High photocatalytic activity of carbon doped TiO2 prepared by fast combustion of organic capping ligands
As a semiconductor photocatalyst, TiO2 has been extensively investigated for decades because of its outstanding stability, environmental friendliness and low cost.1-3 Many efforts have been made to improve its photocatalytic efficiency by extending its absorption to visible light or promoting photogenerated electron-hole separation on its surface. 4-6 Especially, improving absorption in visible range is very important for application considering that only about 4% solar light can be used to excite TiO2.7 Since Asahi et al. demonstrated band-gap narrowing and high photocatalytic activity under visible light of nitrogen doped TiO2,8 non-metal doping has been widely applied to modify band-gap structure of TiO2 to initiate visible light activity9.
In addition to nitrogen10, 11, carbon has also been well accepted as an efficient doping element to make TiO2 sensitive to visible light. Khan12 et al. firstly reported that TiO2–xCx (x ~ 0.15) absorbed light at wavelengths below 535 nm and performed water splitting at an applied potential of 0.3 V. Sakthivel and Kisch found good photocatalytic property of carbon doped TiO2 in degradation of 4-chlorophenol under diffuse indoor daylight.13 Various methods have been developed to synthesize carbon doped TiO2 and enhanced visible light activity of the doped photocatalyst has been verified by many groups subsequently.14-18 Among these methods, heating precursor containing carbon species is the most commonly used one for doping TiO2 with carbon19-21 . However, the existing states of carbon in these photocatalysts have triggered a lot of arguments. Carbon was proposed to substitute a lattice atom in some reports,12, 15, 22 while interstitial carbon atoms were believed to dominate in other works.16, 23 Recently, Kisch and co-workers have attributed the visible light activity of some “carbon-doped” TiO2 to the sensitization of TiO2 by aromatic carbon compound.24 Moreover, no visible activity or detrimental effect of C doping on photocatalytic activity of TiO2 under UV light was observed in some cases.25-27
Herein we report an easy way to fabricate carbon doped TiO2 with high photocatalytic activity for hydrogen production via fast combustion of organic capping reagents. During surface modification of TiO2 with carbon via heating oleylamine wrapped ultrathin TiO2 nanosheets,28 we noticed that elevating calcination temperature or heating rate in air would dope carbon into TiO2 lattice. By varying the temperature ramp-up rates, we prepared several carbon doped TiO2 photocatalysts. A high and stable hydrogen generation rate was observed on those prepared under a high heating rate, which benefits fast combustion of oleylamine ligands. In contrast, those prepared under a low heating rate exhibited a relative low activity under UV light and no activity under visible light. We think fast and slow combustion of oleylamine ligands may lead to different carbon doping states in the final products.
Materials The following chemicals were used as received without further purification. Titanium (IV) isopropoxide (Ti(OC3H8)4 , 95%) was purchased from Alfa Aesar. Oleylamine (OAm, 70%) was purchased from Sigma Aldrich. NaOH (≥96.0%), H2PtCl6 (Pt ≥37.0%), Na2SO4 (>99%), ethanol (>99.7%), methanol (>99.5%) and hexane (>97%) were purchased from Sinopharn Chemical Reagent Co., Ltd. Deionized water was purified by using a highpurity water system (Millipore Milli-Q, resistivity>18.2 MΩ·cm) before it was used. Synthesis of carbon-doped TiO2 nanomaterials. Oleylamine wrapped ultrathin TiO2 nanosheets were first synthesized by a thermal decomposition method reported previously.28 Carbon-doped TiO2 nanomaterials were then prepared by calcining the as-obtained TiO2 -oleylamine nanosheet precursor in a muffle furnace (F47920-33-80, Thermo Scientific Co., USA) at different temperatures in air. For fast heating rate, TiO2 -oleylamine precursor was placed into the muffle furnace which was preheated to certain temperature, and then calcined for 0.5 (500 °C) or 1 h (400 °C). The samples produced via fast combustion are designated by F-500-0.5 and F-400-1, respectively. For normal heating process (low heating rate), the precursor was firstly heated to 300 °C at a ramp rate of 10 °C min-1, held for 1 h, then heated to 450 or 500 °C and help for 1 h. The corresponding samples are denoted as N-300-450 and N-300-500, respectively. Preparation of Pt-loaded photocatalysts Approximately 50mg of carbon-doped TiO2 materials was suspended with sonication in 8 mL of deionized water and 0.8 mL of H2PtCl6 (10 g L-1) aqueous solution. The pH value of the solution was adjusted to 12 by 10 M NaOH solution. Then the suspension was stirred