Photocatalysis with Pt-Au-ZnO and Au-ZnO Hybrids: Effect of Charge

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Photocatalysis with Pt-Au-ZnO and Au-ZnO Hybrids: Effect of Charge Accumulation and Discharge Properties of Metal Nanoparticles. Joseph F. S. Fernando, Matthew P. Shortell, Konstantin L. Firestein, Chao Zhang, Konstantin Larionov, Zakhar I. Popov, Pavel B. Sorokin, Laure Bourgeois, Eric R. Waclawik, and Dmitri Golberg Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b00401 • Publication Date (Web): 29 May 2018 Downloaded from http://pubs.acs.org on May 29, 2018

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Photocatalysis with Pt-Au-ZnO and Au-ZnO Hybrids: Effect of Charge Accumulation and Discharge Properties of Metal Nanoparticles

Joseph F. S. Fernando,†* Matthew P. Shortell,† Konstantin L. Firestein,† Chao Zhang,† Konstantin V. Larionov,‡ Zakhar I. Popov,‡ Pavel B. Sorokin,‡ Laure Bourgeois,

¶

Eric R.

Waclawik† and Dmitri V. Golberg†,#* †School of Chemistry, Physics and Mechanical Engineering, Science and Engineering Faculty, Queensland University of Technology (QUT), Brisbane, Queensland, 4000, Australia ‡

Inorganic Nanomaterials Laboratory, National University of Science and Technology MISIS,

Leninsky prospect 4, Moscow, 119049, Russian Federation ¶

Monash Centre for Electron Microscopy, Department of Materials Science and Engineering,

Monash University, Victoria 3800, Australia #

International Center for Materials Nanoarchitectonics (MANA), National Institute for Materials

Science (NIMS), Namiki 1-1, Tsukuba, Ibaraki 3050044, Japan

ACS Paragon Plus Environment

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Abstract Metal-semiconductor hybrid nanomaterials are becoming increasingly popular for photocatalytic degradation of organic pollutants. Herein, a seed-assisted photodeposition approach is put forward for the site-specific growth of Pt on Au-ZnO particles (Pt-Au-ZnO). A similar approach was also utilized to enlarge the Au nanoparticles at epitaxial Au-ZnO particles (Au@Au-ZnO). An epitaxial connection at the Au-ZnO interface was found to be critical for the site-specific deposition of Pt or Au. Light on-off photocatalysis tests, utilizing a thiazine dye (toluidine blue) as a model organic compound, were conducted and confirmed the superior photodegradation and mineralization properties of Pt-Au-ZnO hybrids compared to Au-ZnO. In contrast, Au-ZnO type hybrids were more effective toward photoreduction of toluidine blue to leuco-toluidine blue. It was deemed that photoexcited electrons of Au-ZnO (Au, ~ 5 nm) possessed high reducing power owing to electron accumulation and negative shift in Fermi level/redox potential; however, exciton recombination due to possible Fermi level equilibration slowed down the complete degradation of toluidine blue. In the case of Au@Au-ZnO (Au, ~15 nm), the photodegradation efficiency was enhanced, and the photoreduction rate reduced compared to Au-ZnO. Pt-Au-ZnO hybrids showed better photodegradation and mineralization properties compared to both Au-ZnO and Au@Au-ZnO owing to a fast electron discharge. However, photoexcited electrons lacked the reducing power for the photoreduction of toluidine blue. The ultimate photodegradation efficiency of Pt-Au-ZnO, Au@Au-ZnO and Au-ZnO were 84%, 66% and 39%, respectively. In the interest of effective metal-semiconductor type photocatalysts, the present study points out the importance of choosing the right metal, depending on whether a photoreduction and/or photodegradation process is desired.

Keywords: photocatalysis, Fermi-level equilibration, Au-ZnO, Pt-Au-ZnO, hybrid nanoparticles, photodeposition


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Introduction Metal-semiconductor hybrid nanoparticles are an interesting class of materials having great potential as the active elements in a variety of optical and optoelectronic applications, such as photovoltaics, photocatalysts, photodetectors and biomedical sensing.1-4 For example, composites of n-type metal-oxide semiconductors with noble metal nanoparticles have exhibited improved photocatalytic properties compared to bare semiconductor particles.5,

6

When metal

nanoparticles, such as Au, Ag or Cu, are loaded onto TiO2 or ZnO semiconductors, and subjected to UV light illumination, a transfer of photoexcited electrons to metallic constituents and subsequent equilibration of the Fermi levels of the two components take place.7,

8

The stored

electrons within metals can participate in redox reactions when acceptor species are present in a solution.5, 9 However, under continuous illumination, Fermi-level equilibration may subsequently lead to exciton recombination and retardation of photocatalysis.7 On the contrary, Pt nanoparticles sustain an Ohmic contact with the solvent, and therefore, promote fast electron discharge to the electrolyte.7 As a consequence, Pt-TiO2/ZnO type hybrids should continue to minimize exciton recombination due to the absence of Fermi-level equilibration, although the reduction power of photoexcited electrons may be comparatively low. The effect of these peculiar properties of different metal nanoparticles toward the photocatalytic degradation and/or photoreduction processes involving organic dye molecules needs to be more deeply understood. Effective photocatalysts have been utilized to enhance hydrogen/oxygen evolution redox reactions,10,

11

mineralization of organic pollutants,12,

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and organic synthesis reactions.14

Photoelectrochemical studies, have demonstrated the enhanced photocurrent and photovoltage properties in metal (Au, Pt, Ir,)-TiO2 composite films.15 Organic redox dyes, such as thionine8 and methyl-viologen,7 have been used for titration and counting of photoexcited electrons in semiconductor systems in deaerated solutions. Apart from the type of metal, the size of the metal nanoparticles has also a considerable effect on the magnitude of Fermi level shift in illuminated metal-semiconductor composites. For instance, Kamat et al. determined an apparent Fermi level shift in TiO2/Au composites; this increased from 20 mV for the 8 nm nanoparticles to 60 mV for 3 nm Au nanoparticles under the Nernstian equilibrium with ( / ) redox couple.16 It is also

interesting to note that the capacitance of Au monolayer protected clusters increased from 0.31 to 3.41 aF as the core size increased from Au38 to Au4033.17


