radicals generated by ZnO NPs or ZnO/metal NP mixtures

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under light irradiation were measured using ESR spectroscopy (Bruker EMX ESR spectrometer,

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Billerica, MA). A Xenon lamp with a simulated sunlight filter was used to create simulated

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sunlight (380 mW/cm2). Spin traps TEMPONE and TEMPONEH•HCl were used to measure the

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electrons and holes generated in ZnO NPs or ZnO/metal NP mixtures after exposure to simulated

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sunlight. The spin trap BMPO was used to verify the formation of [•OH] and [O2 ] formed by the

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photoirradiation of ZnO NPs or ZnO/metal NP mixtures. The amount of [O21] generated during

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light irradiation was measured with 4-oxo-TEMP, and CTPO was used to measure oxygen

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consumption in the free radical generation process.

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ESR samples were prepared in 50 µl solution, put into quartz capillary tubes, and sealed. The

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capillary tubes were inserted into the ESR cavity and irradiated with simulated sunlight, then the

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spectra of different spin adduct or spin labels were recorded at selected time intervals.

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3. Results and Discussion

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3.1. Light induced ZnO/metal heteronanoparticle formation.

•–

8


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ZnO and metal NPs (Pt, Au, Ag) were characterized by acquiring their TEM images, UV-Vis

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absorption spectra, hydrodynamic diameter and zeta potential (Figures S1 and S2 and Table S1).

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In solution, both ZnO NPs and metal NPs have a negative surface charge, which protects them

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from local aggregation (Table S1). We found that ZnO/metal heteronanoparticles were formed in-

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situ during the irradiation of mixed solution of ZnO NPs and metal NPs (Figure 1). The ZnO

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surface remained clear when solution of ZnO/metal NP mixtures were not irradiated with light

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(Figure S3). The morphologies of Pt and Au NPs on ZnO NPs remained unchanged from the

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original Pt and Au NPs (Figures 1a and 1b), while Ag NPs on ZnO became irregular and larger,

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likely due to redox reaction reformation (Figure 1c). Pt, Au, and Ag particles deposited on ZnO

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retained good crystalline structure and attached very close to the ZnO surface (Figures 1d-1f). To

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verify the formation and stability of the ZnO/metal heterojunctions in solution, we examined the

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UV-Vis absorption spectra of the ZnO/metal NP mixtures after light irradiation. For each of the

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mixtures (ZnO/Pt, ZnO/Au and ZnO/Ag), light irradiation caused considerable change in the

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absorption spectra of the mixtures, including a red shift, damping of the plasmonic band, and an

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increase in the absorption of longer wavelengths (Figures 1g-1i). These results support the

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formation of ZnO/metal heterojunction induced by irradiation. In addition, we measured the UV-

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Vis absorption spectra of each mixture’s supernatant after centrifugation (Figure S4). The

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ZnO/metal NP (5 nm) mixtures were irradiated with simulated sunlight for 15 min, compared to

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non-irradiated control mixtures, and centrifuged at 8,000 rpm for 10 min. The 5 nm metal NPs

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were too small to be collected after 10 minutes of centrifugation at 8,000 r/min, but once they

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anchored on ZnO NPs by light irradiation, the hybrid NPs were readily centrifuged to the bottom

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of the vial and left a colorless supernatant. These results again support the formation of ZnO/metal

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hybrids and the strong coagulation effect of simulated sunlight irradiation on ZnO NPs and metal 9



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NPs. Using this method, we may fabricate diverse semiconductor/metal hybrid nanostructures with

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changeable metal component (Au, Ag, Pt, Pd), size, shape and concentrations.

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Figure 1. Formation of ZnO/metal hybrid NPs after simulated sunlight irradiation. TEM images

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of ZnO NPs mixed with (a) Pt 5 nm, (b) Au 5 nm, (c) Ag 5 nm. High-resolution TEM images of

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ZnO NPs mixed with (d) Pt 5 nm (e) Au 5 nm, and (f) Ag 5 nm. UV-Vis absorption spectra of the

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suspensions containing ZnO NPs and (g) Pt 5 nm, (h) Au 5 nm, and (i) Ag 5 nm with or without

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simulated sunlight irradiation.

