A novel and facile approach to obtain NiO nanowire-in-nanotube structured nanofibers with enhanced photocatalysis

NiO nanowire-in-nanotube structured nanofibers were easily and directly fabricated via one-pot uniaxial electrospinning followed by calcination process for the first time. Firstly, Ni(CH3COO)2/PVP composite nanofibers were prepared by a conventional electrospinning method, and then NiO nanowire-in-nanotube structured nanofibers were successfully synthesized by two-stage calcination procedure of Ni(CH3COO)2/PVP composite nanofibers which was determined to be the key process for preparing NiO nanowire-in-nanotube structured nanofibers. The NiO nanowire-in-nanotube structured nanofibers have pure cubic phase structure with space group of Fm3̄m, and the outer diameter and wall thickness of nanotubes and nanowire diameter are 130 ± 0.99 nm, 30 nm and 40 nm, respectively. Preliminarily, it is satisfactorily found that NiO nanowire-in-nanotube structured nanofibers used as photocatalyst for water splitting exhibit higher H2 evolution rate of 622 μmol h−1 than counterpart NiO hollow nanofibers of 472 μmol h−1 owing to its novel nanostructure. The possible formation mechanism of NiO nanowire-in-nanotube structured nanofibers is proposed. To evaluate the universality of this novel preparative technique, taking Co3O4 as an example, it is found that Co3O4 nanowire-in-nanotube structured nanofibers are also successfully fabricated via this novel method. The special nanowire-in-nanotube structure of the one-dimensional nanomaterials makes them have promising applications in catalysis, lithium-ion battery, drug delivery, etc. This manufacturing strategy has some advantages over other methods to form nanowire-in-nanotube structured nanofibers, such as easy, highly efficient and cost effective. The design idea and synthetic technique provide a novel perspective to create other nanowire-in-nanotube structured nanomaterials.


Introduction
One-dimensional (1D) nanostructured materials have received increasing attention owing to their special properties, which have wide applications in various elds, such as magnetism, optics, photocatalysis, Li-ion battery, etc 1 . Many methods, such as hydrothermal method, electrochemical etching/deposition, solid state reaction, microwave synthesis, electrospinning, [2][3][4][5][6] have been used to prepare these 1D nanostructured materials. Among these methods, electrospinning is a straightforward and effective method to prepare 1D nanomaterials with diameters ranging from micrometer to nanometer, 7 including nanobers/ nanobelts, hollow nanobers, coaxial nanobers/nanobelts, Janus nanobers/nanobelts, nanowire-in-microtube structured core/shell bers. [8][9][10][11][12][13][14][15][16] Recently, the nanocomposites with a hollow cavity between the shell and core have aroused widespread interest due to their special structure and large specic surface area, which have potential applications in biological medicine, sensor, lithiumion batteries, adsorption and catalysis, etc [17][18][19] . Usually, the preparation process of these specially structured 1D nanocomposites contains three steps. The rst step is the synthesis of core materials, and then the other two layers including intermediate layers and shells are gradually coated on the core materials to form triple-layered composite materials. Finally, the intermediate layers are removed by calcination or extraction using appropriate solvent, and the nanocomposites with void structure between the inner and outer substances are obtained. Tingting Wang et al. have prepared uniform yolk-shell architectures by high-temperature calcination of core/shell/shell structured nanomaterials to remove the middle layers. 20 Nian Liu et al. have fabricated yolk-shell Si@void@C structure by using HF to remove SiO 2 sacricial layer in the Si@SiO 2 @C structure. 21 In addition to the above three-step method, Zhao Yong et al. have reported the fabrication of nanowire-inmicrotube structured core/shell bers by multiuidic coaxial electrospinning approach. Firstly, three coaxial capillaries were assembled as the spinneret, and a chemically inert middle uid was introduced to work as a spacer between the outer and inner uids, and then a three-layered core/shell structure was formed. Subsequently, the middle layer of the as-prepared bers was selectively removed, thus nanowire-in-microtube structured core/shell bers were obtained with a hollow cavity between the shell and the core materials. 16 However, the aforementioned preparation methods for nanocomposites with hollow cavity are mostly complicated and costly, and usually, the structural uniformity of the products is not satisfactory. Therefore, it is urgent to nd a simple and efficient method to form nanocomposites with void structure in order to simplify the productive process and reduce costs.