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There are several ways to synthesize metal-semiconductor hybrid nanomaterials. Metal nanoparticle-seeded (Au, Pt, Ag etc.) heterogeneous nucleation and growth is one popular approach to prepare two and/or three component hybrid structures, such as Au-ZnO, Pt-ZnO, Ag-ZnO and Au-Ni-ZnO.18-21 Metal-seeded growth consistently results in an epitaxial junction at the metal-semiconductor interface, which has proven to be important for an efficient charge transfer.22 Photodeposition23-25 also provides a convenient way to deposit single or multiple metal islands at semiconductor nanocrystals. However, the metal-semiconductor interface is typically non-epitaxial.24 A combination of the above methods might give the possibility to prepare three component hybrid structures with multiple functionalities (optical, optoelectronic and/or magnetic),21,

26

while still retaining the epitaxial interface between the metallic and

semiconductor components. One of the most popular ways to gauge the photocatalytic activity of metal-semiconductor particles is the monitoring of the degradation/color loss kinetics of organic dye molecules, such as methylene blue,27 thionine,28 methyl orange29 and so on. The color loss in such cases could be due to a simple redox conversion and/or complete degradation and mineralization of the dye. A study by Pawinrat et al. found that Au-ZnO hybrids were more efficient at degradation of methylene blue compared to Pt-ZnO.30 Similarly, a separate study also found that ZnO-Au was more effective at photodegradation of methyl orange.31 In contrast, a recent study has reported that Pt-TiO2 thin films have better methylene blue photodegradation properties over Au-TiO2 under UV illumination.32 Moreover, Pt-based TiO2 photocatalysts have shown superior hydrogen evolution rate compared to Au-TiO2, which has been attributed to better electron capture and charge separation properties of Pt.11, 33 While the particle type, their sizes, morphologies and the ratio between metal/semiconductor components may have influenced the differences in observations in the above mentioned studies, clearly, there must be an effect reflecting the different charge accumulation/discharge properties of Pt and Au. For instance, photodegradation of an organic dye requires a large number of free charge carriers, and for a photoreduction the excited electrons must reach the required redox potential. Therefore, if one considers the particular case of decolorization of an organic dye molecule (such as methylene blue), the origin of color loss could be due to photoreduction, photodegradation or even both, depending on the charge accumulation/discharge properties of metals. A clear perception of the correct mechanism


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is required to customize metal-semiconductor hybrid nanoparticles for appropriate photocatalytic applications. Herein, we employed Pt/Au-ZnO hybrids as a model photocatalytic system to compare and study on how different charge accumulation/discharge properties of metal nanoparticles govern the photoreduction and/or photodegradation/photooxidation pathways of organic dyes. Pt-AuZnO three component hybrid structures with peculiar morphologies have been synthesized via a combination of seeded growth and photodeposition, where Pt nanoclusters were deposited onto Au-seeded ZnO structures. The same technique was extended to enlarge the Au particles at AuZnO. Toluidine blue was used as a model compound to perform photocatalytic experiments. Light on-off photocatalysis tests confirmed the superior photodegradation properties of Pt-AuZnO hybrid structures. On the other hand Au-ZnO type catalysts displayed rapid photoreduction properties. The effect of charge accumulation/discharge properties of Pt and Au on the photoreduction and photodegradation rates are then discussed.

Experimental Materials Zinc acetate dihydrate (Zn(CH3COO)2.2H2O), oleylamine (CH3(CH2)7CH=CH(CH2)7CH2NH2), dodecanol (CH3(CH2)11OH), gold (III) chloride (HAuCl4), platinum (II) chloride (H2PtCl4) and (2,2,6,6-Tetramethylpiperidin-1-yl)oxyl (TEMPO) were purchased from “Sigma Aldrich”. Absolute ethanol (CH3CH2OH) was purchased from “Ajax Finechem”. Toluidine blue O (C15H16ClN3S) was purchased from “Proscitech”, Australia. All chemicals were used without further purification.

Synthesis of Pt-Au-ZnO hybrid nanoparticles Two distinct methods were employed to synthesize three component Pt-Au-ZnO hybrid nanoparticles: 1) Pt deposition at Au-seeded pre-synthesized Au-ZnO hybrid nanoparticles (AuZnO); and 2) Pt deposition at ZnO nanoparticles using a multi-step photodeposition technique (i.e. Au photodeposition at ZnO (p-Au-ZnO) followed by photodeposition of Pt nanoparticles). The Au-ZnO hybrid nanoparticles were synthesized using a Au seed-assisted heterogeneous nucleation process, as previously described.34 The p-Au-ZnO hybrid nanoparticles were synthesized using the photodeposition method reported in our previous work.24 Method 1 was as follows: A colloid solution of Au-ZnO hybrid nanoparticles (0.1 mg/mL) was prepared in


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absolute ethanol. Concurrently, a solution of H2PtCl4 (0.5 mM) was made in absolute ethanol. Then 125 µL of H2PtCl4 solution was added to 1.25 mL of the Au-ZnO colloid in a disposable cuvette. The reactant mixture was illuminated using a UV diode (365 ± 10 nm) for 8 min. The power and intensity incident on the cuvette were approximately 64 mW and 35 mW/cm2, respectively. A custom-built setup attached to an Ocean Optics HR4000 spectrometer was used to collect in situ ultraviolet-visible (UV-Vis) spectra, which were used to monitor the Pt deposition process. As synthesized product was separated by centrifugation (12,000 RPM, 4 min) and washed several times with ethanol before further use. Samples prepared by method 1 were tagged as Pt-Au-ZnO. In Method 2, Au-ZnO was replaced with p-Au-ZnO. All other steps for Pt deposition were identical to Method 1. Samples prepared by Method 2 were tagged as Ptp-Au-ZnO.

