Name: tuning oxide properties by oxygen vacancy control during growth and annealing
Uploaded: 2023-06-09T15:00:00
Description: Watch this Scientific Journal Video about Tuning Oxide Properties by Oxygen Vacancy Control During Growth and Annealing at JoVE.com

The authors thank J. Geyti from the Technical University of Denmark for his technical assistance. F. Trier acknowledges support by research grant VKR023371 (SPINOX) from VILLUM FONDEN. D. V. Christensen acknowledges the support of Novo Nordisk Foundation NERD Programme: New Exploratory Research and Discovery, Superior Grant NNF21OC0068015.

1. Controlling properties by varying growth conditions
<ol>
	<li>Preparation of high-quality surfaces of SrTiO3
	<ol>
		<li>Purchase mixed terminated SrTiO3 substrates (e.g., of 5 mm x 5 mm x 0.5 mm in size) with a typical surface angle of 0.05&#176;&#8211;0.2&#176; with respect to the (001) crystal planes. 
		NOTE: The miscut angle determines the flatness of the surface, which is important for epitaxial growth on the substrate, as well as for the resulting properties at the ...

<ol>
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Z., 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=On+the+origin+of+metallic+conductivity+at+the+interface+of+LaAlO3/SrTiO3.">On the origin of metallic conductivity at the interface of LaAlO3/SrTiO3.</a> Applied Surface Science. 258 (23), 9242-9245 (2012).</li><li>Trier, F., 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=Degradation+of+the+interfacial+conductivity+in+LaAlO3/SrTiO3+heterostructures+during+storage+at+controlled+environments.">Degradation of the interfacial conductivity in LaAlO3/SrTiO3 heterostructures during storage at controlled environments.</a> Solid State Ionics. 230, 12-15 (2013).</li><li>Christensen, D. 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The methods described here rely on using the oxygen content to control oxide properties, and the oxygen partial pressure and operating temperature are, thus, critical parameters. If the total oxidation state of the system is tuned in a way where the system remains in a thermodynamic equilibrium with the surrounding atmosphere (i.e., changed pO2 at high temperature), the changes can be reversible. However, in the case of SrTiO3-based heterostructures, interfacial oxygen vacancies are typically formed...

