Photocatalytic H2Production using High Surface Area ZnONanorods

Transcription

1 Photocatalytic H2Production using High Surface Area ZnONanorods

2 Contents 1. Introduction to photocatalytic water splitting 2. Semiconducting electron acceptors 3. Why nanorods? 4. Main project aims 5. Synthesis of nanorods 6. Increasing stability of ZnO nanorods 7. Synthesis of novel semiconducting materials 8. Conclusions and questions?

3 Introduction to Photocatalysis Need a clean source of hydrogen to fuel the hydrogen economy Current hydrogen production processes are typically run at a great carbon cost (SMR // Electrolysis) with the exception of ThermochemicalCycles which require vast amounts of heat typically from Nuclear Harnessing sunlight to reduce water to hydrogen mitigates carbon cost Utilise a tuned photocatalyst/chromophore and a large surface area semiconductor to split water hv H 2 O Cat Oxidation e - e - e - e - Donor Chromophore Acceptor Cat Reduction 2H + 2H + + ½O 2 H 2

4 Semiconducting Acceptors Semiconducting Acceptor materials usually absorb strongly in the UV region and so need a photosensitive dye to increase absorption Pt centred dye The band gap needs to be sufficient to successfully split water to hydrogen 2H + (aq)+2e - H 2 H 2 O (l) ½O 2 +2e - +2H + (aq) H 2 O (l) H 2(g) +½O 2(g) E=-0.41V E=-0.82V E=-1.23V Semi-conductor Titanium Dioxide Zinc Oxide Zinc Stannate Band Gap 3.2 ev 3.37 ev ev Titanium Dioxide (TiO2) is most common but let down by its low electron mobility Zinc Oxide (ZnO) looks to counter this issue as well as other ternary oxides

5 Effect of Nano-structures Look to increase the surface area to maximise light absorbed and overall efficiency Low surface area semiconducting structure High surface area semiconducting structure Uniform nanorods present high surface areas for dyes to be absorbed to As the amount of dye absorbed is increased increases photocatalytic performance and water splitting to hydrogen However, it is vital that the dye absorbed takes the form of a monolayer

6 Main Project Aims The three main aims of this project are to: Synthesise uniform ZnO nanorods - Look at improving on growth mechanism and optimising growth time for uniform nanorods Increase the stability of ZnO nanorods - Look at improving the lifetime of ZnO when used for water splitting and also through dye attachment Produce a novel semiconducting material - Zinc Stannate(Zn2SnO4) looks to be a promising new candidate for future solar applications in both Dye Sensitized Solar Cells (DSSCs) and water splitting due to it being more acid stable and having a tuneable band gap

7 Synthesis of Nanorods Various synthesis routes were examined from the literature as well as varying growth times to reliably reproduce ZnO nanorods ZnOseeds were produced by thermally decomposing Zinc Acetate on glass substrates using temperatures above 250 C Zinc Nitrate was added as a source of Zinc and HMT was added to provide basic conditions for rod growth Rod growth was carried out at 90 C and growth time was altered for a number of scenarios It was found that a growth time at up to 16 hours produced rods 200 nm in diameter and 1.5 µm in length Rough calculation: 1 m² 22 m²

8 Stability of ZnO Nanorods One of the principal failings of ZnO as a semiconducting acceptor material is its relatively short lifetime when in operation low acid stability One way to counter this issue is to add a buffer region between the ZnO nanorods and the Pt-centred dye Proposed that silanisationof ZnO using APTES will provide a buffer region as well as allowing the attachment of dye Creates electrostatic attraction The addition of an amine group reduces acid etching due to carboxyl groups in dye Zn O Zn O Zn O O O Si NH3 O O + - With Silane No Silane With Silane No Silane Before After

9 Synthesis of Zinc Stannate Can be produced in numerous ways using a variety of conditions In this case used high pressure, hydrothermal route using an autoclave Unfortunately encountered problems The reaction wasn t able to be carried out stably and led to the production of less favoured products chiefly zinc and tin oxide Also led to the destruction of various lab equipment PTFE liners, glassware Products were characterised using XRD and found to be unsuccessful

10 Conclusion Zinc oxide nanorods were shown to be produced reliably and repeatedly using the hydrothermal growth method Rods were able to be successfully silanised using APTES and produced more acid stable rods when immersed in TEA for hydrogen production Dye was able to be successfully absorbed onto both ZnO nanorodswith and without silane group Unfortunately Zinc Stannatewas unable to be produced, however not all synthesis routes have been exhausted

11 Future Work Need to ascertain as to whether the dye absorbed takes the form of a monolayer Look at ways to improve acid stability of absorbed dye Try alternative methods of calculating the quantity of dye absorbed onto nanorods Explore alternative synthesis route for Zinc Stannate formation Novel idea of reacting already grown ZnO nanorodsto Zinc Stannate using ZnO rods as a template for novel semiconductor Look at the dye absorption mechanism for Zinc Stannate

12 References Greene, L. E., Law, M., Tan, D. H., Montano, M., Goldberger, J., Somorjai, G., & Yang, P., (2005), General route to vertical ZnOnanowirearrays using textured ZnOseeds, NanoLetters Journal, Volume 5, Issue 7, pp Silberzan, P., Leger, L., Ausserre, D., & Benattar, J. J., (1991), Silanationof silica surfaces. A new method of constructing pure or mixed monolayers, Langmuir Journal, Volume 7, Issue 8, pp Zhang, J., Du, P., Schneider, J., Jarosz, P., & Eisenberg, R., (2007), Photogenerationof hydrogen from water using an integrated system based on TiO2 and platinum (II) diimine dithiolate sensitizers, Journal of the American Chemical Society, Volume 129, Issue 25, pp Howie, W. H., Claeyssens, F., Miura, H., & Peter, M., (2008), Characterization of solid-state dyesensitized solar cells utilizing high absorption coefficient metal-free organic dyes, Journal of the American Chemical Society, Volume, 130, Issue 4, pp Tan, B., Toman, E., Li, Y., & Wu, Y., (2007), Zinc stannate(zn 2 SnO 4 ) dye-sensitized solar cells, Journal of the American Chemical Society, Volume 129, Issue 14, pp