Synthesis of Au@Au-ZnO hybrid nanoparticles For comparative photocatalytic studies, Au-ZnO hybrid nanoparticles with enlarged Au nanoparticles were also synthesized (Au@Au-ZnO). A photodeposition approach was utilized to enlarge the Au nanoparticles attached to Au-ZnO hybrid nanoparticles. Initially, a 0.5 mM ethanolic solution of Au precursor salt (HAuCl4) was activated by illumination with UV light for 8 min.24 Then 125 µL of activated precursor salt was added to 1.25 mL of ethanolic Au-ZnO colloid (0.1 mg/mL) in a disposable cuvette. The reactant mixture was illuminated using a UV diode (365 ± 10 nm) for 20 s (4 x 5 s UV pulses with a 30 s gap between the pulses). As synthesized product was separated by centrifugation and washed several times in ethanol before further use.

Nanoparticle characterization Nanomaterials were characterized using X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), high-angle annular dark field scanning TEM imaging (HAADFSTEM) and energy dispersive X-ray spectroscopy (EDX) mapping. STEM images and EDX maps of Pt-Au-ZnO nanomaterials were acquired using a JEOL 2100F microscope equipped with JEOL STEM bright field (BF) and HAADF detectors and a JEOL 50 mm2 Si(Li) EDX detector. DF-STEM images and EDX maps of all other nanomaterials were acquired using a JEOL 2100 microscope equipped with JEOL STEM detectors (BF/ADF) and Oxford X-Max 80 mm2 silicon drift EDX detector (SDD). High resolution TEM images of all nanomaterials were


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collected using a JEOL 2100 microscope operating at 200 kV. TEM samples were prepared by adding a drop of diluted colloid solution (in ethanol) onto formvar/carbon coated 200 mesh Cu grids. XPS measurements were performed using a Kratos AXIS Supra photoelectron spectrometer. XPS spectra were analyzed using CasaXPS software and were calibrated to C 1s (284.8 eV).

Photocatalytic measurements Photocatalytic properties of hybrid nanoparticles were measured by monitoring the degradation of an organic dye molecule, toluidine blue (TB, λmax = 625 nm). In a typical experiment, 1.25 mL of the catalyst (0.1 mg/mL in ethanol – concentrations of all catalysts were verified by collecting UV-Visible spectra – Figure S2) was mixed with 50 µL of TB (0.5 mM) in a 1 cm UV cuvette. The mixture was allowed to equilibrate for 30 min in the dark. Then the mixture was illuminated with an UV diode (365 ± 10 nm) for 20 min. The power and intensity incident on the cuvette were approximately 48 mW and 37 mW/cm2. It should be noted here that ethanol acts as a hole scavenger during the photocatalytic reaction. The decoloration of the dye was monitored using in situ UV-Visible spectrometry (Ocean Optics HR4000CG-UV-NIR). At least 75 data points were collected per min to improve the accuracy of results. Light on-off catalytic tests were conducted by first illuminating the samples for 6 min, then followed by a light-off time of 5 min. Two such cycles were performed per sample. After the second cycle the sample was illuminated for 8 min before turning off the light. An electron spin spectroscopy (ESR) study was performed to further confirm the electron discharge properties of as synthesized hybrid nanoparticles. The experiments were conducted under the same conditions as above, except 50 µL of ethanolic TEMPO solution (3 mM) was added in place of TB. ESR spectra were collected using MT MiniScope MS 400 spectrometer. Spectra were recorded under the following conditions: room temperature, modulation amplitude 0.2 mT, and microwave power 20 mW. ESR spectra were recorded before and after 20 minutes of UV illumination of the reaction solution. All photocatalytic measurements were conducted in ambient air.


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Results and discussion Synthesis and characterization of Pt-Au-ZnO particles In this study, we utilized the metal nanoparticle seeded method and photodeposition method to prepare both two and three component hybrid nanostructures. Table 1 is a summary of notations used to distinguish each hybrid system during the following discussion. Table 1. Summary of notations used to identify each hybrid system and their synthetic routines. Sample ID

Starting materials

Synthetic approach

Experimental conditions

Au-ZnO

Au seeds, Zn(AC)2.2H2O

Au - seeded growth of ZnO

175 °C, 2h

Au@Au-ZnO

Au-ZnO, HAuCl4

Photodeposition of Au at Au-ZnO

24 °C, 365 nm UV (35 mW/cm2)

Pt-Au-ZnO

Au-ZnO, H2PtCl4

Photodeposition of Pt at Au-ZnO

‘as above’

Pt-p-Au-ZnO

ZnO, HAuCl4, H2PtCl4

Multi-step photodeposition at ZnO

‘as above’

The Au-ZnO hybrid nanoparticles were first prepared using a Au seed-assisted heterogeneous nucleation method.34 While the Au nanoparticle size can be varied to further tune the size and number of ZnO petals attached per Au nanoparticle, we employed ~ 4 nm Au particles to prepare Au-ZnO hybrids with a 1:1 particle ratio between Au and ZnO nanoparticles. Figure S1a shows a low magnification TEM image of Au-ZnO hybrids. ZnO nanoparticles have a hexagonal nanopyramidal shape where the Au nanoparticle is attached to the center of the hexagonal basal facet. Figure S1b displays a high resolution TEM image of the Au-ZnO basal surface. The observed lattice spacing of 0.28 nm is attributed to the 101 0 planes of ZnO. The Fast Fourier Transform (FFT) pattern obtained from the marked region in Figure S1b revealed that the ZnO pyramid has the (0001) basal facet exposed, hence the growth direction of ZnO pyramid was along [0001] axis, perpendicular to the basal facet. A heterojunction in a hybrid nanoparticle is strategically formed in a way to minimize the overall energy of the system.35 In the Au-ZnO hybrid system the smallest lattice mismatch is between the (101 1)ZnO and (111)Au planes, which is ~5%. The


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high resolution TEM image in Figure S1d confirms that (111) planes of Au are in fact parallel to 101 1 planes of ZnO, and therefore, we could justify that the growth of ZnO at Au is epitaxial in this case. A photodeposition approach was then employed to deposit Pt nanoparticles onto the AuZnO hybrid particles. Illumination of Au-ZnO hybrid with UV light (~365 nm) results in the excitation of ZnO valence band electrons to the conduction band, and the electrons are rapidly transferred to the Au islands (as Au could act as an electron sink).31 It is expected that photoexcited electrons in Au possess sufficient reducing power to transform [PtCl4]- ions to Pt metal (E0 ([PtCl4]-/Pt)ethanol vs NHE = + 0.49 V). In situ UV-Visible spectrometry can be used to monitor the formation of Pt nanoparticles. Figure 1 shows the evolution of UV-Vis extinction spectra during the photodeposition of Pt at Au-ZnO. The extinction peak at ~530 nm, before commencing UV illumination, is due to the surface plasmon resonance of Au nanoparticles. It can be seen that the Au plasmon peak broadens, and the whole spectrum shifts upwards during illumination. The peak broadening is an indication of Pt formation, because Pt absorption spectra are typically featureless and are yellow-brown in color due to d-d transitions.7 The color of the colloid solution indeed changed from a pale pink to a yellowish brown color after 8 min of UV illumination. The positive shift in extinction is partly attributed to the light scattering by newly forming Pt nanoparticles. The loss of extinction peak intensity at ~370 nm (ZnO bandgap absorption) could be ascribed to slight etching of ZnO nanoparticle surface by weakly acidic H2PtCl4 used for photodeposition.

