The authors would like to thank Sabanci University and Swiss Federal Laboratories for Materials Science and Technology (Empa) for all the support provided.

1. Fabrication and operation of the photocatalytic deposition reactor
CAUTION: When UV lamps are turned on, use UV-C protective glasses.
<ol>
	<li>Fabrication of the photocatalytic deposition reactor
	<ol>
		<li>Cover the inner surface of a polyvinyl chloride (PVC) pipe (diameter x length = 15 cm x 55 cm; other materials can also be used) with a thick, polished, and adhesive-backed aluminum foil. Install five 55 W UV-C lamps (see Table of Materials) on the i...

<ol>
	<li>Okitsu, K., Mizukoshi, Y., Ashokkumar, M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Catalytic+Applications+of+Noble+Metal+Nanoparticles+Produced+by+Sonochemical+Reduction+of+Noble+Metal+Ions.">Catalytic Applications of Noble Metal Nanoparticles Produced by Sonochemical Reduction of Noble Metal Ions.</a> Handbook of Ultrasonics and Sonochemistry. , 325-363 (2016).</li><li>Shakoori Oskooie, M., Menceloglu, Y. Z., Unal, S., Hayat Soytas, S. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Rapid+Microwave-assisted+Synthesis+of+Platinum+Nanoparticles+Immobilized+in+Electrospun+Carbon+Nanofibers+for+Electrochemical+Catalysis.">Rapid Microwave-assisted Synthesis of Platinum Nanoparticles Immobilized in Electrospun Carbon Nanofibers for Electrochemical Catalysis.</a> ACS Applied Nano Materials. , (2018).</li><li>Kaplan, B. Y., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Graphene:+A+Promising+Catalyst+Support+for+Oxygen+Reduction+Reaction+in+Polymer+Electrolyte+Membrane+Fuel+Cells.">Graphene: A Promising Catalyst Support for Oxygen Reduction Reaction in Polymer Electrolyte Membrane Fuel Cells.</a> ECS Meeting Abstracts. , (2018).</li><li>Iliev, V., Tomova, D., Bilyarska, L., Eliyas, A., Petrov, L. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Photocatalytic+properties+of+TiO2+modified+with+platinum+and+silver+nanoparticles+in+the+degradation+of+oxalic+acid+in+aqueous+solution.">Photocatalytic properties of TiO2 modified with platinum and silver nanoparticles in the degradation of oxalic acid in aqueous solution.</a> Applied Catalysis B: Environmental. 63, 266-271 (2006).</li><li>Bumajdad, A., Madkour, M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Understanding+the+superior+photocatalytic+activity+of+noble+metals+modified+titania+under+UV+and+visible+light+irradiation.">Understanding the superior photocatalytic activity of noble metals modified titania under UV and visible light irradiation.</a> Physical Chemistry Chemical Physics. 16, 7146 (2014).</li><li>&#350;anl&#305;, L. I., Bayram, V., Yarar, B., Ghobadi, S., G&#252;rsel, S. A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Development+of+graphene+supported+platinum+nanoparticles+for+polymer+electrolyte+membrane+fuel+cells:+Effect+of+support+type+and+impregnation-reduction+methods.">Development of graphene supported platinum nanoparticles for polymer electrolyte membrane fuel cells: Effect of support type and impregnation-reduction methods.</a> International Journal of Hydrogen Energy. 41, 3414-3427 (2016).</li><li>I&#351;&#305;kel &#350;anl&#305;, L., Bayram, V., Ghobadi, S., D&#252;zen, N., G&#252;rsel, S. A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Engineered+catalyst+layer+design+with+graphene-carbon+black+hybrid+supports+for+enhanced+platinum+utilization+in+PEM+fuel+cell.">Engineered catalyst layer design with graphene-carbon black hybrid supports for enhanced platinum utilization in PEM fuel cell.</a> International Journal of Hydrogen Energy. 42, 1085-1092 (2017).</li><li>Dom&#237;nguez-Dom&#237;nguez, S., Arias-Pardilla, J., Berenguer-Murcia, &. #. 1. 9. 3. ;., Morall&#243;n, E., Cazorla-Amor&#243;s, D. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Electrochemical+deposition+of+platinum+nanoparticles+on+different+carbon+supports+and+conducting+polymers.">Electrochemical deposition of platinum nanoparticles on different carbon supports and conducting polymers.</a> Journal of Applied Electrochemistry. 38, 259-268 (2008).</li><li>Pan, X., Xu, Y. -. J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Defect-Mediated+Growth+of+Noble-Metal+(Ag,+Pt,+and+Pd)+Nanoparticles+on+TiO2+with+Oxygen+Vacancies+for+Photocatalytic+Redox+Reactions+under+Visible+Light.">Defect-Mediated Growth of Noble-Metal (Ag, Pt, and Pd) Nanoparticles on TiO2 with Oxygen Vacancies for Photocatalytic Redox Reactions under Visible Light.