The oxygen content plays a vital role in the properties of oxide materials. Oxygen has a high electronegativity and, in the fully ionic limit, attracts two electrons from neighboring cations. These electrons are donated to the lattice when an oxygen vacancy is formed. The electrons can be trapped and form a localized state, or they can become delocalized and capable of conducting a charge current. The localized states are typically located in the band gap between the valence and conduction band with a total angular momentum that can be nonzero1,2,3. The localized states can, thus, form localized magnetic moments and have a large impact on, for instance, the optical and magnetic properties1,2,3. If the electrons become delocalized, they contribute to the density of itinerant charge carriers. In addition, if an oxygen vacancy or other defects are formed, the lattice adapts to the defect. The presence of defects can, thus, naturally lead to local strain fields, symmetry breaking, and a modified electronic and ionic transport in oxides.
Controlling the oxygen stoichiometry is, therefore, often key to tune, for instance, the optical, magnetic, and transport properties of oxide materials. A prominent example is that of SrTiO3 and SrTiO3-based heterostructures, where the ground state of the material systems is very sensitive to the oxygen content. Undoped SrTiO3 is a nonmagnetic insulator with a band gap of 3.2 eV; however, by introducing oxygen vacancies, SrTiO3 changes the state from insulating to metallic conducting with an electron mobility exceeding 10,000 cm2/Vs at 2 K4. At low temperatures (T &#60; 450 mK), superconductivity may even be the favored ground state5,6. Oxygen vacancies in SrTiO3 have also been found to render it ferromagnetic7 and result in an optical transition in the visible spectrum from transparent to opaque2. For more than a decade, there has been a large interest in depositing various oxides, such as LaAlO3, CaZrO3, and &#947;-Al2O3, on SrTiO3 and examining the properties arising at the interface8,9,10,11,12,13. In some cases, it turns out that the properties of the interface differ markedly from those observed in the parent materials. An important result of the SrTiO3-based heterostructures is that the electrons can be confined to the interface, which makes it possible to control the properties related to the density of itinerant electrons using electrostatic gating. In this way, it becomes possible to tune, for instance, the electron mobility14,15, superconductivity11, electron pairing16, and magnetic state17 of the interface, using electric fields.
The formation of the interface also enables a control of the SrTiO3 chemistry, where the deposition of the top film on SrTiO3 can be used to induce a redox reaction across the interface18,19. If an oxide film with a high oxygen affinity is deposited on SrTiO3, oxygen can transfer from the near-surface parts of SrTiO3 to the top film, thereby reducing SrTiO3 and oxidizing the top film (see Figure 1).
<img alt="Figure 1" class="xfigimg" src="/files/ftp_upload/58737/58737fig01v2.jpg" /> 
Figure 1: Oxygen vacancy formation in SrTiO3. Schematic illustration of how oxygen vacancies and electrons are formed in the interface-near region of SrTiO3 during the deposition of a thin film with a high oxygen affinity. Reprinted figure with permission from a study by Chen et al.18. Copyright 2011 by the American Chemical Society. <a href="https://www.jove.com/files/ftp_upload/58737/58737fig1large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
In this case, oxygen vacancies and electrons are formed near the interface. This process is expected to be the origin of the conductivity formed during the deposition at the interface between SrTiO3 and room-temperature-grown metal films or oxides such as amorphous LaAlO318,20 or &#947;-Al2O310,21,22,23. Thus, the properties of these SrTiO3-based interfaces are highly sensitive to the oxygen content at the interface.
Here, we report the use of postdeposition annealing and variations in the pulsed laser deposition parameters to control the properties in oxide materials by tuning the oxygen content. We use &#947;-Al2O3 or amorphous LaAlO3 deposited on SrTiO3 at room temperature as examples on how the carrier density, electron mobility, and sheet resistance can be changed by orders of magnitude by controlling the number of oxygen vacancies. The methods offer some benefits beyond those obtained with electrostatic gating, which is typically used to tune the electrical9,11,14&#160;and in some cases the magnetic15,17&#160;properties. These benefits include forming a (quasi-)stable final state and avoiding the use of electric fields, which requires electrical contact to the sample and may cause side effects.
In the following, we review general approaches for tuning the properties of oxides by controlling the oxygen content. This is done in two ways, namely, 1) by varying the growth conditions when synthesizing the oxide materials, and 2) by annealing the oxide materials in oxygen. The approaches can be applied to tune a range of properties in many oxide and some monoxide materials. We provide a concrete example on how to tune the carrier density at the interface of SrTiO3-based heterostructures. Ensure that a high level of cleanliness is exercised to avoid contamination of the samples (e.g., by using gloves, tube furnaces dedicated to SrTiO3, and nonmagnetic/acid resistant tweezers).

