K.B. acknowledges funding from the fellowship program of the German National Academy of Sciences Leopoldina, grant LPDS 2016-06 and the European Space Agency. Furthermore, she would like to thank Dr. Leopold Summerer, the Advanced Concepts Team, Alan Dowson, Dr. Jack van Loon, Dr. Gabor Milassin and Dr. Robert Lindner (ESTEC), Robbert-Jan Noordam (Notese) and Prof. Harry B. Gray (Caltech) for their great support. M.H.R. is grateful for generous support from Prof. Nathan S. Lewis (Caltech). K.B. and M.H.R. acknowledge support from the Beckman Institute of the California Institute of Technology and the Molecular Materials Research Center. The PhotoEChem Team greatly acknowledges funding from the German Aerospace Center (Deutsches Zentrum f&#252;r Luft- und Raumfahrt e.V.) for the project no. 50WM1848. Furthermore, M.G. acknowledges funding from the Guangdong Innovative and Entrepreneurial Team Program titled &#34;Plasmonic Nanomaterials and Quantum Dots for Light Management in Optoelectronic Devices&#34; (no. 2016ZT06C517). Furthermore, the author team greatly acknowledges the effort and support from the ZARM Team with Dieter Bischoff, Torsten Lutz, Matthias Meyer, Fred Oetken, Jan Siemers, Dr. Martin Castillo, Magdalena Thode and Dr. Thorben K&#246;nemann. It is also thankful for enlightening discussions with Prof. Yasuhiro Fukunaka (Waseda University), Prof. Hisayoshi Matsushima (Hokkaido University) and Dr. Slobodan Mitrovic (Lam Research).

