Energy Platform for Directed Charge Transfer in the Cascade Z‐Scheme Heterojunction: CO2 Photoreduction without a Cocatalyst

Abstract A universal strategy is developed to construct a cascade Z‐Scheme system, in which an effective energy platform is the core to direct charge transfer and separation, blocking the unexpected type‐II charge transfer pathway. The dimension‐matched (001)TiO2‐g‐C3N4/BiVO4 nanosheet heterojunction (T‐CN/BVNS) is the first such model. The optimized cascade Z‐Scheme exhibits ≈19‐fold photoactivity improvement for CO2 reduction to CO in the absence of cocatalysts and costly sacrificial agents under visible‐light irradiation, compared with BVNS, which is also superior to other reported Z‐Scheme systems even with noble metals as mediators. The experimental results and DFT calculations based on van der Waals structural models on the ultrafast timescale reveal that the introduced T as the platform prolongs the lifetimes of spatially separated electrons and holes and does not compromise their reduction and oxidation potentials.

heterojunction was synthesized through a two-step hydroxyl-induced assembly strategy. First, 15CN/BVNS heterojunction was synthesized as described above, and then the T was assembled on it as follows. A proper amount of T and 15CN/BVNS were dispersed in 60 mL of ethanol and ultrasonicated for 30 min. Then, the above mixture was refluxed at 80ºC for 2 h. The obtained samples were washed with deionized water several times and dried at 60 ºC in an oven. This sample was named as yT-15CN/BVNS, where y (3, 5, and 7), which is determined by the mass ratio percentage of T to BVNS.
Synthesis of α-Fe2O3. In a typical procedure, 10 mL of 5 wt% ammonia solution was added into a 50 mL Teflon-lined autoclave. Then, a weighing bottle containing a mixture of 0.8 g Fe(NO3)3·9H2O and 8 mL n-butyl alcohol was placed in the autoclave with a support to separate with the ammonia solution. The Teflon-lined autoclave was kept at 140 ºC for 6 h in an oven. After being cooled naturally to the room temperature, the sample was washed for several times with ethanol and deionized water in turn and then dried in an oven at 80 ºC. Finally, the samples were calcined in air at 400ºC for 2h.

Synthesis of CN coupled Fe2O3.
In the typical synthesis, a proper amount of CN and Fe2O3 were dispersed in 60 mL of ethanol, and ultrasound for 30 min. Then the above mixture was refluxed at 80ºC for 2 h. Afterwards, the obtained samples were washed with deionized water for several times, and dried at 60ºC in an oven. This sample was represented as CN/Fe2O3. Synthesis of (001) facet-exposed TiO2 nanosheets (T) coupled CN/Fe2O3 heterojunction. In a typical procedure, the CN/Fe2O3 heterojunction was synthesized as described above, and then the T was assembled on it as follows. A proper amount of T and CN/Fe2O3 were dispersed in 60 mL of ethanol, and ultrasonicated for 30 min. Then, the above mixture was refluxed at 80ºC for 2 h. Afterwards, the obtained samples were washed with deionized water for several times, and dried at 60ºC in an oven. This sample was represented as T-CN/Fe2O3.

Synthesis of WO3.
In a typical procedure, 0.22 g of polyethylene oxide-polypropylene oxide-polyethylen (P123) (Pluronic, M=5800) was dissolved in 14.44 g of ethanol absolute under vigorous magnetic stirring. Then, a certain amount of deionized water and 1.5 mL of ethylene glycol was added to the above mixture and stirred for 2 h to form a clear solution. Finally, the solution was sealed and kept in a brown reagent bottle at least 48 h before using. The solution was named as solution P. 0.44 g of WCl6 was added to 16.83 g of solution P, and stirred for 20 min to obtain a yellow solution. The solution was then transferred to a 50 mL of Teflon-lined autoclave to be kept under 110°C for 3 h, and naturally cooled to room temperature. After the solvothermal treatment, the precipitate was washed with absolute ethanol, then dried in a vacuum oven at 80°C for 12 h. After that, the obtained sample was calcined for 1 h at 400ºC and represented as WO3.