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3.2. Enhanced antibacterial activity of ZnO/metal NPs under light irradiation.

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The photo-enhanced antibacterial activity of ZnO NPs, metal NPs, and ZnO/metal NP was

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measured by the survival rate of Gram-positive S. aureus and Gram-negative E. coli. Figure 2 10


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shows the survival rate of S. aureus (Figure 2a and 2b) and E. coli (Figure 2c and 2d) after

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simulated sunlight irradiation in the presence of ZnO NPs, metal NPs or ZnO/metal NPs. It can be

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seen that simulated sunlight irradiation alone decreased the survival rate of both S. aureus (75.2%)

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and E. coli (49.6%), as compared to control without sunlight irradiation. Adding ZnO NPs further

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decreased the survival rate of both S. aureus (58.5%) and E. coli (42.7%). For S. aureus, adding

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metal NPs did not affect their survival rate, while adding ZnO/metal NP exhibited much enhanced

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decrease in the survival rate of S. aureus, with ZnO/Pt NP (27.2%), ZnO/Au NP (34.5%), ZnO/Ag

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NP (46.7%). For E. coli, adding metal NPs, except Ag NPs, did not affect their survival rate, while

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adding ZnO/metal NPs, except ZnO/Ag NPs, further decreased the bacterial survival rate, with

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ZnO/Pt NP (24.3%), ZnO/Au NP (15%). It was interesting that ZnO/Ag NP (31.8%) did not

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induced enhanced decrease in the survival rate of E. coli compared with Ag NPs (30%). Previous

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studies have suggested that GSH capped Ag NPs have a more intense effect on E. coli than on S.

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aureus, because the penetration of colloid Ag NPs into the cytoplasm of E. coli, while in S. aureus

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the thick cell wall prevents the penetration of the NPs into the cytoplasm.30-31 In our study, Ag NPs

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decreased the bacterial survival rate of E. coli, but not S. aureus, likely due to the difference in the

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cell wall thickness between Gram positive and Gram-negative bacteria. After light irradiation of

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ZnO/Ag NP mixture, Ag NPs are attached to the surface of ZnO NPs, which decreased the chance

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of Ag NPs entering the bacterial cytoplasm of E. coli and diminish the antibacterial effect of silver.

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The photo-enhanced antibacterial activity of ZnO/metal NPs may be accounted for, at least in part,

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by the enhanced production of charge carrier and ROS due to the interfacial charge transfer from

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ZnO NPs to metal NPs.6

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Figure 2. Antibacterial activity of ZnO/Metal NPs under simulated sunlight irradiation. Bacterial

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survival rate of (a and b) Gram-positive S. aureus and (c and d) Gram-negative E. coli after

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irradiation with simulated sunlight in the presence of ZnO, metal NPs (5 nm) or ZnO/metal NPs

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(5 nm) for 10 min. The concentrations of ZnO and metal NPs (5 nm) for S. aureus are 1.25 µg/ml

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and 50 ng/ml, and for E. coli are 0.625 µg/ml and 25 ng/ml. n=3.

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3.3. Enhanced photocatalytic activities by mixing ZnO with metal NPs.

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Rhodamine B (RhB) and methylene blue (MB) were selected to evaluate the photocatalytic activity

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of ZnO/metal hybrid nanostructures. Three kinds of metal NPs (Pt, Au, and Ag) and Pt NPs of 12


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different sizes and different mass ratios to ZnO, were mixed with ZnO NPs. Both RhB and MB

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are resistant to degradation if photocatalysts are not present. Pure ZnO NPs showed considerable

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ability to degrade both RhB and MB, but mixing ZnO NPs with metal NPs (Pt, Au, Ag) enhanced

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RhB and MB degradation even further (Figure 3a). The smaller the size of the Pt NPs, the higher

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the photocatalytic degradation rate of the ZnO/Pt hybrid nanostructures (Figure 3b). Our previous

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work verified that, compared to ZnO NPs alone, the lower Fermi level of metal in ZnO/metal

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hybrid nanostructures facilitates electron transfer from the conduction band of ZnO NPs to metal

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NPs, enhancing the separation of charge carriers and ROS generation.18-19 The photocatalytic

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enhancement was associated with the production of photo-generated charge carriers and ROS.

