Name: photochemical oxidative growth of iridium oxide nanoparticles on cdse@cds nanorods
Uploaded: 2016-02-11T15:00:00.000+00:00
Duration: 5 min 41 s
Description: Watch this Scientific Journal Video about Photochemical Oxidative Growth of Iridium Oxide Nanoparticles on CdSe@CdS Nanorods at JoVE.com

This research was supported by the I-CORE Program of the Planning and Budgeting Committee and The Israel Science Foundation (Grant No. 152/11). We thank the Schulich Faculty of Chemistry and the Technion - Israel Institute of Technology for the renovated laboratory and startup package. We also thank the Royal Society of Chemistry for permission in adapting materials from http://dx.doi.org/10.1039/C4TA06164K for use in this manuscript. Dr. Kalisman thanks the Schulich postdoctoral fellowship for their support. We thank Dr. Yaron Kauffmann for his assistance with HR-TEM and HAADF as well as Dr. Kamira Weinfeld for her assistance with XPS characterization.

1. Synthesis of Quantum Dots21
<ol>
	<li>Preparation of TOP:Se Precursor
	<ol>
		<li>Combine 58 mg of Se powder with 0.360 g of Tri-n-octylphosphine (TOP) in a vial with a septum.</li>
		<li>Sonicate the TOP:Se mixture until it is clear with no solids.</li>
	</ol>
	</li>
	<li>Synthesis of CdSe
	<ol>
		<li>Combine 3.0 g trioctylphosphine oxide (TOPO), 280 mg n-octadecylphosphonic acid (ODPA), and 60 mg CdO with a 3 mm x 8 mm cylindrical stir bar in a 25 ml 3-neck round bottom flask equipped with a thermocouple (inserted in a custom glass adapter), a reflux condenser with a T-joint (center neck), and a rubber septum. Assemble all glass-to-glass joints with high temperature vacuum grease. Connect the T-joint to a Schlenk line on one end that can be switched between a clean inert gas and vacuum, while connecting the other end to a bubbler. 
		Caution: CdO is very toxic and should be weighed and added to the round bottom flask inside an enclosed environment such as a glovebox.</li>
		<li>Place the round bottom flask apparatus in a heating mantle, and purge with inert gas.</li>
		<li>Heat the solids in the round bottom flask to 150 &#176;C, making sure to start stirring vigorously once the compounds melt (around 60-80 &#176;C). In order to avoid overshooting the target temperature, heat to 100 &#176;C and then 150 &#176;C once the heating rate slows down or stabilizes.</li>
		<li>Degas the mixture under vacuum (at 150 &#176;C) for at least 1 hr while still stirring. Make sure the T-joint is not open to the bubbler or the oil will get sucked into the flask and Schlenk line. When moving from gas to vacuum be careful to transition slowly to avoid more than a rolling boil.</li>
		<li>Refill the flask with inert gas (and continue to flow gas over the sample) and increase the temperature to 350 &#176;C. The solution should turn clear as it heats up, and excess solids on the flask wall can be collected by careful swirling of the flask.</li>
		<li>Inject 1.5 g of TOP into the flask through the septum. Allow the solution temperature to stabilize before proceeding. 
		Note: Try to minimize the amount of air/moisture injected by keeping the TOP in a septa vial under inert atmosphere, and injecting it as quickly as possible.</li>
		<li>Inject all of the TOP:Se mixture (prepared in section 1.1) into the flask through the septum, using a wide needle to inject the TOP:Se as quickly and uniformly as possible.</li>
		<li>Let the reaction proceed for the desired time, and remove from the heating mantle.
		<ol>
			<li>For very small seeds remove from heat just before injecting. For larger seeds remove the flask from heat immediately after injecting TOP:Se or after waiting up to 3 min. Longer wait result in larger seeds.</li>
		</ol>
		</li>
		<li>Let the reaction cool to approximately 100 &#176;C and inject approximately 5 ml of degassed toluene. Transfer the solution to a vial under inert atmosphere for cleaning. 
		Note: To simplify this process 10 ml of toluene can be placed in a 20 ml vial with a septa under constant flow of inert gas. Use approximately half of this toluene to inject into the cooling mixture, and then transfer the cooled mixture back into this vial.</li>
		<li>Cleaning of seeds
		<ol>
			<li>Put the solution in a 50 ml centrifuge tube.</li>
			<li>Add methanol (approximately 5 ml) to precipitate out the seeds from the toluene mixture.</li>
			<li>Centrifuge at 3,400 x g for 5 min.</li>
			<li>Decant clear supernatant and re-dissolve the pellet in toluene (5-10 ml).</li>
			<li>Repeat steps 1.2.10.2 through 1.2.10.4 at least three times total.</li>
		</ol>
		</li>
		<li>Dilute a small aliquot of the seeds in toluene in order to measure Ultraviolet-Visible (UV-Vis) absorbance between 350-800 nm. Use the peaks to determine the concentration and size of the CdSe seeds as described in the literature22.</li>
	</ol>
	</li>
</ol>
2. Synthesis of Seeded Rods 21
<ol>
	<li>Preparation of TOP:S Precursor
	<ol>
