Xuedan Song

Rational design and fabrication of sulfur-doped porous graphene with enhanced performance as a counter electrode in dye-sensitized solar cells

Exploring cost-effective counter electrodes (CEs) with high electrocatalytic activity and excellent electrochemical stability is one of concerned issues for practicable applications of dye-sensitized solar cells (DSSCs). Graphene (G), featuring unique and intriguing physicochemical properties, has emerged as one of the most promising candidates. Nevertheless, the relationships between the electrochemical activity and the intrinsic structure of G need to be further understood. Herein, we report a facile yet effective strategy for engineering sulfur-doped porous graphene (SPG) using sulfur powder as the sulfur source and pore-forming agent. The as-made SPG as the CE for DSSCs achieves a high power conversion efficiency of 8.67%, which is superior to Pt (7.88%), and robust electrochemical stability. The influence of annealing temperature on SPG is analyzed, and SPG prepared at 900 °C shows the best photovoltaic and electrochemical performance. Both experimental and theoretical efforts first elucidate that highly exposed rich edge sites and interconnected porous channels, as well as low ionization energy derived from sulfur species within the G matrix play vital roles in enhanced reaction kinetics and triiodide reduction activity. The present work will inspire the construction of porous graphene with surface-enriched active sites and interconnected networks for advanced energy applications.

Interaction between Formaldehyde and Luminescent MOF [Zn(NH2bdc)(bix)]n in the Electronic Excited State

The hydrogen bond between formaldehyde and the luminescent metal-organic framework (MOF) [Zn(NH2bdc)(bix)]n was investigated using density functional theory and time-dependent density functional theory. The frontier molecular orbitals and electronic configuration demonstrate that the origin of the luminescence can be attributed to ligand-to-ligand charge transfer. Examination of the hydrogen bond behavior in the electronic excited state, with comparison of the electronic transition energies, bond distances, binding energy, (1)H-NMR chemical shifts, and infrared spectra with those of the ground state, demonstrate that the hydrogen bond is stronger when in the electronic excited state. Strengthening of the hydrogen bond weakens the radioactive transition of [Zn(NH2bdc)(bix)]n, which thus leads to a luminescence decrease or quenching phenomenon, meaning that the luminescent MOF [Zn(NH2bdc)(bix)]n may be applied to the detection of formaldehyde.

Scrutinizing Defects and Defect Density of Selenium-Doped Graphene for High-Efficiency Triiodide Reduction in Dye-Sensitized Solar Cells

Understanding the impact of the defects/defect density of electrocatalysts on the activity in the triiodide (I3- ) reduction reaction of dye-sensitized solar cells (DSSCs) is indispensable for the design and construction of high-efficiency counter electrodes (CEs). Active-site-enriched selenium-doped graphene (SeG) was crafted by ball-milling followed by high-temperature annealing to yield abundant edge sites and fully activated basal planes. The density of defects within SeG can be tuned by adjusting the annealing temperature. The sample synthesized at an annealing temperature of 900 °C exhibited a superior response to the I3- reduction with a high conversion efficiency of 8.42 %, outperforming the Pt reference (7.88 %). Improved stability is also observed. DFT calculations showed the high catalytic activity of SeG over pure graphene is a result of the reduced ionization energy owing to incorporation of Se species, facilitating electron transfer at the electrode-electrolyte interface.

A sensor for formaldehyde detection: luminescent metal–organic framework [Zn2(H2L)(2,2′-bpy)2(H2O)]n

Density functional theory and time-dependent density functional theory methods have been used to investigate the hydrogen bonding between the Metal–organic framework [Zn2(H2L)(2,2′-bpy)2(H2O)]n and formaldehyde in the electronically excited state. The calculated geometric configuration, 1H NMR chemical shift and IR spectra of the hydrogen-bonded complex demonstrated that the hydrogen bond was strengthened in the excited state S1. The strengthening of the hydrogen bond in the S1 state would lead to a luminescence decreasing phenomenon of [Zn2(H2L)(2,2′-bpy)2(H2O)]n, and the fluorescent rate constant of [Zn2(H2L)(2,2′-bpy)2(H2O)]n was decreased when encapsulating formaldehyde into it. Taken together, these results indicated that [Zn2(H2L)(2,2′-bpy)2(H2O)]n could be used for the detection of formaldehyde.

