WZ gallium phosphide nanowires

Figure 1b shows a scanning electron microscopy image of a typical nanowire array. The WZ GaP nanowires are grown from a nanoimprint-patterned array of gold particles; this gives an ordered array of nanowires with 495 nm pitch and 90 nm diameter. The wires are grown with optimized parameters for the WZ crystal structure (see Methods section for more details). Figure 1c shows a high-resolution transmission electron microscopy (TEM) image of an as-grown p-GaP wire. The GaP wires have an almost perfect WZ crystal structure with a very low stacking fault density of <1 μm−1. Figure 1d compares the current density–voltage (I–V) behaviour of ZB planar (100)-oriented p-GaP single-crystalline substrate and WZ nanowire p-GaP electrodes. The nanowires used in this experiment are of optimized geometry, with lengths and diameters of ∼2.0 μm and ∼150 nm, respectively. The nanowire length is controlled by adjusting the growth time of the core. The diameter is adjusted by the growth of a shell on the nanowire surface (see Methods section for more details), as this growth method enables the shell to maintain the WZ crystal structure forming single-crystal nanowires. The planar ZB GaP surface is not insulated during experiments; however, it is expected from absorption measurements performed on nanowires, after transfer onto a poly-dimethyl-siloxane (PDMS) film (Supplementary Figs 3 and 4), that <15% of the current is due to the substrate. An amorphous molybdenum sulfide (MoS x ) catalyst35,36,37 is deposited on both planar and nanowire samples, the catalyst will enhance transfer of charges from the semiconductor to the electrolyte, and stabilize reaction intermediates, which will reduce surface recombination. The MoS x catalyst is deposited on the samples before all following experiments (unless stated otherwise). The V OC ; fill factor (ff); I SC ; and energy conversion-efficiency (η%) measured for the ZB GaP planar and nanowire-array WZ p-GaP electrodes with and without the MoS x catalyst are listed in Table 1. We note that our reference planar ZB GaP sample already shows similar V OC and I SC values compared with recently reported best values for planar ZB GaP21. The WZ GaP nanowire sample has a higher V OC , I SC and ff than the planar sample, resulting in a much higher efficiency of 1.4%. This is due to the direct bandgap of WZ GaP decreasing the absorption depth; and the nanowire geometry, decreasing reflection and bulk recombination losses29.

Table 1 Planar and nanowire performance. Full size table

In the following sections, the steps involved in the optimization of the GaP nanowires will be discussed. This includes the study of nanowire geometry, an electrochemically produced passivation layer and a new scheme for platinum catalyst deposition.

Nanowire geometry

In Fig. 2, we show that the nanowire geometry is strongly influencing the attainable I SC in a PEC cell. We independently varied both the nanowire length and the nanowire diameter by switching between vapour–liquid–solid growth, which mainly increases the nanowire length and vapour–solid growth, which mainly increases the nanowire diameter (Fig. 2a,b). A larger nanowire length increases the solar light absorption, but if length increases too far it becomes detrimental due to an increased series resistance, which we independently measured by impedance measurements as is shown in Supplementary Fig. 2. The effect of an increased series resistance is a decrease in both V OC and I SC . As is shown in Supplementary Fig. 3, the solar light absorption in our WZ GaP nanowires saturates below a 2-μm length, due to the direct bandgap decreasing the absorption depth, resulting in an optimum nanowire length of 2 μm. The optimum nanowire diameter is found to be 150 nm, which is determined by a trade-off between a decreasing series resistance, an increased solar light absorption and an increasing reflection loss (because of an increasing average refractive index of the layer) as nanowire diameter increases. In addition, we expect that when the nanowire diameter starts to exceed twice the space charge region thickness, calculated as 30 nm at 0 V (versus RHE) in Supplementary Fig. 1, bulk recombination starts to decrease the overall efficiency. A more detailed study of nanowire length and diameter can be found in the Supplementary Note 1 and Supplementary Figs 2 and 4.

Figure 2: Optimization of the nanowire geometry. (a) Scanning electron microscopy (SEM) images of WZ GaP nanowires grown for 6, 14 and 22 min with lengths of 0.73, 1.65 and 2.16 μm, respectively. Scale bar, 200 nm for all images. (b) SEM images of zinc-doped WZ GaP nanowires grown for 16 min (2 μm), with an additional shell grown for 5, 10 and 30 min with diameters of 120, 150 and 215 nm, respectively. Scale bar, 200 nm for all images. (c) The trend observed in the short circuit current (I SC ) when length is changed. (d) The trend observed in the I SC when diameter is changed. Data were collected by linear sweep voltammetry, performed under chopped 100 mW cm−2 AM1.5 illumination, in aqueous solution pH 0 with HClO 4 as supporting electrolyte. The error bars were calculated as two s.d. away from the average value taken from three or more experiments carried out on separate samples with the same specifications. (The data point for length 0 is for a planar (111) p-GaP ZB substrate). Full size image

