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Engineering Silicon Carbide Nanowires for Solar Fuels Production

Periodic Reporting for period 1 - Solarfuels (Engineering Silicon Carbide Nanowires for Solar Fuels Production)

Reporting period: 2015-08-28 to 2017-08-27

• What is the problem/issue being addressed?
By 2020, the European Union (EU) aims to reduce greenhouse gas emissions by 20-30% and increase renewable energy share to 20%. This scenario has imposed urgent needs to develop fossil fuel alternatives like solar fuels. To produce solar fuels, the coupled reduction of CO2 and H2O is one of the most promising processes. However, the generation of efficient, stable and low-cost material for CO2/H2O reduction remains a big challenge. Silicon carbide nanowires (SiC NW) exhibit the unique properties of large surface-to-volume ratio, tuneable transport properties and quantum size effects, which is very promising for the reduction of CO2/H2O to produce solar fuels. To date, the studies on SiC NW for CO2/H2O reduction are limited due to the lack of (1) large-scale production techniques, (2) in situ characterization of the growth mode, and (3) there are no economical devices available for the evaluation of SiC NW. (Figure 1) This project aims to address these challenges by developing method for large-scale synthesis of SiC NW, studying its growth mechanism via in situ mass spectrometry and building SiC NW-based solar fuel device.

• Why is it important for society?
The addressing of the above issues allows the production of SiC nanowires in large scale and generation of solar fuels in an efficient and low-cost way. This makes a step closer to meet the targets on the reduction of CO2 emission and the increase of renewable energy share in the EU and worldwide, and to ultimately achieve a low-carbon economy and a sustainable society.

• What are the overall objectives?
The overall objectives are to develop SiC nanowires for solar fuels production including:
1) To synthesise aligned SiC nanowires in large scale;
2) To study the morphology and growth mechanism of SiC using in-situ characterisation techniques;
3) To evaluate aligned SiC nanowires as photocatalysts.

• Conclusions of the action
1) Vertically-aligned SiC nanowires can be synthesised in gram scale from the developed carbon nanotube template method;
2) The diameter and length of SiC nanowires are tuneable by controlling the structure of carbon nanotubes;
3) The growth mechanism of SiC nanowires was revealed by using in-situ mass spectrometry;
4) Photocatalytic evaluation showed that the developed SiC nanowires are photocatalytically-active under visible light;
5) The estimation of band structure for SiC nanowires indicated they are promising for the visible-light-driven photocatalytic reduction of CO2 to produce solar fuels.
"• Work performed and overview of the results
1) We have developed a method (Figure 2) for the synthesis of aligned SiC nanowire;
2) These aligned SiC nanowires can be produced in gram scale with tuneable diameter and length;
3) We have studied the growth mechanism using in-situ mass spectrometry (Figure 3);
4) We have estimated the band structure for SiC nanowires;
5) We have conducted photocatalytic evaluation which demonstrated that the as-produced SiC nanowires are photocatalytically active under visible light (Figure 4).

• The exploitation and dissemination of the results
The results have been exploited including the presentation and the publication of research results and have been disseminated through publication in peer-review journal, presenting in conference/workshops and social networking services.
1)The results have been published on Applied Catalysis B: Environmental, 2017, 218, 267-276.
2)The results have also been disseminated via:
a) Poster presentation in ""Nanomaterials by Design and Williams Grand Prix Engineering Limited joint workshop"" (Grover, UK, 2016 May), poster presentation in ""NT16"" (Vienna, Austria, 2016 Aug), an international conference on low-dimensional materials, and poster presentation to a Chinese Delegation of Government Officials from Guangdong province (Oxford, UK, 2017 Jun);
b) Invited talks at Fuzhou University (Fuzhou, China, 2017 May) and at South China University of Technology (Guangzhou, China, 2017 Jun);
c) Self-archived at University of Oxford as green open access.
3) In addition, the results have been disseminated to wider audience through posting on social networking service including google scholar, LinkedIn, ResearchGate, and personal research website, etc.
• Progress beyond the state of the art
Vertically-aligned silicon carbide nanowires (VASiCs) are promising in energy and environmental applications. However, the production of aligned SiC nanowires in gram scale with tuneable diameter and length has not yet available. Previous reports also speculated that both, CO and CO2, are formed as by-products influencing the evolution of the SiC nanowires during the conversion reaction. But the effect has not been experimentally investigated yet. Studies on aligned SiC nanowires so far were focused on the synthesis, structure while their applications are still limited.

Synthesis: Recently, the host group developed an aerosol-assisted chemical vapour deposition method for the large-scale synthesis of aligned multiwalled carbon nanotube (MWCNTs) carpets with tuneable diameter and length. These large area flexible carpets of aligned MWCNTs enable the large-scale production of aligned SiC nanowires with tailored structure. By using these MWCNTs as template and the infiltrated SiO2 as Si source, we have produced VASiCs in gram scale. In addition, the diameter and length of VASiCs are tuneable by controlling the structure of MWCNTs template. As a result, we have made progress in this area by delivering a method for the synthesis of VASiCs in gram scale with tuneable structure for the first time.

Mechanism: We have also analysed the chemistry of a VASiCs CVD reactor using a quadrupole mass spectrometer to monitor the formation of residual by-product gases. These profiles of gas evolution and furnace temperature were then used to study and establish the growth mechanism of VASiCs. The in-situ MS studies revealed that the growth of VASiCs only involve the CO as the only by-product while the widely assumed CO2 was not detected. findings help to understand the growth scenarios for the VASiCs, which are important for the diagnostics of the production line.

Applications: We have conducted photocatalytic studies with VASiCs to degrade dye pollutant using both UV–vis light and visible light showing efficient photocatalytic activity. Besides, VASiCs showed good stability and can be recycled without significant loss of photoactivity. These results demonstrated that the developed VASiCs are efficient, stable, recyclable, and visible-light-driven photocatalysts.

• Potential impact
The developed synthesis method will enable the production of SiC nanowires with controlled structure in large scale. The revealed growth mechanism helps to understand the growth of SiC and control the production line of SiC. The conducted photocatalytic evaluation will contribute to the design of SiC nanowires photocatalysts. Through this project, we have made advances in the synthesis, design, and energy/environmental applications of SiC, which have potential impact in achieving sustainable energy and low-carbon society.
Figure 4. Schematic diagram showing the morphology of aligned SiC nanowires and their photocatalytic
Figure 3. Real-time monitoring of the gas residuals from the VASiCs synthes.
Figure 2. Schematic illustration on the synthesis of VASiCs in large scale.
Figure 1. The overview of problem/issue being address and the overall objectives.