Elucidating triplet-sensitized photolysis mechanisms of sulfadiazine and metal ions effects by quantum chemical calculations

Sulfadiazine (SDZ) mainly proceeds triplet-sensitized photolysis with dissolved organic matter (DOM) in the aquatic environment. However, the mechanisms underlying the triplet-sensitized photolysis of SDZ with DOM have not been fully worked out. In this study, we investigated the mechanisms of triplet-sensitized photolysis of SDZ(0) (neutral form) and SDZ(-) (anionic form) with four DOM analogues, i.e., fluorenone (FL), thioxanthone (TX), 2-acetonaphthone (2-AN), and 4-benzoylbenzoic acid (CBBP), and three metal ions (i.e., Mg(2+), Ca(2+), and Zn(2+)) effects using quantum chemical calculations. Results indicated that the triplet-sensitized photolysis mechanism of SDZ(0) with FL, TX, and 2-AN was hydrogen transfer, and with CBBP was electron transfer along with proton transfer (for complex SDZ(0)-CBBP2) and hydrogen transfer (for complex SDZ(0)-CBBP1). The triplet-sensitized photolysis mechanisms of SDZ(-) with FL, TX, and CBBP was electron transfer along with proton transfer, and with 2-AN was hydrogen transfer. The triplet-sensitized photolysis product of both SDZ(0) and SDZ(-) was a sulfur dioxide extrusion product (4-(2-iminopyrimidine-1(2H)-yl)aniline), but the formation routs of the products for SDZ(0) and SDZ(-) were different. In addition, effects of the metal ions on the triplet-sensitized photolysis of SDZ(0) and SDZ(-) were different. The metal ions promoted the triplet-sensitized photolysis of SDZ(0), but inhibited the triplet-sensitized photolysis of SDZ(-).

Pseudohalogen-Based 2D Perovskite: A More Complex Thermal Degradation Mechanism Than 3D Perovskite

(MA)2Pb(SCN)2I2, a new pseudohalogen-based 2D perovskite material, was reported as a very stable and promising photo-absorber in PSCs previously. However, the later researchers found that MA2Pb(SCN)2I2 was not as stable as claimed. Thus, it is very critical to clarify the controversy and reveal the degradation mechanism of MA2Pb(SCN)2I2. On the other hand, a large number of studies have indicated that adding a small amount of SCN- improves surface topography and crystallinity. However, whether SCN- ions can be incorporated into a 3D perovskite film remains debatable. In this work, the thermal degradation pathway of (MA)2Pb(SCN)2I2 is revealed by thermal gravimetric and differential thermal analysis coupled with quadrupole mass spectrometry and density functional theory calculations. The decomposition of (MA)2Pb(SCN)2I2 has been proved experimentally to be more complex than that of MAPbI3, involving four stages and multi-reactions from room temperature to above 500 °C. By combining the experimental results and theoretical calculations, it is found that 2D (MA)2Pb(SCN)2I2 actually is unstable when serving as photo-absorber in PSCs. Moreover, the role of SCN- in improving the crystallinity of 3D perovskite has also been discussed in detail.

