Anomalous Photocathodic Behavior of CdS within the Urbach Tail Region Agnieszka Podborska, † Bartlomiej Gawel, † Lukasz Pietrzak, ‡ Iwona B. Szyman ´ ska, § Jeremiasz K. Jeszka, ‡ Wieslaw Lasocha, †,| and Konrad Szacilowski* ,†,⊥ Centrum Nanochemii Nieorganicznej nanoInchem, Wydzial Chemii, Uniwersytet Jagiellon ´ski, ul. R. Ingardena 3, 30-060 Krako ´w, Poland, Centrum Badan ´ Molekularnych i Makromolekularnych Polskiej Akademii Nauk, ul. H. Sienkiewicza 112, 90-363 Lo ´dz ´, Poland, Wydzial Chemii, Uniwersytet Mikolaja Kopernika, ul. Gagarina 7, 87-100 Torun ´, Poland, Instytut Katalizy i Fizykochemii Powierzchni Polskiej Akademii Nauk, ul. Niezapominajek 8, 30-239 Krako ´w, Poland, and Wydzial Metali Niez ˙elaznych, Akademia Go ´rniczo-Hutnicza, al. A. Mickiewicza 30, 30-059 Krako ´w, Poland ReceiVed: NoVember 6, 2008; ReVised Manuscript ReceiVed: February 10, 2009 A series of cadmium sulfide materials have been prepared using microwave-assisted hydrolysis of cadmium-thiourea complexes in water and nonaqueous solvents. Materials are characterized using X-ray diffraction, scanning electron microscopy, diffuse reflectance spectroscopy, and various photoelectrochemical techniques. Sulfur-doped samples of CdS exhibit a pronounced photoelectrochemical photocurrent switching effect within low-energy tails of their absorption spectra. A mechanism of the photocurrent switching process is presented on the basis of experimental investigations and quantum-chemical calculations. Introduction A bottom-up approach is a promising alternative for construc- tion of electronic nanodevices. 1-5 Molecular electronics and molecular logic devices may constitute a feasible alternative for classical, silicon-based electronics. 6-10 Among new materials suitable for construction of nanoscale switches and other devices, wide band gap semiconductors are materials of choice because of their stability and functional versatility. 11-13 Wide band gap semiconductors have found numerous applications in photo- voltaic cells, 14-20 electrochromic displays, 21 and photocatalysis. 22-26 Recently, several applications of nanocrystalline semiconductors as optoelectronic switches and logic gates were reported. 11,27-36 Photoactivity of wide band gap semiconductors can be significantly enhanced via bulk or surface modification. This may result in photosensitization, novel luminescent properties, a change in (photo)chemical reactivity, and others. 12,14,20,26,30,37-40 There are two main strategies of semiconductor modification: bulk doping and surface modification. In principle, bulk doping yields materials of higher chemical and photochemical stability, while surface modification offers enormous versatility of systems due to unification of cooperative properties of solids with the structural versatility of molecules. The surface complex formed via chemisorption of various transition metal complexes on surfaces of mesoporous and nanocrystalline wide band gap semiconductors usually plays the role of light-harvesting antenna, while the semiconducting structure acts as a charge separation device and provides mechanical support for the photosensitizer. 14,41,42 Classical semiconductor devices are built on the basis of bulk modification of semiconducting structures via p- and n-doping. On the other hand, almost all of the wide band gap optoelec- tronic devices reported so far are built on the basis of surface- modifiedtitaniumdioxide 11,27-34 andsemiconductorcomposites. 35,36,43-52 In most of the systems, the photocurrent switching processes areassociatedwithselectiveexcitationofap-typecomponent 35,36,46-49,51,52 or activation of alternative electron transfer pathways, 11,28-34,43-45,50 usually involving oxygen or other electron acceptors. Photo- current switching phenomena for other semiconductors (CdS, CdTe, and Se) are rarely reported, and there are no mechanistic data on these processes. 53-58 In the case of CdS monocrystals, these effects are usually attributed to surface contamination and damages, especially associated with elemental sulfur resulting from oxidation of the surface. Similar effects were observed after contamination with aluminum. 58 Single phase, bulk-doped semiconductors should offer supe- rior performance and stability with respect to surface modified semiconductors and semiconducting composites. There are several nanoelectronic devices based on switchable wide band gap semiconductor structures. 59-66 Furthermore, numerous oxide semiconductors are used as novel memory components 67,68 and for flexible macroelectronic devices. 69 On the other hand, photocurrent reversal processes in solar cells result in decreased photocurrent efficiencies and poor overall performance. 54 These parasitic processes should be avoided, which implies a good understanding of the underlying photoelectrochemical processes. Therefore, this study concentrates on the photoelectrochemistry and switching phenomena of cadmium sulfide. Cadmium sulfide is an n-type semiconductor, mainly due to sulfur vacancies. 70 It can be conveniently prepared by several chemical methods. Simple precipitation of CdS from aqueous solutions using soluble sulfides may lead, however, to unho- mogeneous samples of variable crystallinity. These materials may require subsequent thermal processing in order to yield crystalline materials. Other methods require various soluble cadmium coordination compounds with sulfur-containing ligands. Controlled decomposition of these complexes results in high- quality nanocrystalline powders and thin films. 71 Chemical bath deposition, 72 electrodeposition, 73 and self-propagating combus- * Corresponding author. E-mail: szacilow@chemia.uj.edu.pl. Fax +4812 632 0515. † Uniwersytet Jagiellon ´ski. ‡ Centrum Badan ´ Molekularnych i Makromolekularnych, Polskiej Aka- demii Nauk. § Uniwersytet Mikolaja Kopernika. | Instytut Katalizy i Fizykochemii Powierzchni, Polskiej Akademii Nauk. ⊥ Akademia Go ´rniczo-Hutnicza. J. Phys. Chem. C 2009, 113, 6774–6784 6774 10.1021/jp809794s CCC: $40.75  2009 American Chemical Society Published on Web 03/26/2009