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ESR technique is an effective way to obtain specific information on the photoinduced generation

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of electron/hole pairs and ROS.14-15, 32 Therefore, ESR was used to determine how well each metal

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NPs mixed with ZnO NPs generate ROS and charge carriers during light irradiation.

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Figure 3. Photocatalytic activity of ZnO and ZnO/metal NPs on the degradation of RhB and MB

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under simulated sunlight. Photodegradation rate of RhB and MB by mixing ZnO NPs with

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different metal NPs at a fixed size of 5 nm (a) and Pt NPs of different sizes (b). The concentrations 13



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of RhB and MB are 2 µg/mL. The concentrations of ZnO and different metal NPs are 25 µg/mL

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and 1 µg/mL, respectively.

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3.4. Enhanced generation of charge carriers by mixing ZnO with metal NPs.

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It is well-documented that ZnO NPs can absorb light that matches their band gap energy and

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generate electron/hole pairs and ROS.29 ESR spectroscopy coupled with spin trapping and spin

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labeling techniques are an effective way to obtain specific information on the generation of ROS

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and electron/hole pairs during photocatalytic processes.14-15,

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specific for different radicals and charge carriers and make it possible to both identify the radicals

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and charge carriers and compare their relative amounts.33 We used this ESR technique to determine

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how well metal NPs mixed with ZnO NPs could generate ROS and charge carriers during light

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irradiation.

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32

Spin traps and labels are very

The spin trapping agents 4-oxo-2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPONE) and 1-

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Hydroxy-2,2,6,6-tetramethyl-4-oxo-piperidine•HCl

(TEMPONE-H•HCl)

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identify photoinduced electrons and holes, respectively. TEMPONE has a stable ESR signal with

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a triplet spectrum. It does not react with unirradiated ZnO NPs, photoexcited holes, or ROS.6, 34

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Photogenerated electrons can reduce TEMPONE to hydroxylamine TEMPONE, an ESR-silent

284

product. In contrast, TEMPONE-H•HCl can be oxidized to TEMPONE by the holes generated in

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the valence bands of photoexcited semiconductors. This means that photoinduced electrons and

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holes can be easily monitored by observing the changes in the ESR spectra of TEMPONE. Figure

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4a shows the ESR spectra of TEMPONE during different reactions that occurred under simulated

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sunlight irradiation. Compared with ZnO alone, the ZnO/Pt NP mixtures (5 nm, 30 nm, or 50 nm)

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significantly reduced the ESR signal intensity of TEMPONE. For example, more than 78% of the

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ESR signal intensity of TEMPONE was reduced by ZnO/Pt 5 nm within 15 minutes, while ZnO 14


were

selected

to

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NPs alone produced only a 15% reduction in the same period of time. In addition, Pt NPs mixing

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with ZnO enhanced the reduction of TEMPONE in a size-dependent manner: the smaller the size

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of the Pt NP, the greater the reduction of TEMPONE ESR signal intensity, indicating greater

294

electron generation (Figure 4a). The longer the ZnO/Pt NP mixture was irradiated, the more the

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TEMPONE signal intensity was reduced (Figure 4b).

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The mass ratio of Pt/ZnO NPs also influenced the production of electrons. Small mass ratios

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of Pt/ZnO NPs (0.4% and 4%) reduced TEMPONE signal intensity by 97% and 95%, respectively;

298

but a Pt/ZnO ratio of 40% reduced TEMPONE signal intensity by only 67% (Figure 4c). Au and

299

Ag NPs can also reduce TEMPONE’s ESR signal intensity by increasing electron generation. At

300

the same size and mass ratio, Ag NPs showed the highest activity followed by Pt and Au NPs

301

(Figure 4d).

302 15



303

Figure 4. Enhanced production of electrons from ZnO/metal NP mixtures under simulated sunlight

304

irradiation. (a) ESR spectra of TEMPONE with ZnO or ZnO/Pt NP mixtures under light irradiation.

