On the role of metal particle size and surface coverage for photo-catalytic hydrogen production: A case study of the Au/CdS system

doi:10.1016/j.apcatb.2015.09.039

Applied Catalysis B: Environmental

Volume 182, March 2016, Pages 266-276

https://doi.org/10.1016/j.apcatb.2015.09.039 Get rights and content

Highlights

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Active CdS-based photo-catalysts were prepared via dispersion of Au ions using KI.
•
Photocatalytic H₂ production from water-ethanol mixture is evaluated under visible light.
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Highest H₂ production rate is obtained for ethanol–water electrolyte-Au/CdS system.
•
A significant growth of Au particles was observed during the photocatalytic process, without affecting the reaction rate.
•
A reaction mechanism involving both organic and inorganic agents is proposed.

Abstract

Photo-catalytic hydrogen production has been studied on Au supported CdS catalysts under visible light irradiation in order to understand the effect of Au particle size as well as the reaction medium properties. Au nanoparticles of size about 2–5 nm were deposited over hexagonal CdS particles using a new simple method involving reduction of Au³⁺ ions with iodide ions. Within the investigated range of Au (between 1 and 5 wt.%) fresh particles with mean size of 4 nm and XPS Au4f/Cd3d surface ratio of 0.07 showed the highest performance (ca. 1 molecule of H₂/Au_atom s⁻¹) under visible light irradiation (>420 nm and a flux of 35 mW/cm²). The highest hydrogen production rate was obtained from water (92%)-ethanol (8%) in an electrolyte medium (Na₂S–Na₂SO₃). TEM studies of fresh and used catalysts showed that Au particle size increases (almost 5 fold) with increasing photo-irradiation time due to photo-agglomeration effect yet no sign of deactivation was observed. A mechanism for hydrogen production from ethanol–water electrolyte mixture is presented and discussed.

Graphical abstract

Introduction

The effective use of solar energy is important for the establishment of a sustainable economy. Apart from solar energy utilization using photovoltaic technology and direct solar heating, one approach of solar energy utilization is the use of sunlight to generate energy carriers such as hydrogen from renewable sources (water or water/bio-ethanol mixture where the latter is used in small amounts to quench e–h recombination rates) using semiconductor photo-catalysts [1], [2]. Moreover, ethanol can be synthesized from biomass, e.g., fermentation or hydrolyses of celluloses and this is not the case for other fossil fuels [3]. One of the important challenges to realize this is the development of photo-catalysts which can absorb sunlight and convert these renewables to hydrogen with efficiency that warrant scaling up of the process. A range of catalysts from simple binary metal oxides and metal sulfides to more complicated catalysts have been developed to achieve this objective [4], [5], [6]. TiO₂ [7], [8], [9], [10], [11], [12], [13] and CdS [14], [15], [16], [17], [18] attract special interest in this regard because of the relative simplicity of their chemical structure in addition to their stability.

TiO₂ is one of the most stable and active photo-catalyst known, [6], [13], [18], [19] though it suffers from the limitation of its light absorption range mostly in the UV region of solar spectrum (anatase E_g = 3.2 eV, rutile E_g = 3.0 eV). This leaves ca. 95% of incoming sunlight not utilized. On the other hand, CdS has narrower band gap (E_g = 2.4 eV) allowing for visible light absorption below 515 nm (20–25% of the solar spectrum) [15], [16]. CdS has two different stable structures; cubic and hexagonal. It has been recognized early on, that the hexagonal structure is far more active than the cubic one [20] despite the fact that both have a direct bang-gap [21]. However, CdS suffers from the limitation of its own oxidation by valence band holes produced during the photoexcitation process [22], [23]. This problem is circumvented by introducing sacrificial agents with higher redox potential than the valence band of CdS to prevent it from being oxidized. The process can be represented by the equations given below.(Cd²⁺S²⁻) + hν (<515 nm) → (Cd²⁺S²⁻) + e + h (Excitation)(Cd²⁺S²⁻) + e + h + R → Cd²⁺ + S⁻ + R⁻ (Photocorrosion)(Cd²⁺S²⁻) + e + h + R + SA → (Cd²⁺S²⁻) + R⁻ + SA_ox (Photocatalysis)where R represents an electron acceptor species and SA represents a sacrificial agent.

CdS can either be used in presence of metals as electron traps [24] or coupled with other semiconductors to prevent electron-hole recombination [25], [26]. Other catalyst fabrication methods, have shown improved CdS based catalysts, especially core shell structures with other stable semiconductors, with the objective to overcome the inherent photo-corrosion of CdS [27]. CdS has usually been coupled with noble metals such as Au, Pt, Rh, Ru and Pd acting as co-catalyst [28], [29], [30], [31] .

Several researchers have focused on CdS modification with other metal sulfides (WS₂, MoS₂) which has shown improved photo-catalytic activity as compared to CdS alone [32], [33]. CdS nanoparticles are often incorporated within a layered metal oxides to suppress particle growth and formation of nano-hetero-junctions which quickly transfer electrons through the nanostructure while the recombination between the photo-induced electron and the hole is effectively suppressed [34], [35], [36], [37]. Three component nano-junctions (CdS–Au–TiO₂) mimicking the natural photosynthesis has also been designed and tested [38], [39]. CdS/TiO₂ nanotubes where the former is incorporated into the later has been demonstrated to show higher activity as compared to CdS alone [25]. CdS (core)-TiO₂ (shell) structure were also studied and has shown to possess up to seven times higher photo- catalytic activity as compared to CdS [40], [41], [42]. Graphene regarded as an ideal conductive material for nanoparticles forming hybrid structure with CdS has also shown up to five times enhancement in the photo-catalytic activity [43], [44], [45].

