Seite - 133 - in Photovoltaic Materials and Electronic Devices

The high-resolution transmission electron microscopy (HRTEM) image of the heterojunctions displays two types of lattice fringes, as shown in Figure 2b. One set of the fringes spacing is ca. 0.35 nm, corresponding to the (101) plane of the anatase crystal structure of TiO2. Another set of the fringes spacing measures ca. 0.28nm,whichcorrespondsto the (110) latticespacingof theBiOI. It indicates that heterojunctions are composed of TiO2 nanofibers and BiOI nanosheets with exposed {001} facets. Theexposed{001} facetsmayhaveexcellentphotocatalyticactivity for BiOI under visible-light irradiation as reported [32]. Therefore, the surface reactivity mayalsobe improvedbydecoratingTiO2 nanofiberswithBiOInanosheets.   Figure  1.  (a)  SEM  images  of  TiO2  nanofibers;  (b)  BiOI/TiO2‐C10;  (c)  BiOI/TiO2‐C20;  and    (d)  BiOI/TiO2‐C30  at  low  magnification  and  high  magnification  (insets).    Figure  2.  (a)  TEM;  and  (b)  HRTEM  images  of  BiOI/TiO2‐C30.  2.2.  Structure  Characterization  Figure  3  shows  the  X‐ray  diffraction  (XRD)  patterns  of  pure  TiO2  nanofibers,  p‐BiOI/n‐TiO2  NFs  and  BiOI  nanosheets.  For  TiO2  nanofibers,  all  peaks  are  attributed  to  the  anatase  of  TiO2  (JCPDS  No.  21‐1272)  and  the  rutile  of  TiO2  (JCPDS  No.  21‐1276).  For  p‐BiOI/n‐TiO2  NFs,  besides  the  characteristic  peaks  of  TiO2  (solid  and  hollow  diamonds),  there  are  some  new  strong  patterns  that  can  be  indexed  as  tetragonal  phase  of  BiOI  (JCPDS  No.  73‐2062).  The  diffraction  peaks  of  BiOI  (solid  circles)  are  gradually  intensified  as  the  SILAR  cycles  increased  from  0  to  30,  as  shown  in  Figure  3.  No  other  characteristic  peaks  of  impurities  are  observed.  In  particular,  the  domination  of  (110)  plane  in  the  pattern  suggests  that  the  exposed  facets  of  BiOI  nanosheets  are  mainly  {001}.  This  result  is  consistent  with  SEM  and  TEM  analyses.  Figure2. (a)TEM;and(b)HRTEMimagesofBiOI/TiO2-C30. 2.2. StructureCharacterization Figure 3 shows the X-ray diffraction (XRD) patt rns of pure TiO2 n nofibers, p-BiOI/n-TiO2 NFsandBiOInanosheets. ForTiO2 nanofibers,allpeaksareattributed totheanataseofTiO2 (JCPDSNo. 21-1272)andtherutileofTiO2 (JCPDSNo. 21-1276). Forp-BiOI/n-TiO2 NFs,besides thecharacteristicpeaksofTiO2 (solidandhollow diamonds), there are some new strong patterns that can be indexed as tetragonal phase of BiOI (JCPDS No. 73-2062). The diffraction peaks of BiOI (solid circles) are gradually intensified as the SILAR cycles increased from 0 to 30, as shown in Figure3. Noothercharacteristicpeaksof impuritiesareobserved. Inparticular, the domination of (110) plane in the pattern suggests that the exposed facets of BiOI nanosheetsaremainly {001}. This result is consistentwithSEMandTEManalyses. 133