Rational Design of S-Scheme Heterojunction toward Efficient Photocatalytic Cellulose Reforming for H2 and Formic Acid in Pure Water.

Photocatalytic cellulose reforming usually requires harsh conditions due to its sluggish kinetics. Here, a hollow structural S-scheme heterojunction of ZnSe and oxygen vacancy enriched TiO2 , namely, h-ZnSe/Pt@TiO2 , is designed and fabricated, with which the photocatalytic reforming of cellulose for H2 and formic acid is realized in pure water. H2 and formic acid productivity of 1858 and 372 µmol g-1 h-1 and a steady H2 evolution for 300 h are achieved with α-cellulose. Comparable photocatalytic activity can also be achieved using various cellulose sources. It is experimentally proven that the photogenerated charge transfer follows an S-scheme mechanism, which not only promotes the charge separation but also preserves the higher reductive and oxidative abilities of the ZnSe and TiO2 , respectively. Furthermore, the polyhydroxy species produced during cellulose degradation are favored to adsorb on the oxygen vacancy enriched TiO2 surface, which promotes the photocatalytic reforming process and is accounted to the preservation of formic acid as the major solution-phase product. In addition, sequential reactions of oxidation of aldehydes and elimination of formic acid of the cellulose degradation process are revealed. This work provides a photocatalytic strategy to sustainably produce hydrogen and value-added chemicals from biomass under the most environmentally benign condition, i.e., pure water.

Two-Dimensional Ultrathin Graphic Carbon Nitrides with Extended π-Conjugation as Extraordinary Efficient Hydrogen Evolution Photocatalyst.

Construction of 2D graphic carbon nitrides (g-CNx ) with wide visible light adsorption range and high charge separation efficiency concurrently is of great urgent demand and still very challenging for developing highly efficient photocatalysts for hydrogen evolution. To achieve this goal, a two-step pyrolytic strategy has been applied here to create ultrathin 2D g-CNx with extended the π-conjugation. It is experimentally proven that the extension of π-conjugation in g-CNx is not only beneficial to narrowing the bandgap, but also improving the charge separation efficiency of the g-CNx . As an integral result, extraordinary apparent quantum efficiencies (AQEs) of 57.3% and 7.0% at short (380 nm) and long (520 nm) wavelength, respectively, are achieved. The formation process of the extended π-conjugated structures in the ultrathin 2D g-CNx has been investigated using XRD, FT-IR, Raman, XPS, and EPR. Additionally, it has been illustrated that the two-step pyrolytic strategy is critical for creating ultrathin g-CNx nanosheets with extended π-conjugation by control experiments. This work shows a feasible and effective strategy to simultaneously expand the light adsorption range, enhance charge carrier mobility and depress electron-hole recombination of g-CNx for high-efficient photocatalytic hydrogen evolution.

Tuning the current rectification behavior of Rh2-based molecular junctions by varying their supramolecular structures.

Molecular junctions with similar backbones, tunable chemical structures and controllable length are critical for the systematic study of the structure-functionality relationships of their charge transport behavior. Taking advantage of the feasibility and tunability of stepwise fabrication, we built series of asymmetric supramolecular SAMs on gold using Rh2(O2CCR3)4 (Rh2, R = CH3, H, and F) as the building blocks and conjugated N,N'-bidentate ligands (pyrazine (LS), 4,4'-bipyridine (LM) and 1,2-bis(4-pyridyl)ethene (LL)) as the bridges. By varying the Rh2 units and bridging ligands, series of supramolecules with similar backbone and tunable chemical structures were assembled on gold. Their charge transport behavior was examined using conductive-probe atomic force microscopy. Notably, current rectification diminishes gradually as the degree of conjugation of the bridging ligands gets larger from LS to LL due to the decrease in the energy gap between the donor and the acceptor in π(Rh2)-π(L) conjugated MO arrays. Additionally, current rectification can be enhanced when the charge transport mechanistic transits from tunneling in dimers to hopping in tetramers. Unlike charges hopping along the MO arrays in tetramers, charges tunnel through the frontier MOs in dimers. The occupied frontier MOs of dimers localize near the center of the supramolecules or delocalize on the donor and acceptor, which contributes to the weakening of the asymmetric charge tunneling. This work reveals that the frontier MO configurations of these supramolecules could be adjusted by varying their chemical structures, and consequently realize tuning of their charge transport behavior, which deepens the understanding of the charge transport behavior and benefits the establishment of the structure-functionality relationship of Rh2-based molecular junctions.