</a> The Journal of Physical Chemistry C. 117, 17996-18005 (2013).</li><li>Zhang, Y., Zhang, N., Tang, Z. -. R., Xu, Y. -. J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Graphene+Oxide+as+a+Surfactant+and+Support+for+In-Situ+Synthesis+of+Au-Pd+Nanoalloys+with+Improved+Visible+Light+Photocatalytic+Activity.">Graphene Oxide as a Surfactant and Support for In-Situ Synthesis of Au-Pd Nanoalloys with Improved Visible Light Photocatalytic Activity.</a> The Journal of Physical Chemistry C. 118, 5299-5308 (2014).</li><li>Abdolhosseinzadeh, S., Asgharzadeh, H., Sadighikia, S., Khataee, A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=UV-assisted+synthesis+of+reduced+graphene+oxide-ZnO+nanorod+composites+immobilized+on+Zn+foil+with+enhanced+photocatalytic+performance.">UV-assisted synthesis of reduced graphene oxide-ZnO nanorod composites immobilized on Zn foil with enhanced photocatalytic performance.</a> Research on Chemical Intermediates. 42, 4479-4496 (2016).</li><li>Abdolhosseinzadeh, S., Sadighikia, S., G&#252;rsel, S. A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Scalable+Synthesis+of+Sub-Nanosized+Platinum-Reduced+Graphene+Oxide+Composite+by+an+Ultraprecise+Photocatalytic+Method.">Scalable Synthesis of Sub-Nanosized Platinum-Reduced Graphene Oxide Composite by an Ultraprecise Photocatalytic Method.</a> ACS Sustainable Chemistry &amp; Engineering. 6, 3773-3782 (2018).</li><li>Zhang, N., Yang, M. -. Q., Liu, S., Sun, Y., Xu, Y. -. J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Waltzing+with+the+Versatile+Platform+of+Graphene+to+Synthesize+Composite+Photocatalysts.">Waltzing with the Versatile Platform of Graphene to Synthesize Composite Photocatalysts.</a> Chemical Reviews. 115, 10307-10377 (2015).</li><li>Han, C., Zhang, N., Xu, Y. -. J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Structural+diversity+of+graphene+materials+and+their+multifarious+roles+in+heterogeneous+photocatalysis.">Structural diversity of graphene materials and their multifarious roles in heterogeneous photocatalysis.</a> Nano Today. 11, 351-372 (2016).</li><li>Abdolhosseinzadeh, S. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=.">.</a> Bandgap Modulation of Graphene Oxide and Its Application in the Photocatalytic Deposition of Metallic Nanoparticles. , (2017).</li><li>Abdolhosseinzadeh, S., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Fast+and+fully-scalable+synthesis+of+reduced+graphene+oxide.">Fast and fully-scalable synthesis of reduced graphene oxide.</a> Scientific Reports. 5, 10160 (2015).</li><li>Ma, Y., Wei, X. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Determination+of+platinum+in+waste+platinum-loaded+carbon+catalyst+samples+using+microwave-assisted+sample+digestion+and+ICP-OES.">Determination of platinum in waste platinum-loaded carbon catalyst samples using microwave-assisted sample digestion and ICP-OES.</a> AIP Conference Proceedings. 1829 (1), 020008 (2017).</li><li>Sevilla, M., Sanch&#237;s, C., Vald&#233;s-Sol&#237;s, T., Morall&#243;n, E., Fuertes, A. B. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Highly+dispersed+platinum+nanoparticles+on+carbon+nanocoils+and+their+electrocatalytic+performance+for+fuel+cell+reactions.">Highly dispersed platinum nanoparticles on carbon nanocoils and their electrocatalytic performance for fuel cell reactions.</a> Electrochimica Acta. 54, 2234-2238 (2009).</li><li>de S&#225;, D. S., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Intensification+of+photocatalytic+degradation+of+organic+dyes+and+phenol+by+scale-up+and+numbering-up+of+meso-+and+microfluidic+TiO2+reactors+for+wastewater+treatment.">Intensification of photocatalytic degradation of organic dyes and phenol by scale-up and numbering-up of meso- and microfluidic TiO2 reactors for wastewater treatment.</a> Journal of Photochemistry and Photobiology A: Chemistry. 364, 59-75 (2018).</li><li>Kononova, O. N., Leyman, T. A., Melnikov, A. M., Kashirin, D. M., Tselukovskaya, M. M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Ion+exchange+recovery+of+platinum+from+chloride+solutions.">Ion exchange recovery of platinum from chloride solutions.</a> Hydrometallurgy. 100, 161-167 (2010).</li></ol>

