Crystal structures and electrical properties of Sr/Fe‐modified KNbO₃ ferroelectric semiconductors with narrow bandgap

Abstract

In the process of exploring ferroelectric semiconductors, a new system of (1−x) KNbO3–xSrFeO3−δ (x = 0.00‐0.20) was successfully synthesized via solid‐state reaction. The crystal structures, ferroelectric, dielectric, optical, and electrical properties were systematically characterized. The orthorhombic phase with Amm2 space group is detected in all the ceramics. In addition, the orthorhombic and tetragonal phases coexist in 0.80KNbO3‐0.20SrFeO3‐δ ceramic. The decrease in oxygen octahedron distortion induces a weak ferroelectric polarization. The existence of long‐range ferroelectric polarization order in all the ceramics is verified and the bandgap of the ceramics can be tuned to ~2.18 eV. The improved short‐circuit photocurrent density (Jsc) and open‐circuit voltage (Voc) of the poled 0.95KNbO3‐0.05SrFeO3−δ ceramic at 30 kV/cm are ~6.90 nA/cm2 and 0.04 V, respectively. The activation energies for electrical conductivity of the grains and grain boundaries from 0.90KN–0.10SF ceramic are 0.67 and 0.77 eV, respectively, which indicate the doubly ionized oxygen vacancies. This work provides a new way to tune the optical bandgap/ferroelectric properties of KNbO3‐based ceramics for potential application in ferroelectric photovoltaic and energy fields.

Enhanced Visible Photocatalytic Hydrogen Evolution of KN-Based Semiconducting Ferroelectrics via Band-Gap Engineering and High-Field Poling

“…The valences of K, Nb, and O ions in pure KN are in accordance with a previous report (Figure S3a−c). 39 The XPS spectra of 0.99KN−0.01BNB indicate the presence of Ba, Ni, and Bi ions (Figure S3g−i). To analyze the valence state of the Ni element in materials, the narrow-scan XPS spectrum of Ni 2p can be deconvoluted into two pairs of peaks, as shown in Figure S3h.…”

Bandgap narrowing and polarization enhancement in (K,Na,Li)(Nb,Sb,Ta)O 3+ x% Fe 2O 3 lead-free ceramics for photovoltaic applications

“…In Fig. 6, we further compare the E g and P r of the KNLNST+1.3% Fe 2 O 3 ceramics fabricated in our work with those of the ferroelectric materials reported in the literature [6,9,14,[16][17][18]34,35,[37][38][39][40][41][42][43][44][45][46][47][48][49][50][51][52][53][54][55][56]. The detailed data for these materials are summarized in Table 1.…”

“…Hence, the development of ferroelectric materials with both large P r and narrow E g is an urgent priority. Consequently, several new ferroelectric materials including Bi 0.98 Ca 0.02 Fe 0.95 Mn 0.05 O 3 (E g = 2.41 eV and P r = 92.5 μC/cm 2 ) [12], 0.8KNbO 3 -0.2SrFeO 3−δ (E g ≈ 2.18 eV and P r ≈ 2.5 μC/cm 2 ) [17], and PbTiO 3 -BiFeO 3 -Bi(Ni 1/2 Ti 1/2 )O 3 (PT-BF-BNT; E g = 2.25 eV and P r =32 μC/cm 2 ) [18] with narrow E g and large P r have recently been developed. However, the bandgaps of these reported ferroelectric materials are still above 2.0 eV, which is not beneficial to the absorption of visible light.…”

Visible-light photocatalytic hydrogen production in a narrow-bandgap semiconducting La/Ni-modified KNbO3 ferroelectric and further enhancement via high-field poling

“…Several recent studies have suggested that ferroelectricity and semiconducting behaviour can be simultaneously achieved in KN ceramics via Ni, Co and Fe modification. 12–14 The results of these studies provide a reference for the preparation of semiconducting KNbO 3 -based ferroelectrics. However, for most narrow-bandgap semiconducting ferroelectrics, further boosting the PHE activity through external electric fields is difficult because of the large leakage current and weakened ferroelectricity.…”