Synthesis of CN coupled WO3.
In the typical synthesis, a proper amount of CN and WO3 were dispersed in 60 mL of ethanol, and ultrasound for 30 min. Then the above mixture was refluxed at 80ºC for 2 h. Afterwards, the obtained samples were washed with deionized water for several times, and dried at 60ºC in an oven. This sample was represented as CN/WO3. Synthesis of (001) facet-exposed TiO2 nanosheets (T) coupled CN/WO3 heterojunction. In a typical procedure, the CN/WO3 heterojunction was synthesized as described above, and then the T was assembled on it as follows. A proper amount of T and CN/WO3 were dispersed in 60 mL of ethanol, and ultrasonicated for 30 min. Then, the above mixture was refluxed at 80ºC for 2 h. Afterwards, the obtained samples were washed with deionized water for several times, and dried at 60ºC in an oven. This sample was represented as T-CN/WO3.

Synthesis of SnO2 nanoparticles.
In the typical synthesis, 2 g of SnCl4·5H2O was dissolved in deionized water and NaOH solution was added dropwise to it under continuous stirring. At the beginning, a white cloudy suspension was formed in the acidic pH range which slowly disappeared with the addition of more NaOH solution. After the pH value reached about 12, solution became transparent. This solution was transferred to 100 mL Teflon-lined stainless steel autoclave and calcined at 200°C for 12 h. The product was cooled to room temperature naturally and washed three times with distilled water to remove impurities. The purified white powder was dried in an oven overnight at 60°C to obtain SnO2 nanoparticles.

Synthesis of SnO2 coupled CN/BVNS heterojunction.
In a typical procedure, the 15CN/BVNS heterojunction was synthesized as described before, and then the SnO2 was assembled on it as follows. A proper amount of SnO2 and 15CN/BVNS were dispersed in 60 mL of ethanol and ultrasonicated for 30 min. Then, the above mixture was refluxed at 80ºC for 2 h. The obtained samples were washed with deionized water several times and dried at 60ºC in an oven. This sample was named as SnO2-15CN/BVNS.

Preparation of 5T/BVNS/15CN and 5T/15CN/BVNS electrodes.
Both 5T/BVNS/15CN and 5T/15CN/BVNS were obtained by blade-coating the dispersion of these components on FTO glass in sequence, followed by calcination at 300 °C for 30 min in air. In a typical procedure, for 5T/BVNS/15CN photoelectrode synthesis, BVNS, CN and T were firstly dispersed in a mixture solution of Nafion and ethanol with the volume ratio of 1:9 under vigorous stirring to make individual solution, respectively. Then a certain amount of T droplets was dropped on a FTO glass substrate, followed by a blade-coating procedure. Then, the CN was dispersed on the T coated FTO glass, and last the BVNS was dispersed on the substrate. Finally, the sample was calcined at 300 °C for 30 min in a muffle furnace, denoted as 5T/15CN/BVNS. Similarly, 5T/BVNS/15CN electrode was obtained by a same method but with a different coating sequence, swapping CN and BVNS.

