This work was supported by the University of Helsinki and the Jane and Aatos Erkko Foundation. S.H. thanks Erasmus+ EU funds for the fellowship.

1. Synthesis of Ag-Pd/ZrO2 NPs 
NOTE: In this procedure, the Pd mol% in Ag-Pd corresponded to 1%, and the Ag-Pd loading on ZrO2 corresponded to 3 wt.%.
<ol>
	<li>Place 1 g of ZrO2 powder in a 250 mL beaker.</li>
	<li>Add 50 mL of an AgNO3 (aq) (0.0059 mol/L) and 9.71 mL of a K2PdCl4 (aq) (0.00031 mol/L) solutions to the beaker under vigorous magnetic stirring (500 rpm) at room temperature.</li>
	<li>Add 10 mL of lysine (0.53 M) aqueous solution.</li>
	<li>Keep the mixture under vigorous stirring (500 rpm) for 20 min.</li>
	<li>After 20 min, use a pipette to add to the suspension 10 mL of a freshly prepared NaBH4 (aq) (0.035 M) solution dropwise, at a rate of 1 mL/min. Keep the suspension under stirring (500 rpm) throughout the process.</li>
	<li>Let the mixture stir for 30 min at room temperature.</li>
</ol>
2. Separation and purification of the catalyst
<ol>
	<li>Transfer the suspension to centrifuge tubes and separate the solids from the mixture by centrifugation at 3,260 x g for 10 min.</li>
	<li>Carefully remove the liquid phase with a pipette and add 15 mL deionized water to the tubes.
	<ol>
		<li>Shake vigorously until dispersion of the solid is obtained. If good dispersion is not achieved, place the tubes in an ultrasonic bath for 5 min.</li>
		<li>Centrifuge the dispersion at 3,260 x g for 10 min.</li>
	</ol>
	</li>
	<li>Repeat the washing steps (2.2. to 2.2.2.) two more times using deionized water, then once using ethanol instead of water.</li>
	<li>Remove the ethanol and dry the solid in an oven at 60 &#176;C for 12 h.</li>
	<li>Characterize the prepared Ag-Pd/ZrO2 NPs by a variety of microscopy, elemental, and spectroscopic techniques.</li>
</ol>
3. Synthesis of Ag/ZrO2 NPs 
NOTE: In this procedure, Ag loading on ZrO2 corresponded to 3 wt.%.
<ol>
	<li>Place 1 g of ZrO2 powder in a 250 mL beaker.</li>
	<li>Add 50 mL of an AgNO3 (aq) (0.0059 mol/L) solution to the beaker under vigorous magnetic stirring (500 rpm) at room temperature.</li>
	<li>Add 10 mL of lysine (0.53 M) aqueous solution.</li>
	<li>Keep the mixture under vigorous stirring (500 rpm) for 20 min.</li>
	<li>After 20 min, use a pipette to add to the suspension 10 mL of a freshly prepared NaBH4 (aq) (0.035 M) solution dropwise, at a rate of 1 mL/min. Keep the suspension under stirring (500 rpm) throughout the process.</li>
	<li>Let the mixture stir for 30 min under room temperature.</li>
</ol>
4. Separation and purification of the catalyst
<ol>
	<li>Transfer the suspension to centrifuge tubes and separate the solids from the mixture by centrifugation at 3,260 x g for 10 min.</li>
	<li>Carefully remove the liquid phase with a pipette and add 15 mL deionized water to the tubes.
	<ol>
		<li>Shake vigorously until the dispersion of the solid is observed. If good dispersion is not achieved, place the tubes in an ultrasonic bath for 5 min.</li>
		<li>Centrifuge the dispersion at 3,260 x g for 10 min.</li>
	</ol>
	</li>
	<li>Repeat the washing steps (4.2. to 4.2.2.) two more times using deionized water, then once using ethanol instead of water.</li>
	<li>Remove the ethanol and dry the solid in an oven at 60 &#176;C for 12 h.</li>
	<li>The prepared Ag/ZrO2 NPs can then be characterized by a variety of microscopy, elemental, and spectroscopic techniques.</li>
</ol>
5. Investigation of plasmonic catalytic performance towards the nitrobenzene reduction under LSPR excitation (light illumination)
<ol>
	<li>Place 30 mg of catalyst in a 25 mL round-bottom flask along with a magnetic stirring bar.</li>
	<li>Add 5 mL of a solution of nitrobenzene (0.03 mol/L) in isopropyl alcohol (IPA) to the reactor.</li>
	<li>Then, add 11.22 mg of KOH powder (0.0002 mol).</li>
	<li>Purge the reactor by bubbling the suspension with an argon flow for 1 min. Immediately after purging, seal the flask.</li>
	<li>Place the reactor in an oil bath heated at 70 &#176;C above a temperature-controlled magnetic stirrer (500 rpm).</li>
	<li>Irradiate the tube using 4 LED lamps with a wavelength of 425 nm as the light source, and a light intensity of 0.5 W/cm2. The distance from the lamps to the reaction flask should be 7 cm.</li>
	<li>Let the reaction proceed for 2.5 h at 70 &#176;C under vigorous magnetic stirring (500 rpm).</li>
	<li>Then, turn the light off, open the reactor and use a syringe and a needle to collect a 1 mL sample. Filter it through a 0.45 &#956;m filter, to remove the catalyst particulates, into a gas chromatography vial.</li>
</ol>
6. Reaction in the absence of LSPR excitation (dark conditions)
<ol>
	<li>Follow the same steps as described in 5, but without light irradiation. Wrap the reaction tube with aluminium foil to prevent any light exposure.</li>
</ol>
7. Gas chromatography (GC) analysis preparation
<ol>
	<li>Prepare an IPA solution containing approximately 30 mmol/L nitrobenzene (NB), 30 mmol/L of aniline (AN), and 30 mmol/L of azobenzene (AB).</li>
	<li>Run a GC analysis of the solution using a suitable method. Different methods can be tested by varying the column temperature and gas flow programs. The selected method should be able to separate the peaks corresponding to IPA, NB, AN, and AB in the minimum period of retention time.</li>
	<li>Once the method has been selected, prepare a set of solutions of 50 mM, 25 mM, 10 mM, 5 mM and 2.5 mM NB in IPA, and another set of solutions of AN and AB in IPA with the same concentrations.</li>
	<li>Run a GC analysis of the prepared solutions. Each chromatogram should present 2 peaks: the higher one corresponds to IPA and the lower one corresponds to NB, AN, or AB. For each chromatogram, note down the retention time and peak area of all the peaks.</li>
	<li>Trace the calibration curves of NB, AN, and AB by plotting the concentration versus peak area of each sample.</li>
</ol>
8. GC analysis
<ol>
	<li>Run a GC analysis on the samples collected in steps 5. and 6. with the same method used for steps 7.2. and 7.4.</li>
	<li>For each chromatogram, note down the retention time and peak area and use the calibration curves plotted previously to determine the concentration of NB, AN, and AB in the samples.</li>
	<li>Calculate the nitrobenzene conversion as well as the aniline and azobenzene selectivity using the equations: 
	<img alt="Equation 1" src="/files/ftp_upload/61712/61712eq1.jpg" /> 
	<img alt="Equation 2" src="/files/ftp_upload/61712/61712eq2.jpg" /> 
	<img alt="Equation 3" src="/files/ftp_upload/61712/61712eq3.jpg" /> 
	Where <img alt="Equation 4" src="/files/ftp_upload/61712/61712eq4.jpg" /> is the initial NB concentration (0.03 mol/L), and CNB, CAN, CAB&#160; correspond to the NB, AN, and AB concentrations, respectively, after 2.5 hours reaction by the GC analysis.</li>
</ol>