<table border="0" cellpadding="0" cellspacing="0" style="width:1260px;" width="1260">
	<tbody>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">SrTiO3</td>
			<td style="width:264px;">Crystec</td>
			<td style="width:264px;"></td>
			<td style="width:516px;">Single crystalline (001) oriented, 0.05-0.2 degree miscut angle</td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:216px;">LaAlO3</td>
			<td style="width:264px;">Shanghai Daheng Optics and Fine Mechanics Co.Ltd.</td>
			<td style="width:264px;"></td>
			<td>Single crystalline</td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:216px;">Al2O3</td>
			<td style="width:264px;">Shanghai Daheng Optics and Fine Mechanics Co.Ltd.</td>
			<td style="width:264px;"></td>
			<td>Single crystalline</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">Chemicals and gases</td>
			<td style="width:264px;">Standard suppliers</td>
			<td style="width:264px;"></td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">Silver paste</td>
			<td style="width:264px;">SPI Supplies, Structure Probe Inc</td>
			<td style="width:264px;"></td>
			<td>05001-AB, High purity silver paint</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">Ultrasonicator</td>
			<td style="width:264px;">VWR</td>
			<td style="width:264px;"></td>
			<td>USC500D HF45kHz/100W</td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:216px;">Wedge wire bonder</td>
			<td style="width:264px;">Shenzhen Baixiangyuan Science &#38; Technology Co.,Ltd.</td>
			<td style="width:264px;"></td>
			<td>HS-853A Aluminum wire bonder</td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:216px;">Pulsed laser deposition</td>
			<td style="width:264px;">Twente Solid State Technologies (TSST)</td>
			<td style="width:264px;"></td>
			<td style="width:516px;">PLD from TSST with software version V3.0.29, equipped with a 248 nm KrF 
			nanosecond laser (Compex Pro 205 F) from Coherent</td>
		</tr>
		<tr height="105">
			<td height="105" style="height:105px;width:216px;">Resistance measurement setup</td>
			<td style="width:264px;">Custom made</td>
			<td style="width:264px;"></td>
			<td style="width:516px;">Based on the following electrical instruments and custom written software: 
			Keithley 6221 DC and AC current source 
			Keithley 2182A nanovoltmeter 
			Keithley 7001 switch system with a matrix card 
			Keithley 6487 picoammeter</td>
		</tr>
		<tr height="84">
			<td height="84" style="height:84px;width:216px;">Hall measurements</td>
			<td style="width:264px;">Cryogenics</td>
			<td style="width:264px;"></td>
			<td style="width:516px;">Based on the following electrical instruments and custom written software: 
			Keithley 2400 DC current source 
			Keithley 2182A nanovoltmeter 
			Keithley 7001 switch system with a matrix card</td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:216px;">Furnace</td>
			<td style="width:264px;">Custom made</td>
			<td style="width:264px;"></td>
			<td style="width:516px;">Custom written software control of a FTTF 500/70 tube furnace from Scandia Ovnen AS and a eurotherm 2216e temperature controller</td>
		</tr>
	</tbody>
</table>

tuning oxide properties by oxygen vacancy control during growth and annealing

Electrical, optical, and magnetic properties of oxide materials can often be controlled by varying the oxygen content. Here we outline two approaches for varying the oxygen content and provide concrete examples for tuning the electrical properties of SrTiO3-based heterostructures. In the first approach, the oxygen content is controlled by varying the deposition parameters during a pulsed laser deposition. In the second approach, the oxygen content is tuned by subjecting the samples to annealing in oxygen at elevated temperatures after the film growth. The approaches can be used for a wide range of oxides and nonoxide materials where the properties are sensitive to a change in the oxidation state.
The approaches differ significantly from electrostatic gating, which is often used to change the electronic properties of confined electronic systems such as those observed in SrTiO3-based heterostructures. By controlling the oxygen vacancy concentration, we are able to control the carrier density over many orders of magnitude, even in nonconfined electronic systems. Moreover, properties can be controlled, which are not sensitive to the density of itinerant electrons.

Controlling properties by varying growth conditions 
Varying the deposition parameters during the deposition of oxides can lead to a large change in the properties, in particular for SrTiO3-based heterostructures, as shown in Figure 2.
<img alt="Figure 2" class="xfigimg" src="/files/ftp_upload/58737/58737fig2v2.jpg" /> 
Figure 2: Controlling the transpor...

Watch this Scientific Journal Video about Tuning Oxide Properties by Oxygen Vacancy Control During Growth and Annealing at JoVE.com

Tuning Oxide Properties by Oxygen Vacancy Control During Growth and Annealing

Oxide materials show many exotic properties that can be controlled by tuning the oxygen content. Here, we demonstrate the tuning of oxygen content in oxides by varying the pulsed laser deposition parameters and by performing postannealing. As an example, electronic properties of SrTiO3-based heterostructures are tuned by growth modifications and annealing.

Electrical, optical, and magnetic properties of oxide materials can often be controlled by varying the oxygen content. Here we outline two approaches for ...

