K, Na and Cl co-doped TiO₂ nanorod arrays on carbon cloth for efficient photocatalytic degradation of formaldehyde under UV/visible LED irradiation

Abstract

For applicable air purifiers to decompose formaldehyde, it is important to develop TiO₂ based photocatalysts that can be irradiated under visible light and can be anchored on a soft carbon cloth substrate. Herein, we report that rutile TiO₂ nanorod arrays were grown on carbon cloth (TiO₂@CC) by a seed assisted hydrothermal method and subsequently heated with a KCl and NaCl covering to promote K, Na, and Cl ions doping into the TiO₂ nanorods. K, Na, and Cl doping tunes the energy band structure of rutile TiO₂, while the carbon fibers facilitate the electron transfer, improving its photocatalytic activity. Thus, the alkali metal (K, Na) and halogen (Cl) co-doped rutile TiO₂ nanorod arrays (KNCTC) exhibited better photocatalytic degradation of gaseous formaldehyde (HCHO) under UV and visible light irradiation than pure TiO₂. TiO₂ nanorod arrays anchored on soft carbon cloth facilitate the design of an air purifier for degrading formaldehyde.

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1. Introduction

Nowadays, indoor air pollutants have raised pressing concerns with their adverse effects and the fact that they can even be hazardous to human health. Formaldehyde is one of the leading volatile organic compounds (VOCs) among indoor pollutants emitted from decoration materials and furniture. Commonly used air purification methods include adsorption, biocatalysis and photocatalytic oxidation (PCO).^1–4 PCO is an effective approach and the main direction of the latest generation of air purification technologies to decompose chemical pollutants into harmless molecules and even to eliminate gaseous pollutants at low concentrations. To develop applicable and efficient photocatalysts for air purifiers, two primary factors should always be considered: 1) gaining the visible light response of photocatalysts and 2) anchoring the nanostructured catalysts on a soft carbon substrate.

Semiconductor photocatalysis has received extensive attention for eliminating environment pollutants and providing green energy, ever since Fujishima and Honda discovered the photocatalytic water splitting phenomenon of TiO₂ semiconductor electrodes.⁵ Such TiO₂ based photocatalysts have broad application prospects in sewage treatment, air cleaning and virus sterilization.^6–10 TiO₂ is considered to be the most promising photocatalyst for the treatment of air pollutants due to its strong oxidation ability of photogenerated carriers. Its band gap varies for different crystal structures. The band gap is 3.2 eV for anatase and 3.0 eV for rutile. Our recent studies have shown that the anatase TiO₂ nanowires have good photocatalytic activity under ultraviolet irradiation.^9,10 Rutile TiO₂ nanorods achieved photocatalytic water splitting under ultraviolet irradiation.^11,12 Several strategies have been adopted to improve the visible light response, such as introducing impurities or defects on the surface,^11–13 depositing precious metals, other metal oxides, and sulfides,^14–18 and doping with metal and nonmetal ions.^19–27 In the literature, anion and cation co-doping can improve the structural stability of TiO₂.^28,29 On the one hand, doping alkali metal ions (such as Li⁺, Na⁺, and K⁺) shifts the absorption edge toward a longer wavelength and decreases the band gap. On the other hand, doping halogen ions (F⁻ and Cl⁻) narrows the band gap and shifts the adsorption edge toward a higher wavelength. Therefore, alkali and halogen element co-doping could be explored for efficient catalysts for the photocatalytic degradation of formaldehyde.

On the other hand, the vertically oriented growth of 1D nanostructures on flexible carbon fiber cloth has attracted widespread attention. Carbon cloth has an excellent carrier transport performance and high specific surface area, and is easier to recycle and reuse than powdered catalysts.^30–34 The growth of vertically oriented 1D nanostructures on the flexible carbon fiber cloth tends to build nanostructure arrays and heterojunctions, enhancing the photocatalytic activity. As for the design of the photocatalytic reactor, nanostructured catalysts anchored on a soft substrate not only provide a large surface area, but also simplify the filling process compared with particulate catalysts, facilitating the design of air purifiers.

