Preparation of the p-InP photoelectrodes

Single crystal p-InP wafers with the orientation (111A) were obtained from AXT Inc. (Geo Semiconductor Ltd. Switzerland) with a Zn doping concentration of 5 × 1017 cm−3. The preparation of an ohmic back contact involved the evaporation of 4 nm Au, 80 nm Zn and 150 nm Au on the backside of the wafer which was then heated to 400 °C for 60 s. The 0.5 cm2 polished indium face of (111A) p-InP was furthermore etched for 30 s in bromine (0.05% (w/v))/methanol solution, rinsed with ethanol and ultrapure water and dried under nitrogen flux. All solutions were made from ultrapure water and analytical grade chemicals with an organic impurity level below 50 ppb. Subsequent cyclic voltammetric and chronoamperometric measurements were performed in a standard three-electrode potentiostatic arrangement whereas a carbon electrode was used as counter electrode and an Ag/AgCl (3 M) was employed as reference electrode. All potentials are converted to those vs. reversible hydrogen electrode (RHE). Moreover, the p-InP surface was photoelectrochemically conditioned in 0.5 M HCl, realized by potentiodynamic cycling under illumination (100 mW/cm2) between −0.44 V and +0.31 V at a scan rate of 50 mV/s while purging with nitrogen of 5.0 purity. Illumination occurred with a white-light tungsten halogen lamp (Edmund Optics) through a quartz window of the borosilicate glass cell. The light intensity was adjusted with a calibrated silicon reference photodiode.

A thin Rh layer was photoelectrochemically deposited from a solution of 5 mM RhCl 3 , 0.5 M NaCl and 0.5 vol% 2-propanol for 5 s at a constant potential of V dep = + 0.01 V and a light intensity of 100 mW/cm2 using the same settings as for the photoelectrochemical conditioning procedure. The electrodeposition resulted in the formation of a nanocrystalline thin film or a nanostructured surface morphology if the rhodium was deposited through a polystyrene mask applying SNL (see below).

To compare the current–voltage characteristics and solar-to-hydrogen conversion efficiency of the photocathodes under terrestrial and microgravity conditions, sample electrodes were also tested in 1 M HClO 4 electrolyte solution upon illumination with a W-I white-light source (100 mW/cm2) under terrestrial conditions in the laboratory in a quartz glass cell. Samples for the tests in the Drop Tower facility were prepared one week prior to testing and stored under N 2 atmosphere in the dark. XPS analysis of the stored samples did not show changes of the surface composition in comparison to freshly prepared samples (see below).

Fabrication of rhodium nanostructures

SNL16 was employed to fabricate rhodium nanostructures on the InP substrate. For creating the masks, mono-dispersed beads of polystyrene (PS) sized 784 nm obtained at a concentration of 5% (w/v) from Microparticles GmbH were dissolved in MiliQ water and further diluted. For the final solution of 600 μl, 300 μl of the PS-beads dispersion was mixed with 300 μl ethanol containing 1% (w/v) styrene and 0.1% sulphuric acid (v/v). The solution was applied onto the air–water interface using a Pasteur pipette with a curved tip. In order to raise the area of the monocrystalline structures, the petri dish was gently turned, resulting in the transformation of multiple smaller domains into larger ones. The solution was carefully distributed to cover 50% of the water surface with a hcp monolayer, while leaving place for stress relaxation and avoiding formation of cracks in the lattice during the next preparation steps. The photoelectrochemically conditioned p-InP electrodes were delicately placed under the floating closed-packed PS sphere mask in the petri dish. Residual water was gently removed by pumping and evaporation with the mask being subsequently deposited onto the electrode. After the surface was dried with N 2 , rhodium was photoelectrochemically deposited through the PS spheres as described above. The samples were furthermore rinsed with MiliQ water and dried under a gentle flow of N 2 . The PS spheres were removed from the surface by placing the electrodes for 20 min under gentle stirring in a beaker with toluene. The electrodes were further cleaned by rinsing the sample with acetone and ethanol for 20 s. In order to remove residual carbon from the surface, O 2 -plasma cleaning was employed for 6 min at a process pressure of 0.16 mbar, 65 W and gas inflows of O 2 and Ar of 2 sccm and 1 sccm, respectively.

