Hydrogen bonding contributes to the extreme flexibility of vanadium pentoxide paper made from high aspect ratio nanofibers. Credit: Burghard; Wiley.

Last fall I wrote about a small, highly focused conference in Germany I attended called, “Generation of Inorganic Functional Materials Implementation of Biomineralization Principles.” The idea behind biomineralization is to adapt natural processes to synthesize new materials and engineer them into new configurations or engineer new functionalities.

A common point of reference is nacre—also called mother of pearl. The organic-inorganic composite secreted by mollusks has a layered structure comprised of platelets of the aragonite form of calcium carbonate held together with mortar-like organic substance, such as chitin or various proteins. The composite structure of brittle platelets bound together by an elastic biopolymer make nacre an exceptionally strong natural material. The aragonite platelets have a characteristic high-aspect ratio morphology that helps make the layered structure possible.

Paper, too, is made of high-aspect ratio constituents, usually fibers. Examples exist that are based on a platelet-biopolymer type of structure (vermiculate, clay, alumina, and some research on carbon nanotube and graphene oxide papers), but few display the characteristics that make paper so useful, especially flexibility.

This appears no longer to be the case. A newly published article reports some fascinating work on “paper” made of vanadium pentoxide nanofibers and its remarkable properties. The article is “Hydrogen-bond reinforced vanadia nanofiber paper of high stiffness,” by Zaklina Burghard, et al. (I met Burghard, postdoctoral researcher of materials science at the University of Stuttgart, Germany, at the poster session of the aforementioned conference.)

According to the paper, vanadium pentoxide (V2O5) differs from other transition metal oxides in that it can be synthesized into crystalline nanofibers with extremely high aspect ratios. The group reports making fibers with diameters 1-10 nanometers and lengths ranging from 100 nanometers to tens of micrometers.

The ribbon like fibers are made by a polycondensation process in an aqueous solution and are composed of two V2O5 layers with a layer of water in between. They have a rectangular cross section with oxygen groups and water bonded to the surface, both of which contribute to the bonding between fibers, similar to the role of biopolymer in nacre. The paper is made by slow drying vanadia nanofiber sols, which are then floated off the substrate.

The resulting paper is a dark orange color and can be made with a high degree of fiber alignment. The paper has extraordinary flexibility and can be bent or rolled as shown in the image. Burghard reports rolling cylinders with diameters as tight as 1 millimeter.

Because of the hydrogen bonds between the fibers, the team investigated the sensitivity of the papers to water content. They found that drying the paper at 40˚C was an important first step, which was followed by an annealing heat treatment at 100˚C or 150˚C. Without the drying step, the papers cracked. The annealing removes the weakly adsorbed water between the nanofibers, and by removing it slowly, the fibers are mobile enough to pack tightly, “most likely through the structure-directly capability of the hydrogen bonded between the V2O5 fibers.”

The importance of optimized thermal processing is evident in the tensile strength of the paper. The tensile strength of as-prepared paper is about 76 MPa. After drying, it is about 132 MPa and, after annealing, it pushes toward 200 MPa. The team suggests that the slow drying interlocks the basal planes of the fibers and increases the hydrogen-bond density between -OH groups on the surface. The article explains, “The excellent mechanical performance… can be attributed to the alternating layer structure of the vanadia paper, comprising a stiff inorganic oxide component combined with “flexible” layers of water in between, strongly resembling the brick-and-mortar architecture of structure biomaterials like nacre.”

The ordered structure is also reflected in the electrical conductivity. Conductivity increases after drying, and decreases slightly after annealing with the removal of ionic contributions to the overall conductivity. The results also correlated in expected ways with fiber alignment and in-plane-and out-of-plane directions.

Because of the dramatic flexibility of the paper and its promising properties, especially mechanical, the vanadia paper could be used in a range of applications relating to energy and electronics. The authors suggest the a wide range of potential applications including stretchable electronics, energy storage, flexible electrodes in chemical sensors, actuators, electrochromic devices, batteries, and supercapacitors. They note that the toughness of the paper could reduce the crack formation from swelling that tends to occur during ion intercalation of electrodes, for example.

The article is “Hydrogen-bond reinforced vanadia nanofiber paper of high stiffness,” by Zaklina Burghard, et al., Advanced Materials, doi: 10.1002/adma.201300135.