Aerosol-Based Self-Assembly of a AgâZnO Hybrid Nanoparticle

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Aerosol-Based Self-Assembly of Ag-ZnO Hybrid Nanoparticle Cluster with Mechanistic Understanding for Enhanced Photocatalysis Li-Ting Chen, Ung-Hsuan Liao, Je-Wei Chang, Shih-Yuan Lu, and De-Hao Tsai Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b00577 • Publication Date (Web): 01 Apr 2018 Downloaded from http://pubs.acs.org on April 1, 2018

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Aerosol-Based Self-Assembly of Ag-ZnO Hybrid Nanoparticle Cluster with Mechanistic Understanding for Enhanced Photocatalysis Li-Ting Chen, Ung-Hsuan Liao, Je-Wei Chang, Shih-Yuan Lu, De-Hao Tsai* Department of Chemical Engineering, National Tsing Hua University, Hsinchu, Taiwan, R.O.C.

KEYWORDS: Zinc oxide, silver, nanoparticle, photocatalysis, degradation, self-assembly, stability, aerosol, colloid, interface

ACS Paragon Plus Environment

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ABSTRACT

A gas-phase controlled synthetic approach is demonstrated to fabricate Ag-ZnO hybrid nanostructure as a high-performance catalyst for photo-degradation of water pollutants. The degradation of rhodamine B (RhB) was used as representative, which were tested and evaluated with respect to the environmental pH and the presence of dodecyl sulfate corona on the surface of catalyst. The results show that a raspberry-structure Ag-ZnO hybrid nanoparticle cluster was successfully synthesized via gas-phase evaporation-induced self-assembly. The photodegradation activity increased significantly (20×) by using the Ag-ZnO hybrid nanoparticle cluster as catalyst. A surge of catalytic turnover frequency of ZnO nanoparticle cluster (> 20×) was observed through the hybridization with AgNPs. The dodecyl sulfate corona increased the photocatalytic activity of the Ag-ZnO hybrid nanoparticle cluster especially at the acidic and neutral pH environments (maximum 6×), and the enhancement in catalytic activity was attributed to the improved colloidal stability of ZnO-based nanoparticle cluster under the interaction with RhB. Our work provides a generic route of facile synthesis of the Ag-ZnO hybrid nanoparticle cluster with mechanistic understanding of interface reaction for enhancing photocatalysis toward degradation of water pollutants.


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INTRODUCTION

Zinc oxide, a II–IV semiconductor with a direct bandgap energy of 3.37 eV, is highly attractive as a photocatalyst for a variety of energy and environmental applications (e.g., H2 evolution via water splitting; water purification systems for degradation of various types of pollutants).1-13 The ZnO with a nanoscale dimension, for example, the ZnO nanoparticle (NP), is especially useful due to the large surface area, low cost, ready availability, and ease of handling in the aqueous solution.2,5,8,9,12-15 In comparison to the widely-used TiO2 NP, ZnO NP is a suitable alternative as it exhibits an equal high photocatalytic activity.5,16-18 Moreover, ZnO NP has a higher electron mobility and a relatively lower production cost than TiO2-NP, which has shown the promise for the potential large-scale photocatalytic applications.9,16 However, ZnO NP exhibits limitations from the prospects of photon-induced energy transfer, absorption of light irradiation and photocorrosion, all of which restrict its potential use for the applications in photocatalysis.5-10,17-20 Firstly, the ZnO NP adsorbs light irradiation mainly with a wavelength of < 387 nm, which is only useful for the photocatalysis under ultraviolet irradiation (i.e., insufficient use of the sunlight). Besides, the rapid recombination of the photo-induced electrons and holes usually occurs accompanied with the effect of photocorrosion during the photocatalytic reaction in aqueous phase, limiting the efficiency of ZnO NP in photocatalysis. In this regard, hybridization of the noble metal plasmonic NP with ZnO NP has shown the promise, which improves photon-induced energy transfer by suppressing the recombination of electronhole pairs in the ZnO NP and expands its absorption window to the visible light irradiation.2,4,7,8,10,11,15,17,20-23


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Colloidal stability and interface reaction of noble metal-ZnO hybrid nanostructure are two important factors interplaying to the photocatalytic activity,3,24 which affect the abilities of the absorption of light irradiation and the energy transfer, respectively. For example, ligandinduced aggregation results in a fast precipitation of nanocatalyst and the subsequent loss of the ability of light absorption. Surface dissolution of ZnO NP affects the interactions with targeted molecules in the media and the corresponding ability in energy transfer.9 Therefore, developing a suitable method to synthesize noble metal-ZnO hybrid nanostructure stably dispersed in the aqueous media and providing mechanistic understanding of the ligand-nanoparticle interaction and colloidal stability versus the photocatalytic activity are both highly important. In this work, we demonstrate a facile gas-phase synthetic method to fabricate noble metal-ZnO hybrid nanostructure. Here, silver NP (AgNP) is used as the representative noble metal NP due to its substantial interest of the metal-enhanced optical properties generated by surface plasmon resonance (SPR) of AgNP.4,6,20,22,23,25-28 Using an evaporation-induced self-assembly (EISA),29,30 a stable, isotropic aqueous solution containing ZnO and Ag precursors is nebulized as droplets in a carrier air. The fast evaporation of droplets triggers self-assembly to create aerosol particles of dried precursor crystallites, and then the dried precursor crystallites converted to Ag-ZnO hybrid nanostructure via direct gas-phase calcination.25,29-31 In comparison to the traditional synthetic approaches reported previously,4,11,14,16,17,32,33 the gas-phase synthetic method demonstrates the following superior advantages: (1) A continuous synthetic process with an ability in the tuning of chemical composition and oxidation state;25,29-31 (2) Avoiding the issues related to the restrictions arising from solution-based chemistry used for creating homogenous distribution of active component in the hybrid nanostructure (e.g., restriction in the properties of solvent, addition of surfactants during the synthesis).25,29-31


