Introduction

The discharge of industrial sewage and organic wastewater pollutes the environment and threatens the health of wildlife and human [1, 2]. The development of society and the energy crisis has increased the demand for clean energy [3,4,5]. Photocatalysis is an excellent solution for sewage treatment and environmental protection [6,7,8].

The application of TiO2 as a photocatalyst has attracted significant attention due to its fast, non-selective, and complete degradation of organics with respect to biodegradation technique. Meanwhile, TiO2 is also superior to the traditional chemical oxidation methods due to its high photocatalytic activity, low cost, and non-toxicity [9,10,11,12]. Therefore, TiO2 has broad application prospects in environmental pollution control. The catalytic activity of TiO2 is often closely related to its microscopic morphology. The research on TiO2 at the micro- or nanometer scale has attracted extensive attention. One of the technical difficulties of using TiO2 nanopowders in water or air treatments is the recovery and reuse of TiO2, which normally bears a very high cost. TiO2 powder particles can also become agglomerated in water, which reduces their specific surface area and greatly reduces the density of the reactive sites [13]. Therefore, researchers have attempted to immobilize TiO2 nanomaterials on other solid substrates, such as SiO2 [14,15,16], Al2O3 [17, 18], activated carbon [19, 20], and carbon fibers [21, 22]. Compared with other basic materials, CFs have unique properties, including good flexibility, excellent mechanical strength, and acid and alkali corrosion resistance [23,24,25].

Nano-TiO2 can be introduced to the surface of CFs through various methods, such as chemical vapor deposition, surface spraying, sol–gel, and supercritical synthesis [26,27,28,29,30,31]. These methods are complex and require specialized equipment. In this work, TiO2 nanomaterials were generated on the surface of CFs through the TiC intermediates. TiC was converted to titanate nanowires through a hydrothermal reaction and an ion exchange process. The titanate was converted into TiO2 after calcination. The process is simple, easy to control. TiO2 was successfully fixed on CFs.

In recent years, TiC has been found to have the electronic structure and the catalytic activity similar to platinum [32]. The anisotropic structures of nanomaterials can provide favorable carrier transport channels and have therefore attracted the attention of photoelectric catalysis through semiconductors [33, 34]. The TiC transition layer, formed on the surface of CFs, offers a strong substrate for TiO2 grown on CFs. The uniformly distributed TiO2 can effectively prevent the shedding of the TiO2 thin film and minimize the agglomeration of nanoparticles. Thus, its catalytic performance can be effectively optimized and maintained. TiC, which is a transition metal carbide, shows similar chemical stability and catalytic performance to precious metals. After hydrothermal reaction with NaOH, sodium titanate nanowires with controllable micromorphology were obtained on the surface of CFs. After ion exchange and calcination, CFs@TiC/TiO2 composite materials were obtained. The photocatalytic degradation of RhB dye and reduction of Cr(VI) was studied, and the influence of different morphology of the heterogeneous structure of CFs@TiC/TiO2 on the degradation of RhB and reduction of Cr(VI) were discussed.

Experimental

Preparation of CFs@TiC samples

The synthesis procedure is illustrated in Fig. 1. CFs (Japan T700, approximately 7 µm in diameter) were used as a carbon source, and TiH2 powder (Shanghai Aladdin Reagent Co., Ltd., 200 mesh) was used as Ti raw material. The CFs were resin-coated, and it is necessary to purify the CFs. The resin-coated CF fiber bundle was placed into the Soxhlet extractor, and the resin on the fiber surface was removed by reflux with acetone at 80 °C for 72 h. The dispersed pure fibers were collected and vacuum-dried for 24 h at 105 °C. TiH2 particles and KCl (analytical pure) were fully mixed before the CFs were dispersed in the mixture in an alumina crucible. The molar ratio of Ti/C was 0.5, and the mass of KCl was 10 times the weight of the total reactants. The alumina crucible was placed in a tube furnace purged with argon as a protective gas. The temperature was programmed to rise at a rate of 10 °C/min until 1000 °C and hold for 3 h. The sample was cooled to room temperature and thoroughly washed with boiling distilled water several times to remove the excess salt. The fibers were vacuum-dried for 24 h.

