Colloidal properties of the metal-free semiconductor graphitic carbon nitride

Highlights

• Establishing a colloidal background for carbon nitride science is targeted.

• Colloidal aspects of carbon nitride synthesis will be reviewed.

• The role of colloid chemistry and interfaces for hybrid formation will be revealed.

• Carbon nitride as Pickering stabilizer will be discussed.

• Carbon nitride-polymer-colloid relation will be established.

Abstract

The metal-free, polymeric semiconductor graphitic carbon nitride (g-CN) family is an emerging class of materials and has striking advantages compared to other semiconductors, i.e. ease of tunability, low cost and synthesis from abundant precursors in a chemical environment. Efforts have been done to improve the properties of g-CN, such as photocatalytic efficiency, designing novel composites, processability and scalability towards discovering novel applications as a remedy for the problems that we are facing today. Despite the fact that the main efforts to improve g-CN come from a catalysis perspective, many fundamental possibilities arise from the special colloidal properties of carbon nitride particles, from synthesis to applications. This review will display how typical colloid chemistry tools can be employed to make ‘better g-CNs’ and how up to now overseen properties can be levered by integrating a colloid and interface perspective into materials chemistry. Establishing a knowledge on the origins of colloidal behavior of g-CN will be the core of the review.

Introduction

The colloidal state is of great importance and adds additional possibilities, from biological matter to artificial systems, and colloids are integral part of daily life (in cosmetics [1], beverages [2], paints and binders through heterophase reactions) [3]. Colloids are considered as entities that holds the key to understand the functions of biological systems as well as organized, complex systems that are not living beings [4].

Polymeric carbon nitride [5], a metal free semiconductor made from most simple starting compounds in a sustainable fashion, is an organic semiconductor full of possibilities. Metal-free graphitic carbon nitride (g-CN) received significant attention due to facile adjustability of morphology and composition, its low cost, high stability as well as low toxicity [6]. Another advantage of polymer-based g-CN semiconductor systems is its covalent character, i.e. it can be further modified by organic pre- or post-modification techniques [7].

Light irradiation excites an electron (e⁻) from valance band to conduction band while forming a positively charged hole (h⁺), and in carbon nitride, both are accessible to chemistry [8]. The photochemical activity depends on the efficient charge separation, where a reductive pathway through e⁻ and an oxidative pathway through h⁺ is accomplished. Yet, generally fast recombination (coupling of e⁻ and h⁺) rates prevent higher efficiencies. In order to overcome this problem, semiconductors are synthesized in special colloidal morphologies with high surface area, so that e⁻ or h⁺ can be more effectively separated. A splendid trend to employ g-CN as heterogeneous photocatalyst has started in 2009 [9], when the researchers have demonstrated the ability of g-CN for hydrogen evolution from water under visible light. General applications of g-CN can be considered as water splitting, CO₂ photoreduction and pollutant degradation, however many novel dimensions are being reported. Bandgap positions of tailored g-CN materials play a crucial role for the photocatalytic activity. For hydrogen evolution from water, conduction band of g-CN must be more negative than the water reduction potential to form H₂, likewise the valance band of g-CN should be more positive than the water oxidation potential to form O₂ [10]. H₂ evolution relies on successful charge separation, and excited electron is responsible for the formation of elemental hydrogen (metal deposition on g-CN helps for electron transfer and sacrificial agents for hole termination are used) [11]. CO₂ photoreduction via g-CN targets to decrease atmospheric CO₂ levels by conversion into carbon monoxide, methane, methanol, methanal and formic acid [12]. Similarly, redox potentials play a crucial role for the composition of the photoreduction products (excited electron-driven process on g-CN), and higher selectivities are strived. Pollutant degradation (such as aqueous dyes) is governed by the formation of reactive oxygen species (hole forms OH^. from water and electron forms ^.O₂⁻ in the presence of O₂), and photocatalytic disinfection properties of g-CN is also attributed to the formation of aforementioned reactive oxygen species [12].

In many of these applications colloidal processes already take place, and partly unusual behaviors were reported. This comes from the fact that high surface area photocatalysts are of course interface-controlled structures, and that photocharge generation and surface migration automatically come with an interplay with colloidal stability, partly with high complexity as phase specific charge pairs are involved. This is why in this review we decided to focus on the colloidal properties of polymeric semiconductor g-CN particles, as a better knowledge and awareness of these properties that would allow improvements in synthetic pathways, but also in dispersion or interface related applications and hybrid architectures.