1. Introduction

Low dimensional metal oxide nano-structured materials continue to draw increasing attention and fifind uses in an increasing number of potential applications in many fifields such as catalysis, electronics and photonics [1–6]. Therein, owing to the urgent need for a clean and comfortable environment, various nano-structured semiconductor materials have been exploited and used as heterogeneous photocatalysts for various environmental-related applications including the remediation of environmental pollutants through photocatalytic reaction [7–11]. Among various oxide semiconductor photocatalysts TiO2, WO3 and ZnO et al. have been proven to be the considerable common used photocatalysts, due to the high photocatalytic activities, characteristic of stabilities, nontoxic, insoluble in water and low cost [12].   

Since photocatalytic reactions mainly take place on the surface of the catalyst, a high surface-to-volume ratio is of great signififi- cance for increasing the decomposition rate of organic pollutions. Compared with nanopowders and thin fifilms, nanofifibers have higher surface-to-volume ratio, even which are expected to solve these problems of the poor recuperability and reutilization limitation for nanopowders and low contact area for thin fifilms. Electrospinning is a mature method to produce continuous fifibers with diameters ranging from tens to hundreds of nanometers [13–15], and some papers have reported the preparation of TiO2 nanofifibers or their composites by electrospinning for the application in semiconductor devices [16], dye-sensitized solar cells [17], photo-decomposition of organic pollutants [18], electrochemical capacitors [19] and anode materials in batteries [20].    

It is well known that metallic and nonmetallic elements doping can improve the photo-activity of TiO2, and TiO2 nanofifibers doped by nitrogen, iron, copper, erbium and et al. respectively have been reported [21–23]. 

However, to the best of our knowledge, doped TiO2 nanofifibers by coaxial electrospining have not been investigated systematically. In this paper, N, Fe or W doped TiO2 nanofifibers membranes were prepared by electrospinning. The different conditions of the preparation, the morphologies and the crystal structure of the nanofifibers were inspected systematically.

2. Experimental section

2.1. Materials and instruments

Tetrabutyl titanate (TBT, Tianjin Guangfu Institute of Fine Chemicals, CP), acetic acid (Hunan Huihong Reagent Co., AR), polyvinylpyrrolidone (PVP, 360000, Missouri, Sigma–Aldrich, AR), anhydrous ethanol (Hunan Huihong Reagent Co., AR), N,N-dimethyl formamide (DMF, Tianjin Guangfu Institute of Fine Chemicals, AR), carbamide (Taishan Yueqiao reagent Plastic Co., AR), Fe(NO3)3 9H2O(Tianjin Guangfu Institute of Fine Chemicals, AR), phenol (Chongqing Chuandong Chemical Co., AR), nickel foam (Changsha Liyuan Materials Co.), WCl6 (Belgium, ACROE, excellent level of pure), and so on 1003-type electrostatic spinning equipment (Tianjin Dong Wen high-pressure power plant), Rigaku TTRIII-type X-ray diffraction instrument, JSM-5600LV-type scanning electron microscope, ESCALAB 250 X-ray photoelectron spectroscopy (XPS), CEL-HXF300- type xenon lamp parallel light source (Beijing Zhong Jiao Jin Yuan Technology Co., 50 W, 320–2500 nm).

2.2. Preparation of electrospinning of nanofifibers

2.2.1. Preparation of electrospinning of TiO2 nanofifibers

1.00 g PVP as the template was dissolved in 6 ml ethanol under vigorous stirring at the room temperature for several hours, and which was titled solution A; 1.5 g Tetrabutyl titanate was dissolved in a mixture of 4 ml ethanol and 1 ml ice acetic acid under vigorous stirring at the room temperature for several hours, and which was titled the solution B. Mix solution A and solution B evenly, and subsequently the mixture was ejected from the stainless steel capillary with a voltage of 10 kV. The distance between the capillary and collector was 15 cm. The humidity range was set within 10 20%. The moving rate of the pump is 0.001 mm/s. The fifibers mats were collected on the nickel foam, and followed by the calcination at 500  C for 4 h with a heating rate of 2  C/min.

2.2.2. Preparation of electrospinning of N doped TiO2 nanofifibers

Such a method can also be used to prepare N/TiO2 nanofifibers photocatalysts by replacing TiO2 precursor with carbamide/TiO2 precursor which was obtained by a certain quantity of carbamide dissolved in appropriate amount of ethanol, and then it was added into solution B with the doped mole ratio of 1%, 2%, 3%, 5%, 10% respectively and the other steps were the same as Section 2.2.1.