Structural and optical characterization

Soft Tapping Mode Atomic Force Microscopy (TM-AFM) was used for the characterization of the surface morphology after each treatment step using a Bruker Dimension Icon AFM. In order to optimize the tapping (mode) frequency and experimental parameters such as gain, set point and cantilever tuning, ScanAsyst mode was used. ScanAsyst-Air tips (silicon nitride) were employed with a rotated (symmetric) geometry and a nominal tip radius of 2 nm. Peakforce Quantitative Nanomechanical parameters provide information on the height, adhesion and deformation of the sample surface.

Reflectance spectra of the thin-film and nanostructured photoelectrodes were obtained in air using a Cary 5000 UV/vis/NIR with an integrating sphere that include diffuse reflectivity measurement.

SEM images were obtained with a FEI Nova NanoSEM 450 microscope.

HRTEM analysis was performed with a Philips CM-12 electron microscope with twin objective lenses as well as a CCD camera (Gatan) system and an Energy-dispersive spectroscopy of X-rays system to measure the sample composition. For sample preparation, the thin rhodium film deposited on the p-InP substrate was scratched off and placed onto an amorphous carbon-coated (ca. 50 Å thickness) copper grid. The grid was then transferred to an electron microscope. A point number of grids was prepared from each sample in order to ensure the reproducibility of the preparative procedure.

Photoelectron spectroscopy

XPS analysis was performed using a system from VG Scienta with a base pressure below 8 × 10−9 mbar equipped with a Scienta R3000 analyser and a monochromatic Al K α (1486.6 eV) X-ray source. The analyser was operating at a pass energy fixed at 200 eV for survey scans and 50 eV for regional spectra acquisition. The used slit sizes were 3 mm and 0.4 mm for the survey and region scans, respectively. The measured surface area was 5 × 1 mm2.

The binding energy scale was calibrated by calibrating the position of the C 1s peak at 284.8 eV. The background photoelectron intensity was subtracted by the Shirley method27. The area under the principal peaks of each element in the XPS spectra and atomic sensitivity factors were used for calculations of atomic concentrations of the elements in approximately top 3–12 nm of the sample surface, depending on a sample.

Prior to testing the p-InP–Rh photocathodes in the drop tower, it was investigated whether they could be prepared in advance of the drop experiments and then stored under nitrogen atmosphere until used. Two samples were prepared and stored under nitrogen atmosphere by 4 days. The XPS spectra after four days of storage did not significantly change from the ones of freshly prepared samples. Furthermore, structural investigations of the electrode surface prior and after the drop did not suggest any changes of the surface morphology caused by the capsule deceleration process.

Photoelectrochemical experiments in microgravity

Microgravity environments were realized at the Drop Tower facility at the Centre of Applied Space Technology and Microgravity (ZARM), Bremen. With the ZARM Catapult System, 9.3 s of microgravity could be generated18. Here, the capsule was launched upwards from the bottom of the tower by a hydraulically controlled pneumatic piston-cylinder system and was decelerated again in a container which was placed onto the cylinder system during free fall of the capsule. The approached minimum g level was about 10−6g.

For the photoelectrochemical experiments in the drop tower, a custom-made two-compartment photoelectrochemical cell was used (filling volume of each cell: 250 mL). Each cell consisted of two optical windows made of quartz glass (diameter: 16 mm) through which the working electrode was illuminated. Photoelectrochemical measurements in the two cells were carried out in a three-electrode arrangement with a Pt counter electrode and an Ag/AgCl (3 M) reference electrode in HClO 4 (1 M) with the addition of 1% (v/v) isopropanol to reduce the surface tension of the electrolyte and favour gas bubble release. XPS measurements and photoelectrochemical measurements in terrestrial conditions did not show any effect of the isopropanol on the (photoelectrochemical) properties of the photoelectrodes. The light intensity of 70 mW/cm2 was provided by a W-I white-light source (Edmund Optics). All experiments were carried out under ambient pressure.

Two cameras (Basler AG; acA2040-25gc and acA1300-60gm NIR, lens types: 35 mm Kowa LM35HC 1” Sensor F1.4 C-mount and Telecentric High Resolution Type WD110 series Type MML1-HR110, respectively) were attached to each cell via optical mirrors (monochromatic camera, side) and beamsplitters (colour camera, front, see Fig. 1c) to record the gas bubble formation in microgravity conditions. Data were stored during each drop on a Matrox 4Sight GPm integrated unit in the drop capsule. Single pictures were recorded at a frame rate of 25 fps (front camera) and 60 fps (side camera).

