Changbai Long

Nitrogen self-doped graphitic carbon nitride as efficient visible light photocatalyst for hydrogen evolution

Nitrogen self-doped graphitic carbon nitride (C3N4+x) was successfully synthesized by the co-thermal condensation of the precursor with a nitrogen-rich additive. The resultant self-doped semiconductor was characterized by X-ray photoelectron spectroscopy (XPS), which indicated that the nitrogen atom substituted the sp2 carbon atom. The photocatalytic hydrogen evolution was systematically evaluated under visible light irradiation (λ > 400 nm). The average hydrogen evolution rate for C3N4+x was 1.8 times higher than that of pristine graphitic carbon nitride, and the superiority lay in greatly improved optical, emission and electronic properties of the nitrogen modified carbon nitride. This study filled the research gap of self-doping for 2D polymeric carbon nitride and will stimulate intensive investigations in the further improvement of photocatalytic hydrogen evolution.

Structure, Phase Transition Behaviors and Electrical Properties of Nd Substituted Aurivillius Polycrystallines Na0.5NdxBi2.5–xNb2O9 (x = 0.1, 0.2, 0.3, and 0.5)

New high temperature Aurivillius piezoelectrics Na0.5Nd(x)Bi(2.5-x)Nb2O9 (NDBNx, x = 0.1, 0.2, 0.3, and 0.5) with Nd substitution for Bi at the A site were synthesized using a solid state reaction process. Crystal structures of NDBN0.2 and NDBN0.5 were refined with the Rietveld method with powder X-ray diffraction, and they crystallized in the orthorhombic space group A21am [a = 5.48558(8) Å, b = 5.46326(9) Å, c = 24.8940(4) Å, and Z = 4 for NDBN0.2 and a = 5.46872(5) Å, b = 5.46730(5) Å, c = 24.80723(25) Å, and Z = 4 for NDBN0.5], at room temperature. The refinement results and Raman spectroscopy of NDBNx verified that Nd occupied both the A site in the perovskite layers and the cation site in the (Bi2O2)(2+) layers. The Nd substitution induced an enhancement in cation disordering between the A site and the (Bi2O2)(2+) layer and an increase in the degree of the relaxation behavior for NDBNx. The ferroelectric to paraelectric phase transition temperature (Tc) of NDBNx ranged from 735 to 764 °C. Furthermore, the isovalent substitution of Nd for Bi had a great influence on microstructure (grain size and shape), defect concentration (mainly oxygen vacancies), preferred grain orientation (texture), and distortion of the octahedron. The coaction between these effects determined the structure characteristics, phase transition behaviors, and electrical properties of NDBNx.

Phase transformation (cubic to rhombohedral): the effect on the NO2 sensing performance of Zn-doped flower-like In2O3 structures

Cubic In2O3 (bcc-In2O3) was transformed into a mixture of bcc-In2O3 and rhombohedral In2O3 (rh-In2O3) by Zn doping. The Zn-doped flower-like In2O3 structures consisted of many thin sheets with a length of 0.4–1 μm, and cubes with a length of 200 nm, while the size of the microflowers was 1–3.5 μm. The Zn doping concentration significantly affected the phase transformation and the overall morphology of In2O3. Furthermore, the analysis of N2 adsorption–desorption measurements showed that the Zn-doped flower-like In2O3 structures (sample S5) adsorbed the largest amount of N2 and had the biggest surface area (46.41 m2 g−1), which contributed to an improvement in gas sensing performance. Finally, sensors based on the mixture of bcc- and rh-In2O3 structures exhibited a much higher response to NO2 than the pure bcc-In2O3 (sample S1), and the Zn-doped flower-like In2O3 structures (sample S5) exhibited the highest response of 27.4 ± 2.5 for 5 ppm NO2. Thus, the gas sensing performance of In2O3 was enhanced significantly by the phase transformation.

Pt-decorated zinc oxide nanorod arrays with graphitic carbon nitride nanosheets for highly efficient dual-functional gas sensing.