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The authors have nothing to disclose.

The &#64257;ndings described in this method demonstrate that the intrinsic catalytic activity of Pd (or other catalytic but not plasmonic metal) can be signi&#64257;cantly enhanced by LSPR excitation via visible-light irradiation in bimetallic alloyed NPs35. In this case, Ag (or another plasmonic metal) is capable of harvesting energy from visible-light irradiation via LSPR excitation. The LSPR excitation leads to the formation of hot charge carriers (hot electrons and holes) and localized heating5,14,15,16,17,18,19. While localized heating can contribute to enhanced reaction rates, the LSPR-excited charge carriers can participate in the vibrational or electronic activation of surface adsorbates5,14,15,16,17,18,19. This allows for not only increased reaction rates but also changes in reaction selectivity due to selective activation of adsorbates or molecular orbitals at the metal-molecule interface, for example20,21,22,23,24,25. The method described herein effectively allows for the merging of plasmonic and catalytic properties in alloyed nanoparticle systems to extend the applicability of plasmonic catalysis to metals that are important in catalysis but do not support LSPR excitation in the visible range. Although the method described here focused on Ag and Pd as the plasmonic and catalytic metals, it can also be applied and adapted to other plasmonic catalytic combinations such as (Ag-Pt, Au-Pd, Au-Pt, etc.). Moreover, the plasmonic and catalytic properties of the bimetallic alloyed NPs can be further tuned by varying the relative molar ratios of the plasmonic and catalytic components. For instance, increasing the amount of Pd would make the nanoparticles more catalytic, while an increase in the Ag content leads to an increase in the optical properties. The synthesis method can also be adapted to achieve core-shell systems via the sequential deposition and reduction of precursors, for example36. It is noteworthy that there is also the possibility to extend the scope on the choice of plasmonic components to earth-abundant materials that can also be employed as supports. Examples include metal nitrides (TiN and ZrN) and some oxides (MoO3) which support LSPR excitation in the visible and near-IR ranges37,38,39,40.
In addition to the scope of the catalytic materials, the method presented in this paper can be applied to several types of liquid phase transformations that include other reductions, oxidations, and coupling reactions, for example18. Another advantage of this method is that the lamp&#39;s wavelength and number can be changed, which makes possible the study of the impact of the light&#8217;s intensity and wavelength on the photocatalytic reaction. Wavelength-dependent photocatalytic reactions have been used to correlate the plasmonic properties of photocatalysts to their performance5,14,15,16,17,18,19. It has been established increased plasmonic catalytic performances are observed when the light wavelength has a better matching to the LSPR extinction position5,14,15,16,17,18,19.
Finally, to be sure that the results are correct and representative, it is important to pay attention to some crucial steps of the protocol. When synthesizing the NPs, the amount of metal precursors added in the reactor must be precisely known. Indeed, a small error on the Pd content, which is exceptionally low, can result in a dramatic change in the catalytic properties. After the synthesis, the drying temperature should not exceed 60 &#176;C, as it would result in possible oxidation of the silver or aggregation of the NPs, once again interfering with the catalytic activity. The atmosphere of the photocatalytic reaction should also be controlled with great care. In our case, if the reactor is opened, the presence of an ambient atmosphere will put an end to the reaction. Thus, if these issues are well controlled, the method presented here can be used to study the plasmonic catalytic activity and selectivity of various plasmonic catalysts toward a wide range of chemical reactions. This can enable a better understanding of plasmonic catalysis and aid to the design of catalytic systems having target activities and selectivity for a reaction of interest under mild and environmentally friendly conditions.