Figure 1. Evolution of UV-Visible extinction spectrum during photodeposition of Pt at Au-ZnO hybrid nanoparticles.


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The structure and morphology of hybrid nanoparticles were studied using TEM imaging. Comparison of TEM images in Figure 2a and 2b reveals that Pt nanoparticles indeed deposited at Au-ZnO hybrid particles (oxidation state of Pt is later confirmed by XPS). Magnified TEM image in Figure 2c clearly shows that Pt nanoparticles formed cluster-like agglomerates, predominantly around the Au nanoparticle attached to the ZnO basal surface. This is an indication that photoexcited electrons were accumulated at the Au nanoparticles and were available to reduce the Pt salt, followed by nucleation and autocatalytic growth. It is interesting to note that Pt is deposited as small nanoparticles instead of growing a shell around Au to form a ‘core-shell’ type structure. The reason behind this type of growth can be explained by considering the crystal structures of Au and Pt. Typically, the counterparts of epitaxially grown core/shell heterostructures would have similar crystal structures and lattice parameters within 13%.36 When there is a large lattice mismatch (>3%), the second material would exhibit anisotropic growth on a seed particle to produce multicomponent heterojunctions.37 Figures 2d and 2e present high resolution TEM images acquired from a Pt nanoparticle cluster attached to the base of a Au-ZnO hybrid particle. We clearly identified lattice fringes with the spacings of 0.22 nm and 0.19 nm, which can be attributed to the (111) and (200) planes of Pt, respectively. The lattice mismatch between (111) planes of Au and Pt is ~5.3 %, whereas between the (200) planes it is ~5.6 %. From these results it is clear that, although Au and Pt nanoparticles have the same crystal structure (face centered cubic), the smallest lattice mismatch between the two structures is ~5%. Therefore, we can expect heterogeneous nucleation and anisotropic growth of Pt nanoparticles at the Au seeds. After initial nucleation and growth, Pt nanoparticles themselves could act as seeds for further deposition of Pt, leading to cluster-like structures. The Pt nanoparticle cluster tends to lie along the (0001) basal facet of a ZnO particle (cluster size ~18 nm in [100] and ~ 8 nm in [001] directions), possibly because the basal facet is acting as an additional support for the structure. The diameter of an individual Pt nanoparticle in the cluster is ~3 nm. It should be highlighted here that a cluster made of small nanoparticles, as in this case, would have a larger surface area compared to a single nanoparticle with similar atomic weight, potentially increasing the number of active sites for photocatalysis. The inset of Figure 2f depicts the FFT pattern generated from the boxed region in Figure 2e. Analysis of selected spots in the FFT pattern (Figure 2f) identified reflections corresponding to Au(111), Au(200) and Pt(111), further


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confirming the presence of both Au and Pt in the metal nanoparticle cluster attached to ZnO base.

Figure 2. (a) TEM image of Au-ZnO hybrid nanoparticles. (b) TEM image of Pt-Au-ZnO hybrid nanoparticles. (c) Magnified TEM image of Pt-Au-ZnO hybrids. (d-e) High resolution TEM images of a Pt nanoparticle cluster attached to the Au nanoparticle at ZnO basal surface. f) FFT pattern obtain from the marked region in panel e (inset represents the actual FFT pattern, while the main panel displays selected reflections after masking the rest). High-angle annular dark-field (HAADF) STEM imaging combined with EDX mapping was employed to identify each chemical component in Pt-Au-ZnO hybrid nanoparticles. Figure


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3a displays the HAADF-STEM image acquired from a Pt-Au-ZnO sample. In principle, ZnO, Au and Pt should have distinct contrast in the dark field image owing to their different atomic numbers. As expected, triangular shape ZnO exhibits the lowest brightness (compare Figure 3a with Figures 3b and 3c). In this analysis, a notable difference in contrast for Pt/Au could not be identified, possibly because the difference in their atomic numbers is only 1. However, the presence of both Pt and Au was confirmed by the EDX maps in Figures 3d and 3e. Figure 3f shows the overlaid EDX map of Pt-Au-ZnO hybrid nanoparticles.

Figure 3. (a) HAADF-STEM image of Pt-Au-ZnO hybrid nanoparticles, (b-e) EDX maps for individual elements in the hybrid system; and (f) corresponding EDX map overlay.


