Multi-functional MXene quantum dots enhance the quality of perovskite polycrystalline films and charge transport for solar cells.

Recently, two-dimensional (2D) transition metal carbides/nitrides (MXenes) find applications in perovskite solar cells (PSCs), due to their high conductivity, tunable electronic structures, and rich surface chemistry, etc. However, the integration of 2D MXenes into PSCs is limited by their large lateral sizes and relatively-small surface volume ratios, and the roles of MXenes in PSCs are still ambiguous. In this paper, zero-dimensional (0D) MXene quantum dots (MQDs) with an average size of 2.7 nm are obtained through clipping step by step combining a chemical etching and a hydrothermal reaction, which display rich terminals (i.e., -F, -OH, -O) and unique optical properties. The 0D MQDs incorporated into SnO2 electron transport layers (ETLs) of PSCs exhibit multifunction: 1) increasing the electrical conductivity of SnO2, 2) promoting better alignments of energy band positions at the perovskite/ETL interface, 3) improving the film quality of atop polycrystalline perovskite. Particularly, the MQDs not only tightly bond with the Sn atom for decreasing the defects of SnO2, but also interact with the Pb2+ of perovskite. As a result, the defect density of PSCs is significantly decreased from 5.21 × 1021 to 6.4 × 1020 cm-3, leading to enhanced charge transport and reduced nonradiative recombination. Furthermore, the power conversion efficiency (PCE) of PSCs is substantially improved from 17.44% to 21.63% using the MQDs-SnO2 hybrid ETL compared with the SnO2 ETL. Besides, the stability of the MQDs-SnO2-based PSC is greatly enhanced, with only ～4% degradation of the initial PCE after storage in ambient condition (25 °C, RH: 30-40%) for 1128 h, as compared to that of the reference device with a rapid degradation of ～60% of initial PCE after 460 h. And MQDs-SnO2-based PSC also presents higher thermal stability than SnO2-based device with continuous heating for 248 h at 85 °C. The unique MQDs exhibited in this work might also find other exciting applications such as light-emitting diodes, photodetectors, and fluorescent probes.

Dilute Au-Containing Ag Nanosponges as a Highly Active and Durable Electrocatalyst for Oxygen Reduction and Alcohol Oxidation Reactions.

Zero-dimensional nanoparticles (NPs) have been demonstrated as the promising class of catalysts for various chemical and electrochemical reactions. However, the emerging Au-Ag NP catalysts suffer from single functionality, limited activity enhancement, and unsatisfactory stability problems. Here, we report a facile kinetically controlled solution method to prepare a new class of Au-Ag nanoporous sponges (NSs) composed of three-dimensional networks without using additional stabilizing agents at room temperature. The unexpected shift of the d-band center in our Au-Ag NSs was observed for the first time in Au-Ag bimetallic systems, which effectively activates the Au-Ag NSs for electrochemical reactions. The robust electronic effect coupled with abundant accessible active sites from the hierarchically porous architecture make the bare Au-Ag NSs a superior multifunctional catalyst for oxygen reduction, ethylene glycol (EG) oxidation, and glucose oxidation reactions compared to the commercial Pt/C electrocatalyst in alkaline medium. The optimized AuAg3.2 NSs deliver a mass activity of 1.26 A mgAu-1 toward oxygen reduction reaction, which is ∼8.2 times as high as that of the Pt/C electrocatalyst, simultaneously showing outstanding stability with negligible activity decay after 10 000 cycles. For the anodic reactions, these AuAg3.2 NSs show extremely high activity and stability toward both EG and glucose catalytic oxidation reactions with a higher mass activity of 7.58 and 1.48 A mgAu-1, about 3- and 18.5-fold enhancement than Pt/C, respectively. This work provides important insights into the structural design, performance optimization, and cost reduction to promote the practical applications of liquid fuel cells.

NaCl-assisted one-step growth of MoS2-WS2 in-plane heterostructures.

Transition metal dichalcogenides (TMDs) have attracted considerable interest for exploration of next-generation electronics and optoelectronics in recent years. Fabrication of in-plane lateral heterostructures between TMDs has opened up excellent opportunities for engineering two-dimensional materials. The creation of high quality heterostructures with a facile method is highly desirable but it still remains challenging. In this work, we demonstrate a one-step growth method for the construction of high-quality MoS2-WS2 in-plane heterostructures. The synthesis was carried out using ambient pressure chemical vapor deposition (APCVD) with the assistance of sodium chloride (NaCl). It was found that the addition of NaCl played a key role in lowering the growth temperatures, in which the Na-containing precursors could be formed and condensed on the substrates to reduce the energy of the reaction. As a result, the growth regimes of MoS2 and WS2 are better matched, leading to the formation of in-plane heterostructures in a single step. The heterostructures were proved to be of high quality with a sharp and clear interface. This newly developed strategy with the assistance of NaCl is promising for synthesizing other TMDs and their heterostructures.

Atomic-scale investigation of a new phase transformation process in TiO2 nanofibers.

