An unusual network of α-MnO2 nanowires with structure-induced hydrophilicity and conductivity for improved electrocatalysis

doi:10.1016/S1872-2067(21)63793-2

Chinese Journal of Catalysis

Volume 42, Issue 10, October 2021, Pages 1724-1731

https://doi.org/10.1016/S1872-2067(21)63793-2 Get rights and content

Abstract

Nanowires with anisotropic morphologies have been applied in various scientific and technological areas. It is also widely employed to fabricate nanowires into high-dimensional superstructures (arrays, networks etc.) to overcome the shortcomings of low-dimensional nanowires. However, typical strategies for constructing these superstructures are restricted to complicated and harsh synthetic conditions, not to mention unique 3D structures with advanced properties beyond common superstructures. Herein, we report an unusual network of α-MnO₂ nanowires with structure-induced hydrophilicity and conductivity. In the network, the nanowires are interconnected from all directions by nodes, and the 3D network structure is formed from the endless connection of nodes in a node-by-node way. The unique network structure brings about high hydrophilicity and conductivity, both of which are positive factors for an efficient electrocatalyst. Accordingly, the α-MnO₂ network was tested for electrocatalytic water oxidation and showed significantly enhanced activity compared with isolated α-MnO₂ nanowires and 3D α-MnO₂ microspheres. This study not only provides a synthetic route toward an advanced network structure but also a new idea for the design of materials for electrochemistry with both efficient mass diffusion and charge transfer.

Graphical Abstract

An unusual nanowire network of α-MnO₂ was synthesized with high hydrophilicity and conductivity for improved electrocatalysis. A referential-valuable protocol to evaluate the mass diffusion and charge transfer capabilities of electrocatalysts has also been established.

Introduction

With their anisotropic morphologies, nanowires have intrinsic features such as high surface-to-volume ratios, high exposures of low-energy facets and directional charge-transfer abilities, which have guaranteed their applications in photonics, electronics, sensing and catalysis [1, 2, 3, 4]. However, the free accumulation of nanowires on devices results in the lack of internal spaces, largely limiting their applications. To assemble functional devices based on nanowires, the organization of nanowires into integrated arrangements is of scientific and technological significance [⁵]. Thus, the assembly of nanowires into diverse architectures has resulted in the synthesis of nanowire-based 3D superstructures such as arrays and networks [6, 7, 8]. Nevertheless, there are two general lines of problems in the creation of 3D superstructures based on nanowire substructures. First, typical strategies for constructing these superstructures are restricted to complicated and harsh synthetic conditions. Second, unique 3D structures with advanced properties beyond common arrays and networks are rarely reported. Thus, using simple methods under temperate conditions to direct synthesis novel 3D superstructures of nanowires is still important and challenging.

Among various applications, the assembly of low-dimensional materials into superstructures are receiving great attention in the area of electrocatalysis [9, 10, 11]. For heterogeneous electrocatalysts, their performance can be tuned by three major factors, which are the surface intrinsic activity, the mass diffusion efficiency and the charge transfer ability [12, 13, 14, 15, 16]. The superstructure of low-dimensional substructures not only retains the intrinsic activities of the substructures but also creates abundant accessible sites for substances [17, 18]. In the area of electrocatalysis, water oxidation is receiving overwhelming attention for its production of protons and electrons, which are utilized in the cathode for H₂ production, N₂ reduction, CO₂ reduction and so on [19, 20, 21, 22]. Inspired by the CaMn₄O₅ complex as the water oxidation center in the photosystem II, considerable efforts have been made to Mn-based electrocatalysts for water oxidation [23, 24, 25, 26, 27, 28]. Among them, α-MnO₂ is considered one of the most promising materials. In α-MnO₂, [MnO₆] octahedra units are regularly patterned to form 2 × 2 tunnels, which provide a large number of pathways for ions and expose abundant active sites [29, 30].

With these considerations, we report an unusual assembly of α-MnO₂ nanowires synthesized from simple hydrothermal reaction for electrocatalytic water oxidation. First, the nanowires are pseudo single crystals with high aspect ratios. Second, the nanowires are interconnected from all directions by nodes, starting at one node and ending at another node. Third, the 3D network structure is formed from the endless connection of nodes in a node-by-node way. The connecting nodes make our structure different from common networks, which is typically formed by the stacking of random nanowires [³¹]. The connecting nodes in our superstructure result in a 3D endless connection of long wires, which would bring about abundant open spaces for efficient mass diffusion and also fast charge transfer from wire to wire. The α-MnO₂ network was compared with common isolated α-MnO₂ nanowires and 3D urchin-like α-MnO₂ spheres. The α-MnO₂ network displayed much better electrocatalytic activity for water oxidation than the two references because of its significantly enhanced hydrophilicity and conductivity.