Herein, K, Na, and Cl co-doped rutile TiO₂ nanorod arrays on carbon cloth (TiO₂@CC) were prepared and then were used for the photocatalytic degradation of formaldehyde. Rutile TiO₂ nanorod arrays were grown on CC by a seed-assisted hydrothermal method, and subsequently heated with a KCl and NaCl covering to promote K, Na, and Cl doping into the TiO₂ nanorods, tuning the band structure of rutile TiO₂. Such approaches employed in this work could provide a pathway to grow efficient photocatalysts on a flexible substrate.

2. Experimental section

2.1 Chemicals

Titanium butoxide (C₁₆H₃₆O₄Ti, 99%), titanium tetrachloride (TiCl₄, 99.5%), absolute ethanol, hydrochloric acid (HCl, 37%), nitric acid (HNO₃, 68%) and sulphuric acid (H₂SO₄, 98%) were purchased from Sinopharm Chemical Reagent Co.

2.2 Preparation of rutile TiO₂@CC

Rutile TiO₂ nanorod arrays were grown on a carbon cloth (CC) substrate by a seed-assisted hydrothermal method.³⁵ Firstly, carbon cloth was immersed in an acid solution (H₂SO₄ : HNO₃ = 3 : 1) for 24 h, cleaned ultrasonically in deionized water, and then dried at 80 °C. Secondly, the well-cleaned carbon cloth was immersed into a solution (dissolving 220 μL TiCl₄ in 10 mL of 2.4 M HCl) for 12 h, then dried at 80 °C. The dried carbon cloth was heated at 400 °C for 30 min, forming TiO₂ nanoparticles coated on CC as seeding sites. Thirdly, 1 mL titanium butoxide was added dropwise to 24 mL of 6 M hydrochloric acid. The mixture was stirred ultrasonically until a clear solution was achieved. This clear solution and the CC-coated TiO₂ nanoparticles were transferred into a 100 mL Teflon-lined autoclave and heated at 160 °C for 5.5 h. Finally, after the autoclave was cooled down to room temperature, the CC was removed from the autoclave, rinsed with deionized water and ethanol, and dried at 80 °C to obtain TiO₂@CC (Fig. S1†). TiO₂@CC was annealed in air at 550 °C for 1 h to obtain rutile TiO₂ nanorod arrays on carbon cloth, denoted as TC.

2.3 Preparation of K, Na and Cl doped TiO₂@CC

K, Na and Cl co-doped TiO₂ nanorod arrays were achieved by a thermal diffusion method with molten salt. NaCl and KCl with a mass ratio of 1 : 1 were ground with a pestle and mortar. The TiO₂@CC covered with the NaCl and KCl mixture was annealed in air at 550 °C for 1 h. Then TiO₂@CC was sintered with deionized water and dried at 80 °C to obtain K, Na and Cl doped TiO₂@CC, denoted as KNCTC.

In a similar annealing process, TiO₂@CC covered with KCl was annealed at 550 °C to obtain the K and Cl doped TiO₂, denoted as KCTC, while TiO₂@CC was covered with NaCl to obtain Na and Cl doped TiO₂, denoted as NCTC.

2.4 Characterization of rutile TiO₂ nanorods

The catalysts were characterized with X-ray diffraction (XRD, Shimadzu 7000), transmission electron microscopy (TEM, Libra 200FE, Zeiss), scanning electron microscopy (SEM, JSM-7800F10100), X-ray photoelectron spectroscopy (XPS, Thermo Fisher ESCALAB 250Xi), Raman spectroscopy (LabRAM HR Evolution), UV-vis diffuse reflection spectroscopy (Agilent Cary 5000), photoluminescence (PL, Hitachi F-7000), and electron paramagnetic resonance (EPR, Bruker EMX nano). Electrochemical tests were performed on an AUTOLAB workstation (PGSTAT 302N) using a three-electrode system at room temperature. An FTO conductive glass film with an area of 1 cm² was used as the working electrode, the electrolyte was a 0.5 M Na₂SO₄ solution, and the light source was a 500 W xenon lamp with a 365 nm cut-off filter.