		<li>Combine 1.2 g of S with 15 g of TOP in a vial with a stir bar.</li>
		<li>Stir until clear with no solids (usually at least 24 hr).</li>
		<li>Measure 0.62 g of this mixture into a vial with a septum.</li>
	</ol>
	</li>
	<li>Preparation of TOP:CdSe Precursor
	<ol>
		<li>Measure the appropriate volume of CdSe seeds from step 1 (based on UV-Vis peak) into a vial with a septum. 
		Note: For a calculated concentration of 5x10-5 with 2.25 nm seeds (both values calculated from the UV-Vis spectra22), use 300 &#181;l of solution.</li>
		<li>Evaporate the toluene using a vacuum line until the seeds are dry. Do not leave under vacuum for more than 5-10 min once dry, as this can degrade the quality of the seeds.</li>
		<li>Re-dissolve all of the dried seeds in 0.5 g of TOP.</li>
	</ol>
	</li>
	<li>Synthesis of CdSe@Cds
	<ol>
		<li>Combine 60 mg propylphosphonic acid (PPA), 3.35 g TOPO, 1.080 g ODPA, and 230 mg CdO with a 3 mm x 8 mm cylindrical stir bar in a 25 ml 3-neck round bottom flask equipped with a thermocouple (inserted in a custom glass adapter), a reflux condenser with a T-joint (center neck), and a rubber septum. Assemble all glass-to-glass joints with high temperature vacuum grease. Connect the T-joint to a Schlenk line on one end that can be switched between a clean inert gas and vacuum, while connecting the other end to a bubbler. 
		Caution: CdO is very toxic and should be weighed and added to the round bottom inside an enclosed environment, such as a glovebox. PPA is regulated in some countries and can be replaced by butylphosphonic acid (BPA, 72 mg) or hexylphosphonic acid (HPA, 80 mg), though BPA and HPA usually result in shorter rods.</li>
		<li>Place the round bottom flask apparatus in a heating mantle, and purge with inert gas.</li>
		<li>Heat the solids in the round bottom flask to 120 &#176;C, making sure to start stirring vigorously once the compounds melt (around 60-80 &#176;C). In order to avoid overshooting the target temperature, heat to 90 &#176;C and then 120 &#176;C once the heating rate slows down or stabilizes.</li>
		<li>Degas the mixture under vacuum (at 120 &#176;C) for at least &#189; hr while still stirring.
		<ol>
			<li>Make sure the T-joint is not open to the bubbler or the oil will get sucked into the flask and Schlenk line. When moving from gas to vacuum be careful to transition slowly to avoid more than a rolling boil. Use a cold trap with liquid nitrogen (LN2) for a better vacuum.</li>
		</ol>
		</li>
		<li>Refill the flask with inert gas (and continue to flow gas over the sample) and increase the temperature to 320 &#176;C. The solution should turn clear as it heats up, and excess solids on the flask wall can be collected by careful swirling of the flask.</li>
		<li>Cool back down to 120 &#176;C and degas under vacuum as in step 1.2.4.</li>
		<li>Refill and reheat the flask as in step 1.2.5.</li>
		<li>Inject 1.5 g of TOP into the flask through the septum. Allow the solution temperature to stabilize at 340 &#176;C before proceeding. 
		Note: Try to minimize the amount of air/moisture injected by keeping the TOP in a septa vial under inert atmosphere, and injecting it as quickly as possible.</li>
		<li>Inject the TOP:S mixture into the flask through the septum, using a wide needle to inject the TOP:S as quickly and uniformly as possible. Start a timer.</li>
		<li>Exactly 20 sec after injecting the TOP:S, inject the TOP:CdSe mixture into the flask through the septum, using a wide needle to inject the TOP:CdSe as quickly and uniformly as possible. 
		Note: The temperature should have dropped to below 330 &#176;C by this point due to the addition of RT TOP solutions.</li>
		<li>Set the temperature to 320 &#176;C and let the reaction proceed for the desired time (8-15 min), and remove from the heating mantle.</li>
		<li>Let the reaction cool away from the heating mantle and inject approximately 5 ml of degassed toluene when the temperature reaches approximately 100 &#176;C. Transfer the solution to a vial under inert atmosphere for cleaning. 
		Note: To simplify this process 10 ml of toluene can be placed in a 20 ml vial with a septa under constant flow of inert gas. Use approximately half of this toluene to inject into the cooling mixture, and then transfer the cooled mixture back into this vial.</li>
		<li>Cleaning of rods
		<ol>
			<li>Put the solution in a 50 ml centrifuge tube.</li>
			<li>Add methanol (approximately 5 ml) to precipitate out the seeds from the toluene mixture.</li>
			<li>Centrifuge at 3,400 x g for 5 min.</li>
			<li>Decant clear supernatant and re-dissolve the pellet in approximately 10 ml hexane.</li>
			<li>Add 1-2 ml each of n-octylamine and nonanoic acid to the solution. The solution should be transparent.</li>
			<li>Add 5 ml methanol and centrifuge for 5 min at 3,400 x g.</li>
			<li>Repeat steps 2.3.13.4 through 2.3.13.6 at least twice more.</li>