NiO is p-typed semiconductor with wide band gap (3.6-4.0 eV), 22 which has been widely used in catalysis, battery cathode, electrochromic lms, etc. Mengzhu Liu et al. reported the formation of multilayer NiO nanostructures by electrospinning and compared the properties of multilayer NiO and NiO powders. The result shows that multilayer NiO exhibits much higher sensing signal than NiO powders due to its higher surface area. 23 Xiong Wang et al. reported the formation, improved photocatalytic properties and excellent electrochemical performance of hierarchically structured NiO macroporous microspheres with large surface area. 24 Hence, NiO with special structure possesses many excellent performances, which has been reported in the above literatures, especially it has been conrmed that NiO nanotubes have higher photocatalytic property than ordinary NiO solid nanobers because of the larger specic surface area. 25 Nanowire-in-nanotube structured nanobers theoretically have larger specic surface area than counterpart NiO nanotubes. For this reason, the fabrication of NiO nanowire-in-nanotube structured nanober, as a novel and special morphology, is an important and essential subject to research. By now, no reports on the synthesis of NiO nanowirein-nanotube structured nanober are found in the references.
In this work, we design a novel and simple strategy to form NiO nanowire-in-nanotube structured nanobers by two-stage calcination procedure of electrospinning-made Ni(CH 3 COO) 2 / PVP composite nanobers. The products were characterized systematically, and their photocatalytic water splitting activity was initially investigated, and some meaningful results were achieved. Fabrication of NiO nanowire-in-nanotube structured nanobers 1.0000 g of Ni(CH 3 COO) 2 $4H 2 O was dissolved in 7.0000 g of DMF, and then 2.0000 g of PVP was added into the above solution under magnetic stirring for 12 h to form uniform green-transparent spinning solution with a certain viscosity. In the electrospinning solution, the mass ratio of Ni(CH 3 COO) 2 -$4H 2 O, PVP and DMF was xed as 10 : 20 : 70. Subsequently, the electrospinning was carried out at ambient temperature by using a conventional single-spinneret electrospinning setup with the positive direct current (DC) voltage of 13 kV and the spinning distance of 18 cm. Then, Ni(CH 3 COO) 2 /PVP composite nanobers were obtained on the collector through the above process with the volatilization of solvent.

Experimental sections
The as-prepared Ni(CH 3 COO) 2 /PVP composite nanobers were heat-treated from ambient temperature (20 C) to 200 C with a heating rate of 1 C min À1 and then remained for 2 h at 200 C (rst-stage calcination, named as pre-oxidation process), aer that, the temperature was raised to 450 C with the same heating rate of 1 C min À1 and remained for 2 h (second-stage calcination, denoted as oxidation process). Thereaer, the temperature was reduced to 200 C at a cooling rate of 1 C min À1 followed by natural cooling down to room temperature, and thus NiO nanowire-in-nanotube structured nano-bers were successfully obtained.

Comparative and conditional experiments
In order to obtain the optimum preparation parameters for NiO nanowire-in-nanotube structured nanobers, the inuence of different preparation conditions such as pre-oxidation temperature, pre-oxidation duration time, heating rate, oxidation temperature, oxidation duration time and various inorganic salts on the morphology of the sample were studied, and a series of conditional experiments were conducted and detailedly listed in Table 1.
Samples S1-S3 were obtained at different pre-oxidation temperatures by two-stage calcination of Ni(CH 3 COO) 2 /PVP composite nanobers. S4-S6 were prepared by changing the pre-oxidation duration time via two-stage calcination of Ni(CH 3 COO) 2 /PVP composite nanobers. Samples S7-S9 were obtained in the same conditions except for the different heating rates. S10-S13 were synthesized at different oxidation temperatures by one-stage calcination of Ni(CH 3 COO) 2 /PVP composite nanobers without undergoing pre-oxidation. S14 and S15 were fabricated by using different kinds of inorganic salts via twostage calcination of corresponding composite nanobers.
