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Of primary importance is the development of efficient means to harness the huge energy potential of solar irradiation and use it to drive chemical reactions or convert it into stored chemical energy.  To meet this goal, our support from ACS-PRF has been directed towards the aerosol synthesis of photocatalysts that are i) compositionally- and/or ii) architecturally-diverse. Two avenues in research in particular have flourished, as summarized below.

Part A.  As outlined in the proposal, we sought to validate nitrogen-doped M_1xO_y/M_2xO_y composites prepared by an electrospray method as a new class of Z-scheme photocatalysts in which the nitrogen-doped unit would enhance visible light absorption while composite formation would enhance electron-hole separation (see Figure 1A).  Initial synthetic targets included nitrogen-doped In₂O₃/TiO₂ and nitrogen-doped TiO₂/SnO₂.  Urea, NH₄OH, and NH₄Cl were found to be suitable nitrogen sources; however, the high concentrations required for the syntheses inhibited the formation of high-quality samples by electrospray.  Thus, a solution based route towards these composites has been developed instead. 

Both targets have been successfully synthesized by adding preformed nanoparticles of the undoped M_2xO_y component to a sol-gel precursor solution for the nitrogen-doped M_1xO_y component.  After gelation and solvent removal, the composite is annealed to crystallize the nitrogen-doped M_1xO_y phase.  Electron microscopy and X-ray powder diffraction (XRD) confirmed the formation of the composites, while EPR confirmed dopant incorporation (Figures 1B-E).  Analysis by X-ray photoelectron spectroscopy (XPS) is underway.  The photodegradation of rhodamine B has been selected as a model system to evaluate their photocatalytic potential.  Preliminary results indicate a strong dependence on the ratio of the two composite components under UV-visible irradiation, with the nitrogen-doped component serving as a sensitizer at low concentrations.  At higher concentrations, performance decreases presumably due to the greater number of recombination sites associated with the nitrogen-doped phase.  A full paper will be submitted after catalyst testing is completed. 

Part B.  Owing to the synthetic challenges encountered when electrospray was employed, we switched to ultrasonic spray pyrolysis as a means of aerosol generation for the synthesis of compositionally and architecturally-diverse photocatalysts.  Bi₂WO₆ was selected as the initial synthetic target given its promise as a visible light photocatalyst to be incorporated into Z-scheme composites for overall water splitting. 

To produce compositionally complex materials such as Bi₂WO₆, precursors are typically selected so that upon pyrolysis, only the desired material and small gaseous byproducts are generated.  This selection leads to the formation of polycrystalline microspheres.  However, precursors capable of metathesis were selected (BiOCl  +  Na₂WO₆  à  Bi₂WO₆  +  NaCl) in order to generate a non-transient byproduct salt for templating.  Surprisingly, Bi₂WO₆ nanoplates were produced rather than porous particles, which contrast greatly with the microspheres obtained when conventional precursors are employed (Figure 2).  Differential scanning calorimetry and thermal gravimetric analysis of the various precursor combinations indicate that the different decomposition pathways and generated byproducts account for the varying particle structures.  The synthesis of shape-controlled particles provides a platform for elucidating structure-function properties.  Thus, both the Bi₂WO₆ nanoplates and microspheres were evaluated as photocatalysts for the degradation of rhodamine B and compared to Bi₂WO₆ prepared by solid-state heating of the parent oxides.  While the nanoplates had the highest surface area, the microspheres facilitated the greatest rate of dye degradation.  XPS analysis and dye adsorption studies indicate that the expression of hydrophilic surfaces, as is the case with the Bi₂WO₆ microspheres, enhances photocatalytic performance.  A manuscript has been submitted and the versatility of this approach to other shape-controlled nanostructures is being probed.

Broader Impact.  The current ACS-PRF grant has been effectively used to attract and support the work of one postdoctoral associate and two graduate students.  The PI and postdoctoral associate presented results from these projects at the spring and fall 2009 MRS conferences, respectively.  The postdoctoral associate has now started her independent career as a faculty member at a PUI.  One graduate student recently completed his master's degree in chemistry.  The second graduate student presented results at the 2010 CERMACS conference.  These preliminary results formed the basis of the recently funded NSF CAREER Award proposal (DMR-0955028), which will financially support this work further.