Section snippets

Synthesis of the α-MnO₂ nanowire network (α-MnO₂-NWN)

In a typical procedure, 2 mmol Mn(NO₃)₂·4H₂O was dissolved in 2 mL water. 1 mmol (NH₄)₃PO₄·3H₂O was dissolved in 25 mL water. White suspensions were obtained by adding the latter aqueous solution into the former aqueous solution dropwise under stirring. Then, nitric acid was added into the above mixture to obtain a clear and transparent solution with pH at 4.0. Next, the obtained solution was sealed in a 100 mL Teflon-lined stainless steel autoclave and heated at 220 °C for 24 h and then

Synthesis and characterization of the samples

The α-MnO₂-NWN was synthesized by the oxidative hydrolysis of manganese nitrate with the presence of ammonium phosphate under fine-controlled hydrothermal conditions. The overall morphology of the assembly is shown in Fig. 1. Specifically, the concentration of the substances, the reaction temperature and time and especially the pH values of the solution are key factors for the formation of the unusual network structure. The effects of the above mentioned conditions on the morphology of the

Conclusions

In summary, an unusual nanowire network of α-MnO₂ was synthesized via a mild hydrothermal reaction without any surfactants. In the network, the nanowires are interconnected from all directions by nodes, and the 3D network structure is formed from the endless connection of nodes in a node-by-node way. From comparative studies with isolated α-MnO₂ nanowires and α-MnO₂ microspheres, the unique network structure brought about much better hydrophilicity and conductivity, both of which are positive

Electronic supporting information

Supporting information is available in the online version of this article.

References (47)

Q. Wu et al.
Chin. J. Catal.
(2021)
K. Huang et al.
Chin. J. Catal.
(2020)
H.C. Nguyen et al.
Chem. Sci.
(2020)
Y. Yuan et al.
Joule
(2019)
Y. Gao et al.
J. Cryst. Growth
(2005)
E. Laviron
J. Electroanal. Chem. Interfacial Electrochem.
(1979)
H.-W. Liang et al.
Acc. Chem. Res.
(2013)
J. Li et al.
Adv. Sci.
(2017)
C. Couteau et al.
Nanophotonics
(2015)
R. Yan et al.
Nat. Photonics
(2009)

M. Rauber et al.

Nano Lett.

(2011)

Y.P. Zhu et al.

Angew. Chem. Int. Ed.

(2017)

Z. Jiang et al.

Adv. Mater.

(2019)

Y. Zhao et al.

Adv. Energy Mater.

(2017)

D. Voiry et al.

Nat. Rev. Chem.

(2018)

J. Qi et al.

ChemCatChem

(2019)

H.S. Ahn et al.

J. Am. Chem. Soc.

(2016)

W. Tu et al.

Adv. Mater.

(2018)

H.Y. Kim et al.

ACS Catal.

(2019)

S. Wan et al.

Adv. Mater.

(2017)

L. Huang et al.

Adv. Mater.

(2019)

M.P. Browne et al.

Energy Environ. Sci.

(2019)

C.L. Bentley et al.

J. Am. Chem. Soc.

(2019)

Cited by (13)

