Construction of stable Ta3N5/g-C3N4 metal/non-metal nitride hybrids with enhanced visible-light photocatalysis

In this paper, a novel Ta3N5/g-C3N4 metal/non-metal nitride hybrid was successfully synthesized by a facile impregnation method. The photocatalytic activity of Ta3N5/g-C3N4 hybrid nitrides was evaluated by the degradation of organic dye rhodamine B (RhB) under visible light irradiation, and the result indicated that all Ta3N5/g-C3N4 samples exhibited distinctly enhanced photocatalytic activities for the degradation of RhB than pure g-C3N4. The optimal Ta3N5/g-C3N4 composite sample, with Ta3N5 mass ratio of 2%, demonstrated the highest photocatalytic activity, and its degradation rate constant was 2.71 times as high as that of pure g-C3N4. The enhanced photocatalytic activity of this Ta3N5/g-C3N4 metal/metal-free nitride was predominantly attributed to the synergistic effect which increased visible-light absorption and facilitated the efficient separation of photoinduced electrons and holes. The Ta3N5/g-C3N4 hybrid nitride exhibited excellent photostability and reusability. The possible mechanism for improved photocatalytic performance was proposed. Overall, this work may provide a facile way to synthesize the highly efficient metal/metal-free hybrid nitride photocatalysts with promising applications in environmental purification and energy conversion

Revealing the activity of Co3Mo3N and Co3Mo3N0.5 as electrocatalysts for the hydrogen evolution reaction

The hydrogen evolution reaction (HER) from water is governed by electrocatalysts used. Multiple factors such as crystal structure, composition and morphology dictate the final catalytic performance. However, as multicomponent materials are developed to replace noble metals in the HER, it has become increasingly difficult to identify intrinsically active materials. Hence, there is an imperative for phase-pure catalysts to be synthetized and tested without obscuring contributions from impurities or substrates. Herein, we demonstrate that phase-pure, unsupported Co3Mo3N achieves a competitively low overpotential (OVP) of 108 ± 8 mV at 10 mA cm‒2 in 0.5 M H2SO4. Density functional theory (DFT) reveals weakly binding metal sites as the catalytic centres for the HER in the nitride. Remarkably, the N-deficient Co3Mo3N0.5 shows similar electrochemical properties but has limited chemical stability under cathodic bias. Thus, even though nitrogen sites play only a minor role in catalytic performance, their occupancy is crucial for the stability of nitride catalysts in the corrosive electrolyte. The composite of Co3Mo3N on Ni-foam sustains 10 ± 0.7 mA cm‒2 at applied potential of just 20 mV over extended time, highlighting the utility of nitrides for future design of stable and active HER catalytic systems

Open X-Embodiment:Robotic learning datasets and RT-X models

Large, high-capacity models trained on diverse datasets have shown remarkable successes on efficiently tackling downstream applications. In domains from NLP to Computer Vision, this has led to a consolidation of pretrained models, with general pretrained backbones serving as a starting point for many applications. Can such a consolidation happen in robotics? Conventionally, robotic learning methods train a separate model for every application, every robot, and even every environment. Can we instead train "generalist" X-robot policy that can be adapted efficiently to new robots, tasks, and environments? In this paper, we provide datasets in standardized data formats and models to make it possible to explore this possibility in the context of robotic manipulation, alongside experimental results that provide an example of effective X-robot policies. We assemble a dataset from 22 different robots collected through a collaboration between 21 institutions, demonstrating 527 skills (160266 tasks). We show that a high-capacity model trained on this data, which we call RT-X, exhibits positive transfer and improves the capabilities of multiple robots by leveraging experience from other platforms. The project website is robotics-transformer-x.github.io