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The photon-induced degradation of pollutants in water is chosen as the representative system to study the photocatalytic activity of the Ag-ZnO hybrid nanostructure. Rhodamine B (RhB) is used as a convenient representative pollutant ligands in water,6,7,11 and sodium dodecyl sulfate (SDS) is chosen as an example of ligand in the media due to their high availability in the commercial detergent products. Complementary characterization approaches, including differential mobility analysis (DMA), high-resolution transmission electron microscopy (HRTEM), scanning electron microscopy (SEM) and X-ray diffractometry (XRD) are employed to provide information of physical size, elemental composition, morphology and crystallinity, respectively. Our objective is to fabricate Ag-ZnO hybrid nanostructure with desired material properties (controlled morphology, particle size, and elemental composition), by which we can effectively improve the photocatalytic activity of Ag-ZnO hybrid nanostructure for a variety of energy and environmental applications. Through a systematic kinetic study of the degradation of RhB (i.e., the representative photocatalytic reaction), we aim to develop a mechanistic understanding of ligand-nanoparticle interaction of the synthesized Ag-ZnO hybrid nanostructure in the aqueous phase and establish a correlation to its photocatalytic activity. To our knowledge, our work reports the first study of mechanistic understanding of the photocatalysis by the noble metal-ZnO hybrid nanostructure from the prospects of interface reaction and colloidal stability of photocatalyst.


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Figure 1. Schematic diagram of gas-phase synthetic method to fabricate noble metal-ZnO hybrid nanostructure.

EXPERIMENTAL SECTION Materials Zinc nitrate (Zn(NO3)2‧6H2O, ≧99%, Alfa Aesar, Haverhill, MA, USA) and silver nitrate

(AgNO3, purity≧99.8%, Sigma-Aldrich, St. Louis, MO, USA) are used as the precursors of ZnO and Ag, respectively. Sodium dodecyl sulfate, SDS (≈ 96.5%. Sigma-Aldrich) and rhodamine B (RhB. ≧98%, ACROS organics, Thermo Fisher Scientific, NJ, USA) are prepared in the form of aqueous solution without further purification. Glacial acetic acid (Macron, Center Valley, PA, USA) and ammonia hydroxide (>98.5%; Sigma-Aldrich) are used for the tuning of solution pH.


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Biological grade 18.2 MΩ•cm deionized water (Millipore, Billerica, MA, USA) is used to prepare the aqueous solutions for particle synthesis and photocatalytic activity measurements. Aerosol-based Synthesis of Nanoparticles The aerosol-based synthetic system includes a nebulizer, a drying unit, and a gas-phase temperature-programmed flow reactor (see Figure 1). Details of the experimental setups have been described in the previous publications.25,29,31 Briefly, an aqueous solution of Zn precursor and Ag precursor mixes together to form a homogeneous precursor solution. Then a nebulizer (Model 3076, TSI Inc., Shoreview, MN, USA) is employed to spray the precursor solution using a compressed filtered air at a flow rate of 1.5 L/min. The droplets of the nebulized precursors are converted to aerosols and delivered to a quartz-made flow reactor (inner diameter: 2.2 cm; length: 45 cm). The flow reactor is placed in a tube furnace (HS-40, Huahsing, Taiwan, ROC) with a heated length of 25 cm at the operating temperature of 500 °C, where the Zn precursor and Ag precursor are thermally decomposed to ZnO and Ag, respectively. The synthesized nanoparticles are delivered downstream for sample collection via an aerosol in-line filter holder (All Field Enterprise Corp., Taipei, Taiwan, ROC) equipped with a 0.2 µm mixed cellulose ester membrane filter (Advantec, Tokyo, Japan). Table 1 summarizes a list of samples with different molar fractions of ZnO (nZnO) and Ag (nAg), and the presence of SDS in the hybrid nanostructure. Here CAg and CZn are mass concentrations of Ag precursor and ZnO precursor in the aqueous solution, respectively.

Table 1. List of samples of ZnO-based hybrid nanostructures. NPC: nanoparticle cluster

Sample I.D.

