Enhanced hydrogen evolution rates at high pH with a colloidal cadmium sulphide-platinum hybrid system

We demonstrate enhanced hydrogen generation rates at high pH using colloidal cadmium sulphide nanorods decorated with Pt nanoparticles. We introduce a simplified procedure for the decoration and subsequent hydrogen generation, reducing both the number of working steps and the materials costs. Different Pt precursor concentrations were tested to reveal the optimal conditions for the efficient hydrogen evolution. A sharp increase in hydrogen evolution rates was measured at pH 13 and above, a condition at which the surface charge transfer was efficiently mediated by the formation of hydroxyl radicals and further consumption by the sacrificial triethanolamine hole scavenger.

Both climate change and the finite supply of fossil resources have raised the pressing needs to secure clean and yet reliable energy resources in the not-too-distant future.For a thorough solution, the green energy ought to be storable at sufficiently high energy density.2][3] Several approaches in the green production of H 2 have been addressed, such as mimicking the way of nature using artificial photosynthesis. 4Photocatalytically active nanocomposites based on colloidal semiconductors have been widely researched for their application in light-induced H 2 generation. 5One material of focus is cadmium sulphide (CdS), a low cost and highly abundant semiconductor. 61][12] A number of hybrid semiconductor-metal colloidal systems have been developed over the recent years, aiming to improve the light-harvesting properties and charge separation. 5,13,14][20] In this letter, we report enhanced hydrogen generation rates over a colloidal CdS-Pt system at high pH.We compare the hydrogen production at four different pH values ranging from 11 to 14, and show a large increase in the hydrogen generation rates at pH higher than 13.Different to previous literature approaches, we employed a simple one-step procedure for the simultaneous lightdriven deposition of the Pt co-catalyst and the hydrogen generation from water, which reduces the number of working steps and eliminates possible loss of material during the purification and phase transfer stages.We optimized this approach towards higher H 2 production capacity in terms of the most optimal Pt precursor concentrations, which were tested in the broad range of 0.4 to 16 wt.% relative to the amount of CdS.Nanorods of CdS were synthesized in organic medium consisting of trioctylphosphine oxide (TOPO)/trioctylphosphine (TOP)/tetradecylphosphonic acid (TDPA), according to a previously published procedure. 9The synthesis yields uniform nanorods of 100-150 nm length and approximately 5.5 nm width, as presented in the transmission electron microscopy image in the inset of Fig. 1(a).Their high monodispersity is further reflected in the absorption spectrum (Fig. 1(a)), with a number of well-resolved optical transitions and the first excitonic peak located at 456 nm (equivalent to a bandgap of 2.72 eV).The blue shift of the excitonic peak compared to bulk CdS (bandgap 2.42 eV) is typical for the quantum confinement effect, which in this case is determined by the diameter of the nanorod.For the H 2 evolution experiments, the as-produced nanorods were transferred to aqueous phase by ligand exchange with L-cysteine, and the solutions were set to an optical density, OD = 1.0 at 456 nm, corresponding to a particle concentration of 5.0 × 10 −8 mol/l.Photodeposition of Pt on CdS nanorods was carried out by reducing chloroplatinic acid hexahydrate under simulated solar irradiation (Abet Technologies Sun 2000, AM 1.5, 1 sun) and argon atmosphere, in the presence of the reducing agent ascorbic acid in a molar ratio of 2:1 to the amount of the Pt precursor, and of 10% triethanolamine (TEA) as a hole scavenger.Before starting the irradiation, the pH of the solutions was adjusted to pH values varying from 11 to 14 by addition of 1 M NaOH.Since the decoration of Pt is completed after 30 min of illumination, 14 detection of H 2 evolution started at that point.The one-step procedure for the photodeposition of the Pt, followed by the evolution of hydrogen, essentially reduces the number of working steps normally involved in the purification and phase transfer stages, and as such eliminates possible loss of material.
Fig. 1(b) shows the H 2 evolution kinetics on a colloidal CdS-Pt system with 7 wt.% of Pt precursor, presented here in mmol of H 2 per g CdS over a period of 4 h continuous solar irradiation.The linear trend indicates high stability of the CdS-Pt photocatalyst despite a long period of irradiation. 21With increasing pH, the H 2 evolution rates grew steadily, with the largest increase from pH 12 (3.6 mmol h −1 g −1 ) to pH 13 (21.8mmol h −1 g −1 ).While the former is slightly lower compared to the H 2 evolution in the presence of SO 3 2− hole scavenger (5 mmol h −1 g −1 ), 15,19 a remarkable six-fold increase in H 2 generation rates for pH 13 and 14 in comparison with pH 11 and 12 follows the recently reported trend for Ni-decorated CdS nanorods. 22This strong increase in efficiencies also favourably compares to the previously reported CdS/ZnSe/Pt hybrid system by Acharya et al. (0.2 mmol h −1 g −1 ) 18 and the CdS/Pt system published by Wang et al. (13.8 mmol h −1 g −1 ), 17 and is ascribed to a fast removal process of the photohole.Instead of the slower direct photohole transfer to TEA at low pH, 23 the OH − ions at high pH effectively promote the scavenging of photohole transfer by forming • OH radicals.These free • OH radicals in turn diffuse quickly to target the TEA in an • OH radicals-mediated oxidation, a widely observed general pathway in photocatalytic reactions as observed by us, 24,25 and others. 26The faster photohole consumption results in higher net charge separation, and hence more photoelectrons are available for H 2 evolution.Since Pt is a more efficient co-catalyst, the onset high evolution rates for CdS-Pt were observed at lower pH 12-13 compared to CdS-Ni (pH 14.0-14.7).
To optimize the amount of decoration of Pt cocatalyst, the concentration of the Pt precursor chloroplatinic acid hexahydrate has been varied in the range of 0.4 to 16 wt.% with respect to the amount of CdS.The data on the corresponding H 2 evolution rates at pH 13 are presented in Fig. 2 In conclusion, we present a strong improvement of the H 2 generation rates upon increasing pH in the range of 11 to 14 for colloidal CdS-Pt hybrid system, and ascribe this effect to the enhanced charge separation as mediated by the efficient formation of hydroxyl radicals.In particular, we demonstrate a six-fold increase in hydrogen generation efficiencies for pH values changing from pH 12 to 13, indicating a threshold concentration of OH − ions, which needs to be exceeded for the effective H 2 generation.Furthermore, we simplify the process of H 2 evolution compared to our previous work by combining the Pt co-catalyst deposition and H 2 evolution into a single-step procedure.To optimize