1. Preparation of p-InP photoelectrodes
<ol>
	<li>Use single crystal p-InP (orientation (111 A), Zn doping concentration of 5 &#215; 1017 cm-3) as the photoabsorber. For the back contact preparation, evaporate 4 nm Au, 80 nm Zn and 150 nm Au on the backside of the wafer and heat it to 400 &#176;C for 60 s.</li>
	<li>Apply Ag paste to attach the ohmic contact to a thin-plated Cu wire. Thread the wire to a glass tube, encapsulate the sample and seal it to the glass tube using black, chemical resistant epoxy.</li>
	<li>In order to remove native oxides, etch the 0.5 cm2 polished indium face of p-InP for 30 s in 10 mL of bromine/methanol solution (0.05 % w/v), rinse the surface with ethanol and ultrapure water for 10 s each and dry the sample under nitrogen flux. Prepare the solutions from ultrapure water and analytical grade chemicals with an organic impurity level below 50 ppb. 
	CAUTION: Bromine causes acute toxicity upon inhalation, skin corrosion and acute aquatic toxicity. Wear protective equipment such as safety glasses, gloves and lab coat. Work under the fume hood. Methanol is flammable, causes acute toxicity (oral, dermal and inhalation) and is known to cause specific target organ toxicity. Wear protective equipment such as safety glasses, gloves and a lab coat. Work under the fume hood.</li>
	<li>Subsequently, condition the p-InP electrode photoelectrochemically in a standard three-electrode potentiostatic arrangement. Use a borosilicate glass cell with a quartz window as a photoelectrochemical cell in order to illuminate the sample with a white-light tungsten halogen lamp (100 mW/cm2) during the procedure.</li>
	<li>Adjust the light intensity with a calibrated silicon reference photodiode.</li>
	<li>Prepare a 0.5 M HCl solution and purge it in the photoelectrochemical cell with nitrogen of 5.0 purity for 15 min.</li>
	<li>Use potentiodynamic cycling between -0.44 V und +0.31 V at a scan rate of 50 mV s-1 for 50 cycles to photoelectrochemically condition the sample under continuous illumination. 
	CAUTION: Hydrochloric acid causes serious eye damage, skin corrosion and it is corrosive to metals. Furthermore, it possesses specific target organ toxicity following single exposure. Wear protective equipment such as safety glasses, gloves and a lab coat. Work under the fume hood.</li>
</ol>
2. Fabrication of rhodium nanostructures
<ol>
	<li>Employ shadow nanosphere lithography (SNL)11,12 for the formation of rhodium nanostructures on the p-InP photoelectrode. In order to create the polystyrene masks on the p-InP electrode, obtain mono-dispersed beads of polystyrene (PS) sized 784 nm at a concentration of 5% (w/v) and dissolve them in ultrapure water.</li>
	<li>To obtain the final volume of 600 &#956;L, mix 300 &#956;L of the polystyrene bead dispersion with 300 &#956;L of ethanol containing 1% (w/v) styrene and 0.1% sulphuric acid (v/v).</li>
	<li>Apply the solution onto the water surface using a Pasteur pipette with a curved tip. In order to increase the area of the monocrystalline structures, turn the Petri dish gently. Carefully distribute the solution to cover 50% of the air-water interface with a hcp monolayer. Leave place for stress relaxation and avoid forming cracks in the lattice during the next preparation steps.</li>
	<li>Protect the Cu wire of the photoelectrochemically conditioned p-InP electrodes with parafilm. Place them delicately under the floating closed packed PS sphere mask by carefully taping them to a microscope slide, preventing the samples from rotating. Gently remove residual water with a pipette and by evaporation, causing the mask to be subsequently deposited onto the electrode surface.</li>
	<li>Take the electrode out of the Petri dish and gently dry the surface with N2. Store the electrode under nitrogen until rhodium photoelectrodeposition (e.g., in a desiccator). 
	NOTE: The protocol can be paused here for up to one week.</li>
</ol>
3. Photoelectrodeposition of rhodium nanoparticles
<ol>
	<li>For the photoelectrochemical deposit of rhodium nanoparticles through the PS sphere mask, place the electrode in an electrolyte solution containing 5 mM RhCl3, 0.5 M NaCl and 0.5% (v/v) 2-propanol and apply a constant potential of Vdep = +0.01 V for 5 s under simultaneous illumination with a W-I lamp (100 mW/cm2). Electrochemical specifications such as the electrochemical cell, reference and counter electrode are the same as for the photoelectrochemical conditioning procedure.</li>
	<li>Rinse the photoelectrode with ultrapure water and dry it under a gentle flow of N2.</li>
	<li>In order to remove the PS-spheres from the electrode surface, place the electrodes for 20 min under gentle stirring in a beaker with 10 mL of toluene (the electrode should be covered with toluene). Subsequently, rinse the electrode with acetone and ethanol for 20 s each.</li>
	<li>Remove residual carbon from the surface by O2-plasma cleaning for 6 min at a process pressure of 0.16 mbar, 65 W and gas inflows of O2 and Ar of 2 sccm and 1 sccm, respectively.</li>
	<li>Prepare the samples up to one week prior to tests in the drop tower and store them until the experiments under N2 atmosphere in the dark (e.g., in a glove bag or desiccator). 
	NOTE: The protocol can be paused here for about 1-2 weeks.</li>
</ol>
4. Photoelectrochemical experiments in microgravity
<ol>
	<li>For the experiments in microgravity environment, contact one of the major drop tower facilities, (e.g., the Centre of Applied Space Technology and Microgravity (ZARM), Bremen Germany). 
	NOTE: By employing a catapult system, 9.3 s of microgravity environment can be generated at ZARM with an approached minimum g-level of about 10-6 m&#183;s-2 13. A hydraulically controlled pneumatic piston-cylinder system is used to launch the drop capsule (Figure 1A) upwards from the bottom of the tower. The capsule is decelerated again in a container which is placed onto the cylinder system during the time of free fall.</li>
	<li>Use a two-compartment photoelectrochemical cell (filling volume of each cell: 250 mL) for the photoelectrochemical experiments in order to carry out two experiments in microgravity environment in parallel. The front of each cell should consist of an optical quartz glass window (diameter: 16 mm) for illuminating the working electrode (see Figure 1B).</li>
	<li>Employ a three-electrode arrangement in each cell for the photoelectrochemical measurements with a Pt counter electrode and an Ag/AgCl (3 M KCl) reference electrode in HClO4 (1 M). Add 1% (v/v) isopropanol to the electrolyte in order to reduce the surface tension and enhance gas bubble release. Use a W-I white light source for illuminating each cell compartment through the optical windows. 
	CAUTION: Concentrated perchloric acid is a strong oxidizer. Organic, metallic and non-organic salts formed from oxidation are shock sensitive and pose a great fire and explosion hazard. Wear safety glasses, gloves and a protective lab coat. Work under the fume hood and minimize bench top storage time.</li>
	<li>For gas bubble investigations, attach two cameras to each cell via optical mirrors and beamsplitters (e.g., a color camera at the front and a monochromatic camera at the side, see Figure 1) to record gas bubble evolution during free fall of the experiment. For each drop, store the recorded data on an integrated board computer in the drop capsule. Record single pictures at a frame rate of (e.g., 25 fps (color camera) and 60 fps (monochromatic camera)).</li>
	<li>The drop capsule is equipped with several boards (Figure 1). Mount the photoelectrochemical set-up and the cameras onto an optical board and attach it to one of the middle boards in the capsule. Use the remaining boards for installment of additional equipment such as potentiostats, light sources, shutter controls and the board computer. Attach a battery supply at the bottom board of the capsule to power the set-up during free fall (Figure 1).</li>
	<li>Write an automated drop sequence for the experimental steps which should be controlled and carried out in microgravity environment. The program should be started prior to each drop. Upon reaching microgravity environment, the sequence should automatically start cameras, illumination sources and the electrochemical experiment for the duration of 9.3 s while simultaneously immersing the working electrode into the electrolyte using a pneumatic system (see Figure 1, Table 1).</li>
	<li>Investigation of light-assisted hydrogen production on the samples in photoelectrochemical measurements (e.g., cyclic voltammetry and chronoamperometry).
	<ol>
		<li>Control the electrochemical parameters by the two potentiostats in the capsule. For optimal resolutions in J - V measurements, use scan rates (dE/dt) of 218 mV/s to 235 mV/s in order to run 3 scan cycles in cycling voltammetry experiments, using voltage ranges of +0.25 V to -0.3 V vs Ag/AgCl (3 M KCl). Employ the initial potential, Ei = +0.2 V vs Ag/AgCl (3 M KCl) and the finishing potential, Ef = +0.2 V vs Ag/AgCl (3 M KCl). To compare the recorded J - V measurements, take the second scan cycle of each experiment for analysis.</li>
		<li>In chronoamperometric measurements, use the time scale of generated microgravity environment, 9.3 s, to record the photocurrent produced by the sample. Apply potential ranges of -0.3 V to -0.6 V vs Ag/AgCl (3 M KCl) to compare produced photocurrents.</li>
	</ol>
	</li>
	<li>At the end of each drop, when the drop capsule is decelerated again to zero velocity, use the drop sequence to let the sample be removed from the electrolyte and cameras, potentiostats and illumination sources be switched off.</li>
	<li>After retrieving the capsule from the deceleration container, remove the capsule protection shield. Remove the samples from the pneumatic stative, rinse them with ultrapure water and dry them under gentle nitrogen flux. Store them under N2 atmosphere until optical and spectroscopic investigations are carried out.</li>
	<li>Exchange the electrolyte in the two cells, ensure the function of all instruments before equipping the cells with new samples and prepare the capsule for another drop experiment.</li>
</ol>