3. Results and discussions

3.1. The SEM analysis of the catalysts

Fig. 1 shows the SEM images of the precursor of TiO2 nanofifibers (A) and TiO2 nanofifibers (B). From Fig. 1, it can be seen that these fifibers with smooth surfaces were randomly oriented, and the size of TiO2 nanofifibers decreased after calcined at 500  C, which can be explained by the fact of the burning of the organics in the precursor of TiO2. The sizes of the TiO2 nanofifibers are less than 100 nm from image B.

Fig. 2 shows the SEM images of the precursor of 5% N/TiO2 nanofifibers (A) and 5% N/TiO2 nanofifibers (B). From Fig. 2, it can be seen that the surfaces of the precursor of 5% N/TiO2 nanofifibers are smoother than that of 5% N/TiO2 nanofifibers. After calcined at 500  C, the surfaces of the nanofifibers become rough, and which provides an indirect proof of the dopant of N element. It was also found that the sizes of 5% N/TiO2 nanofifibers decreased after calcined at 500  C. The sizes of the 5% N/TiO2 nanofifibers are less than 100 nm from image B.

Fig. 3 shows the SEM images of the precursor of Fe/TiO2 nanofibers (A) and Fe/TiO2 nanofifibers (B). It can be observed from Fig. 3 (A) that the obtained polymeric fifibers exhibit smooth surfaces due to the amorphous nature of the PVP. The fifiber diameter was uniform and the average value was about 50 nm determined by measuring dozens of fifibers. These fifibers were ultra long and in a random orientation, which was caused by the instability of the spinning jet. Compared with 5% N/TiO2 nanofifibers in Fig. 2, the surfaces of the precursor of Fe/TiO2 nanofifibers are not as smooth as the precursor of TiO2 nanofifibers. After calcined at 500  C, the tubers on the surfaces of the precursor become more obvious, and which also provides an indirect proof for the dopant of Fe element. From the images, the sizes of Fe/TiO2 nanofifibers increase after calcined at 500  C. The sizes of some of the Fe/TiO2 nanofifibers are larger than 100 nm from image B. Electrospun nanofifibers with iron mostly present this kind of morphology [24].

Fig. 4 shows the SEM images of the precursor of W/TiO2 nanofibers (A) and W/TiO2 nanofifibers (B). From Fig. 4, it can be seen that these fibers with smooth surfaces were randomly oriented, and there is no obvious decrease on the size of W/TiO2 nanofifibers after calcined at 500  C. The sizes of the W/TiO2 nanofifibers are larger than 100 nm from image B.

3.2. The XRD analysis of the catalystsFig. 5 shows the XRD patterns for the precursor of TiO2 nanofifi- bers (B) and TiO2 nanofifibers (A). From pattern A, some distinct peaks for TiO2 can be observed. From the left to right: 2h at 25.26 , 37.8 , and 48.0 are corresponding respectively to the diffraction peaks of anatase TiO2 crystal faces 101, 111, and 200 [25]. 2h at 27.41 is corresponding to the diffraction peaks of rutile TiO2 crystal face 110 [26]. From the pattern B, 2h between 22 and 24 are corresponding to the diffraction peaks of PVP, which are consistent with literature reports. After calcined at 500  C for 4 h, PVP and other organics were burned out, and the anatase began to convert to rutile.

Fig. 6 shows the XRD patterns of (A) 1% Fe/TiO2 nanofifibers and (B) 4% N/TiO2 nanofifibers. From Fig. 6 there is no characteristic peaks of Fe or N, which indicates that maybe atoms of Fe or N have doped into the crystal lattices of TiO2. Compared with TiO2 nanofibers, Fe or N doping is easier to promote the transformation of

4. Conclusions

Pure TiO2 nanofifibers and doped TiO2 nanofifibers were fabricated using a sol–gel method and coaxial electrospinning technique. From the XRD patterns, the XPS spectra, and the SEM imagines of TiO2 nanofifibers, N /TiO2 nanofifibers, Fe /TiO2 nanofifibers and W/ TiO2 nanofifibers, it can be concluded that the dopants have different inflfluence on the surface topographies, the crystal structures and the transformation of anatase to rutile of TiO2 nanofifibers. The surfaces of the precursor of Fe/TiO2 and N/TiO2 nanofifibers are smoother than that of Fe/TiO2 and N/TiO2 nanofifibers respectively. However there is no obvious difference between W/TiO2 nanofifibers and its precursor in the surface topographies and sizes; Fe doping can make bigger topography changes, higher degree of crystallization and easier transformation from anatase to rutile than that of N or W doping; The maximum doping concentration of W in TiO2 nanofifibers is between 5% and 10% when calcined at 500  C for 4 h.