With SDS

CZn (%)

CAg (%)

nZnO

nAg


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ZnO-NPC

No

10

0

1

0

1Ag-10ZnO-NPC

No

10

1

0.85

0.15

10Ag-10ZnO-NPC

No

10

10

0.36

0.64

SDS-ZnO-NPC

Yes

10

0

1

0

SDS-1Ag-10ZnO-NPC

Yes

10

1

0.85

0.15

SDS-10Ag-10ZnO-NPC

Yes

10

10

0.36

0.64

Nanomaterial Characterization The number-based mobility size distributions of the synthesized aerosol nanoparticles are characterized in-situ by DMA.25,31,34 Firstly, the synthesized NPs in the form of aerosol (1.5 L/min) are charge-neutralized by a soft X-ray-based aerosol neutralizer (Model 308801, TSI Inc.) and delivered to an electrostatic classifier (Model 3081, TSI Inc.) with a sheath flow of dry and filtered air carrying NPs downstream at a flow rate of 10 L/min. By varying the electric field, NP of a specific mobility size, dp,m, exits the electrostatic classifier and is counted by a condensation particle counter (CPC, model 3775, TSI Inc.).25,31,34-36 The step size of the DMA measurement is 2 nm, and the step time of the measurement is 10 s. A field emission-scanning electron microscope (FE-SEM, Hitachi SU8010, Hitachi, Tokyo, Japan) is used to identify the morphology and the primary diameter of NPs at an acceleration voltage of 10 kV. High-resolution transmission electron microscopy (HR-TEM, JEM-2100F, JEOL, Japan) equipped with an energy dispersive spectrometer (EDS) is operated at an


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acceleration voltage of 200 kV to obtain the crystalline structure and elemental mapping of the synthesized NP. Electrostatic-directed deposition is employed for collecting particles on a TEM grid or a silicon chip for the image analysis. The operating sample flow rate is ≈ 1.5 L/min, and the electric field used for aerosol deposition is -(5-10) kV/cm. Zeta potential (ZP) is measured using a Zetasizer Nano (Malvern Instruments, Westborough, MA, USA) in backscatter configuration (θ = 173˚) at a laser wavelength of 633 nm and a constant temperature of 25.0 ˚C. A palladium dip cell module (Malvern Instruments) is used, and the buffer solution used for the ZP measurement is 2 mmol/L NaCl aqueous solution (i.e., with specified pH).

Photocatalytic activity measurements Photocatalytic activity of the as-prepared samples is evaluated by measuring the efficiency of RhB degraded in the aqueous solution under light irradiation. The photocatalytic reaction is carried out in a closed plastic cuvette at ≈ 25 °C with a solution volume (V0) of 1.5 mL. 2 mg of the synthesized NPs is ultrasonically dispersed in 20 mL of DI water. Then the colloid is diluted to a concentration of 90 mg L-1, reacted with the RhB at an initial concentration of RhB (CRhB,t=0) of 25 µmol/L (0.0012wt%), and adjusted to the specific pH for the subsequent measurement. A 300W simulated solar light, (Xe arc lamp. UXL-302-O. Ushio corp., Cypress, CA, USA) is employed as the light source. The vertical distance between the light source and the surface of the cuvette is constant at 30 cm. The time of irradiation (t) ranged from t = 30 min to t = 150 min. The concentration of RhB (CRhB,t) is monitored by measuring the UV–vis absorption of the


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suspensions versus t at a 30-min interval using ultraviolet-visible light absorbance spectrometer (U-3300, Hitachi, Ltd.). The kinetics of the photo-degradation of RhB in the aqueous solution were analyzed using the 1st-order reaction kinetics,25

ln(

CRhB,t=0 ) = kt CRhB,t

(1).

Here k is the rate constant of RhB degradation, which has shown to be dependent of the projected area of light illumination and the light irradiance. The turnover frequency, k*, is defined as the number of RhB converted (∆NRhB,t) on the basis of unit mass of ZnO, WZnO (i.e., the active phase) per minute.

∆N RhB,t C RhB,t=0 N avV0 (I 0 − I t ) − (I 0 − I * ) k* = = [ ] (2) I0 WZnO * t WZnO * t . Here, It is the peak intensity of UV–vis absorption of the RhB (λRhB = 553 nm) at t =150 min in the presence of catalyst, and I0 is the It at t =0 min. I* is the peak intensity of UV–vis absorption of the RhB (λRhB= 553 nm) at t =150 min using the control sample (i.e., without the catalyst). Nav is the Avogadro's number.

RESULTS AND DISCUSSION

Gas-phase synthesis and characterization of Ag-ZnO hybrid nanostructure


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Firstly, TGA was employed to determine the required temperature (T) to thermally decompose the Ag and Zn precursors to Ag and ZnO, respectively. Figure 2a shows the changes of sample mass versus the temperatures of the dried Ag and Zn precursors measured by the TGA. A mass loss of ≈ 36% was observed at the range of 370 °C to 480 °C for the curve of Ag precursor (green), indicating that the Ag precursor was thermally decomposed to Ag when T > 480 °C (i.e., theoretical value of the mass loss was 37%).31 For the curve of Zn precursor (blue), the mass decreased by 36% at the range of 60 °C to 216 °C, which was correlated to the removal of lattice water (i.e., the theoretical value of the mass loss was 37%). By further increasing T to 350 °C, a 35% of the mass loss was identified, indicating a transformation of Zn(NO3)2 to ZnO. The results of TGA show that both Zn and Ag precursors were able to be thermally decomposed when T > 480 °C. Therefore, T = 500 °C was chosen as the temperature for the gas-phase synthesis of Ag-ZnO hybrid nanostructure. XRD analyses (Figure 2b) show clear diffraction patterns of ZnO for all synthesized samples. The diffraction peaks at 31.8°, 34.4°, 36.3°, 47.5°, 56.6°, 62.9° and 68° correspond to the ZnO crystal planes of the (100), (002), (101), (102), (110), (103) and (112), respectively. For 1Ag-10ZnO-NPC and 10Ag-10ZnO-NPC, the diffraction peaks at 2θ of 38.1°, 44.3° and 64.4° represent the (111), (200) and (220) crystal planes of the Ag, respectively. The results of XRD analyses confirm a successful thermal decomposition of Ag and Zn precursors to metallic Ag and ZnO crystallites at T = 500 °C, respectively. The crystallite sizes of ZnO (dc,ZnO) were calculated as 29 nm for ZnO-NPC, 20 nm for 1Ag-10ZnO-NPC, and 16 nm for 10Ag-10ZnO-NPC [i.e., using ZnO (101)]. The crystallite sizes of Ag (dc,Ag) were calculated as 14 nm and 22 nm for 1Ag-10ZnO-NPC and 10Ag-10ZnO-NPC, respectively [i.e., using Ag (111)]. The results show that increasing CAg resulted in an increase of dc,Ag and a decrease of dc,ZnO.