Among the several applications of metal nanoparticles (NPs), catalysis deserves special attention. Catalysis plays a central role in a sustainable future, contributing to less energy consumption, better utilization of raw materials, and enabling cleaner reaction conditions1,2,3,4. Thus, progress in catalysis can provide tools for enhancing the atomic efficiency of chemical processes, making them cleaner, more economically viable, and more environmentally friendly. Metal NPs encompassing silver (Ag), gold (Au) or copper (Cu) can display interesting optical properties in the visible range that arise from the unique way these systems interact with light at the nanoscale via the localized surface plasmon resonance (LSPR) excitation5,6,7,8. In these NPs, referred to as plasmonic NPs, the LSPR comprises the resonant interaction between the incident photons (from an incoming electromagnetic wave) with the collective motion of electrons5,6,7,8. This phenomenon takes place at a characteristic frequency which is dependent on the size, shape, composition, and dielectric constant of the environment9,10,11. For example, for Ag, Au, and Cu, these frequencies can range from the visible to the near-IR, opening up possibilities for the utilization of solar energy to excite their LSPR5,6,7,8,12,13.
Recently, it has been demonstrated that the LSPR excitation in plasmonic NPs can contribute to accelerate the rates and control the selectivity of molecular transformations5,14,15,16,17,18,19. This gave birth to a field called plasmonic catalysis, which focus on using energy from light to accelerate, drive, and/or control chemical transformations5,14,15,16,17,18,19. In this context, it has been established that the LSPR excitation in plasmonic NPs can lead to the formation of energetic hot electrons and holes, referred to as LSPR-excited hot carriers. These carriers can interact with adsorbed species through electronic or vibrational activation15,16. In addition to increased reaction rates, this process can also provide alternative reaction pathways not accessible via traditional thermochemically-driven processes, opening up new avenues for the control over reaction selectivity20,21,22,23,24,25. Importantly, it is worth noting that the plasmon decay can also lead to thermal dissipation, leading to a temperature increase in the vicinity of the NPs which can also contribute to speed up reaction rates15,16.
Due to these interesting features, plasmonic catalysis has been successfully employed towards a variety of molecular transformations18. Nevertheless, an important challenge remains. While plasmonic NPs such as Ag and Au display excellent optical properties in the visible and near-IR ranges, their catalytic properties are limited in terms of the scope of transformations. In other words, they do not display good catalytic properties for several of transformations. Additionally, metals that are important in catalysis, such as palladium (Pd) and platinum (Pt), do not support LSPR excitation in the visible or near-IR ranges. To bridge this gap, bimetallic NPs containing a plasmonic and catalytic metal represents an effective strategy20,26,27,28,29. In these systems, the plasmonic metal can be employed as an antenna to harvest energy from the light excitation through the LSPR, which is then used to drive, accelerate, and control molecular transformations at the catalytic metal. Therefore, this strategy enables us to extend plasmonic catalysis beyond traditional plasmonic metal NPs20,26,27,28,29.
This protocol describes the facile synthesis of bimetallic silver-palladium (Ag-Pd) alloyed NPs supported on ZrO2 (Ag-Pd/ZrO2) that can act as a plasmonic-catalytic system for plasmonic catalysis. The Ag-Pd/ZrO2 NPs were prepared by co-impregnation of the corresponding metal precursors on the ZrO2 support followed by simultaneous reduction30. This approach led to the formation of bimetallic NPs around 10 nm in size (diameter) directly at the surface of the ZrO2 support. The NPs were composed of 1 mol% of Pd to minimize the utilization of the catalytic metal while maximizing the optical properties of the resulting Ag-Pd NPs. A protocol for the application of the Ag-Pd/ZrO2 NPs in plasmonic catalysis was demonstrated for the reduction of nitrobenzene. We employed 425 nm LED illumination for the LSPR excitation. Gas chromatography was performed to monitor the conversion and selectivity of the reduction reaction under the dark and light irradiation conditions. LSPR excitation led to enhanced catalytic performance and control over selectivity in Ag-Pd/ZrO2 NPs relative to purely thermally driven conditions. The method described in this protocol is based on a simple photocatalytic reaction setup coupled with gas chromatography and can be adapted to a wide range of molecular transformations and NPs compositions. Thus, this method makes possible the characterization of photocatalytic activity, in terms of conversion and reaction selectivity, of different NPs and for a myriad of liquid-phase transformations. We believe this article will provide important guidelines and insights to both newcomers and more experienced scientists in the field.

<table><tbody><tr><td>2-Propanol (anhydrous, 99.5%)</td><td>Sigma-Aldrich</td><td>278475</td><td>CAS Number 67-63-0</td></tr><tr><td>Aniline (for synthesis)</td><td>Sigma-Aldrich</td><td>8.22256</td><td>CAS Number 62-53-3</td></tr><tr><td>Azobenzene (98%)</td><td>Sigma-Aldrich</td><td>424633</td><td>CAS Number 103-33-3</td></tr><tr><td>Ethanol</td><td>Honeywell</td><td>32221</td><td>CAS Number 64-17-5</td></tr><tr><td>Hydrochloric acid (37%)</td><td>VWR</td><td>PRLSMC310066</td><td>CAS Number 7647-01-0</td></tr><tr><td>L-Lysine (crystallized, &amp;ge;98.0% (NT))</td><td>Sigma-Aldrich</td><td>62840</td><td>CAS Number 56-87-1</td></tr><tr><td>Nitric acid (65%)</td><td>Merck</td><td>100456</td><td>CAS Number 7697-37-2</td></tr><tr><td>Nitrobenzene</td><td>Sigma-Aldrich</td><td>8.06770</td><td>CAS Number 98-95-3</td></tr><tr><td>Potassium hydroxide</td><td>Fisher</td><td>10448990</td><td>CAS Number 1310-58-3</td></tr><tr><td>Potassium tetrachloropalladate (II) (98%)</td><td>Sigma-Aldrich</td><td>205796</td><td>CAS Number 10025-98-6</td></tr><tr><td>Silver nitrate (ACS reagent, &amp;ge;99.0%)</td><td>Sigma-Aldrich</td><td>209139</td><td>CAS Number 7761-88-8</td></tr><tr><td>Sodium borohydride (fine granular for synthesis)</td><td>Sigma-Aldrich</td><td>8.06373</td><td>CAS Number 16940-66-2</td></tr><tr><td>Zirconium (IV) oxide (nanopowder, &amp;lt;100 nm particle size (TEM))</td><td>Sigma-Aldrich</td><td>544760</td><td>CAS Number 1314-23-4</td></tr></tbody></table>

preparation of silver-palladium alloyed nanoparticles for plasmonic catalysis under visible-light illumination