In this work, well-aligned ZnO nanorods were grown on the substrate of exfoliated g-C3N4 nanosheets via a microwave-assisted hydrothermal synthesis, and then Pt/ZnO/g-C3N4 nanostructures were obtained after the deposition of Pt nanoparticles. The growth of vertically ordered ZnO nanorods was occurred on g-C3N4 nanosheets through the bonding interaction between Zn and N atoms, which was confirmed by XPS, FT-IR data and molecular orbital theory. The Pt/ZnO/g-C3N4 nanostructures sensor exhibited the remarkable sensitivity, selectivity, and fast response/recovery time for air pollutants of ethanol and NO2. The application of Pt/ZnO/g-C3N4 nanostructures could be used as a dual-functional gas sensor through the controlled working temperature. Besides, the Pt/ZnO/g-C3N4 nanostructures sensor could be applied to the repeating detection of ethanol and NO2 in the natural environment. The synergistic effect and improved the separation of electron-hole pairs in Pt/ZnO/g-C3N4 nanostructures had been verified for the gas sensing mechanism. Additionally, Pt/ZnO/g-C3N4 nanostructures revealed the excellent charge carriers transport properties in electrochemical impedance spectroscopy (EIS), such as the longer electron lifetime (τn), higher electron diffusion coefficient (Dn) and bigger effective diffusion length (Ln), which also played an important role for Pt/ZnO/g-C3N4 nanostructures with striking gas sensing activities.

Effect of lanthanum and tungsten co-substitution on the structure and properties of new Aurivillius oxides Na0.5La0.5Bi2Nb2-xWxO9

New Aurivillius ferroelectrics Na0.5La0.5Bi2Nb2−xWxO9 (NLBN-W, x = 0, 0.01, 0.02, 0.03, and 0.05) by lanthanum (La3+) and tungsten (W6+) co-substitution were prepared by using a solid-state reaction process. The crystal structure of Na0.5La0.5Bi2Nb1.98W0.02O9 (NLBN-W-0.02) was refined by the Rietveld method with powder X-ray diffraction at room temperature and it was confirmed to be a two-layer Aurivillius oxide with an orthorhombic space group A21am (a = 5.48754(8) A, b = 5.47967(9) A and c = 24.8523(4) A). The ferroelectric to paraelectric phase transition temperature (Tc) of NLBN-W-0.02 is around 501 °C, with prominent relaxation behavior. The NLBN-W-0.02 ceramics have a high piezoelectric coefficient (d33 = 31 pC N−1) and large remnant polarization (Pr = 15.1 μC cm−2). The decrease in Pr for the NLBN-W-0.02 ceramics after 107 switching cycles at 40 and 120 °C is only 2 and 9%, respectively.

Fast economical synthesis of Fe-doped ZnO hierarchical nanostructures and their high gas-sensing performance

The Fe-doped ZnO with excellent porous features was successfully fabricated via a fast economical solution combustion synthesis. The morphology and the porous nature were characterized by SEM, TEM and BET – N2 adsorption–desorption analysis. The results of XRD Rietveld refinements and Raman spectra demonstrate that the dopant was successfully incorporated into the nanostructure ZnO lattice. The gas-sensing tests show that the sensors based on Zn0.97Fe0.03O have high sensitivity, quick response, excellent selectivity and long-time stability for detecting ethanol vapor. AC impedance spectroscopy and DC electrical conductivity measurements were used to investigate the gas-sensing mechanism, and the results demonstrate that the O− (300–400 °C) on the surface is the most reactive species with ethanol gas.

A candidate for lead-free ultrahigh-temperature piezoelectrics: the excellent electro-mechanical properties of Aurivillius oxides, Ca1−5xLi2xNd2x□xBi2Nb2−2xScxWxO9−1.5x

Modified ultrahigh-temperature piezoelectric ceramics, Ca1−5xLi2xNd2x□xBi2Nb2−2xScxWxO9−1.5x (CBNO-LiNd-ScW-x, x = 0, 0.01, 0.02, 0.03, 0.04; □ represented A-site vacancies), were synthesized by a traditional solid-state reaction process. XRD Rietveld refinement, TEM, HRTEM and Raman spectroscopy were used to characterize the crystal structures and domain structures of the prepared ceramics. The lithium/neodymium (Li/Nd) and scandium/tungsten (Sc/W) co-substitution as well as the existence of small amounts of A-site vacancies significantly enhanced the ferroelectric and electro-mechanical properties of the CBNO-LiNd-ScW-x ceramics. The Curie temperature (Tc) decreased from 941 °C to 832 °C as the x values increased in CBNO-LiNd-ScW-x, because of an enhancement in the cation disordering between the A site and the bismuth oxide layer. The CBNO-LiNd-ScW-0.01 ceramic with an ultrahigh Tc of 908 °C had a high piezoelectric activity (d33) of 20.6 pC N−1, a large field-induced strain (S33) of 0.27 × 10−3 and a high remnant polarization (Pr) of 6.1 μC cm−2, due to the perfect growth in the ferroelectric domains and the contribution from the reversal of the defect dipoles (VCa′′–VO•• and ScNb′′–VO••); interestingly, it still has a high d33 of 19.5 pC N−1 after up to 800 °C thermal annealing. In addition, the CBNO-LiNd-ScW-0.01 ceramic had a relatively high resistivity of >106 Ω cm at 600 °C and >105 Ω cm at 700 °C.

