Control of valence and conduction band energies in layered transition metal phosphates via surface functionalization

Abstract

Layered transition metal phosphates and phosphites (TMPs) are a class of materials composed of layers of 2D sheets bound together via van der Waals interactions and/or hydrogen bonds. Explored primarily for use in proton transfer, their unique chemical tunability also makes TMPs of interest for forming large-scale hybrid materials. Further, unlike many layered materials, TMPs can readily be solution exfoliated to form single 2D sheets or bilayers, making them exciting candidates for a variety of applications. However, the electronic properties of TMPs have largely been unstudied to date. In this work, we use first-principles computations to investigate the atomic and electronic structure of TMPs with a variety of stoichiometries. We demonstrate that there exists a strong linear relationship between the band gap and the ionic radius of the transition metal cation in these materials, and show that this relationship, which opens opportunities for engineering new compositions with a wide range of band gaps, arises from constraints imposed by the phosphorus-oxygen bond geometry. In addition, we find that the energies of the valence and conduction band edges can be systematically tuned over a range of ∼3 eV via modification of the functional group extending from the phosphorus. Based on the Hammett constant of this functional group, we identify a simple, predictive relationship for the ionization potential and electron affinity of layered TMPs. Our results thus provide guidelines for systematic design of TMP-derived functional materials, which may enable new approaches for optimizing charge transfer in electronics, photovoltaics, electrocatalysts, and other applications.

Sn(II)-Containing Phosphates as Optoelectronic Materials

“…Furthermore, metal vacancies would be expected to p-type dopants in these compounds, while the very high chemical stability of the phosphate polyanions could prevent the formation of compensating defects on the polyanion lattice. However, we note that there has to date been very limited investigation of phosphates as electronic materials [22].…”

“…It is also of interest to explore the range of properties that can be realized, as applications will no doubt benefit from chemical tuning [22].…”

First-principles design of nanostructured hybrid photovoltaics based on layered transition metal phosphates

“…Building off of previously demonstrated surface functionalization substitution 9 , we replace these -OH groups with a range of organic groups. Figure 4 shows the effects of –R substitution on the valence and conduction band edges of Zr(RPO 3 ) 2 .…”

“…Crystalline, layered TMP superlattices of this type have been previously synthesized using solution-based processing methods 8 , but to date have not been widely investigated in the context of electronic or optical applications. Our recent work suggests high tunability of TMP band energies through simple surface functionalization 9 . Using these relationships, we optimize a new hybrid TMP-organic superlattice based on zirconium phosphate, α-Zr(HPO 4 ) 2 , one of the most well-studied layered TMPs, and we show that this new material is predicted to exhibit very low exciton recombination, high electron and hole mobilities, and a large open-circuit voltage of 1.7 eV.…”

“…To optimize the energy alignment of the organic and inorganic layers so that there are no barriers for electron (hole) transfer from the organic region into the inorganic electron (hole) conducting layer, we use relationships identified in our previous work to independently tune the band gap and the energies of the band edges in the alternating TMP layers 9 . In particular, large variations in the band gap can be obtained via in-plane cation substitution, while independent and precise control of the energies of the band edges is achieved through substitution of the out-of-plane –R functional groups.…”