			<li>Re-dissolve pellet in 10 ml of toluene. If the pellet does not easily dissolve, more cleaning steps are likely needed, in which case repeat steps 2.3.13.4 through 2.3.13.6.</li>
			<li>Add approximately 7 ml of IPA, 1 ml at a time, until the solution is slightly cloudy even when mixed.</li>
			<li>Centrifuge for 30 min at 2,200 x g in order to separate longer rods from everything else.</li>
			<li>Re-dissolve pellet in 10-15 ml of toluene.</li>
		</ol>
		</li>
		<li>Dilute a small aliquot of the seeds in toluene in order to measure UV-Vis absorbance and/or photoluminescence (PL) of the rods. 
		Note: Any aliquot volume is acceptable as long as the dilution factor is known, however, a typical dilution factor would be 20. For PL, the absorption should be at or below 0.1 at the chosen excitation wavelength (typically use 450 nm).&#160;</li>
	</ol>
	</li>
</ol>
3. Transfer of Seeded Rods to Aqueous Solution
<ol>
	<li>Preparation of Methanol Solution
	<ol>
		<li>Pour approximately 10 ml of methanol into a centrifuge tube.</li>
		<li>Add approximately 250 mg mercaptoundecanoic acid (MUA) and 400 mg tetramethylammonium hydroxide (TMAH).</li>
		<li>Vortex or let sit until all solids fully dissolved.</li>
	</ol>
	</li>
	<li>Ligand Exchange
	<ol>
		<li>Add methanol (5-10 ml, or enough to precipitate the rods) to &#188; to &#189; of rods synthesized from step 2 in a centrifuge tube. 
		Note: The volume of rods used will be dependent on the amount of toluene used to dissolve the rods for storage. If 10-15 ml are used as suggested in step 2.3.13.11, then 3-6 ml of the rod solution should be appropriate.</li>
		<li>Centrifuge at 3,400 x g for 5 min.</li>
		<li>Decant the clear supernatant.</li>
		<li>Add all of the methanol solution from section 3.1 to the pellet.</li>
		<li>Vortex or shake by hand to fully dissolve. Allow the solution to sit at least 1 hr to allow maximum ligand exchange to occur. 
		Note: The solution should be allowed to sit for at least 1 hr even if it appears to dissolve immediately and completely</li>
		<li>Separate the solution into two halves in two centrifuge tubes. 
		Note: Save the centrifuge tube used for step 3.1 and transfer half of the solution into there.</li>
		<li>Add 20 ml of toluene to each half. If there is a phase separation between the alcohol and toluene add methanol drop wise until the phases recombine.</li>
		<li>Centrifuge at 7,700 x g for 15 min.</li>
		<li>Very carefully decant the clear supernatant from pellet.</li>
		<li>Invert the centrifuge tube carefully in order to dry the sample.</li>
		<li>Add 5 ml ultrapure water to the pellet and store in a vial wrapped well in Al foil (or other opaque covering). 
		Note: Once moved from toluene to water, the rods are not photostable, which is why they are covered. Even if kept in the dark the rods should be used as soon as possible, and it is not recommended to store rods in water for more than one month.</li>
	</ol>
	</li>
</ol>
4. Growth of Iridium Nanocrystalline Particles
<ol>
	<li>Sodium Hydroxide
	<ol>
		<li>In a plastic vial weigh out 1,450 mg NaOH. Dissolve NaOH in 20 ml ultrapure water. Solution should be clear with no solids.&#160;</li>
	</ol>
	</li>
	<li>Sodium Persulfate
	<ol>
		<li>In a plastic vial weigh out 950 mg NaS2O8. Dissolve NaS2O8 in 20 ml ultrapure water. Solution should be clear with no solids.</li>
	</ol>
	</li>
	<li>Sodium Nitrate
	<ol>
		<li>In a plastic vial weigh out 300 mg NaNO3. Dissolve NaNO3 in 18 ml ultrapure water. Solution should be clear with no solids.</li>
	</ol>
	</li>
	<li>Iridium Precursor Solution
	<ol>
		<li>In a plastic vial weigh out 50 mg Na3IrCl6. Dissolve Na3IrCl6 in 5.0 ml ultrapure water. Solution should be transparent brown (like scotch) with no solids.</li>
	</ol>
	</li>
	<li>Preparation of sample
	<ol>
		<li>Place a spectroscopic stirrer in a standard polystyrene cuvette. 
		Note: Because the solution is very basic, quartz and other glass cuvettes should not be used.</li>
		<li>Add 0.20 ml iridium precursor solution from step 4.4.1.</li>
		<li>Add 0.50 ml nitrate solution from step 4.3.1.</li>
		<li>Add 0.30 ml of seeded rods in water from section 3 (trace toluene from the ligand exchange may cause clouding of the cuvette wall).</li>
		<li>Add 0.50 ml of persulfate solution from step 4.2.1.</li>
		<li>Add 0.50 ml of sodium hydroxide solution from step 4.1.1.</li>
	</ol>
	</li>
	<li>Illumination of sample
	<ol>
		<li>Place the cuvette in a holder with stirring capabilities.</li>
		<li>Illuminate with 450 nm light at 100 mW for up to 4 hr. The solution should turn green and later blue.</li>
	</ol>
	</li>
	<li>Collecting sample
	<ol>
		<li>Pour solution (but not stir bar) into a centrifuge tube.</li>
		<li>Centrifuge at 7,700 x g for 10 min.</li>
		<li>Carefully decant the supernatant from the pellet, which should be green or blue depending on the reaction time selected. 
		Note: The pellet can now be collected or dispersed in a polar solvent through sonication for use in other experiments.</li>
	</ol>
	</li>
</ol>

