Nanophase Evolution at Semiconductor/ Electrolyte Interface in Situ Probed by Time-Resolved High-Energy Synchrotron X-ray Diffraction Yugang Sun,* ,† Yang Ren, ‡ Dean R. Haeffner, ‡ Jonathan D. Almer, ‡ Lin Wang, § Wenge Yang, § and Tu T. Truong † † Center for Nanoscale Materials and ‡ X-ray Science Division, Advanced Photon Source, Argonne National Laboratory, 9700 South Cass Avenue, Argonne, Illinois 60439 and § HPSync, Geophysical Laboratory, Carnegie Institute of Washington, 9700 South Cass Avenue, Argonne, Illinois 60439 ABSTRACT Real-time evolution of nanoparticles grown at the semiconductor/electrolyte interface formed between a single crystalline n-type GaAs wafer and an aqueous solution of AgNO 3 has been studied by using high-energy synchrotron X-ray diffraction. The results reveal the distinct nucleation and growth steps involved in the growth of anisotropic Ag nanoplates on the surface of the GaAs wafer. For the first time, a quick transit stage is observed to be responsible for the structural transformation of the nuclei to form structurally stable seeds that are critical for guiding their anisotropic growth into nanoplates. Reaction between a GaAs wafer and AgNO 3 solution at room temperature primarily produces Ag nanoplates on the surface of the GaAs wafer in the dark and at room temperature. In contrast, X-ray irradiation can induce charge separation in the GaAs wafer to drive the growth of nanoparticles made of silver oxy salt (Ag 7 NO 11 ) and silver arsenate (Ag 3 AsO 4 ) at the semiconductor/electrolyte interface if the GaAs wafer is illuminated by the X-ray and reaction time is long enough. KEYWORDS Time-resolved X-ray diffraction, in situ probing, galvanic reaction, silver nanoplates S emiconductor/electrolyte interfaces are important in a broad range of applications including photovoltaics (e.g., photoelectrochemical cells), 1-3 photocatalysis, 4,5 and chemical/biological sensing. 6,7 If a semiconductor is reactive toward an electrolyte that is in contact with the semiconductor, reaction(s) at the semiconductor/electrolyte interface might result in the formation of nanoparticles on the semiconductor to modify properties of the interface, thus affecting its performance. For example, illuminating TiO 2 nanoparticles immersed in a solution of noble metal ions with ultraviolet (UV) light can induce photoreduction of metal ions on the TiO 2 surfaces, resulting in the deposition of metal nanoparticles on the TiO 2 nanoparticles. 8 Placing an n-type semiconductor wafer (e.g., Si, GaAs, InP, Ge) in contact with an aqueous solution of noble metal ions (e.g., AgNO 3 , AuCl 4 - , PtCl 6 2- ) usually initiates a spontaneous galvanic reaction between the wafer and the metal ions, leading to the deposition of metal nanoparticles on the surface of the wafer. 9-12 Modification of the semiconductor surfaces with metal nanoparticles can significantly influence charge trans- fer processes and chemical reactions at the semiconductor/ electrolyte interfaces when the composite semiconductors are used as functional components in the aforementioned applications. 4,13-16 In general, effect of the metal nanopar- ticles strongly relies on their size, morphology, composition, and crystalline structure, which are determined by the reaction conditions and growth time. 17 As a result, develop- ing time-resolved techniques that are capable of in situ probing the evolution of nanoparticles formed from reac- tions at semiconductor/electrolyte interfaces is very impor- tant for understanding the dependence of nanoparticles on the reaction conditions as well as for controlling the param- eters and properties of the resulting nanoparticles. In the past decade, many in situ experiments have been attempted by monitoring the variation of tailorable properties (e.g., optical absorption and scattering, electrical conductivity) of the nanoparticles. However, intrinsic parameters (e.g., crys- talline structure, chemical environment, size, and shape) of the nanoparticles that determine their tailorable properties are difficult to probe in the real time because characterizing these parameters usually requires high vacuum techniques (such as electron microscopy and X-ray photoelectron spec- troscopy) that are not compatible with liquid electrolytes. Most recently, thin cells have been specially designed for studying liquid samples in transmission electron microscope (TEM). 18,19 Preliminary results show that this in situ TEM technique can monitor variations of nucleation population and particle size during the growth of nanoparticles. The obtained images, however, cannot reveal the detailed struc- tural and morphological information of the nanoparticles. * To whom correspondence should be addressed. E-mail: ygsun@anl.gov. Received for review: 07/14/2010 Published on Web: 08/03/2010 pubs.acs.org/NanoLett © 2010 American Chemical Society 3747 DOI: 10.1021/nl102458k | Nano Lett. 2010, 10, 3747–3753