Exciting new material uses solar energy to remove man-made dye pollutants from water

A novel composite material has been developed by scientists in the Energy Safety Research Institute (ESRI) at Swansea University which shows promise as a catalyst for the degradation of environmentally-harmful synthetic dye pollutants, which are released at a rate of nearly 300,000 tonnes a year into the world's water.

This novel, non-hazardous photocatalytic material effectively removes dye pollutants from water, adsorbing more than 90 % of the dye and enhancing the rate of dye breakdown by almost ten times using visible light.

The researchers, led by Dr. Charles W. Dunnill and Dr. Daniel Jones at the Energy Safety Research Institute in Swansea University, reported their discovery in the Nature open access journal Scientific Reports.

By heating the reaction mixture at high pressures inside a sealed container, the composite is synthesized by growing ultra-thin "nanowires" of tungsten oxide on the surface of tiny particles of tantalum nitride. As a result of the incredibly small size of the two material components - both the tantalum nitride and tungsten oxide are typically less than 40 billionths of a metre in diameter - the composite provides a huge surface area for dye capture.

The material then proceeds to break the dye down into smaller, harmless molecules using the energy provided by sunlight, in a process known as "photocatalytic degradation". Having removed the harmful dyes, the catalyst may simply be filtered from the cleaned water and reused.

While the photocatalytic degradation of dyes has been investigated for several decades, it is only relatively recently that researchers have developed materials capable of absorbing the visible part of the solar spectrum - other materials, such as titanium dioxide, are also able to break down dyes using solar energy, but their efficiency is limited as they only absorb higher energy, ultra-violet light. By making use of a much greater range of the spectrum, materials such as those used by the ESRI team at Swansea University team are able to remove pollutants at a far superior rate.

Both of the materials used in the study have attracted significant interest in recent years. Tungsten oxide, in particular, is considered one of the most promising materials for a range of photocatalytic applications, owing to its high electrical conductivity, chemical stability and surface activity, in addition to its strong light absorbance. As a low band-gap semiconductor, tantalum nitride is red in colour due to its ability to absorb almost the entire spectrum of visible light, and therefore extracts a high amount of energy from sunlight to power the degradation processes.

However, the true potential of the two materials was only realised once they were combined into a single composite. Due to the exchange of electrons between the two materials, the test dye used within the study was broken down by the composite at around double the rate achieved by tantalum nitride on its own, while tungsten oxide alone was shown to be incapable of dye degradation. In contrast to other leading photocatalytic materials, many of which are toxic to both humans and aquatic life, both parts of the composite are classed as non-hazardous materials.

The scientists responsible for the study believe that their research provides just a taster of the material's potential. "Now that we've demonstrated the capabilities of our composite, we aim to not just improve on the material further, but to also begin work on scaling up the synthesis for real-world application." said Dr. Jones. "We're also exploring its viability in other areas, such as the photocatalysed splitting of water to generate hydrogen."

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In addition to Drs. Dunnill and Jones, co-authors of the paper are Drs. Virginia Gomez, James McGettrick and Serena Margadonna and PhD students Bertrand Rome, Francesco Mazzali and Aled Lewis, who are all fellow researchers in the College of Engineering at Swansea University, in collaboration with Dr. Joseph Bear from the Materials Chemistry Centre at University College London and Dr. Waheed Al-Masry from the Department of Chemical Engineering at King Saud University, Saudi Arabia.

The Energy Safety Research Institute is positioned to discover and implement new technology for a sustainable, affordable, and secure energy future and is housed on Swansea University's new world class Bay Campus. ESRI provides an exceptional environment for delivering cutting edge research across energy and energy safety related disciplines with a focus on renewable energy, hydrogen, carbon capture and utilisation as well as new oil and gas technologies.

Notes

• The scientific report 'Active removal of waste dye pollutants using Ta3N5/W18O49 nanocomposite fibres ' published in Nature can be found here: https://www.nature.com/articles/s41598-017-04240-4.

• Financial support for the study was provided by the Welsh Government Sêr Cymru Programme and the Flexis project, which is part-funded by the European Regional Development Fund (ERDF) through the Welsh Government, as well as through collaboration with King Saud University.

• For information about the Dunnill Research Group go to: http://www.esri-swansea.org/en/the-team.htm?id=5, http://www.Charles.Dunnill.me. Twitter: @Charlie_Dunnill

• Swansea University is a world-class, research-led, dual campus university. The University was established in 1920 and was the first campus university in the UK. It currently offers around 350 undergraduate courses and 350 postgraduate courses to circa 20,000 undergraduate and postgraduate students.

The University's 46-acre Singleton Park Campus is located in beautiful parkland with views across Swansea Bay. The University's 65-acre science and innovation Bay Campus, which opened in September 2015, is located a few miles away on the eastern approach to the city. It has the distinction of having direct access to a beach and its own seafront promenade. Both campuses are close to the Gower Peninsula, the UK's first Area of Outstanding Natural Beauty.

Swansea is ranked the top university in Wales and is currently The Times and The Sunday Times 'Welsh University of the Year'. It is also ranked within the top 350 best universities in the world in the Times Higher Education World University rankings.

The results of the Research Excellence Framework (REF) 2014 showed the University has achieved its ambition to be a top 30 research University, soaring up the league table to 26th in the UK, with the 'biggest leap among research-intensive institutions' (Times Higher Education, December 2014) in the UK.

The University has ambitious expansion plans as it moves towards its centenary in 2020, as it continues to extend its global reach and realising its domestic and international ambitions.

Swansea University is a registered charity. No.1138342. Visit http://www.swansea.ac.uk

For more information, please contact Janis Pickwick, Swansea University Public Relations Office. Tel: 01792 295050, or email: j.m.pickwick@swansea.ac.uk

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