Carbon nanomaterials with variable bandgaps exhibit wide spectral absorption, and photoluminescent nanodots have attracted much interest. In this work, carbon nanodots (CNDs) are grafted onto the surface of TiO2 nanotubes to enhance the photocatalytic properties. The CNDs increase light absorption, trap and shuttle photo-generated electrons, and enhance the pollutant adsorptivity. In addition, the synergistic photothermal effect of the CNDs-based nanocomposite facilitates photocatalysis. The CNDs-based nanocomposites with improved photothermal performance and efficient photocatalytic characteristics have large potential in environment and energy applications.
Owing to the distinctive morphology and vertically oriented nanostructure, TiO2 nanotube arrays (TiNTs) have aroused scientific interests for the good performance in photocatalysis and photoelectrochemistry.1,2 However, pure TiNTs have a wide bandgap which restricts optical energy conversion, and measures to maximize the utilization of solar energy have been proposed.3–6
Carbon nanodots (CNDs) have outstanding optical properties, dispersibility in water, excellent biocompatibility, and low toxicity.7–9 Recently, researchers have exploited these attractive properties by preparing nanocomposites comprising CNDs.10–15 The four common functions of CNDs are trapping of photo-generated electrons, extension of light absorption, enrichment of pollutants on composites,16 and upconversion.17,18 CNDs also exhibit photothermal effects, and Ge et al.19 have reported red luminescent CNDs with a photothermal conversion efficiency of 38.5% when irradiated with the 671 nm laser. The strong photothermal effect is a key factor in the photocatalytic performance, but up to now it has not been studied in details. Herein, photo-degradation of methylene blue (MB) by the TiO2-CNDs nanocomposite is investigated, and the role played by the photothermal effect in the degradation of organic pollutants is determined.
The TiNTs were fabricated by electrochemical anodization of titanium (Ti) foils (99.6% pure). The chemical reagents used in the experiments were of analytical grade and used without purification. After cleaning with acetone, ethanol, and deionized water, the Ti sheets were chemically etched by dipping into a solution containing NH4F (0.5 wt. %), glycerol and water (Vglycerol:Vwater = 17:3). Anodization was performed on a two-electrode electrochemical cell at a constant voltage of 50 V for 5 h.20 After anodization, the samples were rinsed with deionized water, dried in air, and annealed at 450 °C for 2 h.
The CNDs were prepared by the modified Kang's method.17 In short, graphite rods were used as the anode and cathode simultaneously, and the electrolyte was an ethanol/H2O solution (volume ratio of 99.5:0.5) containing 0.5 g of NaOH. Since CNDs are produced with a small current density emitted light with longer wavelengths under irradiation,14 electrochemical oxidation was carried out at a current intensity of 20 mA cm−2 for 10 h. After the reaction, a suitable amount of anhydrous MgSO4 (5 wt. %) was added to the solution, stirred for 1 h, and stored for 24 h. The solid salts were then removed by centrifugation, and after ethanol was evaporated at a reduced pressure, the CND powders were obtained.
The TiNTs sheets were placed in a Teflon-sealed autoclave and immersed in CND solutions with different concentrations of 25, 50, and 75 μg/ml. The autoclave was placed in an oven at 120 °C for 10 h. For comparison, some TiNT sheets were processed under the same hydrothermal conditions without CNDs. The samples were washed with deionized water and dried at room temperature prior to the photocatalytic experiments. Degradation of these samples is presented in Fig. S1 in supplementary material. The samples prepared with the 50 μg/ml CND solution are the most efficient, and so, these CNDs/TiNTs were chosen to study the photothermal effect.
X-ray diffraction (XRD) was conducted on a X'PERT diffractometer (Philips Corporation, Holland), and X-ray photoelectron spectroscopy (XPS) was performed on a K-Alpha spectrometer (Thermo Fisher Scientific Corporation, USA). The microstructure and morphology of the samples were examined by scanning electron microscopy (SEM, Hitachi-3400N, Japan) and high-resolution transmission electron microscopy (HR-TEM, JEOL JEM-4000EX). The UV-Vis absorption spectra were acquired on a UV-Vis spectrophotometer (UV-2550, Shimadzu). The photoluminescence (PL) measurements were conducted on an Edinburgh FLS-920 PL spectrometer, and Raman scattering was performed on a T64000 triple Raman system.
The CNDs were nearly spherical in shape as shown in Fig. 1(a), and the particle diameter is about 5 nm. The inset in Fig. 1(a) confirms the crystalline structure of the CNDs with an interplanar distance of 0.32 nm. It deviates slightly from the lattice spacing of the (002) plane of bulk carbon, i.e., 0.338 nm of JCPDS card No. 41-1487 and 0.329 nm of JCPDS card No. 46-0943. Nevertheless, when the structure is ultra-small (<10 nm), there are surface stress and strain at surface and an interplanar distance (002) of 0.32 nm has been observed from CNDs.14,21 Figure 1(b) shows the PL spectra of the CNDs, and the emission spectra range from 400 to 800 nm according to the excitation. The PL quantum yield of the CNDs is determined to be 0.7%, and the low value may be attributed to the efficient non-radiative pathways and vibrational relaxation enabled by the high frequency core and surface modes.