Jianhua Hou a,* , Muyi Yang a , Qian Dou a , Qing Chen b , Xiaozhi Wang a , Chuangang Hu c,* , Rajib Paul d,e,*

a School of Environmental Science and Engineering, Yangzhou University, Yangzhou 225000, China 

b Chinese Research Academy of Environment Sciences, Beijing 100012, China 

c State Key Laboratory of Organic-Inorganic Composites, College of Chemical Engineering, Beijing University of Chemical Technology, Beijing 100029, China 

d Department of Macromolecular Science and Engineering, Case school of Engineering, Case Western Reserve University, Cleveland, OH 44106, USA 

e Advanced Materials and Liquid Crystal Institute, Kent State University, Kent, OH 44242, USA

ABSTRACT

Herein, simultaneously adjustable N-vacancy defect containing and C-doped graphitic carbon-nitride nanosheets (g-CN-X) in accordion-like architectural structures have been synthesized through ethanol containing steam process for the first time. The band-gap energy of the g-CN-X could be precisely tuned enhancing visible light absorption up to the whole visible spectrum region. Henceforth, the optimized catalyst (g-CN-10) has demonstrated an impressive hydrogen evolution rate of 27.6 mmol h− 1 g− 1 which is 16.2 times higher than the bulk-gCN, with an apparent quantum yield of 9.1 % at 420 nm wavelength. Furthermore, the g-CN-10 has showed CO2 photoreduction yield to CO production rate of 226.1 μmol h-1g− 1 , about 28.6 times higher than bulk g-CN (7.9 μmol h− 1 g− 1 ). Both the performances are distinctly higher than all other previous reports. Theoretical calculations show that the defect sites lead to a more localized charge density distribution and promote photocatalytic active spots, which uplift the light absorption efficiency and improve the transport of charge carriers involved in the photocatalysis, demonstrating the importance of defect engineering to achieve highly efficient and multifunctional photocatalysis.

1. Introduction

Developing photocatalysts with high photo-conversion efficiency along with advanced catalytic performance is required urgently in order to confront the ever-increasing requirement for energy and chemical resources through environment-friendly but low-cost processes [1]. Renewable energy conversion and utilization for various electrocatalytic energy applications have become popular and effective way to address the ensuing energy crisis and environmental concerns [2]. In this regard, solar light assisted energy production and conversion of CO2 are being researched very seriously in recent years. Polymeric graphitic carbon nitride (g-C3N4) is a promising metal-free electrode material for capturing solar-light due to its moderate bandgap (~2.7 eV) and high chemical stability [3]. Bulk-g-C3N4 can be prepared in large quantity by thermolysis of various N- and C-rich precursors, but it shows insufficient photo-absorption, high excitonic photoelectron recombination [4], low specific surface area, and low quantum yield [5].

To solve this issue, nano structuring and surface modification of gC3N4 are adapted without considerable progress [6]. As for example, the bulk-g-C3N4 was performed through post-synthetic exfoliation methods, such as ultrasonication, thermal agitation and chemical treatment for synthesizing g-C3N4 nanosheets [7–9]. Recently, Wang et al reported a facile and green approach to produce g-C3N4 nanosheets using watersteam reforming reaction [10]. However, due to the quantum confinement effect, the band-gap of randomly oriented g-C3N4 nanosheets was increased resulting in poor photo-absorption ability. As such, the related photo-induced charge recombination was high which could not be addressed by simple morphological regulation [11]. Henceforth, establishing a simple, scalable and low-cost process for producing photocatalytically efficient g-C3N4 nanosheets is highly desirable to optimize its electronic as well as optical characteristics for achieving highly efficient photocatalysis.

