Tailoring TiO2 Nanotube‐Interlaced Graphite Carbon Nitride Nanosheets for Improving Visible‐Light‐Driven Photocatalytic Performance

Abstract Rapid recombination of photoinduced electron–hole pairs is one of the major defects in graphitic carbon nitride (g‐C3N4)‐based photocatalysts. To address this issue, perforated ultralong TiO2 nanotube‐interlaced g‐C3N4 nanosheets (PGCN/TNTs) are prepared via a template‐based process by treating g‐C3N4 and TiO2 nanotubes polymerized hybrids in alkali solution. Shortened migration distance of charge transfer is achieved from perforated PGCN/TNTs on account of cutting redundant g‐C3N4 nanosheets, leading to subdued electron–hole recombination. When PGCN/TNTs are employed as photocatalysts for H2 generation, their in‐plane holes and high hydrophilicity accelerate cross‐plane diffusion to dramatically promote the photocatalytic reaction in kinetics and supply plentiful catalytic active centers. By having these unique features, PGCN/TNTs exhibit superb visible‐light H2‐generation activity of 1364 µmol h−1 g−1 (λ > 400 nm) and a notable quantum yield of 6.32% at 420 nm, which are much higher than that of bulk g‐C3N4 photocatalysts. This study demonstrates an ingenious design to weaken the electron recombination in g‐C3N4 for significantly enhancing its photocatalytic capability.


Experimental section 1.1 Preparation of photocatalyst
Ultralong TiO 2 nanotubes (TNTs) were synthesized by a one-step hydrothermal process according to a previous report. [1] Typically, P25 powder (0.1 g) was dispersed into NaOH solution (15 mL, 10 M) with continuous stirring for 5 min, and then the solution was transferred into 25 mL Teflon-lined stainless steel autoclave with a magnetic stirrer. The autoclave was put inside a silicon oil bath on a hot plate and the reaction temperature was set at 130 °C for 24 h. After the reaction, the autoclave was taken out from oil bath and cooled to room temperature. The product, sodium titanate, was collected by centrifugation, washed with deionized water for several times to reach a pH value of 9. After that, the obtained precipitates were washed with HCl solution (0.1 M) and stirred at room temperature for overnight. The purified precipitates were rinsed with water several times followed by centrifugation. The obtained powders were dried in a vacuum drying chamber at 60 °C for 12 h. The final anatase TiO 2 nanotubes were obtained after annealing at 500 C for 1 h.
The g-C 3 N 4 /TNTs were prepared by heating the mixture of TNTs and melamine. In a typical procedure, obtained TiO 2 nanotubes (100 mg) and melamine (6 g) were mixed in methanol (100 mL) followed by an ultrasonic treatment at 40 °C for 30 min. The mixture was stirred at room temperature for 4 h. The white solid was obtained through drying at 60 °C.
Then, this white precursor in an alumina crucible was annealed at 550 °C inside a muffle furnace with aluminized paper to shelter from oxygen for 3 h. The final products were collected for use without further treatment.
The PGCN/TNTs were prepared by hydrolyzing bulk g-C 3 N 4 /TNTs in alkaline conditions. Briefly, g-C 3 N 4 /TNTs powder (1 g) was mixed with NaOH solution (20 mL, 3 M). The mixture was treated under ultra-sonication for 2 h. Then, PGCN/TNTs were dialyzed to remove excess NaOH using a membrane with molecular weight cutoff of 2000 Da (D306-50, S3 Biodesign Inc., U.S.A.) against water until reaching neutral pH. Finally, the solid PGCN/TNTs were collected with freeze drying.