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The authors have nothing to disclose.

For the preparation of photoelectrodes, it is important to minimize oxygen exposure between the etching and conditioning procedure and to purge the 0.5 M HCl before usage for about 10 - 15 min with nitrogen. Once the samples are conditioned, they can be stored under nitrogen atmosphere in 15 mL conical tubes for a few hours to allow sample transport and/ or preparation time of the polystyrene particle masks. In order to achieve a homogenous arrangement of PS spheres on the electrode substrate, it is important to form a c...

Our atmosphere on Earth is formed through oxygenic photosynthesis, a 2.3-billion-year-old process converting solar energy into energy-rich hydrocarbons, releasing oxygen as a by-product and using water and CO2 as substrates. Currently, artificial photosynthetic systems following the concept of the energetic Z-scheme of catalysis and charge transfer in natural photosynthesis are realized in semiconductor-electrocatalyst systems, showing hitherto a solar-to-hydrogen conversion efficiency of 19 %1,2,3. In these systems, semiconductor materials are employed as light absorbers which are coated with a thin, transparent layer of electrocatalysts4. Intense research in this field is promoted by the global quest for renewable energy systems with hydrogen and long-chain hydrocarbons making excellent candidates for an alternative fuel supply. Similar obstacles are also faced on long-term space missions, where a resupply of resources from Earth is not possible. A reliable life-support hardware is required, employing an efficient air revitalization unit providing about 310 kg of oxygen per crew member per year, not accounting for extravehicular activities5. An efficient solar water-splitting device, capable of producing oxygen and hydrogen or reduce carbon dioxide solar-assisted and in a monolithic system would provide an alternative, lighter route to currently employed technologies on the ISS: the air revitalization unit is comprised of a separated system with an alkaline electrolyzer, a solid amine carbon dioxide concentrator and a Sabatier reactor for the reduction of CO2.
Unprecedentedly, we realized efficient solar-hydrogen production in microgravity environment, provided by a 9.3 s during free-fall at the Bremen Drop Tower (ZARM, Germany)6. Using p-type indium phosphide as a semiconducting light-absorber7,8 coated with a nanostructured rhodium electrocatalyst, we overcame substrate and product mass transfer limitations to and from the photoelectrode surface, which is an obstacle in reduced gravity environments due to the absence of buoyancy9,10. The application of shadow nanosphere lithography11,12 directly on the photoelectrode surface allowed the formation of rhodium catalytic 'hot spots', which prevented hydrogen gas bubble coalescence and the formation of a froth layer in proximity of the electrode surface.
Herein, we provide experimental details of the p-InP photoelectrode preparation including surface etching and conditioning, followed by the application of shadow nanosphere lithography on the electrode surface and the photoelectrodeposition of rhodium nanoparticles through the polystyrene spheres. Furthermore, the experimental set-up in the drop capsule at the Bremen Drop Tower is described and details of the experimental sequence during the 9.3 s of free fall are provided. Sample installment and handling before and after each drop are outlined as well as the preparation of the drop capsule and its equipment to operate illumination sources, potentiostats, shutter controls and video cameras upon command.