2.5 Evaluation of photocatalytic performance

The photocatalytic performance was evaluated by measuring the changes of formaldehyde concentration in a continuous-flow reactor in an ambient environment, as shown in Fig. S2.† Three pieces of TiO₂@CC (20 mm × 6.5 mm) were placed into a ϕ3 mm quartz tube. Prior to the degradation reaction, the formaldehyde flow rate was well-controlled to achieve an adsorption–desorption equilibrium. An LED lamp (365 nm or 420 nm) was used as the light source. The efficiency of the photocatalytic degradation of formaldehyde was calculated by comparing the formaldehyde concentration change at the inlet and outlet of the quartz tube. The formaldehyde concentration was continuously monitored by the formaldehyde sensor (Dart Sensor Ltd, UK). The formaldehyde degradation rate was calculated according to the following equation:where C₀ and C_t are the concentration of HCHO at the inlet and outlet stream, respectively.

3. Results and discussion

3.1 Structural characterization

Fig. 1(a) shows the XRD patterns of TC, NCTC, KCTC, and KNCTC. The main diffraction peaks at the 2θ position of 27.5°, 36.4°, 39.4°, 41.5°, 44.2°, 54.6°, 56.8°, 62.9°, 64.3°, 69.2°, and 69.9° can be readily indexed to the (110), (101), (200), (111), (210), (211), (220), (002), (310), (301), and (112) planes of the rutile phase of TiO₂ (JCPDS 87-0710). The weak peaks at 25.5°, 48.3° are attributed to the (101), (200) planes of anatase phase of TiO₂ (JCPDS 71-1166). The peaks at 62.9, 69.2, 69.9° may correspond to the (204), (116), and (220) planes of the anatase phase (JCPDS 71-1166). In all the catalysts, the hump peak at 26.1° corresponds to the (002) plane of the carbon cloth substrate, which appears to be weaker than that of pure carbon cloth. The XRD results show that the catalysts are composed mainly of the rutile phase and a small amount of the anatase phase of TiO₂. With Na, K, and Cl element doping, the 2θ position for the rutile (110) peak increases slightly, indicating that the lattice constant decreases a little after element doping. It may be inferred that K⁺ and Na⁺ substitute Ti⁴⁺ and Cl⁻ substitutes O²⁻. The Raman spectra in Fig. 1(b) show the typical E_g (487.1 cm⁻¹) and A_1g (649.8 cm⁻¹) modes corresponding to the P42/mnm group of rutile. The broad band at 281.5 cm⁻¹ is due to the multi-photon scattering of rutile phase TiO₂, while the weak peak at 185.5 cm⁻¹ is attributed to anatase phase TiO₂.^36–38 This indicates that the catalysts are composed mainly of the rutile phase of TiO₂, which is consistent with the XRD observations.

Fig. 1 XRD patterns (a) and Raman spectra (b) of TC, NCTC, KCTC, and KNCTC.

SEM images in Fig. 2(a–e) show that the TiO₂ nanorod arrays are uniformly grown on the carbon cloth, completely covering the surface of carbon fiber. Fig. 2(f) shows the cross-section of KNCTC, in which the thickness of the TiO₂ nanorod arrays is about 2 μm and the TiO₂ nanorods are a length of about 2 μm and a width of about 100 nm. The TEM image of KNCTC in Fig. 2(g) further shows that some pores exist in the TiO₂ nanorods, indicating that it is a porous material. The HRTEM images of KNCTC, as shown in Fig. 2(h) and (i), demonstrate that the distances between the lattice fringes, 0.32 nm and 0.20 nm, can be assigned to the (110) and (210) of rutile TiO₂, respectively. The EDS element mapping of KNCTC, as shown in Fig. 2(j–o), indicates that Ti, O, Na, K, and Cl are uniformly distributed on the background of the carbon fiber. The results confirm that uniform rutile TiO₂ nanorod arrays are successfully grown on the carbon cloth and K, Na, and Cl elements are doped in the TiO₂ nanorods.