  Until now, many strategies have been developed to regulate electronic structure of g-C3N4, including the defect engineering [12], modification of surface vacancy [13], heteroatom-doping [5], heterojunction formation [14,15], etc. For example, Li et al [13] reported that g-C3N4 nanosheets rich in N vacancies could reduce the excitonic recombination resulting in enhanced photo-catalytic reduction of CO2 producing CO in an attractive rate of 6.61 μ mol h− 1 g− 1 which was much higher than bulk-g-CN (2.89 μ mol h− 1 g− 1 ). In another study, Liu et al. proposed that carbon doping (pyridine heterocycle) in g-C3N4 could reduce down the energy-gap by increasing the π electron density, promoting efficient photo-catalysis [16]. Furthermore, the sulfur-doped holey [5] or Se-modified g-C3N4 [17] nanosheets exhibited extendedrange visible light absorption driven photocatalytic hydrogen production activity. Nevertheless, the suitable heteroatom dopants and their particular location might promote enhanced charge separation, suitable photo-absorption, and advanced photocatalysis, which have not yet been systematically explored [18].

The steam reforming reaction [CnHm + n H2O → n CO + (n + m/2) H2)] provides H2, CO, or other useful chemicals converting hydrocarbons effectively. Wang et al [10] performed the C/N-steam reforming reactions with bulk C3N4 to prepare few-layered g-C3N4 [CN(s) + H2O (g) → CO(g) + H2(g) + NO(g)] and demonstrated photocatalytic H2 production rate of 261.1 μmol h− 1 . Here we have developed a technique where the bulk g-C3N4 utilizes the synergistic effect of hydrocarbons and steam for the selective etching of C/N atoms by regulating the ratio of water to ethanol. The water vapor reformation reaction of ethanol would actually release more H2, CO2 and CO gases would get inserted into the layer gap of bulk g-CN [C2H5OH + 2H2O → CO + CO2 + 5 H2)]. We have demonstrated that this method not only can produce fewlayered g-CN nanosheets through the release of H2O, CO, NO and H2 gas bubbles, but also introduce adjustable defects into g-C3N4 lattice providing homogeneously distributed nitrogen vacancies and C-doping in an accordion-like hierarchical structure for efficient photocatalytic CO2 conversion to CO and CH4 (226.1 µmol h-1g− 1 CO and 4.0 µmol h1 g− 1 CH4) along with H2 production (27.6 mmol h-1g− 1 for 5 wt% Pt) with superior performances than all other previous reports. Through detailed analysis, the enhanced photocatalytic activities have been attributed to the enhanced surface area, the efficient photo-absorption, and low charge recombination through defect engineering bringing forward a great promise in green energy production and recycling.

2. Experimental section

2.1. Materials

Dicyandiamide (C2H4N4), thiourea (CH4N2S), absolute ethyl alcohol, triethanolamine (TEOA), chloroplatinic acid hexahydrate (H2PtCl6⋅6H2O), cobalt chloride (CoCl2⋅6H2O), acetonitrile (C2H3N), bipyridine (C10H8N2) and sodium sulphate (Na2SO4) were bought from Sinopharm Chemical Reagents Co., ltd. The reagents were of analytical grade, and deionized (DI) water was also used.

2.2. Synthesis

Using dicyandiamide and thiourea as raw materials, 50 g of dicyandiamide and 50 g of thiourea are ground to make them thoroughly mixed. Next, the mixture was heated in a tubular furnace with air conditions from 20 ℃ to 550 ℃ at a rate of 5 ℃ min− 1 for 2 h to obtain bulk-g-CN. Then, 2 g bulk-g-CN is fed into a tubular furnace via N2 carrier gas in the atmosphere of different proportions of ethanol solutions (0, 2.5, 5, 10, and 20 % ethanol solutions). After heating to 500 ℃ (5℃ min− 1 ) and holding for 5 h, g-C3N4 with fewer layers was synthesized without further treatment and named as g-CN-X (X = 0, 2.5, 5, 10, 20). With the increase of the proportion of ethanol solution, the yield decreased gradually, 68.8 % of g-CN-0, 48.8 % of g-CN-2.5, 40.5 % of gCN-5, 30.1 % of g-CN-10 and 25.6 % of g-CN-20 respectively. The corresponding color varies from bud yellow to orange.