Localized surface plasmon resonance (LSPR) in plasmonic nanoparticles (NPs) can accelerate and control the selectivity of a variety of molecular transformations. This opens possibilities for the use of visible or near-IR light as a sustainable input to drive and control reactions when plasmonic nanoparticles supporting LSPR excitation in these ranges are employed as catalysts. Unfortunately, this is not the case for several catalytic metals such as palladium (Pd). One strategy to overcome this limitation is to employ bimetallic NPs containing plasmonic and catalytic metals. In this case, the LSPR excitation in the plasmonic metal can contribute to accelerate and control transformations driven by the catalytic component. The method reported herein focuses on the synthesis of bimetallic silver-palladium (Ag-Pd) NPs supported on ZrO2 (Ag-Pd/ZrO2) that acts as a plasmonic-catalytic system. The NPs were prepared by co-impregnation of corresponding metal precursors on the ZrO2 support followed by simultaneous reduction leading to the formation of bimetallic NPs directly on the ZrO2 support. The Ag-Pd/ZrO2 NPs were then used as plasmonic catalysts for the reduction of nitrobenzene under 425 nm illumination by LED lamps. Using gas chromatography (GC), the conversion and selectivity of the reduction reaction under the dark and light irradiation conditions can be monitored, demonstrating the enhanced catalytic performance and control over selectivity under LSPR excitation after alloying non-plasmonic Pd with plasmonic metal Ag. This technique can be adapted to a wide range of molecular transformations and NPs compositions, making it useful for the characterization of the plasmonic catalytic activity of different types of catalysis in terms of conversion and selectivity.