The method presented here allows for controlling the amount of oxygen vacancies in oxide thin films both during and after the deposition. The main advances of this approach is that electrical and magnetic properties can be tuned by modifying the amount of oxygen vacancies. Oxygen vacancies serve as functional defects in most oxide materials, and the properties of many oxides can therefore be controlled systematically by defect engineering using this approach.Demonstrating the procedure will be Shinhee, Carlos, and Eric, a post-doc and two PhD students from our laboratory. To begin, purchase mixed-terminated strontium titanate substrates with a typical surface angle of 0.05 to 0.2 degrees with respect to the crystal planes. Clean the desired number of substrates by ultrasonication in acetone for five minutes.Then ultrasonicate the substrates for 20 minutes at 70 degrees Celsius in clean water, which dissolves strontium oxide or forms strontium hydroxide complexes at surface domains terminated with strontium oxide, while leaving the chemically stable, titanium dioxide terminated domains unchanged. In the meantime, prepare aqua regia solution by slowly adding hydrochloric acid into water and then adding nitric acid into the solution. Next, ultrasonicate the substrates in an acidic solution containing hydrochloric acid, nitric acid, and water at 70 degrees Celsius for 20 minutes in a fume hood to selectively etch strontium oxide due to the basic nature of strontium oxide surface domains, the acidity of titanium dioxide, and the presence of the strontium hydroxide complexes.Remove the residual acid from the substrates by ultrasonication in 100 milliliters of clean water for five minutes at room temperature in a fume hood. Then bake the substrates in an atmosphere of one bar of oxygen for one hour at 1, 000 degrees Celsius with a heating and cooling rate of 100 degrees Celsius per hour in a ceramic tube furnace to relax the substrate surface into a state with low energy. To deposit a thin film on the substrates, mount the substrates on a heater or chip carrier, depending on whether NC2 transport measurements are to be performed during the deposition.Next, place the titanium dioxide terminated substrate 4.7 centimeters from the single crystalline alumina target for a typical deposition of gamma alumina on strontium titanate at room temperature. Prepare for ablating from a single crystalline alumina target in an oxygen pressure of 10 to the power minus five millibar. Tune the properties using the oxygen content by using an oxygen deposition pressure in the range of 10 to the power minus six to 0.1 millibars, or by varying other deposition parameters.After incubation, observe the substrate for the desired thickness of gamma alumina deposition. Next, remove the sample from the deposition chamber, and stop any electrical measurements. Then store the sample in a vacuum.The sample degradation is slowest when stored in a vacuum or nitrogen. Mount the sample on a chip carrier using silver paste. Then electrically connect the sample to the chip carrier using wedge wire bonding of aluminum wires in the Van der Pauw geometry.Next, place the chip carrier with the sample in a closed furnace. Then using a connector and wires with thermally resistant insulation, electrically connect the chip carrier to the measurement equipment, and start the sheet resistance measurements. Then place the chip carrier equipped with the sample in a closed furnace and flush thoroughly with the gas used for the annealing while checking whether the sample resistance is sensitive to a change in the atmosphere.Anneal the sample using the desired annealing profile, depending on the thickness of the top film and the desired rate of oxygen incorporation. Abort the annealing when a desired change in the sheet resistance has occurred. Using this setup, the development of the sheet resistance in oxide heterostructures such as gamma alumina strontium titanate and lanthanum aluminate strontium titanate can be monitored in-situ during pulsed laser deposition.When the measurement environment is changed by measuring ex-situ or via in-situ oxygen flushing, significant changes in the sheet resistance of the strontium titanate based heterostructures can be observed. In samples where gamma alumina is deposited on strontium titanate, the electron mobility stays largely unchanged at room temperature, but changes dramatically at two Kelvin when the deposition pressure is varied. The properties of oxide heterostructures can also be tuned after deposition using annealing.The final state is determined by the annealing time and annealing temperature and atmosphere. The sheet conductants of heterostructures composed of strontium titanate capped with gamma alumina or amorphous lanthanum aluminate are measured at various annealing temperatures. The fastest decrease in the conductance was observed for the amorphous lanthanum aluminate strontium titanate heterostructures.For the strontium titanate heterostructures, the carrier density is controlled by controlling the annealing and oxygen. Consecutive annealing steps result in a steady decrease of carrier density and a transition from a metallic conducting to an insulating interface. Changing the conducting state in the strontium titanate heterostructure can enable different properties.Here, attempts to write nanowires using a conducting atomic force microscope were not possible before annealing. However, after annealing, conducting lines can be written and erased at the interface. Using this approach, we can systematically change the magnetic and electronic properties of oxide heterostructures, and in this way, study the role of oxygen vacancies in determining these properties.