The authors have nothing to disclose.

Nanoparticles are the most widely used form of noble-metal-based catalysts. In almost all cases, NNPs are deposited either on a conductive or a semiconductive support material. This hybridization is mostly done by the reduction of the cations of the noble metal in the presence of the intended substrate (material). Hence, a successful synthesis method for the production of NNP-based nanocomposite should meet at least two main requirements: 1) the reduction of the cations should be efficient and complete; 2) the deposition...

Metallic nanoparticles, especially noble metals (e.g., Pt, Au, Pd) have vast applications in catalysis1. In general, decreasing the size of the nanoparticles (NPs) increases their catalytic activity while maintaining the cost (weight) constant, but it also makes their application more difficult. NPs (usually smaller than 10 nm) have great tendencies to aggregation, which degrades their catalytic activity; however, immobilization on suitable substrates can mostly resolve this problem. Furthermore, depending on the application type (e.g., electrocatalysis), it is sometimes necessary to immobilize the NPs on conductive substrates2,3. NPs can also be hybridized with semiconductors to form a Schottky barrier and avoid (delay) the electron-hole recombination (acting as electron traps)4,5. Hence, in most of the applications, the noble metal NPs (NNPs) are deposited either on a conductive (e.g., graphene) or a semiconductive (e.g., TiO2) substrate. In both cases, metal cations are usually reduced in the presence of the substrate, and the reduction technique differs from one method to another.
For the deposition of NNPs via a reduction of their cations, electrons (with proper electrical potential) should be provided. That can be done in two ways: by oxidation of other chemical species (a reducing agent)6,7 or from an external power source8. In any case, for the homogeneous deposition of monodispersed NPs, it is necessary to impose a strict control on the generation and transfer of the (reducing) electrons. This is very difficult when a reducing agent is used since there is virtually no control over the reduction process once the reactants (cations and reducing agent) are mixed. Furthermore, NPs can form anywhere and not necessarily on the target substrate. When using an external power source, the control over the number of the provided electrons is much better, but NPs can only be deposited on an electrode surface.
Photocatalytic deposition (PD) is an alternative approach, which offers more control over the number of the (photo)generated electrons since it is directly related to the dose of the illuminated photons (with a proper wavelength). In this method, the substrate material has a dual role; it provides the reducing electrons9 and stabilizes the formed NPs10. Moreover, NPs form only on the substrate since the electrons are generated by the substrate. A proper electrical connection between composite components (made by the photocatalytic reduction method) is also guaranteed11. Nevertheless, in conventional photocatalytic deposition methods in which the whole batch of reactants (photocatalyst and metal cation) is illuminated simultaneously, there is no control over the nucleation of the NNPs. Indeed, once a few particles (nuclei) are formed, they act as preferred transfer sites for the photogenerated electrons5 and act as a preferred growth site. This superior electron transfer promotes the growth of the existing particles and disfavors the formation of new nuclei, which results in the formation of large NNPs. This problem can be addressed by the pulsed illumination of UV light in a special continuous-flow reactor (Figure 1) that has recently been developed by our group12. The unique feature of this reactor is that it allows researchers to control both NP-size-determining factors, namely, nucleation and growth. In this reactor, a very small portion of reactants is illuminated for a very short period of time, promoting the formation of nuclei (more nuclei are formed) and restricting the growth (smaller particles are attained). In this method, by controlling the illumination dose (i.e., by adjusting the exposure duration [changing the length of uncovered parts of the reaction tube; Figure 1C] or intensity of the incident light [number of the lamps]), a very precise control over the number of photogenerated electrons and, consequently, on the reduction process (NNP deposition) can be exerted.
<img alt="Figure 1" class="xfigimg" src="/files/ftp_upload/58883/58883fig1.jpg" /> 