Figure 1A shows digital photographs of the solid samples containing the pure ZrO2 oxide (left) and the Ag-Pd/ZrO2 NPs (right). This change in color from white (in ZrO2) to brown (Ag-Pd/ZrO2) provides the initial qualitative evidence on the deposition of Ag-Pd NPs at the ZrO2 surface. Figure 1B shows the UV-visible absorption spectra from the Ag-Pd/ZrO2 NPs (blue trace) as well as ZrO2 (black trace) and Ag/ZrO2 NPs (red trace). Here, the ZrO2 support and Ag/ZrO2 NPs were employed as reference materials. ZrO2 did not display any bands in the visible range. Therefore, it should not contribute to any photocatalytic activity. A signal centered at 428 nm could be detected for the Ag/ZrO2 NPs (red trace). This signal is assigned to the LSPR dipolar mode in Ag NPs9. The Ag-Pd/ZrO2 NPs displayed a peak centered at 413 nm which is slightly blue-shifted and lower in intensity relative to the Ag/ZrO2 NPs. The blue shift could be assigned to the change in material permittivity upon alloying with Pd31. Also, the decrease in the peak intensity is evidence on the formation of alloyed Ag-Pd NPs, as it is well established that the addition of a non-plasmonic metal to a plasmonic nanoparticle leading to core-shell or alloyed systems lead to the damping in the intensity of the LSPR peak32. It is important to note that in this case, we kept the Pd wt. % in the Ag-Pd NPs low (~1 %) so that the LSPR peak is not completely suppressed and the Ag-Pd samples still retain optical properties (LSPR excitation) in the visible range and therefore are active for plasmonic catalysis.
<img alt="Figure 1" class="xfigimg" src="/files/ftp_upload/61712/61712fig1.jpg" /> 
Figure 1: Optical characterization of the catalysts. (A) Digital photography of the solid ZrO2 supports (left) and Ag-Pd/ZrO2 catalyst (right). (B) UV-Visible extinction spectra of ZrO2, Ag/ZrO2, and Ag-Pd/ZrO2 catalysts. The spectra were recorded using an integration sphere in Diffuse Reflectance Spectra (DRS) mode. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/61712/61712fig1large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
During the synthesis of the catalysts, the amount of Ag and Pd salt used were calculated in order to reach 3 wt. % metal loading on the support, and a composition of 99% Ag and 1% Pd by weight (wt.%) for Ag-Pd/ZrO2. To verify the composition of the catalysts, an Atomic Emission Spectroscopy (AES) study was conducted. Calculated amounts of Ag/ZrO2 and Ag-Pd/ZrO2 were digested in concentrated nitric acid. The obtained solutions were then analyzed by AES and the amount of Ag initially present in the catalysts was deduced from calibration curves. To determine the Pd content of Ag-Pd/ZrO2, the same process was employed, except that the catalyst was digested using aqua regia. The AES results revealed that the metal loading was 2.6 wt.% for both catalysts, while the composition of the Ag-Pd was 1 wt.% Pd as expected.
Figures 2 show scanning (SEM, Figure 2A) and transmission electron microscopy (TEM, Figure 2B) of the Ag-Pd/ZrO2 NPs. The Ag-Pd NPs at the surface of the ZrO2 supports are difficult to be identified from SEM images (Figure 2A) due to their small NPs sizes. However, the formation of Ag-Pd NPs with mean particle size around 10 nm (Figure 2C) in diameter can be identified from the TEM images (some of them are indicated by the arrows in Figure 2B for clarity). They displayed a spherical shape and a relatively uniform dispersion over the surface of the ZrO2 supports.
<img alt="Figure 2" class="xfigimg" src="/files/ftp_upload/61712/61712fig2.jpg" /> 
Figure 2: Morphological analysis of the Ag-Pd/ZrO2 catalyst. (A) SEM image of the Ag-Pd/ZrO2 catalyst. (B) TEM image of the Ag-Pd/ZrO2 catalyst. The white arrows depict examples of regions containing Ag-Pd NPs. (C) Histogram of the size distribution of Ag-Pd NPs on the Ag-Pd/ZrO2 catalyst. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/61712/61712fig2large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
After the synthesis of Ag-Pd NPs supported on ZrO2, this method focused on the application as alloyed systems in plasmonic catalysis. Specifically, it describes the utilization of the reduction of nitrobenzene as a model transformation in the liquid phase as illustrated in Figure 3. This probe reaction is interesting as the reduction of nitrobenzene can lead to the formation of azobenzene and aniline33,34. Therefore, this model transformation enables the simultaneous investigation of conversion percentages and reaction selectivity as a function of the light illumination (LSPR excitation) in plasmonic catalysis. Here, the reaction was performed in the presence of isopropanol as the solvent and KOH. Also, 70 &#176;C was employed as the reaction temperature, four 425 nm LED lamps were employed as the light illumination source, and 2.5 h was the reaction time (as described in section 5 of the protocol). In addition to the use of Ag-Pd/ZrO2 NPs as plasmonic catalysts, blank reactions (absence of catalyst), and Ag/ZrO2 NPs as reference catalysts to demonstrate the role of Pd in the alloyed bimetallic NPs were also described.
<img alt="Figure 3" class="xfigimg" src="/files/ftp_upload/61712/61712fig3.jpg" /> 
Figure 3: Schematic representation of the model reaction. Scheme of the photocatalyzed nitrobenzene reduction used as model reaction. Under LSPR excitation, this reaction leads to the formation of azobenzene and aniline as products. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/61712/61712fig3large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
Figures 4 show a scheme (Figure 4A) and a digital photograph (Figure 4B) of the reactor and lamps setup employed in the plasmonic catalysis investigation. The setup used for LSPR excitation was made of four 425 nm LED lamps equally spaced around the reactor, at a distance of 7 cm. The reactor was positioned in the center of the system, immersed in an oil bath over a temperature-controlled magnetic stirrer. This enables control over the temperature and more uniform illumination of the reaction mixture from all directions.
<img alt="Figure 4" class="xfigimg" src="/files/ftp_upload/61712/61712fig4.jpg" /> 
Figure 4: Representation of the photocatalytic reaction set-up. (A) Top-view scheme and (B) digital photography of the light reaction setup including the reactor in an oil bath surrounded by four 425 nm LED lamps positioned at 7 cm distance from the reactor. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/61712/61712fig4large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
After the reaction proceeds, the conversion and selectivity for the formation of azobenzene and aniline can be measured by gas chromatography. Figures 5 show the chromatograms obtained at the end of the reaction catalyzed by Ag-Pd/ZrO2 NPs that was carried out under LSPR excitation (Figure 5A) and dark conditions (Figure 5B). In this case, one must ensure to use a GC method that enables the separation of nitrobenzene, azobenzene, and aniline in different retention times to correctly identify these molecules, while calibration curves for each molecule were employed to perform their quantification. Moreover, the reaction mixture can also be analyzed by gas chromatography-mass spectrometry (GC-MS) to confirm the formation of azobenzene and aniline and also for any other products that could be formed.
<img alt="Figure 5" class="xfigimg" src="/files/ftp_upload/61712/61712fig5.jpg" /> 
Figure 5: Chromatograms of the reaction mixture. GC chromatograms obtained from the reaction mixture after 2.5 h catalysis by Ag-Pd/ZrO2 under LSPR excitation (light irradiation) (A) and dark (B) conditions. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/61712/61712fig5large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
Table 1 and Figures 6 depict the conversion percentages for the nitrobenzene reduction (Figure 6A) and the selectivity towards azobenzene and aniline (Figure 6B) under light illumination for the alloyed Ag-Pd/ZrO2 NPs as well as for Ag/ZrO2 NPs. In the absence of any catalysts (blank reactions), no nitrobenzene conversion was detected both in the presence and absence of light illumination. For Ag/ZrO2 NPs, while no conversion was detected in the dark, a 36% conversion was observed under LSPR excitation. A 56% selectivity towards azobenzene (18% selectivity towards aniline) was detected. This result indicates that the Ag alone can catalyze this reaction under LSPR excitation. For the bimetallic Ag-Pd/ZrO2 NPs, no significant conversion was detected under dark conditions (2.2%). Interestingly, under LSPR excitation, the conversion % corresponded to 63%, with a 73% selectivity towards azobenzene (27% selectivity towards aniline). This observation demonstrates the potential of the bimetallic configuration in plasmonic-catalytic nanoparticles not only to increase conversion under LSPR excitation but also to control reaction selectivity.
<table border="1" fo:keep-together.within-page="1">
	<tbody>
		<tr>
			<td rowspan="2">Catalyst</td>
			<td rowspan="2">Condition</td>
			<td rowspan="2">Conversion %</td>
			<td colspan="2">Selectivity %</td>
		</tr>
		<tr>
			<td>Aniline</td>
			<td>Azobenzene</td>
		</tr>
		<tr>
			<td rowspan="2">AgPd/ZrO2 (2.56 %)</td>
			<td>Light</td>
			<td>63</td>
			<td>27</td>
			<td>73</td>
		</tr>
		<tr>
			<td>Dark</td>
			<td>2.2</td>
			<td>ND</td>
			<td>ND</td>
		</tr>
		<tr>
			<td rowspan="2">Ag/ZrO2 (2.61 %)</td>
			<td>Light</td>
			<td>36</td>
			<td>18</td>
			<td>56</td>
		</tr>
		<tr>
			<td>Dark</td>
			<td>0</td>
			<td>ND</td>
			<td>ND</td>
		</tr>
		<tr>
			<td rowspan="2">Blank</td>
			<td>Light</td>
			<td>0</td>
			<td>ND</td>
			<td>ND</td>
		</tr>
		<tr>
			<td>Dark</td>
			<td>0</td>
			<td>ND</td>
			<td>ND</td>
		</tr>
	</tbody>
</table>
Table 1: Summary of the conversion and selectivity for the nitrobenzene reduction. Conversion&#160; and product selectivity for nitrobenzene reduction reaction under LSPR excitation and dark conditions. Peaks were not detected (ND) if their area was less than 10&#160;000 counts. Ag-Pd/ZrO2 and Ag/ZrO2 were employed as catalysts and a blank reaction without any catalyst was also analyzed. Reaction conditions: catalyst (30 mg), solvent (IPA, 5 mL), base (KOH, 0.2 mmol/L) and reactant (nitrobenzene, 0.15 mmol/L), under Ar atmosphere, 2.5 h at 70 &#176;C.
<img alt="Figure 6" class="xfigimg" src="/files/ftp_upload/61712/61712fig6.jpg" /> 
Figure 6: Conversion percentage and selectivity under light illumination. (A) Nitrobenzene conversion under 425 nm light irradiation and in the dark for the reaction catalyzed by Ag-Pd/ZrO2 (blue bar) and Ag/ZrO2 (red bar). (B) Aniline and azobenzene selectivity under light irradiation for the reaction catalyzed by Ag-Pd/ZrO2 (blue bars) and Ag/ZrO2 (red bars). <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/61712/61712fig6large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>

Watch this Scientific Journal Video about Preparation of Silver-Palladium Alloyed Nanoparticles for Plasmonic Catalysis under Visible-Light Illumination at JoVE.com

Preparation of Silver-Palladium Alloyed Nanoparticles for Plasmonic Catalysis under Visible-Light Illumination

Presented here is a protocol for the synthesis of silver-palladium (Ag-Pd) alloy nanoparticles (NPs) supported on ZrO2 (Ag-Pd/ZrO2). This system allows for harvesting energy from visible light irradiation to accelerate and control molecular transformations. This is illustrated by nitrobenzene reduction under light irradiation catalyzed by Ag-Pd/ZrO2 NPs.