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An XPS study was then performed to understand the chemical state of metal nanoparticles attached to ZnO particles. Figure 4a shows the high resolution XPS spectrum of Au-ZnO particles obtained within the binding energy region of 72 eV to 97 eV. Deconvolution of the Au 4f doublet identified two pairs of binding energy peaks. The two obvious peaks at 84.0 eV and 87.7 eV are attributed to the Au (0) 4f7/2 and Au(0) 4f5/2 electronic states, respectively. These values corroborate that Au nanoparticles in the Au-ZnO hybrids are predominantly metallic.38, 39 While the binding energy of Au (0) 4f7/2 electronic state is comparable to that of bulk metallic Au, it should be noted here that the ultimate binding energy value is governed by positive and negative shifts imposed by a combination of factors including nanoparticle size, structural arrangement, and nature and type of interaction with the metal oxide support.40 The two other peaks centered at 86.1 eV and 89.7 eV are attributed to the presence of surface Au(III) ions. Another notable feature alongside Au 4f region is the overlapped Zn 3p doublet. The two peaks at 89.1 eV and 92.0 eV are attributed to Zn(II) 3p3/2 and Zn(II) 3p1/2, respectively.40 Moreover, high resolution XPS spectrum obtained from the Zn 2p region (Figure S3) shows Zn(II) 2p3/2 and Zn(II) 2p1/2 doublet with an energy separation of 23.0 eV which is typical for ZnO. Figure 4b depicts the high resolution XPS spectrum acquired after Pt deposition at Au-ZnO particles. A careful comparison of the two spectra in Figure 4a and 4b revealed that there were no significant changes to the peak positions of Au 4f and Zn 3p doublets. The XPS signal from Au(0) electronic state was relatively lower compared to the signal from surface Au(III) ions, possibly due to the fact that Au nanoparticles are now enveloped by Pt nanoparticles. As expected, the Pt 4f doublet is apparent in the XPS spectrum shown in Figure 4b. The deconvolution of the peaks revealed two pairs of Pt 4f doublets, in which the dominant peaks at 71.2 eV and 74.5 eV are attributed to Pt (0) 4f7/2 and Pt (0) 4f5/2 electronic states, respectively, confirming the presence of metallic Pt.41 The two tenuous peaks at 72.4 eV and 75.7 eV are attributed to the surface bound Pt(II) ions. XPS data analysis confirms that metallic Pt nanoparticles are deposited at Au-ZnO hybrid nanoparticles. Findings from XPS analysis are in good agreement with TEM, STEM and EDX results, and reaffirm the effective synthesis of three-component Pt-Au-ZnO hybrid nanostructures.


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Figure 4. High resolution XPS spectra of hybrid nanoparticles. (a) Au-ZnO; binding energy region 72 eV to 97 eVs and (b) Pt-Au-ZnO; binding energy region 68 eV to 98 eV. The epitaxial relationship between Au and ZnO in Au-ZnO hybrid particles was particularly important to prepare Pt-Au-ZnO three component hetero-nanostructures. One might argue that multi-step photodeposition of metal nanoparticles is a more feasible route to synthesize Pt-Au-ZnO hybrids; this would involve photodeposition of Au at ZnO (p-Au-ZnO) followed by photodeposition of Pt. However, p-Au-ZnO hybrids prepared by photodeposition typically have a non-epitaxial relationship between Au and ZnO components,24 and the rate of electron transfer at the non-epitaxial interface is expected to be relatively slow.21, 22 Figure 5a displays a TEM image of a Pt-p-Au-ZnO hybrid product obtained after photodeposition of Pt at p-Au-ZnO hybrid particles. It is apparent that Pt nanoparticles are not only deposited at Au islands, but also widely distributed over ZnO surfaces. The deposition pattern itself evidences that the electron transfer at the p-Au-ZnO non-epitaxial interface is inefficient. Then, a theoretical estimate was performed to better understand the difference in photodeposition pattern


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of Pt nanoparticles at the different Au/ZnO types (see Supporting Information for calculation method). Due to the large sizes of Au particles (with respect to the atomic scale) observed in the experiment, Au nanoparticles were represented as a flat slab (in a periodic supercell 8×8 Å2) with the (111) surface. The epitaxially grown particles (Au-ZnO) were simulated as chemically connected Au(111) and

ZnO(011) slabs, whereas in the case of non-epitaxially deposited Au

particle, a freestanding Au(111) slab was considered. The back ZnO surface was terminated by hydrogen atoms in order to exclude dangling bonds. The inhomogeneous character of charge distribution in Au, Au-ZnO and ZnO with adsorbed Pt particle is presented in Figure 5b. The work function difference between Au and ZnO42 leads to charge redistribution in Au (inset in Figure 5b) and p-doping of Au slab in the case of O-terminated ZnO slab in the interface area and n-doping in the case of Zn-termination. The dipole moment arising from such charge transfer can be responsible for the attraction of negatively charged [PtCl4]- ions to Au surface. Furthermore, DFT calculations show that binding energy between [PtCl4]- and Au-ZnO is stronger than on pure Au by 1.04 eV per molecule, which results in the selective deposition of Pt at the Au islands of ZnO. The Bader analysis43 shows that [PtCl4]- become more negatively charged on Au-ZnO surface in comparison with pure Au on 0.05e. In the case of non-epitaxially deposited Au, such dipole effect cannot be expected, and this leads to the uniform arrangement of Pt nanoparticles both on Au and ZnO.


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Figure 5. (a) TEM image of Pt-p-Au-ZnO hybrid nanoparticles. (b) Charge difference between Pt-Au, Pt-Au-ZnO and Pt-ZnO and corresponding constituent parts (Pt and Au, Pt and Au-ZnO, and Pt and ZnO).

Synthesis and characterization of Au@Au-ZnO particles To perform a fair comparison of photocatalytic performances, Au-ZnO hybrids with enlarged Au nanoparticles were also prepared (Au@Au-ZnO, see Table 1). The idea was to keep equivalent noble metal content in both Pt-Au-ZnO and Au@Au-ZnO samples. In our previous work34 we demonstrated that a Au-seeded method cannot be used to prepare 1:1 Au:ZnO hybrids with larger Au nanoparticles (>7 nm), because large seed particles may provide multiple, stable ZnO-nucleation sites, and thus generate heterotrimers, tetramers, and oligomers. Therefore, a simple photodeposition approach was employed here to enlarge the Au nanoparticle in Au-ZnO hybrids. Figure 6a shows the evolution of an UV-Visible extinction spectrum during Au photodeposition. There is an apparent increase in the extinction at 530 nm, indicating the deposition of Au nanoparticles at Au-ZnO. Typically, the enlarged Au particles were ~15 nm in diameter, so the diameter of Au islands was at least increased by ~10 nm (Figure 6b). The size of the Au nanoparticles may be controlled using different concentrations of Au precursor salt. The elemental composition (Zn, O, and Au) was confirmed by the overlaid EDX map shown in


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Figure 6d (obtained from the region in Figure 6c). Figure 6e presents a high resolution TEM image of the enlarged Au particle attached to a ZnO basal facet. Unlike Pt deposition at Au-ZnO hybrid particles, Au has clearly deposited as a uniform layer on the pre-existing Au nanoparticle as the lattice mismatch is 0%. A clear boundary between pre-existing Au and photodeposited Au was not identified.