Figure 1
figure 1

Schematic diagram of the material synthesis preparation process

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Synthesis of CFs@TiC/TiO2 composite materials

The 0.1 g of TiC-modified CFs and 20 mL of 1 mol/L sodium hydroxide solution were placed in a 25-mL PL high-pressure hydrothermal reactor. The heating rate was 3 °C/min, and the reaction temperature was 240 °C, held for different durations (2 h, 4 h, 6 h, 10 h). After the reaction, the sample was cooled down to room temperature at a controlled cooling rate of 5 °C/min. The fibers were washed repeatedly with distilled water until the pH became neutral. Then, the fibers were placed into a beaker containing 10 mL of 1 moL/L hydrochloric acid solution. The fibers were soaked for 1 h at room temperature followed by washing until neutral. Finally, the fibers were placed into a crucible in a muff furnace. The temperature was raised to 400 °C at a rate of 5 °C/min, holding for 3 h before being cooled and collected.

Structural and morphological characterization

An X-ray diffractometer (D/max2550pc, PIGAKV Co., Ltd.) with a copper target and a scanning step of 0.02° was used to analyze and detect the material crystal structure. The phase structure of the fibers was analyzed using a microlaser Raman spectrometer (LabRAM HR800). The chemical composition and element binding energies were analyzed by XPS (k-alpha, Thermo Fisher Scientific). An electron scanning electron microscope (Helios Nanolab G3 UC, FEI) was used to observe the surface microstructure of the fibers. A scanning probe microscope (NanoManVS + Multimode, Veeco) was used to observe the 3D morphology of the sample surface at the nanoscale. A xenon lamp (PLS-SXE300, Perfect Light, wavelength > 400 nm, UV filter: UVCUT400) was adopted as the light source for photocatalytic reactions. The load of the photocatalysts was 50 mg in a 50 mL solution. The concentration of the RhB solution was 20 mg/L, and the concentration of Cr(VI) solution was 10 mg/L. In total, 0.1 mL of citric acid (concentration of 100 g/L) was added as the trapping agent. The concentration of Cr(VI) was measured by the diphenylcarbazide spectrophotometry method. To eliminate the adsorption effects, the sample solution was stirred in the dark for 30 min before the measuring of the photodegradation rate under the light.

Results and discussion

Structural characteristics

At present, considerable research focuses on obtaining a uniform and compact TiC film layer on the surface of CFs. Such films protect CFs or improve their surface properties to achieve their superior performance in composite materials, especially in enhancing their mechanical properties. In this work, we carefully choose TiC as the transition layer, which could bridge the catalyst and CFs and provide Ti raw material for TiO2. It also offers a large active surface area. In the process of TiC synthesis, the bidirectional diffusion of C and Ti was most likely to cause the formation of a porous or uneven TiC film structure. Existing work [35] has shown that a layer of fragmented graphite structure exists on the surface of CFs, which helps the diffusion of atoms and can provide possibilities for surface reactions.

TiH2 is decomposed into H and Ti raw materials at 400 °C. At 1000 °C, the Ti atoms continue to move toward CFs within the molten salt liquid phase and to form the TiC film layer on the CFs surface. Figure 2a shows the XRD spectra of the composite materials at different stages. The XRD showed peaks of 2θ angles at 35.9°, 41.7°, 62.4°, 72.3°, and 76.14°, which are formed on the surface of CFs. These peaks are associated with the crystal planes of (111), (200), (220), (311), and (222) of the typical cubic structure of a TiC crystal, shown in the red curve in Fig. 2a. The formation of TiC hinders the X-ray penetration, and the intensity of amorphous CFs diffraction peak was reduced. The formed TiC film was then subject to the hydrothermal reaction, acid treatment, and calcination. Part of the TiC in CFs@TiC was transformed into anatase TiO2 (named as CFs@TiC/TiO2) composite material, identified by the anatase TiO2 diffraction peaks of (101) and (200) at 25.3° and 48.0° [36] (blue curve in Fig. 2a), respectively.

