Page - 134 - in Photovoltaic Materials and Electronic Devices

Materials  2016,  9,  90  4  of  12    Figure  3.  XRD  patterns  of  different  samples.  2.3.  Composition  and  Chemical  States  X‐ray  photoelectron  spectroscopy  (XPS)  measurements  are  also  performed  to  further  investigate  the  chemical  compositions  and  chemical  states  of  p‐BiOI/n‐TiO2  NFs.  The  typical  high  resolution  XPS  spectrum  of  Bi  4f  is  shown  in  Figure  4a.  The  peaks  at  158.82  eV  and  164.14  eV  correspond  to  Bi  4f3/2  and  Bi  4f1/2,  respectively,  which  indicates  a  normal  state  of  Bi3+  in  BiOI/TiO2‐C30  [33].  Figure  4b  reveals  the  high‐resolution  XPS  spectrum  of  I  3d.  The  two  peaks  at  630.3  eV  and  618.8  eV  are  attributed  to  I  3d3/2  and  I  3d5/2,  respectively,  which  indicates  that  the  chemical  state  of  iodine  is  I−1  in  BiOI/TiO2‐C30  [34].  The  deconvolution  of  the  O  1s  spectrum  in  Figure  4c  implies  that  more  than  one  chemical  state  of  O  1s  exists  in  the  BiOI/TiO2‐C30.  The  peaks  with  lower  binding  energies  at  529.8  eV  and  531.5  eV  correspond  to  the  stronger  Bi‐O  and  Ti‐O  bond,  respectively.  The  higher  bonding  energy  of  532.9  eV  might  be  caused  by  adsorbed  water  and  surface  hydroxyl  groups  (OOH),  which  may  also  lead  to  an  enhanced  photocatalytic  property  [35].  The  splitting  between  Ti  2p1/2  and  Ti  2p3/2  are  both  5.7  eV  for  TiO2  and  BiOI/TiO2‐C30,  suggesting  a  normal  state  of  Ti4+  in  pure  TiO2  nanofibers  and  BiOI/TiO2‐C30  [36,37].  However,  for  BiOI/TiO2‐C30,  the  binding  energy  of  Ti  2p3/2  locates  at  458.7  eV,  which  is  about  0.4  eV  higher  than  that  of  pure  TiO2  nanofibers  (458.3  eV).  This  can  be  explained  as  follow:  when  p  type  BiOI  nanosheets  are  deposited  on  n  type  TiO2  nanofibers,  the  electrons  in  TiO2  nanofibers  would  diffuse  to  BiOI,  forming  p‐n  heterojunctions;  thus,  in  the  space  charge  region,  TiO2  is  positively  charged  which  could  increase  the  binding  energy  of  electrons  in  Ti  2p  chemical  states.  Similar  results  have  been  observed  in  the  heterojunctions  of  p‐MoO3  nanosheets/n‐TiO2  nanofibers  [26].  Figure3. XRDpatternsofdifferentsamples. 2.3. CompositionandChemicalStates X-rayphotoelectronsp ct oscopy(XPS)mea ure entsarealsoperformedto further investigate the chemical compositions and chemical states of p-BiOI/n-TiO2 NFs. ThetypicalhighresolutionXPSspectrumofBi4f is showninFigure4a. The peaks at 158.82 eV and 164.14 eV correspond to Bi 4f3/2 an Bi 4f1/2, respectively, which indicate a ormal state of Bi3+ in BiOI/ iO2-C30 [3 ]. Figure 4b reveals the high-resolution XPS spectrum of I 3d. The two peaks at 630.3 eV and 618.8 eV are attributed to I 3d3/2 and I 3d5/2, respectively, which indicates that the chemical stat of iodine is I´1 in BiOI/TiO2-C30 [34]. The deconvolution of the O1sspectruminFigure4c implies thatmore thanonechemical stateofO1sexists in the BiOI/TiO2-C30. The peaks with lower binding energies at 529.8 eV and 531.5 eV correspond to the stronger Bi-O and Ti-O bond, respectively. The higher bonding energy of 532.9 eV might be caused by adsorbed water and surface hydroxyl groups (OOH),whichmayalsoleadtoanenhancedphotocatalyticproperty[35]. Thesplitting betweenTi2p1/2 andTi2p3/2 areboth5.7eVforTiO2 andBiOI/TiO2-C30,suggesting a normal state of Ti4+ in pure TiO2 nanofibers and BiOI/TiO2-C30 [36,37]. However, for BiOI/TiO2-C30, the binding energy of Ti 2p3/2 locates at 458.7 eV, which is about 0.4 eV higher than that of pure TiO2 nanofibers (458.3 eV). This can be explained as follow: when p type BiOI nanosheets are deposited on n type TiO2 nanofibers, the electrons in TiO2 nanofibers would diffuse to BiOI, forming p-n heterojunctions; thus, in the space charge region, TiO2 is positively charged which could increase 134