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P. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Luminescence+Studies+of+Individual+Quantum+Dot+Photocatalysts.">Luminescence Studies of Individual Quantum Dot Photocatalysts.</a> J. Am. Chem. Soc. 135 (35), 13049-13053 (2013).</li><li>Vaneski, A., Schneider, J., Susha, A. S., Rogach, A. L. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Aqueous+synthesis+of+CdS+and+CdSe/CdS+tetrapods+for+photocatalytic+hydrogen+generation.">Aqueous synthesis of CdS and CdSe/CdS tetrapods for photocatalytic hydrogen generation.</a> APL Materials. 2 (1), 012104 (2014).</li></ol>

The authors have nothing to disclose.

The synthesis of CdSe seeds and CdSe@CdS seeded rods has been well studied21,24,25. Slight modifications to the amounts, temperatures, and times for steps of the synthesis of these substrate particles can be used to tune their length, diameter, and/or morphology. The synthetic protocol described herein yields highly photoluminescent seeded-rods of uniform dimensions.
The ligand exchange procedure allows for the use of seeded rods in polar environments, in this case water. At the fin...

Photocatalysis presents an attractive and promising solution for renewable energy generation and other environmental applications such as water treatment and air purification1-3. Overall water splitting, driven by solar energy, could be a source of clean and renewable hydrogen fuel; however, despite decades of research, systems that are sufficiently stable and efficient for practical use have not yet been realized.
Both photodeposition and semiconductor-mediated photocatalysis rely on the same mechanism of separating photo-generated electron-hole pairs and driving them to the surface where they can initiate redox reactions. The similarities between these two processes make photodeposition an attractive synthetic tool for the field of photocatalysis4-6. This method is expected to take photocatalyst production to new and unexplored frontiers. It might potentially offer pristine control over the spatial arrangement of the different components in a heterostructures, and advance the ability to construct sophisticated nanoparticle systems. Ultimately the method will bring us one step closer to realizing an efficient photocatalyst for direct solar-to-fuel energy conversion.
We investigated the growth of IrO2 as a co-catalyst, as it is known to be an efficient catalyst for water oxidation7-11. A tunable structure of quantum dot (CdSe) embedded in a rod (cadmium sulfide)12,13 was used as our photocatalyst substrate14,15. It is currently undetermined whether the oxidative pathway occurs via a mediated pathway, or by a direct hole attack. Here, our knowledge and control over the photogenerated holes in the semiconductor heterostructure can be harnessed for a mechanistic study of oxidation reactions. This is made possible by the substrate architecture, which facilitates localization of confined holes16,17 and formation of a distinct oxidation reaction site on the rod. The use of nanoscale materials with localized charge carrier can be exploited for mechanistic studies of redox reactions by simple examination of the products. In this way photodeposition can be used as a unique probe of both reduction and oxidation reaction pathways. This is one example of the new and exciting possibilities afforded by the combination of photodeposition and cutting edge colloidal synthesis18-20.
The quest to develop an efficient photocatalyst for water splitting and renewable energy conversion has become an important thrust within the materials community. This has spurred worldwide interest in CdS, which is known to be highly active for hydrogen production, though it is hampered by photochemical instability. Our work here treats the Achilles heel of the material. IrO2 decorated CdSe@CdS rods demonstrate remarkable photochemical stability under prolonged illumination in pure water.

