Photocatalytic splitting of water over semiconductor materials  has been considered as a critical approach to achieve the aim  of conversing solar energy to hydrogen energy, which is regard  as potential response to the global energy crisis and  environmental pollution.1-3 With the outstanding stability,  good photostability, nontoxicity, and low cost, TiO2  have been  chosen as a promising semiconductor photocatalyst for  hydrogen generation utilizing solar energy.4,5 However, TiO2 photocatalyst is limited to the UV-light irradiation for  photocatalytic activation and have no effective absorption in  the visible light region (λ> 420 nm) due to a large band gap of  3.2 eV. As is well known, UV light accounts for only around 4%  of the entire solar spectrum while 45% of the solar energy lie  in the visible light region. Therefore, it is essential to develop  novel visible-light responsive photocatalysts for water splitting  with high activity and good stability as an alternative to UVactive photocatalysts.

Recently, iron-based cubic spinel semiconductor with a  chemical formula of MFe2O4  (M = Ca, Mg, Ni, Cu, Zn), in which  divalent ions M2+ is incorporated into the lattice of the  (Fe2+Fe2 3+O4 ) group, exhibits a relatively high photocatalytic  properties for H2  evolution through water oxidation reactions. 6-10 In particular, n-type semiconductor zinc ferrite  (ZnFe2O4 ) with a typical Eg  of about 1.9 eV show effective  absorption of sunlight, high photochemical stability, high  catalytic activity, good durability and low cost. Despite this, the  photocatalytic activity of individual ZnFe2O4  nanoparticles is  pretty poor due to the rapid recombination of photo generated charge and the large scale aggregation of nanoparticles with the ferromagnetic property. Carbon  materials like graphene, reduced graphene oxide (rGO), carbon  nanotube, activated carbon and carbon nonofiber are appropriate as a supporting substrate to hinder the aggregation of semiconductor nanoparticles, while its superior  electrical conductivity could achieve the quick transfer of  electrons and inhibit the recombination of photo-generated  carries efficiently, resulting in the enhancement of the  photocatalytic activity.11-16 Therefore, it is significant and  meaningful to develop novel ZnFe2O4 -carbon nanocomposites  with effective and enhanced photocatalytic performance for  hydrogen generation.

Recently, semiconductor photocatalysts with porous, hollow and flower-like spherical structures have attracted  much attention due to the excellent photocatalytic activity.17-19 .  In this study, carbon-incorporated porous ZnFe2O4 nanospheres have been successfully synthesized by a facile  one-step solvothermal reaction and the subsequent heat  process. The phase structure and morphological information of  the as-prepared photocatalysts were characterized by X-ray  diffraction (XRD) and Field-emission scanning electron  microscopy (FESEM). The existence of graphite-like carbon on  the porous nanosphere was proved by Fourier transform  infrared (FTIR) spectra, X-ray photoelectron spectroscopy (XPS) and Raman spectra. The photocatalytic activity for H2  production of the obtained ZnFe2O4 -carbon nanocomposite  under visible light irradiation (λ> 420 nm) and the photogenerated electron transfer between ZnFe2O4  and  graphite-like carbon were also investigated.

Experimental

Typically, Zn(NO3)2•6H2O, proper stoichiometry of Fe(NO3)3•9H2O  and a desired amount of polyvinylpyrrolidone (PVP) were  completely dissolved into a 30mL ethanol–ethyleneglycol (EG)  mixed solution (the volume ratio of ethanol and EG is 3:7). The  mixtures were uniformly mixed under continuous magnetic stirring  for several minutes, and the homogeneous solution was transferred  into a 50mL Teflon-lined stainless autoclave and maintained at  180 °C for 24 h. The precipitates were obtained by centrifugation  separation, washed several times with deionized water and ethanol  alternately, and then dried in air at 80 °C for 12h. Finally, the  ZnFe2O4 -carbon nanocomposites were acquired by sintering the  precipitates at 500°C for 2h under N2  atmosphere with a heating  rate of 2 °C /min. The as-prepared ZnFe2O4 -carbon nanocomposites  with different amounts of PVP (0.05, 0.1, 0.15 or 0.2g) were named  ZFO-C1, ZFO-C2, ZFO-C3 and ZFO-C4, respectively. As a comparison,  the pure ZnFe2O4  sample without adding PVP obtained in the same  process is denoted as ZFO-C0.  The phase structure of the obtained products were were  analyzed using an X-ray powder diffractometer (XRD, D8 Advance  diffractometer, Bruker Corporation, Germany) using Cu Kα (λ =  1.5419 Å). Simultaneous thermogravimetric and differential thermal  analyses (TGA/DTA) of the samples were performed on a thermal  analyzer (STA 8000) in air with a heating rate of 10 °C min−1The  morphologies and particle sizes of the samples were observed by  Field-emission scanning electron microscopy (FESEM, Zeiss SUPRA  55) and transmission electron microscopy (TEM, Tecnai G2 F20 STWIN). BET surface area and pore structure of samples were  performed on a Quadrasorb SI-MP surface area and porosity  analyzer. Prior to the BET analysis, the powder was degassed at 120  ◦C for 5h to remove the adsorbed H2O from the surface. FT-IR  spectra were taken on a KBr disk in the frequency range of 4000- 400 cm−1 by using a FT-IR spectrophotometer (Spectrum 100). Raman spectra were recorded on a laser micro-Raman  spectrometer (LabRAM HR Evolution) with the exciting wavelength  at 532 nm. The chemical binding energies of the respective ions in  the samples were measured using X-ray photoelectron spectroscopy (XPS) (ESCALAB 250Xi). The UV–vis diffuse reflection  spectra were obtained for the dry pressed disk samples on a UV–vis  spectrophotometer (Cary 5000) by BaSO4  as the reflectance sample  The photoluminescence emission (PL) spectra at room temperature  were characterized by a fluorescence spectrophotometer (ELS980- S2S2-stm) with a 150 W Xe lamp as the excitation source.

