The goal of this work was to observe the direct dissociation of a water molecule induced by a photon. This should be be achieved by employing small metal clusters as novel photocatalysts. The method used for analyzing their catalytic activity was the investigation of metal-water clusters in the gas phase by means of time-resolved photoelectron spectroscopy (TRPES). This method yields information about the evolution of excited electronic states of the clusters. The occurrence of new peaks in the TRPES spectra indicates a dissociation of the investigated system.
The first step in the search for novel photocatalysts was to produce small metal clusters in the gas phase, which has been achieved by employing a pulsed arc cluster ion source (PACIS). In order to investigate the photocatalytic activity, water molecules were subsequently adsorbed to the clusters. The range of metal-water clusters produced with the here presented experimental setup were determined via mass spectrometry. The success of this method is evident in section 5.2, where mass spectra of water-metal clusters are presented. Several metals were tested, amongst which small gold clusters were identified as the most promising candidates. The case of silver and copper clusters showed the limitations of the PACIS to produce pure metal-water clusters. Since for these materials the atom-water clusters are less stable than atom-hydroxide clusters, mostly the latter has been observed. A further limitation of investigating these types of clusters with the current experimental setup lies on the mass resolution. The distinction between hydroxide and metal-water, which have a mass difference of one atomic mass unit with respect to each other, is only possible for small clusters. This sets the size limit for a conclusive investigation of pure metal-water clusters to the mass of about 500 atomic mass units.
The next step to investigate of the electronic ground state of the produced metal-water clusters with photoelectron spectroscopy. The ground state of the bare metal cluster is shifted in energy to a higher value upon water adsorption. This shift can be interpreted as stabilization of the probed state. As an example, mea-surements for various sizes of gold clusters with different amounts of adsorbed water molecules provided a comprehensive picture of this adsorption process. The shift in
The final step in this investigation was to search for excited states and to screen the time-resolved measurements for dissociation products. Excited states of various metal-water clusters have been measured. In the cases of AgOH-(H2O)m
(m=7...10) and Au-(H2O)3, long lived excited states were observed, which represents a precondition for water dissociation to occur. However, in these cases no evidence of dissociation could be found.
The main obstacle for the dissociation to occur is that the H-OH bond is stronger (5.1 eV [98]) then the available photon energies (maximal 4.65 eV), which inhibits the simple breaking of the bond. However, a closer thermodynamic and kinetic inspection of the feasibility of water dissociation with the aid of small metal clusters provides insight into this process and helps to search for possible candidates for photo induced water dissociation.
In this work, these evaluations have been performed for the case of the Au-(H2O)m
system. Considering this system as a free semiconductor, comparisons with the model for photocatalytic water dissociation have been made. If the ground state has a binding energy below the value -5.7 eV, the hydroxide reduction is possible, after the excitation of one electron out of this state. Since a photon energy of 3.1 eV has been used to excite the electron, it ended up in a state above the energy of -4.6 eV, where the proton reduction is possible. However, these requirements for the overall dissociation of water are for photocatalysts in bulk water. In the case of the Au-(H2O)m cluster system, PES measurements suggest that these requirements might be fulfilled with many adsorbed water molecules (~24), which is reasonably close to the bulk limit. Nevertheless, for isolated clusters with only a few adsorbed water molecules, the potentials for oxidation and reduction may even lie at different energy values. For the evaluation of the kinetic situation of Au-(H2O)m clusters TRPES measurements were performed, which have shown that the adsorbed water molecules are essential for the existence and evolution of the bound excited states.
Thus being a transition from the ground state into a state within the water shell.
Therefore the kinetic requirement would be fulfilled due to the fact that the excited
electron is immediately transferred to the water molecules. These two evaluations suggests that a systematic analysis of the thermodynamic and kinetic situation of clusters can lead to systems where water dissociation could be possible.
The Au3-(H2O)m (m=0...3) cluster system represents a yet more interesting example to study water dissociation with the aid of metal clusters. The bare Au3
-cluster dissociates upon absorption of one photon. Adsorbing one water molecule resulted in a dramatic reduction of the time constant of this dissociation. The TR-PES spectra measured here suggest that, during the dissociation of the gold cluster, the water molecule had also dissociated. The reason for this water dissociation to occur lies on the fact that the binding energy of the resulting dissociation products combined is higher then the binding energy of the H-OH bond in water. A theoreti-cal investigation is currently taking place to shed light on this claimed dissociation.
Further experiments showed that the adsorption of additional water molecules seems to close the dissociation channel, most probably due to the introduction of new and fast relaxation channels, like internal conversion for instance, where the energy is lost to vibrational modes of the metal-water cluster system.
The investigation of photocatalysts described in literature has been realized with solid state catalysts in aqueous environments. There, the individual dissocia-tion processes are not directly accessible by any experimental method. A very recent idea to obtain a photocatalyst for water dissociation is to employ porous nanoparti-cles consisting of a semiconducting material like TiO2 or ZnO2 [102]. This material allows for generation of charge carriers and reduction and oxidation catalysts are embedded within the material to achieve an efficient overall water splitting. In terms of structure, these porous nanoparticles can be envisioned as being constituted of small building blocks of the semiconducting material.
As a future research idea, the experimental setup presented in this thesis can be used to produced these building blocks and to investigate their relaxation time constants. The adsorption of water molecules to them might reveal their influence on the excited state lifetimes. In the next step, reduction or oxidation centers might be incorporated into these building blocks, allowing access to the single processes of water dissociation. A comparison between this suggested investigation and the excited state lifetime measurements of actual catalysts could provide new insight in the way such catalysts work. These results might yield invaluable information to improve the design of photocatalysts that could render them applicable to the energy industry.