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Figure 2. Gas-phase synthesis of Ag-ZnO hybrid nanostructure and the characterization of morphology and particle size. (a) Determination of required temperature using TGA. (b) XRD patterns of ZnO-NPC, 1Ag-10ZnO-NPC and 10Ag-10ZnO-NPC. (c) The representative SEM images. 1: ZnO-NPC; 2: 10Ag-10ZnO-NPC. The scale bars are 500 nm. (d) Mobility size distributions of ZnO-NPC, 1Ag-10ZnO-NPC, and 10Ag-10ZnO-NPC in-situ measured by DMA.

Fig. 2c shows the representative SEM micrographs of the NPs synthesized using the gasphase synthetic method. Here the large particle cluster (≈500 nm in diameter) was chosen in the SEM images for clarity in presentation (additional SEM images and HR-TEM images are shown


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in the Supporting Information). For ZnO-NPC (Figure 2c-1), the ZnO crystallites aggregated as a spherical nanoparticle cluster (NPC). After the addition of Ag precursor (10Ag-10ZnO-NPC; Figure 2c-2), AgNPs (i.e., with a relative strong electron density and a diameter of ≈ 22 nm) were homogenously deposited on the surface of the nanoparticle cluster to form a raspberry-like hybrid nanostructure. From the mobility size distribution (Figure 2d), the peak dp,m (dp,m*) was constant at (62-66) nm for ZnO-NPC, 1Ag-10ZnO-NPC and 10Ag-10ZnO-NPC. The results of DMA indicate that increasing CAg did not result in an increase of the particle size of the Ag-ZnO hybrid nanostructure. The morphology of the synthesized ZnO-based NPC was able to be controlled via the tuning of precursor concentration (i.e., from spherical to raspberry-like). The HR-TEM image analysis was employed for visualization of crystalline structure of AgZnO hybrid NPC. Clearly both the crystalline structures of Ag and ZnO were identified on the surface of 1Ag-10ZnO-NPC (Figure 3a-1) and 10Ag-10ZnO-NPC (Figure 3a-2). The EDS elemental mapping was employed to examine the spatial distribution of AgNP and ZnO NP in the hybrid nanostructure (i.e., at a molecular level). For the sample of 1Ag-10ZnO-NPC (Figure 3b-1), Ag was distributed with Zn with a partial segregation on the outer surface of hybrid nanostructure. For 10Ag-10ZnO-NPC (Fig. 3b-2), a raspberry-like structure was shown in the image, where the Ag was homogeneously dispersed on the outer surface of the hybrid nanostructure with a significantly higher Ag density than 1Ag-10ZnO-NPC.


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Figure 3. HR-TEM analyses with EDS elemental mapping of Ag-ZnO hybrid NPCs. (a) Representative HR-TEM images. 1: 1Ag-10ZnO-NPC. 2: 10Ag-10ZnO-NPC. The scale bars are 2 nm. (b) HR-TEM images with elemental mapping. 1: 1Ag-10ZnO-NPC. 2: 10Ag-10ZnO-NPC.

Based on the results of Figure 2-3, a cartoon depiction shown in Figure 4 demonstrates the proposed mechanism of the gas-phase synthesis of the Ag-ZnO hybrid NPC. Firstly, sub-micron aqueous droplets containing a mixture of Ag and Zn precursors are created via nebulization. The


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fast evaporation of the droplets progressively concentrates the precursor, which induces precipitation of solid precursor crystallites in the droplet. Due to the low colloidal stability, the solid Zn precursors aggregate during the droplet evaporation process, where Ag precursors are progressively precipitated as particles on the surface of Zn precursors. After gas-phase thermal treatment at T = 500 °C, the aerosols of dried precursor crystallites are thermally decomposed and sintered to become the raspberry-structure Ag-ZnO hybrid NPCs. As a summary, the experimental results confirm the formation mechanism of Ag-ZnO-NPC via the proposed evaporation-induced self-assembly in gas phase.29,30,37 To our knowledge, this is the first published study on the fabrication of ZnO NPC homogeneously decorated with noble metal nanoparticles to form a raspberry-structure hybrid nanoparticle cluster using the gas-phase selfassembly approach with a mechanistic understanding in the synthetic process.