Localized surface plasmon resonance (LSPR) in plasmonic nanoparticles (NPs) can accelerate and control the selectivity of a variety of molecular ...

preparation-silver-palladium-alloyed-nanoparticles-for-plasmonic

Nanyang Technological University

Research

JoVE Journal

Chemistry

5.4K Views.  University of Helsinki. Hier wordt een protocol gepresenteerd voor de synthese van zilver-palladium (Ag-Pd) legering nanodeeltjes (NPs) ondersteund op ZrO2 (Ag-Pd/ZrO2). Dit systeem maakt het mogelijk om energie te oogsten uit zichtbare lichtbestraling om moleculaire transformaties te versnellen en te beheersen. Dit wordt geïllustreerd door nitrobenzeenreductie bij lichte bestraling gekatalyseerd door Ag-Pd/ZrO2 NPs.

Video: Preparation of Silver-Palladium Alloyed Nanoparticles for Plasmonic Catalysis under Visible-Light Illumination

Gyroid Nickel Nanostructures from Diblock Copolymer Supramolecules

Nanoporous metal foams possess a unique combination of properties - they are catalytically active, thermally and electrically conductive, and furthermore, have high porosity, high surface-to-volume and strength-to-weight ratio. Unfortunately, common approaches for preparation of metallic nanostructures render materials with highly disordered architecture, which might have an adverse effect on their mechanical properties. Block copolymers have the ability to self-assemble into ordered nanostructures and can be applied as templates for the preparation of well-ordered metal nanofoams. Here we describe the application of a block copolymer-based supramolecular complex - polystyrene-block-poly(4-vinylpyridine)(pentadecylphenol) PS-b-P4VP(PDP) - as a precursor for well-ordered nickel nanofoam. The supramolecular complexes exhibit a phase behavior similar to conventional block copolymers and can self-assemble into the bicontinuous gyroid morphology with two PS networks placed in a P4VP(PDP) matrix. PDP can be dissolved in ethanol leading to the formation of a porous structure that can be backfilled with metal. Using electroless plating technique, nickel can be inserted into the template&#39;s channels. Finally, the remaining polymer can be removed via pyrolysis from the polymer/inorganic nanohybrid resulting in nanoporous nickel foam with inverse gyroid morphology.

This article describes the preparation of well-ordered nickel nanofoams via electroless metal deposition onto nanoporous templates obtained from self-assembled diblock copolymer based supramolecules.

Nanoporous metal foams possess a unique combination of properties - they are catalytically active, thermally and electrically conductive, and furthermore, ...

Preparation of Silica Nanoparticles Through Microwave-assisted Acid-catalysis

Microwave-assisted synthetic techniques were used to quickly and reproducibly produce silica nanoparticle sols using an acid catalyst with nanoparticle diameters ranging from 30-250 nm by varying the reaction conditions. Through the selection of a microwave compatible solvent, silicic acid precursor, catalyst, and microwave irradiation time, these microwave-assisted methods were capable of overcoming the previously reported shortcomings associated with synthesis of silica nanoparticles using microwave reactors. The siloxane precursor was hydrolyzed using the acid catalyst, HCl. Acetone, a low-tan &#948; solvent, mediates the condensation reactions and has minimal interaction with the electromagnetic field. Condensation reactions begin when the silicic acid precursor couples with the microwave radiation, leading to silica nanoparticle sol formation. The silica nanoparticles were characterized by dynamic light scattering data and scanning electron microscopy, which show the materials&#39; morphology and size to be dependent on the reaction conditions. Microwave-assisted reactions produce silica nanoparticles with roughened textured surfaces that are atypical for silica sols produced by St&#246;ber&#39;s methods, which have smooth surfaces.

Silica nanoparticles were prepared using acid-catalysis of a siloxane precursor and microwave-assisted synthetic techniques resulting in the controlled growth of nanomaterials ranging from 30-250 nm in diameter. The growth dynamics can be controlled by varying the initial silicic acid concentration, time of the reaction, and temperature of reaction.

Microwave-assisted synthetic techniques were used to quickly and reproducibly produce silica nanoparticle sols using an acid catalyst with nanoparticle ...

Synthesis, Characterization, and Functionalization of Hybrid Au/CdS and Au/ZnS Core/Shell Nanoparticles

Plasmonic nanoparticles are an attractive material for light harvesting applications due to their easily modified surface, high surface area and large extinction coefficients which can be tuned across the visible spectrum. Research into the plasmonic enhancement of optical transitions has become popular, due to the possibility of altering and in some cases improving photo-absorption or emission properties of nearby chromophores such as molecular dyes or quantum dots. The electric field of the plasmon can couple with the excitation dipole of a chromophore, perturbing the electronic states involved in the transition and leading to increased absorption and emission rates. These enhancements can also be negated at close distances by energy transfer mechanism, making the spatial arrangement of the two species critical. Ultimately, enhancement of light harvesting efficiency in plasmonic solar cells could lead to thinner and, therefore, lower cost devices. The development of hybrid core/shell particles could offer a solution to this issue. The addition of a dielectric spacer between a gold nanoparticles and a chromophore is the proposed method to control the exciton plasmon coupling strength and thereby balance losses with the plasmonic gains. A detailed procedure for the coating of gold nanoparticles with CdS and ZnS semiconductor shells is presented. The nanoparticles show high uniformity with size control in both the core gold particles and shell species allowing for a more accurate investigation into the plasmonic enhancement of external chromophores.