305

ESR signal intensity of TEMPONE decreased by ZnO or ZnO mixed with (b) Pt NP of different

306

sizes or (c) Pt 5 nm at different mass ratios to ZnO or (d) Pt 5 nm, Au 5 nm or Ag 5 nm.

307 308

TEMPONE-H•HCl is ESR silent. It can be oxidized by photogenerated holes to form a

309

TEMPONE radical that has a three-line ESR spectrum and a relative intensity ratio of 1:1:1. We

310

examined TEMPONE-H•HCl oxidation in ZnO suspensions with different sizes and different

311

concentrations of Au, Ag, and Pt NPs to understand their effect on photogenerated holes. When

312

TEMPONE-H•HCl was exposed to simulated sunlight in the presence of ZnO, an ESR spectrum

313

with three lines was observed, indicating the oxidation of TEMPONE-H•HCl to TEMPONE and

314

the production of holes from photoexcited ZnO (Figure 5a). All the mixtures tested, no matter what

315

size nanoparticles or mass ratios were used, greatly increased TEMPONE signal intensity

316

compared to ZnO alone, suggesting that all the mixtures enhanced the production of

317

photogenerated holes.

318

The oxidation rate of TEMPONE-H•HCl by irradiated ZnO/metal NP mixtures is determined

319

by the metal composition, size, and mass ratio of the metal NP to the ZnO NP and the irradiation

320

time. Smaller Pt NPs oxidized TEMPONE-H•HCl more quickly (Figure 5a). Longer irradiation

321

times resulted in stronger ESR signal intensity in a linear fashion (Figure 5b). The Pt/ZnO mass

322

ratios had little effect on the oxidation rate of TEMPONE-H•HCl, but retarded its initial oxidation

323

time by increasing the Pt/ZnO mass ratio (Figure 5c). The ability of the mixtures to oxidize

324

TEMPONE-H•HCl was the same as their ability to reduce TEMPONE: Ag>Pt>Au (Figure 5d).

325

These observations are consistent with the fact that electrons and holes are simultaneously

326

produced in pairs from photoexcited ZnO NPs. 16


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327 328

Figure 5. Enhanced generation of holes by mixing ZnO NPs with metal NPs under simulated

329

sunlight irradiation. (a) ESR spectra of TEMPONE generated by ZnO or ZnO/Pt NP mixture under

330

light irradiation. ESR signal intensity of TEMPONE generated by ZnO or ZnO mixed with (b) Pt

331

NPs of different sizes or (c) Pt 5 nm at different mass ratios to ZnO or (d) Pt 5 nm, Au 5 nm or Ag

332

5 nm.

333 334

3.5. Enhanced generation of ROS by mixing ZnO with metal NPs.

335

The highly energetic holes and electrons of photoexcited ZnO NPs can react with surrounding

336

oxygen-containing molecules to produce ROS, which has been proposed as the main mechanism

337

of semiconductor photocatalytic activity.8 In this publication, we identified and quantified the ROS

338

species generated by ZnO/metal NP mixtures under simulated sunlight irradiation. The spin trap

339

used to detect [•OH] and [O2 ] was 5-tertbutoxycarbonyl-5-methyl-1-pyrroline N-oxide (BMPO).

•–

17



Page 18 of 31

340

Figure 6 shows the ESR spectra generated by irradiating ZnO NPs and ZnO/metal NPs with

341

simulated sunlight. No ESR signal was observed in unirradiated samples or samples without

342

catalysts (data not shown). When ZnO NPs were irradiated with simulated sunlight, a four-line

343

spectrum with relative intensities of 1:2:2:1 was observed with the hyperfine splitting parameters

344

of aN= 13.56, aβH= 12.30, aγH= 0.66 that are characteristic for the adduct of BMPO and [•OH]

345

([BMPO/•OH]).33

346

It is worth noting that the ESR spectra of [BMPO/·OH] and [BMPO/·OOH] overlap, so it is

347

hard to distinguish whether there is an [O2 ] signal generated by a photo-irradiated ZnO/metal NP

348

mixture using the ESR spectra. We used dimethyl sulfoxide (DMSO) to scavenge [•OH], and

349

superoxide dismutase (SOD) to scavenge [O2 ], to determine the composition of the ESR

350

spectrum and found that (1) adding DMSO significantly decreased ESR signal intensity and

351

changed the ESR spectrum, and (2) adding 5 U/ml SOD slightly decreased the ESR signal

352

intensity by 11% after 5 min of light irradiation, indicating that [O2 ] was also generated but in a

353

much less content compared to [•OH] in photoexcited ZnO/metal NP mixtures (Figure S5).