A novel Ni<inf>3</inf>S<inf>2</inf>/MnO<inf>2</inf>@N,F-CQDs composite with spherical-chain-like morphology for efficient oxygen evolution reaction in aqueous alkaline solutions
2023, Journal of Solid State Chemistry
Developing earth-abundant transition metal-based oxygen evolution reaction (OER) electrocatalysts is extremely essential for accomplishing efficient electrocatalytic overall water splitting. Herein, we synthesized a Ni₃S₂/MnO₂@N,F-CQDs catalyst with a microstructure composed of interlaced nanowires in a spherical-chain shape by treating a nickel foam substrate with sulfur and co-regulating the morphology and properties of manganese dioxide with carbon dots. The catalyst exhibited excellent OER electrocatalytic performance and stability under alkaline conditions, requiring only a low overpotential of 226 mV at 10 mA cm^-2 and a Tafel slope of 105.76 mV dec^-1. Furthermore, the catalyst remained stable after 3000 cycles of Cyclic Voltammetry (CV) testing and multistep chronopotentiometric (CP) measurements at the current density of 10, 50 and 100 mA cm⁻² for 12 h, respectively. The enhanced performance of Ni₃S₂/MnO₂@N,F-CQDs catalyst was mainly attributed to the practical improvement of charge interface transport performance induced by carbon dots and interface layer Ni₃S₂ which increased active catalytic sites of the electrode and reduced charge transfer resistance. These results suggest a promising approach for developing efficient OER electrocatalysts.
Facile cubic Nd doped MnO nanostructure synthesis as effective electrocatalyst for oxygen evolution reaction
2023, Journal of Electroanalytical Chemistry
Development of an effective electrocatalyst for the electrochemical water splitting to store electrical energy as H₂ fuel and improve sluggish oxygen evolution reaction (OER) is the need of the time. For H₂ production and making it more accessible, developing a low-cost fabrication method for an efficient OER catalyst with characteristics including a large surface area, an abundance of active sites, and exceptional stability is necessary. In this study, neodymium-doped manganese oxide (Nd-MnO) with a larger specific surface area (32.6 m²/g), small size particles (84 nm), and most crucially high concentration of oxygen vacancies fabricated via a simple solution reduction method using NaBH₄ as a reductant. Nd-MnO has an overpotential of 394 mV and a Tafel slope value of 84 mV/dec reaching 10 mA/cm², superior to RuO₂ and MnO. The potential results of the Nd-MnO are due to a unique structure consisting of nanocubes that may enhance OH ion mass diffusion/transport and offer a large number of active sites for catalysis of OER, as well as oxygen vacancies which are also validated by DFT that may enhance the electronic conductivity and provide H₂O adsorption on the surface of neighboring Mn³⁺ sites.
The role of manganese-based catalyst in electrocatalytic water splitting: Recent research and progress
2023, Materials Today Physics
Electrocatalytic water splitting is an effective way to produce hydrogen, which is considered as a promising strategy to solve energy crisis and promote energy conversion. But there is still a lack of stable and efficient catalyst. Transition metal (such as Mn, Fe, Co, and Ni) compounds are considered as potential catalyst materials for electrocatalytic water splitting. Among them, manganese-based compounds have the characteristics of low price and abundant reserves. And manganese-based catalysts have attracted extensive attention of researcher due to their diversity of polyvalence and structure as well as morphology. It reviewed that the classification summary and modification method of manganese-based catalysts on the aspect of performance improvement of electrocatalytic water splitting. The classification and summary of manganese-based catalysts from the aspects of single atom and alloy, compound as well as compound composites are summarized in detail, and the current methods and strategies for improving catalytic performance are prominently introduced, emphasizing the regulation and optimization of the catalytic reaction by manganese-based compounds. Finally, in the last part of the review, the further experimental design strategies, the importance of advanced characterization methods and the future development direction of manganese-based catalysts are emphasized. Further understanding of the mechanism by which manganese-based catalysts improve performance will point the way for the design of highly efficient electrocatalysts for water splitting and other possible applications.
Heterostructured ZIF-67@α-MnO<inf>2</inf> based nanofibrous membranes for highly effective removal of low-molecular-weight pollutants from wastewater
2022, Separation and Purification Technology
Citation Excerpt :
Owing to large specific active surface area and tunable structure, manganese oxides (MnOX) showed also satisfactory catalytic activation as Fenton-like catalyst for the activation of proxymonosulfate (PMS) [25,26]. Besides, manganese oxides (MnOX) with nanowires and nanorods structure possessed high exposures of low-energy facets and directional charge-transfer abilities, which provided efficient electron transport compared with those of bulky structures [27]. Additionally, the construction of heterostructured catalyst by combining MOFs with manganese oxides could enhance catalytic performance in a synergistic way [13,28].
In this work, a thorn ball-like three-dimensional (3D) heterostructured ZIF-67@α-MnO₂ catalyst consisting of ZIF-67 nanocages impaled by α-MnO₂ nanorods were robustly embedded in polyacrylonitrile nanofibers by electrospinning technique for highly efficient removal of low-molecular-weight pollutants (LMWPs, like dyes and antibiotics) from wastewater. Benefiting from abundant active sites and enhanced charge transfer abilities provided by ZIF-67@α-MnO₂ nanoparticles, the optimal ZIF-67@α-MnO₂ nanofibrous membranes (NFMs) showed a significant response towards acid fuchsin (AF) in terms of comparable adsorption capacity (2495 mg/g) and impressive catalytic activity (degradation degree of 99 % AF within 20 min). In addition, the resultant membranes possessed satisfactory versatile catalytic activity toward tetracycline (95.3 %), norfloxacin (98.4 %) and doxycycline hyclate (89.2 %). Besides, the resultant membranes had high permeability of 1302 L·m⁻²·h⁻¹ driven by gravity and could efficiently treat 370.5 L·m⁻² AF (20 ppm), which was significantly higher than that of pure ZIF-67 NFMs (141.8 L·m⁻²). The successful fabricating of such membrane with extraordinary adsorption and catalytic performance provided a promising approach for high-performance catalytic membranes.
Self-templated formation of hierarchical hollow β-MnO<inf>2</inf> microspheres with enhanced oxygen reduction activities
2022, Colloids and Surfaces A: Physicochemical and Engineering Aspects
Citation Excerpt :
As one of the most important classes of functional oxides, MnO2 has been researched intensively because of its multiple crystalline phases (e.g., δ-, α-, γ-, and β-), low toxicity, natural abundance, and novel physicochemical properties [9–11]. In recent years, realizing shape-controlled preparation of MnO2 crystals has become more and more fascinating, since their performance is usually shape-dependent [12–15]. In particular, great advances have been achieved in synthesizing hollow MnO2 materials with various phases, such as δ-MnO2 hollow microspheres [16], hierarchical hollow α-MnO2 framework [17], γ-MnO2 hollow microspheres [18], and hollow β-MnO2 nanospheres [19].
Herein, hierarchical hollow β-MnO₂ microspheres assembled by nanorod subunits are facilely fabricated using a one-step hydrothermal approach, which serve as electrocatalyst for oxygen reduction reaction (ORR). The influences of the reaction temperature and H₂SO₄ concentration on the crystalline phase and morphology of products are studied. Impressively, detailed characterizations reveal that the hollow β-MnO₂ microspheres can be formed through a self-templated process by phase transformation, in which solid γ-MnO₂ microspheres are first generated and then convert to hollow β-MnO₂ microspheres. Owing to their unique hollow shape and robust O₂ adsorption ability, the hollow β-MnO₂ microspheres show superior ORR performance (a more positive half-wave potential of ~ 0.69 V, a larger limiting current density of 5.04 mA cm⁻¹, and better stability) compared with commercial β-MnO₂ particles. The insights afforded in this study offer positive guidance for the rational design of hollow Mn-based materials.
Effect of the hydrothermal synthesis temperature on the capacitive performance of α-MnO<inf>2</inf> particles
2022, International Journal of Electrochemical Science
A hydrothermal method was used to synthesise α-MnO₂ particles, with manganese sulfate as the metal precursor and potassium permanganate as the oxidising agent. The α-MnO₂ samples synthesised by hydrothermal treatment at 120 °C (α-120) and 140 °C (α-140) for 2 h exhibited different sample morphologies. The sample morphology consisted of a mixture of rose-like microflower and needles, and X-ray diffraction (XRD), scanning electron microscopy (SEM), Brunauer‒Emmett‒Teller (BET) characterisation and Fourier transform infrared spectroscopy (FT-IR) were carried out on both the α-120 and α-140 samples. The results show that the only MnO₂ phase obtained in the synthesis was α-MnO₂. The electrochemical properties of the samples were analysed by cyclic voltammetry (CV) using a 0.1 M Na₂SO₄ electrolyte solution at scan rates ranging from 5 to 100 mV s^‒1. The specific capacitance of the system was calculated from the CV curves. The α-120 and α-140 samples had specific surface areas of 128 m² g^‒1 and 95 m² g^‒1, respectively, and specific capacitances at a scan rate of 5 mV s^‒1 of 112.8 F g^‒1 and 34.86 F g^‒1, respectively. The specific capacitance decreased as the scan rate increased for both samples.