Characterization methods X-ray diffraction (XRD) patterns were collected by using a Rigaku D/max-RA X-ray diffractometer operating at 40 kV and 30 mA with the Cu Ka radiation and Ni lter (l ¼ 0.15418 nm). The scanning electron microscope (FESEM, JSM-7610F) and transmission electron microscope (TEM; JEM-2100 Plus) were used to observe the morphologies and sizes of the samples. The elemental analysis was performed by X-MaxN80 energy dispersive X-ray spectrometer (EDS) attached to SEM. Thermogravimetric and differential scanning calorimetry (TG-DSC) analysis was carried out on a Q600 thermal analyzer in air atmosphere. The specic surface area of products was measured by ASAP 2020 instrument.

Hydrogen production measurements
Experiments for photocatalytic water splitting into hydrogen were performed in a Labsolar-IIIAG photocatalytic system device (Beijing Bofeilai Technology Co., Ltd) by external visible light irradiation. The light source was a xenon lamp (300 W, PLSSXE300/300UV, China) equipped with a cut off lter L38 (380 < l < 750 nm). Before testing, 0.1 g of the photocatalyst (NiO nanowire-in-nanotube structured nanobers and NiO hollow nanobers), 25 mL of methyl alcohol and 75 mL of tap water were successively added into a 200 mL quartz cuvette to ensure uniform dispersion of the sample under vigorous magnetic stirring. Then the suspension was degassed by evacuation. Throughout the experiment, the amount of produced gas was sampled intermittently, and the hydrogen content was measured by gas chromatography (GC7900, Tianmei Techcomp Ltd., thermal conductivity detector, using nitrogen as carrier gas).

Results and discussion
Thermal analysis TG and DSC curves of Ni(CH 3 COO) 2 /PVP composite nanobers, as seen in Fig. 1. Due to the volatilization of the residual solvent and the surface adsorbed water, the Ni(CH 3 COO) 2 /PVP composite nanobers lose about 12.55% of their initial weight when the temperature arises from 20 C to 100 C accompanied by a wide endothermic peak at 50 C in the DSC curve. With the continuous rise in temperature to 250 C, Ni(CH 3 COO) 2 /PVP composite nanobers slightly lose their weight due to the oxidation of Ni(CH 3 COO) 2 to form NiO. When the temperature reaches up to 250 C, Ni(CH 3 COO) 2 is intensively decomposed to form NiO, gaseous H 2 O and CO 2 , together with obvious weight loss in TG curve and an exothermic peak at 260 C in DSC curve. Aerwards, PVP begins to decompose at 290 C, and the decomposition process is completed at 324 C. Major weight loss and heat release occur in this process. No weight loss in TG curve and thermal peak in DSC curve are detected when temperature is over 324 C, meaning that stable inorganic oxide can be obtained above 324 C, and the total weight loss percentage is 85%.   This journal is © The Royal Society of Chemistry 2018 standard diffraction lines (PDF#73-1523) and no impurity peaks are detected, meaning that pure cubic phase NiO with the space group of Fm 3m is obtained. In order to study the crystalline phases of products undergone the pre-oxidation process, the XRD patterns of samples S10-S12 were also gained, as seen in Fig. 2b-d. One can see that only amorphous peak at ca. 22 is found, indicating that no crystalline NiO is formed or the weight percentages of crystalline NiO in these samples do not exceed 5% (limit of XRD detection) at 150 C to 250 C.

