Regular Article
Controllable synthesis of porous silver cyanamide nanocrystals with tunable morphologies for selective photocatalytic CO2 reduction into CH4

https://doi.org/10.1016/j.jcis.2021.02.087Get rights and content

Abstract

The conversion of CO2 driven by solar energy into carbon-containing fuel has huge potential applications. However, most photocatalysts can only promote the two-electron reduction process to generate CO, and it is difficult to produce eight-electron CH4, which is more valuable and can store more solar energy. Herein, we prepared porous silver cyanamide nanocrystals with tunable morphologies via a facile synthesis strategy. Compared with ball/rectangular plate (RAP) and ball/leaf-like plate (SAP), the ball/porous leaf-like plate (LAP) can provide more internal reaction microenvironment, which is not only conducive to the transport of photoinduced charge carriers, but also can expand the active sites for photocatalytic CO2 reduction. Moreover, the suitable band gap of LAP sample can capture more visible light to provide more photoinduced electron-hole pairs to participate in the reduction reaction. 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. This work provides a promising way to design new photocatalysts for various applications in solar energy conversion.

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 Csingle bondN and Ctriple bondN 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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