Associate Professor Chen Hongyu

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Chen, Hongyu (陈虹宇)
Division of Chemistry & Biological Chemistry,
School of Physical & Mathematical Sciences
Nanyang Technological University
21 Nanyang Link, SPMS-CBC-03-02,
Singapore 637371

E-mail:
Web Site: http://www.ntu.edu.sg/home/hongyuchen/


Nanosynthesis.

      The Nanotechnology depicted in fictions involves complex machinery with extremely sophisticated components, whereas the Nanotechnology today utilizes simple nanostructures for applications, such as nanospheres, nanowires, and nanosheets. This huge gap of synthetic capability is an obvious bottleneck for next stage development of nanotechnology, and yet it is largely overlooked by the research community, particularly funding agencies and journal editors.

      From Stone Age to Information Age, at each step the synthetic advance was THE critical factor for the transitions in Human society. The advances were initially serendipitous but became less so in the modern era. Without the coherent efforts by numerous organic chemists, for example, the modern pharmaceutical industry and curing of mass diseases would not have been possible. Now the questions are, whether serendipitous discoveries are sufficient to take us to the future Nanotechnology; and whether we should organize ourselves towards coherent synthetic advances?

      The current emphasis on application in the nanotechnology community is not wrong, but it should not prevent long-term vision targeting critical issues in this emerging field. The reality is, this field is morphed mainly by funding and journal prestige, not the passion and curiosity of a few individuals. Not every step of synthetic advance will lead to something useful and window-dressing as application is often meaningless. Without enough rewards, the leading scientists have shifted away from synthesis to exploring more usage out of the existing nanostructures. The field has shown signs of plateauing and it WILL run dry without an expanding foundation of synthetic skills.

      There are often confusions when I talk about synthetic skills and complex nanostructures. If we fix our eyes on the gap towards future Nanotechnology, there are plenty of research directions beyond nanoparticles of different components and shapes.

      I devoted my career in the past 10 years to advancing the synthetic skills at the nanoscale, a sub-branch of nanotechnology that I call nanosynthesis. My research group has studied 1) the formation of nanoparticle clusters, including dimers,1 trimers,1a,c linear aggregates,2 and globular aggregates with precise internal structures;1c,3 2) the formation of hybrid nanostructures, including inorganic-inorganic,4 inorganic-polymer,2a,5 and inorganic-organic6 multi-component systems, as well as core-shell,2a,4d,i,5b,d,6-7 eccentric,4e,g,5a and Janus4c,e,g,5a structural types; 3) the formation of new nanowire conformations, including rings,8 spirals,9 single helix,10 double helix,11 and Boerdijk-Coxeter-Bernal structures;11-12 and 4) other types of nanostructures, including multi-layer hollow structures,4a,5b,13 hybrid vesicles,14 dendritic structures,4b,f etc.

Mechanistic Study

      The bottleneck for expanding synthetic methodology, in my opinion, is to understand the fundamental processes in nanosynthesis. The traditional way of mechanistic study in this field, is to identify the key reaction condition (e.g., choice of reactants, rate of reaction) and propose a broad, often vague, mechanism (e.g., oriented attachment, kinetic control). Without understanding WHAT exactly the kinetic control is and HOW exact it influences the detailed steps of synthesis, for example, it would be impractical to design new synthetic routes and overcome obstacles.

      I believe that it would more useful for the field if one provides complete, step-by-step scenario explaining the synthetic processes, supported by evidences and sound chemical logic. This, of course, will expose the researcher to an array of questions, which are often too many to answer given the lack of knowledge in the current field. At the very least, it serves as a working hypothesis where the imperfections can be gradually improved. In contrast, one cannot even start arguing about a broad and vague mechanism. Analogies can be found in Organic Chemistry: Detailed mechanistic proposals are useful but difficult to defend, whereas broad explanations are easy to publish but useless in terms of guiding synthesis.

      With inexplicit and stringent expectations, complete mechanistic proposals (e.g., the VLS mechanism) are extremely rare in the literature. What the field needs are the appreciation for details, the recognition of new types of explanation, and perhaps also the tolerance for imperfections (out of many detailed steps). Without clearing the vagueness in mechanistic discussions, and without funding and recognition, there will be hardly grounds for debates and improvements.