Figure 1: The fabricated photocatalytic deposition reactor. (A) The reactor. (B) Inside the illumination chamber. (C) A quartz tube with 5 cm x 1 cm illumination exposure length. <a href="https://www.jove.com/files/ftp_upload/58883/58883fig1large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
Despite the great potential of the PD method for the controlled deposition of NNPs, its application is limited to semiconducting materials. Fortunately, it is possible to open a wide band gap in graphene (one of the best-conducting substrates13) by its simple chemical functionalization. Afterward, these functional groups (FGs) can be mostly removed and the resulting graphene will still be conductive enough for most of the applications. Among numerous functionalized derivatives of graphene, graphene oxide (GO), which exhibits considerable semiconducting properties14, is the most promising candidate for this purpose. This is mainly due to the fact that GO&#39;s production has the highest production yield among the others. Nevertheless, since GO consists of different types of FGs, its chemical composition varies continuously under UV illumination. We have recently shown that by a selective removal of weakly bonded FGs (partial reduction; PRGO), the chemical structure and electronic properties of GO can be stabilized, which is an essential requirement for homogeneous depositions of the NNPs12. In this report, we describe the structure of the reactor and provide detailed information for its replication and operation. The deposition mechanism (working mechanism of the reactor) and possible optimization strategies are also discussed in great detail. To validate the applicability of the developed PD reactor for both types of common substrates (conductor and semiconductor) and different NNPs, the deposition of platinum on PRGO and TiO2, as well as of gold on TiO2, is demonstrated. It is noteworthy that by a proper selection of the metal, photocatalyst and precursor materials (e.g., salt, hole scavenger), and the dispersion media, several other metallic particles (such as Ag and Pd15) can also be deposited. In principle-since, in the photodeposition of NNPs, the cations of the metal are reduced by the photoexcited electrons-the energy level of the semiconductor&#39;s conduction band minimum (CBM) should match with (be more negative than) the reduction potential of the aimed cations. Due to the extensive technical production aspects, the synthesis of PRGO is also described in detail. For further information regarding the chemical structure and electronic properties of PRGO, please refer to previous work12.
The detailed structure of the reactor is schematically depicted in Figure 2. The reactor has two main components: a UV illumination and a reservoir compartment. The illumination section consists of a quartz tube, which is exactly fixed along the central axis of a cylindrical tube with a polished aluminum liner. The reservoir consists of a 1 L sealed-cap glass bottle with gas and liquid (reactants) inlets and outlets. Use a silicon septum with an open-top screw cap for inserting the tubes. To take samples during the reaction without letting oxygen enter the reactor, an outlet with a valve is also installed. It should be mentioned here that the samplings on specific time intervals are not a part of the nanocomposite production process, and sampling only needs to be done once to obtain the concentration-time curves for each set of synthesis parameters (the application of these curves will be discussed in the Discussion section). The reservoir is placed inside an ice-water bath while being vigorously mixed on a magnetic stirrer. A magnetic pump circulates the reactant from the reservoir to the reaction chamber (illumination section) and back to the reservoir. A magnetic one is used since high flow rates are necessary (the flow rate in this work = 16 L&#183;min-1) and peristaltic pumps (or other similar pumps) can hardly provide those flows. When using a magnetic pump, care should be taken to completely fill the impeller casing (pump housing) with the reactant liquid and evacuate any trapped air (oxygen source). The trapped air can also decrease the pump&#39;s real flow rate.
For a pulsed excitation of the photocatalyst material, specific lengths of the quartz tube are covered by a thick aluminum foil, leaving equal lengths between them uncovered (Figure 2). The duration of the pulsed excitation can be adjusted by changing the length of the uncovered parts (exposure length). The optimum exposure length is determined by various parameters, such as the quantum yield of the photocatalyst and the intended NP loading (concentration of the precursors; see Discussion).

