The method of optically induced photocurrent transients is a powerful tool for probing the mechanisms of photoinduced charge transfer at the semiconductor/electrolyte interface. Using this technique on the Ti02/aqueous interface, we conclude that water oxidation occurs via an outer-sphere process for electrolyte pH <12 and an inner-sphere mechanism at more basic pH's. Hydrogen isotope effects on the observed transient substantiate this finding. These isotope effects along with selective poisoning studies suggest a "Ti-OH" surface species as the key intermediate for the inner-sphere channel, along with the cleavage of a hydrogen bond as the rate-limiting process (H/D kinetic isotope effect of ~10) in basic electrolyte. These studies further indicate a strong crystal face dependence on the observed photoelectrochemistry.

Crystallographic methods provide the details of molecular structure. Many chemistry texts show ball-and-stick pictures. electron densitv maps. and stereo diagrams based on crystsl structure det&minations. Most chemistry texts have sections devoted to crystals and the determination of their structures by diffraction methods. Many of these eo well beyond introducing the 14 Bravais lattices and the ~r a~~ law. By the time theycomplete their undergraduate training, most chemists have heard the statement: "This configuration has been proved by X-ray crystallographic analysis." Yet probably few have an idea of how that crystal structure determination is done. Though laboratory experience is often the hest way to learn. few crvstalloera~hic experiments are simple enoueh for most undkgrad;aies to perform. ~xperiments designed for the ohvsical cbemistw laboratorv are usuallv restricted to dem&trating the diffraction phenomenon for a cubic crystal using the powder method (1-4). Measurement of the diffraction angles, 28, and use of the Bragg law leads to the identification of the Bravais lattice and the determination of the single lattice constant needed to define the cubic unit cell. This may allow the determination of interatomic distances, since the atomic positions in most very simple crystals, such as NaC1, many pure metals, and even the somewhat more complex diamond, are fixed a t crystallographic special positions in the unit cell by symmetry constraints. Thus. unit cell dimensions alone mav be used to calculate interatomic distances. For example, "the 1.5445-A carboncarbon bond length in diamond is just (,/3)/4 times the 3.5669-A lattice constant. Most atom positions, however, are not fixed by the crystal's symmetry. The information needed to determine the atom positions and displacements caused by thermal motion comes from the relative intensities of the diffracted reflections.Several other experiments require specialized equipment, considerable crystallographic expertise on the partof the instructor, and frequently several weeks of class or lahoratorv time. A sinele crvstal camera can he utilized to determine the lattice coktants of an orthorhombic crystal and determine restrictions on nossible soace erouvs from the observed -.systematic extinctions (51, b A this requires both personnel and equipment available only in crystallographic laboratories. Several possibilities exist for advanced undergraduate students where a large block of time and the necessary specialiled equipment and/or computer programs are available (fi-8). They do closel\~ simulate real rrystal structure determinations; hut most-chemistry students will probably not take a course where these are used.This experiment attempts to overcome the shortcoming of the simpler experiments by using both diffraction-angle and diffraction-intensitv information to determine the lattice constant and a lattice independent molecular parameter, while still emolovine standard X-rav powder diffraction . " -. .techniques. The experiment requires only a little more st...

ChemInform Abstract It is demonstrated that the method of optically induced photocurrent transients is a powerful tool for the study of the mechanisms of photoinduced charge transfer at the semiconductor/electrolyte interface. The results obtained for the (001) nTiO2/aqueous electrolyte interface suggest a model for water oxidation which involves two pH sensitive pathways. In the pH range 0-11, water oxidation appears to occur via an outer-sphere channel. From pH 12 to pH 15.5 (10 M NaOH) hydroxide oxidation follows an inner-sphere reaction channel. Based on the transient response as a function of hydrogen isotope, along with the activity of nonelectroactive competitive ligands (e.g., MeCN), the inner-sphere pathway is suggested to occur via a "Ti-OH" surface species as the key intermediate.