In the case of TiO₂, charge carriers are more stable in anatase than in rutile and thus partially explain the activity of the former [46]. There are no data available in the case of pure CdS linking charge carrier life time to structural and catalytic performance. Although in some cases semiconductors alone have been found to exhibit photo-catalytic activity, however in most studies metals have been found to increase the photo-catalytic activity considerably [47]. Metals (e.g., Pt, Pd, Ru, Rh and Au) are generally effective for facilitating H₂ evolution as a result of the H⁺ reduction. Au has recently been the preferred co-catalyst for water splitting because it is less active for back reaction of H₂ oxidation,[48] have suitable work function [49] and is highly resistant to oxidation [50]. Moreover, under visible light irradiation, electrons can be transferred from Au particles to the conduction band of the semiconductor due to surface plasmon resonance which could be responsible for visible water splitting [51]. Au can also enhance the activity via plasmon resonance energy transfer rather than hot electron transfer where in this case both increase in charge carriers (because CdS and Au absorption overlap) and a decrease in charge carrier recombination rate could occur [52]. Au/CdS (hexagonal phase) photo-catalysts were thus selected as the main focus of this study.This study focuses on the effect of Au loading and the nature of the aqueous media on the enhancement of photo-catalytic activity. To the best of our knowledge, there are only three studies reported about Au/CdS [53], [54], [55] while other metals were used for most other studies [55], [56], [57], [58], [59], [60], [61]. Almost all of these studies have been performed using inorganic electrolyte (0.1 M Na₂S, 0.02 M Na₂SO₃) as reaction medium. There is no work reported on hydrogen production from a mixture of an inorganic electrolyte and organic sacrificial agent, ethanol in our case. This study also probe into the catalyst structure after the reaction in order to probe into the restructuring of the material that has occurred.

Section snippets

Catalyst preparation

CdS nanoparticles were synthesized by the sol–gel method using equimolar amounts of Cd(NO₃) ₂·4H₂O (Sigma–Aldrich, ≥99.0%) and Na₂S·9H₂O (Kanto Chemicals, ≥95.5%). Briefly, 2.1 g of Cd(NO₃)₂ was dissolved in 50 mL of isopropanol in a beaker and stirred vigorously. After 45 min, 50 mL of an aqueous Na₂S·9H₂O solution (0.136 M) was added drop wise over a period of one hour using a burette. The mixture was further stirred for two hours. The orange precipitate obtained was vacuum-filtered, washed five

Results and discussion

XRD results indicate that the as-prepared CdS crystallizes mainly in the cubic phase with <10% of the hexagonal phase (Fig. 1a). The cubic phase remains the dominant CdS one on heating in air up to 400 °C, while a further increase in temperature to 500 °C leads to complete oxidation to white CdSO₄ in less than one hour. However, heating the as-prepared cubic CdS at 600 °C in inert environment (ca. 120 mL/min of N₂ flow rate) prevented the oxidation of CdS to CdSO₄ and converted cubic CdS to its

Conclusions

Au nanoparticles deposited on CdS by reductive deposition method with KI were prepared and used as a model to test the metal particle size activity as well as the effect of reaction medium. Mean particles sizes between 2 nm to 5 nm were obtained by changing the initial nominal Au wt.% (from 1 to 5 wt.%). The crystallinity of the Au/CdS photo-catalyst monitored by XRD could also be monitored by S 2p and Cd 3d XPS core levels where a decrease in the FWHM of 0.5 eV for Cd 3d and 0.4 eV for S2p for the

Acknowledgments

This work was conducted under the financial support of Uniservices Ltd, University of Auckland, New Zealand and the Higher Education Commission of Pakistan. We would like to thank Morgan Allison for his assistance in the laboratory work.

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On the role of metal particle size and surface coverage for photo-catalytic hydrogen production: A case study of the Au/CdS system

Highlights

Abstract

Graphical abstract

Introduction

Section snippets

Catalyst preparation

Results and discussion

Conclusions

Acknowledgments

Int. J. Hydrogen Energy

Catal. Today

Int. J. Hydrogen Energy

J. Catal.

Chem. Phys. Lett.

Prog. Solid State Chem

Appl. Surf. Sci.

Int. J. Hydrogen Energy

J. Photochem. Photobiol. A: Chem.

J. Catal.

Sol. Energy Mater. Sol. Cells

Electrochem. Commun.

Catal. Today

Int. J. Hydrogen Energy

Catal. Commun.

J. Catal.

Int. J. Hydrogen Energy

Int. J. Hydrogen Energy

Int. J. Hydrogen Energy

J. Catal.

Appl. Surf. Sci.

J. Photochem. Photobiol. A

Adv. Free Radical Biol. Med.

Surf. Sci.

J. Catal.

J.Catal.

Current status of hydrogen production techniques by steam reforming of ethanol: a review

Energy Fuels

The photoreaction of TiO2 and Au/TiO2 single crystal and powder with organic adsorbates

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The photoreaction of TiO₂ and Au/TiO₂ single crystal and powder with organic adsorbates

HYdrogen production by tuning the photonic band gap with the electronic band gap of TiO₂

Photocatalytic H₂ production from bioethanol over Au/TiO₂ and Pt/TiO₂ photocatalysts under UV irradiation—A comparative study

Dual cocatalysts loaded type I CdS/ZnS Core/Shell nanocrystals as effective and stable photocatalysts for H₂ evolution

Photocatalytic H₂ evolution on cds loaded with WS₂ as cocatalyst under visible light irradiation