Molybdenum sulfide catalyst

To fully realize the potential of GaP for water reduction, a suitable catalyst is required to promote charge transfer, thereby suppressing charge-carrier recombination. As previously mentioned, nanowires have a large surface area, and therefore a low current density, meaning that an earth-abundant catalyst such as MoS x (refs 35, 36, 37, 38) should yield promising results. Figure 3a shows the I–V characteristics for nanowires without catalyst (black line, top panel), with MoS x as catalyst (blue line, middle panel) and with platinum as catalyst (red line, bottom panel). Before this MoS x deposition, an electrochemically produced oxide (EPO) passivation layer is formed. This EPO layer has been shown to improve the I SC of uncatalysed nanowires from 1.5 to 4.1 mA cm−2 (see Table 1, Supplementary Note 2 and Supplementary Fig. 5). The MoS x catalyst is deposited for an optimum deposition time of 30 s (Supplementary Fig. 6). The reaction of the precursor with the semiconductor surface also results in the formation of sulfide, which is widely known to passivate III–V semiconductors39,40,41. Even so, the presence of the EPO improves the overall efficiency from 1.37% (Fig. 1d) to 1.5% (Fig. 3a, middle panel), demonstrating that the EPO is a more effective passivation layer than sulfide, and will be much more important for other catalysts (that are not produced with their own passivation layer). V OC , I SC , ff and η% values for the nanowires, catalysed by MoS x, with and without the EPO can be found in Table 1. This combination of nanowire, oxide and catalyst has already achieved the current record in V OC of 0.71 V (versus RHE) for GaP, and has achieved a much higher I SC (6.4 mA cm−2) than has yet been reported for GaP.

Figure 3: Platinum catalyst deposition. (a) Linear sweep voltammograms of nanowire samples with no catalyst (black), Molybdenum sulfide deposited photochemically (blue) and platinum deposited photochemically (red), (b) TEM images of a section of a single nanowire, of optimized geometry, after platinum has been deposited PEC for 3 × 60 s; scale bar, 50 nm, and a zoomed-in image of the same wire, with the Platinum particles, Gallium oxide and GaP nanowire clearly labelled; scale bar, 20 nm. (c) The trend of the short circuit current when consecutive platinum depositions are performed on the same nanowire sample (black points) and when a single long deposition is performed on a nanowire sample (red point). The error bars were calculated as two s.d. away from the average value taken from three or more experiments carried out on separate samples with the same specifications. (d) Long-term chronoamperometric measurement performed on nanowires after platinum catalyst deposition performed under 100 mW cm−2 AM1.5 illumination, in aqueous solution pH 0 with HClO 4 as supporting electrolyte. Full size image

Platinum catalyst

As platinum is well known to be the best catalyst for water reduction, this catalyst is implemented to explore the full potential of WZ GaP. The best performance should be achieved with a uniform particle distribution, and an average particle size of 2–5 nm (refs 42, 43). We have achieved this by a simple and cheap electroless photodeposition method (as outlined in the Methods section). Longer deposition times, as expected, result in larger platinum particles, but remarkably the number of platinum particles decreases as deposition time is increased. When the deposition time is increased from 60 to 180 s, the number of particles per 0.01 μm2 decreases from 100 to 34 (Supplementary Fig. 7, Supplementary Table 1 and Supplementary Note 3). This shows that the platinum deposition is a dynamic process in which larger particles are growing while smaller particles are dissolved, typical for Ostwald ripening. By interrupting the deposition process, and performing chronopotentiometry on the sample, the platinum particles are exposed to hydrogen gas, which adsorbs onto their surface44, changing the properties of the platinum particles, and therefore the Ostwald ripening effect during the following deposition step. Multiple deposition steps lead to a, close to optimum, particle size of 5±3 nm and a uniform particle distribution over the wire, as can be seen in Fig. 3b. There is also the added benefit in the multi-deposition case of a slightly thicker EPO layer, produced during the chronoampeometry step, improving surface passivation, as discussed in Supplementary Note 3. The trend in I SC achieved by the interrupted deposition process is shown in Fig. 3c (black points). By performing three depositions of 60 s, a high I SC of up to 10.9 mA cm−2 can be achieved. The same deposition time, without the interruptions, results in a lower I SC of only 6.7 mA cm−2 (Fig. 3c red point). This is due to the poor uniformity and large particle size caused by the long continuous deposition (Supplementary Table 1). When more than three 60 s depositions are performed, the platinum particles no longer have optimum size and coverage, leading to light scattering and a decrease in current. The I–V characteristics for nanowires with the optimum platinum catalyst deposition can be seen in Fig. 3a (red line, bottom panel). With this deposition procedure, record high I SC and V OC values of 9.78 and 0.76 V (versus RHE), respectively, are obtained. However, the ff remains relatively low, below 0.4 for all samples, due to the large surface area of the nanowires. The best sample, with a ff of 0.39, nevertheless resulted in a record efficiency of 2.90% for a GaP large bandgap PEC cell. Higher I SC s of up to 10.9 mA cm−2 were recorded for other samples (Supplementary Fig. 8); however, the overall efficiency was best in the sample used for the data in Fig. 3a. The measured I SC of >10 mA cm−2 corresponds to >80% of the theoretical maximum current of 12.5 mA cm−2. For this high efficiency WZ p-GaP device, with this level of platinum coverage, merely tens of milligrams of platinum are required for every square metre of device area. III/V devices have been shown to work well under >10 times concentrated light2, by combining our device geometry with light concentrators, the amount of platinum catalyst can be cut even further.

Stability measurement

Figure 3c shows a 7 hour chronoampeometry measurement on nanowires catalysed by platinum in the presence of the EPO layer. The current starts to decreases after 5 hours, most likely due to the loss of catalyst particles as is observed by others21, demonstrating the promising capabilities of this system. Several gas samples were taken during this experiment, and measured by gas chromatography, giving a 97±3% Faradaic efficiency (Supplementary Fig. 9) for the hydrogen evolution reaction. This stability is not as high as is required for a commercial device, but is already higher than others have reported for unpassivated III/V PEC devices2,17 due to the conformal coverage of the EPO and catalyst particles.