XRD analysis
Furthermore, it can be observed from Fig. 2e that the XRD patterns of S13 prepared by one-stage calcination are consistent with those of PDF standard card of NiO (PDF#73-1523), implying that pure phase NiO is also acquired.  2 /PVP composite nanobers and samples S1-S3 and S13 obtained by different calcination process. One can see that Ni(CH 3 COO) 2 / PVP composite nanobers have smooth surface and uniform dispersity, as indicated in Fig. 3a. Fig. 3b-d reveals the morphologies of samples S1-S3 obtained at different preoxidation temperatures (S1: 150 C, S2: 200 C, S3: 250 C) by two-stage calcination are NiO nanowire-in-nanotube structured nanobers that consist of a shell of nanotube and a core of nanowire. With the increase of the pre-oxidation temperature, the shell of the NiO nanowire-in-nanotube structured nano-bers is gradually thickened and becomes rough, and the space between the nanowire and the nanotube is gradually reduced, as seen in Fig. 3b-d. Fig. 3e and f indicate the morphology of S13 prepared by onestage calcination of Ni(CH 3 COO) 2 /PVP composite nanobers without pre-oxidization process. It is observed that hollow structured nanobers, rather than nanowire-in-nanotube structured nanobers, are obtained. Therefore, it can be concluded that pre-oxidation process plays an important role in the formation of NiO nanowire-in-nanotube structured nano-bers. Furthermore, SEM observation demonstrates that the diameters of Ni(CH 3 COO) 2 /PVP composite nanobers, S1, S2, S3 and S13 are 293 AE 1.43 nm, 160 AE 3 nm, 130 AE 0.99 nm, 111 AE 1.33 nm and 120 AE 3.17 nm, respectively. It is found that with the increase of the pre-oxidation temperature, the diameters of the samples are gradually decreased. To investigate the inuence of other conditions on the morphology of the products, we choose 200 C as the optimal pre-oxidation temperature in the subsequent work. Fig. 4 reveals the SEM images of NiO nanowire-in-nanotube structured nanobers obtained at different pre-oxidation duration time of 1 h, 2 h, 4 h, 6 h (S4, S2, S5, S6) by two-stage calcination of Ni(CH 3 COO) 2 /PVP composite nanobers. It is easy to nd that all these samples are NiO nanowire-innanotube structured nanobers. With the increase of preoxidation duration time, the surface of nanowire-in-nanotube structured nanobers gradually becomes rough. It is also found that the diameters of S4, S5 and S6 respectively are 138 AE 3.36 nm, 98 AE 0.79 nm and 95 AE 0.35 nm, indicating that the diameters of the samples are gradually reduced with the increase of the pre-oxidation duration time. Thus, 2 h is selected as the optimum pre-oxidation duration time in the following study.

SEM and TEM analyses
The SEM images of samples S7, S2, S8, S9 fabricated at different heating rate (0.5 C min À1 , 1 C min À1 , 3 C min À1 , 5 C min À1 ) by two-stage calcination of Ni(CH 3 COO) 2 /PVP composite nanobers, are displayed in Fig. 5. When the heating Fig. 2 XRD patterns of samples S2 (a), S10 (b), S11 (c), S12 (d) and S13 (e) with PDF standard card of NiO. NiO nanowire-in-nanotube structured nanofibers S1 (b), S2 (c), S3 (d) and NiO hollow nanofibers S13 (e); TEM image of NiO hollow nanofibers S13 (f). rate is 0.5 C min À1 , NiO solid nanobers, rather than nanowire-in-nanotube structured nanobers, are acquired. Fig. 5b demonstrates the appropriate heating rate (1 C min À1 ) is benecial to form nanowire-in-nanotube structure. However, over high heating rate (3 C min À1 , 5 C min À1 ) will lead to the structural damage of the products, as indicated in Fig. 5c and d. The above analyses indicate that the heating rate has a great impact on the formation of NiO nanowire-in-nanotube structured nanobers. Thus, the heating rate of 1 C min À1 is the best condition for the preparation of NiO nanowire-in-nanotube structured nanobers. Fig. 6 shows the SEM images of samples obtained by oxidizing Ni(CH 3 COO) 2 /PVP composite nanobers at 150 C (a), 200 C (b) and 250 C (c) for 2 h. The samples are solid nano-bers regardless of the oxidation temperature of 150 C, 200 C and 250 C, indicating that the PVP in original composite nanobers does not decomposed and the nanowire-innanotube structure is unformed at the ranges of calcination temperatures. Nevertheless, the diameters of these samples are slightly decreased with the increased calcination temperature, which are measured to be 142 AE 4.7 nm (S10), 127 AE 2.22 nm (S11) and 125 AE 1.8 nm (S12), respectively. This is maybe because the Ni(CH 3 COO) 2 on the surface of the nanobers is decomposed to NiO, gaseous H 2 O and CO 2 , which causes slight decrease in diameter, whereas the Ni(CH 3 COO) 2 in the inner nanobers is barely decomposed due to the non-contact with abundant oxygen.
