INTRODUCTIONGoethite (α-FeOOH) is a semiconductor (bandgap 2.10À2.50 eV 1,2 ). It is abundant at and near Earth's surface and forms as nanoparticles as the result of mineral weathering and neutralization of acid mine drainage. During nucleation, crystal growth, and aggregation, nanogoethite particles can adsorb oxyanions, toxic metal cations, and organic molecules. In sunlight, nanogoethite can photochemically oxidize adsorbed organic molecules, leading to its reductive dissolution. 3 The goethite band positions and bandgap determine the generation of electrons and holes and, thus, the chemical/photochemical redox behavior. Usually, the bandgap of semiconductor nanomaterials (such as nano-CdSe 4 and ZnS 5 ) increases with decreasing particle size due to quantum size effects. 6 For example, goethite nanorods have shorter UVÀvis absorption wavelengths than do micrometer-scale rods, 3 implying a blue-shift in the bandgap. In contrast, it was also reported that the bandgap of a nanogoethite is smaller than that of bulk goethite. 7 To investigate this apparent inconsistency, we systematically explored the particle size dependence of the bandgap of goethite. The size-dependence of the goethite bandgap predicts the sizedependence of its reactivity in the environment 3,8 and is also relevant for technical applications (e.g., lithium battery electrodes and solar energy devices 9,10 ). ' EXPERIMENTAL SECTIONSyntheses of Nanogoethite. The 8.7 nm goethite nanoparticles (size obtained from Rietveld analysis, as described below) were synthesized using the method in ref 11. Eighteen milliliters of 2 M Fe(NO 3 ) 3 3 9H 2 O was mixed with 70 mL of 1 M NaOH, and then with 70 mL of deionized (DI) water. The reacted mixture was aged at room temperature for 49 days. The suspension was centrifuged to separate the precipitates from the solution. Precipitates were redispersed in DI water to remove salts and then recovered by centrifugation. The centrifugation/ washing cycle was repeated six times. The final product was dried at 40°C overnight.The 10.1 nm goethite sample was synthesized as follows. Five milliliters of 5 M KOH was mixed with 250 mL of 0.1 M Fe(NO 3 ) 3 3 9H 2 O in a beaker under magnetic stirring. The mixture was aged at 60°C for 70 h. Next, the cooled mixture was centrifuged and washed, as above, and the product was dried at 80°C for 4 h, then at 50°C for 2 h.The 16.6 nm goethite sample was synthesized by reacting 30 mL of 0.5 M Fe(NO 3 ) 3 3 9H 2 O with 125 mL of 2.5 M KOH, followed by aging at 60°C for 100 h. 11 The aged suspension was treated by repeated centrifugation/washing and then dried at 40°C overnight to obtain the final product.The 26.8 nm and the 38.2 nm goethite samples were synthesized by reacting 20 mL of 5 M KOH with 200 mL of 0.1 M Fe(NO 3 ) 3 3 9H 2 O, followed by aging at 40°C for 42 or 72 h, respectively. The aged suspensions were centrifuged, washed, and dried as above.ABSTRACT: Rod-shaped goethite nanoparticles with average particle sizes (equivalent spherical diameters) of between ∼9 and 38 nm wer...

No abstract

No abstract

d i a t e e f f e c t o f changes i n s p u t t e r i n g parameters on gas t r a p p i n g , as \ > / e l l ' a s measur.e t h e t o t a l gas t r a p p e d , a s e n s i t i v e mass f l o w i n d i c a t o r was i n s t a l l e d i n t h e k r y p t o n s u p p l y o f t h e d y n a m i c a l l y pumped system. Data r e l a t i n g k r y p t o n c o n t e n t t o s u b s t r a t e b i a s c o n d i t i o n s and d e p o s i t i o n r a t e a r e d i s c u s s e d i n l i g h t o f t h e t h e o r y o f C a r t e r , C o l l i g o n and Leck on i o n a b s o r p t i o n i n t h e presence o f s p u t t e r i n g. K r y p t o n c o n t e n t s > 2 a t. % were found i n t h e m e t a l s s t u d i e d , a l l o f which were d e p o s i t e d a t r a t e s exceedi n g .l nm/sec. Computed k r y p t o n c o n c e n t r a t i o n s agreed w i t h c o n c e n t r a t i o n s ' i n samples t h a t were a n a l y z e d c h e m i c a l l y .