<table><tbody><tr><td>12.7 mm XZ Dovetail Translation Stage with Baseplate, M4 Taps (4 x)</td><td>Thorlabs</td><td>DT12XZ/M</td><td></td></tr><tr><td>Beam splitters (2 x)</td><td>Thorlabs</td><td>CM1-BS013</td><td>50:50 400-700nm</td></tr><tr><td>Beamsplitters (2 x)</td><td>Thorlabs</td><td>CM1-BS014</td><td>50:50 700-1100nm</td></tr><tr><td>Ohmic back contact: 4 nm Au, 80 nm Zn, 150 nm Au</td><td>Out e.V., Berlin, Germany</td><td>https://www.out-ev.de/english/index.html</td><td>Company provides custom made ohmic back contacts</td></tr><tr><td>Glass tube, ca. 10 cm, inner diameter about 4 mm</td><td>E.g., Ga&amp;szlig;ner Glasstechnik</td><td></td><td>Custom made</td></tr><tr><td>p-InP wafers, orientation 111A, Zn doping concentration: 5 x 10^17 cm^-3</td><td>AXT Inc. Geo Semiconductor Ltd. Switzerland</td><td></td><td>Custom made</td></tr><tr><td>Photoelectrochemical cell for terrestrial experiments</td><td>E.g., glass/ materials workshop</td><td></td><td>Custom made</td></tr><tr><td>Matrox 4Sight GPm (board computer)</td><td>Matrox imaging</td><td></td><td>Ivy Bridge, 7 x Cable Ace power I/O HRS 6p, open 10m, Power Adapter for Matrox 4sight GPm, Samsung 850 Pro 2,5&quot; 1 TB, Solid State Drive in exchange for the 250Gb hard drive</td></tr><tr><td>2-propanol</td><td>Sigma Aldrich</td><td>I9516-500ML</td><td></td></tr><tr><td>35mm Kowa LM35HC 1&quot; Sensor F1.4 C-mount (2 x)</td><td>Basler AG</td><td></td><td></td></tr><tr><td>Acetone</td><td>Sigma Aldrich</td><td>650501-1L</td><td></td></tr><tr><td>Ag/AgCl (3 M KCl) reference electrode</td><td>WPI</td><td>DRIREF-5</td><td></td></tr><tr><td>Aluminium breadboard, 450 mm x 450 mm x 12.7mm, M6 Taps (2 x)</td><td>Thorlabs</td><td>MB4545/M</td><td></td></tr><tr><td>Beaker, 100 mL</td><td>VWR</td><td>10754-948</td><td></td></tr><tr><td>Black epoxy</td><td>Electrolube</td><td>ER2162</td><td></td></tr><tr><td>Bromine</td><td>Sigma Aldrich</td><td>1.01945 EMD Millipore</td><td></td></tr><tr><td>Colour camera (2 x)</td><td>Basler AG</td><td>acA2040-25gc</td><td></td></tr><tr><td>Conductive silver epoxy</td><td>MG Chemicals</td><td>8331-14G</td><td></td></tr><tr><td>Copper wire</td><td>E.g., Sigma Aldrich</td><td>349224-150CM</td><td></td></tr><tr><td>Ethanol</td><td>Sigma Aldrich</td><td>459844-500ML</td><td></td></tr><tr><td>Falcon tubes, 15 mL</td><td>VWR</td><td>62406-200</td><td></td></tr><tr><td>Glove bags</td><td>Sigma Aldrich</td><td>Z530212</td><td></td></tr><tr><td>Hydrochloric acid (1 M)</td><td>Sigma Aldrich</td><td>H9892</td><td></td></tr><tr><td>Magnetic stirrer</td><td>VWR</td><td>97042-626</td><td></td></tr><tr><td>Methanol</td><td>Sigma Aldrich</td><td>34860-100ML-R</td><td></td></tr><tr><td>Microscope slides</td><td>VWR</td><td>82003-414</td><td></td></tr><tr><td>MilliQ water</td><td></td><td></td><td></td></tr><tr><td>NIR camera (2 x)</td><td>Basler AG</td><td>acA1300-60gm</td><td></td></tr><tr><td>Nitrogen, grade 5N</td><td>Airgas</td><td>NI UHP300</td><td></td></tr><tr><td>&amp;Oslash; 1&quot; Stackable Lens Tubes (6 x)</td><td>Thorlabs</td><td>SM1L03</td><td></td></tr><tr><td>O2 Plasma Facility</td><td></td><td></td><td></td></tr><tr><td>OEM Flange to SM Thread Adapters (4 x)</td><td>Thorlabs</td><td>SM1F2</td><td></td></tr><tr><td>Parafilm</td><td>VWR</td><td>52858-000</td><td></td></tr><tr><td>Pasteur pipette</td><td>VWR</td><td>14672-380</td><td></td></tr><tr><td>Perchloric acid (1 M)</td><td>Sigma Aldrich</td><td>311421-50ML</td><td></td></tr><tr><td>Petri dish</td><td>VWR</td><td>75845-546</td><td></td></tr><tr><td>Photoelectrochemical cell for microgravity experiments</td><td>E.g., glass/ materials workshop</td><td></td><td></td></tr><tr><td>Polystyrene particles, 784 nm, 5 % (w/v)</td><td>Microparticles GmbH</td><td>0.1-0.99 &amp;micro;m size (50 mg/ml): 10 ml, 15 ml, 50 ml</td><td></td></tr><tr><td>Potentiostats (2 x)</td><td>Biologic</td><td>SP-200/300</td><td></td></tr><tr><td>Pt counter electrode</td><td>ALS-Japan</td><td>12961</td><td></td></tr><tr><td>Rhodium (III) chlorid</td><td>Sigma Aldrich</td><td>520772-1G</td><td></td></tr><tr><td>Shutter control system (2 x)</td><td></td><td></td><td></td></tr><tr><td>Silicon reference photodiode</td><td>Thorlabs</td><td>FDS1010</td><td></td></tr><tr><td>Sodium chlorid</td><td>Sigma Aldrich</td><td>567440-500GM</td><td></td></tr><tr><td>Stands and rods to fix the cameras</td><td>VWR</td><td></td><td></td></tr><tr><td>Sulphuric acid (0.5 M)</td><td>Sigma Aldrich</td><td>339741-100ML</td><td></td></tr><tr><td>Telecentric High Resolution Type WD110 series Type MML1-HR110</td><td>Basler AG</td><td></td><td></td></tr><tr><td>Toluene</td><td>Sigma Aldrich</td><td>244511-100ML</td><td></td></tr><tr><td>Various spare beakers and containers for leftover perchloric acid etc for the drop tower</td><td>VWR</td><td></td><td></td></tr><tr><td>W-I lamp with light guides (2 x)</td><td>Edmund Optics</td><td>Dolan-Jenner MI-150 Fiber Optic Illuminator</td><td></td></tr><tr><td>CM-12 electron microscope with a twin objective lens, CCD camera (Gatan) system and an energy dispersive spectroscopy of X- rays (EDS) system)</td><td>Philips</td><td></td><td></td></tr><tr><td>Dimension Icon AFM, rotated symmetric ScanAsyst-Air tips (silicon nitride), nominal tip radius of 2 nm</td><td>Bruker</td><td></td><td></td></tr></tbody></table>

