60th Annual Report on Research 2015

In the meanwhile, several inorganic low-dimensional crystals (analog to the prototypical organic graphene) are known. A technological important example is two-dimensional (2D) silica films, called silicatene. The molecular structure is well described in the literature [Freund, Goodman]. However, much less is known about the surface chemistry of silicatene. Therefore, the general object of the current two-year project, which is in its first year, is to further characterize the surface chemistry of silicatene.

Specifically we proposed as our objective to synthesize 2D zeolite-like films, deposit Mo clusters on the porous zeolite material, and use thiophene as a probe molecule to map the catalytic activity of the system for hydrodesulphurization catalysis. Note that these thin films are grown on Mo single crystal supports. Mo is a well-known catalyst for hydrodesulphurization. The original proposal pointed out that our project will focus on the synthesis of these films which is a significant challenge.

Obviously, a necessary prerequisite of our project is to first synthesize 2D silica films and then aluminum doping these films to fabricate 2D zeolites. Developing that skill will open up for our group a large variety of possible future projects on a new class of materials (inorganic graphene, silicatene). Surface science joins more and more with materials science. Therefore, synthesis skills become important for the next generation of surface chemists which we educate in our lab. The participating students will gain those skills. Hydrodesulphurization catalysis is petroleum related.

Unfortunately and in contrast to epitaxial graphene, the nanofabrication of silicatene in ultra-high vacuum is rather tricky and cumbersome; some variations of preparation receipts can be found in the literature. We proposed to proceed along the following strategy
1) Synthesize/fabricate 2D zeolite-like films (“Al-doped silica films”), which requires first to synthesize 2D crystalline silica; 2) Use thiophene to probe catalytic activity by means of surface chemistry techniques; 3) Deposit Mo on/in the porous support targeting hydrodesulphurization–related (HDS) surface chemistry as petroleum-related research.

In the meanwhile we have succeeded with the first project step namely synthesizing silicatene. Despite the complex synthesis, the success of the nanofabrication is, in our opinion however, consistent to the observation of a hexagonal LEED (low energy electron diffraction) pattern. Amorphous films do not generate a LEED pattern; the Mo substrate has a rectangular unit cell opposed to the hexagonal pattern of silicatene. Whereas STM (scanning tunneling microscopy) micrographs provide more details, LEED, AES, and XPS sample the macroscopic surface structure important for catalysis.

However, the synthesis is further complicated by reports that oxygen-rich and oxygen-poor structures can be made which apparently cannot be distinguished using LEED. Similarly, atomic defects and Si clusters on top of these films are not easy to identify. Therefore, as a further characterization we collected thermal desorption spectroscopy data of water since the hydrophobicity of the film depend e.g. on the defect density. Indeed, it turned out the hydrophobicity depends on the very details of the film preparation and defect density. Only well-ordered films are perfectly hydrophobic. Thus, water TDS may be used as a very sensitive chemical probe technique for film quality when STM characterization is not available. Water TDS is more sensitive than standard LEED. Besides LEED, the silica films were characterized by XPS (X-ray photoelectron spectroscopy) and Auger electron spectroscopy, in agreement with prior studies. We succeeded to fabricate silicatene.

Our preparation variation, did consist of a thorough UHV cleaning of the Mo(112) support and subsequent formation of an oxygen overlayer structure by oxygen exposure (method I). Next, Si vapor deposition and high temperature (>1150 K) oxygen annealing cycles were applied, until the typical hexagonal LEED pattern appeared. Literature describes receipts where the last annealing(/flashing) step was done in UHV (method II). In addition, we used another similar synthesis variation (method III), a one-step process, which also was described in the literature before.  Only method III resulted in our project in perfectly hydrophobic silicatene.

Because samples have now been made successfully the next steps of studying thiophene and HDS surface chemistry should precede much faster.

The first draft supported by this grant was just submitted for publication. This draft concerns sample preparation and characterization by water adsorption. Experiments with thiophene are on the way right now.

Fig. 1: (color online) water TDS and LEED images of silicatene. Although the quality of the LEED images may be comparable, according to TDS, one silicatene surface is hydrophobic (0-th order desorption, low temperature edges lining up) the other one hydrophilic.