The synthesis of uniform gold nanoparticles coated with semiconductor shells of CdS or ZnS is performed. The semiconductor coating is conducted by first depositing a silver sulfide shell and exchanging the silver cations for zinc or cadmium cations.

Plasmonic nanoparticles are an attractive material for light harvesting applications due to their easily modified surface, high surface area and large ...

Multifunctional Hybrid Fe2O3-Au Nanoparticles for Efficient Plasmonic Heating

One of the most widely used methods for manufacturing colloidal gold nanospherical particles involves the reduction of chloroauric acid (HAuCl4) to neutral gold Au(0) by reducing agents, such as sodium citrate or sodium borohydride. The extension of this method to decorate iron oxide or similar nanoparticles with gold nanoparticles to create multifunctional hybrid Fe2O3-Au nanoparticles is straightforward. This approach yields fairly good control over Au nanoparticle dimensions and loading onto Fe2O3. Additionally, the Au metal size, shape, and loading can easily be tuned by changing experimental parameters (e.g., reactant concentrations, reducing agents, surfactants, etc.). An advantage of this procedure is that the reaction can be done in air or water, and, in principle, is amenable to scaling up. The use of such optically tunable Fe2O3-Au nanoparticles for hyperthermia studies is an attractive option as it capitalizes on plasmonic heating of gold nanoparticles tuned to absorb light strongly in the VIS-NIR region. In addition to its plasmonic effects, nanoscale Au provides a unique surface for interesting chemistries and catalysis. The Fe2O3 material provides additional functionality due to its magnetic property. For example, an external magnetic field could be used to collect and recycle the hybrid Fe2O3-Au nanoparticles after a catalytic experiment, or alternatively, the magnetic Fe2O3 can be used for hyperthermia studies through magnetic heat induction. The photothermal experiment described in this report measures bulk temperature change and nanoparticle solution mass loss as functions of time using infrared thermocouples and a balance, respectively. The ease of sample preparation and the use of readily available equipment are distinct advantages of this technique. A caveat is that these photothermal measurements assess the bulk solution temperature and not the surface of the nanoparticle where the heat is transduced and the temperature is likely to be higher.

Multifunctional Hybrid Fe2O3-Au Nanoparticles for Efficient Plasmonic Heating

We describe the synthesis and properties of multifunctional Fe2O3-Au nanoparticles produced by a wet chemical approach and investigate their photothermal properties using laser irradiation. The composite Fe2O3-Au nanoparticles retain the properties of both materials, creating a multifunctional structure with excellent magnetic and plasmonic properties.

One of the most widely used methods for manufacturing colloidal gold nanospherical particles involves the reduction of chloroauric acid (HAuCl4) to ...

Engineering

Colloidal Synthesis of Nanopatch Antennas for Applications in Plasmonics and Nanophotonics

We present a method for colloidal synthesis of silver nanocubes and the use of these in combination with a smooth gold film, to fabricate plasmonic nanoscale patch antennas. This includes a detailed procedure for the fabrication of thin films with a well-controlled thickness over macroscopic areas using layer-by-layer deposition of polyelectrolyte polymers, namely poly(allylamine) hydrochloride (PAH) and polystyrene sulfonate (PSS). These polyelectrolyte spacer layers serve as a dielectric gap in between silver nanocubes and a gold film. By controlling the size of the nanocubes or the gap thickness, the plasmon resonance can be tuned from about 500 nm to 700 nm. Next, we demonstrate how to incorporate organic sulfo-cyanine5 carboxylic acid (Cy5) dye molecules into the dielectric polymer gap region of the nanopatch antennas. Finally, we show greatly enhanced fluorescence of the Cy5 dyes by spectrally matching the plasmon resonance with the excitation energy and the Cy5 absorption peak. The method presented here enables the fabrication of plasmonic nanopatch antennas with well-controlled dimensions utilizing colloidal synthesis and a layer-by-layer dip-coating process with the potential for low cost and large-scale production. These nanopatch antennas hold great promise for practical applications, for example in sensing, ultrafast optoelectronic devices and for high-efficiency photodetectors.

A protocol for the colloidal synthesis of silver nanocubes and fabrication of plasmonic nanoscale patch antennas with sub-10 nm gaps is presented.

We present a method for colloidal synthesis of silver nanocubes and the use of these in combination with a smooth gold film, to fabricate plasmonic ...

In Situ Synthesis of Gold Nanoparticles without Aggregation in the Interlayer Space of Layered Titanate Transparent Films

Combinations of metal oxide semiconductors and gold nanoparticles (AuNPs) have been investigated as new types of materials. The in situ synthesis of AuNPs within the interlayer space of semiconducting layered titania nanosheet (TNS) films was investigated here. Two types of intermediate films (i.e., TNS films containing methyl viologen (TNS/MV2+) and 2-ammoniumethanethiol (TNS/2-AET+)) were prepared. The two intermediate films were soaked in an aqueous tetrachloroauric(III) acid (HAuCl4) solution, whereby considerable amounts of Au(III) species were accommodated within the interlayer spaces of the TNS films. The two types of obtained films were then soaked in an aqueous sodium tetrahydroborate (NaBH4) solution, whereupon the color of the films immediately changed from colorless to purple, suggesting the formation of AuNPs within the TNS interlayer. When only TNS/MV2+ was used as the intermediate film, the color of the film gradually changed from metallic purple to dusty purple within 30 min, suggesting that aggregation of AuNPs had occurred. In contrast, this color change was suppressed by using the TNS/2-AET+ intermediate film, and the AuNPs were stabilized for over 4 months, as evidenced by the characteristic extinction (absorption and scattering) band from the AuNPs.

In Situ Synthesis of Gold Nanoparticles without Aggregation in the Interlayer Space of Layered Titanate Transparent Films

Here, we present a protocol for the in situ synthesis of gold nanoparticles (AuNPs) within the interlayer space of layered titanate films without the aggregation of AuNPs. No spectral change was observed even after 4 months. The synthesized material has expected applications in catalysis, photo-catalysis, and the development of cost-effective plasmonic devices.