2.3. Characterization

The X-ray diffraction (XRD) were performed on AXS D-8 ADVANCE by using Cu Kα radiation (λ = 1.5406 Å) with scanning speed of 6◦ per min. Fourier Transform Infrared Spectroscopy (FT-IR) spectra were analyzed by TENSOR27 spectrometer. The Brunauer-Emmett-Teller (Wagered) (ASAP 2460, Micromeritics) was operated to explore the pore size and particular surface area. To determine the chemical composition of material and surface element state was texted by X-ray photoelectrons spectroscopy (XPS) (ESCALAB 250Xi, Thermo Fisher Scientific). 13C solid-state NMR spectra for Bulk-g-CN and g-CN-X were obtained on a Bruker AVANCE III 400 MHz WB solid nuclear magnetic resonance spectrometer. To study the surface morphology and nanostructures of synthesized material the scanning electrons microscopy (SEM) (Hitachi, S-4800) and high resolution transmission electrons microscopy (HRTEM) (Tecnai G2 F30S-Twin, FEI) was used at high resolutions. The contents of Bulk-g-CN of the as-prepared photocatalysts were characterized using thermal gravimetric analysis (TGA) with a Pyris 1 TGA analyzer (PerkinElmer). Electron Paramagnetic Resonance (EPR) was measured using A300-10/12 Bruker EPR spectrometer. UV–visible diffuse reflectance spectroscopy (UV–vis DRS) (TU-1901, PERSEE) was used to investigate the optical properties of material. Photoluminescence (PL) was studied by Renishaw in Via. Time-resolved photoluminescence (TRPL) decay curves were measured by using a time correlated single-photon counting technique (FLS980 Series of Fluorescence Spectrometers) with a 325 nm pulsed laser diode. Electrochemical impedance spectra (EIS) and transient photocurrent response were tested by CHI760E electrochemical workstation with a standard three-electrode system with Ag/AgCl as the reference electrode, Pt as counter electrode, and ITO conductive glass containing 10 mg catalyst sample as working electrode in 0.2 M Na2SO4 solution. The EIS frequency ranged from 0.01 to 105 Hz, AC voltage magnitude was 5 mV

2.4. Photocatalytic tests

The H2 production reactions were accomplished in a Pyrex top-irradiation reaction vessel associated to a glass closed gas circulation system. The 20 mg sample was dispersed into 50 mL triethanolamine aqueous solution (10 mL triethanolamine, 40 mL water). The amount of Pt co-catalyst was 1–5 wt% using H2PtCl6 as precursor by in situ photodeposition. After the reactor is sealed, air is pumped out of the system. The reaction was carried out under 300 W Xeon lamp with 420 nm cutoff filter (λ > 420 nm) and no filter, and the amount of hydrogen produced was analyzed by online gas chromatograph (GC-7920). The quantum efficiency of g-CN-10 was calculated by using QD 420 cut-off filter.

The temperature of solution make with reactants was preserved at 6 ℃ by a stream of cooling water in the reaction. The amount of prepared H2 was investigated by gas chromatography furnished with a thermal conductive detector with high-purity Ar as carrier gas. We also evaluate the pH of solution before and after water splitting, no obvious change was observed, the pH values are 11.0 and 10.9, respectively. This result indicates that the water-splitting system has admirable stability.

Reactions of photocatalytic CO2 reduction were performed in a custom made reactor connected to a close-loop gas flow system (CEL-PAEM-D8, Beijing China Education Au-light Co., ltd,). Took 20 mg catalyst powder was mixed by ultrasound for 10 min in the homogeneous solution representing 12 mL of de-ionized H2O, 6 mL of (TEOA), 12 μ mol of CoCl2, 45 mg of bipyridine, and 18 mL of MeCN in a 200 mL reactor with a quartz glass cover. Later on air in this system was cleared away, the high purity of CO2 was presented into the system until the pressure achieved 0.08 MPa and circulated for 60 min to gain uniform

delivery of CO2 gas. The temperature of photocatalytic reduction of CO2 was adjusted at 15 ℃ with the help of cooling water circulation in the system which can enhanced the adsorption ability of CO2, and a 300 W Xe lamp (CEL-HXF300, China Education Au-light, Beijing) was used as the source of light. Ar is utilized as the carrier gas. The gas chromatograph (GC-7920, China Education Au-light, Beijing) equipped with a thermal conductivity detector (TCD, China Education Au-light, Beijing) and a hydrogen flame ionized detector (FID, China Education Au-light, Beijing) with a capillary column was used to find the amount of the gas product.