•–

• –

•–

354

Adding 5 nm, 30 nm, and 50 nm of Pt NPs greatly increased ESR signal intensity, suggesting

355

that mixing Pt NPs with ZnO NPs produced more [•OH] and [O2 ] (Figures 6a and 6b). The

356

amount of [•OH] and [O2 ] produced by smaller Pt NPs mixed with ZnO NPs is much higher than

357

the amount produced by larger Pt NPs. The generation of [•OH] and [O2 ] also depends on the

358

concentration of metal NPs. When the mass ratio of Pt/ZnO NPs increased from 0.4% to 4%, the

359

ESR signal increased; but when the mass ratio was increased from 4% to 40% the ESR signal

360

decreased (Figure 6c). Au and Ag NPs also increase ROS ([•OH] and [O2 ]) generation in the

361

same irradiation/time-dependent manner (Figure 6d). ZnO/Pt mixtures produced the most ROS

• –

•–

•–

•–

18


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362

followed by ZnO/Au mixtures and ZnO/Ag mixtures. ESR signal intensity generated from

363

photoexcited ZnO/Pt NPs is about three times higher than that generated by ZnO/Ag NPs and

364

seven times higher than ZnO NPs alone.

365 •–

366

Figure 6. Enhanced generation of [•OH] and [O2 ] from ZnO/metal NP mixtures under light

367

irradiation. (a) ESR spectra of [•OH] and [O2 ] generated from ZnO NPs or ZnO mixed with Pt

368

NPs of different sizes. ESR signal intensity of [•OH] and [O2 ] generated from ZnO NPs or ZnO

369

mixed with (b) Pt NPs of different sizes or (c) 5 nm Pt at different mass ratios to ZnO NPs or (d)

370

Pt 5 nm, Au 5 nm, or Ag 5 nm.

•–

•–

371

Spin trap 4-oxo-2,2,6,6-tetramethylpiperidine (4-oxo-TEMP) was used to quantify [O21]

372

production in ZnO and ZnO/metal NP mixtures under simulated sunlight irradiation (Figure 7).

373

No ESR signal was observed in unirradiated control samples or samples without catalysts (data 19



374

not shown). A triplet spectrum, characteristic of the product of 4-oxo-TEMP and [O21], was

375

observed in photoirradiated pure ZnO or ZnO/metal NP mixtures. We tested the effect of mixing

376

NPs of different sizes, concentrations, and compositions with ZnO NPs on the generation of [O21],

377

and the results were similar to those produced by irradiated ZnO/metal NPs on the generation of

378

[•OH] and [O2 ].

•–

379

The triplet ESR signal was greatly enhanced by mixing ZnO with Au, Pt, or Ag at different

380

sizes and concentrations. We found that small-sized Pt NPs promoted more [O21] generation. For

381

example, ZnO/Pt 5 nm produced five times the [O21] signal intensity and ZnO/Pt 50 nm produced

382

twice the [O21] signal intensity than ZnO alone after 5 minutes of light irradiation (Figures 7a and

383

7b). Longer irradiation times resulted in stronger [O21] signals. Adding Pt NPs not only increased

384

the generation of [O21], it affected its rate of formation. For example, increasing the mass ratio of

385

Pt/ZnO from 0.4% to 4% significantly increased [O21] generation, but continuing to increase the

386

mass ratio from 4% to 40% reduced the [O21] signal and delayed its production (Figure 7c).

387

Different metal NPs enhanced the generation of [O21] at different rates (Figure 7d). Photoexcited

388

ZnO/Pt 5 nm produced the strongest ESR signal, while ZnO/Ag 5 nm resulted in the fastest

389

production of [O21] (Figure 7d).