Figure 4. The cartoon depiction of the gas-phase synthesis of the raspberry-structure Ag-ZnO hybrid NPC.


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UV-vis spectroscopy was employed to identify absorption wavelength (λ) of the synthesized ZnO-based NPCs. As shown in Figure 5, an absorbance band of ZnO with a peak at λ1≈ 370 nm was observed for all three ZnO-based NPC samples. For 1Ag-10ZnO-NPC, an additional shoulder-type absorbance at λ2 ≈ 520 nm was observed, which indicates the absorption by the SPR of AgNP distributed in the hybrid nanostructure. For 10Ag-10ZnO-NPC, the intensity of the SPR absorbance (i.e., peaked at λ2 ≈ 528 nm) further enhanced in comparison to 1Ag-10ZnONPC. The results indicate that the absorption of visible light irradiation is proportional to the amount of Ag in the hybrid nanostructure, implying that the spectral range of ZnO-NPC for photocatalysis could be effectively expanded through the hybridization with AgNP.

Figure 5. UV-vis absorption spectra of ZnO-NPC, 1Ag-10ZnO-NPC and 10Ag-10ZnO-NPC.


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Ligand interactions with Ag-ZnO hybrid nanostructure Zeta potential (ZP) measurement was employed to investigate the interactions between the ZnO-based NPCs and the ligands presenting in the media. Figure 6a shows the ZP profiles of ZnO-NPC versus pH under the interactions of RhB and SDS. Prior to the interactions, the isoelectric point (IEP) of ZnO-NPC was (8-9). The IEP decreased from (8-9) to < 2 after the interaction with SDS, which indicates that SDS was bound to the surface of ZnO-NPC (i.e., via electrostatic attraction under neutral pH environment38 and also hydrophobic interaction39). After the interaction with RhB, the ZP profiles of ZnO-NPC and SDS-ZnO-NPC are similar with an IEP of ≈3, close to the pKa of RhB. The results of Figure 6a indicate that both SDS and RhB were able to adsorb to ZnO-NPC, and the corresponding surface potentials of ZnO-NPC were mainly determined by the adsorption of RhB. Note that the ZP of ZnO-NPC was not increased with the acidity, and the unusual ZP profile was attributed to the surface dissolution of ZnO.40,41 For 1Ag-10ZnO-NPC (Figure 6b), the ZP profile changed significantly after interacting with SDS and RhB. For 10Ag-10ZnO-NPC (Figure 6c), the addition of SDS (i.e., SDS-10Ag-10ZnONPC) did not alter the ZP. In contrast, the IEP of 10Ag-10ZnO-NPC and SDS-10Ag-10ZnONPC changed from < 2 to ≈4 after interacting with RhB. As a summary, the results of Figure 6bc imply that (1) SDS was able to bind with Ag-ZnO-NPC to form a dodecyl sulfate corona via hydrophobic interaction39 and increase the negative ZP of Ag-ZnO-NPC, (2) RhB adsorbed to Ag-ZnO-NPC with or without the presence of dodecyl sulfate corona, which decreased the ZP of Ag-ZnO-NPC at neutral and acidic pH environments, and (3) hybridization with AgNP increased the negative ZP of ZnO-NPC at neutral and acidic pH conditions. Note that the ZP of SDS-bound 1Ag-10ZnO NPC and 10Ag-10ZnO-NPC only decreased by ≈10 mV after adjusting the pH from


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2 to 10, implying a reduction of packing density of dodecyl sulfates on the surface of Ag-ZnO NPCs by the increase of surface deprotonation.


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Figure 6. Zeta potential analyses of ZnO-based NPC samples over various pH and molecular interactions (SDS, RhB). (a) ZnO-NPC. (b) 1Ag-10ZnO-NPC. (c) 10Ag-10ZnO-NPC.

Photocatalysis of Ag-ZnO hybrid nanostructures Figure 7a shows ln(CRhB,t=0/CRhB,t) versus t under acidic environments (i.e., at pH 3) based on the results of the normalized UV-vis absorbance, CRhB,t/CRhB,t=0, versus t catalyzed by the samples of ZnO-NPC, 1Ag-10ZnO-NPC and 10Ag-10ZnO-NPC (additional data of UV-vis spectra and CRhB,t/CRhB,t=0 versus t were shown in the Supporting Information). A linear correlation of ln(CRhB,t=0/CRhB,t) versus t was observed, indicating that the degradation of RhB follows a first-order reaction. From the slope of Figure 7a, k was ≈ 7×10-4 min-1, ≈ 1.8×10-3 min-1 and ≈ 3.2 ×10-3 min-1 catalyzed by ZnO-NPC, 1Ag-10ZnO-NPC and 10Ag-10ZnO-NPC, respectively. On the contrary, the k was ≈ 6×10-4 min-1 for the control sample (RhB-only). The results illustrate that the Ag-ZnO hybrid NPCs promoted the photo-degradation of RhB in the aqueous solution. By calculation, the k* was (0±0.4)×1017 g-1min-1 for ZnO-NPC, (1.6±0.1)×1017 for 1Ag-10ZnO-NPC, and (9.6±0.1)×1017 g-1min-1 for 10Ag-10ZnO-NPC. The results of k* indicate that the photocatalytic activity of ZnO-NPC was proportional to the amount of AgNP in the hybrid NPC (nAg) under the acidic pH environment, and the hybridization of AgNP significantly increased the turnover frequency of ZnO-NPC. Note that photocatalytic activity (k and k*) was assumed to be independent of the size of AgNP over the size range we study (dc,Ag = 14-22 nm).