Figure 2
figure 2

a XRD of CFs,CFs@TiC,CFs@TiC/TiO2 and b Raman analysis of the CFs@TiC/Na2Ti6O13, CFs@TiC/H2Ti6O13, CFs@TiC/TiO2

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Figure 2b shows the corresponding Raman spectra of samples at different stages. The Raman peaks without hydrochloric acid treatment mainly include four strong peaks at 275, 443, 700, and 915 cm−1 and three weak peaks at 192, 384, and 823 cm−1. The Raman peak located at 275 cm−1 belongs to the Ti–O telescopic vibration, which involves two-coordinate O and is the characteristic vibration peak of titanate. The Raman peak located near 443 cm−1 belongs to the Ti–O bending vibration involving the tri-coordinate O. The Raman peak at 700 cm−1 is associated with the bending vibration of Ti–O in two-coordinate O. The Raman peak located near 915 cm−1 involves the stretching of Ti–O atoms, some of which coordinate with sodium ions to form Ti–O–Na+. After hydrochloric acid treatment, the vibration peak of Ti–O–Na+ at 915 cm−1 disappears due to the proton exchange. After heat treatment, the strong absorption peak of anatase TiO2 (192 cm−1) was observed, and the other existing peaks exhibited a blue shift.

XPS analysis

The electronic structure of the material surface and the chemical state of elements were analyzed and studied by XPS. The relevant chemical elements, such as Ti, O, and C, were monitored in the entire reaction process, shown in Fig. 3.

Figure 3
figure 3

X-ray photoelectron spectroscopy analysis high-resolution scan of a O 1s and b Ti 2p of CFs@TiC/TiO2 and Anatase TiO2, and c C 1s of CFs@TiC/TiO2 and CFs@TiC, respectively

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The XPS high-resolution scan of O 1s for CFs@TiC/TiO2 is decomposed into three peaks at 530.0, 531.6, and 532.4 eV, which are attributed to the lattice O of anatase TiO2, C–O, and the O atom near the oxygen vacancy. Two chemical states of O1s for pure anatase TiO2 at 529.3 eV and 530.4 eV can be found, corresponding to Ti–O and hydroxyl group, respectively. The Ti–O binding energy was increased by 0.7 eV, attributed to the C-doping in CFs@TiC/TiO2. The XPS spectrum of Ti 2p shows two major peaks with binding energies of 458.5 eV (2p 3/2) and 464.2 eV (2p 1/2). The difference in binding energy with an approximate separation of 5.7 eV between Ti2p ½ and Ti2p 3/2 is in good agreement with the standard binding energy value [37]. Compared to pure anatase TiO2 (458.1 eV and 463.8 eV), the binding energies for both peaks were increased by 0.4 eV. This is because of the existence of the positively charged carbon in CFs@TiC/TiO2, which may push the Ti atom toward the adjacent oxygen atom in the TiO2 crystal. This will shorten the Ti–O bond and increase the binding energy of Ti2p. The small peak at 461.3 eV belongs to the Ti3+ oxidation state, which is associated with O vacancy or defect in the sample. From C 1s for CFs@TiC/TiO2, the C bond peaks of C=O, C–O, and C–C sp2 substrates are identified at 288.4 eV, 286.7 eV, and 284.8 eV. The peak at 281.6 eV is attributed to the binding energy of C-Ti. Compared to the sample of CFs@TiC, this peak disappeared mainly due to the conversion of TiC into TiO2. From the co-existence of these chemical bonds, it is expected that some interstitial carbon in the TiO2 lattice could form the Ti–O–C, which helps to promote the absorption in the visible light region and the charge carrier transfer required for photoexcitation. Carbon doping can improve the visible light absorption efficiency which in turn improves the photocatalytic performance.

SEM/AFM analysis

The morphology, size, location, and stability of active components of the catalyst have a close relationship and great influence on its catalytic activity [38, 39]. The growth process of sodium titanate nanowires on CFs was observed by SEM (Fig. 4). The effect of reaction time on the microstructure was studied.

Figure 4
figure 4

SEM images (at different magnifications) of samples prepared with different reaction times in hydrothermal reaction (a 2 h, b 4 h, c 6 h, d 10 h)

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After the molten salt reaction, the TiC was treated with NaOH basic solution in a hydrothermal reactor. On the surface of TiC, thin lamellar Na2Ti6O13 was formed which is wedged into the fiber surface. This suggests that the growth of Na2Ti6O13 at the interface junction between Na2Ti6O13 and TiC is faster than on the Na2Ti6O13 surface. The lamellar structure grows and thickens after the reaction time was 4 h or longer. At the reaction time of 6 h, the bottom of the lamellar structure cross-linked with contraction presents a floral structure. Once the reaction time was extended to 10 h, the neighboring Na2Ti6O13 crystals interact to form a networked structure. Nanowires are also connected between the fibers.