      Over the years, my research group endeavours to provide detailed mechanistic explanations for various nanosynthesis phenomena, including 1) the colloidal assembly of nanoparticles1a,c and the choice between linear and globular aggregates;2a,c,e,f,3 2) the fundamental reasons underlying the hollowing of Stöber silica (and perhaps also other types of oxides);4a,15 3) the influence of surface energy in guiding the growth of Janus nanoparticles;4e,g,5a 4) the controlled seeded growth and surface nucleation;4h,i,16 5) the diffusion of small molecules/ions among nanoparticles7 and into nanoparticle shells;5c 6) the thermodynamics (overall driving force) and kinetics (detailed pathways) of ring formation;8a-c 7) the controlled coalescence among nanowires;8d 8) the room temperature growth of ultrathin nanowire forest;4j,17 9) the thermodynamics and kinetics (critical barriers) of polymer vesicle formation;14,18 and quite a few other minor directions.

Applications

      There is no doubt that applications are essential, both for returning to the society and for advancing the research field. But it is curious that only a tiny fraction of the application-oriented research leads to useful products with real impact to society. It is not difficult to recognize that many researchers use application as merely justifications for the advances in more fundamental areas, i.e., as "window-dressing". As such, many "applications" are proof-of-concept demonstrations of known concepts and often completely out of context from the real obstacles in application.

      The field needs new concepts, new approaches, new understandings, and breakthroughs in performance, and less in the different embodiments of old concepts and approaches. Unfortunately, the over-emphasis on application has caused the community to value "window-dressing" applications over fundamental studies.

      Over the years, we have tried our best to make complex nanostructures useful, hopefully by exploiting the synergy within the complex structure. While the successes are few, we are proud that we have been taking our own path: 1) make of nano-stirbars that can stir inside picoliter emulsion droplets using commercial hot-plate stirrer;2f 2) use of ultrathin nanowire forest to achieve rapid flow-through in a fix-bed catalysis;17b 3) a level platform for screening bimetallic nanoparticles for catalysis;1d 4) use of ultrathin nanowire forest to achieve 13 times higher catalytic activity per electrode area for ethanol fuel cell (in collaboration with Prof. Liu Bin).19

Reviews.

      Reviews are great opportunities to consolidate knowledge and to contemplate the key challenges in the field. I am surprised to find that there are very few review articles summarizing the different mechanistic views, despite the huge number of review articles summarizing synthetic phenomena.

      Hence, we focus our efforts on reviewing mechanisms in nanosynthesis. The first review entitled "Emerging Chirality in Nanoscience"20 summarizes and categorizes the known chirality-forming mechanisms in the literature, in particular regarding to the critical symmetry-breaking step. The mechanisms are compared to the basic processes in the simplest forms to help understanding.

      In our second review entitled "Thermodynamics versus Kinetics in Nanosynthesis",21 we advocate the distinction between thermodynamically and kinetically controlled scenarios. Great endeavours are made to articulate the multiple concurrent processes in typical nanoscience phenomena, so that the mechanistic proposals in the literature are brought into a common framework for easy contrast and comparison.

References

(1)          (a) Chen, G.; Wang, Y.; Tan, L. H.; Yang, M. X.; Tan, L. S.; Chen, Y.; Chen, H. J. Am. Chem. Soc. 2009, 131, 4218;(b) Wang, X.; Li, G.; Chen, T.; Yang, M.; Zhang, Z.; Wu, T.; Chen, H. Nano Lett. 2008, 8, 2643;(c) Wang, Y.; Chen, G.; Yang, M.; Silber, G.; Xing, S.; Tan, L. H.; Wang, F.; Feng, Y.; Liu, X.; Li, S.; Chen, H. Nat. Commun. 2010, 1, 87;(d) Tan, R. L. S.; Song, X.; Chen, B.; Chong, W. H.; Fang, Y.; Zhang, H.; Wei, J.; Chen, H. Nanoscale 2016, 8, 3447.