View all citing articles on Scopus

Available online 20 June 2021

This work was supported by the Starting Research Funds of Shaanxi Normal University, the National Natural Science Foundation of China (21773146, 21872092).

^†: Contributed equally to this work.

View full text

ArticleAn unusual network of α-MnO2 nanowires with structure-induced hydrophilicity and conductivity for improved electrocatalysis

Abstract

Graphical Abstract

Introduction

Section snippets

Synthesis of the α-MnO2 nanowire network (α-MnO2-NWN)

Synthesis and characterization of the samples

Conclusions

Electronic supporting information

Chin. J. Catal.

Chin. J. Catal.

Chem. Sci.

Joule

J. Cryst. Growth

J. Electroanal. Chem. Interfacial Electrochem.

Acc. Chem. Res.

Adv. Sci.

Nanophotonics

Nat. Photonics

Nano Lett.

Angew. Chem. Int. Ed.

Adv. Mater.

Adv. Energy Mater.

Nat. Rev. Chem.

ChemCatChem

J. Am. Chem. Soc.

Adv. Mater.

ACS Catal.

Adv. Mater.

Adv. Mater.

Energy Environ. Sci.

J. Am. Chem. Soc.

Article
An unusual network of α-MnO₂ nanowires with structure-induced hydrophilicity and conductivity for improved electrocatalysis

Synthesis of the α-MnO₂ nanowire network (α-MnO₂-NWN)