<table><tbody><tr><td>Sulfur (S)</td><td>Sigma</td><td>84683</td><td></td></tr><tr><td>Selenium (Se)</td><td>Sigma</td><td>229865</td><td></td></tr><tr><td>Cadmium Oxide (CdO)</td><td>Sigma</td><td>202894</td><td>Highly Toxic</td></tr><tr><td>n-Octadecylphosphonic acid (ODPA)</td><td>Sigma</td><td>715166</td><td></td></tr><tr><td>Propylphosphonic acid (PPA)</td><td>Sigma</td><td>305685</td><td>Highly regulated in some countries and regions</td></tr><tr><td>Butylphosphonic acid (BPA)</td><td>Sigma</td><td>737933</td><td>Alternative to PPA</td></tr><tr><td>Hexylphosphonic acid (HPA)</td><td>Sigma</td><td>750034</td><td>Alternative to PPA</td></tr><tr><td>Trioctylphosphonic oxide (TOPO)</td><td>Sigma</td><td>346187</td><td></td></tr><tr><td>Tri-n-octylphosphine, 97% (TOP)</td><td>Sigma</td><td>718165</td><td>Air sensitive</td></tr><tr><td>Spectrochemical Stirbar</td><td>Sigma</td><td>Z363545</td><td></td></tr><tr><td>Sodium Hydroxide</td><td>Sigma</td><td>S5881</td><td></td></tr><tr><td>Methanol</td><td>Sigma</td><td>322415</td><td></td></tr><tr><td>Toluene</td><td>Sigma</td><td>244511</td><td></td></tr><tr><td>Hexane</td><td>Sigma</td><td>296090</td><td></td></tr><tr><td>Octylamine</td><td>Sigma</td><td>74988</td><td></td></tr><tr><td>Nonanoic Acid</td><td>Sigma</td><td>N5502</td><td></td></tr><tr><td>Isopropanol</td><td>Sigma</td><td>278475</td><td></td></tr><tr><td>Mercaptoundecanoic Acid (MUA)</td><td>Sigma</td><td>674427</td><td></td></tr><tr><td>Tetramethylammonium Hydroxide (TMAH)</td><td>Sigma</td><td>T7505</td><td></td></tr><tr><td>Apiezon H Grease (high temperature grease)</td><td>Sigma</td><td>Z273562</td><td></td></tr><tr><td>Sodium Persulfate</td><td>Sigma</td><td>216232</td><td></td></tr><tr><td>Sodium Nitrate</td><td>Sigma</td><td>229938</td><td></td></tr><tr><td>Sodium Hexachloroiridate(III) hydrate</td><td>Sigma</td><td>288160</td><td></td></tr><tr><td>Mounted 455 nm LED</td><td>Thorlabs</td><td>M455L3</td><td></td></tr><tr><td>Cuvette Holder</td><td>Thorlabs</td><td>CVH100</td><td></td></tr><tr><td>25 ml 3-neck Round Bottom Flask</td><td>Chemglass</td><td>CG-1524-A-02</td><td></td></tr><tr><td>Liebig Condensor</td><td>Chemglass</td><td>CG-1218-A-20</td><td></td></tr><tr><td>T-Joint Adapter</td><td>Chemglass</td><td>AF-0509-10</td><td></td></tr></tbody></table>

photochemical oxidative growth of iridium oxide nanoparticles on cdse@cds nanorods

We demonstrate a procedure for the photochemical oxidative growth of iridium oxide catalysts on the surface of seeded cadmium selenide-cadmium sulfide (CdSe@CdS) nanorod photocatalysts. Seeded rods are grown using a colloidal hot-injection method and then moved to an aqueous medium by ligand exchange. CdSe@CdS nanorods, an iridium precursor and other salts are mixed and illuminated. The deposition process is initiated by absorption of photons by the semiconductor particle, which results with formation of charge carriers that are used to promote redox reactions. To insure photochemical oxidative growth we used an electron scavenger. The photogenerated holes oxidize the iridium precursor, apparently in a mediated oxidative pathway. This results in the growth of high quality crystalline iridium oxide particles, ranging from 0.5 nm to about 3 nm, along the surface of the rod. Iridium oxide grown on CdSe@CdS heterostructures was studied by a variety of characterization methods, in order to evaluate its characteristics and quality. We explored means for control over particle size, crystallinity, deposition location on the CdS rod, and composition. Illumination time and excitation wavelength were found to be key parameters for such control. The influence of different growth conditions and the characterization of these heterostructures are described alongside a detailed description of their synthesis. Of significance is the fact that the addition of iridium oxide afforded the rods astounding photochemical stability under prolonged illumination in pure water (alleviating the requirement for hole scavengers).

Transmission electron micrographs (TEM) were collected in order to see the distribution of iridium oxide on the seeded rods (Figure 1). TEM samples were prepared by pipetting a drop of dissolved particles onto a TEM grid. X-ray diffraction (XRD, Figure 2) and X-ray photoelectron spectra (XPS, Figure 3) were used to characterize the observed growth as a mix of crystalline IrO2 and Ir2O3. Preparation of XRD ...

Watch this Scientific Journal Video about Photochemical Oxidative Growth of Iridium Oxide Nanoparticles on CdSe@CdS Nanorods at JoVE.com

Photochemical Oxidative Growth of Iridium Oxide Nanoparticles on CdSe@CdS Nanorods

A protocol for the photochemical oxidative growth of small crystalline iridium oxide nanoparticles on the surface of CdSe@CdS seeded rod nanoparticles is presented.

We demonstrate a procedure for the photochemical oxidative growth of iridium oxide catalysts on the surface of seeded cadmium selenide-cadmium sulfide ...

photochemical-oxidative-growth-iridium-oxide-nanoparticles-on-cdsecds

Research

JoVE Journal

Chemistry

9.5K Views.  Technion – Israel Institute of Technology. A protocol for the photochemical oxidative growth of small crystalline iridium oxide nanoparticles on the surface of CdSe@CdS seeded rod nanoparticles is presented.

