Lab profile: Junwang Tang, University College London

Modern society depends upon a reliable energy supply, but the challenge facing us is to make that supply sustainable. Solar energy is readily available but its low intensity and intermittence require efficient technologies to convert into a useful and storable supply. As a chemist specializing in heterogeneous catalysis, Junwang Tang has always been interested in solving this problem and developing clean technologies for recycling raw materials.

He received his PhD in Physical Chemistry from DICP in 2001 and then joined NIMS in Japan as a JSPS fellow following. He subsequently moved to Imperial College London, before joining University College London (UCL) in 2009, where he is now Director of the UCL Materials Hub and Professor of Materials Chemistry and Engineering in the Department of Chemical Engineering where he leads a research effort focusing on catalysts for small molecule activation and microwave catalysis for renewable energy and environmental purification applications. He was elected a Fellow of the Royal Society of Chemistry in 2014 and received the IPS (International Conference on Photochemical Conversion and Storage of Solar Energy) Scientist Award in 2018. Tang also serves on the Editorial Board of Materials Today Advances.

Junwang Tang talked to Materials Today about his current research and future plans.

How long has your group been running?

Since May 2009, so it has been more than 10 years.

What's the make up of your group?

Currently, we have 22 members in our group including postdocs, PhD students, visiting scholars, and several masters’ students.   

What are the major themes of your research?

The major themes of my research are:

• Photocatalysis, including the activation of small molecules (H₂O, N₂, CH₄ and Benzene) and water treatment;
• Microwave catalysis, involving waste to valuable chemicals, e.g. polyethylene terephthalate (PET) plastic depolymerization;
• Fundamental understanding by time resolved spectroscopies, e.g. time-resolved infrared spectroscopy (TRIR) and -ime-resolved absorption spectroscopy (TAS);
• Microwave intensified chemical processes, e.g. continuous nanoparticle synthesis and graphene production.

How and why did you come to work in these areas?

Modern society is heavily dependent on energy and the environment. The challenge of today is to make this sustainable. To solve this problem, we have to find renewable and widespread energy sources and develop clean technologies for recycling raw materials. My MSc and PhD in heterogeneous catalysis put me in an ideal position for tackling these challenges.

Solar energy is the best available renewable energy source while its low intensity and intermittence require an efficient technology to not only convert but also store it in transportable chemicals. This is the big drive behind the photocatalytic activation of small molecules, as this photochemical process will store solar energy as fuels or chemicals. I have been in this field since 2002, active in the discovery of efficient and robust photocatalysts and in the in-depth understanding of the underlying mechanisms.

On the other hand, microwave catalysis provides an efficient pathway for raw materials/waste recycling, promising a clean technology for the cycling of the Earth’s resources. I started this project in 1998 when I was a PhD student.

What facilities and equipment does your lab have?

My laboratory benefits from:

(i) Advanced techniques for catalyst synthesis apart from common facilities (e.g. furnaces, hydrothermal device etc.) including:

• Facilities for sol-gel, dip-coating, electrophoresis, aerosol-assisted chemical vapor deposition (AACVD) synthesis;
• Facilities for the deposition of nanoparticle and molecular catalysts, e.g. chemical bath deposition, chemical vapor deposition;
• Prototype fluidic system for massive production of catalysts;

(ii)  Advanced ex-situ and in-situ characterization tools:

Spectrophotometers, time-resolved photoluminescence, time-resolved absorption spectroscopy, intensity modulated photocurrent spectroscopic facilities, temperature-programmed adsorption/desorption facilities;

(iii) Advanced in-situ testing tools for evaluation of the performance of photocatalysts and microwave catalysts:

• Systems equipped with solar simulators for small molecule conversion (water to H₂, N₂ to ammonia, CH₄ to C²⁺ and CO₂ to alcohols) and product monitoring;
• Impedance frequency analyzers with potentiostats; 
• Brunauer-Emmett-Teller surface area analyzer (BET), Gas chromatography–mass spectrometry (GC-MS), and High Performance Liquid Chromatography (HPLC);
• Quantum efficiency and solar-to-fuel conversion efficiency measurements;
• Photoelectrochemical devices;
• Microwave catalysis system operated in high pressure or fluidic mode.

Do you have a favorite piece of kit or equipment?

The time-resolved spectroscopies (e.g. the in-house-built TAS, Figure 1) are my favorite as they can tell me how fast charges move in solid catalysts and how easily reactants can take these charges away.

What do you think has been your most influential work to date?

There are a few pieces of work that I believe are significant. In particular, my work on the fundamental understanding of photocatalysis [J Am. Chem. Soc. 42 (2008) 13885] and the discovery of highly active catalysts for small molecule transformation under ambient conditions [Nature Catalysis 1 (2008) 889].

The former work (Figure 2) illustrates for the first time the extremely sluggish kinetics of water oxidation on artificial catalysts, ca. 6 orders of magnitude slower than the reduction half-reaction of water splitting. It also gives preliminary experimental evidence of four-hole chemistry for water oxidation.

The other work is related to the first report of the conversion of methane (CH₄) to high-value chemicals such as alcohols under ambient conditions (room temperature) and qwith high selectivity (97%) using single atom Fe ions on a TiO₂ catalyst driven by light.