Characterizations
The morphologies of the as-prepared samples were examined by field-emission scanning electron microscopy (FE-SEM, JSM-7600F, JOEL) at an acceleration voltage of 5 kV. The detailed structure analysis was conducted with transmission electron microscopy (TEM, JEOL JEM2100F) operating at 100 kV. Crystallographic information was collected by powder Xray diffraction (XRD, D8 Advance diffractometer, Bruker) with Cu Kα radiation (λ = 1.5406 Å). The nitrogen adsorption/desorption isotherm was measured using a Micromeritics ASAP 2020 sorptometer. Thermogravimetric analysis (TGA Q500, TA) was carried out in air atmosphere from 40 to 900 C with a ramping rate of 10 °C min −1 . Fourier transform infrared (FT-IR) spectra were recorded on a Nicolet 6700 spectrometer using the KBr pellet as the background. X-ray photoelectron spectroscopy (XPS) measurements were performed on an AXIS Ultra DLD (Kratos, USA) using monochromatic Al Kα X-ray source (anode HT = 15 kV) operating at a vacuum more than 10 -7 Pa. Atomic force microscopy (AFM) was recorded by a Veeco Nanoscope IVa Multimode system. UV−vis diffuse reflection spectra (DRS) were recorded on a Shimadzu UV-2550 UV−vis spectrophotometer at normal temperature from 250 to 800 nm, in which fine BaSO 4 was used as the reflectance standard. The photoluminescence (PL) measurements were carried out using a Fluoromax 4P spectrofluorometer (Horiba) with a laser (λ = 380 nm) at room temperature. The timeresolved fluorescence measurements were recorded on an Edinburgh FLS980 at an excitation wavelength of 345 nm.

Photocatalytic measurements
In a typical experiment of water splitting for hydrogen evolution, photocatalyst (50 mg) and methanol (20 mL) serving as the sacrificial electron donor were added to ultrapure water (60 mL) under stirring. Then, H 2 PtCl 6 aqueous solution was added as the precursor for the co-S4 catalyst Pt, which was in-situ photoreduced during the photocatalytic reaction (~3 wt% Pt). Pt serves as a co-catalyst in photocatalytic water splitting for its excellent electron extraction from the conductive band of the main catalyst. Finally, the sealed quartz tube was sideirradiated under visible light by using a 300 W xenon lamp equipped with a 400 nm cutoff filter. The reaction temperature was carefully maintained at room temperature. During the visible light irradiation, the evolved gas was collected at the given time intervals and analyzed with Shimadzu GC gas chromatography equipped with a thermal conductive detector (TCD) and high-purity Ar carrier gas.
The photocatalytic activity of each sample was evaluated in terms of the degradation of rhodamine B (RhB, 10 mg/L). The sample (20 mg) was added into a Pyrex photocatalytic reactor containing RhB solution (100 mL). A 300 W xenon lamp (with 400 nm cutoff filter) was used as visible light source. Prior to the irradiation, the suspension was stirred in the dark for 30 min to achieve the adsorption-desorption equilibrium. Aeration was performed using an air pump to ensure a constant supply of oxygen. At the given time intervals, the analytical samples were taken from the mixture and immediately centrifuged to remove the photocatalysts.

AQE measurements and wavelength dependent experiments
The apparent quantum efficiency (AQE) was detected using six LEDs at centered wavelengths of 380, 400, 420, 460, 500 and 460 nm. The photo intensity was confirmed by optical power meter (Advantest, Q8230). The irradiation area was controlled as 19.6 cm 2 . The photocatalytic reaction was controlled as one hour. The AQY was calculated based on the following equation: where N e is the amount of reaction electrons, N p is the incident photons, M is the amount of H 2 molecules, N A is Avogadro constant, h is the Planck constant, c is the speed of light, S is S5 the irradiation area, P is the intensity of the irradiation, t is the photoreaction time, and λ is the wavelength of the monochromatic light.

Photoelectrochemical testing
Electrochemistry impedance spectroscopy (EIS) and photocurrent intensity response          Figure S11a shows the enlarged view of transient photocurrent from GCN/TNTs and PGCN/TNTs. It was found that the photocurrent decaying rate of PGCN/TNTs is faster than that of GCN/TNTs. The electron transfer time (τ d ) can be fitted to one major single S14 exponential decay process, [13] and it is 0.568 and 0.091 s, respectively. By combining the results of PL lifetime (τ n ), the electron diffusion length (L n ) was obtained according to the following calculation formula: d n n τ τ L = L where L is the length of the semiconductor. It turned out that the L n of PGCN/TNTs is 2.9 times higher than that of GCN/TNTs.