390

20


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391 392

Figure 7. Enhanced generation of [O21] from ZnO/metal NP mixtures under light irradiation. (a)

393

ESR spectra of [O21] generated from ZnO NPs or ZnO mixed with Pt NPs of different sizes. ESR

394

signal intensity of [O21] generated from ZnO NPs or ZnO mixed with (b) Pt NPs of different sizes

395

or (c) 5 nm Pt at different mass ratios to ZnO NPs or (d) Pt 5 nm, Au 5 nm or Ag 5 nm.

396

3.6. Enhanced oxygen consumption in photoexcited ZnO/metal NP mixture.

397

The oxygen dissolved in solutions is the most common electron acceptor during ROS generation.

398

ESR oximetry in conjunction with spin label CTPO was used to monitor dissolved oxygen

399

consumption during the photoirradiation of ZnO/metal NP mixtures. Spin label oximetry is based

400

on the biomolecular collision between oxygen and a spin label.35 Because O2 is paramagnetic, the

401

collision of a spin label with O2 produces a spin exchange, which results in shorter relaxation

402

times, broader line widths, and increased resolution of the ESR spectrum of the spin label.36

21



403

CTPO was chosen as the spin label for oxygen quantification because of its super-hyperfine ESR

404

spectrum, making it very sensitive to oxygen variation at low concentrations. A lower

405

concentration of dissolved oxygen results in a more super-hyperfine spectrum. Before a ZnO/Pt 5

406

nm mixture was irradiated, the ESR spectrum of CTPO was a smooth line without super-hyperfine

407

structures because the sample was saturated with oxygen (Figure 8a). After 5 min of light

408

irradiation, the super-hyperfine structures appeared and were clearly split over longer irradiation

409

times, suggesting continued oxygen decrease in the sample. The equation to calculate the oxygen

410

concentration is:

411

[O2](mM) = 0.23 ― 0.746k

412

where the K parameter is calculated from the super-hyperfine structures using the equation K =

413

(X+Y)/2Z (Figure 8a [inset]).37

414

Figure 8b compares the oxygen consumption of ZnO NPs or ZnO/Pt NPs of different sizes

415

during photoirradiation. Concentrations of oxygen in the control did not change with

416

photoirradiation. In the ZnO NP sample, the oxygen concentration slightly decreased with light

417

irradiation, while in the ZnO/Pt 5 nm sample, the concentration of oxygen decreased 80% after 15

418

min of photoirradiation. The oxygen consumption pattern was the same as the photoexcitation of

419

electrons pattern and generated [O21]: ZnO/Pt 5 nm > ZnO/Pt 30 nm> ZnO/Pt 50 nm. This indicates

420

that the enhanced production of [O21] was linked to the quicker consumption of dissolved

421

molecular oxygen. All three metal NP greatly enhanced the ability of ZnO to reduce oxygen

422

concentration under irradiation, with Ag more active than Au and Pt (Figure 8c). This is consistent

423

with Ag’s greater ability to promote the generation of electrons and degrade MB.

22


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424 425

Figure 8. Oxygen consumption during light irradiation of ZnO and a ZnO/metal NP mixture. (a)

426

Time-dependent evolution of the ESR spectrum of 0.1 mM CTPO during irradiation with

427

simulated sunlight in the presence of a ZnO/Pt 5 nm mixture. (b) Oxygen consumption during light

428

irradiation of CTPO alone (control), CTPO with ZnO NPs, CTPO with ZnO NPs/Pt NPs of

429

different sizes. (c) Dissolved oxygen consumption during the photoirradiation of CTPO alone

430

(control), CTPO with ZnO/Pt 5 nm, ZnO/Au 5 nm, or ZnO/Ag 5 nm mixtures. The concentration

431

of ZnO is fixed at 25 µg/ml and the concentration of the metal NPs is 1 µg/ml.