Crystallography of phase transformation combining transmission electron microscopy (TEM) with in situ heating techniques and X-ray diffraction (XRD) can provide critical information regarding solid-state phase transitions and the transition-induced interfaces in TiO2 nanomaterials theoretically and experimentally. Two types of reduced titanium oxides (Ti3O5, Ti6O11) are found during ex situ and in situ heating of TiO2 (B) nanofibers with a specific morphology of the {100} single form (SF) in air and vacuum. The results indicate that the phase transformation process from TiO2 (B) follows the TiO2 (B) → Ti3O5 → Ti6O11 → anatase sequence for the nanofibers with the {100} SF. The occurrence of such a phase transition is selective to the morphology of TiO2 (B) nanofibers. The corresponding orientation relationships (COR) between the four phases are revealed according to the TEM characterization. Four types of coherent interfaces, following the CORs are also found. They are TiO2 (B)/Ti3O5, TiO2 (B)/Ti6O11, Ti6O11/anatase and TiO2 (B)/anatase respectively. The habit plane for the TiO2 (B) to Ti3O5 transition is calculated as the {100}TB by using the invariant line model. The detailed atomic transformation mechanism is elucidated based on the crystallographic features of the four phases.

Activity Trends of Binary Silver Alloy Nanocatalysts for Oxygen Reduction Reaction in Alkaline Media.

The electrocatalytic activity of Pt-based alloys exhibits a strong dependence on their electronic structures, but a relationship between electronic structure and oxygen reduction reaction (ORR) activity in Ag-based alloys is still not clear. Here, a vapor deposition based approach is reported for the preparation of Ag75 M25 (M = Cu, Co, Fe, and In) and Agx Cu100-x (x = 0, 25, 45, 50, 55, 75, 90, and 100) nanocatalysts and their electronic structures are determined by valence band spectra. The relationship of the d-band center and ORR activity exhibits volcano-shape behaviors, where the maximum catalytic activity is obtained for Ag75 Cu25 alloys. The ORR enhancement of Ag75 Cu25 alloys originates from the 0.12 eV upshift in d-band center relative to pure Ag, which is different from the downshift in the d-band center in Pt-based alloys. The activity trend for these Ag75 M25 alloys is in the order of Ag75 Cu25 > Ag75 Fe25 > Ag75 Co25 . These results provide an insight to understand the activity and stability enhancement of Ag75 Cu25 and Ag50 Cu50 catalysts by alloying.

Dynamic Behavior of the Triboelectric Charges and Structural Optimization of the Friction Layer for a Triboelectric Nanogenerator.

Seeking to increase the triboelectric charge density on a friction layer is one of the most basic approaches to improve the output performance of triboelectric nanogenerators (TENGs). Here, we studied the storage mechanism of triboelectric charge in the friction layer and discussed the function of carrier mobility and concentration in the charge-storing process. As guided by these results, a kind of composite structure is constructed in the friction layer to adjust the depth distribution of the triboelectric charges and improve the output performance of TENGs. To further elucidate this theory, a simple TENG, whose negative friction layer is a composite structure by integrating polystyrene (PS) and carbon nanotubes (CNTs) into polyvinylidene fluoride (PVDF), was fabricated, and its performance test was also carried out. Comparing with a pure PVDF friction layer, the composite friction layer can raise the triboelectric charge density by a factor of 11.2. The extended residence time of electrons in the friction layer is attributed to a large sum of electron trap levels from PS.

Ag-Cu nanoalloyed film as a high-performance cathode electrocatalytic material for zinc-air battery.

A novel Ag50Cu50 film electrocatalyst for oxygen reduction reaction (ORR) was prepared by pulsed laser deposition (PLD) method. The electrocatalyst actually is Ag-Cu alloyed nanoparticles embedded in amorphous Cu film, based on transmission electron microscopy (TEM) characterization. The rotating disk electrode (RDE) measurements provide evidence that the ORR proceed via a four-electron pathway on the electrocatalysts in alkaline solution. And it is much more efficient than pure Ag catalyst. The catalytic layer has maximum power density of 67 mW cm(-2) and an acceptable cell voltage at 0.863 V when current densities increased up to 100 mA cm(-2) in the Ag50Cu50-based primary zinc-air battery. The resulting rechargeable zinc-air battery exhibits low charge-discharge voltage polarization of 1.1 V at 20 mAcm(-2) and high durability over 100 cycles in natural air.

Predictable and controllable dual-phase interfaces in TiO2(B)/anatase nanofibers.

The TiO2(B)/anatase dual-phase nanofiber exhibits enhanced photocatalytic activity when interfaces form during phase transformation of TiO2. To precisely control the formation of coherent interfaces between TiO2(B) and anatase, a thorough knowledge of phase transformation particularly from TiO2(B) to anatase (TA) is required. In our study, a crystallographic model in conjunction with transmission electron microscopy (TEM) was employed to investigate the phase transformation. The coherent interface with a crystallographic orientation relationship of [001]TB//[100]TA, (200)TB//(002)TA, and (020)TB//(020)TA predicted by the crystallography model was also observed by TEM experimentally. In addition, two types of incoherent interfaces that may deteriorate photocatalytic activity were examined and can be eliminated via an accurate tuning of calcination. The fundamental knowledge acquired from this work, therefore, provides a new insight to synthesize more efficient dual-phase TiO2 photocatalysts.

Atomic mechanism of predictable phase transition in dual-phase H2 Ti3 O7 /TiO2 (B) nanofiber: an in situ heating TEM investigation.

The phase transition from H(2)Ti(3)O(7) to TiO(2) (B) in a 1D single nanocrystal of H(2)Ti(3)O(7) was observed by in situ heating in a transmission electron microscope experimentally. The results indicate a typical monoclinic-to-monoclinic crystallographic orientation relationship between the two phases. Moreover, the fundamental building blocks and invariant deformation element model were both adopted to reveal the atomic mechanism and predict the crystallographic orientation relationship quantitatively for the phase transition. The prediction was precisely consistent with TEM results.