Adatoms?

A few weeks ago, I attended a talk given by Professor John Yates at the local Michigan Catalysis Society meeting. John Yates (University of Virginia) is a well respected figure in the area of surface chemistry and catalysis. I hadn’t heard him speak before, but I was looking forward to it since, according to a friend of mine, John could make a talk on life insurance interesting. I was not disappointed. On a side note, he looks pretty damn good for someone in his 70's. His talk centered on self-assembled thiol complexes on gold surfaces. Interested parties might read the following paper: P. Maksymovych, D. C. Sorescu and J. T. Yates, Jr., “Gold-Adatom-Mediated Bonding in Self-Assembled Short-Chain Alkanethiolate Species on the Au(111) Surface,” Phys. Rev. Lett. 97, 146103 (2006).

John spent a fair amount of time discussing gold “adatoms.” What are adatoms, you ask?

No. Not this guy.

An adatom is an “adsorbed atom” and is usually described as a single atom sitting on a crystal surface. Adatoms have been around for years, but I’d never heard of the term before, despite the fact that I’ve apparently been making them for years. Simply exposing Pt to a hydrogen molecule, for example, will lead to the formation of hydrogen adatoms on the Pt surface. Of course, many of us who work on catalyst development would not think of these as adatoms, since in our line of work, exposure to even tiny amounts of hydrogen would essentially cover the entire surface of the platinum with a layer of hydrogen atoms, which doesn’t really fit the adatom definition.

What I found interesting was that Au, Ag, and Cu have the ability to generate their own adatoms. Due to structural strains in the crystal packing of these metals, individual atoms tend to pop out of the bulk to form single atoms on the surface. Assuming I understand this correctly, these adatoms tend to appear in a regularly repeating two dimensional pattern across the surface. Molecules (such as thiols) can bind to these adatoms, creating a surface layer with unique chemical properties. (Note: It’s possible that exposure to these binding molecules actually helps cause the adatoms to appear on the surface in the first place, but I don’t know. Perhaps someone else can answer that question.) Interesting chemistry indeed.
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I see that I passed the one year anniversary of this blog back in January. It’s unfortunate that it occurred during a dry spell, but I’m happy the blog is still going. The percentage of blogs that last more than a year is fairly low, so I can’t complain.

First Week Back At Work

Well, I'm a week into my new job and things are beginning to settle down.

A little.

Even though I'm working for the same company as before, I'm working at a totally different location on a totally different project under a whole new set of rules and regulations. Although I know I'm working on fuel cells, my specific duties have yet to be fully determined. One of my initial assignments is to get the various testing facilities up and running, but I can see that I'm going to be needed in many different areas. The manager of the project has apparently decided to let me decide where I should be focusing my efforts. Hopefully I don't disappoint. I'm certainly not going to be bored. Here are the pros and cons so far:

Pros
--I'm getting paid. Yeah!!!!!!!! (Come on, what did you expect?)
--The project looks like fun. There's a lot of interesting chemistry happening here.
--Sorry, that's all I've got.

Cons
--Long commute. It's a 55 minute commute each way now, but it could be worse, so I won't complain too much. Some of my coworkers have even longer commutes. Such is your fate when you work for a company in the midst of a significant downsizing.
--No phone number, computer logon ID, email address, or computer! I have to request these things and the response time is yet to be determined. I'm currently borrowing a computer for making notes and browsing the net, but someone else has to log me in on their account every day for this to work. The lack of ID means I can't be the first person to arrive in the morning or the last person out at night, else I'll be setting off alarms.
--There are no vending machines here! Not even for soda! Not enough people here to support them I suppose, but I was under the impression that OSHA regulations required their presence at all work facilities. ;)

I previously wrote that I needed more structure in my life in order to keep up with this blog. Beware of what you wish for! I definitely have structure in my life now, but at the expense of free time. We'll see how this translates into blog entries.

Photo Chemistry

A few weeks ago, my wife began taking a photography class at a local college. Although her real interest is in the area of digital photography, course prerequisites require her to take an initial course using real film. One of the things the course is designed to teach is how to develop your own film, so she's getting some hands-on experience in some basic chemistry. She may never use any of this knowledge, but she does seem to be enjoying the class.

In any case, she's reached the section of the textbook which covers the chemical aspects of film development and she's already asked me for help on her homework questions. Unfortunately, the questions turned out to be a little bit more difficult than I expected. This was due in part to the fact that the chemistry questions were written by someone who obviously was not a chemist, which always adds to the degree of difficulty. This meant I spent a lot of time trying to determine exactly what the instructor "thought" he was asking and what type of answer he was hoping for. (Now I don't mean to suggest that the non-chemist is always to blame in situations like this. Sometimes it's the chemist who is the problem. I recall trying to answer a Trivial Pursuit question many years ago which read "Glass is made out of what?" The answer, of course, was "sand", but all I could think of were answers like "silica" or other more esoteric chemical terms, despite being given the hint not to think like a chemist.)

The other difficulty arose when I came to the realization that I knew less about the chemistry of film than I thought. I knew that silver halide salts (the main ingredient in film) are light sensitive, decomposing to black silver metal upon exposure to light. After all, that's why silver salts are always shipped in dark brown bottles. And I had known about the role of sodium thiosulfate in the fixer (dissolving and removing unreacted silver halide) since I was a freshman. But what did the developer do? And what is a stop bath?

Apparently, the initial exposure to light only reduces a small fraction of the silver halide in the film -- not nearly enough to make a negative. The developer operates by magnifying the amount of silver reduced to the metallic state. The developer contains organic compounds (such as hydroquinone or p-aminophenol) which are good at reducing silver halides in the film, but only when catalyzed by the presence of small metallic silver clusters (such as Ag₄^o) which were formed during the initial exposure to light. The grain size of the silver halide particles are an important determiner of the amount of silver reduced by the developer, and this fact is used to produce films with different speeds.

And the stop bath? It's basically a solution of acid which lowers the pH of the developer and essentially halts the further reduction of silver halide.

At least my wife is experiencing the joys of using graduated cylinders in her lab. Maybe I'll make a chemist out of her yet!