Combinations of metal oxide semiconductors and gold nanoparticles (AuNPs) have been investigated as new types of materials. The in situ synthesis of AuNPs ...

Ligand-Mediated Nucleation and Growth of Palladium Metal Nanoparticles

The size, size distribution and stability of colloidal nanoparticles are greatly affected by the presence of capping ligands. Despite the key contribution of capping ligands during the synthesis reaction, their role in regulating the nucleation and growth rates of colloidal nanoparticles is not well understood. In this work, we demonstrate a mechanistic investigation of the role of trioctylphosphine (TOP) in Pd nanoparticles in different solvents (toluene and pyridine) using in situ SAXS and ligand-based kinetic modeling. Our results under different synthetic conditions reveal the overlap of nucleation and growth of Pd nanoparticles during the reaction, which contradicts the LaMer-type nucleation and growth model. The model accounts for the kinetics of Pd-TOP binding for both, the precursor and the particle surface, which is essential to capture the size evolution as well as the concentration of particles in situ. In addition, we illustrate the predictive power of our ligand-based model through designing the synthetic conditions to obtain nanoparticles with desired sizes. The proposed methodology can be applied to other synthesis systems and therefore serves as an effective strategy for predictive synthesis of colloidal nanoparticles.

The main goal of this work is to elucidate the role of capping agents in regulating the size of palladium nanoparticles by combining in situ small angle x-ray scattering (SAXS) and ligand-based kinetic modeling.

The size, size distribution and stability of colloidal nanoparticles are greatly affected by the presence of capping ligands. Despite the key contribution ...

A Continuous-flow Photocatalytic Reactor for the Precisely Controlled Deposition of Metallic Nanoparticles

In this work, a novel photocatalytic reactor for the pulsed and controlled excitation of the photocatalyst and the precise deposition of metallic nanoparticles is developed. Guidelines for the replication of the reactor and its operation are provided in detail. Three different composite systems (Pt/graphene, Pt/TiO2, and Au/TiO2) with monodisperse and uniformly distributed particles are produced by this reactor, and the photodeposition mechanism, as well as the synthesis optimization strategy, are discussed. The synthesis methods and their technical aspects are described comprehensively. The role of the ultraviolet (UV) dose (in each excitation pulse) on the photodeposition process is investigated and the optimum values for each composite system are provided.

For a continuous and scalable synthesis of noble-metal-based nanocomposites, a novel photocatalytic reactor is developed and its structure, operation principles, and product quality optimization strategies are described.

In this work, a novel photocatalytic reactor for the pulsed and controlled excitation of the photocatalyst and the precise deposition of metallic ...

Photodeposition of Pd onto Colloidal Au Nanorods by Surface Plasmon Excitation

A protocol is described to photocatalytically guide Pd deposition onto Au nanorods (AuNR) using surface plasmon resonance (SPR). Excited plasmonic hot electrons upon SPR irradiation drive reductive deposition of Pd on colloidal AuNR in the presence of [PdCl4]2-. Plasmon-driven reduction of secondary metals potentiates covalent, sub-wavelength deposition at targeted locations coinciding with electric field &#8220;hot-spots&#8221; of the plasmonic substrate using an external field (e.g., laser). The process described herein details a solution-phase deposition of a catalytically-active noble metal (Pd) from a transition metal halide salt (H2PdCl4) onto aqueously-suspended, anisotropic plasmonic structures (AuNR). The solution-phase process is amenable to making other bimetallic architectures. Transmission UV-vis monitoring of the photochemical reaction, coupled with ex situ XPS and statistical TEM analysis, provide immediate experimental feedback to evaluate properties of the bimetallic structures as they evolve during the photocatalytic reaction. Resonant plasmon irradiation of AuNR in the presence of [PdCl4]2- creates a thin, covalently-bound Pd0 shell without any significant dampening effect on its plasmonic behavior in this representative experiment/batch. Overall, plasmonic photodeposition offers an alternative route for high-volume, economical synthesis of optoelectronic materials with sub-5 nm features (e.g., heterometallic photocatalysts or optoelectronic interconnects).

A protocol for anisotropic photodeposition of Pd onto aqueously-suspended Au nanorods via localized surface plasmon excitation is presented.

A protocol is described to photocatalytically guide Pd deposition onto Au nanorods (AuNR) using surface plasmon resonance (SPR). Excited plasmonic hot ...

Aqueous Synthesis of Plasmonic Gold-Tin Alloy Nanoparticles

This protocol describes the synthesis of Au nanoparticle seeds and the subsequent formation of Au-Sn bimetallic nanoparticles. These nanoparticles have potential applications in catalysis, optoelectronics, imaging, and drug delivery. Previously, methods for producing alloy nanoparticles have been time-consuming, require complex reaction conditions, and can have inconsistent results. The outlined protocol first describes the synthesis of approximately 13 nm Au nanoparticle seeds using the Turkevich method. The protocol next describes the reduction of Sn and its incorporation into the Au seeds to generate Au-Sn alloy nanoparticles. The optical and structural characterization of these nanoparticles is described. Optically, prominent localized surface plasmon resonances (LSPRs) are apparent using UV-visible spectroscopy. Structurally, powder X-ray diffraction (XRD) reflects all particles to be less than 20 nm and shows patterns for Au, Sn, and multiple Au-Sn intermetallic phases. Spherical morphology and size distribution are obtained from transmission electron microscopy (TEM) imaging. TEM reveals that after Sn incorporation, the nanoparticles grow to approximately 15 nm in diameter.

Author Spotlight: Designing Sustainable Nanomaterials for Advancing Synthesis and Element Mixing

Here, the synthesis of gold (Au) seeds is described using the Turkevich method. These seeds are then used to synthesize gold-tin alloy (Au-Sn) nanoparticles with tunable plasmonic properties.

This protocol describes the synthesis of Au nanoparticle seeds and the subsequent formation of Au-Sn bimetallic nanoparticles. These nanoparticles have ...

Preparation of Silver-Palladium Alloyed Nanoparticles for Plasmonic Catalysis under Visible-Light Illumination

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