(2)          (a) Xing, S.; Tan, L. H.; Yang, M.; Pan, M.; Lv, Y.; Tang, Q.; Yang, Y.; Chen, H. J. Mater. Chem. 2009, 19, 3286;(b) Chen, T.; Wang, H.; Chen, G.; Wang, Y.; Feng, Y.; Teo, W. S.; Wu, T.; Chen, H. ACS Nano 2010, 4, 3087;(c) Yang, M.; Chen, G.; Zhao, Y.; Silber, G.; Wang, Y.; Xing, S.; Han, Y.; Chen, H. PCCP 2010, 12, 11850;(d) Shen, X.; Chen, L.; Li, D.; Zhu, L.; Wang, H.; Liu, C.; Wang, Y.; Xiong, Q.; Chen, H. ACS Nano 2011, 5, 8426;(e) Wang, H.; Chen, L.; Shen, X.; Zhu, L.; He, J.; Chen, H. Angew. Chem. Int. Ed. 2012, 51, 8021;(f) Chong, W. H.; Chin, L. K.; Tan, R. L. S.; Wang, H.; Liu, A. Q.; Chen, H. Angew. Chem. Int. Ed. 2013, 52, 8570.

(3)          Urban, A. S.; Shen, X.; Wang, Y.; Large, N.; Wang, H.; Knight, M. W.; Nordlander, P.; Chen, H.; Halas, N. J. Nano Lett. 2013, 13, 4399.

(4)          (a) Wong, Y. J.; Zhu, L.; Teo, W. S.; Tan, Y. W.; Yang, Y.; Wang, C.; Chen, H. J. Am. Chem. Soc. 2011, 133, 11422;(b) Pan, M.; Xing, S.; Sun, T.; Zhou, W.; Sindoro, M.; Teo, H. H.; Yan, Q.; Chen, H. Chem. Commun. 2010, 46, 7112;(c) Chen, T.; Chen, G.; Xing, S.; Wu, T.; Chen, H. Chem. Mater. 2010, 22, 3826;(d) Feng, Y.; Wang, Y.; Wang, H.; Chen, T.; Tay, Y. Y.; Yao, L.; Yan, Q.; Li, S.; Chen, H. Small 2012, 8, 246;(e) Feng, Y.; He, J.; Wang, H.; Tay, Y. Y.; Sun, H.; Zhu, L.; Chen, H. J. Am. Chem. Soc. 2012, 134, 2004;(f) Pan, M.; Sun, H.; Lim, J. W.; Bakaul, S. R.; Zeng, Y.; Xing, S.; Wu, T.; Yan, Q.; Chen, H. Chem. Commun. 2012, 48, 1440;(g) Xing, S.; Feng, Y.; Tay, Y. Y.; Chen, T.; Xu, J.; Pan, M.; He, J.; Hng, H. H.; Yan, Q.; Chen, H. J. Am. Chem. Soc. 2010, 132, 9537;(h) Feng, Y.; Wang, Y.; He, J.; Song, X.; Tay, Y. Y.; Hng, H. H.; Ling, X. Y.; Chen, H. J. Am. Chem. Soc. 2015, 137, 7624;(i) Sun, H.; He, J.; Wang, J.; Zhang, S.-Y.; Liu, C.; Sritharan, T.; Mhaisalkar, S.; Han, M.-Y.; Wang, D.; Chen, H. J. Am. Chem. Soc. 2013, 135, 9099;(j) He, J.; Wang, Y.; Fan, Z.; Lam, Z.; Zhang, H.; Liu, B.; Chen, H. Nanoscale 2015, 7, 8115.

(5)          (a) Chen, T.; Yang, M.; Wang, X.; Tan, L. H.; Chen, H. J. Am. Chem. Soc. 2008, 130, 11858;(b) Xing, S.; Tan, L. H.; Chen, T.; Yang, Y.; Chen, H. Chem. Commun. 2009, 1653;(c) Sun, H.; Shen, X.; Yao, L.; Xing, S.; Wang, H.; Feng, Y.; Chen, H. J. Am. Chem. Soc. 2012, 134, 11243;(d) Zhu, L.; Wang, H.; Shen, X.; Chen, L.; Wang, Y.; Chen, H. Small 2012, 8, 1857.

(6)          Sindoro, M.; Feng, Y.; Xing, S.; Li, H.; Xu, J.; Hu, H.; Liu, C.; Wang, Y.; Zhang, H.; Shen, Z.; Chen, H. Angew. Chem. Int. Ed. 2011, 50, 9898.