Based on the observed morphological evolution, the growth process of sodium titanate in the hydrothermal reaction is proposed as follows. Small sodium titanate crystal seeds were first formed on the surface of the CFs. As the reaction progresses, the sodium titanate sheet grew and thickened. The nanowires at the bottom became close together, grew into each other, and acted as pestles similar to flowers. The other lamellar structures acted as petals and presented a floral structure. After the reaction time is prolonged, the petals on the surface grew together to form a network-like structure. Hence, the microstructures of the reaction products can be effectively controlled by simply controlling the reaction time. Active sites are concentrated in angles. To obtain the most catalytic active sites, the reaction time is determined to be 4 h.

SEM and AFM were both used to characterize the morphology of the fibers after each step of the reaction.

TiH2 decomposes into Ti and H2 at high temperatures. The presence of the KCl molten salt helps the distribution and reaction of Ti in the reaction system as it provides the liquid system needed for the reaction. This condition lowers the reaction energy barrier and reaction temperature between Ti and CFs. The C atoms were diffused outwardly, whereas the Ti atoms were moving toward the surface of CFs. The two-way diffusion of C and Ti forms a porous TiC structure on the surface of the CFs, as shown in Fig. 5d. The effective diameter of a single fiber increases from the original 7–9 µm, which proves the porous nature of the TiC with reduced packing density directly observed from the SEM images. Such a sponge-like structure provides a large number of reactive sites for the conversion of TiC to sodium titanate nanowires in the next reaction, as shown in Fig. 5g. The appearance of the porous structure provides a useful substrate for developing nanocatalysts.

Figure 5
figure 5

SEM and AFM images of a–c CFs, d–f CFs@TiC, g–i CFs@TiC/Na2Ti6O13 (4 h), j–l CFs@TiC/TiO2 (4 h)

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The cross-sectional view of SEM images of the CFs, CFs@TiC, and CFs@TiC/TiO2 is inserted in Fig. 5a, d, g, and j. The thicknesses of the TiC layer, Na2Ti6O13 layer, and TiO2 layer on CFs surfaces can be measured directly as 2.3, 0.60, and 0.35 µm, respectively.

The surface structure of a catalyst is an important affecting factor of its performance. The changes in the surface structure of the fibers during the entire reaction process can also be observed from the atomic force microscope. Figure 5b, e, h, and k reveals such changes. As shown in Fig. 5b, uneven grooves are present on the surface of CFs. This condition is conducive to the growth of TiC. Once the TiC was formed, the uneven granular structure can be observed on the surface of CFs, shown in Fig. 5e. As such, the interface area between TiC and CFs increases. Such a structure is favorable for the growth and support of TiO2. Acicular sodium titanate nanowires were grown on the surface of the CFs by hydrothermal reaction with NaOH. Proton exchange and heat treatment converted the sodium titanate to the TiO2 forming a multilayer heterogeneous nanostructure on CFs surface, which preserved the TiC nanostructures and maintained a high density of surface catalytic center.

Photocatalytic decomposition of RhB and reduction of Cr(VI)

The photocatalytic degradation of RhB and Cr(VI) solutions at room temperature under the visible light (wavelength > 400 nm) was studied to evaluate the photocatalytic activity of the prepared photocatalysts, including CFs, CFs@TiC, and CFs@TiC/TiO2. Repetitive experiments were also carried out to test the reusability of the catalysts. The experimental results are shown in Fig. 6a–d.

Figure 6
figure 6

a Catalytic degradation of RhB, b repeating experiments of RhB with CFs@TiC/TiO2, c catalytic reduction of Cr(VI), d repeating experiments of Cr(VI) with CFs@TiC/TiO2

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Before catalytic degradation, the tested aqueous solution was stirred in the dark for 30 min to achieve adsorption equilibrium. Neither CFs nor CFs@TiC degrades RhB and reduces Cr(VI) significantly. After partial TiC is converted into TiO2, a heterogeneous structure of TiC and TiO2 was formed on the surface of CFs, which greatly increases the degradation rate of RhB and the reduction rate of Cr(VI). The CFs@TiC/TiO2 was used repeatedly for three times. The results show that the degradation rate and efficiency were maintained constant. Therefore, TiO2, supported by CFs, can be reused under our experimental conditions with good chemical and physical stabilities.

By comparing the microstructure and XPS analysis of the CFs@TiC/TiO2 before and after the reaction, shown in Figs. 7 and 8, it is not difficult to find that the microstructure of the material was well preserved without the obvious phenomenon of membrane fracture and peeling. At the same time, XPS analysis showed no significant changes in the element chemical states on the composite surface. These are important properties for the stability of catalytic performance after repeating tests.