Simple and efficient fabrication of multi-stage color-changeable photonic prints as anti-counterfeit labels.

Color changeable photonic prints (CCPPs) show their potential applications in high-level information storage and anti-counterfeiting, but usually suffer from the complex fabrication process and limited color variation. Here, a simple and efficient method is developed to generate CCPPs with multilevel tunable color contrasts by packing the solvent responsive photonic crystals with diverse cross-linking degrees and desired way. The key to the successful fabrication is to create and control over the optical response of each part of the CCPPs through altering the cross-linking degree of PCs and thus the affinity between the CCPPs and solvents. A CCPPs based anti-fake label with the encrypted information functionality which originates from reversible color change between dried state and swelling with the mixture of acetic acid and ethanol is investigated. Compared with conventional CCPPs, the as-prepared CCPPs can reveal multistage information depending on the volume fraction of ethanol. This work provides a new insight for the simple fabrication of CCPPs and will facilitate their applications in the information protection and high-level anti-counterfeiting.

Understanding the charge transport properties of redox active metal-organic conjugated wires.

Layer-by-layer assembly of the dirhodium complex [Rh2(O2CCH3)4] (Rh2) with linear N,N'-bidentate ligands pyrazine (LS) or 1,2-bis(4-pyridyl)ethene (LL) on a gold substrate has developed two series of redox active molecular wires, (Rh2LS) n @Au and (Rh2LL) n @Au (n = 1-6). By controlling the number of assembling cycles, the molecular wires in the two series vary systematically in length, as characterized by UV-vis spectroscopy, cyclic voltammetry and atomic force microscopy. The current-voltage characteristics recorded by conductive probe atomic force microscopy indicate a mechanistic transition for charge transport from voltage-driven to electrical field-driven in wires with n = 4, irrespective of the nature and length of the wires. Whilst weak length dependence of electrical resistance is observed for both series, (Rh2LL) n @Au wires exhibit smaller distance attenuation factors (ß) in both the tunneling (ß = 0.044 Å-1) and hopping (ß = 0.003 Å-1) regimes, although in (Rh2LS) n @Au the electronic coupling between the adjacent Rh2 centers is stronger. DFT calculations reveal that these wires have a π-conjugated molecular backbone established through π(Rh2)-π(L) orbital interactions, and (Rh2LL) n @Au has a smaller energy gap between the filled π*(Rh2) and the empty π*(L) orbitals. Thus, for (Rh2LL) n @Au, electron hopping across the bridge is facilitated by the decreased metal to ligand charge transfer gap, while in (Rh2LS) n @Au the hopping pathway is disfavored likely due to the increased Coulomb repulsion. On this basis, we propose that the super-exchange tunneling and the underlying incoherent hopping are the dominant charge transport mechanisms for shorter (n ≤ 4) and longer (n > 4) wires, respectively, and the Rh2L subunits in mixed-valence states alternately arranged along the wire serve as the hopping sites.

Control of the rectifying effect and direction by redox asymmetry in Rh2-based molecular diodes.

Four asymmetrical self-assembled monolayers (SAMs) consisting of two subunits with different Rh2 building blocks present a pronounced rectifying behavior. The rectification ratio (RR) increases on increasing the redox potential difference between the two Rh2 subunits, and the rectifying direction can be reversed by reordering the subunits in the assembly.

Regulation of local structure and composition of binary disulfide and thiol self-assembled monolayers using nanografting.

Nanografting is used to create spatial confinement, which enables regulation of self-assembly reaction pathways and outcome. The degree and outcome of this regulation is revealed using binary self-assembled monolayers (SAMs) of organothiols and disulfides. In naturally grown systems, these SAMs have more complex morphology when compared with corresponding binary alkanethiol SAMs. Taller molecules form nanodomains of ellipsoidal cap in shape. These domains arrange in various irregular geometries, including 1D worm-like and 2D branches. This observation differs from binary alkanethiol SAMs, where nanodomains are separated and randomly dispersed. During nanografting, more homogeneous morphology was observed compared with naturally grown layers. By varying nanoshaving speed, the nanodomain structure can be regulated from randomly dispersed to more heterogeneous and, finally, to near natural growth. This trend is very similar to mixed alkanethiol systems, where the domain size and separation increase with increasing speed. Different from the alkanethiol systems, the observed structural variations are due to the changes in surface composition, in addition to domain size, shape, and arrangement.