Photocatalytic H 2 production using High Surface Area ZnO Nanorods

Transcription

1 Photocatalytic H 2 production using High Surface Area ZnO Nanorods Mini-project Report 2 Written by James Craven Department of Chemistry Kroto Research Institute May 2011

2 1.0 Introduction Artificial photosynthesis or the photocatalytic production of hydrogen from water has become an increasingly attractive topic in recent years; largely due to the requirement of a clean source of hydrogen to fuel the hydrogen economy [1] [4]. Current sources of hydrogen are either generated from steam methane reforming reactions (SMR), electrolysis or thermochemical cycles, all of which are typically carried out at great carbon cost. Artificial photosynthesis looks to counter this issue by harnessing a small proportion of the 120,000 TW of solar energy reaching the Earth at any given moment and in turn abate the standard carbon cost incurred, thus generating a clean source of hydrogen [4]. hv H 2O Cat Oxidation e - e - e - e - Donor Chromophore Acceptor Cat Reduction 2H + 2H + + ½O 2 H 2 Figure.1: Illustrates the principal elements needed for artificial photosynthesis [2]. Figure.1 highlights the principal elements needed to convert incoming photons in the form of sunlight into hydrogen through the splitting of water. Light absorption takes place at the central chromophore where incoming photons are absorbed and are used to promote electrons to a higher energy molecular orbital leaving behind a positively charged hole in its place. The electron is then passed to an electron accepting medium and in turn a chemically reducing catalyst where a source of aqueous protons are reduced to hydrogen. The positively charged hole is filled with an electron from an electron donating medium which is supplied through the catalytic oxidation of water to oxygen thus completing the cycle [1] [2]. Figure.1 presents a closed loop system whereby hydrogen and oxygen production both take place. However, by examining differing energy molecular orbital s it is possible to omit the production of oxygen by using an electron donating group without catalysing chemical oxidation and therefore presenting an open loop system producing hydrogen alone. 2.0 Semiconducting electron acceptors In both fields of dye sensitized solar cells (DSSCs) and artificial photosynthesis, titanium dioxide (TiO 2 ) remains the most popular material for use as electrodes and semiconducting electron acceptors. This is mainly due to its appropriate band gap, surface structure and - 1 -

3 electron stability. However, TiO2 is susceptible to photobleaching [9] and its electron mobility is low which therefore presents inferior conversion efficiencies which look to be enhanced by the utilisation of alterative semiconducting materials [3]. Zinc oxide (ZnO) presents many advantages over TiO2. It has higher electron mobility and is easier to modify and in turn tailor to the specific platinum (Pt) or ruthenium (Ru) centred dye being used as the central chromophore. As well as this it has a similar band gap to TiO2 and therefore presents the possibility for enhanced water splitting efficiencies [3]. 3.0 ZnO nanostructures In order to maximise the number of photons being absorbed by any given chromophore, which in this case takes the form of a platinum centred dye (Pt-dye), the surface area of the structure on which the Pt-dye sits must be maximised. This can be achieved through employing nanostructures; specifically nanorods [6]. ZnO nanorods were successfully grown and reproduced using the hydrothermal growth mechanism. Arrays of uniform rods were produced on glass slides by creating zinc acetate nucleation sites and reacting at 90 C in a basic environment of 40 ml M zinc nitrate hexahydrate and 40 ml M hexamethylentetramine (HMT) for up to 16 hours [13] [16]. The rods are grown in the <001> plane due to kinetics favouring a lower surface energy in the vertical orientation [7]. Figure.2 shows a typical array of ZnO nanorods grown hydrothermally. Figure.2: Displays a typical array of hexagonal ZnO nanorods ~200nm diameter (1µm left, 5µm right). However, one problem that persists with the use of ZnO nanorods is their acid stability and in turn their relatively short lifetime when used for water splitting. Alternative attachment -2-

4 mechanisms for dye-zno must be explored to counter this issue as well as highlighting more acid stable semiconducting materials for water splitting. 4.0 Silanisation of ZnO As well as maximising the surface area of the dye absorbed it is vital that aggregation is minimised and that the dye absorption takes the form of a monolayer. By silanising the ZnO surface it looks to promote the formation of a monolayer and reduce acid etching by adding a buffer region between the acidic carboxyl group within the Pt-dye and the ZnO surface making the ZnO more acid stable [10] [11]. ZnO nanorods were successfully silanised by immersing in 40 mm APTES (3-aminopropyltriethoxysilane) in methanol for 10 minutes before air drying [11]. Pt-dye was then attached firstly by heating the glass slide on which the ZnO nanorods were grown to 90 C [12] and then immersing the slide in a 1 mm solution of Pt-dye in DMF (dimethylformamide) for 24 hours. This method was carried out for both ZnO nanorods treated and not treated with APTES. The dye was shown to absorb more strongly to the unsilanised ZnO nanorods, however when immersed in a solution of ph mm TEA (triethyl-amine) [17] to test the dyes stability during hydrogen production, the silanised ZnO nanorods were shown to be more stable; the silanised ZnO retaining a slight yellow colour compared to the unsilanised ZnO. Figure.3: Shows ZnO nanorods before (left) and after (right) immersion in 12.6 mm TEA. Figure.3 shows that the silanised ZnO rods are more acid stable for photocatalytic hydrogen production. The fainter yellow colour before immersion may also suggest a monolayer, however the time constraints of this project meant that this was unable to be confirmed

5 5.0 Hydrothermal preparation of Zinc Stannate (Zn 2 SnO 4 ) In addition to the modification of ZnO nanorods, novel semiconducting materials were explored; specifically the ternary oxide zinc stannate (Zn 2 SnO 4 ). Multication oxides such as zinc stannate have the ability to be chemically and physically tuned by varying the relative metal ratio; in this case zinc (Zn) to tin (Sn). This allows criteria such as the band gap, work function and electric resistivity to be tuned; zinc stannate also exhibits the added advantage of a reduced acid etching rate compared to ZnO [14]. It was proposed that zinc stannate was to be prepared by hydrothermally reacting 40 ml 0.25 M zinc acetate dihydrate (Zn(CH 3 COO) 2.2H 2 O) with 160 ml 0.25 M tin tetrachloride pentahydrate (SnCl 4.5H 2 O) in a 300 ml Teflon lined autoclave at 220 C for 24 hours; both solutions being made up using 3 M sodium hydroxide immediately prior to being reacted [15]. However, complications in the synthesis route were encountered which subsequently led to the predominant production of zinc and tin oxide which were characterised by XRD. Alternative routes may need to be explored for successful synthesis of zinc stannate. 6.0 Conclusion ZnO nanorods were successfully synthesised using the hydrothermal growth mechanism as well as being silanised using APTES. The attached silane group was able to improve acid stability of the ZnO nanorods showing improved performance for photocatalytic water splitting. Zinc stannate was unable to be produced using high pressure synthesis, however further reading suggests alternative conditions may promote a more successful route. 7.0 Future work Due to the limited time scale of this project an in depth look into the absorption mechanism of the Pt-dye was unable to be carried out. Quantitative data regarding the quantity of dye absorbed needs to be recorded in order to draw a direct comparison between the volume of dye absorbed and the volume of hydrogen produced, as well as ascertaining whether the dye absorbed takes the form of a monolayer. Although the acid stability was shown to be improved by silanising ZnO, dye desorption was still present in samples. A range of dyes should be absorbed and tested in order to quantify the dyes stabilities and water splitting performance. As well as this further research into synthesis routes for zinc stannate must be conducted in order to carry out similar work to that done with zinc oxide

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