Characterizations
The X-ray powder diffraction (XRD) patterns of the samples were collected by a Bruker D8 advance diffractometer with CuKα radiation. The UV-Vis diffuse reflectance spectra (UV-Vis DRS) of the samples were acquired using Shimadzu UV 2550 spectrophotometer with BaSO4 as a reference. The Fourier-transform infrared (FT-IR) spectra of the samples were recorded with a Bruker Equiox 55 spectrometer, KBr as the diluents. The morphologies of the samples were analyzed by transmission electron microscopy (TEM) on an FEI Tecnai G2 S-Twin instrument with an acceleration voltage of 200 kV. The FEI Tecnai G2 S-Twin equipped with Energy-dispersive X-ray (EDX) Detector was used to acquire element analysis of the samples. The surface photovoltage spectroscopy (SPS) measurements were carried on home-built apparatus, equipped with a lock-in amplifier (SR830, USA) synchronized with a light chopper (SR540, USA). The transient-state surface photovoltage (TPV) responses of the samples were recorded in the air at room temperature. The samples were excited by a radiation pulse of 355 nm with 10 ns width from the second harmonic of a neodymium-doped yttrium aluminum garnet (Nd:YAG) laser (Lab-130-10H, Newport, Co.). The signals were amplified with a preamplifier and registered with a 1 GHz digital phosphor oscilloscope (DPO 4104B, Tektronix). The Raman spectra were recorded with a Renishaw inVia Confocal Raman spectrometer with a 785 nm laser as the excitation source. The electron paramagnetic resonance (EPR) measurements were carried out on a Bruker EMX plus model spectrometer operating at the X-band frequency. The reactive •O2and •OH species were detected by using 5,5-dimethyl-1pyrroline N-oxide (DMPO) as a spin trap under visible-light irradiation. The X-ray photoelectron spectroscopy (XPS) was performed by Kratos-AXIS ULTRA DLD. Al (Mono) was used as the X-ray source.
Hydroxyl radical measurement. Hydroxyl radicals (•OH) are important active species in photocatalytic reaction. Using coumarin as a labeled molecule to detect the content of hydroxyl radicals is an effective method with high sensitivity. The specific method for the hydroxyl radical test is as follows: 0.02 g of the catalyst was placed in 50 mL of coumarin solution at a concentration of 2×10 -4 M. The mixture was stirred for 30 min before the experiment, to ensure that it reached the adsorption-desorption equilibrium. After 1 h irradiation, appropriate amount of the suspension was centrifuged and the supernatant was transferred into a Pyrex glass cell for the fluorescence measurement of 7-hydroxycoumarin by a spectrofluorometer (Perkin-Elmer LS55). To cut off UV-light, a light filter of 420 nm was placed between the light source and the reactor.
Photoelectrochemical and electrochemical measurements. Photoelectrochemical (PEC) and electrochemical reduction measurements were carried out in a traditional three-electrode system. The prepared thin film electrode was used as a working electrode, a platinum plate (99.9%) as the counter electrode, and an Ag/AgCl as the reference electrode. A 0.2 M Na2SO4 solution as the electrolyte. High purity nitrogen gas (99.999%) was bubbled through the electrolyte before and during the experiments. PEC experiments were performed in a quartz cell using a 500 W xenon lamp with a cut-off filter (λ > 420 nm) as the illumination source. An IVIUM V13806 electrochemical workstation was employed to test the photoelectrochemical and electrochemical performance of the series of photocatalysts. All the experiments were performed at room temperature (about 25 ºC). The surface charge transfer efficiency (ηtrans) was calculated using the following equations: Photocatalytic activities for CO2 conversion. 0.1 g of photocatalyst powder was suspended in 5 mL of water with magnetic stirring in a quartz reactor. A 300 W Xenon lamp was used as the light source with a 420 nm cut-off filter. High pure CO2 gas was passed through water and then entered into the reaction setup for reaching ambient pressure. The photocatalyst was allowed to equilibrate in the CO2/H2O system for 20 min and then irradiated for 4 hours. During irradiation, about 0.25 mL of gas produced was taken from the reaction cell at given time intervals for CO and CH4 concentration analysis using a gas chromatograph (GC-7920 with both TCD and FID detectors, Au Light, Beijing), and for O2 evolution analysis using a gas chromatograph (GC-7900 with TCD, Perfect Light, Beijing). The isotope-labeled experiment was carried out using 13 CO2 instead of CO2, and the reduction products were analyzed on a gas chromatography−mass spectrometry (GC-MS, MS5977A, Agilent). The isotopic D2O labelled experiment was conducted under identical photocatalytic CO2 reduction reaction conditions, but H2O was substituted with a mixture of D2O and methanol. The reduction products were analyzed on a gas chromatography−mass spectrometry (GC-MS, MS5977A, Agilent).