432

To summarize the above results, we used ESR spectroscopy to study how Au, Ag, and Pt NPs

433

at different sizes and concentrations changed the formation of electrons, holes, and all three ROS

434

([•OH], [O2 ] and [O21]) by ZnO NPs during photoexcitation and found that all of them enhanced

•–

23



435

the generation of these elements in different and interesting ways. ZnO NPs that were mixed with

436

smaller-sized Pt NPs produced charge carriers and ROS more efficiently than larger Pt NPs. A

437

small amount (0.4% mass ratio) of Pt/ZnO NPs increased the production of electrons and holes,

438

and a 4% mass ratio of Pt/ZnO NPs produced the fastest and strongest generation of all the ROS.

439

Ag NPs promoted charge carrier separation more effectively than Pt NPs and Au NPs, while Pt

440

NPs promoted [•OH], [O2 ] and [O21] production better than Au and Ag NPs at the same diameter

441

(5 nm). In addition, we also compared the effect of metal shape and crystal facets on the generation

442

of ROS under sunlight irradiation. We mixed palladium (Pd) NPs (cubes vs octahedrons) with ZnO

443

NPs under sunlight irradiation (Figure S6) and found a clear difference in the generation of [•OH],

444

[O2 ] and [O21]. ZnO NPs mixed with Pd nanocubes (enclosed by [100] crystal facets) were more

445

active than ZnO NPs mixed with Pd octahedrons (enclosed by [111] crystal facets), suggesting

446

that metal NP shape and crystal facets could also influence semiconductor ROS production. It is

447

also worth noting that the difference in oxygen adsorption capacity of different metal nanoparticles

448

could also have an effect on the oxygen consumption efficiency and ROS generation.38

•–

•–

449

Aligning the ESR results with photocatalytic degradation explains why ZnO/Au, ZnO/Ag, and

450

ZnO/Pt NPs enhance the photocatalytic degradation of RhB and MB: RhB degradation was

451

mediated by the photo-oxidation of ROS and holes whereas MB degradation was dominated by

452

the photo-reduction of electrons. We can therefore conclude that 1) the ability of Pt, Au and Ag

453

NPs to enhance the photoactivity of ZnO correlates closely with their ability to improve charge

454

carrier reactivity and the generation of ROS, and 2) one can fine-tune the generation of ROS and

455

charge carriers from photoexcited metal oxides by mixing the oxides with metal NPs that have

456

different physicochemical properties.

24


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457

Since Brownian motion can lead to collisions between metal NPs and ZnO NPs, we

458

hypothesized that temperature would also affect the inter-particle collision frequency, which

459

would then affect the electron transfer process and ROS production in ZnO/metal NP mixtures. To

460

explore this hypothesis, we compared photoirradiation-induced [•OH] generation in as-prepared

461

ZnO/Pt heteronanoparticles and physically mixed ZnO/Pt NPs at different temperatures. The as-

462

prepared ZnO/Pt heteronanoparticles was prepared by pre-irradiation of ZnO/Pt NPs mixture for

463

15 min. Then we added BMPO and recorded the time-dependent ESR signal intensities of the ROS

464

at temperatures from 10ºC to 40ºC (Figure S7). In physically mixed ZnO/Pt NPs, the ESR signals

465

of the [•OH] increased dramatically as the temperature rose from 10ºC to 40ºC, while in the as-

466

prepared ZnO/Pt heteronanoparticles, hydroxyl radical generation increased when the temperature

467

rose from 10ºC to 20ºC, but stayed constant between 20ºC and 40ºC. The results suggest that,

468

while Brownian motion increased steadily in the physically mixed ZnO/Pt NPs as the temperature

469

rose. This increased the number of collisions between ZnO NPs and Pt NPs and promoted the

470

production of ROS. In addition, we found that when metal NPs were coated with silica, which is

471

an insulator, ROS production of the ZnO/silica-coated-metal mixture was not enhanced, indicating

472

the enhanced ROS generation depends on the direct electron transfer between the semiconductor

473

and metal NPs. (Figure S8).

474

Although the detailed mechanisms need further clarified, we proposed that ZnO/metal

475

heterojunctions formed as a result of particle collisions and photo-induced local dipole interactions.