tuning-oxide-properties-oxygen-vacancy-control-during-growth

Research

JoVE Journal

Chemistry

Atomically Defined Templates for Epitaxial Growth of Complex Oxide Thin Films

Atomically defined substrate surfaces are prerequisite for the epitaxial growth of complex oxide thin films. In this protocol, two approaches to obtain such surfaces are described. The first approach is the preparation of single terminated perovskite SrTiO3 (001) and DyScO3 (110) substrates. Wet etching was used to selectively remove one of the two possible surface terminations, while an annealing step was used to increase the smoothness of the surface. The resulting single terminated surfaces allow for the heteroepitaxial growth of perovskite oxide thin films with high crystalline quality and well-defined interfaces between substrate and film. In the second approach, seed layers for epitaxial film growth on arbitrary substrates were created by Langmuir-Blodgett (LB) deposition of nanosheets. As model system Ca2Nb3O10- nanosheets were used, prepared by delamination of their layered parent compound HCa2Nb3O10. A key advantage of creating seed layers with nanosheets is that relatively expensive and size-limited single crystalline substrates can be replaced by virtually any substrate material.

Various procedures are outlined to prepare atomically defined templates for epitaxial growth of complex oxide thin films. Chemical treatments of single crystalline SrTiO3 (001) and DyScO3 (110) substrates were performed to obtain atomically smooth, single terminated surfaces. Ca2 Nb3 O10- nanosheets were used to create atomically defined templates on arbitrary substrates.

Atomically defined substrate surfaces are prerequisite for the epitaxial growth of complex oxide thin films. In this protocol, two approaches to obtain ...

Tuning a Parallel Segmented Flow Column and Enabling Multiplexed Detection

Active flow technology (AFT) is new form of column technology that was designed to overcome flow heterogeneity to increase separation performance in terms of efficiency and sensitivity and to enable multiplexed detection. This form of AFT uses a parallel segmented flow (PSF) column. A PSF column outlet end-fitting consists of 2 or 4 ports, which can be multiplexed to connect up to 4 detectors. The PSF column not only allows a platform for multiplexed detection but also the combination of both destructive and non-destructive detectors, without additional dead volume tubing, simultaneously. The amount of flow through each port can also be adjusted through pressure management to suit the requirements of a specific detector(s). To achieve multiplexed detection using a PSF column there are a number of parameters which can be controlled to ensure optimal separation performance and quality of results; that is tube dimensions for each port, choice of port for each type of detector and flow adjustment. This protocol is intended to show how to use and tune a PSF column functioning in a multiplexed mode of detection.

Here, we present a protocol for the operation and tuning of parallel segmented flow chromatography columns to enable multiplexed detection.

Active flow technology (AFT) is new form of column technology that was designed to overcome flow heterogeneity to increase separation performance in terms ...

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).

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 ...

TiO2-coated Hollow Glass Microspheres with Superhydrophobic and High IR-reflective Properties Synthesized by a Soft-chemistry Method