Based on the above experimental results, it can be concluded that the optimum preparation conditions for NiO nanowire-innanotube structured nanobers are as following: 200 C for 2 h with a heating rate of 1 C min À1 for the pre-oxidation process, and 450 C for 2 h with the same heating rate of 1 C min À1 for oxidation process. Fig. 7 displays the TEM image, EDS line-scan analysis, EDS spectrum, and histogram of diameters distribution of NiO nanowire-in-nanotube structured nanobers obtained under the optimum preparation conditions. As illustrated in Fig. 7a, the diameters of nanowire and nanotube in the NiO nanowire-in-nanotube structured nanobers are about 40 and 130 nm, respectively. In order to further prove the nanowire-in-nanotube structure and compositions, TEM-EDS line scan analysis was carried out, where Ni element represents NiO, as presented in Fig. 7b. It is found that elemental Ni locates in the whole nanowire-in-nanotube structured nano-ber, and the two edges of the nanotube and the nanowire have larger amount of Ni than the space between the nanowire and   Paper nanotube, which is consistent with the structure of nanowire-innanotube structured nanobers. Furthermore, Fig. 7c depicts O and Ni are the main elements in NiO nanowire-in-nanotube structured nanobers. The average outer diameter of NiO nanowire-in-nanotube structured nanobers is 130 AE 0.99 nm (Fig. 7d).
In order to demonstrate the universality of this fabrication method, Ni(CH 3 COO) 2 $4H 2 O is respectively replaced by Ni(NO 3 ) 2 $6H 2 O and Co(CH 3 COO) 2 $4H 2 O, using the identical optimum pre-oxidation and oxidation conditions. Fig. 8a and b respectively demonstrate the XRD patterns of pure phase NiO and Co 3 O 4 nanostructures fabricated by two-stage calcination of Ni(NO 3 ) 2 /PVP and Co(CH 3 COO) 2 /PVP composite nanobers. It can be obviously found that NiO and Co 3 O 4 nanowire-innanotube structured nanobers are obtained, as seen in Fig. 9a and d. Fig. 9c shows that O and Ni are the main elements in NiO nanowire-in-nanotube structured nanobers. The presence of Co and O corresponds to Co 3 O 4 nanowire-in-nanotube structured nanobers, as seen in Fig. 9f. The diameters of NiO and Co 3 O 4 nanowire-in-nanotube structured nanobers are 120 AE 3.17 nm and 107 AE 0.56 nm, respectively, as shown in Fig. 9b and e. The above analyses demonstrate that this technique is of certain universality for preparing inorganic metallic oxide nanowire-in-nanotube structured nanobers.
Possible formation mechanism of NiO nanowire-in-nanotube structured nanobers Fig. 10a shows the ow diagram of heat-treatment procedure for preparing NiO nanowire-in-nanotube structured nanobers. According to the above results, possible formation mechanism of the NiO nanowire-in-nanotube structured nanobers is proposed, as indicated in Fig. 10b. Firstly, Ni(CH 3 COO) 2 /PVP composite nanobers are formed by the traditional electrospinning process using the spinning solution containing PVP, DMF and Ni(CH 3 COO) 2 . At the moment, the composite nano-ber is a solid nanober, and PVP plays the role of ber framework. Secondly, Ni(CH 3 COO) 2 /PVP composite nanobers are calcined at 200 C for 2 h (namely pre-oxidation process), and then the temperature is raised to 450 C and keeps for 2 h (namely oxidation process). In the pre-oxidation process, Ni(CH 3 COO) 2 on the surfaces of Ni(CH 3 COO) 2 /PVP composite nanobers rst begins to gradually decompose and oxidize to NiO rather than that in the inside of the composite nanobers   because the Ni(CH 3 COO) 2 on the ber surfaces could directly contact with oxygen. As a consequence, a NiO shell together with PVP is formed on the surface of each nanober. As the preoxidation time goes by, the Ni 2+ ions near the surface of composite nanober are gradually attracted to the NiO shell so that the NiO crystal can grow up. Thus, the concentration of Ni 2+ near the surface of composite nanober becomes lower and lower, and the NiO shell turns to denser and denser. Aerwards, in the oxidation process, PVP begins to decompose at 290 C and produce CO 2 and H 2 O, accompanied by the decomposition and oxidization of the residual Ni(CH 3 COO) 2 to NiO. In this process, pre-generated NiO takes place the role of PVP as ber framework. Moreover, a space between the surface and core of each nanober is formed because almost all of the Ni 2+ ions near the surface of composite nanober move to the ber surface, leaving only PVP which is eliminated by oxidation process. On the other hand, the Ni 2+ ions in the core of each nanober are oxidized to form a NiO nanowire in the outer NiO nanotube. Based on the above formation mechanism of NiO nanowire-in-nanotube structured nanobers, it is rational that the thickness of the nanotube of each NiO nanowire-innanotube structured nanober is increased with raising preoxidation temperature or extending pre-oxidation duration time, as seen in Fig. 3 and 4, because more Ni 2+ ions can move to the surface of the composite nanober.