Fig. 2 SEM images of carbon cloth (a), TiO₂ nanorod arrays@CC (b), NCTC (c), KCTC (d), and KNCTC (e). A SEM image of the KNCTC cross-section (f). A TEM image (g) and HRTEM images (h and i) of KNCTC. EDS element mapping of KNCTC (j–o).

X-ray photoelectron spectroscopy (XPS) was performed to study the chemical states of all the catalysts. Fig. 3(a) shows the XPS scan spectra of TC and KNCTC. The Ti 2p and O 1s peaks for TC can be seen, and the additional peaks K 2p, Na 1s, and Cl 2p for KNCTC appear compared with CC, indicating that the K, Na, and Cl elements have been successfully introduced into the catalysts. The atomic ratio of Ti to O is about 1 : 1.77, demonstrating that the surface of TiO₂ is oxygen deficient. In the Ti 2p spectra shown in Fig. 3(b), two peaks at a binding energy of 457.7 eV and 463.4 eV are assigned to the spin–orbital splitting doublet of 2p_3/2 and 2p_1/2, respectively, corresponding to Ti⁴⁺ in TiO₂.^39,40 With the element doping, the binding energy of Ti 2p shifts by 0.2 eV to a lower binding energy due to the electron transfer from alkali metals Na and K to Ti and O of TiO₂; this could also be partly attributed to the further reduction of Ti by the addition of alkali metal ions.^41,42 The O 1s peak, as shown in Fig. 3(c), can be fitted to the peaks at 528.9 eV, 530.4 eV, and 531.5 eV, which correspond to oxygen in the Ti–O lattice and physiosorbed H₂O and O₂, respectively. The Ti–O peak shifts to the lower binding energy of 0.1 eV after element doping, which is attributed to the influence of K, Na alkali metal and Cl element doping on lattice oxygen. The Na 1s spectrum in Fig. 3(d) shows that the Na 1s peak is located at 1070.4 eV, indicating that Na atoms exist in TiO₂ in the form of Na⁺.^41,42 The K 2p spectrum as shown in Fig. 3(e) shows that 2p_3/2 and 2p_1/2 peaks are located at 294.7 eV and 291.9 eV, respectively, indicating that K atoms exist in TiO₂ as K⁺.^41,42 The Cl 2p spectrum in Fig. 3(f) shows that the 2p_3/2 and 2p_1/2 peaks are located at 198.5 eV and 196.9 eV, respectively, indicating that Cl atoms exist in the form of O–Ti–Cl.^43–45 The atomic ratios for KNCTC are 1, 1.77, 0.06, 0.26, and 0.02, for Ti, O, K, Na, and Cl, respectively. This shows that element doping induces oxygen deficiency to fulfill the electric neutrality.

Fig. 3 XPS scan spectra (a), Ti 2p (b), O 1s (c), Na 1s (d), K 2p (e), and Cl 2p (f) of TC and KNCTC.

3.2 UV-vis and PL spectra

Fig. 4(a) shows the UV-vis reflectance spectra of all catalysts, with a band-edge adsorption around 434.6 nm, and the absorption edge red shifts with the doping of elements. The plots of the transformed Kubelka–Munk function versus the photon energy give the band gap as 2.99 eV, 2.99 eV, 2.99 eV, and 2.98 eV, for TC, NCTC, KCTC, and KNCTC, respectively, indicating that K, Na, and Cl co-doping narrows the band gap. Among those catalysts, the three-element doping narrows the band gap the most and increases visible light absorption. The results imply that element doping can tune the band structure of TiO₂ and increase the harvesting of visible light. Fig. 4(b) shows the PL spectra with the excitation wavelength of 295 nm, and the fluorescence band is about 409.2 nm. In Fig. 4(b), obvious fluorescence quenching occurs after element doping, and KNCTC can achieve effective separation of electron–hole pairs.

