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

Schneider, Julian; Vaneski, Aleksandar; Pesch, Georg R.; Susha, Andrei S.; Yang Teoh,Wey; Rogach, Andrey L.

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Published: 01/12/2014

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Published version (DOI):10.1063/1.4904070

Publication details:Schneider, J., Vaneski, A., Pesch, G. R., Susha, A. S., Yang Teoh, W., & Rogach, A. L. (2014). Enhancedhydrogen evolution rates at high pH with a colloidal cadmium sulphide-platinum hybrid system. APL Materials,2(12), [126102]. https://doi.org/10.1063/1.4904070

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Enhanced hydrogen evolution rates at high pH with a colloidal cadmium sulphide–platinum hybrid systemJulian Schneider, Aleksandar Vaneski, Georg R. Pesch, Andrei S. Susha, Wey Yang Teoh, and Andrey L.Rogach

Citation: APL Materials 2, 126102 (2014); doi: 10.1063/1.4904070View online: https://doi.org/10.1063/1.4904070View Table of Contents: http://aip.scitation.org/toc/apm/2/12Published by the American Institute of Physics

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APL MATERIALS 2, 126102 (2014)

Enhanced hydrogen evolution rates at high pH witha colloidal cadmium sulphide–platinum hybrid system

Julian Schneider,1 Aleksandar Vaneski,1 Georg R. Pesch,2Andrei S. Susha,1 Wey Yang Teoh,2 and Andrey L. Rogach1,a1Department of Physics and Materials Science and Centre for Functional Photonics (CFP),City University of Hong Kong, 83 Tat Chee Avenue, Kowloon, Hong Kong2Clean Energy and Nanotechnology (CLEAN) Laboratory, School of Energy andEnvironment, City University of Hong Kong, 83 Tat Chee Avenue, Kowloon, Hong Kong

(Received 15 October 2014; accepted 1 December 2014; published online 15 December 2014)

We demonstrate enhanced hydrogen generation rates at high pH using colloidalcadmium sulphide nanorods decorated with Pt nanoparticles. We introduce a simpli-fied procedure for the decoration and subsequent hydrogen generation, reducingboth the number of working steps and the materials costs. Different Pt precursorconcentrations were tested to reveal the optimal conditions for the efficient hydrogenevolution. A sharp increase in hydrogen evolution rates was measured at pH 13 andabove, a condition at which the surface charge transfer was efficiently mediatedby the formation of hydroxyl radicals and further consumption by the sacrificialtriethanolamine hole scavenger. C 2014 Author(s). All article content, except whereotherwise noted, is licensed under a Creative Commons Attribution 3.0 UnportedLicense. [http://dx.doi.org/10.1063/1.4904070]

Both climate change and the finite supply of fossil resources have raised the pressing needs tosecure 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. In this respect, hydrogen asa solar fuel (or more specifically a renewable energy carrier) is particularly attractive, as supportedby the state-of-the-arts technologies in clean conversion and opportunities for storage.1–3 Several ap-proaches in the green production of H2 have been addressed, such as mimicking the way of natureusing artificial photosynthesis.4 Photocatalytically active nanocomposites based on colloidal semi-conductors have been widely researched for their application in light-induced H2 generation.5 Onematerial of focus is cadmium sulphide (CdS), a low cost and highly abundant semiconductor.6 Recentadvancements in colloidal synthesis have allowed precise manipulation of the size, shape, and surfacefunctionalities of nanoscale CdS,7–9 which when decorated with co-catalysts, such as platinum, nickel,or cobaloxime resulted in good H2 evolution activity.10–12 A number of hybrid semiconductor-metalcolloidal systems have been developed over the recent years, aiming to improve the light-harvestingproperties and charge separation.5,13,14 For the CdS-Pt colloidal system, it has been demonstrated thatthe amount, as well as the size and position of the co-catalyst particles can have significant influence onthe H2 evolution efficiencies.10,15–17 Other factors such as the chemical nature and the redox potentialof surfactants and sacrificial hole scavengers, and solution pH are relevant factors determining ratesof H2 evolution.18–20

In this letter, we report enhanced hydrogen generation rates over a colloidal CdS-Pt system athigh 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 pre-vious literature approaches, we employed a simple one-step procedure for the simultaneous light-driven deposition of the Pt co-catalyst and the hydrogen generation from water, which reduces thenumber of working steps and eliminates possible loss of material during the purification and phase

aAuthor to whom correspondence should be addressed. Electronic mail: andrey.rogach@cityu.edu.hk

2166-532X/2014/2(12)/126102/4 2, 126102-1 ©Author(s) 2014

126102-2 Schneider et al. APL Mater. 2, 126102 (2014)

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 solarirradiation (AM 1.5, 1 sun).