Figure 6. (a) Evolution of UV-Visible extinction spectrum during photodeposition of Au at AuZnO hybrid nanoparticles. (b) Magnified TEM image of Au@Au-ZnO hybrid nanoparticles. (c) and (d) DF-STEM image and the corresponding overlaid EDX map of Au@Au-ZnO particles. (e) High resolution TEM image of the enlarged Au nanoparticle attached to ZnO.

Photocatalysis experiments As noted earlier, improved photocatalytic properties have been reported for metal nanoparticlemodified ZnO structures, such as Au-ZnO,20, 22 Pt-ZnO,19, 31 Au-Ni-ZnO21 and so on. Efficiency enhancement is largely attributed to the lowering of exciton recombination in ZnO particles by


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transferring photoexcited electrons to the metal nanoparticles.33 In the present study, we evaluated the photocatalytic properties of Au-ZnO, Au@Au-ZnO and Pt-Au-ZnO hybrid nanoparticles and demonstrated how the type of metal nanoparticle influence the photo-reduction and photo-oxidation/degradation of toluidine blue dye molecule. It is quite common to utilize colored organic dye molecules, such as methylene blue27, thionine28 and methyl orange29 as model compounds to study photocatalysis. Typically, photocatalytic efficiency is measured by monitoring the kinetics of decoloration using UV-Visible spectrometry. However, one should keep in mind that color-loss does not necessarily mean that the dye is fully degraded or mineralized. For instance, thiazine type dyes, such as Methylene blue, can undergo rapid photoreduction to a leuco form (leuco-methylene blue - which is colorless) upon illumination in combination with a semiconductor, such as TiO2 or ZnO.44 The leuco-form is O2 sensitive and can be re-oxidized back to original form upon termination of UV light. Hence, the reduction and oxidation reactions may compete with each other depending on the intensity of light and dissolved oxygen content. However, prolonged illumination of the dye solution above the threshold light intensity (in the presence of a suitable photocatalyst) would generate reactive radical species that completely mineralize the dye.45 We exploited the above mentioned features of a thiazine type dye (Toluidine blue (TB)) to comprehend the charge distribution and, in turn, photocatalytic properties of Au-ZnO, Au@Au-ZnO and Pt-Au-ZnO hybrid nanoparticles. Figure 7a depicts the change in absorption at 625 nm over time, when an ethanolic mixture of TB and photocatalyst was UV illuminated (365 ± 10 nm, diode) for up to 20 min. The irradiance at the cuvette was ~37 mW /cm2. It can be clearly discerned that decoloration of TB was notably more rapid in Au-ZnO and Au@Au-ZnO catalysts compared to Pt-Au-ZnO catalyst during the first few seconds. Figure 7b shows the first 10 s of the time-resolved UV-Visible spectra in Figure 7a. The initial rate of change in absorption (see Supporting Information) was calculated to be 0.052, 0.034 and 0.0007 cm-1/s for Au-ZnO, Au@Au-ZnO and Pt-Au-ZnO, respectively. Despite the remarkably lower photoreduction rate of the Pt hybrids, after 20 min of UV illumination, the conversion efficiency of Pt-Au-ZnO was distinctly greater compared to ZnO hybrids with Au (see Figure 7a). The photodegradation efficiency of Pt-Au-ZnO, Au@AuZnO and Au-ZnO were 84%, 66% and 39%, respectively. Moreover, after the initial rapid decrease of TB λmax in Au-ZnO and Au@Au-ZnO there is a slow increase in λmax indicating the simultaneous reduction and re-oxidation of TB. When Au-ZnO was used as the catalyst, the blue


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color of TB reappeared upon turning off the light; this is reflected as an increase in absorbance at 625 nm (Figure 7a). This observation further confirms that the dye was in the reversible leucotoluidine blue (TB2-) state and only a small fraction of the dye was degraded and mineralized. To elucidate the possible mechanism for aforementioned observations, a test light on-off photocatalysis experiment was conducted.

Figure

7.

(a)

Time-resolved

UV-Visible

spectra

collected

during

photocatalytic

reduction/degradation of TB for Au-ZnO, Au@Au-ZnO and Pt-Au-ZnO. (b) First 10 seconds of the time-resolved UV-Visible spectra in Figure 7a. (c) Time-resolved UV-Visible spectra for light on-off photocatalytic reduction/degradation of TB. (TB, λmax = 625 nm) and (d) UV-Visible extinction spectra collected at discrete times when Pt-Au-ZnO was used as the catalyst. Figure 7c shows the time-resolved UV-Visible spectra (at 625 nm) for light on-off photocatalytic reduction/degradation of TB for Au@Au-ZnO and Pt-Au-ZnO. It can be clearly seen that there is a substantial regeneration of TB, during light-off period, when Au@Au-ZnO was used as the catalyst. Hence, the decoloration of the reactant mixture was partly due to the


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photoreduction of TB to TB2- according to reactions 1 and 2,8 and during light-off period TB2has re-oxidized to TB. ℎ() + → ℎ + .

(1)

2 . → + (2)

In contrast, when Pt-Au-ZnO was used as the photocatalyst, during all light-off periods, there was no regeneration of TB, implying that TB totally degraded. The conversion efficiency (complete degradation of TB) of the photocatalyst was calculated after turning off the light and allowing the reaction to attain a steady state. For Pt-Au-ZnO the conversion efficiency was calculated to be 55%, 76% and 88% after UV illumination for 8, 16 and 24 min, respectively. The conversion efficiency of Au@Au-ZnO was comparatively lower and was calculated to be 30%, 51% and 77% at the same time intervals. An important finding from the above results is that photogenerated electrons of both Au@Au-ZnO and Au-ZnO samples have sufficient reducing power to convert TB to TB2-, while Pt-Au-ZnO hybrid lacks the required redox potential. The work function of the solid photocatalyst and the redox potential of the electrolyte species may be considered to elucidate the charge transfer mechanism. The work functions of ZnO, Au and Pt on the absolute potential scale (Eabs) are 5.20 V,46 5.32 V47 and 5.65 V,33 respectively. The reduction potential of TB/TB2redox couple on the Eabs was calculated to be 4.47 V (absolute reduction potential calculated according to Ref48 – refer Supporting Information). When the hybrid metal-semiconductor system is placed in contact with an electrolyte, electrons flow between the metal, semiconductor and redox couple until the Fermi level (Ef) of the electrons in the solid becomes equal to the redox potential of the electrolyte.49,