2.5. The AQY measurement

Monochromatic LED lamp was used to measure apparent quantum yield (AQY) for H2 production at desired 420 nm, which attained using bandpass filter. The spectroradiometer (ILT 950) were used to control intensities at 44.71 mW cm− 2 on controlled irradiation area at 7.54 cm2 . The AQY was measured as: AQY = Ne Np × 100% = 2MNAhc SPλt × 100%

where Ne is the amount of reaction electrons, Np represents the number of incident photons, M is the number of H2 molecules, NA is Avogadro constant, h is the Planck constant, c is the speed of light, S is the irradiation area, P is the intensity of the light, t is the photoirradiation time, and λ is the wavelength of the monochromatic ligh.

2.6. Computational methods.

Theoretical calculation measured using Vienna Ab initio Simulation Package (VASP) using projector augmented wave (PAW) method. The exchange-functional is treated using the generalized gradient approximation (GGA) of Perdew-Burke-Ernzerhof (PBE) functional. The Monkhorst-Pack K-mesh for the Brillouin-zone integration with K-point separation of 0.04 Å− 1 was applied for the calculation. For the convergence principal of geometry relaxation for plane wave expansion, the cut-off energy was set 500 eV, and the force on every atom approximately keep low at 0.02 eV/A0 . The electronic energy was considered self-consistent when the energy change was smaller than 10-6 eV.

3. Results and discussion.

3.1. Catalysts characterization

The preparation method for N-vacancy defect and C-doped g-CN-X nanosheets is presented in Scheme 1. After the synergistic effect of ethanol and water steam reforming process, the g-CN-X displayed thin nanosheet structures (Fig. 1a-c and S1), which considerably differs from the dense and packed sheets of the bulk-g-CN. In this gas-phase exfoliation method, the yield of g-CN-X decreased (from 68.8 to 25.6 %) with the increase of the proportion of ethanol solution, and the thickness of the corresponding nanosheets also decreased to varying degrees till gCN-10 (Fig. S1b-f). As Compared to pure water steam reforming reaction with bulk-g-CN to obtain the nanosheet structure of g-CN-0 (Fig. S1b), addition of ethanol actually generated nanosheets of g-CN-X with crimps and severe foldings (Fig. S1c-f). The yield was also significantly higher than that of 6 % when the air thermal oxidation method was used in the literature [19]. Particularly, g-CN-X shows a three-dimensional structure composed of nanosheets with different surfaces lose and crimped thin layers as evident by SEM and TEM images (Fig. 1i-n and Fig. S1), which is different from the 2D structure of the oxidation etching method. In particular, the accordion structure composed of sheets of g-CN-10 in Fig. 1m is shown in the green box (inset). Because compared with the single precursor preparation of carbon nitride, the dicyandiamide and thiourea mixture showed multi-peak derivative thermogravimetry (DTG) curves (Fig. S2). The multiple weight loss of the mixture is due to the interaction of different types of gas by-products produced at different polymerization temperatures to form the template and polymerize into g-C3N4 with a multi-layer structure. After the further synergistic effect of ethanol and water steam reforming process, the multitier architecture 3D skeleton of g-C3N4 is still retained, and the thick lamellar structure is evolved into few-layered nanosheets.

  In addition, the HR-TEM image of multi-tier architecture lattice stripe of 0.32 nm, corresponding to the (002) interplane distance of gCN-10 (Fig. 1d). Accordion structure composed of sheets of g-CN-10 (Fig. 1e), and the corresponding EDX mapping image confirms the presence of carbon, nitrogen, and a small amount of oxygen (Fig. 1f-i). Therefore, controllable production of g-C3N4-X nanosheets with unique morphology has been demonstrated through controlling the relative proportion of ethanol in water, which facilitated the suitable seed nucleation, growth during polymerization, subsequent gas-phase stripping and etching [10,20]. The accordion-like structure of g-CN-X promotes the multiple reflection of incident enhances light harvesting rate [21]. Accordion-like hierarchical structure composed of nanosheets could also be effective in preventing interlayer stacking shortened route of mass-transfer process the surface.

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