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Figure 7. Calculations of the reaction rate constants of the RhB photo-degradation (k) catalyzed by ZnO-NPC, 1Ag-10ZnO-NPC and 10Ag-10ZnO-NPC over three different pH conditions. (a) pH 3. (b) pH 6. (c) pH 10.


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At pH 6 (i.e., a relatively neutral pH environment), k was ≈ 6.5×10-4 min-1, (1.9±0.1)×10-3 min-1, and (3.7±0.1)×10-3 min-1 catalyzed by ZnO-NPC, 1Ag-10ZnO-NPC, and 10Ag-10ZnONPC, respectively, based on the slopes shown in Figure 7b. In comparison, the k was ≈ 7.0×10-4 min-1 for the control sample, which indicates that the Ag-ZnO hybrid nanostructures promoted the photo-degradation of RhB in neutral pH environments. By calculation, the k* was (0±0.1)×1017 g-1min-1 for ZnO-NPC, (1.8±0.2)×1017 g-1min-1 for 1Ag-10ZnO-NPC, and (11.5±0.7)×1017 g-1min-1 for 10Ag-10ZnO-NPC, indicating that the hybridization with AgNP improved the photocatalytic activity of the ZnO-NPC under a neutral pH environment. Note that the catalytic activity of the ZnO-NPC was shown to be negligible (i.e., close to the activity of the control sample) under both acidic and neutral pH environments. The lack of photocatalytic activity of the ZnO-NPC was attributed to its low colloidal stability, which induced fast precipitation of the ZnO-NPC prior to the photocatalysis and dramatically reduced the amount of ZnO-NPC under light irradiation for photocatalysis. In a basic pH environment (pH 10; see Figure 7c), k was (18.9±3.2) ×10-3 min-1, (10.3±0.4)×10-2 min-1 and (2.8±0.6)×10-3 min-1 catalyzed by ZnO-NPC, 1Ag-10ZnO-NPC, and 10Ag-10ZnO-NPC, respectively. In contrast, the k was ≈ 7×10-4 min-1 for the control sample. The results show that a surge of photocatalytic activity was observed for the ZnO-NPC and 1Ag10ZnO-NPC by tuning the acidity to basic pH conditions, and the enhancement could be mainly attributed to the increase of dispersibility of ZnO NPC in the solution. By calculation, the k* was (9.2±0.4)×1017 g-1min-1 for ZnO-NPC, ≈ 9.2×1017 g-1min-1 for 1Ag-10ZnO-NPC, and (8.9±1.2)×1017 g-1min-1 for 10Ag-10ZnO-NPC. The results show that the addition of Ag did not improve k* of ZnO-NPC at the basic pH environment, implying that the dispersibility of ZnO-


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NPC is the dominant factor to the catalytic activity. Note that the increase of the catalytic activity of ZnO-NPC and 1Ag-10ZnO-NPC could also be partially attributed to the formation of abundant hydroxyl ions reacting with the holes of ZnO to form hydroxyl radicals. As a result, the recombination of electron-hole pair can be reduced by the abundant hydroxyl ions in the basic pH environment.7,11,19,21 Table 2 shows a summary of k and k* for the ZnO-NPC, 1Ag-10ZnONPC, and 10Ag-10ZnO-NPC over different pH environments.

Table 2. A summary of k and k* for the ZnO-NPC, 1Ag-10ZnO-NPC, and 10Ag-10ZnO-NPC over different pH environments k (min-1 )(×10-3)

k* (g-1 min-1) (×1017)

pH3

pH6

pH10

pH3

pH6

pH10

ZnO-NPC

≈ 0.7

≈ 0.65

(18.9 ± 3.2)

(0 ± 0.4)

(0 ± 0.1)

(9.2 ± 0.4)

1Ag-10ZnONPC

≈ 1.8

(1.9 ± 0.1)

(10.3 ± 0.4)

(1.6 ± 0.1)

(1.8 ± 0.2)

(9.2 ± 0)

10Ag10ZnO-NPC

≈ 3.15

(3.7 ± 0.1)

(2.8 ± 0.6)

(9.6 ± 0.6)

(11.5 ± 0.7)

(8.9 ± 1.2)

Control

≈ 0.6

≈ 0.7

≈ 0.7

Figure 8a and 8b show the ln(CRhB,t=0/CRhB,t) versus t at pH 3 and pH 6, respectively (i.e., based on the results of CRhB,t/CRhB,t=0 versus t catalyzed by the samples of SDS-ZnO-NPC, SDS1Ag-10ZnO-NPC and SDS-10Ag-10ZnO-NPC; see Supporting Information). Linear correlations of ln(CRhB,t=0/CRhB,t) versus t were observed, indicating that the degradation of RhB followed a