Theoretical studies of perovskite solar cells

© 2017 Dr Yecheng ZhouPerovskite solar cells (PSCs) are solar cells implemented with perovskite absorber. Within 5 years of development, the power conversion efficiency (PCE) of PSCs has reached 22.1%. It has been proposed that the ferroelectric polarization of perovskites may affect the electronic performance of PSCs. CH3NH3PbI3 (MAPbI3) is one of the most widely used perovskites in PSCs. Methylammonium ion (MA+) is believed to be involved in inducing ferroelectric polarization as single MA+ in vacuum shows a large dipole moment. On the other hand, MA+ ions are found to be disordered at room temperature. However, the behavior of MA+ alignment, and the connection between MA+ alignment and polarization are not fully understood. A better understanding of these phenomena are crucial in improving the PCE of PSCs. In this thesis, studies of the relationship between the performance and MA+ alignment, and polarisation have been undertaken. Band structures and effective masses of MAPbI3 in the $\alpha$ phase and the $\beta$ phase were calculated with MA+ orientated differently. It is found that these structures have comparable energies. This is in agreement with the observed disordered MA+ in experiments. The influence of the orientation of MA+ ions on band gap is smaller than 0.1 eV. There is a significant effect of orientation on the effective mass due to the large dipole moment of MA+. However, overall reduced effective masses of differently orientated MA+ unit cells are comparable. It is also found that the electronic properties of the $\alpha$ phase are similar to that of the $\beta$ phase. This is the reason why MAPbI3 PSCs are able to work stably near their phase transition point. These results explained that the orientation of MA+ and phase transition near room temperature have little influence on solar cell performance. The energy landscapes for MA+ reorientations in the $\alpha$ phase and the $\beta$ phase unit cells and super cells are investigated in detail. The MA+ reorientation energy barrier depends on the initial and final orientations, it also depends on the orientations of its neighboring MA+. The energy barrier is smaller for MA+ rotating from anti-parallel to parallel. This suggests MA+ prefers parallel alignment. It is also found that the rotational energy barrier in the $\alpha$ phase is lower than that in the $\beta$ phase. The polarization induced by MA+ rotation is about 6-8 $\mu C cm^{-2}$, which is about three times higher than that produced by lead ion relaxation. Our work suggests that polarization in MAPbI3 is mainly from MA+ orientations, which gives us a new understanding of the polarization of perovskite materials. To study the influence of the specific microscopic characteristics of PSC's on solar cell power conversion performance, setting up of an appropriate transport model for numerical simulations is required. Existing models to simulate solar cell efficiency are unsuitable to study PSCs, as they are developed for silicon solar cells. Also, they tend to be overparameterised, which compromises their predictive capacity. In this thesis numerical models have been developed and implemented in efficient home-made codes. Using these models, it is found that PCEs increase with charge carrier lifetimes, mobilities and diffusion lengths. The open-circuit voltage ($V_{oc}$) depends on the intensity of the exciting radiation and charge carrier lifetimes. Diffusion length and light intensity determine the saturated circuit current ($J_{sc}$). Additionally, three theoretical guidelines are proposed for PSC fabrication and optimization. It is theoretically shown that concentrator PSCs may offer advantages. We thus argue that the model developed here provides a framework for numerical modeling of perovskite--based cells and the optimization of their performance. Hysteresis is the performance difference between the forward and backward measurements. There is considerable debate about the reasons for the observed hysteresis. Polarization and ion migration are two possible reasons causing hysteresis in J-V curves for PSCs. However there are no quantitative hysteresis simulations, also no theory to show the possibility of hysteresis induced by polarization. By considering two screening relaxation fields in numerical models, I quantitatively reproduced experimental hysteresis in J-V curves. It is theoretically shown that both polarization and ion migration can induce screening fields and then produce hysteresis. Lastly, two possible methods to reduce hysteresis in PSCs are proposed. One consists of reducing defects in thin films and at interfaces; The other consists of using polarisable materials as a charge transfer layer to compensate for the field induced by polarization or (and) ion migration. This work shows for the first time the role of polarization and suggests two ways to eliminate hysteresis. Our numerical work shows that polarization and ion migration are both possible mechanisms inducing hysteresis in the J-V curves. In order to distinguish between the role of polarization and ion migration in perovskite materials, two-dimensional periodic thin film models for relaxations of ion migration and polarization of MA+ are investigated. Dynamic simulations show both ion migration and MA+ polarization are able to build screening fields and promote charge transfer in PSCs. Lifetimes of these two relaxation mechanism increase with the thin film thickness, and decrease with the initial applied external field. The relaxation lifetime of ion migration obtained with a model using experimental length scale and field strength is estimated to be in the range from 1 ms to 1.5 s, which is comparable to measurement delay times. While, the relaxation lifetimes of MA+ orientations are estimated to be about several hundred ns to several $\mu$s, which significantly differ from experimental delay times. This polarization relaxation lifetime is much shorter than others simulation results. The discrepancy is due to the different model that has been used in this work. Others have used three-dimensional periodic bulk models, which led to longer relaxation lifetimes compared to the two-dimensional periodic thin film models. However, three-dimensional periodic bulk models can not consider surface and interface charges, results from three-dimensional periodic models are thus misleading. Charge behaviors in thin films should be mimicked by two-dimensional periodic thin film models. Our results indicate that the hysteresis in PSCs is induced by ion migration rather than the polarization of MA+ due to the fast theoretical relaxation time of MA+ polarization compared to experiments. This finding provides us a better understanding of hysteresis in PSCs. The influence of the density of the conduction band states ($N_c$) and the density of the valence band states ($N_v$) on PSC performance are elucidated. Firstly, $N_c$ and $N_v$ of silicon, CdTe, and typical perovskites are calculated from DFT calculations based on two different methods. It is found that $N_c$ $N_v$ of CdTe and typical perovskites are much lower than that of silicon. Using our developed numerical models, the lower $N_c$ $N_v$ is expected to produce higher output voltage including $V_{oc}$. The lower $N_c$ $N_v$ in perovskite will result in 100 mV higher $V_{oc}$ and 10\% higher PCEs compared to solar cells with the same parameters except for $N_c$ $N_v$. This provides a new guideline for finding and developing new photovoltaic materials

