Concurrence of negative in-plane piezoelectricity and photocatalytic properties in 2D ScAgP₂S₆ monolayers

Abstract

Apart from poly(vinylidene fluoride) and its copolymers, 2D multilayered CuInP 2 S 6 (CIPS) is experimentally known to show negative out-of-plane piezoelectric coefficient (d 33 ) [2019 Sci. Adv. 5 1-10], which is attributable to weak interlayer van der Waals binding. However, CIPS monolayers exhibit positive in-plane piezoelectricity (d 11 ) [Phys. Status Solidi RRL, 14 2000321], as per DFT calculations. We ascribe it to strong in-plane interatomic bonding. Weakening of the in-plane interatomic bonding may even induce a negative in-plane piezoelectric coefficient, which is exceptionally rare, as observed in this work based on DFT calculations. Much smaller in-plane elastic constants found in ScAgP 2 S 6 monolayers relative to that in CIPS monolayers indicate much weaker in-plane interatomic bonding, which induces negative in-plane piezoelectric coefficient, d 22 = −2.81 pm V −1 , in ScAgP 2 S 6 monolayers. In addition to its all-around stability, ScAgP 2 S 6 monolayer is a semiconducting showing a bandgap of 2.04 eV, thereby making it suitable for electronics and solar energy harvesting. Upon the application of 4% biaxial strain on this flexible and stretchable 2D material, d 22 has been found to be drastically increased to −591 pm V −1 . The factors causing such an enhancement in d 22 have been ascertained through the maximum rise in the Born effective charges and maximum drop in the elastic constants. With the further drop in elastic constants relative to ScAgP 2 S 6 monolayer, d 22 reaches −3.45 pm V −1 and −13.48 pm V −1 in ScAgP 2 Se 6 and ScAgP 2 Te 6 monolayers respectively. 2D ScAgP 2 S 6 monolayer is found to be a promising photocatalyst as well, since its band edges straddle the water redox potential over the entire range in pH (0-14), in addition to having a high absorption coefficient (≈10 5 cm −1 ) in the visible region. It is understood that 2D ScAgP 2 S 6 will substantially expand the scope for its applications in piezoelectricity and visible light harvesting at the nanoscale.

Photocatalytic and Piezocatalytic Properties of h-NbP and h-NbN Monolayers for Green Hydrogen Production: Insight from Density Functional Theory Calculations

“…Understanding the mechanical behavior of materials under strain and stress conditions requires a more in-depth understanding of their elastic properties. The elastic coefficients of a material can be calculated using Hooke's law 39,60…”

“…In photocatalytic splitting, the CBM of the photocatalyst should be above the reduction potential (E H + /Hd 2 ), and the VBM should be below the (E Od 2 /Hd 2 O ) oxidation potential so that the corresponding HER and OER events can be initiated. 39 The band edges relative to redox potentials are also shown, for varying pH from 0 to 14, in Figure 6(a, b). The presence of a vertical intrinsic electric field creates two distinct vacuum levels in a photocatalyst (see Figure 7), as opposed to the traditional model in which the top and bottom surfaces of the monolayer are associated with a single vacuum level.…”

“…Understanding the mechanical behavior of materials under strain and stress conditions requires a more in-depth understanding of their elastic properties. The elastic coefficients of a material can be calculated using Hooke’s law , σ ij = C ijkl ε kl where σ ij , ε kl , and C ijkl are the stress, strain, and elastic tensor, respectively . As these coefficients are calculated by DFT algorithms, periodic boundary conditions in 3D are assumed.…”

“…In this aspect, 2D materials with broken out-of-plane symmetry have an intrinsic electric field that has been shown to significantly improve the separation of photoinduced e – - h + pairs, leading to a lower carrier recombination rate. − In addition, the 1.23 eV bandgap criterion for overall photocatalytic water splitting due to redox potential difference can be overcome by this intrinsic electric field. , To this extent, the photocatalytic potential of 2D piezoelectric materials is revealed. These findings have prompted theoretical research into the use of various 2D piezoelectric materials for photocatalytic water splitting, including MX and M 2 X 3 (M = In, Ga, Al; X = S, Se, Te), Janus MoSSe, X 2 PAs (X = Si, Ge, and Sn), AgBiP 2 Se 6 , ScAgP 2 S 6 , etc. Although these are promising, low optical absorption efficiency remains a bottleneck, so the search for novel 2D photocatalysts that can drive the photocatalytic process by absorbing the visible range of the solar spectrum continues.…”

Two-dimensional semiconductor materials with high stability and electron mobility in group-11 chalcogenide compounds: MNX (M = Cu, Ag, Au; N = Cu, Ag, Au; X = S, Se, Te; M ≠ N)

“…According to the Born-Huang criteria, 6,[71][72][73][74] . These values were comparable to phosphorene, 75 Mg 2 C 76 and single-layer SnSe, 77 and were smaller than those of typical two-dimensional materials such as graphene (∼340 N m −1 ), 78 MoS 2 (∼140 N m −1 ) 79 and BN (∼267 N m −1 ), 80 indicating that the MNX monolayers were relatively soft.…”