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first-order reaction catalyzed by the SDS-bound ZnO-based NPCs. From the slope of Figure 8a (pH 3), k was calculated as (4.3±0.8)×10-3 min-1, (6.7±1.6)×10-3 min-1, and (9.9±2.3)×10-3 min-1 for ZnO-NPC, 1Ag-10ZnO-NPC, and 10Ag-10ZnO-NPC, respectively. At pH 6 (Figure 8b), k was calculated as ≈ 0.7×10-3 min-1, ≈ 3.0×10-3 min-1 and ≈ 6.4×10-3 min-1 catalyzed by ZnO-NPC, 1Ag-10ZnO-NPC, and 10Ag-10ZnO-NPC, respectively. In comparison, the k of the control sample was ≈ 1.6×10-3 min-1 at pH 3 and ≈ 3.0×10-4 min-1 at pH 6. The results indicate that the SDS-bound ZnO-based NPCs promoted the photo-degradation of RhB in acidic and neutral pH environments. Note that the reaction rate constants of the RhB photo-degradation catalyzed by the ZnO-based NPCs were shown to increase by the presence of dodecyl sulfate corona under the acidic and neutral environments (i.e., a maximum of 6-fold enhancement), and the enhancement was mainly attributed to the increase of colloidal stability of the ZnO-based NPCs. We then investigate the effect of the amount of AgNP on the turnover frequency of SDSbound ZnO-based NPCs. At pH 3, the k* was (3.4±0.7)×1017 g-1min-1 for SDS-ZnO-NPC, (6.4±0.5)×1017 for SDS-1Ag-10ZnO-NPC, and (20.7±3.0)×1017 g-1min-1 for SDS-10Ag-10ZnONPC. The k* under pH 6 was ≈ (1.0±0.5)×1017 g-1min-1 for SDS-ZnO-NPC, ≈ 4.7×1017 g-1min-1 for SDS-1Ag-10ZnO-NPC, and (20.7±0.1)×1017 g-1min-1 for SDS-10Ag-10ZnO-NPC. The results of k* indicate that the photocatalytic activity of SDS-bound ZnO-NPC significantly increased by the hybridization with AgNP under both acidic and neutral pH environments (6× and 20×, respectively). The increase of k* was proportional to nAg, which is correlated to the extent of electrostatic repulsion (i.e., as evidenced by the zeta potential analysis, Figure 6).


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Figure 8. Calculations of the reaction rate constants of the RhB photo-degradation catalyzed by SDS-ZnO-NPC, SDS-1Ag-10ZnO-NPC and SDS-10Ag-10ZnO-NPC over three different pH conditions. (a) pH 3. (b) pH 6. (c) pH 10.


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In the basic pH environment (Figure 8c), k was calculated as (13.9±2.3)×10-3 min-1, (18.1±6.2)×10-3 min-1 and (7.7±0.5)×10-3 min-1for SDS-ZnO-NPC, SDS-1Ag-10ZnO-NPC and SDS-10Ag-10ZnO-NPC, respectively, which were shown to be significantly higher than the control sample (k ≈ 9.0×10-4 min-1). The results also show that the photo-degradation rate significantly increased by the use of the ZnO-based NPC as catalyst (max. 20×; in the presence of SDS corona). The k* was calculated as ≈ (8.2±0.5)×1017 g-1min-1 for ZnO-NPC, (10.3±0.4)×1017 g-1min-1 for 1Ag-10ZnO-NPC, and (19.3±1.2)×1017 g-1min-1 for 10Ag-10ZnONPC. The results show that hybridization of AgNP increased k* of ZnO-NPC by up to 3× under the basic pH environment. Table 3 shows a summary of k and k* for the SDS-ZnO-NPC, SDS1Ag-10ZnO-NPC, and SDS-10Ag-10ZnO-NPC over three different pH environments.

Table 3. A summary of k and k* for the SDS-ZnO-NPC, SDS-1Ag-10ZnO-NPC, and SDS10Ag-10ZnO-NPC over different pH environments

k (min-1 )( ×10-3)

SDS-ZnONPC

k* (g-1 min-1) ( ×1017)

pH3

pH6

pH10

pH3

pH6

pH10

(4.3 ± 0.8)

≈ 0.7

(13.9 ± 2.3)

(3.4 ± 0.7)

(1.0 ± 0.5)

(8.2 ± 0.5)


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SDS-1Ag10ZnO-NPC

(6.7 ± 1.6)

≈3

(18.1 ± 6.2)

(6.4 ± 0.5)

(4.7 ± 0.01)

(10.3 ± 0.4)

SDS-10Ag10ZnO-NPC

(9.9 ± 2.3)

≈ 6.4

(7.7 ± 0.5)

(20.7 ± 3.0)

(20.7 ± 0.1)

(19.3 ± 1.2)

≈ 1.6

≈ 0.3

≈ 0.9

Control

Figure 9 shows a summary plot of k* versus the absolute value of ZP (ZPabs), an indicator to the electrostatic repulsion between the ZnO-based NPC during the photocatalysis (i.e., after the interaction with RhB). Three zones were clearly identified in Figure 9 for both the unbound and the SDS-bound ZnO-based NPCs: Zone 1 (ZPabs < 5 mV), Zone 2 (5 mV < ZPabs < 21 mV), and Zone 3 (ZPabs > 21 mV). A very low k* was observed in Zone 1, indicating an insufficient electrostatic repulsion to stabilize ZnO-based NPC for the photocatalysis. The k* was proportional to ZPabs in Zone 2 for both the unbound and SDS-bound ZnO-based NPCs, indicating that the photocatalytic performance ZnO-based NPC was dominated by the electrostatic repulsion between the individual ZnO-based NPCs in the solution. At Zone 3, k* reached a plateau value, ≈ 10*1017 g-1*min-1 for the unbound ZnO-based NPC and ≈ 20*1017 g1