experimental methods for efficient solar hydrogen production in microgravity environment

Long-term space flights and cis-lunar research platforms require a sustainable and light life-support hardware which can be reliably employed outside the Earth's atmosphere. So-called 'solar fuel' devices, currently developed for terrestrial applications in the quest for realizing a sustainable energy economy on Earth, provide promising alternative systems to existing air-revitalization units employed on the International Space Station (ISS) through photoelectrochemical water-splitting and hydrogen production. One obstacle for water (photo-) electrolysis in reduced gravity environments is the absence of buoyancy and the consequential, hindered gas bubble release from the electrode surface. This causes the formation of gas bubble froth layers in proximity to the electrode surface, leading to an increase in ohmic resistance and cell-efficiency loss due to reduced mass transfer of substrates and products to and from the electrode. Recently, we have demonstrated efficient solar hydrogen production in microgravity environment, using an integrated semiconductor-electrocatalyst system with p-type indium phosphide as the light-absorber and a rhodium electrocatalyst. By nanostructuring the electrocatalyst using shadow nanosphere lithography and thereby creating catalytic 'hot spots' on the photoelectrode surface, we could overcome gas bubble coalescence and mass transfer limitations and demonstrated efficient hydrogen production at high current densities in reduced gravitation. Here, the experimental details are described for the preparations of these nanostructured devices and further on, the procedure for their testing in microgravity environment, realized at the Bremen Drop Tower during 9.3 s of free fall.

Etching the p-InP surface in Br2/ methanol for 30 s with consecutive photoelectrochemical conditioning of the sample by cycling polarization in HCl is well established in the literature and discussed (e.g., by Schulte &#38; Lewerenz (2001)14,15). The etching procedure removes the native oxide remaining on the surface (Figure 2) and electrochemical cycling in HCl causes furthermore a conside...

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Experimental Methods for Efficient Solar Hydrogen Production in Microgravity Environment

Efficient solar-hydrogen production has recently been realized on functionalized semiconductor-electrocatalyst systems in a photoelectrochemical half-cell in microgravity environment at the Bremen Drop Tower. Here, we report the experimental procedures for manufacturing the semiconductor-electrocatalyst device, details of the experimental set-up in the drop capsule and the experimental sequence during free fall.

Long-term space flights and cis-lunar research platforms require a sustainable and light life-support hardware which can be reliably employed outside the ...