at 50 °C for 5 h under ambient atmosphere, followed by adding 2 mL of methanol and irradiated using a 300 W Xenon lamp for 2 h (~ 600 mW cm-2 , CEL-HXF300, Beijing Aulight Co., Ltd.). The Pt-loaded photocatalysts were washed with water and ethanol and centrifuged, followed by drying in an electric oven at 60 °C for overnight. Photocatalytic hydrogen generation An online photocatalytic hydrogen generation system (AuLight, Beijing, CEL-SPH2N) was used to conduct the photocatalytic hydrogen evolution experiments at ambient temperature (25 °C). In a typical photocatalytic experiment, 50 mg of Pt-loaded photocatalyst was suspended in 100 mL of aqueous solution containing 40% of methanol in volume. Prior to irradiation, the suspension was degassed with vacuum pump for 10 min to completely remove the dissolved oxygen and to assure the reaction system in an inertial condition. A magnetic stirrer was applied at the bottom of the reactor to keep a good dispersion of the photocatalysts throughout the whole experiment. The hydrogen evolved was analysed by gas chromatograph (GC) using a thermal conductivity detector (TCD) with nitrogen as a carrier gas. Once the photocatalytic reaction of a testing cycle in 5 h was finished, the reactor was replenished with 2 mL of methanol and degassed in vacuum before starting the subsequent cycles. The above mentioned Xenon lamp (300 W, a total light intensity of 600 mW cm-2) was used as simulated light source. Visible light was acquired by equipping Xenon lamp with a 400 nm longpass filter (UVCUT400, AuLight, Beijing, λ>400 nm, ~ 550 mW cm-2). The apparent quantum efficiency (QE) was measured under the same photocatalytic reaction conditions with a 365 nm (~ 90 mW cm-2) and a 420 nm band-pass filter (~ 30 mW cm-2). The QE was calculated according to the following equation:
Photocurrent measurement of PEC cells
Transient photocurrent response was performed on a Zennium electrochemical workstation (ZAHNER, Germany) in a standard three-electrode system with the as-prepared samples as the working electrodes with an active area of ca. 1 cm2 , a Pt wire as the counter electrode and Ag/AgCl (saturating KCl) as the reference electrode, which includes a UV light source (365 nm) and the corresponding control system. 1 M Na2SO4 aqueous solution was used as the electrolyte. For working electrodes, FTO glass (2cm × 1.2 cm) was immersed in an ethanolic dispersion of carbon-doped photocatalysts and followed by drying in the air. This process was repeated several times until the coated photocatalyst reaches ca. 0.4 mg cm-2 on FTO glass. The amperometric I-t curves were recorded under an illumination of 10 mW cm-2 for three 50 seconds light-on-off cycles.
The morphologies of carbon-doped photocatalysts were observed by transmission electron microscopy (TEM) images recorded using a JEOL JEM1011 TEM operated at 100 kV and high-resolution TEM (HRTEM) images recorded on a JEOL 2010F operated at 200 kV. Samples were prepared by drop casting a dispersion drop on a 300 mesh carbon-coated copper TEM grid followed by drying at ambient atmosphere. All X-ray diffraction (XRD) patterns were recorded on a PANalytical Empyrean diffractometer equipped with a Cu Kα radiation (λ= 1.5406 Å). Each Fourier transform infrared spectrometer (FTIR) spectrum was collected on a Tensor 27 FT-IR Spectrometer (Bruker, Germany) after 32 scans at a resolution of 4 cm-1 from 400 to 4000 cm-1. Raman spectra were obtained using a Renishaw InVia Reflex spectrometer (Wotton-under-Edge, UK), operating with an excitation laser wavelength of 532 nm. The diffraction grating gave the spectra with a spectral resolution of 2 cm-1. The specific surface areas of the photocatalysts were determined by Quadrasorb SI-MP (Quantachrome Instrument). All samples were degassed at 60 °C for 10 h before N2 adsorption. The carbon contents of the samples were investigated using an electron probe microanalyzer (EPMA) (Shimadzu, EPMA-1720, Japan). X-ray photoelectron spectroscopy (XPS) was obtained with an ESCALab220i-XL electron spectrometer using 300 W Mg Kα radiation. All binding energies were referenced to the C 1s neutral carbon peak, which was assigned to the value of 284.8 eV to compensate for surface charge effects. Diffuse reflectance UVvisible spectra were recorded with Hitachi U-3010 spectrophotometer and photoluminescence (PL) spectra were measured at room temperature on an Edinburgh Instruments FLS920 spectrometer with 375 nm laser light source. The electron paramagnetic resonance (EPR) spectra of carbon doped nanomaterials were recorded using an ELEXSYS ESR spectrometer (Bruker) operating at 9.7 GHz with a modulation frequency of 50 kHz and a super-high Q microwave cavity. All ESR samples were placed in Quartz ESR tubes with an inner diameter of about 5.8 mm.