*min-1 for the SDS-bound ZnO-based NPC (i.e., independent of ZPabs). As a summary, our

results illustrate that the electrostatic repulsive force between the ZnO-based NPCs in solution is the most critical factor to the photocatalytic activity especially when ZPabs 11 has shown to reduce photocatalytic activity due to the effect of surface dissolution of ZnO.24 On the other hand, Ag-ZnO-NPC exhibits a higher electrostatic repulsion than the ZnO-NPC especially under the interaction with RhB in the acidic and neutral pH environments (Fig. 10b). Besides, the presence of AgNP in the NPC is beneficial for the utilization of visible light irradiation. Therefore, the photocatalytic activity of Ag-ZnO-NPC will increase significantly from the prospect of enhancing the absorption of light irradiation during the photocatalysis. However, the adsorption of RhB neutralizes the surface charge of Ag-ZnO-NPC to gradually induce the particle aggregation and the subsequent loss in the ability of the light absorption. Note that surface dissolution of AgNP is found in the acidic environment (pH ≦3),27,28 which will


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gradually affect the photocatalytic activity of Ag-ZnO-NPC by reducing its ability of absorbing the light irradiation. Figure 10c and 10d demonstrate cartoon depictions of the photocatalysis of RhB degradation using ZnO-based NPCs with dodecyl sulfate corona as catalyst. In comparison to the ZnO-NPC (no SDS), the SDS-ZnO-NPC is more stable under the interaction with RhB in the acidic and neutral pH environments (Figure 10c-1). At the basic pH environment, the SDS-ZnO-NPC (Figure 10c-2) is also stably dispersed for the photocatalysis. For the Ag-ZnO-NPC (Figure 10d), the presence of SDS corona improves the colloidal stability over various pH environments, including the decrease of surface dissolution of AgNP in the acidic environment.38 Besides, the dodecyl sulfate corona can improve the adsorption of RhB without disrupting the colloidal stability of Ag-ZnO-NPC, a critical factor for the absorption of light irradiation. Hence the photocatalytic activity of Ag-ZnO-NPC will increase significantly by the dodecyl sulfate corona.


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Figure 10. Cartoon depictions of the photocatalysis of RhB degradation using ZnO-based NPCs as catalyst. (a) ZnO-NPC. 1: Acidic and neutral pH; 2: Basic pH. (b) Ag-ZnO-NPC. (c) SDSZnO-NPC. 1: Acidic and neutral pH; 2: Basic pH. (d) SDS-Ag-ZnO-NPC.


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CONCLUSIONS A facile aerosol-based self-assembly approach is demonstrated to successfully synthesize a

raspberry-structure Ag-ZnO hybrid nanoparticle cluster. The photocatalytic activity of the synthesized nanostructure is highly dependent of the ligand-nanoparticle interaction and the corresponding colloidal stability of NPs, which are determined by the acidity of the solution, the amount of AgNP in the hybrid nanoparticle cluster, and the presence of dodecyl sulfate corona on the surface of the hybrid nanostructure. The photo-degradation rate is able to significantly increase by the use of the Ag-ZnO hybrid nanoparticle cluster as catalyst (max. 20×). The dodecyl sulfate corona enhances the photocatalytic activity of the ZnO and Ag-ZnO hybrid nanoparticle cluster especially at the acidic and the neutral pH environments (maximum 6×). The hybridization of AgNP significantly increases the turnover frequency of ZnO-NPC (> 20×). The improved photocatalytic activity is mainly attributed to the enhanced colloidal stability, where the electrostatic repulsive force between the ZnO-based NPCs in solution is demonstrated as the most critical factor especially when the absolute zeta potential below 21 mV. The work demonstrates a prototype method to fabricate stable noble metal-ZnO hybrid nanoparticle clusters with a mechanistic understanding for the optimization of photocatalytic activity in aqueous solutions, which has shown the promise for a variety of aqueous-based photocatalytic technology (e.g., H2 evolution via water-splitting).


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Supporting Information.

Page 32 of 40

Additional SEM and HR-TEM images, UV-visible spectra and

data of photocatalysis of RhB degradation. This material is avail-able free of charge via the Internet at http://pubs.acs.org.

* AUTHOR INFORMATION Corresponding Author *

De-Hao Tsai

[email protected]

Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT The authors thank Ministry of Science and Technology (MOST) of Taiwan, R.O.C. (MOST 1052221-E-007-129-MY3, MOST 106-3113-E-007-002) and Office of Naval Research Global, U.S.A. (ONRG N62909-17-1-2040) for financial support. The authors also thank Profs. HsingWen Sung and Hsin-Lung Chen at NTHU, and Hsin-Chia Ho and Yen-Liang Lin at Industrial


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Technology Research Institute of Taiwan, R.O.C., for the experimental support and helpful advice.

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Table of Contents Graphic and Synopsis


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Aerosol-Based Self-Assembly of a AgâZnO Hybrid Nanoparticle

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