Double-side solar hydrogen evolution nanopaper

Highlights

• A g-C₃N₄/nanocellulose nanopaper that is highly porous, crystalline, flexible and chemically stable has been fabricated.

• The hybrid nanopaper has an excellent mechanical property and hydrogen production activity.

• The hybrid nanopaper is able to produce H₂ from both sides.

• This work leads the way to a plethora of free-standing nanopapers.

Abstract

Carbon nitride, regarded as a promising, environmentally friendly and sustainable photocatalyst for solar hydrogen generation, has shown a gradual improvement of photocatalytic activity with an apparent quantum yield up to 60%. However, it is still challenging to achieve flexible, efficient and scalable carbon nitride solar hydrogen evolution devices, limiting its practical application. Herein we report a visible-light-driven double-side hydrogen evolution nanopaper that is highly porous, crystalline and chemically stable. The nanopaper was fabricated via vacuum filtration of electrostatically self-assembled carbon nitride and nanocellulose. This nanopaper shows excellent mechanical properties with tensile strength of 18.5 MPa and Young’s modulus of 414 MPa, a high hydrogen evolution rate of 3.9 mmol g⁻¹ h⁻¹ (corresponding to 6.5 μmol cm⁻² h⁻¹), and remarkable photostability over 32-h photocatalytic tests.

Introduction

Since the discovery of photocatalytic hydrogen evolution over graphitic carbon nitride (g-C₃N₄) by Wang et al. [1], it has attracted increasing attention because of its high abundance, good chemical stability, nontoxicity, and visible-light response [[2], [3], [4], [5], [6], [7], [8]]. The original g-C₃N₄ shows extremely low visible-light-driven hydrogen evolution activity due to the limited optical absorption, low surface area and fast photoexcited carrier recombination [1]. Considerable work has been carried out to address these issues [[9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29], [30], [31], [32], [33], [34], [35]]. The visible-light-driven hydrogen evolution activity of g-C₃N₄ can be greatly enhanced by rational tailoring of its structure [[9], [10], [11]], and optical and electronic properties [[12], [13], [14], [15], [16]]. More specifically, by doping [[17], [18], [19], [20]], amorphization or polymerization synthesis of g-C₃N₄ [[21], [22], [23], [24]], its visible-light response can be extended to 680 nm or even further [[25], [26], [27]]. Moreover, the creation of porous structures and/or heterojunctions not only promotes the photogenerated carrier separation, but also provides more catalytic sites for hydrogen evolution reaction [[28], [29], [30]]. Tight control over these parameters enables the synthesis of highly active g-C₃N₄. For example, a high solar hydrogen production activity with an apparent quantum yield (AQY) of 60% at 420 nm has recently been achieved over a novel g-C₃N₄ composed of internal triazine-heptazine donor-acceptor heterostructures [35].

Generally, g-C₃N₄ is synthesized by thermal polymerization of cyanamide or urea at an elevated temperature (typically 550 °C), forming a condensed yellow solid [[1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29], [30], [31], [32], [33], [34], [35], [36]]. To date, their photocatalytic hydrogen evolution activity has been tested in the form of milled powders, but not thin films, due to the lack of reliable methods for the fabrication of high-quality and highly active g-C₃N₄ films. This is a major obstacle for their practical applications in solar hydrogen evolution because it is difficult to maintain a stable dispersion of powdered g-C₃N₄ based photocatalysts in solution and recycle the expensive noble metal (typically Pt) co-catalyst deposited on the g-C₃N₄ surface. Therefore, the ability to fabricate high-quality and highly active g-C₃N₄ films is a key requirement towards their practical applications in solar hydrogen evolution. Common strategies for g-C₃N₄ thin-film fabrication involve physical methods under non-equilibrium conditions, such as chemical vapor deposition and pulsed-laser deposition [[36], [37], [38]]. A general drawback of these methods, however, is that the resulting films suffer from poor crystallinity and nitrogen deficiency. An alternative strategy is to use solution-based thin-film fabrication methods, in which a g-C₃N₄ suspension is screen-printed or spin-coated on a solid substrate (e.g., ITO or FTO glass) [[39], [40], [41], [42]]. Due to the poor particle-particle and particle-substrate contact, it is very difficult to achieve high-quality thin films using the milled g-C₃N₄ powders as raw materials. Since g-C₃N₄ is a polymer composed of heptazine repeating units bridged by the tertiary nitrogen, the carbon-nitride bonds in the linking bridges can be easily cleaved by a strong oxidizing acid like nitric acid, yielding a stable g-C₃N₄ colloid. Using the colloid as a coating material, a g-C₃N₄ thin-film on FTO electrodes has recently been fabricated [43]. Unfortunately, the structural destruction of bulk g-C₃N₄ required for the production of the colloid causes a hypsochromic shift of the absorption into the UV and generates a large number of surface defects, which can act as carrier recombination centers.

Although several attempts have been made to fabricate g-C₃N₄ films [[36], [37], [38], [39], [40], [41], [42], [43], [44]], to the best of our knowledge, no free-standing solar hydrogen evolution thin films (nanopapers) have been achieved. To address this issue, we report highly porous, crystalline and chemically stable hybrid nanopapers (HNPs) composed of g-C₃N₄ and nanocellulose (also called cellulose nanofibers, CNFs). Moreover, we also demonstrate the feasibility of using such flexible and lightweight HNPs for solar hydrogen evolution. The HNPs were fabricated via vacuum filtration of the electrostatically assembled g-C₃N₄ and CNFs, which offer excellent photoactive and mechanical functions, respectively. CNFs were selected as building blocks for several reasons. Firstly, they are ultra-light, earth-abundant, biodegradable, and renewable materials that can be easily derived from any plant wastes. In other words, they are environmentally friendly and sustainable materials [45]. Secondly, they possess good light transmitting ability [46] and allow visible light to reach g-C₃N₄ nanoparticles. Thirdly, the nanosized dimension and high aspect ratio of CNFs lend mechanical flexibility to the stiff and fragile g-C₃N₄ as the nanofibers can wrap around the large g-C₃N₄ particles driven by electrostatic interaction, and then self-assemble into a layered microstructure through hydrogen bonds [[47], [48], [49]]. Unlike the previously developed thin-film fabrication methods, the one developed here does not damage the fine structure of the pristine g-C₃N₄, thus maintaining its photoactivity. A HNP that exhibits excellent mechanical property with tensile strength of 18.5 MPa and Young’s modulus of 414 MPa, high hydrogen evolution rate of 3.9 mmol g⁻¹ h⁻¹ (i.e., 6.5 μmol cm^-2 h⁻¹), and remarkable long-term photostability has been achieved.