The photocatalytic H2  production reactions were carried out a  top-irradiation-type reactor connected to a gas-closed circulation  and evacuation system (AULIGHT CEL-SPH2N). The photocatalytic  activity evaluation of as-prepared samples were performed by  dispersing 50 mg of the photocatalysts in an aqueous solution  containing CH3OH (100 mL, 10 vol%) as a sacrificial electron donor.  The reaction solution was evacuated to remove the air completely  prior to irradiation with a 300W xenon-lamp (CEL-HXF300)  equipped with an optical UV-IR cutoff filter (λ > 420 nm). The  temperature of the reaction mixture was maintained at around 6 °C  by a continuous flow of cooling water. The amounts of the evolved  H2  was analyzed by on line gas chromatography (GC7920) equipped with a thermal conductive detector (TCD), using N2  as the carrier  gas. The H2  generation experiments were performed for 5 hours, and then the photocatalysts were separated from the mixed  solution by the centrifugation process. Another aqueous solution  containing CH3OH (100 mL, 10 vol%) and the separated  photocatalysts were added again into the reaction vessel to start a  new 5 hours cycle.  

Results and Discussion

Fig.1 shows the powder X-ray diffraction (XRD) pattern of the  carbon-incorporated ZnFe2O4  photocatalysts synthesized with  different mass ratio of PVP. It can be seen that all of the recorded  diffraction peaks could be well indexed to the cubic phase of spinel  ZnFe2O4  (JCPDS NO.89-1010), and no diffraction peaks derived from  any other impurities could be observed. The peaks at 2θ values of  30.05°, 35.36°, 42.85°, 53.28°, 56.67° and 62.16° can be attributed  to the reflection of (2 2 0), (3 1 1), (4 0 0), (4 2 2), (5 1 1) and (4 4 0)  planes, respectively. Moreover, it worth noting that no obvious  peaks related to carbon are observed from the XRD patterns of  ZnFe2O4 -carbon nanocomposites, and the existence of carbon on  the ZnFe2O4  nanospheres were proved by other characterization  methods.

The carbon content of as-prepared ZnFe2O4 -carbon  nanocomposites was measured and calculated by TG-DTA analysis.  Fig.2 displays the TG curve of ZnFe2O4 -C3 samples from room  temperature to 800 °C during the calcination process in air. As is  shown, the initial weight loss appearing between room  temperature and 470 °C may be ascribed to the removal of  absorbed water and organic compound. With the calcination  temperatures increasing, there emerges a steep decrease of about  17.5% in mass in the range of 470–770 °C corresponding to the  combustion of carbon.20 Based on the TG results, the carbon  content of ZnFe2O4 -carbon samples with increasing the addition  amount of PVP were determined as 6.2%, 11.7%, 17.5% and 20.1%,  respectively.  

Conclusions

In summary, carbon-incorporated porous ZnFe2O4  nanospheres  have been successfully prepared via a simple hydrothermal  route and the subsequent heat process with PVP as carbon  source. FESEM observations demonstrated that the ZnFe2O4 nanospheres with porous structure and diameter in the range  of 100-200nm were composed of numerous primary particles.  Due to excellent electron transfer property, the presence of  graphite-like carbon in the ZnFe2O4 -carbon nanocomposite  exhibits an effective restraint of the recombination of the  photo-excited electron–hole pairs. Compared with pure  ZnFe2O4 , the photocatalytic hydrogen evolution rate of carbonincorporated ZnFe2O4  nanocomposites photocatalyst in the presence of methanol sacrificial reagent under visible light  irradiation is greatly improved. Together considering the  magnetic separation ability and stability, ZnFe2O4 -carbon  nanospheres are a promising candidate as visible-light excited  photocatalyst for photocatalytic hydrogen generation.