Surface Chemistry-Modulated Photo electrochemistry of Colloidal Nanocrystal Layers for Solar Water Splitting

The on-going demands for a clean and sustainable energy and its utilization in an environmental-friendly and optimized manner define a vector that guide the current scientific era. Harvesting the abundant solar light and converting it to a hydrogen fuel by photo-electrochemical water splitting with non-toxic semiconducting photocatalyst materials is a decent route to comply these demands. The efficiency and performance issues of these materials can be confronted by going down to the nanoscale region, accompanied with wise structural engineering of the nanocrystals and of their interfacial properties. In the case of photoelectrochemical-catalyzed water splitting, the role of the interfaces of nanocrystal photoelectrodes is focal and can be optimized by, for example, the incorporation of suitable functional molecules on the surfaces of the materials. The focus is on how the in- scale dimensions and wide diversity and versatility of chemical structures and physicochemical properties of organic molecules should be utilized to control and optimize the electronic and energetic characteristics of the nanocrystal materials and their performance in photoelectrochemical water splitting.

A sustainable energy economy, of which the society strive to minimize the usage of carbon- based fossil sources of energy, is oriented by the collection of energy from renewable and abundant sources and its conversion to feasible types of energy that can be harnessed for the daily basic uses, such as electricity supply and transportation [1,2]. A convenient way to accumulate the unexploited abundant energies, for example: solar, wind, thermo or waves, is by converting them into chemical energy in the form of chemical bonds and materials that plays the role of the energy carrier and fuel. Hydrogen is a common example for a chemical accumulator of solar light that can be further consumed to produce electrical energy in an environment-friendly manner by oxidation in fuel cells, with water as the sole by- product [3,4]. Solar light is converted directly to hydrogen by photoelectrochemical (PEC) water splitting [5,6]. This process is carried out in a photoelectrochemical cell with semiconducting photoanodes promoting water oxidation to oxygen (OER), [7-10] and with photocathodes allowing the reduction of water and the formation of hydrogen (HER) [11-13]. The overall reaction is photocatalytic electrolysis of water with the ability to enhance the photocatalytic activity by an additional applied bias [5,14,15]. The photocatalytic water splitting reaction is conducted by semiconductor materials as the light absorbers and photocatalysts in the form of thin films or as nanocrystals (NCs). The photocatalyst component in the form of NCs can be devised as colloidal suspensions in solution [16-18] or as thin layers on photoelectrodes in an electrochemical cell [19-21].

The implementation of semiconducting NCs as the photocatalyst in PEC has several advantages. First, NCs can be tuned to the solar spectrum by the structure, and size in addition to the chemical composition [15,22,23]. Second, the size of the NCs helps to overcome the issue of the collection efficiency of the photogenerated carriers. When the photocatalyst is devised in a form of a thin layer, an unfavorable ratio of the carrier diffusion length in the specific semiconducting material over the light absorption thickness of the photocatalyst layer may cause an inefficient collection of the photo-generated carriers and to lower photocurrent density. Upon preparation of the photocatalyst in the form of a NCs layer, the dimensions of the NCs and of the result thin layer can be optimized to correspond with the required diffusion length of the charge carriers that can facilitate a short-enough collection length and a more efficient photo response [24-27]. Third, the surface area of nanostructures is commonly large and allows an increased exposure of catalytic sites in the interfaces with the solution and thus to accelerated reaction kinetics [13,28]. In general, the hydrogen generation in a photoelectrochemical cell is associated with the development of photocurrent due to light absorption by the photocatalyst materials (Figure 1). The absorbed photons form excited electron-hole pairs (excitons) that relax to the lowest conduction band state and to the highest valance band state, respectively. The existence of an electric field at the interfaces of the semiconducting photocatalyst with the metal substrates and with the liquid due to band bending, and the applied external overpotential, enable charge separation over the substrate/photocatalyst/electrolyte bi-junctions [15,29]. The minority carriers (excited electrons, in the case of p-type semiconductors, such as: Cu₂O) are transferred from the photocatalyst layer to the electrolyte solution and promote water reduction to hydrogen via a 2 electron reaction process. The majority carriers (i.e. the generated holes in p-type semiconductors) are delivered to the substrate and transferred to the anode for completion of the full electrochemical water splitting reaction by water oxidation to oxygen [30]. As the applied overpotential increases, the electron-hole separation efficiency and result photocurrent increase, and the kinetics of the electrochemical reduction of water are faster [31] reduction potential, at the interfaces of, for example, Cu₂O NCs photocathode/liquid junctions are schematically presented.

Figure 1: An illustration of PEC hydrogen generation reaction with the photo-excited charge separation by solar light. The relevant excited charge carriers’ energies versus standard water reduction potential, at the interfaces of, for example, Cu2O NCs photocathode/liquid junctions are schematically presented.