(7)         Wang, H.; Xu, J.; Wang, J.; Chen, T.; Wang, Y.; Tan, Y. W.; Su, H.; Chan, K. L.; Chen, H. Angew. Chem. Int. Ed. 2010, 49, 8426.

(8)          (a) Xu, J.; Wang, H.; Liu, C.; Yang, Y.; Chen, T.; Wang, Y.; Wang, F.; Liu, X.; Xing, B.; Chen, H. J. Am. Chem. Soc. 2010, 132, 11920;(b) Chen, L.; Wang, H.; Xu, J.; Shen, X.; Yao, L.; Zhu, L.; Zeng, Z.; Zhang, H.; Chen, H. J. Am. Chem. Soc. 2011, 133, 9654;(c) Chen, L.; Yu, S.; Wang, H.; Xu, J.; Liu, C.; Chong, W. H.; Chen, H. J. Am. Chem. Soc. 2013, 135, 835;(d) Xu, J.; Wang, Y.; Qi, X.; Liu, C.; He, J.; Zhang, H.; Chen, H. Angew. Chem. Int. Ed. 2013, 52, 6019.

(9)          Zhu, L.; Shen, X.; Zeng, Z.; Wang, H.; Zhang, H.; Chen, H. ACS Nano 2012, 6, 6033.

(10)        Yong Wang, J. H., Xiaoke Mu, Di Wang, Bowei Zhang, Youde Shen, Ming Lin, Christian KĘ╣bel, Yizhong Huang, and Hongyu Chen under preparation 2016.

(11)       Wang, Y.; Wang, Q.; Sun, H.; Zhang, W.; Chen, G.; Wang, Y.; Shen, X.; Han, Y.; Lu, X.; Chen, H. J. Am. Chem. Soc. 2011, 133, 20060.

(12)        Zhu, Y.; He, J.; Shang, C.; Miao, X.; Huang, J.; Liu, Z.; Chen, H.; Han, Y. J. Am. Chem. Soc. 2014, 136, 12746.

(13)        Yao, L.; Liu, C.; Chong, W. H.; Wang, H.; Chen, L.; Chen, H. Small 2015, 11, 232.

(14)        Sun, H.; He, J.; Xing, S.; Zhu, L.; Wong, Y. J.; Wang, Y.; Zhai, H.; Chen, H. Chemical Science 2011, 2, 2109.

(15)        Song, X.; Ding, T.; Yao, L.; Lin, M.; Siew Tan, R. L.; Liu, C.; Sokol, K.; Yu, L.; Lou, X. W.; Chen, H. Small 2015, 11, 4351.

(16)        Yuhua Feng, Y. W., Xiaohui Song, Shuangxi Xing and Hongyu Chen Chemical Science 2016, submitted.

(17)        (a) He, J.; Wang, Y.; Feng, Y.; Qi, X.; Zeng, Z.; Liu, Q.; Teo, W. S.; Gan, C. L.; Zhang, H.; Chen, H. ACS Nano 2013, 7, 2733;(b) He, J.; Ji, W.; Yao, L.; Wang, Y.; Khezri, B.; Webster, R. D.; Chen, H. Adv. Mater. 2014, 26, 4151.

(18)        (a) Liu, C.; Yao, L.; Wang, H.; Phua, Z. R.; Song, X.; Chen, H. Small 2014, 10, 1332; (b) Liu, C.; Chen, G.; Sun, H.; Xu, J.; Feng, Y.; Zhang, Z.; Wu, T.; Chen, H. Small 2011, 7, 2721.

(19)        Zhenhui Lam, C. L., Jiazang Chen, Hsin-Yi Wang, Huabin Tao, Weichang Xu, Liping Zhang, Bin Liu, Hongyu Chen under preparation 2016.

(20)        Wang, Y.; Xu, J.; Wang, Y.; Chen, H. Chem. Soc. Rev. 2013, 42, 2930.

(21)        Wang, Y.; He, J.; Liu, C.; Chong, W. H.; Chen, H. Angew. Chem. Int. Ed. 2015, 54, 2022.