Video: Photochemical Oxidative Growth of Iridium Oxide Nanoparticles on CdSe@CdS Nanorods

Compact Quantum Dots for Single-molecule Imaging

Single-molecule imaging is an important tool for understanding the mechanisms of biomolecular function and for visualizing the spatial and temporal heterogeneity of molecular behaviors that underlie cellular biology 1-4. To image an individual molecule of interest, it is typically conjugated to a fluorescent tag (dye, protein, bead, or quantum dot) and observed with epifluorescence or total internal reflection fluorescence (TIRF) microscopy. While dyes and fluorescent proteins have been the mainstay of fluorescence imaging for decades, their fluorescence is unstable under high photon fluxes necessary to observe individual molecules, yielding only a few seconds of observation before complete loss of signal. Latex beads and dye-labeled beads provide improved signal stability but at the expense of drastically larger hydrodynamic size, which can deleteriously alter the diffusion and behavior of the molecule under study.
Quantum dots (QDs) offer a balance between these two problematic regimes. These nanoparticles are composed of semiconductor materials and can be engineered with a hydrodynamically compact size with exceptional resistance to photodegradation 5. Thus in recent years QDs have been instrumental in enabling long-term observation of complex macromolecular behavior on the single molecule level. However these particles have still been found to exhibit impaired diffusion in crowded molecular environments such as the cellular cytoplasm and the neuronal synaptic cleft, where their sizes are still too large 4,6,7.
Recently we have engineered the cores and surface coatings of QDs for minimized hydrodynamic size, while balancing offsets to colloidal stability, photostability, brightness, and nonspecific binding that have hindered the utility of compact QDs in the past 8,9. The goal of this article is to demonstrate the synthesis, modification, and characterization of these optimized nanocrystals, composed of an alloyed HgxCd1-xSe core coated with an insulating CdyZn1-yS shell, further coated with a multidentate polymer ligand modified with short polyethylene glycol (PEG) chains (Figure 1). Compared with conventional CdSe nanocrystals, HgxCd1-xSe alloys offer greater quantum yields of fluorescence, fluorescence at red and near-infrared wavelengths for enhanced signal-to-noise in cells, and excitation at non-cytotoxic visible wavelengths. Multidentate polymer coatings bind to the nanocrystal surface in a closed and flat conformation to minimize hydrodynamic size, and PEG neutralizes the surface charge to minimize nonspecific binding to cells and biomolecules. The end result is a brightly fluorescent nanocrystal with emission between 550-800 nm and a total hydrodynamic size near 12 nm. This is in the same size range as many soluble globular proteins in cells, and substantially smaller than conventional PEGylated QDs (25-35 nm).

We describe the preparation of colloidal quantum dots with minimized hydrodynamic size for single-molecule fluorescence imaging. Compared to conventional quantum dots, these nanoparticles are similar in size to globular proteins and are optimized for single-molecule brightness, stability against photodegradation, and resistance to nonspecific binding to proteins and cells.

Single-molecule imaging is an important tool for  understanding the mechanisms of biomolecular function and for visualizing the  spatial and temporal ...

Engineering

Harvesting Solar Energy by Means of Charge-Separating Nanocrystals and Their Solids

Conjoining different semiconductor materials in a single nano-composite provides synthetic means for the development of novel optoelectronic materials offering a superior control over the spatial distribution of charge carriers across material interfaces. As this study demonstrates, a combination of donor-acceptor nanocrystal (NC) domains in a single nanoparticle can lead to the realization of efficient photocatalytic1-5 materials, while a layered assembly of donor- and acceptor-like nanocrystals films gives rise to photovoltaic materials.
Initially the paper focuses on the synthesis of composite inorganic nanocrystals, comprising linearly stacked ZnSe, CdS, and Pt domains, which jointly promote photoinduced charge separation. These structures are used in aqueous solutions for the photocatalysis of water under solar radiation, resulting in the production of H2 gas. To enhance the photoinduced separation of charges, a nanorod morphology with a linear gradient originating from an intrinsic electric field is used5. The inter-domain energetics are then optimized to drive photogenerated electrons toward the Pt catalytic site while expelling the holes to the surface of ZnSe domains for sacrificial regeneration (via methanol). Here we show that the only efficient way to produce hydrogen is to use electron-donating ligands to passivate the surface states by tuning the energy level alignment at the semiconductor-ligand interface. Stable and efficient reduction of water is allowed by these ligands due to the fact that they fill vacancies in the valence band of the semiconductor domain, preventing energetic holes from degrading it. Specifically, we show that the energy of the hole is transferred to the ligand moiety, leaving the semiconductor domain functional. This enables us to return the entire nanocrystal-ligand system to a functional state, when the ligands are degraded, by simply adding fresh ligands to the system4.
To promote a photovoltaic charge separation, we use a composite two-layer solid of PbS and TiO2 films. In this configuration, photoinduced electrons are injected into TiO2 and are subsequently picked up by an FTO electrode, while holes are channeled to a Au electrode via PbS layer6. To develop the latter we introduce a Semiconductor Matrix Encapsulated Nanocrystal Arrays (SMENA) strategy, which allows bonding PbS NCs into the surrounding matrix of CdS semiconductor. As a result, fabricated solids exhibit excellent thermal stability, attributed to the heteroepitaxial structure of nanocrystal-matrix interfaces, and show compelling light-harvesting performance in prototype solar cells7.