When semiconducting NCs are deposited as thin films on metal electrodes, a metal/NCs/liquid bi-junction is created. It is expected that under the applied electric field and its direction the photo-exited charge would dominantly transfer across the bi-junction rather than by hoping between NCs in parallel to the metal substrate. Second, the metal/NCs/liquid bi-junction can be described as composed of two interfaces, as reflected in Figure 1:

1. The metal/NCs interface that can be defined as a metal-semiconductor junction.
2. The NCs/liquid interface that can be referred as a semiconductor-electrolyte solution interface.

The two interfaces are different in their physical essence, where interface I has solid-state characteristics and interface II is characterized as an electrochemical interface. Both interfaces have an important role in the behavior and properties of metal-semiconductor-solution junction: interface I as a solid state metal-semiconductor diode with its typical electronic behavior and characteristics [32]; and interface II as a semiconductor electrode in solution for electro-catalysis, where the electrode reactions and the kinetics depend strongly on the physicochemical properties of the electrochemical interface [33]. In the case that depicted in Figure 1-a band bending and a Schottky barrier is formed in both interfaces that regulate the charge transport through the bi-junction [34]. Therefore, both interfaces would inevitably have an impact on the photocurrent response and on the efficiency of the water splitting reaction. In order to get the optimum performance, the two interfaces should be investigated and handled separately and in accordance to their physical essence and characteristic behavior.

Colloidal NCs are originally covered with a monolayer of organic ligands as a result of their synthesis method. Accordingly, when the colloidal NCs are grafted on a metal electrode, an organic monolayer is present in both interfaces. It is stressed that the organic layers have a significant role in the modulation of the electrostatics of the interfaces and of the total charge transport across the metal/NCs/liquid bi-junction [21,32]. Specifically, in the case of the metal/organic-monolayer/semiconductor junction, the organic ligands contribute to the passivation of interface states by intermolecular interactions [35] on the alignment of energy levels by surface polarization and on the charge transport by their chemical structure. At the semiconductor-electrolyte interface, the organic layer influences interfacial physico-chemical properties, such as double-layer organization, hydrophobicity of the interface, electrical passivation [36] and electrochemical kinetics [33]. In accordance, a NCs thin layer photoelectrode can be treated as a modular system that allows a systematic investigation of the effect of the interfacial chemistry on the PEC activity. When NCs are grafted on a metal substrate, the chemistry of the two interfaces, i.e. Au/NCs and NCs/water (Figure 1), can be modified separately and controllably by desired ligands. First and prior to the deposition of the NCs on the metal substrate, a layer of selected type of organic ligands is assembled. After deposition of the NCs, the ligands of the colloidal NCs at the NCs/Liquid interface can be exchanged by different methodologies. The versatility of the molecular structure and the ability to synthesize molecules with a desired chemical structures and properties pave a wide route of implying specific physico-chemical properties to organic ligands and to interfaces upon ligand assembly. Therefore, a wise choice of even only the terminal group of the ligands can impose distinct changes to the properties of the interface, such as: electrostatics, charge transport kinetics, hydrophobicity/hydrophilicity, electronic passivation, and conductivity [21]. In addition, intermolecular interactions within the ligands in parallel to the substrate [35] and in perpendicular to it with adjacent phases are affected, as well, and have an impact on the interfacial properties and photo-electro-catalytic performance.

Conversion of sunlight to hydrogen by photo-induced electro-catalyzed water splitting is promoted as a clean and sustainable process for accumulation of an abundant energy (solar, for instance) in a form of an applicable and green fuel. Hydrogen can be further utilized in an environmental-friendly manner in a fuel cell, with water as the major waste. To comply with a zero-waste production and accumulation demand, non-toxic semiconducting photo- catalyst materials should be considered and implemented to the photoelectrodes. In addition, a maximum performance and efficiency should be strived to be obtained, in terms of solar to hydrogen conversion efficiency, optimized optical absorption in the visible region that fits the required potentials for water electrolysis and long-term chemical stability. While complying all of these requirements seems to be a challenging task, the option of using colloidal nanocrystals of abundant and non-toxic semiconductor materials can be considered in order to tackle most of them.

Major parameters of colloidal nanocrystals that can be tuned and tailored for optimized PEC performance are the nanocrystal chemical composition, its structure and size for optimized optical absorption and for charge separation. Additionally, the interfacial chemistry of nanocrystal photocatalysts, when composed as thin layer electrodes, has a prominent impact on the energetic, electrical, electrochemical and catalytic properties of the interfaces, as well as on their chemical passivation and stability. Both organic and in-organic surface molecular ligands are in the game in order to impose changes in the interfacial properties and PEC performance. In addition, organometallic ligands may be considered as molecular co-catalyst agents to be incorporated into the nanocrystal photoelectrode interfaces and to improve the photo response of the semiconducting NCs. Looking from one perspective, with a wise choice of ligands in terms of chemical structure and physico-chemical properties-the surface chemistry can be a game changer and promote the photoelectrochemical response of nanocrystal materials for photoelectrochemical water splitting applications. From a different perspective, photoelectrochemical measurements can be used as an in-situ tool to probe and study molecular scale effects, in the course of research and development of nano-scale chemical systems for molecular electronics and solar conversion applications.

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