This manuscript proposes a soft-chemistry method to develop superhydrophobic and highly IR-reflective hollow glass microspheres (HGM). The anatase TiO2 and a superhydrophobic agent were coated on the HGM surface in one step. TBT and PFOTES were selected as the Ti source and the superhydrophobic agent, respectively. They were both coated on the HGM, and after the hydrothermal process, the TBT turned to anatase TiO2. In this way, a PFOTES/TiO2-coated HGM (MCHGM) was prepared. For comparison, PFOTES single-coated HGM (F-SCHGM) and TiO2 single-coated HGM (Ti-SCHGM) were synthesized as well. The PFOTES and TiO2 coatings on the HGM surface were demonstrated through X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy-dispersive detector (EDS) characterizations. The MCHGM showed a higher contact angle (153&#176;) but a lower sliding angle (16&#176;) than F-SCHGM, with a contact angle of 141.2&#176; and a sliding angle of 67&#176;. In addition, both Ti-SCHGM and MCHGM displayed similar IR reflectivity values, which were about 5.8% higher than the original HGM and F-SCHGM. Also, the PFOTES coating barely changed the thermal conductivity. Therefore, F-SCHGM, with a thermal conductivity of 0.0479 W/(m&#183;K), was quite like the original HGM, which was 0.0475 W/(m&#183;K). MCHGM and Ti-SCHGM were also similar. Their thermal conductivity values were 0.0543 W/(m&#183;K) and 0.0543 W/(m&#183;K), respectively. The TiO2 coating slightly increased the thermal conductivity, but with the increase in reflectivity, the overall heat-insulation property was enhanced. Finally, since the IR-reflecting property is provided by the HGM coating, if the coating is fouled, the reflectivity decreases. Therefore, with the superhydrophobic coating, the surface is protected from fouling, and its lifetime is also prolonged.

TiO2-coated Hollow Glass Microspheres with Superhydrophobic and High IR-reflective Properties Synthesized by a Soft-chemistry Method

This manuscript proposes a soft-chemistry method to synthesize superhydrophobic, TiO2-coated hollow glass microspheres (HGM) with high IR-reflective properties.

This manuscript proposes a soft-chemistry method to develop superhydrophobic and highly IR-reflective hollow glass microspheres (HGM). The anatase TiO2 ...

Aerosol-assisted Chemical Vapor Deposition of Metal Oxide Structures: Zinc Oxide Rods

Whilst columnar zinc oxide (ZnO) structures in the form of rods or wires have been synthesized previously by different liquid- or vapor-phase routes, their high cost production and/or incompatibility with microfabrication technologies, due to the use of pre-deposited catalyst-seeds and/or high processing temperatures exceeding 900 &#176;C, represent a drawback for a widespread use of these methods. Here, however, we report the synthesis of ZnO rods via a non-catalyzed vapor-solid mechanism enabled by using an aerosol-assisted chemical vapor deposition (CVD) method at 400 &#176;C with zinc chloride (ZnCl2) as the precursor and ethanol as the carrier solvent. This method provides both single-step formation of ZnO rods and the possibility of their direct integration with various substrate types, including silicon, silicon-based micromachined platforms, quartz, or high heat resistant polymers. This potentially facilitates the use of this method at a large-scale, due to its compatibility with state-of-the-art microfabrication processes for device manufacture. This report also describes the properties of these structures (e.g., morphology, crystalline phase, optical band gap, chemical composition, electrical resistance) and validates its gas sensing functionality towards carbon monoxide.

Columnar zinc oxide structures in the form of rods are synthesized via aerosol-assisted chemical vapor deposition without the use of pre-deposited catalyst-seed particles. This method is scalable and compatible with various substrates based on either silicon, quartz or polymers.

Whilst columnar zinc oxide (ZnO) structures in the form of rods or wires have been synthesized previously by different liquid- or vapor-phase routes, ...

Fabrication of Spherical and Worm-shaped Micellar Nanocrystals by Combining Electrospray, Self-assembly, and Solvent-based Structure Control