Photocatalytic activities for Overall water splitting. The overall water splitting test was conducted by an integrated system composed of a photoreactor and an online analytic system (Perfectlight, Beijing, Labsolar-6A circulation system). In detail, 0.1 g of photocatalyst powder was dispersed in 100 mL of deionized water with a calculated amount of H2PtCl6 as the cocatalyst precursor in a cubic glass cell and kept stirring. Ahead of the reaction, the mixture was deaerated by evacuation to remove O2 and CO2 dissolved in water. Subsequently, the reactor was exposed under a 300 W Xe lamp with a cutoff filter (λ≥420 nm). The amount of the evolved H2 and O2 in the photocatalysis was analyzed by an online gas chromatograph (GC-2002, TCD, molecular sieve 5 Å, Ar carrier). The gas tight reactor can be guaranteed by measuring an extremely low leakage rate (≤5x10 -5 Pa•L/s), and the amount of O2 leakage for 24 h is less than 1 μmol.
DFT study. All theoretical calculations were performed by the density functional theory (DFT) pseudopotential plane-wave method as implemented in the Vienna Ab initio simulation package (VASP). [2,3] The all-electron projected argument wave (PAW) [4] potential was used to describe the ion-electron interaction. The exchange and correlation potential were described with the Perdew−Burke−Ernzerhof (PBE) [5] of the generalized gradient approximation (GGA). A van der Waals (vdW) interaction was described by the DFT-D2 method of Grimme. [6] The cutoff energy was set as 520 eV, and structure relaxation was performed until the convergence criteria of energy and force reached 1×10 -5 eV and 0.02 eV Å -1 , respectively. The Brillouin zone was sampled with 3×2×1 and 6×4×1 Monkhorst-Pack K-point mesh for the structure optimizations of BiVO4, C3N4, and anatase TiO2 and electronic structure calculations of BiVO4/C3N4/TiO2 heterostructure or with their hydroxylated interface, respectively. And the band structure of the BiVO4/C3N4/TiO2 heterostructure was calculated along the G→X→S→Y→G path. In the DFT-D2 scheme, all force field parameters are obtained based on the PBE functional. The detailed information of the ultrafast interfacial electron transfer (IET) processes is carried out using the semi-classical quantum dynamics method based on semiempirical EH Hamiltonian. [7] The EH method has been widely used to calculate the electronic structure of periodic condensed matter systems. It requires a small number of transferable parameters and can provide a reliable description of the chemical bonding and energy band of both elemental and bulk materials with relatively small computation expense. The IET computational details please refer to the literature for details and the calculation method therein. [8] The optimized the lattice parameters were a=5.214 Å, b=5.084 Å for unit cell of BiVO4, a=10.886 Å, b=15.104 Å for C3N4 monolayer and a=b=3.776 Å, c=9.486 Å for unit cell of anatase TiO2. These values are in good agreement with experiment values. [9,10] The heterostructure was constructed by 2×3×1 unit of BiVO4, C3N4 and 3×4×1 unit of anatase TiO2 monolayer. The anatase TiO2 monolayer was chosen from (001) face and (100) face. The lattice parameters were a= 11.328 Å, b=15.104 Å for (001) TiO2 and a=9.486 Å, b=15.104 Å for (100) TiO2. The mismatch defined as for the parameters a and b of the heterostructure containing (001) TiO2 is only 4.9% and 0.9% which are in acceptable range. The mismatch for the parameters a and b of the heterostructure containing (100) TiO2 is 7.4% and 0.9%. A vacuum zone of 20 Å was used to minimize the interactions between the adjacent systems for the BiVO4/C3N4/TiO2 heterostructure. The interlayer distances between three layers are 2.87 Å and 3.01 Å for the containing (100) TiO2 heterostructures and containing (001) TiO2 heterostructures, respectively.