50

Figures 8a and 8b show schematic representations of

electronic energy levels of Au-ZnO/Au@Au-ZnO and Pt-Au-ZnO hybrid nanoparticles after attaining Fermi level equilibration with the redox couple (TB/TB2-) before UV illumination. The redox potentials of both metal-semiconductor systems studied here are more positive compared to that of TB/TB2-, hence, before UV illumination, Fermi level electrons of both systems were not at the required redox potential for the reduction of TB. It has been demonstrated that when a semiconductor, such as TiO216 or ZnO,7,

8

is attached to Au nanoparticles, and is subjected to

illumination with light at band gap energy, there is a negative shift in the Fermi level. A negative


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Langmuir

shift suggests an increase in the reducing power of the electrons. In the present study, we envisaged that the apparent shift of Fermi level (Ef*) in Au-ZnO and Au@Au-ZnO hybrids upon UV illumination enabled the rapid photoreduction of TB to TB2- (Figure 8c). The rate and extent of photoreduction in Au-ZnO are larger than in Au@Au-ZnO (Figure 7a and 7b) because the apparent Fermi level shift in smaller Au nanoparticles is reportedly higher.16 Photodegradation of a dye molecule should involve 1) light-driven electron-hole separation (generation of free charge carriers) at the semiconductor surface and 2) formation of reactive radical species (responsible for degradation) via the interfacial charge transfer process. The generation of reactive radical species in the presence of free charge carriers (electrons and holes) is well known.45 Under continuous illumination, the apparent negative shift in Fermi level may equilibrate the Fermi levels of ZnO and Au.7 Consequently, there will be an increase in exciton recombination, which may limit the concentration of reactive radical species essential for complete mineralization of TB. In Figure 7a, the photodegradation efficiency of Au@Au-ZnO is higher than Au-ZnO, because larger Au nanoparticles have greater capacitance17 and so would generate more free charge carriers. In contrast to Au, under UV illumination, Pt nanoparticles may not undergo Fermi level equilibration with ZnO, because Pt sustains an Ohmic contact with the surrounding solvent.7 The same theory can be applied to Pt-Au-ZnO, because an Ohmic contact exists between Au and Pt metals. Since there is no negative shift in the Fermi level (compare Figures 8c and 8d), we suggest that photoexcited electrons of Pt-Au-ZnO lacked the reducing power to convert any TB to TB2-. On the other hand, because Pt exhibits a fast electron discharge (i.e. more electron-hole separation), the generation of reactive radical species was enhanced. In the Pt-Au-ZnO system studied herein, generation of surface hydroxyl radicals (OH•) is expected via hole trapping by surface-OH. (Refer supporting information for details). Surface OH• plays the key role in the photodegradation of organic molecules.51,

52

Hence, Pt-Au-ZnO is superior at

photodegrading TB compared to Au-ZnO or Au@Au-ZnO. This explanation also supports the obvious variances in initial decoloration rates for the three different catalysts. Therefore, photocatalytic experiments conducted herein reveal that Pt-Au-ZnO hybrids are more suited for photodegradation/photooxidation type catalysis, while Au-ZnO type hybrids are better suited for rapid photoreduction type photocatalysis under illumination conditions comparable to the UV fraction in natural solar irradiation.


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An ESR study was also performed to further corroborate the efficient electron-hole separation in Pt-Au-ZnO hybrid particles compared to Au-ZnO particles. A typical spin label TEMPO was used for the ESR analysis. TEMPO can be oxidized to oxoammonium cation (TEMPO+) by photogenerated holes.53 TEMPO+ lacks an ESR signal, and therefore, the TEMPO/TEMPO+ couple can be used to monitor the charge discharge properties of hybrid nanoparticles. Figure S4a shows the ESR spectra of TEMPO, obtained before and after UV irradiation of an ethanolic mixture of TEMPO and Pt-Au-ZnO. A clear decrease in the ESR signal was observed after UV irradiation for 20 min, which can be attributed to hole trapping by TEMPO and oxidation to TEMPO+. Upon turning off the UV light the ESR signal reappeared, which indicates that the decrease in ESR signal here is solely due to photo-oxidation of TEMPO to TEMPO+ rather than to photodegradation (Figure S4b). Similarly, oxidation of TEMPO in the presence of BiVO453 and TiO254 photocatalysts has been reported previously. The valence band position of ZnO (~ 7.4 V) is more positive compared to the absolute oxidation potential of TEMPO (~ 5.2 V), thus further confirming the oxidation of TEMPO to TEMPO+ by photogenerated holes in Pt-Au-ZnO system (refer supporting information for calculation of absolute potential). Under the reaction conditions used herein, when Au-ZnO was used as the catalyst, the changes to the ESR signal were minimal (Figure S4c). This result was attributed to increased electron-hole recombination in the Au-ZnO hybrid system due to electron accumulation and Fermi level equilibration process.7 ESR results combined with photocatalysis experiments reported herein, clearly demonstrate the effect of charge accumulation and discharge properties of metal nanoparticles toward photocatalytic properties of Au-ZnO and Pt-Au-ZnO hybrid particles. Photocatalytic properties of Pt-p-Au-ZnO hybrid nanoparticles were also measured. Note that Ptp-Au-ZnO hybrid nanoparticles were synthesized by a multi-step photodeposition technique. Ptp-Au-ZnO hybrid particles showed low overall photocatalytic efficiency compared to Pt-Au-ZnO hybrid particles (Figure S5). As noted previously, the metal-semiconductor interface is nonepitaxial in photodeposited hybrid particles. The charge transfer via the epitaxial interface in PtAu-ZnO is expected to be higher compared to the non-epitaxial interface, and therefore, the better photocatalytic activity. Although, the nature of metal-semiconductor interface has a direct influence on the charge transfer process, it must be noted here that other factors such as particle size, morphology and crystallographic structure could also play a role since Pt-p-Au-ZnO and Pt-


