Oxidized Ti3C2 MXene nanosheets for dye-sensitized solar cells

Porous TiO2 electrodes were prepared by oxidizing two-dimensional titanium carbide nanosheets (Ti3C2 MXene) and the electrodes were tested in dye-sensitized solar cells. The effects of oxidation temperature and duration time together with various thicknesses on the device performance were investigated. A power conversion efficiency of 2.66% was observed

Aggregate-forming semi-synthetic chlorophyll derivatives / Ti₃C₂Tₓ MXene hybrids for photocatalytic hydrogen evolution

Chlorophylls (Chls) are the most abundant natural pigments having excellent opt-electrical and semi-conductive properties. Ti3C2Tx MXene, one of the most extensively studied 2D noble metal-free co-catalyst, features outstanding electrochemical properties. This work compares three aggregate-forming chlorophyll derivatives (Chl-n; n = 1–3), namely, zinc methyl 3-devinyl-3-hydroxymethyl-pyropheophorbide-a (Chl-1), zinc dodecyl 3-devinyl-3-hydroxymethyl-pyropheophorbide-a (Chl-2) and zinc dodecyl 131-deoxo-3-devinyl-131-dicyanomethylene-3-hydroxymethyl-pyropheophorbide-a (Chl-3), as light-harvesting antenna pigments in the MXene-based photocatalytic system for hydrogen evolution under the white light illumination (λ > 420 nm). The hydrogen evolution reaction (HER) of these Chls depends on the peripheral substituent groups at the C13- and/or C17-positions of the chlorin macrocyclic π-system. Differences among these Chl-n sensitized Ti3C2Tx MXene (Chl-n@Ti3C2Tx) are compared in terms of their light-harvesting ability, morphology, charge transfer efficiency and photocatalytic performance. The best HER performance is found to be as high as 122 μmol/h/gcat with the Chl-3@Ti3C2Tx composite. This work leads the direction in synthesizing Chls in Chl/MXene hybrid structure suitable for highly efficient photocatalytic HER

Chlorophyll derivatives/MXene hybrids for photocatalytic hydrogen evolution: Dependence of performance on the central coordinating metals

Development of efficient photocatalytic hydrogen evolution reaction (HER) with illumination of visible light is challenging. In this work, five chlorophyll derivatives (M-Chls; M = H2/Cu/Ni/Co/Zn) with different central ions in its cyclic tetrapyrrole ring including free base, copper, nickel, cobalt, and zinc were synthesized and employed as the effective visible-light harvester for efficient HER. In addition, two-dimensional (2D) noble metal-free co-catalyst Ti3C2Tx MXene was used as an excellent electron capturer due to its outstanding conductivity property. These M-Chls are modified on the surface of Ti3C2Tx MXene with 2D accordion-like morphology by means of a simple deposition process to form noble metal-free Chl/Ti3C2Tx-based photocatalysts for HER. It is found that the best HER performance as high as 49 μmol/h/gcat was achieved with the Co-Chl@Ti3C2Tx hybrid, which was much higher than those of other M-Chl@Ti3C2Tx composites. This research provides a specific way to synthesize low-cost and environmentally friendly natural Chls for developing highly efficient photocatalytic HER through molecular engineering

Effect of the Fabrication Method of Chlorophyll-Ti3C2Tx-Based Photocatalysts on Noble Metal-Free Hydrogen Evolution

Composites composed of a chlorophyll-a derivative and Ti3C2Tx MXene recently showed promising results as photocatalysts for the hydrogen evolution reaction. Herein, this type of composites is prepared by using a layered Ti3C2Tx material via an HCl@LiF etching technique, instead of the previously adopted HF etching technique. The performance of H2 evolution, therefore, showed a fourfold increase in this photocatalytic system, compared with the reported data in our previous works. The underlying reason for such a large improvement of the chlorophyll-MXene photocatalyst performance would be attributed to a more suitable surface chemistry, higher conductivity, fewer defects, higher surface area, and larger interlayer space of Ti3C2Tx introduced by the Li+ ions from LiF in the etching process. Herein, it is revealed that the preparation method of MXenes used as the cocatalysts is key to improve the H2 evolution efficiency in photocatalytic water splitting

Chlorophyll based organic heterojunction on Ti3C2Tx MXenes nano‐sheets for efficient hydrogen production

Z‐scheme process is a photo‐induced electron transfer pathway in natural oxygenic photosynthesis involving the electron transport from photosystem II (PSII) to PSI. Inspired by the interesting Z‐scheme process, here we demonstrated a photocatalytic hydrogen evolution reaction (HER) employing chlorophyll derivatives, Chl‐1 and Chl‐2, on the surface of Ti3C2Tx MXenes with two‐dimensional accordion‐like morphology forming Chl‐1@Chl‐2@Ti3C2Tx composite. Due to the frontier molecular orbital energy alignments of Chl‐1 and Chl‐2, the sublayer Chl‐1 is a simulation of PSI whereas the upper layer Chl‐2 is equivalent to PSII, and the resultant electron transport can take place from Chl‐2 to Chl‐1. Under the illumination of visible light (> 420 nm), the HER performance of Chl‐1@Chl‐2@Ti3C2Tx photocatalysts was found to be as high as 143 μmol/h/gcat , which was substantially higher than that of photocatalysts of either Chl‐1@Ti3C2Tx (20 μmol/h/g) or Chl‐2@Ti3C2Tx (15 μmol/h/g)

Chlorosome‐Like Molecular Aggregation of Chlorophyll Derivative on Ti₃C₂Tₓ MXene Nanosheets for Efficient Noble Metal‐Free Photocatalytic Hydrogen Evolution