A general strategy for the development of charge-separating semiconductor nanocrystal composites deployable for solar energy production is presented. We show that assembly of donor-acceptor nanocrystal domains in a single nanoparticle geometry gives rise to a photocatalytic function, while bulk-heterojunctions of donor-acceptor nanocrystal films can be used for photovoltaic energy conversion.

Conjoining different semiconductor materials in a single nano-composite provides synthetic means for the development of novel optoelectronic materials ...

Seeded Synthesis of CdSe/CdS Rod and Tetrapod Nanocrystals

We demonstrate a method for the synthesis of multicomponent nanostructures consisting of CdS and CdSe with rod and tetrapod morphologies. A seeded synthesis strategy is used in which spherical seeds of CdSe are prepared first using a hot-injection technique. By controlling the crystal structure of the seed to be either wurtzite or zinc-blende, the subsequent hot-injection growth of CdS off of the seed results in either a rod-shaped or tetrapod-shaped nanocrystal, respectively. The phase and morphology of the synthesized nanocrystals are confirmed using X-ray diffraction and transmission electron microscopy, demonstrating that the nanocrystals are phase-pure and have a consistent morphology. The extinction coefficient and quantum yield of the synthesized nanocrystals are calculated using UV-Vis absorption spectroscopy and photoluminescence spectroscopy. The rods and tetrapods exhibit extinction coefficients and quantum yields that are higher than that of the bare seeds. This synthesis demonstrates the precise arrangement of materials that can be achieved at the nanoscale by using a seeded synthetic approach.

A protocol for the seeded synthesis of rod-shaped and tetrapod-shaped multicomponent nanostructures consisting of CdS and CdSe is presented.

We demonstrate a method for the synthesis of multicomponent nanostructures consisting of CdS and CdSe with rod and tetrapod morphologies. A seeded ...

Revealing Dynamic Processes of Materials in Liquids Using Liquid Cell Transmission Electron Microscopy

The recent development for in situ transmission electron microscopy, which allows imaging through liquids with high spatial resolution, has attracted significant interests across the research fields of materials science, physics, chemistry and biology. The key enabling technology is a liquid cell. We fabricate liquid cells with thin viewing windows through a sequential microfabrication process, including silicon nitride membrane deposition, photolithographic patterning, wafer etching, cell bonding, etc. A liquid cell with the dimensions of a regular TEM grid can fit in any standard TEM sample holder. About 100 nanoliters reaction solution is loaded into the reservoirs and about 30 picoliters liquid is drawn into the viewing windows by capillary force. Subsequently, the cell is sealed and loaded into a microscope for in situ imaging. Inside the TEM, the electron beam goes through the thin liquid layer sandwiched between two silicon nitride membranes. Dynamic processes of nanoparticles in liquids, such as nucleation and growth of nanocrystals, diffusion and assembly of nanoparticles, etc., have been imaged in real time with sub-nanometer resolution. We have also applied this method to other research areas, e.g., imaging proteins in water. Liquid cell TEM is poised to play a major role in revealing dynamic processes of materials in their working environments. It may also bring high impact in the study of biological processes in their native environment.

We have developed a self-contained liquid cell, which allows imaging through liquids using a transmission electron microscope. Dynamic processes of nanoparticles in liquids can be revealed in real time with sub-nanometer resolution.

The recent development for in situ transmission electron microscopy, which allows imaging through liquids with high spatial resolution, has attracted ...

Synthesis of Ligand-free CdS Nanoparticles within a Sulfur Copolymer Matrix

Aliphatic ligands are typically used during the synthesis of nanoparticles to help mediate their growth in addition to operating as high-temperature solvents. These coordinating ligands help solubilize and stabilize the nanoparticles while in solution, and can influence the resulting size and reactivity of the nanoparticles during their formation. Despite the ubiquity of using ligands during synthesis, the presence of aliphatic ligands on the nanoparticle surface can result in a number of problems during the end use of the nanoparticles, necessitating further ligand stripping or ligand exchange procedures. We have developed a way to synthesize cadmium sulfide (CdS) nanoparticles using a unique sulfur copolymer. This sulfur copolymer is primarily composed of elemental sulfur, which is a cheap and abundant material. The sulfur copolymer has the advantages of operating both as a high temperature solvent and as a sulfur source, which can react with a cadmium precursor during nanoparticle synthesis, resulting in the generation of ligand free CdS. During the reaction, only some of the copolymer is consumed to produce CdS, while the rest remains in the polymeric state, thereby producing a nanocomposite material. Once the reaction is finished, the copolymer stabilizes the nanoparticles within a solid polymeric matrix. The copolymer can then be removed before the nanoparticles are used, which produces nanoparticles that do not have organic coordinating ligands. This nascent synthesis technique presents a method to produce metal-sulfide nanoparticles for a wide variety of applications where the presence of organic ligands is not desired.