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	<tbody>
		<tr height="21">
			<td height="21" style="height:21px;width:431px;">Chloroplatinic acid solution</td>
			<td style="width:219px;">Sigma Aldrich</td>
			<td style="width:272px;">262587-50ML</td>
			<td style="width:303px;"></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:431px;">Hydrogen tetrachloroaurate(III) hydrate</td>
			<td style="width:219px;">Alfa Aesar</td>
			<td style="width:272px;">12325.03</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:431px;">TiO2 Nanopowder (TiO2, anatase, 99.9%, 100nm)</td>
			<td style="width:219px;">US research nanomaterials</td>
			<td style="width:272px;">US3411</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:431px;">Graphite powder</td>
			<td style="width:219px;">Alfa Aesar</td>
			<td style="width:272px;">10129</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:431px;">Sulfuric acid&#160;</td>
			<td style="width:219px;">Sigma Aldrich</td>
			<td style="width:272px;">1120802500</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:431px;">Hydrogen peroxide</td>
			<td style="width:219px;">Sigma Aldrich</td>
			<td style="width:272px;">H1009-100ML</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:431px;">L-Ascorbic acid</td>
			<td style="width:219px;">Sigma Aldrich</td>
			<td style="width:272px;">A92902-500G</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:431px;">Hydrochloric acid</td>
			<td style="width:219px;">Sigma Aldrich</td>
			<td style="width:272px;">320331-2.5L</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:431px;">Sodium hydroxide</td>
			<td style="width:219px;">Sigma Aldrich</td>
			<td style="width:272px;">S5881-1KG</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:431px;">Potassium permanganate</td>
			<td style="width:219px;">Merck</td>
			<td style="width:272px;">1050821000</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:431px;">Corning&#174;&#160;Silicone Septa for GL45 Screw Cap</td>
			<td style="width:219px;">Sigma Aldrich (Corning)</td>
			<td style="width:272px;">CLS139545SS</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:431px;">Polyvinyl chloride pipe</td>
			<td style="width:219px;">Koctas</td>
			<td style="width:272px;"></td>
			<td>UV-Reactor casing</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:431px;">Fuded silica (Quartz) tube</td>
			<td style="width:219px;">Technical Glass Products</td>
			<td style="width:272px;"></td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:431px;">UV&#8722;C lamps&#160;</td>
			<td style="width:219px;">Philips</td>
			<td style="width:272px;">TUV PL-L 55W/4P HF 1CT/25</td>
			<td></td>
		</tr>
	</tbody>
</table>

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.

XPS is one of the most powerful techniques for confirming the formation of metallic NPs and study their chemical states. For this purpose, both survey spectra and high-resolution spectra (of Pt4f and Au4f) were recorded, which confirms the complete reduction of the metallic cations and successful deposition of the NNPs (Figure 3). For the deconvolution of both Pt4f and Au4f, initially, a Shirley background subtracti...

Watch this Scientific Journal Video about A Continuous-flow Photocatalytic Reactor for the Precisely Controlled Deposition of Metallic Nanoparticles at JoVE.com

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

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 ...

