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Report

Teaser, summary, work performed and final results

Periodic Reporting for period 1 - NanoAID (Advanced In-situ Techniques for the Development of Metal Oxide Nanostructures.)

Teaser

Summary

We set out to develop advanced in-situ experimental approaches to study the formation of nano-crystalline metal oxide thin films and colloidal nanocrystals to be used in solar water splitting devices. At the time of application, there did not exist an established approach to thoroughly study the formation of nanocrystalline thin films at elevated temperatures. Also the fundamental understanding of mechanisms governing colloidal synthesis of complex nanocrystals was limited. Nanocrystalline morphologies of photocatalyst thin films present an opportunity to enhance the performance of photoelectrochemical cells used for solar water splitting. The search for new photoabsorbers with nano-scale morphologies is ongoing, and our goal was to develop tools to accelerate the searches and facilitate optimization of the fabrication process. We planned to build custom in-situ sample cells to study the formation of thin films and nanocrystal seeds using X-ray scattering techniques, which offer the adequate structural information and speed of data acquisition, especially considering synchrotron sources. The cells we used in a number of experimental campaigns to study nanomaterials in the Cu-Mn-V-O systems. The work contributed to the general understanding of the formation mechanism of nanocrystalline thin films and how to control the properties of the products. We have also uncovered mechanisms in colloidal synthesis of nanocrystals, which can be used as seed in production of thin films. The results provide a valuable contribution both to the fundamental understanding of the physico-chemical processes, and to the ongoing search for photocatalysts.

Work performed

Experimental non-ambient cells were designed and build: two high temperature cells for in-situ X-ray scattering measurements on thin films and powders, and one glass cell for in-situ studies of colloidal synthesis by X-ray absorption and scattering. We have studied fundamental thermodynamic properties of the Cu-V-O ternary system, in which a number o promising photocatalysts were identified. We used the acquired knowledge to study the formation of Cu-V-O nanocrystalline thin films, and optimized the process to obtain single-phase products. The resulting thin films with slightly varying crystallite sizes were tested as photoanodes in water cleavage. The results indicated that the performance is not on par with what had been predicted theoretically. We have collaborated with a theoretical group at the host institute to look for the cause, and found the presence of strong excitonic interactions between charge carries, which was confirmed by modelling and experimental data. We also studied the possibility of creating a heterojuction between two nanocrystalline metal oxides to extract photogenerated charge carriers more efficiently. We found limited improvement in optimized samples with the junction. We also studied colloidal synthesis of Cu-V-O nanocrystals at the synchrotron. We identified the reaction mechanism but could not optimize the synthesis to achieve a pure, monodisperse product. We have also studies the synthesis of pure Cu nanocrystals, and identified the two mechanisms of syntheses previously reported in the literature with little explanation of the underlying processes involved in the nucleation. We generalized our results, and are currently working on disseminating the work.

Final results

The cells developed as part of the project go beyond the state of the art in several ways. First, the in-situ heating cell for thin film studies has a passive dilatation-minimizing mechanism, which makes grazing incidence scattering experiments at elevated temperatures easier due to limited calibration. It is also light-weight, and can be attached both on in-house X-ray diffraction instruments and on goniostats available at synchrotron beamlines. The second cell used for in-situ studies of colloidal synthesis offers unrivaled flexibility in terms of the reactions that can be studied. The advanced materials that were used in the construction allow for heating up to 300C, measuring X-ray absorption at energies as low at 5keV, and greatly facilitate sample preparation, which is of key importance with limited beamline access. The cell also includes a mechanism that enables remote modification of the flight-path during the experiments, which enable facile calibration and tailoring the sample environment to the constraints given by the studied sample. The two cells enables us to study the formation of thin films and colloidal synthesis of nanocrystals in precisely and rapidly, which is key for the development of screening approaches in material searches.

We have also proposed a new synthesis approach for thin films with precisely controlled grain sizes at the nanoscale. Using colloidal metal nanocrystals with exactly controlled sizes as seeds, we were able to fine-tune the crystal domain size in the resulting thin films after annealing. The grain size is directly linked to the performance of photocatalysts, and fine-tuning this parameter has been previously very limited.

Moreover, we have identified a significant limitation in the class of copper vanadates and copper-containing oxides for use as photoanodes. Through a combined theoretical and experimental study we revealed that strong excitonic interactions, arising due to the presence of copper in a certain structural arrangement, strongly limit the performance in this class of materials.

We have also contributed to the fundamental understanding of a certain class of colloidal synthesis of nanocrystals. Using the in-sity cell we identified the exact mechanism of nucleation via disproportionation of a metallo-organic complex, commonly used to obtain monodisperse nanocrystals with different shapes.