Recently, a collaborative effort between the University of Cambridge in the UK, the Swiss Federal Institute of Technology in Lausanne, and Nankai University has led to new advancements in the field of photocatalytic water splitting for hydrogen production.

For the first time, a room-temperature liquid-phase epitaxial growth method has been developed, which can be used for the low-cost, high-quality, and large-scale preparation of copper(I) oxide (Cu2O) single-crystal thin films.

Researchers have discovered for the first time the anisotropy of carrier transport in the bulk phase of copper(I) oxide. Specifically, the carrier mobility along the [111] crystal direction is an order of magnitude higher than in other directions, and the diffusion distance is also more than an order of magnitude longer than the average diffusion distance.

In terms of photoelectrochemical water splitting performance at key potentials, it has been improved by 70% compared to the current state-of-the-art flat copper(I) oxide devices.

These results have significantly enhanced the performance record of copper(I) oxide photoelectrodes, bringing them closer to large-scale applications. The series of photoelectric property parameters obtained for the first time in this study also provide important and precise guidance for the design of photoelectric devices based on copper(I) oxide.Photoelectrochemical Hydrogen Production: Providing Solutions for Renewable and Clean Energy

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In recent years, environmental changes caused by human activities have drawn unprecedented attention to the sustainable development and application of energy. Although nuclear fusion technology is highly anticipated, it is still far from being realized in terms of implementation time.

Clean energy technologies such as solar fuels and solar cells will be in great demand in the next few decades or even a century, characterized by their intermittency, instability, and low energy density.

Photoelectrochemical hydrogen production, as a renewable energy technology, offers a solution to directly convert solar energy into hydrogen energy and store it in fuels.

However, this is a huge challenge. To achieve large-scale application of this technology, it is necessary to achieve high efficiency, low manufacturing costs, and high stability simultaneously.The most efficient solar fuel devices currently in use all employ III-V group semiconductor materials for light absorption. However, it cannot be ignored that these materials have limitations in their application, including extremely high costs, complex fabrication processes, and stringent requirements.

At the end of April, at the 2024 Materials Research Society (MRS) Spring Meeting held in Seattle, USA, the project leader responsible for using these materials to split water into hydrogen using solar energy reported that the current cost of hydrogen production is still far from the target.

Currently, industry insiders generally believe that copper oxide materials are the best choice for photocathodes, and the electro-deposition method they use is a low-cost production method recognized in the industry. In addition, this method also has the advantages of low equipment requirements and simple preparation conditions.

At present, the photoelectric performance of copper oxide photoelectrodes can already compete with the performance of photoelectrodes based on mature photovoltaic materials. If further advanced, it is very likely to become the best material for photoelectrochemical water splitting.Discovering the Anisotropy of Charge Carrier Transport

Cuprous oxide is one of the ideal materials for photoelectrocatalytic hydrogen production electrodes, but it is undeniable that oxides generally have the bottleneck problem of short charge carrier transport distances. Moreover, this problem is "innate," and it is difficult to fundamentally solve it even by changing the synthesis method to control the doping concentration or optimizing the crystal.

Therefore, to fully understand cuprous oxide and find a solution, it is necessary to conduct the most basic research on the material and photoelectric properties of cuprous oxide.

At the basic research level, the simpler the material system, the more likely it is to accurately control the variables, thus obtaining more reliable results. Therefore, the team first thought of single-crystal thin films, which have a clear and orderly crystal structure and are less affected by various defects.

They drew on a thin film demolding process and innovatively developed a liquid-phase epitaxial growth method at room temperature, thereby obtaining a very valuable single-crystal cuprous oxide thin film material platform.The lead author of the paper, Pan Linfeng, a postdoctoral researcher at the University of Cambridge, said: "Due to the unique properties of epitaxial growth, by selecting the crystal orientation of the substrate, single-crystal thin films of cuprous oxide with any crystal orientation can be obtained."

Figure | Cuprous oxide thin film with anisotropic photoelectrochemical properties and mobility controlled by surface defects (Source: Nature)

Combining the advanced femtosecond laser transient reflection spectroscopy technology of the Cavendish Laboratory at the University of Cambridge, researchers for the first time precisely quantified the transport distance of charge carriers in the bulk of cuprous oxide thin films with various crystal orientations, thereby discovering the anisotropy of charge carrier transport.

Usually, the time resolution of transient spectroscopy technology is in the range of picoseconds to nanoseconds, which is not enough to observe the most important carrier dynamics parameters in oxide materials.

Due to the limitations of the device's layered structure, the team designed and customized advanced spectral instruments. In the subsequent data interpretation, the team also provided a Hilbert transform, thereby adopting a method of analyzing data using transient absorption.The study indicates that spectral techniques with high temporal and spatial resolution play a significant role in the research of optoelectronic devices. Moreover, it is expected to enhance the attention of scholars in the field of solar fuels towards spectral technology, said Pan Linfeng.

Thanks to these carrier dynamics parameters, researchers have achieved a new understanding of the data based on precise quantitative data, and have made a significant breakthrough in adjusting the crystal structure of semiconductor thin films.