Composites of noble metals with conductor or semiconductor materials have vast applications ranging from medical technology to energy conversion systems, but their conventional synthesis methods are not suitable for commercial production. For instance, microwave reduction is a powerful technique, but it can only process a few milliliters in each batch, while commercial production of graphene/platinum-based fuel cells would need to process several liters of suspension in minutes. To address this problem, we have developed a photocatalytic deposition reactor that is easily upscalable and can operate continuously.In our photodeposition system, small portions of the reactants are illuminated for short, adjustable periods of time. This lets us efficiently control the nucleation and growth processes even when scaling up the reactor. To begin, cover the inner surface of a 15-centimeter-by-55-centimeter polyvinyl chloride pipe with thick, polished, adhesive-backed aluminum foil.Space five, 55-watt UV-C lamps evenly around the inside of the pipe. Then, wrap a 0.5-centimeter-by-55-centimeter quartz tube with pieces of adhesive-backed aluminum foil to form several equally wide windows spaced evenly in the middle of the tube. Leave 2.5 centimeters of the tube exposed at each end.Install the quartz tube in the center of the PVC tube to form the illumination chamber, and connect opaque plastic tubing to each end. Then, mount the pipe vertically in a fume hood, and fix a heavy-duty fan at the lower end for cooling, Next, set up a magnetic drive pump on a raised platform with the bottom of the pump above the top of a magnetic stir plate. Clamp a one-liter separatory funnel just higher than the pump.Use a T-fitting and opaque plastic tubing to connect the separatory funnel outlet to the top of the quartz tube and the magnetic pump inlet. Then, insert the tubing from the bottom of the quartz tube and the pump outlet through the septum of a one-liter bottle equipped with a stir bar so that the ends of the tubing are two to three centimeters above the stir bar. This bottle will be the reservoir.Next, connect a T-fitting to the sampling and evacuation valves and a length of opaque plastic tubing. Insert the tubing into the reservoir, and connect the sampling valve to a syringe fitting. Lastly, run a gas exhaust line from the reservoir to a water bubbler.To begin the synthesis, add two grams of graphite and 100 milliliters of 98%by weight sulfuric acid to a 500-milliliter Erlenmeyer flask equipped with a large magnetic stir bar. Cool the mixture to about zero degrees Celsius in an ice-water bath while stirring. Then, add one gram of potassium permanganate over the course of one minute while stirring.Let the mixture continue stirring for another four minutes. Repeat this process five more times to add a total of six grams over the course of 30 minutes. Once addition is complete, remove the bath, and continue stirring the mixture for six hours.Then, cool the mixture in an ice-water bath while stirring for 15 minutes. Add 250 milliliters of distilled water dropwise to the stirring mixture. Remove the ice-water bath, and add 10 milliliters of hydrogen peroxide dropwise to the stirring mixture.Then, add 20 milliliters of hydrogen peroxide all at once, and continue stirring at room temperature for 30 minutes. Centrifuge the resulting suspension at 3, 500 g for 15 minutes, and discard the supernatant. Stir the solids in one liter of distilled water for 30 minutes, centrifuge them again, remove the supernatant, and check its pH.Wash the solids in this way until the supernatant pH reaches five. Then, combine the washed precipitate with 500 milliliters of one-molar hydrochloric acid, and stir for one hour. Wash the product until the supernatant pH reaches five.Disperse the washed solids in one liter of distilled water, and sonicate the mixture for three hours at about room temperature. Centrifuge the resulting graphene oxide mixture for 15 to 20 minutes at 3, 500 g at room temperature three times, discarding the precipitate each time. Transfer the graphene oxide suspension to a tinted or foil-wrapped glass bottle.Next, weigh three dry crystallization dishes on an analytical balance three times each. Shake the graphene oxide suspension well, and let it settle for one minute. Then, pour exactly 100 milliliters of the suspension into each dish.Keep the dishes in an oven at 70 to 80 degrees Celsius until their contents are completely dry. Then, weigh each dish three times on the same analytical balance, and calculate the dry weight of graphene oxide in each dish. Determine the concentration of the suspension from the average of these values.Then, dilute the suspension to 0.2 grams per liter, and