Fig. 4 UV-vis reflectance (a), and PL (b) spectra of TC, NCTC, KCTC, and KNCTC.

3.3 Electrochemical measurements

Fig. 5(a) shows the transient photocurrent response under the excitation with light wavelength λ > 365 nm. The current response is in consequence of KNCTC > KCTC > NCTC. The photocurrent of KNCTC, KCTC, NCTC, and TC tended to be stabilized, indicating that the transient photocurrent response of KNCTC, KCTC, NCTC, and TC was stable under visible light illumination. The KNCTC electrode has a high transient photocurrent response, which means that more photoelectrons can be generated. The doping of K, Na, and Cl increases the lag after turning off the light, indicating that element doping is beneficial to the effective separation of electron–hole pairs. Electrochemical impedance spectroscopy (EIS) results are shown in Fig. 5(b), including the circuit diagram fitting selection circuit. After element doping, the interface reaction impedance (Rct) decreases, and the impedance is in consequence of KNCTC < NCTC < KCTC < TC. The minimum impedance of KNCTC can achieve effective separation of electron–hole pairs. The Mott–Schottky curves are shown in Fig. 5(c). The flat band potential of TC is −0.32 V, the flat band potential of NCTC is −0.43 V, the flat band potential of KCTC is −0.25 V, and the flat band potential of KNCTC is −0.30 V. The conduction band potentials are −0.32 V, −0.43 V, −0.25 V, and −0.30 V, respectively. Na and Cl doping makes the conduction band negatively shift, and the co-doping of K, Cl and the three elements shifts the conduction band positively. The schematic diagram of the energy band structure is obtained from Fig. 4(a) and 5(c). As shown in Fig. 5(d), the co-doping of the three elements shifts the conduction band positively, which is beneficial to the generation of ˙OH and ˙O₂⁻ to achieve formaldehyde degradation.

Fig. 5 Photocurrent response (a), EIS (b), Mott–Schottky plots (c) and schematic band structure (d) of TC, NCTC, KCTC, and KNCTC.

3.4 Photocatalytic performance

The photocatalytic degradation of formaldehyde was carried out in a quartz tube reactor under room temperature and humidity of 60%, and the inlet concentration of formaldehyde was set to 0.4 ppm. Fig. 6(a) shows the photocatalytic degradation performance of catalysts under 365 nm LED radiation with light intensity of 1.5 W. After the adsorption equilibrium is reached, the light is turned on to degrade the formaldehyde. The outlet concentration of formaldehyde gradually decreases to the minimum value, indicating that the catalyst reaches the limit of the formaldehyde degradation in a short time interval. Then, the outlet formaldehyde concentration rises a little after a period of degradation and eventually reaches equilibrium saturation. After the light is turned on, the maximum formaldehyde degradation rates are 72%, 78%, 86%, and 96% for TC, NCTC, KCTC, and KNCTC, respectively. Then, the degradation rate increases a little after a period of time. Finally, the degradation rate eventually reaches the equilibrium saturation of 58%, 60%, 70%, and 86% after 13 h of light irradiation. As the KNCTC exhibits the highest degradation rate among all the catalysts, the element doping is evidenced to be beneficial to the degradation of formaldehyde. All the catalysts show a similar degradation behavior under 1.7 W 420 nm LED irradiation, as shown in Fig. 6(b). The maximum formaldehyde degradation rates reach 36%, 74%, 92%, and 100%, for TC, NCTC, KCTC, and KNCTC, respectively. Eventually the degradation reaches an equilibrium saturation of 32%, 68%, 80%, and 90% for TC, NCTC, KCTC, and KNCTC, respectively. KNCTC has the highest degradation rate among all the catalysts under visible light. Thus, element doping is also beneficial to enhancing the degradation ability of formaldehyde under visible light. Element doping can induce a supplementary energy level, facilitating the generation of electrons/holes under visible light irradiation.⁴⁶Fig. 6(c) shows the formaldehyde degradation stability of KNCTC under 365 nm LED, with light intensity of 1.5 W. The formaldehyde degradation rate reaches a limit of 88% and an equilibrium saturation of 84% after turning on the light for 1 h and 20 h. Moreover, the formaldehyde can be completely decomposed by switching to a strong light intensity of 4.4 W. The degradation of formaldehyde does not decrease after 20 cycles in 48 h, indicating that the catalyst is quite stable and can be used continuously.