Micellar nanocrystals (micelles with encapsulated nanocrystals) have become an emerging major class of nanobiomaterials. We describe a method of fabricating micellar nanocrystals based on combining top-down electrospray, bottom-up self-assembly, and solvent-based structure control. This method involves first using electrospray to generate uniform ultrafine liquid droplets, each of which functions as a micro-reactor in which self-assembly reaction occurs forming micellar nanocrystals, with the structures (micelle shape and nanocrystal encapsulation) controlled by the organic solvent used. This method is largely continuous and produces high quality micellar nanocrystal products with an inexpensive structure control approach. By using a water-miscible organic solvent tetrahydrofuran (THF), worm-shaped micellar nanocrystals can be produced due to solvent-induced/facilitated micelle fusion. Compared with the common spherical micellar nanocrystals, worm-shaped micellar nanocrystals can offer minimized non-specific cellular uptake, thus enhancing biological targeting. By co-encapsulating multiple nanocrystals into each micelle, multifunctional or synergistic effects can be achieved. Current limitations of this fabrication method, which will be part of the future work, primarily include imperfect encapsulation in the micellar nanocrystal product and the incompletely continuous nature of the process.

The present work describes a method to fabricate micellar nanocrystals, an emerging major class of nanobiomaterials. This method combines top-down electrospray, bottom-up self-assembly, and solvent-based structure control. The fabrication method is largely continuous, can produce high quality products, and possesses an inexpensive means of structure control.

Micellar nanocrystals (micelles with encapsulated nanocrystals) have become an emerging major class of nanobiomaterials. We describe a method of ...

Synthesis of Platinum-nickel Nanowires and Optimization for Oxygen Reduction Performance

Platinum-nickel (Pt-Ni) nanowires were developed as fuel cell electrocatalysts, and were optimized for the performance and durability in the oxygen reduction reaction. Spontaneous galvanic displacement was used to deposit Pt layers onto Ni nanowire substrates. The synthesis approach produced catalysts with high specific activities and high Pt surface areas. Hydrogen annealing improved Pt and Ni mixing and specific activity. Acid leaching was used to preferentially remove Ni near the nanowire surface, and oxygen annealing was used to stabilize near-surface Ni, improving durability and minimizing Ni dissolution. These protocols detail the optimization of each post-synthesis processing step, including hydrogen annealing to 250 &#176;C, exposure to 0.1 M nitric acid, and oxygen annealing to 175 &#176;C. Through these steps, Pt-Ni nanowires produced increased activities more than an order of magnitude than Pt nanoparticles, while offering significant durability improvements. The presented protocols are based on Pt-Ni systems in the development of fuel cell catalysts. These techniques have also been used for a variety of metal combinations, and can be applied to develop catalysts for a number of electrochemical processes.

&#160; The protocol describes the synthesis and electrochemical testing of platinum-nickel nanowires. Nanowires were synthesized by the galvanic displacement of a nickel nanowire template. Post-synthesis processing, including hydrogen annealing, acid leaching, and oxygen annealing were used to optimize nanowire performance and durability in the oxygen reduction reaction.

Platinum-nickel (Pt-Ni) nanowires were developed as fuel cell electrocatalysts, and were optimized for the performance and durability in the oxygen ...

Syntheses, Crystallization, and Spectroscopic Characterization of 3,5-Lutidine N-Oxide Dehydrate

The synthesis of 3,5-lutidine N-oxide dehydrate, 1, was achieved in the synthesis route of 2-amino-pyridine-3,5-dicarboxylic acid. Ochiai first used the methodology for non-substituted pyridines in 1957 in a 12 h process, but no X-ray suitable crystals were obtained. The substituted ring used in the methodology presented here clearly influenced the addition of water molecules into the asymmetric unit, which confers a different nucleophilic strength in 1. The X-ray suitable crystal compound 1 was possible due to the stabilization of the negative charge in the oxygen by the presence of two water molecules where the hydrogen atoms donate positive charge into the ring; such water molecules serve well to construct a supramolecular interaction. The hydrated molecules may be possible for the alkaline system that is reached by adjusting the pH to 10. Importantly, the double methyl substituted ring and a reaction time of 5 h, makes it a more versatile method and with wider chemical applications for future ring insertions.

Syntheses, Crystallization, and Spectroscopic Characterization of 3,5-Lutidine N-Oxide Dehydrate

Herein, we report the synthesis and crystallization of 3,5-lutidine N-oxide dehydrate by a simple protocol that differs from the classical synthesis of pyridine N-oxide. This protocol utilizes different starting material and involves less reaction time to yield a new solvated supramolecular structure, which crystallizes under slow evaporation.