slowly add it to an equal volume of four-molar sodium hydroxide. Reflux the mixture at 90 degrees Celsius for eight hours, and let it cool to room temperature. Wash the partially reduced graphene oxide precipitate until the supernatant reaches pH seven to eight, and then store it in a tinted or foil-wrapped bottle.Prepare 540 milliliters of a 50-milligram-per-milliliter suspension of partially reduced graphene oxide in distilled water, and sonicate it in an ice-water bath for one hour at 40%power. Then, add 60 milliliters of analytical-grade ethanol, and sonicate the mixture in an ice-water bath for another hour. Next, add 169 microliters of 8%by weight aqueous hexachloroplatinic acid to the graphene mixture, and continue stirring at room temperature for 15 minutes.Then, pour the mixture into the separatory funnel of the reactor and stopper it. Open the stopcock to feed the reactant into the system, and immediately close it when the funnel is empty to keep gas bubbles out of the tubing. Run the pump at 16 liters per minute, and flow nitrogen gas through the reactor at a rate that produces a steady stream of bubbles.Stir the suspension at about 1, 000 rpm for 30 minutes to remove dissolved oxygen. Then, reduce the nitrogen flow, and turn on the UV lamps. After five minutes, increase the nitrogen flow, and connect a 20-milliliter Luer-Lok syringe to the sample line.Lightly hold the syringe plunger to keep it from being ejected, and open the sampling valve. Collect 20 milliliters of the suspension, close the sampling valve, and detach the syringe. Open the tube evacuation valve to flush the tubing, and then close the valve and reduce the nitrogen gas flow to its previous level.Centrifuge the sample at 10, 000 g for 10 minutes, and set aside the supernatant for metal cation concentration analysis. Wash the solids twice by stirring in 20-milliliter portions of distilled water and centrifuging under the same conditions. After that, disperse the solids in 50 milliliters of distilled water by gentle sonication for one hour.Dissolve one milligram of ascorbic acid in the dispersion, and stir the mixture at 90 degrees Celsius for one hour. Collect and wash the precipitate as previously described, and store the washed precipitate for further characterization. X-ray photoelectron spectroscopy confirmed that platinum and gold nanoparticles were successfully deposited on reduced graphene oxide and titanium dioxide.Deconvolution of the high-resolution platinum 4f and gold 4f peaks showed no non-metallic components, indicating that the platinum four and gold three cations had been reduced to platinum zero and gold zero, respectively. The illumination dose per exposure, or IDE, was optimized for each composite to achieve a relatively uniform distribution of small noble metal nanoparticles over the substrates. When the IDE was too high, depletion around growing particles disfavored the formation of nuclei nearby, resulting in large particles and wide size distributions.The IDE was adjusted by altering the number of UV lamps and the exposure area of the reactor. When the IDE was too low, photodeposition was unsuccessful, owing to insufficient photoexcited electrons for stable nucleus formation, as seen here. Widening the windows to two centimeters produced the desired photodeposition behavior for the platinum/graphene composite.Exposing the entire reactor tube increased the photodeposition rate without significantly compromising the particle size and monodispersity. The deposition process is controlled by plotting concentration time curves under various conditions to identify the optimal reactor design and IDE. Conducting the photodeposition in the linear concentration time region allows continuous collection of the product after a known circulation time.Although we demonstrate the synthesis of platinum/graphene composites, this method and reactor can be used for deposition of other noble metals on various semiconductors too. This continuous-flow reactor can also be used for other chemical synthesis methods in which the reactions are initiated with light.

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Research

JoVE Journal

Chemistry

9.6K vistas.  École Polytechnique Fédérale de Lausanne (EPFL). Los compuestos de metales nobles con materiales conductores o semiconductores tienen grandes aplicaciones que van desde la tecnología médica hasta los sistemas de conversión de energía, pero sus métodos de síntesis convencionales no son adecuados para la producción comercial. Por ejemplo, la reducción de microondas es una técnica poderosa, pero sólo puede procesar unos pocos mililitros en cada lote, mientras que la producción comercial de pilas de combustible basadas en grafeno/pla...