It has been found that heating rate also strongly affect the morphology of the products. When the heating rate is 0.5 C min À1 , NiO solid nanobers are formed. The reason is that the inside and surface of the composite nanober can fully contact with oxygen to form NiO because there has been sufficient reaction time before the temperature rises to the decomposition temperature of PVP. By contrast, over high heating rate (3 C min À1 , 5 C min À1 ) causes destruction of the morphology of the nanobers. That is because when the heating rate is too high, PVP and Ni(CH 3 COO) 2 decompose so fast that lots of gases are rapidly produced, which impedes nanoparticles from mutually connect to form nanobers.
It has been also discovered that NiO hollow nanobers are formed when pre-oxidation process is not carried out. The possible formation mechanism of NiO hollow nanobers is as following: in the process of heating, lots of voids appear in the surface of the composite nanobers due to the volatilization of residual DMF in the nanobers. With the increase of temperature, Ni(CH 3 COO) 2 on the surface of each nanober begins to decompose, and thus a porous NiO shell are generated on the surface of nanober. Before the NiO shell becomes dense, PVP starts to decompose due to the absence of pre-oxidation process, which causes the Ni 2+ ions inside the nanober are transported to the ber surface by the gases generated from the PVP. 26 Finally, Ni 2+ ions are enriched in the shell of the Photocatalytic activity and mechanism Fig. 11 reects a comparison of the hydrogen production activity from water splitting by using NiO nanowire-in-nanotube structured nanobers and NiO hollow nanobers under visible light illumination (l > 380 nm) with methanol as a sacrice. It can be seen that the hydrogen production rate of NiO nanowirein-nanotube structured nanobers and NiO hollow nanobers is respectively 622 mmol h À1 and 472 mmol h À1 under the same mass. The amount of hydrogen produced by NiO nanowire-innanotube structured nanobers is 1.32 times higher than that of NiO hollow nanobers. Fig. 12 demonstrates nitrogen adsorption-desorption isotherm and pore diameter distribution of NiO nanowire-in-nanotube structured nanobers and NiO hollow nanobers. The specic surface area and pore diameter of NiO nanowire-in-nanotube structured nanobers (20.81 m 2 g À1 , 24 nm) are bigger than those of NiO hollow nanobers (9.95 m 2 g À1 , 12 nm). It has been known that the H 2production activity from water splitting strongly depends on the microstructure of NiO. 27,28 Fig. 14 displays the total surface area of the NiO nanowire-in-nanotube structured nanobers includes the surface area of the embedded nanowires and the inner and outer surface areas of the nanotubes, leading to the fact that NiO nanowire-in-nanotube structured nanobers have bigger surface area than NiO hollow nanobers, which is conrmed by the above specic surface area data. Hence, NiO nanowire-in-nanotube structured nanobers can absorb more light and adsorb more water and methanol molecules than NiO hollow nanobers, resulting in the fact that hydrogen production rate of NiO nanowire-in-nanotube structured nanobers is faster than that of NiO hollow nanobers. On the other hand, bigger pore diameter is benecial to enhance the absorption efficiency of light and accelerate the ow rate of the water molecules, 29 which also causes higher hydrogen production rate of NiO nanowire-in-nanotube structured nanobers. Nonetheless, at the beginning of the reaction, the hydrogen production rate of the NiO hollow nanobers is higher than that of NiO nanowire-in-nanotube structured nanobers. This may be because nanowires in the NiO nanowire-in-nanotube structured nanobers block water molecules and methanol molecules from getting into the NiO nanowire-in-nanotube structured nanobers, and thus the inner surfaces of the NiO nanowire-innanotube structured nanobers are rarely used. As the reaction time increases, water molecules and methanol molecules gradually enter into the NiO nanowire-in-nanotube structured nanobers, the large inner surfaces greatly promote the reaction. Therefore, the H 2 -production activity is accelerated. Fig. 13 shows the UV-vis spectroscopy of the NiO nanowirein-nanotube structured nanobers and NiO hollow nano-bers. It can be seen that NiO nanowire-in-nanotube structured nanobers exhibit stronger absorption in the range of visible light than NiO hollow nanobers, which