It is worth noting, as described at the end of the paper: For the first time in this study, the optoelectronic property parameters tested are of extremely high value for various cuprous oxide optoelectronic devices. Among them, the research strategy for the anisotropy of optoelectronic properties is also universally applicable to various optoelectronic devices such as photovoltaics, detectors, and light-emitting diodes.

Therefore, this study provides a unique material platform for basic research, device construction, and exploration and application in the field of catalysis, including a complete set of templates from material preparation, material characterization, optoelectronic property testing, and property utilization.

At the same time, it provides an important strategy to solve the problem of short charge carrier transport distance in oxide semiconductors.The research team "joins forces" to take research to the extreme

In this study, many of the test data are "first-time", and the researchers hope to do their utmost to make the test standards and data reliable.

Therefore, after obtaining the single-crystal cuprous oxide thin film for the first time, they spent a lot of time optimizing the parameters to improve reproducibility. It turns out that although this approach is time-consuming and labor-intensive, it is very valuable.

This research was jointly completed by four research teams. Among them, the material science and electrical engineering research teams of the University of Cambridge mainly solved the characterization of crystallography, while the researchers from the physics department of the University of Cambridge mainly focused on the research of the photoelectric properties and carrier dynamics of single-crystal thin films.

The research team of the Federal Institute of Technology in Lausanne focused on the development of material preparation methods, material preparation and the construction of photoelectrode devices, and the main contribution of the researchers from Nankai University was to provide indispensable testing and characterization.Additionally, whether it is the characterization of material crystallographic parameters or the characterization of carrier dynamics using spectroscopic techniques, this is the first time they have been applied on this unique material platform.

In fact, various teams have been researching this material for many years, and the researchers conducting the characterization are experts in their respective fields. However, many problems and difficulties still arose during testing and data analysis.

To address this, they adopted communication methods suitable for different time scales, such as instant messaging office software, email, online meetings, and synchronized office work, to maintain efficient and high-quality communication.

"We maintain a very open and honest attitude, and we all strive to do our own work to the extreme, greatly improving the efficiency of problem-solving and providing a strong guarantee for the final high-quality results," said Pan Linfeng.

Recently, the related paper was published in Nature with the title "High carrier mobility along the [111] orientation in Cu2O photoelectrodes" [1].Postdoctoral researchers Linfeng Pan and Linjie Dai from the University of Cambridge are co-first authors.

Professor Samuel D. Stranks from the University of Cambridge, Professor Michael Grätzel from the École Polytechnique Fédérale de Lausanne, Professor Anders Hagfeldt, and Professor Jingshan Luo from Nankai University serve as co-corresponding authors.

Gathering the "last piece of the puzzle" for a comprehensive study of cuprous oxide photocathodes.

Linfeng Pan obtained his master's degree from East China University of Science and Technology, with Professor Huagui Yang as his master's supervisor.He subsequently obtained his Ph.D. at the Swiss Federal Institute of Technology in Lausanne, where he conducted research in the field of oxide solar fuels under the guidance of Professor Michael Grätzel, known as the "Father of Dye-Sensitized Solar Cells," and Professor Anders Hagfeldt.

Starting from 2020, he has been engaged in postdoctoral research in the research group of Professor Samuel D. Stranks at the University of Cambridge, focusing mainly on the anisotropic electronic and optical physics of semiconductors, especially oxide materials.

To date, he has published 26 papers in academic journals, which have been cited more than 3,000 times. During his Ph.D., he achieved remarkable results in both the electron extraction and hole transport ends of photoelectrodes.

At the electron extraction end, by using atomic layer deposition technology (ALD), an efficient coaxial p-n heterojunction was constructed on copper oxide nanowires, significantly improving photon absorption, charge separation, and extraction efficiency. At the same time, the highest photocurrent density and photovoltage in the world at that time were achieved.

On this basis, they also built and demonstrated a full-oxide, bias-free, stand-alone solar water splitting system, achieving a solar-to-hydrogen conversion efficiency of 3%, which was the highest record of its kind at that time. [2]On the hole transport side, Pan Linfeng and his collaborators employed a copper thiocyanate layer (CuSCN) with selective hole transport properties. Due to the presence of tail states in the energy level, the hole transport process has become very smooth. This has led to a significant increase in the fill factor of the photoelectric performance curve, ultimately achieving a record conversion efficiency of 4.5% from solar energy to hydrogen energy [3].

The focus of this study on the light absorption of cuprous oxide is the "last piece of the puzzle" in the comprehensive study of cuprous oxide photocathodes.

"After I reported the results at the MRS Spring Meeting, many colleagues present were very interested in the preparation of single-crystal thin films, and I also gave a detailed introduction to the key points," said Pan Linfeng.

It should be understood that in this study, although the stability of the cuprous oxide photocathode is excellent among all photocathodes, there is still a considerable gap from the requirements for practical application.

Therefore, the researchers plan to combine high spatial resolution spectroscopy technology to conduct in-situ observation and analysis, trying to explore the instability factors of the cuprous oxide photocathode, and make targeted optimizations.When discussing the future prospects of this technology, Pan Linfeng expressed his hope to apply the spectroscopy techniques learned at the Cavendish Laboratory to the research field of solar fuels in the future. While promoting the large-scale application of this technology, he aims to provide effective solutions for the country's "dual carbon" strategic goals.