Experimental Methods for Efficient Solar Hydrogen Production in Microgravity Environment

1. Preparation of p-InP photoelectrodes

1. Use single crystal p-InP (orientation (111 A), Zn doping concentration of 5 × 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 °C for 60 s.
2. 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.
3. 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.
4. 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.
5. Adjust the light intensity with a calibrated silicon reference photodiode.
6. Prepare a 0.5 M HCl solution and purge it in the photoelectrochemical cell with nitrogen of 5.0 purity for 15 min.
7. 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.

2. Fabrication of rhodium nanostructures

1. 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.
2. To obtain the final volume of 600 μL, mix 300 μL of the polystyrene bead dispersion with 300 μL of ethanol containing 1% (w/v) styrene and 0.1% sulphuric acid (v/v).
3. 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.
4. 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.
5. 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.

3. Photoelectrodeposition of rhodium nanoparticles

1. 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 V_dep = +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.
2. Rinse the photoelectrode with ultrapure water and dry it under a gentle flow of N₂.
3. 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.
4. 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.
5. 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.

4. Photoelectrochemical experiments in microgravity

1. 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·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.
2. 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).
3. 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.
4. 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)).
5. 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).
6. 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).
7. Investigation of light-assisted hydrogen production on the samples in photoelectrochemical measurements (e.g., cyclic voltammetry and chronoamperometry).
   1. 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, E_i = +0.2 V vs Ag/AgCl (3 M KCl) and the finishing potential, E_f = +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.
   2. 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.
8. 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.
9. 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.
10. 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.