Figure 7
figure 7

SEM of CFs@TiC/TiO2a before reaction and b after reaction

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Figure 8
figure 8

XPS analysis high-resolution scan of a Ti 2p(a) and b O 1s of CFs@TiC/TiO2 before and after catalytic reaction

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Fundamentals and characteristics of catalysts

The main reaction processes can be described by the following chemical reactions:

$${\text{TiH}}_{2} = = {\text{Ti}} + {\text{H}}_{2}$$
(1)
$${\text{C}} + {\text{Ti}} = = {\text{TiC}}$$
(2)
$${\text{6TiC}} + {\text{14NaOH}} + {\text{17H}}_{{2}} {\text{O}} = = {\text{Na}}_{{2}} {\text{Ti}}_{{6}} {\text{O}}_{{{13}}} + {\text{6Na}}_{{2}} {\text{CO}}_{{3}} + {\text{24H}}_{{2}}$$
(3)
$${\text{Na}}_{{2}} {\text{Ti}}_{{6}} {\text{O}}_{{{13}}} + {\text{2HCl}} = = {\text{H}}_{{2}} {\text{Ti}}_{{6}} {\text{O}}_{{{13}}} + {\text{2NaCl}}$$
(4)
$${\text{H}}_{{2}} {\text{Ti}}_{{6}} {\text{O}}_{{{13}}} = = {\text{3TiO}}_{{2}} + {\text{H}}_{{2}} {\text{O}}_{{}}$$
(5)
$${\text{O}}_{2} + \, e^{ - } \to \cdot{\text{O}}_{2}^{ - }$$
(6)
$$\cdot{\text{O}}_{{2}}^{ - } + {\text{2H}}^{ + } + e^{ - } + hv \to {2}\cdot{\text{OH}}_{{}}$$
(7)
$$\cdot{\text{O}}_{{2}}^{ - } + h^{ + } + \cdot{\text{OH}} + {\text{RhB}} \to {\text{degradation products}}$$
(8)
$${\text{Cr}}({\text{VI}}) + {3}e^{ - } \to {\text{Cr}}({\text{III}})$$
(9)

The photocatalytic degradation mechanism is proposed in Fig. 9 c. The ability of the composite to absorb and utilize the visible light comes down to the fact that the carbon dopant acts as a sensitizer into the TiO2 lattice of the prepared core–shell nanostructured material [37]. The existence of a Ti–O–C bond was confirmed by X-ray photoelectron spectroscopy. The residual TiC phase can be used as an electron emitter because of its high conductivity. The doping of carbon atoms introduces dopant states within the bandgap, which allows titanium dioxide to be excited by visible light. With the help of the Fermi level of titanium carbide, the fast electron transfer can be achieved, which improves the separation of the excited holes and electrons in the TiO2. When the CFs@TiC/TiO2 photocatalyst was irradiated with the visible light, the electrons were excited to the conduction band of the TiO2. They can travel rapidly to the TiC and are subsequently captured by O2 in the solution. At the same time, the holes in the VB react with H2O/OH− to create oxidative radicals. Anatase TiO2 reacts with O2 and H2O to produce superoxide radical and ·OH under the visible light [40]. As shown in Fig. 9a and b, the reactive species required for the redox reaction were confirmed by ESR test results. These active radicals were directly involved in the catalytic oxidation of RhB and the reduction of Cr(VI).

Figure 9
figure 9

a ESR spectra for·OH, b ESR spectra for·O2−, c schematic diagram of catalytic mechanism

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Conclusions

In this work, the CFs@TiC precursor was prepared with CFs as the substrate. TiC/TiO2 heterostructure was grown on the surface of CFs by hydrothermal, proton exchange, and calcination method. By simply controlling the reaction time, the reaction process can be controlled and samples with different micronanostructures can be obtained with shapes including needles, flowers, and networking morphology. The nanostructured materials provide a large surface area with a large number of reactive sites for catalysis, which results in good catalytic activities for both the degradation of RhB and the reduction of Cr(VI) under visible light excitation. Repeated experiments demonstrate the good stability of the photocatalytic activity. The catalytic mechanism and reaction processes are also discussed. The proposed simple and easy-to-control preparation technology has broad application prospects in the fields of water treatment and active liquid filtration. It provides new ideas for the application of CFs.