Design of Photoelectrochemical Materials Via Non-Native Nanostructures and Their Click Assembly into Photoreactors
The development of highly efficient corrosion-resistant photoelectrode materials and their processing technologies are critical in the context of efficient hydrogen production from solar energy through water photolysis. The central physicochemical parameters in meeting this are band-gap, band-edge alignments, band bending, surface electrocatalytic activity, stability in solution and most importantly, abundant availability at low cost. To date, no suitable material (nanostructure/film) has been demonstrated to meet all these requirements. While modulation of properties via variations in chemical composition has been explored extensively in the context of hydrogen generation using photo-electrochemical (PEC) systems, comprehensive modulation of properties via variation of nanostructure has not been pursued. We propose that modulation of desirable material properties can be achieved by stabilization of ‘non-native’ structures which are nanostructures having discrete translational symmetry in the sub-surface regions different from that present in the sub-surface regions of the thermodynamically most stable form of large crystals (1,2,3,4). These non-native structures have different physico-chemical properties (e.g. band-gap, band-edges) and catalytic activity in comparison to bulk ‘native’ structure due to different chemical coordination. Further, charge carrier separation and transport related problems can be addressed through formation of native@non-native isomaterial heterostructure. The problem of stabilization of lattice mismatched heterostructure is circumvented through ‘click’ chemistry. To illustrate the our methodology we have taken TiO2 as a model material and is also being extended to other transition metal oxides.
Materials and Methods
The ‘native’ rutile phase and ‘non-native’ anatase phase has been synthesized using sol-gel method. The heterostructure is made through ‘click’ chemistry of alkyne-azide linkage. Finally the hetero-structures are attached to the substrate using similar click reaction.
Results and Discussion
Anatase and rutile are two polymorphs of TiO2. Anatase is found to be stable in lower dimension nanostructure whie rutile is stable as a higher dimension nanostructure. According to our definition of ‘native’ and ‘non-native’ structure, we identify rutile phase as native structure while anatase as non-native structure.
The order of photoelectrocatalytic(PEC) activity is found to be in the order of rutile@anatase>anatase>rutile. The advantage of iso-material heterostructures is that they have lesser interfacial defects due to better lattice match at the interface between phases. This helps in facilitating electron-hole separation. It can be noted that the interfacial charge carrier transport in TiO2 polymorphic heterojunctions can be explained in two main pathways: 1) due to lower CB minimum, rutile phase acts as a passive electron sink and captures electrons from anatase phase and 2) transport of electrons from the rutile to anatase occurs via lower energy electron trapping sites of anatase phase.
The as synthesized heterostructures have high stability due to covalently bound interfaces which enhances long-term performance of the PEC water splitting. We have demonstrated this in a wide variety of metal-semiconductor heterostructures relevant to photoelectrochemical hydrogen production (5). We also point towards other transition metal oxide heterostuctures relevant to photoelectrical hydrogen generation that are being designed using the heuristics developed using non-native nanostructures.
(1) Non-native nanostructures are catalytically more active than native nanostructures in case of TiO2, (2) Isomaterial stable heterostructures can be formed using click chemistry which are found to have better activity than any individual polymorphs. We attribute this to better electron-hole separation.
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