Herein we present a method to synthesize ligand-free cadmium sulfide (CdS) nanoparticles based on a unique sulfur copolymer. The sulfur copolymer operates as a high temperature solvent and a sulfur source during the nanoparticle synthesis and stabilizes the nanoparticles after the reaction.

Aliphatic ligands are typically used during the synthesis of nanoparticles to help mediate their growth in addition to operating as high-temperature ...

Hydroquinone Based Synthesis of Gold Nanorods

Gold nanorods are an important kind of nanoparticles characterized by peculiar plasmonic properties. Despite their widespread use in nanotechnology, the synthetic methods for the preparation of gold nanorods are still not fully optimized. In this paper we describe a new, highly efficient, two-step protocol based on the use of hydroquinone as a mild reducing agent. Our approach allows the preparation of nanorods with a good control of size and aspect ratio (AR) simply by varying the amount of hexadecyl trimethylammonium bromide (CTAB) and silver ions (Ag+) present in the "growth solution". By using this method, it is possible to markedly reduce the amount of CTAB, an expensive and cytotoxic reagent, necessary to obtain the elongated shape. Gold nanorods with an aspect ratio of about 3 can be obtained in the presence of just 50 mM of CTAB (versus 100 mM used in the standard protocol based on the use of ascorbic acid), while shorter gold nanorods are obtained using a concentration as low as 10 mM.

This paper describes a protocol for the synthesis of gold nanorods, based on the use of hydroquinone as reducing agent, plus the different mechanisms for controlling their size and aspect ratio.

Gold nanorods are an important kind of nanoparticles characterized by peculiar plasmonic properties. Despite their widespread use in nanotechnology, the ...

Synthesis of Hierarchical ZnO/CdSSe Heterostructure Nanotrees

A two-step chemical vapor deposition procedure is here employed to prepare tree-like hierarchical ZnO/CdSSe hetero-nanostructures. The structures are composed of CdSSe branches grown on ZnO nanowires that are vertically aligned on a transparent sapphire substrate. The morphology was measured via scanning electron microscopy. The crystal structure was determined by X-ray powder diffraction analysis. Both the ZnO stem and CdSSe branches have a predominantly wurtzite crystal structure. The mole ratio of S and Se in the CdSSe branches was measured by energy dispersive X-ray spectroscopy. The CdSSe branches result in strong visible light absorption. Photoluminescence (PL) spectroscopy showed that the stem and branches form a type-II heterojunction. PL lifetime measurements showed a decrease in the lifetime of emission from the trees when compared to emission from individual ZnO stems or CdSSe branches and indicate fast charge transfer between CdSSe and ZnO. The vertically aligned ZnO stems provide a direct electron transport pathway to the substrate and allow for efficient charge separation after photoexcitation by visible light. The combination of the abovementioned properties makes ZnO/CdSSe nanotrees promising candidates for applications in solar cells, photocatalysis, and opto-electronic devices.

Here, we prepare and characterize novel tree-like hierarchical ZnO/CdSSe nanostructures, where CdSSe branches are grown on vertically aligned ZnO nanowires. The resulting nanotrees are a potential material for solar energy conversion and other opto-electronic devices.

A two-step chemical vapor deposition procedure is here employed to prepare tree-like hierarchical ZnO/CdSSe hetero-nanostructures. The structures are ...

Facet-to-facet Linking of Shape-anisotropic Colloidal Cadmium Chalcogenide Nanostructures

Here, we describe a protocol that allows for shape-anisotropic cadmium chalcogenide nanocrystals (NCs), such as nanorods (NRs) and tetrapods (TPs), to be covalently and site-specifically linked via their end facets, resulting in polymer-like linear or branched chains. The linking procedure begins with a cation-exchange process in which the end facets of the cadmium chalcogenide NCs are first converted to silver chalcogenide. This is followed by the selective removal of ligands at their surface. This results in cadmium chalcogenide NCs with highly reactive silver chalcogenide end facets that spontaneously fuse upon contact with each other, thereby establishing an interparticle facet-to-facet attachment. Through the judicious choice of precursor concentrations, an extensive network of linked NCs can be produced. Structural characterization of the linked NCs is carried out via low- and high-resolution transmission electron microscopy (TEM), as well as energy-dispersive X-ray spectroscopy, which confirm the presence of silver chalcogenide domains between chains of cadmium chalcogenide NCs.

A protocol detailing how shape-anisotropic colloidal cadmium chalcogenide nanocrystals can be covalently linked via their end facets is presented here.

Here, we describe a protocol that allows for shape-anisotropic cadmium chalcogenide nanocrystals (NCs), such as nanorods (NRs) and tetrapods (TPs), to be ...

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.