Co 3 O 4 nanoparticles assembled on polypyrrole/graphene oxide for electrochemical reduction of oxygen in alkaline media

ABSTRACT The development of highly efficient catalysts for cathodes remains an important objective of fuel cell research. Here, we report Co 3 O 4 nanoparticles assembled on a polypyrrole/graphene oxide electrocatalyst (Co 3 O 4 /Ppy/GO) as an efficient catalyst for the oxygen reduction reaction (ORR) in alkaline media. The catalyst was prepared via the hydrothermal reaction of Co 2+ ions with Ppy-modified GO. The GO, Ppy/GO, and Co 3 O 4 /Ppy/GO were characterized using scanning electron microscopy, Fourier-transform infrared spectroscopy, and X-ray photoelectron spectroscopy. The incorporation of Ppy into GO nanosheets resulted in the formation of a nitrogen-modified GO porous structure, which acted as an efficient electron-transport network for the ORR. With further anchoring of Co 3 O 4 on Ppy/GO, the as-prepared Co 3 O 4 /Ppy/GO exhibited excellent ORR activity and followed a four-electron route mechanism for the ORR in alkaline solution. An onset potential of −0.10 V vs. a saturated calomel electrode and a diffusion limiting current density of 2.30 mA/cm 2 were achieved for the Co 3 O 4 /Ppy/GO catalyst heated at 800 °C; these values are comparable to those for noble-metal-based Pt/C catalysts. Our work demonstrates that Co 3 O 4 /Ppy/GO is highly active for the ORR. Notably, the Ppy coupling effects between Co 3 O 4 and GO provide a new route for the preparation of efficient non-precious electrocatalysts with hierarchical porous structures for fuel cell applications.

Coaxial heterojunction carbon nanofibers with charge transport and electrocatalytic reduction phases for high performance dye-sensitized solar cells

Novel coaxial heterojunction carbon nanofibers, fabricated by electro-spinning a mixture of hydro-pitch and polyacrylonitrile, served as the counter electrode for dye-sensitized solar cells. Their high power conversion efficiency, being comparable to that of Pt CE, was achieved due to their good conductivity and high heteroatom content.

A recognition mechanism study: Luminescent metal-organic framework for the detection of nitro-explosives

This article presents a recognition mechanism for nitro-explosives by the luminescent metal-organic framework 1 (LMOF-1) with the aid of density functional theory (DFT) and time-dependent density functional theory (TDDFT). The behavior of hydrogen bonding between the LMOF-1 and nitro-explosives in the S1 state is closely associated with the fluorescence properties of the LMOF-1. In our research, we calculated the geometric configuration, 1H NMR and IR spectra of the Complex 2 formed by LMOF-1 and nitrobenzene in the S0 and S1 states. The results showed that the hydrogen bond in the S1 state was increased, which was unfavorable for the luminescence of LMOF-1. Furthermore, the fluorescence rate of LMOF-1 decreased after encapsulating nitrobenzene into it. These calculated results collectively suggest that LMOF-1 is a potential fluorescence sensor for the detection of nitro-explosives. This research was aiming to provide a better understanding of the recognition mechanism by LMOFs for nitro-explosives.

Bromine Doping as an Efficient Strategy to Reduce the Interfacial Defects in Hybrid Two-Dimensional/Three-Dimensional Stacking Perovskite Solar Cells

Solar-to-electricity conversion efficiency, power conversion efficiency (PCE), and stability are two important aspects of perovskite solar cells (PSCs). However, both aspects are difficult to simultaneously enhance. In the recent two years, two-dimensional (2D)/three-dimensional (3D) stacking structure, designed by covering the 3D perovskite with a thin 2D perovskite capping layer, was reported to be a promising method to achieve both a higher PCE and improved stability simultaneously. However, when reducing the surface defects of 3D perovskite, the thin 2D capping layer itself may probably introduce additional interfacial defects in a 2D/3D stacking structure, which is thought to be able to trigger trap-assisted nonradiative recombination or ion migration. Thus, efforts should be paid to reduce the interfacial defects of 2D hybrid perovskite when serving as a modification layer in a 2D/3D stacking structure PSCs. Here, we demonstrate that bromine (Br) doping of the 2D perovskite capping layer is an efficient strategy to passivate interfacial defects robustly, by which the photoluminescence lifetime is enhanced notably, whereas the interfacial charge recombination is suppressed a lot. As a result, the PCE is enhanced from 18.01% (3D perovskite) to 20.07% (Br-doped 2D/3D perovskite) along with improved moisture stability.