The synthesis of 3,5-lutidine N-oxide dehydrate, 1, was achieved in the synthesis route of 2-amino-pyridine-3,5-dicarboxylic acid. Ochiai first used the ...

Molten-Salt Synthesis of Complex Metal Oxide Nanoparticles

The development of feasible synthesis methods is critical for the successful exploration of novel properties and potential applications of nanomaterials. Here, we introduce the molten-salt synthesis (MSS) method for making metal oxide nanomaterials. Advantages over other methods include its simplicity, greenness, reliability, scalability, and generalizability. Using pyrochlore lanthanum hafnium oxide (La2Hf2O7) as a representative, we describe the MSS protocol for the successful synthesis of complex metal oxide nanoparticles (NPs). Furthermore, this method has the unique ability to produce NPs with different material features by changing various synthesis parameters such as pH, temperature, duration, and post-annealing. By fine-tuning these parameters, we are able to synthesize highly uniform, non-agglomerated, and highly crystalline NPs. As a specific example, we vary the particle size of the La2Hf2O7&#160;NPs by changing the concentration of the ammonium hydroxide solution used in the MSS process, which allows us to further explore the effect of particle size on various properties. It is expected that the MSS method will become a more popular synthesis method for nanomaterials and more widely employed in the nanoscience and nanotechnology community in the upcoming years.

Here, we demonstrate a unique, relatively low-temperature, molten-salt synthesis method for preparing uniform complex metal oxide lanthanum hafnate nanoparticles.

The development of feasible synthesis methods is critical for the successful exploration of novel properties and potential applications of nanomaterials. ...

U2O5 Film Preparation via UO2 Deposition by Direct Current Sputtering and Successive Oxidation and Reduction with Atomic Oxygen and Atomic Hydrogen

We describe a method to produce U2O5 films in situ using the Labstation, a modular machine developed at JRC Karlsruhe. The Labstation, an essential part of the Properties of Actinides under Extreme Conditions laboratory (PAMEC), allows the preparation of films and studies of sample surfaces using surface analytical techniques such as X-ray and ultra-violet photoemission spectroscopy (XPS and UPS, respectively). All studies are made in situ, and the films, transferred under ultra-high vacuum from their preparation to an analyses chamber, are never in contact with the atmosphere. Initially, a film of UO2 is prepared by direct current (DC) sputter deposition on a gold (Au) foil then oxidized by atomic oxygen to produce a UO3 film. This latter is then reduced with atomic hydrogen to U2O5. Analyses are performed after each step involving oxidation and reduction, using high-resolution photoelectron spectroscopy to examine the oxidation state of uranium. Indeed, the oxidation and reduction times and corresponding temperature of the substrate during this process have severe effects on the resulting oxidation state of the uranium. Stopping the reduction of UO3 to U2O5 with single U(V) is quite challenging; first, uranium-oxygen systems exist in numerous intermediate phases. Second, differentiation of the oxidation states of uranium is mainly based on satellite peaks, whose intensity peaks are weak. Also, experimenters should be aware that X-ray spectroscopy (XPS) is a technique with an atomic sensitivity of 1% to 5%. Thus, it is important to obtain a complete picture of the uranium oxidation state with the entire spectra obtained on U4f, O1s, and the valence band (VB). Programs used in the Labstation include a linear transfer program developed by an outside company (see Table of Materials) as well as data acquisition and sputter source programs, both developed in-house.

U2O5 Film Preparation via UO2 Deposition by Direct Current Sputtering and Successive Oxidation and Reduction with Atomic Oxygen and Atomic Hydrogen

This protocol presents the preparation of U2O5 thin films obtained in situ under ultra-high vacuum. The process involves oxidation and reduction of UO2 films with atomic oxygen and atomic hydrogen, respectively.

We describe a method to produce U2O5 films in situ using the Labstation, a modular machine developed at JRC Karlsruhe. The Labstation, an essential part ...