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7.6K Views.  California Institute of Technology. Dit protocol bevat een stapsgewijze procedure van de constructie van nanogestructureerde foto-elektroden voor efficiënte lichtondersteunde waterstofproductie in microzwaartekrachtomgevingen. Het omvat ook een elektrode testprocedure van hoe deze foto-elektroden te testen op de Bremen Drop Tower, waar 10 tot de min zes g kan worden gegenereerd in 9,2 seconden van de vrije val. Het wordt waargenomen door ons en andere onderzoeksteams dat elektrochemisch gegenereerde gasbellen vasthouden aan he...

Video: Experimental Methods for Efficient Solar Hydrogen Production in Microgravity Environment

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 ...

Engineering

Integration of Light Trapping Silver Nanostructures in Hydrogenated Microcrystalline Silicon Solar Cells by Transfer Printing

One of the potential applications of metal nanostructures is light trapping in solar cells, where unique optical properties of nanosized metals, commonly known as plasmonic effects, play an important role. Research in this field has, however, been impeded owing to the difficulty of fabricating devices containing the desired functional metal nanostructures. In order to provide a viable strategy to this issue, we herein show a transfer printing-based approach that allows the quick and low-cost integration of designed metal nanostructures with a variety of device architectures, including solar cells. Nanopillar poly(dimethylsiloxane) (PDMS) stamps were fabricated from a commercially available nanohole plastic film as a master mold. On this nanopatterned PDMS stamps, Ag films were deposited, which were then transfer-printed onto block copolymer (binding layer)-coated hydrogenated microcrystalline Si (&#181;c-Si:H) surface to afford ordered Ag nanodisk structures. It was confirmed that the resulting Ag nanodisk-incorporated &#181;c-Si:H solar cells show higher performances compared to a cell without the transfer-printed Ag nanodisks, thanks to plasmonic light trapping effect derived from the Ag nanodisks. Because of the simplicity and versatility, further device application would also be feasible thorough this approach.

A viable transfer printing-based methodology to introduce plasmonic metal nanostructures in solar cells is described. Using nanopillar poly(dimethylsiloxane) stamps, an Ag-based ordered nanodisk array was integrated with standard hydrogenated microcrystalline Si solar cells, which led to improved device performances due to plasmonic light trapping.

One of the potential applications of metal nanostructures is light trapping in solar cells, where unique optical properties of nanosized metals, commonly ...

Printing Fabrication of Bulk Heterojunction Solar Cells and In Situ Morphology Characterization

Polymer-based materials hold promise as low-cost, flexible efficient photovoltaic devices. Most laboratory efforts to achieve high performance devices have used devices prepared by spin coating, a process that is not amenable to large-scale fabrication. This mismatch in device fabrication makes it difficult to translate quantitative results obtained in the laboratory to the commercial level, making optimization difficult. Using a mini-slot die coater, this mismatch can be resolved by translating the commercial process to the laboratory and characterizing the structure formation in the active layer of the device in real time and in situ as films are coated onto a substrate. The evolution of the morphology was characterized under different conditions, allowing us to propose a mechanism by which the structures form and grow. This mini-slot die coater offers a simple, convenient, material efficient route by which the morphology in the active layer can be optimized under industrially relevant conditions. The goal of this protocol is to show experimental details of how a solar cell device is fabricated using a mini-slot die coater and technical details of running in situ structure characterization using the mini-slot die coater.

Printing Fabrication of Bulk Heterojunction Solar Cells and In Situ Morphology Characterization

Here, we present a protocol to fabricate organic thin film solar cells using a mini-slot die coater and related in-line structure characterizations using synchrotron scattering techniques.

Polymer-based materials hold promise as low-cost, flexible efficient photovoltaic devices. Most laboratory efforts to achieve high performance devices ...

A Simple, Low-cost, and Robust System to Measure the Volume of Hydrogen Evolved by Chemical Reactions with Aqueous Solutions

There is a growing research interest in the development of portable systems which can deliver hydrogen on-demand to proton exchange membrane (PEM) hydrogen fuel cells. Researchers seeking to develop such systems require a method of measuring the generated hydrogen. Herein, we describe a simple, low-cost, and robust method to measure the hydrogen generated from the reaction of solids with aqueous solutions. The reactions are conducted in a conventional one-necked round-bottomed flask placed in a temperature controlled water bath. The hydrogen generated from the reaction in the flask is channeled through tubing into a water-filled inverted measuring cylinder. The water displaced from the measuring cylinder by the incoming gas is diverted into a beaker on a balance. The balance is connected to a computer, and the change in the mass reading of the balance over time is recorded using data collection and spreadsheet software programs. The data can then be approximately corrected for water vapor using the method described herein, and parameters such as the total hydrogen yield, the hydrogen generation rate, and the induction period can also be deduced. The size of the measuring cylinder and the resolution of the balance can be changed to adapt the setup to different hydrogen volumes and flow rates.

The study of methods to generate on-demand hydrogen for fuel cells continues to grow in importance. However, systems to measure hydrogen evolution from the reaction of chemicals with water can be complicated and expensive. This article details a simple, low-cost, and robust method to measure the evolution of hydrogen gas.