The photoelectrochemical set-up and the cameras were mounted on an optical board (Thorlabs) attached to the capsule. Power supply in the capsule was provided by a battery. Prior to each drop and during the evacuation time of the drop shaft (about 1.5 h), the capsule and the photoelectrochemical cell were set under Ar atmosphere which was maintained during the drop and during capsule recovery after the drop (about 45 min).

For the photoelectrochemical measurements, an automated drop sequence was written which was started prior to each drop. Upon reaching µg conditions, the sequence started cameras, illumination sources and potentiostats while simultaneously immersing the working electrode into the electrolyte using a pneumatic system (see Fig. 1 and Supplementary Figure 1 for more detailed information). Photoelectrochemical measurements such as cyclic voltammetry and chronoamperometric measurements were performed during the 9.3 s of microgravity. At the end of the drop, when the drop capsule was decelerated again to zero velocity, the sample was emersed from the electrolyte and the cameras, potentiostats and illumination source were switched off. The pneumatic system used for immersing and emersing the sample into and out of the electrolyte ensured that surface morphology changes of the electrode resulted not from long-term exposure to the electrolyte prior or after each drop. After retrieving the capsule from the deceleration container and removal of the protection shield, the samples were removed from the pneumatic stative, rinsed with MiliQ water and dried under nitrogen flux. The sample was stored under N 2 atmosphere until the optical and spectroscopic investigations were carried out.

Theoretical simulations

Lumerical FDTD, a commercial electromagnetic simulation software package, was used to optically model the system. To apply the above set of equations to the structures used here, the following set of assumptions is made. The current–voltage curve incorporating mass transport considerations (Eqs. (2) and (3)) is used for the thin-film sample in microgravity environments, and a limiting mass transport current density, j mtl , of ±5 mA/cm2 is assumed for the anodic and cathodic current density respectively; for all other current–voltage curves Eq. (1) is used. These assumptions are based on experimental observations, as discussed above. The catalyst exchange current density, j 0,cat , is assumed to be 0.1 mA/cm2, which is consistent with experimental reports in literature for Rh as a hydrogen evolution catalyst8. For the InP|Rh Schottky junction, the dark current (j 0 ) is assumed to be 10−8 mA/cm2. Due to the InP x O y layer, the ideal equations for the dark current of a Schottky junction did not accurately describe the system, therefore, this value is based on a fit to the experimentally measured current–voltage curves. For the thin-film sample under simulated microgravity conditions, j 0 was assumed to be 10−5 mA/cm2, accounting for enhanced charge recombination processes in the semiconductor due to mass transfer limitations. The f SA values for the thin-film and nanostructured samples are 1.16 and 1.1, respectively, and are based on the surface areas of the catalyst as determined from the AFM data (Fig. 3b).

Due to the nanostructuring of our catalyst, numerical simulations are required to accurately determine the limiting photocurrent density, j L , which is needed to apply the above set of equations to our photocatalytic system. Lumerical FDTD was used to obtain the InP absorption spectrum, f A (λ). The InP absorption spectrum was furthermore weighted with the lamp spectrum which was used in our experiment and via integration, the absorbed photocurrent, j L , according to Eq. (4) was obtained. Here, λ is the wavelength and λ Eg is the wavelength corresponding to the semiconductor band edge which is 925 nm for InP.

$$j_{\mathrm{L}} = {\scriptstyle{\int

olimits_0^{\lambda _{\mathrm{Eg}}}}} f_{\mathrm{A}}\left( \lambda \right) \cdot {\mathrm{AM}}1.5\mathrm{G}\left( \lambda \right){\mathrm{d}}\lambda$$ (4)

In the optical simulations, the device structure is defined as a semi-infinite layer of InP coated with an 8 nm layer of InP x O y and an effective medium layer of Rh (see XPS data discussion above and refs. 20,28), all embedded in water. The Rh|H 2 O effective medium layer is assumed to follow the Maxwell Garnett approximation, whereas the fill fraction of Rh was 0.4. For the thin-film and nanostructured samples, a layer thickness of 20 and 25 nm, respectively, was used. For the nanostructured Rh layer, the pattern is based on the assumption that the polystyrene spheres were hexagonally close-packed on the electrode surface with each sphere resulting in a cylindrical opening in the Rh layer, possessing a radius of 200 nm. These assumptions are based on previous publications (see above) and AFM data on the surfaces (Fig. 3b).

Data availability

All relevant data are available from the authors upon request.