The corresponding paper in Nature Communication is here: http://go.nature.com/2pqTq9t
The light-induced photoelectrochemical (PEC) water splitting has been considered as the most promising strategy to obtain hydrogen fuels. In this process, a suitable semiconductor is needed, however thus far this field is dominated by inorganic materials. Organic semiconductors based on earth-abundant elements, are more promising due to the tunable electronic structures, low cost, and environmental sustainability. Recently, landmark progress has been made in this field by using graphitic carbon nitride (g-C3N4) and conjugated porous polymers (J. Am. Chem. Soc. 2015, 137, 3265) for the direct solar water reduction, suggesting that conjugated carbon-rich materials can be a new family of synthetic semiconductors for solar water splitting.
Acetylenic carbon-rich materials (e.g., graphyne, graphdiyne and related analogues) are predicted to exhibit unique electronic, optical and mechanical properties, because of the extended π-conjugated structure composed by alternative double and triple bonded carbon atoms. The great potential of acetylenic carbon-rich materials as photocatalysts was recently illustrated by the visible-light-driven degradation of water pollutants (i.e., phenol and methyl orange) using poly(diphenylbutadiyne) nanofibers synthesized through an in-solution templating approach (Nat. Mater. 2015, 14, 505). More recently, it was shown that graphdiyne nanosheet synthesized on Cu foil surface can be used as hole-transfer material in CdSe quantum dots based PEC devices for water splitting (J. Am. Chem. Soc. 2016, 138, 3954).
Our initial interest was to explore the synthesis 1,3,5-graphdiyne on external substrates using metallic-Cu catalysed Glaser coupling from commercial available 1,3,5-triethynylbenzene (TEB). The impetus for this idea is derived from our previous experiments showing that metallic copper is able to generate dissolved CuI/II species in polar liquids to catalyse atom transfer radical polymerization (ATRP) on a facing substrate functionalized by monolayer initiator (Polym. Chem., 2015, 6, 2726). In the current paper, we show that the Glaser polycondensation of TEB can also be initiated on external substrates facing to metallic copper in a distance of ca. 0.1 mm. The obtained poly(1,3,5-triethynylbenzene) (PTEB) features interconnected nanofibers with diameter ranging from a few nanometers to tens of nanometers dependant on substrates, including conductive, e.g., copper, graphite, nickel and titanium, and non-conductive, e.g., Kapton®, glass, fused silicon and SiO2. It is worth to note that these are the substrates we have tried, but many other substrates are also possible to be used to grow PTEB nanofibers.
Furthermore, we demonstrate that the acetylenic carbon-rich (i.e., PTEB) nanofibers grown directly on conductive substrates can be used as excellent photocathodes for PEC, affording a maximum photocurrent of ca. 10 µA cm-2 (at 0.3 - 0 V vs. RHE). The performance is superior to those of the state-of-the-art metal-free photocathode materials (e.g., g-C3N4, normally in the range of 0.1-1 µA cm-2), owning to the unique textures of PTEB as photocathode, i.e., the intimate contact with the electrode, tunable film thickness, and interconnected nanofiber morphology. The most notable feature of the acetylenic carbon-rich framework is that the C-C triple bond can be chemically tailored (e.g., thiol-yne reaction, cycloaddition with cyanocontaining acceptor molecules, and metal coordination). Thus, the diversity of alkynes will allow the rational design of a much broader set of acetylenic carbon-rich materials with controlled optical and electronic properties. These strategies will render the further improvement of the PEC performance of PTEB, and thus open up a new pathway in seeking for novel metal-free photocathode materials for PEC water splitting.