Efficient water reduction with gallium phosphide nanowires

Photoelectrochemical hydrogen production from solar energy and water offers a clean and sustainable fuel option for the future. Planar III/V material systems have shown the highest efficiencies, but are expensive. By moving to the nanowire regime the demand on material quantity is reduced, and new materials can be uncovered, such as wurtzite gallium phosphide, featuring a direct bandgap. This is one of the few materials combining large solar light absorption and (close to) ideal band-edge positions for full water splitting. Here we report the photoelectrochemical reduction of water, on a p-type wurtzite gallium phosphide nanowire photocathode. By modifying geometry to reduce electrical resistance and enhance optical absorption, and modifying the surface with a multistep platinum deposition, high current densities and open circuit potentials were achieved. Our results demonstrate the capabilities of this material, even when used in such low quantities, as in nanowires.


Space
charge region width at 0V (vs. RHE) ZB GaP Planar y = -2.34E+15x + 2.48E+15 1.06E+00 6.81E+18 14nm WZ GaP Nanowires y= -1.09E+16x + 1.16E+16 1.06E+00 1.46E+18 30nm where C SC is the capacitance of the space charge region, q is the elementary charge, Ɛ 0 is the permittivity of free space, Ɛ is the dielectric constant of ZB GaP, N is the dopant concentration, V is the applied potential, V FB is the flat band potential and approximate valence band position (vs. RHE) for a p-type semiconductor, k is the Boltzmann constant and T is the temperature in Kelvin. The data from the Mott-Schottky equation allows that calculation of the space charge width, which can be written as; (2) where W is the space charge region width. The table shows; the flat band potential (V FB ), the dopant concentration, and the width of the space charge region at 0V, for the ZB planar and WZ nanowire GaP samples used.   .c, a potential of 0V (vs. RHE) was applied, and the current was measured to be 3x10 -5 A due to the small size of the sample, the measured data can be seen in the inset In the following sections we will discuss the geometry optimization, and catalyst deposition steps studied to reach high efficiency. We first study the effect of wire length. where R is the resistance of the wire, ρ is the resistivity of the material, L is the wire length, and A is the wire cross sectional area. However this is clearly not the only factor affecting performance as the trend for I SC is not the inverse of resistance. This is due to the light absorption increasing with nanowire length (supplementary figure 3). Due to the increased surface area, the flux of electrons through the electrode electrolyte junction per unit area decreases. This causes a decrease in the quasi-Fermi level, and therefore V OC 2-4 . The change in V OC can be calculated by where γ is the actual junction area and I 0 is the saturation current density. This factor, however, only accounts for a small decrease in V OC on the order of 10s of mV, the further drop in voltage is due to the resistance and length of the nanowire. As the nanowire length continues to increase, more voltage is lost due to the increased resistance and surface area, causing the decrease in I SC observed in supplementary figure 2.c, and the change in the I-V curve shape observed in supplementary figure 2.b. It is found that the optimum wire length is 2µm, yielding promising V OC and ffs; this wire length allows for good transport of charge carriers and reasonable absorption of light without too much voltage drop from the increased resistance and surface area.
The I SC, however, should be able to reach a much larger value of up to the previously mentioned value of; 12.5mA/cm 2 .

Wire diameter:
Using the optimized wire length, the effect of the wire diameter is studied by growing a shell on the wires, maintaining the pure WZ crystal structure, with nominally the same dopant concentration as used for the growth of the core. Average wire diameters of 90nm, 120nm, 150nm, 180nm and 215nm are obtained by respective shell growth times of 0, 5, 10, 20 and 30 minutes. Supplementary figure 4.a shows the SEM images of the wires with 5, 10, and 30 minute shell growth times. It is evident from these SEM images that the wire length also increases with shell growth; this is due to unavoidable axial growth through the catalytic gold particle from the initial VLS growth. Supplementary reason for the lower I SC from the thicker nanowires is the axial growth caused by the gold particle during shell growth. As the axial growth is not intentional during this growth phase stacking faults are incorporated, which could lead to recombination of charge carriers, and a lower than expected I SC .

Supplementary note 2. Electrochemically Produced Oxide (EPO)
Due to the large surface area of the 2µm long 150nm wide nanowires, an oxide layer can help to passivate surface states [6][7][8] , reducing surface recombination. A simple method for the production of an oxide layer is to apply a reducing potential to the GaP electrode while under illumination in an aqueous acid. The surface of the GaP will be reduced to gallium metal and phosphine. The also be observed. The fact that the dark current does not increase during the experiment shows that the current under illumination is purely due to the passivating effect of the oxide layer and not due to any surface charging, as that would also cause the dark current to increase.
Supplementary Figure 5.b shows the I-V behavior of the nanowires after the production of the EPO layer, the I SC and V OC are both increased to 4.1mA/cm 2 and 0.75V (vs. RHE) respectively.
The inset in this figure shows a TEM image of a section of a nanowire after the production of the EPO layer. The EPO layer is observed to be approximately 3nm in thickness, and is clearly not evident prior to the electrochemical treatment in figure 1.c in the main text. This EPO passivates surface states [6][7][8] , decreasing surface recombination, leading to the observed increase in current.
In the presence of the EPO the I SC is still limited to ~4mA, so a catalyst should still be implemented to promote charge transfer further.

Supplementary note 3. Catalyst deposition
Supplementary figure 7.a shows chronopotentiometry measurements performed on the nanowire samples after a single 180s deposition (black line), a single 60s deposition (red line) and after 3 consecutive 60s depositions (blue line). During the chronopotentiometry measurements, after a single 60s deposition (red line), an increase in current is observed over time. The increase is, however, not observed after 3 consecutive 60s depositions (blue line), and only a slight increase is observed for the 180s deposition (black line). It can be seen from supplementary figure 5 that the oxide layer increases slightly in thickness after 3 consecutive platinum depositions. When the platinum coverage is low, as is the case for 1x60s (and to a lesser extent for 1x180s), reactions will still occur on the nanowire surface without the aid of the catalyst. This, has in this case, lead to the oxide being reduced back to gallium metal, and exposing the GaP surface, allowing for the production of a thicker (passivating) oxide layer, which will act to reduce surface recombination and therefore increase current. Once the catalyst loading is high enough, as is the case for 3x60s, the catalyst particles are used preferentially for charge transfer from the semiconductor to the electrolyte, allowing for an increased reaction rate.
The preferential use of the catalyst particles for charge transfer will also reduce the chance of oxide layer removal and surface reduction, leading to the reasonable stability observed in figure   3.c.