Fig. 6 Photocatalytic degradation of formaldehyde under 365 nm LED (a) and 420 nm LED (b) of TC, NCTC, KCTC, and KNCTC. The stability test (c) of KNCTC.

3.5 Photocatalytic mechanism

In order to understand the photocatalytic performance of KNCTC, DMPO (5,5-dimethyl-1-pyrrole N-oxide) was used as a free radical trapping agent, and the active free radicals generated on the catalyst surface were identified by EPR technology. Fig. 7 shows the EPR measurement results of KNCTC in DMPO–˙O₂⁻ in a methanol dispersion system and DMPO–˙OH in a water dispersion system. Upon Xe lamp (λ > 365 nm) irradiation, the two samples in the methanol dispersion system show obvious 1 : 1 : 1 : 1 peaks corresponding to ˙O₂⁻, and those in the water dispersion system show 1 : 2 : 2 : 1 peaks corresponding to ˙OH. This indicates that KNCTC produces ˙O₂⁻ and ˙OH to degrade formaldehyde. The EPR experiments show that the hydroxyl radical (˙OH) and superoxide radical (˙O₂⁻) are the main reactive oxygen species (ROS) involved in the photocatalysis. The ROS are generated through the interaction of photoinduced charge carriers with H₂O and O₂ adsorbed on the catalyst surface. Under visible light irradiation, the photoinduced electrons are trapped by O₂ to yield ˙O₂⁻, and ˙O₂⁻ reacts with water to form –OH. HCHO is then oxidized into a formate species with the surface –OH. Holes in the VB directly oxidize –OH and/or water to produce ˙OH. Subsequently, the formate is further oxidized to CO₂ and H₂O by ˙OH.⁹ On the other hand, TiO₂ nanorod arrays on carbon cloth have several advantages: 1) the photoinduced electrons are rapidly injected into the conduction band and transported along the carbon fibers because of their high electrical conductivity; 2) TiO₂ nanorod arrays on carbon cloth have a large surface area and can be easily assembled in the quartz tube reactor; 3) TiO₂ nanorod arrays on soft carbon cloth could be integrated into curtain fabrics for indoor air purification.

Fig. 7 EPR spectra in methanol dispersion for DMPO–˙O₂⁻ and aqueous dispersion for DMPO–˙OH for KNCTC in the dark and under Xe light irradiation (λ > 365 nm).

4. Conclusions

In summary, rutile TiO₂ nanorod arrays were grown on carbon cloth by a seeding assisted hydrothermal technique, and were then doped with K, Na, and Cl by thermal diffusion. The photocatalytic degradation results showed that element doped TiO₂ arrays exhibited an excellent visible light response. Especially, KNCTC showed the enhanced photocatalytic degradation of formaldehyde with good stability under UV and visible LED irradiation. Such rutile TiO₂@CC realized the immobilized integration of catalysts on a flexible substrate with good catalytic performance, providing a more efficient and convenient design for air purifiers than nanoparticles.

Author contributions

The manuscript was written through contributions of all the authors. All the authors have given approval to the final version of the manuscript.

Conflicts of interest

The authors declare no competing financial interest.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (51801164), Fundamental Research Funds for Central Universities (XDJK2020C005), Venture & Innovation Support Program for Chongqing Overseas Returnees (cx2018080).

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: 10.1039/d0cy01918f

‡ These authors contributed equally to this work.

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