There is a growing research interest in the development of portable systems which can deliver hydrogen on-demand to proton exchange membrane (PEM) ...

Non-equilibrium Microwave Plasma for Efficient High Temperature Chemistry

A flowing microwave plasma based methodology for converting electric energy into internal and/or translational modes of stable molecules with the purpose of efficiently driving non-equilibrium chemistry is discussed. The advantage of a flowing plasma reactor is that continuous chemical processes can be driven with the flexibility of startup times in the seconds timescale. The plasma approach is generically suitable for conversion/activation of stable molecules such as CO2, N2 and CH4. Here the reduction of CO2 to CO is used as a model system: the complementary diagnostics illustrate how a baseline thermodynamic equilibrium conversion can be exceeded by the intrinsic non-equilibrium from high vibrational excitation. Laser (Rayleigh) scattering is used to measure the reactor temperature and Fourier Transform Infrared Spectroscopy (FTIR) to characterize in situ internal (vibrational) excitation as well as the effluent composition to monitor conversion and selectivity.

This article describes a flowing microwave reactor that is used to drive efficient non-equilibrium chemistry for the application of conversion/activation of stable molecules such as CO2, N2 and CH4. The goal of the procedure described here is to measure the in situ gas temperature and gas conversion.

A flowing microwave plasma based methodology for converting electric energy into internal and/or translational modes of stable molecules with the purpose ...

Optimization, Test and Diagnostics of Miniaturized Hall Thrusters

Miniaturized spacecraft and satellites require smart, highly efficient and durable low-thrust thrusters, capable of extended, reliable operation without attendance and adjustment. Thermochemical thrusters which utilize thermodynamic properties of gases as a means of acceleration have physical limitations on their exhaust gas velocity, resulting in low efficiency. Moreover, these engines demonstrate extremely low efficiency at small thrusts and may be unsuitable for continuously operating systems which provide real-time adaptive control of the spacecraft orientation, velocity and position. In contrast, electric propulsion systems which use electromagnetic fields to accelerate ionized gases (i.e., plasmas) do not have any physical limitation in terms of exhaust velocity, allowing virtually any mass efficiency and specific impulse. Low-thrust Hall thrusters have a lifetime of several thousand hours. Their discharge voltage ranges between 100 and 300 V, operating at a nominal power of &lt;1 kW. They vary from 20 to 100 mm in size. Large Hall thrusters can provide fractions of millinewton of thrust. Over the past few decades, there has been an increasing interest in small mass, low power, and high efficiency propulsion systems to drive satellites of 50-200 kg. In this work, we will demonstrate how to build, test, and optimize a small (30 mm) Hall thruster capable of propelling a small satellite weighing about 50 kg. We will show the thruster operating in a large space environment simulator, and describe how thrust is measured and electric parameters, including plasma characteristics, are collected and processed to assess key thruster parameters. We will also demonstrate how the thruster is optimized to make it one of the most efficient small thrusters ever built. We will also address challenges and opportunities presented by new thruster materials.

Here, we present a protocol to test and optimize space propulsion systems based on miniaturized Hall-type thrusters.

Miniaturized spacecraft and satellites require smart, highly efficient and durable low-thrust thrusters, capable of extended, reliable operation without ...

CO2 Photoreduction to CH4 Performance Under Concentrating Solar Light

We demonstrate a method for the enhancement of CO2 photoreduction. As the driving force of a photocatalytic reaction is from solar light, the basic idea is to use concentration technology to raise the incident solar light intensity. Concentrating a large-area light onto a small area cannot only increase light intensity, but also reduce the catalyst amount, as well as the reactor volume, and increase the surface temperature. The concentration of light can be realized by different devices. In this manuscript, it is realized by a Fresnel lens. The light penetrates the lens and is concentrated on a disc-shaped catalyst. The results show that both the reaction rate and the total yield are efficiently increased. The method can be applied to most CO2 photoreduction catalysts, as well as to similar reactions with a low reaction rate at natural light.

CO2 Photoreduction to CH4 Performance Under Concentrating Solar Light

We present a protocol for improving the performance of CO2 photoreduction to CH4 by heightening the incident light intensity via concentrating solar energy technology.

We demonstrate a method for the enhancement of CO2 photoreduction. As the driving force of a photocatalytic reaction is from solar light, the basic idea ...