transfer stages. We optimized this approach towards higher H2 production capacity in terms of themost 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 pub-lished procedure.9 The synthesis yields uniform nanorods of 100–150 nm length and approximately5.5 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 numberof well-resolved optical transitions and the first excitonic peak located at 456 nm (equivalent to abandgap 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 thenanorod. For the H2 evolution experiments, the as-produced nanorods were transferred to aqueousphase by ligand exchange with L-cysteine, and the solutions were set to an optical density, OD = 1.0at 456 nm, corresponding to a particle concentration of 5.0 × 10−8 mol/l. Photodeposition of Pt onCdS nanorods was carried out by reducing chloroplatinic acid hexahydrate under simulated solarirradiation (Abet Technologies Sun 2000, AM 1.5, 1 sun) and argon atmosphere, in the presence ofthe 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 wasadjusted to pH values varying from 11 to 14 by addition of 1 M NaOH. Since the decoration of Pt iscompleted after 30 min of illumination,14 detection of H2 evolution started at that point. The one-stepprocedure for the photodeposition of the Pt, followed by the evolution of hydrogen, essentially reducesthe number of working steps normally involved in the purification and phase transfer stages, and assuch eliminates possible loss of material.

Fig. 1(b) shows the H2 evolution kinetics on a colloidal CdS-Pt system with 7 wt. % of Pt pre-cursor, presented here in mmol of H2 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 irradia-tion.21 With increasing pH, the H2 evolution rates grew steadily, with the largest increase from pH 12(3.6 mmol h−1 g−1) to pH 13 (21.8 mmol h−1 g−1). While the former is slightly lower compared tothe H2 evolution in the presence of SO3

2− hole scavenger (5 mmol h−1 g−1),15,19 a remarkable six-foldincrease in H2 generation rates for pH 13 and 14 in comparison with pH 11 and 12 follows the recentlyreported trend for Ni-decorated CdS nanorods.22 This strong increase in efficiencies also favourablycompares 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 fastremoval 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 •OHradicals. These free •OH radicals in turn diffuse quickly to target the TEA in an •OH radicals-mediated

126102-3 Schneider et al. APL Mater. 2, 126102 (2014)

FIG. 2. (a) Hydrogen generation rates (at pH 13) for varying amounts of Pt precursor used for decoration. (b)-(d) TEMimages of CdS nanorods, decorated with 0.4 wt. %, 4 wt. %, and 16 wt. % Pt pecursor concentration, respectively, after 4 hof AM 1.5 irradiation at 1 sun. All scale bars are 100 nm.

oxidation, a widely observed general pathway in photocatalytic reactions as observed by us,24,25 andothers.26 The faster photohole consumption results in higher net charge separation, and hence morephotoelectrons are available for H2 evolution. Since Pt is a more efficient co-catalyst, the onset highevolution 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 precursorchloroplatinic acid hexahydrate has been varied in the range of 0.4 to 16 wt. % with respect tothe amount of CdS. The data on the corresponding H2 evolution rates at pH 13 are presented inFig. 2(a), with the morphology of Pt-decorated CdS nanorods under varying Pt precursor concen-trations, namely, 0.4, 4.0, and 16 wt. % after 4 h of irradiation as shown in Figs. 2(b)-2(d). The datapresented in Fig. 2(a) show the steep increase towards higher H2 evolution rates in the range of thePt precursor concentrations 0.4 to 4 wt. % (optimum 23.4 mmol h−1 g−1), beyond which the activitydecreases to 15.2 mmol h−1 g−1 at 16 wt. % Pt concentrations. TEM images in Figs. 2(b)-2(d) help tocorrelate this behaviour to the observed morphological characteristics of the samples. Platinum parti-cles appear as dark spots in these images (since Pt is a stronger electron absorber), and are attachedto the grey CdS nanorods. The function of the Pt cocatalyst is two-fold: to form Schottky barrierat the Pt-CdS interface that enhances charge separation27 and to facilitate the photoelectron chargetransfer for the reduction of water to H2.21 At high concentrations, Pt deposits counteract as chargerecombination centres. As shown in Fig. 2(d), nanorods decorated with 16 wt. % Pt precursor aresurrounded by large polydisperse agglomerates of Pt particles that combine multiple nanorods inbundles. Such morphologies allow photoholes to diffuse across the nanorods and recombine with thetrapped photoelectrons at the Pt deposits.

In conclusion, we present a strong improvement of the H2 generation rates upon increasing pH inthe range of 11 to 14 for colloidal CdS-Pt hybrid system, and ascribe this effect to the enhanced chargeseparation as mediated by the efficient formation of hydroxyl radicals. In particular, we demonstratea 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 H2generation. Furthermore, we simplify the process of H2 evolution compared to our previous work bycombining the Pt co-catalyst deposition and H2 evolution into a single-step procedure. To optimize

126102-4 Schneider et al. APL Mater. 2, 126102 (2014)

this approach, Pt precursor concentrations ranging from 0.4 to 16 wt. % in relation to the amount ofCdS in solution are examined towards the resulting H2 generation rates. The highest rate was achievedusing 4 wt. % of Pt precursor and reached 23.4 mmol h−1 g−1 at pH 13, which corresponds to a reduc-tion of the expensive Pt precursor by a factor of two compared to our previous work.15 TEM datademonstrate the most favourable morphology of the samples at this Pt concentration, with every CdSnanorods decorated by few tightly attached, monodisperse Pt nanoparticles.

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