New layer-structured ferroelectric polycrystalline materials, Na0.5NdxBi4.5−xTi4O15: crystal structures, electrical properties and conduction behaviors

New layer-structured ferroelectric polycrystalline materials, Na0.5NdxBi4.5−xTi4O15 (x = 0–2.5; NNBTO-x), were prepared, and the tailoring effects of neodymium (Nd) were investigated thoroughly. A relaxation in orthorhombic distortion was caused by the Nb substitution, and the compositions with high x value modification tended to crystallize in tetragonal space group. Rietveld structure refinements revealed that NNBTO-0.5 and NNBTO-1.5 crystallized in the orthorhombic space group A21am at room temperature, and Nd/Bi combinations occupied the cation sites of the (Bi2O2)2+ layers. The Nd substitution had strong impacts on the electrical properties of NNBTO-x. Ferroelectric to paraelectric phase transition temperature (Tc) was lowered from about 670 °C to below 200 °C. Ferroelectric (remnant) polarization (Pr), piezoelectric activity (d33) and field-induced strain (S33) first increased, and then decreased drastically owing to a significant degradation in spontaneous polarization (Ps). In addition, the high temperature resistances of the NNBTO-x specimens were discussed and their conduction mechanisms were explored. The NNBTO-0.5 with a Tc of 670 °C had a Pr of 10.8 μC cm−1, a d33 of 24.5 pC N−1 and a S33 of about 0.04%, due to the decrease in oxygen vacancy concentration and the local lattice distortion.

A ferroelectric polarization contribution from defect dipoles in acceptor Aurivillius oxide, (Na,Bi)0.47(Li,Ce)0.03Bi2Ta1.97Sc0.03O8.97

Double-loop-like characteristic was detected in acceptor Aurivillius ceramic, (Na,Bi)0.47(Li,Ce)0.03Bi2Ta1.97Sc0.03O8.97 (NBTO-LiCeSc-0.03) due to the restoring force provided by defect polarization (PD). The PD was associated with non-centric distributing defect dipoles [(ScTa″-VO••)×], which was verified by impedance spectroscopy measures. Interestingly, the PD could be reoriented with high external electric field at very low frequency, which contributed to the significant enhancement of the polarization of NBTO-LiCeSc-0.03. Air or O2 annealing led to the degradation of the polarization owing to high leakage current and the strong “pinning” effects of space charge and charged defect complexes [e.g., (ScTa″-OO•)′].

Differences in nature of electrical conductions among Bi 4 Ti 3 O 12 -based ferroelectric polycrystalline ceramics

Bismuth titanate Bi4Ti3O12 (BiT), was one of the most promising lead-free high-temperature piezoelectric materials, due to high Curie temperature (675 °C) and large spontaneous polarization (50 µC/cm2); however, extensive studies had revealed that high leakage conductivity interferes with the poling process, hindering its practical applications. In this paper, an electrically insulating property was achieved by a low level Nb donor substitution to suppress a high level of holes associated with high oxygen vacancy concentration. Bi4Ti2.97Nb0.03O12 ceramic showed significant enhancements of electrical resistivity by more than three order of magnitude and activity energy with value >1.2 eV, which are significant for piezoelectric applications of BiT-based materials. However, pure and A2O3-excess (A = Bi, La and Nd; 3 at %) BiT ceramics, were mixed hole and oxygen ion conductors. Schottky barriers were both formed at grain boundary region and the sample-electrode interface, because of the existence of semiconducting bulk. Interestingly, the electron conduction could be suppressed in N2, as a consequence, they became oxide ion conductors with conductivity of about 4 × 10−4 S cm−1 at 600 °C.