The next significant piece of work concerns solar H₂ fuel synthesis from water.  As water oxidation is the rate-determining step in artificial water splitting due to the multi-electron process, I always give it priority. The recently developed tetrahedral Ag₃PO₄ crystals resulted in a record activity for water oxidation, with an internal quantum yield of nearly 100% in the visible light region due to the synergistic effect between high surface energy and smaller hole mass. I also developed a highly polymerized urea-derived graphitic carbon nitride, leading to an extremely high hydrogen production rate (~20 000 µmol h^-1 g^-1) and 26% quantum efficiency, which is currently the second best single-phase photocatalyst in this area. However, it is much more stable than the best material, CdS, and is, therefore, of practical importance. Furthermore, inspired by natural photosynthesis, I very recently designed a double excitation system using an organic semiconductor instead of inorganic materials, which more closely resembles a natural photosynthetic system. This led to the first example of pure water splitting under visible light irradiation by an organic semiconductor-based system.

The last advance is related to my work on microwave-intensified chemical processes. By means of coupling fast microwave heating with prompt mass-transfer in fluidic capillary reactors, metals (e.g. Au), metal oxides (e.g. Fe₂O₃), graphene, and polymers (e.g. poly(glycerol sebacate) or PGS) have been successfully prepared by microwave chemistry. Such a fast and energy-efficient process can also continuously produce functional materials in the metaphase, which shows unique properties in catalysis, drug delivery, tissue engineering, and sensors.

What is the secret to running a successful group?

There are many factors affecting a group but the most important for me is the combination of a flexible research environment and regular brainstorming discussions. These not only promote research projects but also foster bright, independent, and highly motivated researchers in my group.

What's next for your research?

The research sub-areas I have described will be maintained and, more importantly, fundamental understanding will be further enhanced, which will direct materials discovery. In parallel, multidisciplinary collaborations both locally and internationally will be widened and deepened, as a single group cannot solve the challenges effectively. Eventually, these catalysts will be applied to real industrial environments to benefit society financially. 

Key publications

1. J. Tang, J. R. Durrant, D. R. Klug. Mechanism of Photocatalytic Water Splitting in TiO₂. Reaction of Water with Photoholes, Importance of Charge Carrier Dynamics, and Evidence for Four-Hole Chemistry. J Am. Chem. Soc. 42 (2008) 13885-13891. https://doi.org/10.1021/ja8034637
2. J. Xie, R. Jin, A. Li, Y. Bi, Q. Ruan, Y. Deng, Y. Zhang, S. Yao, G. Sankar, D. Ma, J. Tang. Highly selective oxidation of methane to methanol at ambient conditions by titanium dioxide-supported iron species. Nature Catalysis 1 (2008) 889-896. https://doi.org/10.1038/s41929-018-0170-x
3. Y. Wang, A. Vogel, M. Sachs, R.S. Sprick, L. Wilbraham, S.J.A. Moniz, R. Godin, M.A Zwijnenburg, J.R. Durrant, A.I. Cooper, J. Tang. Current understanding and challenges of solar-driven hydrogen generation using polymeric photocatalysts. Nature Energy 4 (2019) 746-760. https://doi.org/10.1038/s41560-019-0456-5
4. C. C. Lau, M.K. Bayazit, P.J.T. Reardon, J. Tang. Microwave Intensified Synthesis: Batch and Flow Chemistry, The Chemical Record, 19 (2019) 172-187. https://doi.org/10.1002/tcr.201800121
5. D. Kong, Y. Zheng, M. Kobielusz, Y. Wang, Z. Bai, W. Macyk, X. Wang, J. Tang. Recent advances in visible-light driven water oxidation and reduction in suspensions systems, Materials Today 21 (2018) 897-922. https://doi.org/10.1016/j.mattod.2018.04.009
6. Y. Wang, H. Suzuki, J. Xie, O. Tomita, D.J. Martin, M. Higashi, D. Kong, R Abe, J. Tang. Mimicking Natural Photosynthesis: Solar to Renewable H₂ Fuel Synthesis by Z-Scheme Water Splitting Systems. Chemical Reviews 118 (2018) 5201-5241. https://doi.org/10.1021/acs.chemrev.7b00286
7. J. Xie, S.A. Shevlin, Q. Ruan, S.J.A. Moniz, Y. Liu, X. Liu, Y. Li, C.C. Lau, Z. Guo, J. Tang. Efficient visible light-driven water oxidation and proton reduction by an ordered covalent triazine-based framework. Energy and Environmental Science 11 (2018) 1617-1624. https://doi.org/10.1039/C7EE02981K
8. Q. Ruan, W. Luo, J. Xie, Y. Wang, X. Liu, Z. Bai, C.J. Carmalt, J. Tang. A Nanojunction polymer photoelectrode for efficient charge transport and separation. Angewandte Chemie International Edition 56 (2017) 8221-8225. https://doi.org/10.1002/anie.201703372
9. R. Godin, Y. Wang, M.A. Zwijnenburg, J. Tang, J.R. Durrant Time-Resolved Spectroscopic Investigation of Charge Trapping in Carbon Nitrides Photocatalysts for Hydrogen Generation. J Am. Chem. Soc. 139 (2017) 5216–5224. https://doi.org/10.1021/jacs.7b01547
10. D.J. Martin, P.J.T. Reardon, S.J.A. Moniz, J. Tang. Visible Light-Driven Pure Water Splitting by a Nature-Inspired Organic Semiconductor-Based System. J Am. Chem. Soc. 136 (2014) 12568-12571. https://doi.org/10.1021/ja506386e