Results and discussion
All of the samples obtained after calcination of TiO2- oleylamine nanosheet precursor in air are yellow powders in appearance, in contrast to grey powders of carbon hybrid photocatalysts we reported previously,28 no matter high or low heating rate was used. However, the activity of these photocatalysts differs from one another significantly. Photocatalytic H2 -production activity was evaluated under simulated solar irradiation using methanol as a sacrificial reagent and Pt as co-catalyst. All the measurements were carried out in an online photocatalytic hydrogen generation system at ambient temperature. Hydrogen production rates from the suspensions with different photocatalysts as a function of time are shown in Fig. 1a. The rates of hydrogen production calculated for F-400-1 and F-500-0.5 are 621 μmol·h-1 and 676μmol·h-1, respectively. The stability of these two photocatalysts was also tested by repeating photocatalytic experiments. After four cycles, both the photocatalysts exhibit no significant loss in activity, indicating their good stability in photocatalytic hydrogen production. In contrast, the hydrogen production rate over N-300-450 is around 338 μmol·h-1 in the first 1 h, but decreases quickly to 212 μmol·h-1 in the second cycle. Similar behaviour and slower hydrogen evolution than that of N-300-450 can be observed for N-300-500. It is apparently that F-400-1 and F-500-0.5 show much higher and more stable photocatalytic activity for water splitting than N- 300-450 and N-300-500. Photocatalytic experiment of commercial P25 powders was also tested and the results are shown in Fig. 1a for comparison. In otherwise identical condition, P25 powders present a slightly lower hydrogen evolution rate than those of fast heating process derived photocatalysts. More importantly, evolved hydrogen from water in P25 suspension decreases gradually with increasing the illumination time. The photocatalytic hydrogen production rates under visible light (λ>400 nm) over these carbon doped TiO2 samples were also measured by equipping the light source with a 400 nm longpass filter, and the results are shown in Fig. 1b. Only the fast heating process derived photocatalysts exhibit visible light activity. Hydrogen evolution rates from samples F-400-1 and F-500-0.5 are detected as 5.2 and 5.9 μmol·h-1, respectively. No hydrogen was detected from water with N-300-450 and N-300-500 after 5 h illumination. The apparent quantum efficiencies of F-500-0.5 are calculated to be 6.02% at 365 nm and 0.14% at 420 nm.
To better understand the key factors leading to such a difference in activity among these carbon doped TiO2 photocatalysts, detailed structural analyses were carried out. The crystalline structures of the photocatalysts were confirmed by powder XRD patterns (shown in Fig. 2a). The peaks at 25.52°, 48.01°, 53.96°, 55.04° and 62.68° can be assigned to the (101), (004), (200), (105), (211) and (204) planes of anatase (space group: I41/amd; tetragonal symmetry, a = 3.7852Å, c = 9.5139 Å, JCPDS card no. 21-1272), respectively.29 No diffraction peaks belonging to potential impurities, rutile or brookite can be discerned. According to the half-width at half maximum (FWHM) of (101) peaks of F- 400-1, F-500-0.5, N-300-450 and N-300-500, the crystallite size of anatase estimated using the Scherrer equation are about 11.0, 12.4, 13.2, 14.2 nm, respectively. The crystallite size slightly increases with the rise of calcination temperature and the prolongation of calcination time.
Fig. 2b shows the FTIR spectra of the photocatalysts. The characteristic absorption bands of adsorbed water molecules including stretching vibration ranging from 3200 to 3400 cm-1 and bending vibration at around 1633 cm-1 are clearly visible in all samples, indicating that water is adsorbed on their surface.30 The strong absorption band at ca. 462 cm-1 originates from the vibration of Ti-O bonds, and the absorption bands at 1384 cm-1 can be assigned to O-H in-plane.