is due to the larger specic area of NiO nanowire-in-nanotube structured nano-bers. Generally, larger specic area provides more highly active sites for H 2 evolution, which facilitates photocatalytic reaction. For crystalline semiconductors, the band gap energies of the samples can be estimated from a plot of (ahn) 2 versus photon energy (hn). The indirect band gap energies of the samples are similar to the intercept of the tangent to the plot, and the band gap of the sample can be calculated by the formula: 22 In which B is the absorption constant for indirect transitions, absorbance (A) is proportional to the absorption coefficient (a). Here, a is replaced by A. The insets of Fig. 13 display that the band gaps of NiO nanowire-in-nanotube structured nanobers and NiO hollow nanobers are 3.4 and 3.7 eV, respectively, which are close to the reported values of NiO (3.6-4.0 eV). 22 Fig. 11 Photocatalytic water splitting activities of NiO nanowire-innanotube structured nanofibers (S2) and NiO hollow nanofibers (S13). The possible mechanism for photocatalytic hydrogen generation over NiO nanostructures is as following: the CB position of NiO can be expressed empirically by the formula: 30 where E g is the band gap energy of the NiO, E C is the energy of free electrons on the hydrogen scale (ca. 4.5 eV), X is the electronegativity of NiO. 31 CB potential of NiO nanowire-innanotube structured nanobers and NiO hollow nanobers are calculated to be À3.6 eV and À3.79 eV, respectively. The VB potential of NiO can be calculated by the formula: Aer calculation, the VB potential of NiO nanowire-innanotube structured nanobers and NiO hollow nanobers are À0.2 eV and À0.09 eV, respectively, as indicated in Fig. 14. The band gap of NiO nanowire-in-nanotube structured nano-bers (3.4 eV) is much smaller than that of NiO hollow nano-bers (3.7 eV). The narrower the band gap of the sample, the easier the electrons are excited in the valence band, which results in the fact that the photocatalytic reaction on NiO nanowire-in-nanotube structured nanobers is more easily to occur than that on NiO hollow nanobers under the same energy of light. Under visible light irradiation, methanol, which acts as a sacricial electron donor, can fast remove the photogenerated holes and/or photo-generated oxygen in an irreversible fashion, thereby restraining electron-hole recombination and/or the reverse reaction of H 2 and O 2 . 32 When NiO nano-bers are exposed to visible light, the energy of a photon is absorbed by an electron in the valence band of NiO nanobers. The photogenerated electron (e À ) is excited to the conduction band and simultaneously leaves behind a positive hole (h + ) in the valence band. Subsequently, OH À and H 2 are produced to reduce a water molecules by a photogenerated electron (e À ). At the same time, the separation efficiency of the electron-hole pairs is enhanced, due to the reaction of the CH 3 OH reagents with the photogenerated hole (h + ). It was proposed that the reaction equation for production-H 2 from water splitting is as follows: 33 NiO + hn / h + + e À

Conclusions
We propose a simple and universal technique to synthesize metallic oxide nanowire-in-nanotube structured nanobers by two-stage calcination of electrospinning-made composite nanobers. NiO nanowire-in-nanotube structured nanobers with space group of Fm 3m were fabricated by this fabrication method for the rst time. The outer diameter and wall thickness of nanotubes and embedded nanowire diameter are 130 AE 0.99 nm, 30 nm and 40 nm, respectively. NiO nanowire-innanotube structured nanobers used as photocatalyst for water splitting exhibit higher H 2 evolution rate of 622 mmol h À1 than NiO hollow nanobers of 472 mmol h À1 under visible light illumination owing to its special nanostructure. This technique  we newly designed and proposed possesses universality and guiding signicance to fabricate other nanowire-in-nanotube structured materials. The step for removing middle layer of the as-prepared bers in conventional methods can be omitted by using our method, so that the preparation procedure is simplied. Moreover, for nanowire-in-nanotube structured materials, the spatial characteristics between embedded nanowire and nanotube may be considered as potential applications in the elds of photocatalysis, drug loading and delivery, sensor, and Li-ion battery.

Conflicts of interest
There are no conicts of interest to declare.