Efficient photocatalytic hydrogen evolution reaction (HER) in the visible‐to‐near infrared region at a low cost remains a challenging issue. This work demonstrates the fabrication of organic‐inorganic composites by deposition of supramolecular aggregates of a chlorophyll derivative, namely, zinc methyl 3‐devinyl‐3‐hydroxymethyl‐pyropheophorbide a (Chl) on the surface of Ti3C2Tx MXene with 2D accordion‐like morphology. This composite material is employed as noble metal‐free catalyst in photocatalytic HER under the white light illumination, where Chl serves as a small molecule organic semiconductor component instead of ordinary inorganic and polymer organic semiconductors such as TiO2 and g‐C3N4, and Ti3C2Tx serves as a co‐catalyst. Different composition ratios of Chl/Ti3C2Tx are compared for their light‐harvesting ability, morphology, charge transfer efficiency, and photocatalytic performance. The best HER performance is found to be as high as 52 ± 5 µmol h−1 gcat−1 after optimization. Such a large HER activity is attributed to the efficient light harvesting followed by exciton transfer in Chl aggregates and the resultant charge separation at the interface of Chl/Ti3C2Tx

Performance improvement of MXene-based perovskite solar cells upon property transition from metallic to semiconductive by oxidation of Ti₃C₂Tₓ in air

The unique properties of MXenes that arise from terminating functional groups and oxidation of MXenes make them attractive for application in photovoltaic devices like perovskite solar cells (PSCs). Here, oxidation of Ti3C2Tx hydrocolloid was carried out to tune its properties desirable for an electron transport layer (ETL) in low-temperature processed PSCs. The calculations of the energy levels were carried out using the Vienna ab initio simulation package (VASP) code based on density functional theory (DFT). Oxidation of Ti_{3}C_{2}T_{x} can generate Ti–O bonds and effectively reduce the macroscopic defects of the film fabricated by spin-coating, while a transition from metallic material to semiconductor occurred after heavy oxidation. A better matching of energy levels between perovskite and ETL layer in the case of a hybrid of oxidized and pristine Ti_{3}C_{2}T_{x} renders a champion power conversion efficiency (PCE) of 18.29%. The improvement in PCE can be attributed to the increased electron mobility in the ETL, which promotes electron transport and reduces the electron–hole recombination. Hence, by presenting a simple method for high performance in PSCs by MXene-derived materials, this work demonstrates the great potential of these materials for applications in low-temperature processed PSCs and other photovoltaic technologies

Intercalation and delamination of layered carbides and carbonitrides

Intercalation and delamination of two-dimensional solids in many cases is a requisite step for exploiting their unique properties. Herein we report on the intercalation of two-dimensional Ti3C2, Ti3CN and TiNbC—so called MXenes. Intercalation of hydrazine, and its co-intercalation with N,N-dimethylformamide, resulted in increases of the c-lattice parameters of surface functionalized f-Ti3C2, from 19.5 to 25.48 and 26.8 Å, respectively. Urea is also intercalated into f-Ti3C2. Molecular dynamics simulations suggest that a hydrazine monolayer intercalates between f-Ti3C2 layers. Hydrazine is also intercalated into f-Ti3CN and f-TiNbC. When dimethyl sulphoxide is intercalated into f-Ti3C2, followed by sonication in water, the f-Ti3C2 is delaminated forming a stable colloidal solution that is in turn filtered to produce MXene ‘paper’. The latter shows excellent Li-ion capacity at extremely high charging rates

Capacitance of two-dimensional titanium carbide (MXene) and MXene/carbon nanotube composites in organic electrolytes

Pseudocapacitive materials that store charges by fast redox reactions are promising candidates for designing high energy density electrochemical capacitors. MXenes - recently discovered two-dimensional carbides, have shown excellent capacitance in aqueous electrolytes, but in a narrow potential window, which limits both the energy and power density. Here, we investigated the electrochemical behavior of Ti3C2 MXene in 1M solution of 1-ethly-3-methylimidazolium bis- (trifluoromethylsulfonyl)-imide (EMITFSI) in acetonitrile and two other common organic electrolytes. This paper describes the use of clay, delaminated and composite Ti3C2 electrodes with carbon nanotubes in order to understand the effect of the electrode architecture and composition on the electrochemical performance. Capacitance values of 85 F g-1 and 245 F cm-3 were obtained at 2 mV s-1, with a high rate capability and good cyclability. In situ X-ray diffraction study reveals the intercalation of large EMI+ cations into MXene, which leads to increased capacitance, but may also be the rate limiting factor that determines the device performance

Electrical and Elastic Properties of Individual Single-Layer Nb₄C₃Tₓ MXene Flakes

2D carbides and nitrides (MXenes) are widely recognized for their exceptional promise for numerous applications. However, physical property measurements of their individual monolayers remain very limited despite their importance for revealing the intrinsic physical properties of MXenes. The first mechanical and electrical measurements of individual single‐layer flakes of Nb4C3Tx MXene, which are prepared via an improved synthetic method are reported. Characterization of field‐effect transistor devices based on individual single‐layer Nb4C3Tx flakes shows an electrical conductivity of 1024 ± 165 S cm−1, which is two orders of magnitude higher than the previously reported values for bulk Nb4C3Tx assemblies, and an electron mobility of 0.41 ± 0.27 cm2 V−1 s−1. Atomic force microscopy nanoindentation measurements of monolayer Nb4C3Tx membranes yield an effective Young's modulus of 386 ± 13 GPa, assuming a membrane thickness of 1.26 nm. This is the highest value reported for nanoindentation measurements of solution‐processable 2D materials, revealing the potential of Nb4C3Tx as a primary component for various mechanical applications. Finally, the agreement between the mechanical properties of 2D Nb4C3Tx MXene and cubic NbC suggests that the extensive experimental data on bulk carbides could be useful for identifying new MXenes with improved functional characteristics