Low Density of Conduction and Valence Band States Contribute to the High Open-Circuit Voltage in Perovskite Solar Cells

Hybrid perovskites are widely used for high-performance solar cells. Large diffusion lengths and long charge carrier lifetimes are considered two main factors for their high performance. Here, we argue that not only large diffusion lengths and long carrier lifetimes but also the low densities of the conduction and valence band states (<i>N</i>_c, <i>N</i>_v) contribute to high-performance perovskite solar cells. We estimated <i>N</i>_c and <i>N</i>_v of silicon, CdTe, and typical perovskites with two different methods. It was found that <i>N</i>_c and <i>N</i>_v of perovskites and CdTe are much lower than that of silicon. Using numerical models, we found that the solar cell of a material with same characteristics but lower <i>N</i>_c and <i>N</i>_v can realize a higher open-circuit voltage (<i>V</i>_oc) and higher power conversion efficiency (PCE). We put forward and proved that the low <i>N</i>_c and <i>N</i>_v in hybrid perovskite is one of the factors for its high performance. This provides a new guideline for finding and developing new photovoltaic candidate materials

Phonon-electron coupling and tunneling effect on charge transport in organic semi-conductor crystals of C-n-BTBT

C-n-[1] benzothieno[3,2-b][1]-benzothiophene (BTBT) crystals show very high hole mobilities in experiments. These high mobilities are beyond existing theory prediction. Here, we employed different quantum chemistry methods to investigate charge transfer in C-n-BTBT crystals and tried to find out the reasons for the underestimation in the theory. It was found that the hopping rate estimated by the Fermi Golden Rule is higher than that of the Marcus theory due to the high temperature approximation and failure at the classic limit. More importantly, molecular dynamics simulations revealed that the phonon induced fluctuation of electronic transfer integral is much larger than the average of the electronic transfer integral itself. Mobilities become higher if simulations implement the phonon-electron coupling. This conclusion indicates that the phonon-electron coupling promotes charge transfer in organic semi-conductors at room temperature. Published by AIP Publishing

Comparative Study of Parameter Extraction from a Solar Cell or a Photovoltaic Module by Combining Metaheuristic Algorithms with Different Simulation Current Calculation Methods

In this paper, single-diode model (SDM) and double-diode model (DDM) parameters of the French RTC solar cell and the Photowatt PWP 201 photovoltaic (PV) module were extracted by combining five metaheuristic algorithms with three simulation current calculation methods (i.e., approximation method, Lambert W method and Newton–Raphson method), respectively. It was found that the parameter-extraction accuracies of the Lambert W (LW) method and the Newton–Raphson (NR) method are always approximately equal and higher than that of the approximation method. The best RMSEs (root mean square error) obtained by using the LW or the NR method on the solar cell and the PV module are 7.72986 × 10−4 and 2.05296 × 10−3 for SDM parameter extraction and 6.93709 × 10−4 and 1.99051 × 10−3 for DDM parameter extraction, respectively. The latter may be the highest parameter-extraction accuracy reported on the solar cell and the PV module so far, which is due to the adoption of more reasonable DDM parameter boundaries. Furthermore, the convergence curves of the LW and the NR method basically coincide, with a convergence speed faster than that of the approximation method. The robustness of a parameter-extraction method is mainly determined by the metaheuristic algorithm, but it is also affected by the simulation current calculation method and the parameter-extraction object. In a word, the approximation method is not suitable for application in PV-model parameter extraction because of incorrect estimation of the simulation current and the RMSE, while the LW and NR methods are suitable for the application for accurately calculating the simulation current and RMSE. In terms of saving computation resources and time, the NR method is superior to the LW method