Developing Photosensitizer-Cobaloxime Hybrids for Solar-Driven H2 Production in Aqueous Aerobic Conditions

Developing photocatalytic H2 production devices is the one of the key steps for constructing a global H2-based renewable energy infrastructure. A number of photoactive assemblies have emerged where a photosensitizer and cobaloxime-based H2 production catalysts work in tandem to convert light energy into the H-H chemical bonds. However, the long-term instability of these assemblies and the need for hazardous proton sources have limited their usage. Here, in this work, we have integrated a stilbene-based organic dye into the periphery of a cobaloxime core via a distinct axial pyridine linkage. This strategy allowed us to develop a photosensitizer-catalyst hybrid structure with the same molecular framework. In this article, we have explained the detailed procedure of the synthesis of this hybrid molecule in addition to its comprehensive chemical characterization. The structural and optical studies have exhibited an intense electronic interaction between the cobaloxime core and the organic photosensitizer. The cobaloxime was active for H2 production even in the presence of water as the proton source. Here, we have developed a simple airtight system connected with an online H2 detector for the investigation of the photocatalytic activity by this hybrid complex. This photosensitizer-catalyst dyad present in the experimental setup continuously produced H2 once it was exposed in the natural sunlight. This photocatalytic H2 production by the hybrid complex was observed in aqueous/organic mixture media in the presence of a sacrificial electron donor under complete aerobic conditions. Thus, this photocatalysis measurement system along with the photosensitizer-catalyst dyad provide valuable insight for the development of next generation photocatalytic H2 production devices.

Developing Photosensitizer-Cobaloxime Hybrids for Solar-Driven H2 Production in Aqueous Aerobic Conditions

We have directly incorporated a stilbene-based organic dye into a cobaloxime core to generate a photosensitizer-catalyst dyad for photocatalytic H2 production. We have also developed a simple experimental setup for evaluating the light-driven H2 production by photocatalytic assemblies.

Developing photocatalytic H2 production devices is the one of the key steps for constructing a global H2-based renewable energy infrastructure. A number ...

Synthesis of Metal Nanoparticles Supported on Carbon Nanotube with Doped Co and N Atoms and its Catalytic Applications in Hydrogen Production

A method for facile synthesis of nanostructured catalysts supported on carbon nanotubes with atomically dispersed cobalt and nitrogen dopant is presented herein. The novel strategy is based on a facile one-pot pyrolysis treatment of cobalt (II) acetylacetonate and nitrogen-rich organic precursors under Ar atmosphere at 800 &#176;C, resulting in the formation of Co- and N- co-doped carbon nanotube with earthworm-like morphology. The obtained catalyst was found to have a high density of defect sites, as confirmed by Raman spectroscopy. Here, cobalt (II) nanoparticles were stabilized on the atomically dispersed cobalt- and nitrogen-doped carbon nanotubes. The catalyst was confirmed to be effective in the catalytic hydrolysis of ammonia borane, in which the turnover frequency was 5.87 mol H2&#183;molCo-1&#183;min-1, and the specific hydrogen generation rate was determined to be 2447 mL H2&#183;gCo-1&#183;min-1. A synergistic function between the Co nanoparticle and the doped carbon nanotubes was proposed for the first time in the catalytic hydrolysis of ammonia borane reaction under a mild condition. The resulting hydrogen production with its high energy density and minimal refueling time could be suitable for future development as energy sources for mobile and stationary applications such as road trucks and forklifts in transport and logistics.

Here, we present a protocol to synthesize Co nanoparticles supported on carbon nanotubes with Co- and N- dopants for hydrogen productions.

A method for facile synthesis of nanostructured catalysts supported on carbon nanotubes with atomically dispersed cobalt and nitrogen dopant is presented ...

Hydrogen Production and Utilization in a Membrane Reactor

Industrial hydrogenation consumes ~11 Mt of fossil-derived H2 gas yearly. Our group invented a membrane reactor to bypass the need to use H2 gas for hydrogenation chemistry. The membrane reactor sources hydrogen from water and drives reactions using renewable electricity. In this reactor, a thin piece of Pd separates an electrochemical hydrogen production compartment from a chemical hydrogenation compartment. The Pd in the membrane reactor acts as (i) a hydrogen-selective membrane, (ii) a cathode, and (iii) a catalyst for hydrogenation. Herein, we report the use of atmospheric mass spectrometry (atm-MS) and gas chromatography mass spectrometry (GC-MS) to demonstrate that an applied electrochemical bias across a Pd membrane enables efficient hydrogenation without direct H2 input in a membrane reactor. With atm-MS, we measured a hydrogen permeation of 73%, which enabled the hydrogenation of propiophenone to propylbenzene with 100% selectivity, as measured by GC-MS. In contrast to conventional electrochemical hydrogenation, which is limited to low concentrations of starting material dissolved in a protic electrolyte, the physical separation of hydrogen production from utilization in the membrane reactor enables hydrogenation in any solvent or at any concentration. The use of high concentrations and a wide range of solvents is particularly important for reactor scalability and future commercialization.

Membrane reactors enable hydrogenation in ambient conditions without direct H2 input. We can track the hydrogen production and utilization in these systems using atmospheric mass spectrometry (atm-MS) and gas chromatography mass spectrometry (GC-MS).

Industrial hydrogenation consumes ~11 Mt of fossil-derived H2 gas yearly. Our group invented a membrane reactor to bypass the need to use H2 gas for ...

Experimental Methods for Efficient Solar Hydrogen Production in Microgravity Environment

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