476

In solution, both ZnO NPs and metal NPs have a negative surface charge, which protects them

477

from local aggregation (Table S1). When ZnO NPs were photo-irradiated, photoexcited electrons

478

and holes were produced and redistributed on ZnO NP surfaces and generated local electrical

479

fields.39 Since Brownian motion can lead to collisions between metal NPs and ZnO NPs, the newly 25



480

charged ZnO particles attracted oppositely charged or uncharged smaller metal particles through

481

dipole interactions. When the mass ratio of metal NPs increased, most charges on ZnO NP surfaces

482

were sacrificed to attract metal particles, leaving less charge to generate ROS and react with spin

483

probes. The formation of solid heterojunctions facilitates electron transfer from conduction bands

484

to metal surfaces because the Fermi levels of Au, Ag, and Pt are lower than the conduction band

485

edge position of ZnO and increase the generation of ROS (Scheme 1, Pathway I). When the metal

486

NPs and ZnO NPs are at a critical interaction distance before they coagulate, electron transfer may

487

occur between them by a tunneling effect that enhances electron/hole separation (Scheme 1,

488

Pathway II). In the case of Pathway III (Scheme 1), the electron transfer will not occur because

489

the distance between the ZnO NPs and the metal NPs is too great.

490

491 492

Scheme 1. Possible pathways of electron transfer from photoexcited ZnO to metal NPs in mixture

493

and enhances generation of reactive oxygen species.

494 26


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495

4. Conclusions

496

In this study, we demonstrated a mixing/irradiation way to directly fabricate ZnO/metal hybrid

497

nanostructures with tunable metal nanocomponent and enhanced antibacterial and photocatalytic

498

activity. The anchored Ag, Au, and Pt NPs significantly increases light-induced electron-hole

499

separation efficiency and the production of ROS including [•OH], [O2 ], and [O21]. The degree to

500

which the hybrid NPs enhance is determined by metal particle size and composition, crystal facet,

501

and the mass ratio of metal to ZnO. Although the detailed mechanisms of this process remain

502

unclear, our hypothesis is that ZnO/metal heterojunctions formed in mixed solutions under

503

simulated sunlight irradiation as a result of particle collisions and photo-induced local dipole

504

interactions. Our work has revealed a new semiconductor photocatalytic enhancement strategy that

505

allows us to fabricate semiconductor/metal hybrid nanostructures easily and tailor the production

506

of charge carriers and ROS for photocatalytic applications.

•–

507 508

Acknowledgements

509

This work was supported by a regulatory science grant under the FDA Nanotechnology CORES

510

Program, and the authors used instruments at the FDA Advanced Characterization Facility (ACF).

511

Additional support came from the National Natural Science Foundation of China (Grant No.

512

51772256), the Program for Innovative Research Team (in Science and Technology) in University

513

of Henan Province (19IRTSTHN026), the Plan for Scientific Innovation Talent of Henan Province

514

(174100510014), and Xuchang University research project (2017YB005; 217ZD003). We are

515

grateful to Dr. John Callahan from CFSAN/FDA for his comments on the manuscript. The authors

516

appreciate Barbara Berman for her scientific writing support. The views presented in this paper do

517

not necessarily represent those of the U.S. Food and Drug Administration. No official support or

518

endorsement by the U.S. Food and Drug Administration is intended or should be inferred. 27



519

ASSOCIATED CONTENT

520

Supporting Information

521

Experimental Sections, Figure S1-S8, Table S1. This material is available free of charge via the

522

Internet at http://pubs.acs.org.

523 524

AUTHOR INFORMATION

525

Corresponding Authors:

526

Email: [email protected] (Yin J.-J.)

527

Email: [email protected] (He W.)

528 529 530

Notes

531

References

532 533 534 535 536 537 538 539 540 541 542 543 544 545 546 547 548 549 550 551 552 553 554 555 556

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557 558 559 560 561 562 563 564 565 566 567 568 569 570 571 572 573 574 575 576 577 578 579 580 581 582 583 584 585 586 587 588 589 590 591 592 593 594 595 596 597 598 599 600 601 602 603


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Light-Induced Assembly of Metal Nanoparticles on ZnO Enhances the

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