Regular ArticleControllable synthesis of porous silver cyanamide nanocrystals with tunable morphologies for selective photocatalytic CO2 reduction into CH4
Graphical abstract
Introduction
The photocatalytic CO2 reduction reaction (CO2RR) driven by solar energy has not only caused the production of value-added chemicals and energy-rich fuels, but also reduced CO2 emissions, which has attracted widespread attention [1], [2]. Since the groundbreaking report of using TiO2 as a photocatalyst, a large number of photocatalysts (such as g-C3N4 nanosheets, semiconductor oxide, metal-organic framework, and metal complex, etc.) have been further investigated for CO2RR [3], [4]. In addition, methane (CH4) with the eight-electron reduction process (E°CH4/CO2 = −0.24 V) is a valuable chemical fuel (combustion heat: 890.03 kJ/mol) [5], [6]. Unfortunately, most of as-prepared samples did not display excellent eight-electron reduction ability, resulting in the yield of CH4 being much lower than that of CO [7], [8]. Therefore, in order to realize the conversion of CO2 to CH4, it is expected to weave a new photocatalyst that can selectively undergo an eight-electron reduction reaction under visible light irradiation.
Recently, silver cyanamide (Ag2NCN), as one of metal pseudochalcogenides, has received significant attention and its NCN2– is equivalent to O2– in metal oxide [9], [10], [11], [12]. Moreover, compared with the O 2p6 state in metal oxides, the electronic state of NCN2– in Ag2NCN is more delocalized at the VB and CB [13], [14]. In this respect, it is very similar to a peptide or chalcogenide semiconductor structure [15], [16]. Interestingly, it is found that the bond distances in Ag2NCN are 1.27 Å and 1.20 Å, respectively, and the CN and CN bond distance respectively are 1.31–1.44 Å and 1.10–1.19 Å coexisted in the single cyanamide ions, which leads to an asymmetrical form [17]. The asymmetric form can generate local dipole fields, which may facilitate the transport of oppositely charged carriers in photocatalytic reactions [18], [19]. In addition, many strategies have been made to improved catalytic activity of photocatalysts by controlling their nanostructures via soft or hard template methods, such as nanosheets, nanospheres, hollow spheres and janus structures, etc [20], [21], [22]. Among the many structures, the porous structure inside the catalyst can offer a short path for the fast transfer of photoinduced charge carriers and furnish more active sites, which should be beneficial to the reduction performance [23], [24]. Therefore, controlling the morphology is effective and critical ways to enhance the photocatalytic performance of Ag2NCN, but still waiting to be researched.
In this work, we design and synthesize silver cyanamide nanocrystals with tunable morphologies. The ball/rectangular plate (RAP), ball/leaf-like plate (SAP) and ball/porous leaf-like plate (LAP) were acquired via using a facile synthesis strategy. Compared with RAP and SAP, the porous structure with random stacking of nanosheets in LAP can provide more accessible active sites and its internal nanosheet structure can effectively transfer photoinduced charge carriers. Consequently, the LAP sample can convert CO2 to CH4 with remarkable activity and high selectivity (nearly 100%) without cocatalyst and photosensitizer under visible light irradiation. Our work provides a facile means to construct a fresh type of catalysts for photocatalytic CO2 reduction.
Section snippets
Synthesis of Ag2NCN nanocrystals
Ag2NCN nanocrystals (ball/rectangular plate (RAP), ball/leaf-like plate (SAP) and ball/porous leaf-like plate (LAP)) were synthesized by a facile synthesis method. The as-obtained LAP is as follows: 0.17 g AgNO3 and 60 mL NH3·H2O (6 wt%) was added in 30 mL H2O to synthesize A solution, and 50 mL cyanamide (1 wt%) was dissolved in 50 mL H2O to weave the B solution, which was dropwise dissolved to A solution under vigorous magnetic stirring and then the dark green nanocrystals are gradually
Structure and composition
Silver cyanamide nanocrystals were synthesized by a chelation process with silver nitrate (AgNO3) and ammonia (NH3·H2O) as A solution, together with cyanamide (H2NCN) as B solution, as shown in Scheme 1. Typically, AgNO3 and NH3·H2O were dissolved and then mixed homogeneously in the aqueous solution. Next, the B solution was dissolved to the stirring A solution, and the nanocrystals were generated at the bottom of the reactor at the same time. Different silver cyanamide nanocrystals are
Conclusions
In summary, silver cyanamide nanocrystals with tunable morphologies are obtained via using a facile preparation strategy. The maximum production rate for CO2-to-CH4 in LAP sample was detected with remarkable activity and high selectivity (nearly 100%) without cocatalyst and photosensitizer under visible light irradiation and the other as-produced samples did not detect any gas products. The enhanced photocatalytic activity was assigned to the efficient separation of photoinduced charge carriers
CRediT authorship contribution statement
Zhuquan Fu: Methodology, Investigation, Formal analysis. Sicheng Yi-Wang: Methodology, Validation. Ziyi Li: Methodology, Validation. Ting Song: Writing - review & editing, Funding acquisition. Bei Long: Methodology, Formal analysis. Atif Ali: Visualization, Writing - review & editing. Guo-Jun Deng: Writing - review & editing, Funding acquisition.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
We are extremely grateful for the financial support of the research fund of the Xiangtan University startup grant (06KZ/KZ08082) and the Science and Technology Innovation Program of Hunan Province (2020RC2076).
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