Seite - 137 - in Photovoltaic Materials and Electronic Devices

2.5. OpticalProperties Figure 6 shows the UV-vis absorption spectra of TiO2, BiOI/TiO2-C10, BiOI/TiO2-C20, BiOI/TiO2-C30 and BiOI converted from corresponding diffuse reflectancespectrabymeansof theKubelka–Munkfunction[28]: F(R)=(1´R)2/2R=α/S (1) R=RSample/RBaSO4 (2) where R,α, and S are the reflectance, absorption coefficient and scattering coefficient, respectively. It can be seen that TiO2 exhibited a typical absorption characteristic of thewidebandgapsemiconductorwithanedgeabout380nm,whilepureBiOI withastrongabsorptionatabout630nminthevisible light region, indicates that it isanarrowbandgapsemiconductoraccordingto theequationEg =1240/λ,where Eg is the band gap (eV) and λ (nm) is the wavelength of the absorption edge in the spectrum. The band gap of TiO2 and BiOI are estimated to be 3.2 eV and 1.9 eV, respectively. It is noted that the absorption edge of p-BiOI/n-TiO2 NFs show significant red-shift from 393 to 500 nm with the increased amount of BiOI in the composite nanofibers. Based on the above, the increased amount of BiOI inp-BiOI/n-TiO2 NFsextends lightabsorbingrange, which is thepreconditionof effectivephotocatalyticactivity. Materials  2016,  9,  90  7  of  12    Figure  6.  UV‐vis  absorption  spectra  of  different  samples.  2.6.  Photocatalytic  Properties  Figure  7a  shows  the  photocatalytic  activities  of  TiO2  NFs,  BiOI/TiO2‐C10,  BiOI/TiO2‐C20,  BiOI/TiO2‐C30  and  the  mechanical  mixture  of  BiOI  and  TiO2  (M‐BT,  the  molar  ratio  of  Bi:Ti  =  0.4:1  based  on  energy  dispersive  X‐ray  (EDX)  analysis  in  Figure  S1)  on  the  degradation  of  methyl  orange  (MO)  under  visible‐light  irradiation  (≥420  nm).  Before  irradiation,  the  adsorption‐desorption  equilibrium  of  MO  in  the  dark  is  established  within  30  min  over  different  samples.  The  time‐dependent  absorbance  spectra  of  different  samples  are  shown  in  Figure  S1.  The  adsorption  of  BiOI/TiO2‐C30  increases  significantly  compared  to  other  samples,  which  might  be  attributed  to  the  high  specific  surface  area.  After  3  h  irradiation,  the  photodegradation  efficiencies  of  MO  for  BiOI/TiO2‐C30  are  about  92%,  in  comparison  to  60%,  66%,  38%  and  almost  none  for  M‐BT,  BiOI/TiO2‐C20,  BiOI/TiO2‐C10  and  TiO2  nanofibers,  respectively.  In  Figure  7b,  the  kinetic  linear  fitting  curves  over  different  photocatalysts  show  that  the  photocatalytic  degradation  of  MO  followed  a  Langmuir‐Hinshelwood  apparent  first‐order  kinetics  model:  In  C/C0  =  − kKt  =  − kappt  (3) where  C0  is  the  initial  concentration  (mg/L)  of  the  reactant;  C  is  the  concentration  (mg/L);  t  is  the  visible‐light  irradiation  time;  k  is  the  reaction  rate  constant  (mg/(L∙min));  and  K  is  the  adsorption  coefficient  of  the  reactant  (L/mg);  kapp  is  the  apparent  first‐order  rate  constant  (min−1).  The  kapp  of  different  samples  are  shown  in  Table  1.  It  is  indicated  that  the  photocatalytic  activities  is  in  the  order  of  BiOI/TiO2‐C30  >  BiOI/TiO2‐C20  >  M‐BT  >  BiOI/TiO2‐C10  >  TiO2.  The  above  illuminates  that  the  construction  of  p‐n  heterojunctions  can  effectively  enhance  the  photocatalytic  properties.  Furthermore,  the  increased  of  the  specific  surface  area  and  the  amount  of  p‐n  heterojuctions  obviously  enhance  the  photocatalytic  activity.  Furthermore,  the  photocatalysis  under  UV‐light  irradiation  (Figure  S2)  also  demonstrates  the  above  point.  Figure6. UV-visabsorptionspectraofdifferentsamples. 2.6. PhotocatalyticProperties Figure 7a shows the photocatalytic activities of TiO2 NFs, BiOI/TiO2-C10, BiOI/TiO2-C20, BiOI/TiO2-C30 and the mechanical mixture of BiOI and TiO2 (M-BT, the molar ratio of Bi:Ti=0.4:1 based on energy dispersive X-ray (EDX) analysis in 137