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Au-ZnO are synthesized using two different synthetic methods. Table 2 shows a summary of the main results obtained in this study. Table 2: Summary of photodegradation efficiencies and photoreduction rates obtained for hybrid nanoparticles. Sample ID

Photodegradation efficiency

Initial photoreduction rate

2

(@365 nm, 37 mW/cm , 20 min)

(cm-1/s)

Au-ZnO (Au,~ 5nm)

39 %

0.052

Au@Au-ZnO (Au, ~ 15 nm)

66 %

0.034

Pt-Au-ZnO

84 %

0.0007

Pt-p-Au-ZnO

65%

0.005

Finally, it is noted that in this study we were not able to synthesize binary Pt-ZnO type particles with analogous size, structure and morphology as for the regarded Au seeded Au-ZnO system. However, Pt-ZnO particles prepared using a photodeposition technique showed low photocatalytic activity compared to Pt-Au-ZnO (Figure S5). In a previous study, photocatalytic efficiency of Pt-ZnO particles was measured to be lower compared to Au-ZnO particles, which was attributed to a shadowing effect by Pt islands photodeposited throughout ZnO particles.31 The Au-ZnO seed-assisted deposition employed herein, enabled site-specific attachment of Pt islands to Au and preserved the light absorbing area of ZnO particles. Combination of electron accumulation and discharge properties of Au and Pt in a Pt-Au-ZnO system should give rise to useful synergistic effects which need to be further investigated. For instance, a study by Amirav et al. showed how Au nanoparticles in a Pt-Au-CdSe@CdS system act as a gate through which electrons can migrate from the semiconductor photocatalyst to the Pt cocatalyst.55 As a result, the efficiency of the photocatalytic water splitting reaction was enhanced. This study further confirms the advantage of coupling Pt-Au to semiconductor photocatalysts. Moreover, the Au component in Pt-Au-semiconductor has the advantage of localized surface plasmon resonance (to utilize visible light for catalysis), field enhancement and local heating.


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Figure 8. Schematic representation of electronic energy levels. (a) Au-ZnO/Au@Au-ZnO before irradiation, (b) Pt-Au-ZnO before irradiation, (c) Au-ZnO/Au@Au-ZnO during irradiation and (d) Pt-Au-ZnO during irradiation.

Evac, vacuum energy; ECB, energy of conduction band

minimum; EVB, energy of valence band maximum; Ef, Fermi level in Au-ZnO/Redox and Pt-AuZnO/redox systems before UV irradiation; E*f, Fermi level in Au-ZnO/Redox system after UV irradiation.

Conclusions Pt-Au-ZnO three component hybrid nanoparticles were synthesized by site-specific photodeposition of Pt at the Au islands of Au-seeded ZnO nanoparticles. The epitaxial relationship at the Au-ZnO hybrid nanoparticles was found to be critical for the site-specific deposition of Pt. When the Au-ZnO interface was non-epitaxial, Pt nanoparticles deposited all over both ZnO and Au components. A similar approach was employed to successfully control


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and enlarge the Au nanoparticles at Au-ZnO hybrids (Au@Au-ZnO). TEM, STEM, EDX and XPS analyses confirmed the presence and crystalline structure of metallic nanoparticles in the hybrid systems. Photocatalysis tests conducted under light on-off conditions revealed that decoloration of Toluidine blue was due to both photoreduction (TB to TB2-) and photodegradation (mineralization) processes; and the nature of metal (Pt or Au) has a major effect in determining the reaction pathway. The Au-ZnO and Au@Au-ZnO exhibited high TB photoreduction efficiency owing to photoexcited electron accumulation and negative shift in the Fermi level or redox potential, yet the TB photodegradation efficiency was poor, possibly due to exciton recombination after Fermi level equilibration. Photoexcited electrons of Pt-Au-ZnO hybrids lacked the required redox potential for the photoreduction of TB, however, exhibited the best TB photodegradation efficiency. This was attributed to the fast electron discharge properties of Pt (i.e. absence of Fermi level shift). Fast electron discharge meant that Pt-Au-ZnO generated more reactive radical species for the photodegradation of TB. The photocatalysis results presented herein are in good agreement with electron accumulation and discharge properties reported for Au/Pt- ZnO structures. The present study reveals an important criterion to consider when selecting metal nanoparticles to prepare metal-semiconductor photocatalysts, depending on whether the desired process is a photoreduction and/or photodegradation / oxidation.

ASSOCIATED CONTENT Supporting Information. Figure S1: TEM analysis of Au-ZnO hybrids; Figure S2: UV-Visible extinction spectra obtained prior to photocatalysis experiments; Figure S3: High resolution XPS spectrum of Zn 2p doublet; Figure S4: ESR spectra of TEMPO collected before and after the photocatalytic reaction; Figure S5:

Time-resolved

UV-Visible

extinction

spectra

collected

during

photocatalytic

reduction/degradation of toluidine blue; S6: Computational method; S7: Calculation of absolute redox potential; S8 Calculation of initial photoreduction rate; S9 Photodegradation mechanism. The Supporting Information is available free of charge on the ACS Publications website.


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AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] *E-mail: [email protected] Author Contributions The manuscript was written with contributions from all the authors. All the authors have given approval to the final version of the manuscript. ACKNOWLEDGEMENTS The authors acknowledge the facilities, and the scientific and technical assistance of staff at Central Analytical Research Facility (CARF) operated by the Institute for Future Environments (IFE), QUT. Access to CARF is supported by generous funding from the Science and Engineering Faculty, QUT. The authors also acknowledge use of facilities within the Monash Centre for Electron Microscopy. D.G. particularly acknowledges Australian Research Council (ARC) for granting an Australian Laureate Fellowship (FL160100089). K.V.L., Z.I.P., P.B.S. and D.G. acknowledge the financial support from the Ministry of Education and Science of the Russian Federation (Increase Competitiveness Program of NUST “MISiS” no. K2-2017-082)

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“Table of Contents/Abstract Graphic”


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Photocatalysis with Pt-Au-ZnO and Au-ZnO Hybrids: Effect of Charge

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