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FIG. 1.(a) Absorption spectrum of CdS nanorods, with an inset showing a TEM image of the as-synthesized rods.(b) Evolution of hydrogen over the colloidal CdS-Pt system (7 wt.% of Pt precursor used) at pH 11 to 14 under simulated solar irradiation (AM 1.5, 1 sun).
(a), with the morphology of Pt-decorated CdS nanorods under varying Pt precursor concentrations, namely, 0.4, 4.0, and 16 wt.% after 4 h of irradiation as shown in Figs.2(b)-2(d).The data presented in Fig. 2(a) show the steep increase towards higher H 2 evolution rates in the range of the Pt precursor concentrations 0.4 to 4 wt.% (optimum 23.4 mmol h −1 g −1 ), beyond which the activity decreases to 15.2 mmol h −1 g −1 at 16 wt.% Pt concentrations.TEM images in Figs.2(b)-2(d) help to correlate this behaviour to the observed morphological characteristics of the samples.Platinum particles appear as dark spots in these images (since Pt is a stronger electron absorber), and are attached to the grey CdS nanorods.The function of the Pt cocatalyst is two-fold: to form Schottky barrier at the Pt-CdS interface that enhances charge separation27 and to facilitate the photoelectron charge transfer for the reduction of water to H 2 .21At high concentrations, Pt deposits counteract as charge recombination centres.As shown in Fig.2(d), nanorods decorated with 16 wt.% Pt precursor are surrounded by large polydisperse agglomerates of Pt particles that combine multiple nanorods in bundles.Such morphologies allow photoholes to diffuse across the nanorods and recombine with the trapped photoelectrons at the Pt deposits.