Novel Spinel Nanomaterials for Photocatalytic Hydrogen Evolution Reactions

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1		Atul Kulkarni	--	2181	2023-08-17 10:55:36	\|
2	Reference format revised.	Lindsay Dong	Meta information modification	2181	2023-08-21 03:15:30	\|

This entry is adapted from the peer-reviewed paper 10.3390/en16124707

The energy demand generated by fossil fuels is increasing day by day, and it has drastically increased after the COVID-19 pandemic as industries and household utilities rejuvenate. Renewable sources are thus becoming more essential as easily available, alternative methods of low-cost energy generation. Among these renewables, solar energy, i.e., solar power, is a promising energy source and can be used for solar-based H₂ evolution because H₂ technology is a leading source of eco-friendly electricity generation, and most of the worldwide efforts to develop this method involve heterogeneous catalysis for H₂ evolution via water splitting and its storage, i.e., using a fuel cell. In the current scenario, there is a need to develop a stable, recyclable, and reusable heterogeneous catalyst system, which is a great challenge.

renewable energy H2 evolution solar power recyclable stable

1. Introduction

Traditionally, non-renewable energy sources termed fossil fuels, including coal, oil, nuclear energy, and natural gas, are used in huge amounts for power generation in industries and domestic settings due to the increased demand for petroleum and automobiles. Attention to renewable energy sources, known as clean energy sources, including wind, solar, water, geothermal, and biomass, is increasing, mainly because of their advantages, such as abundance and negligible cost, whereas non-renewable energy sources are limited due to shortages and high costs ^[1].

Renewable energy sources, i.e., wind, biomass, and solar, are used for photocatalytic H₂ generation ^[2]; however, these sources are regional and seasonal. Therefore, H₂ can be generated by splitting water in H₂ and oxygen (O₂) by using sunlight (photocatalysis) ^[3], thermal and chemical (thermochemical catalysis) ^[4], and electrical (electrocatalytic) methods ^[5]. In addition, being a natural and abundant source of sunlight with no carbon dioxide (CO₂) gas emissions, solar energy is playing a crucial role in overcoming kinetic barriers during heterogeneous catalysis. State-of-the-art methods involve materials where the catalysts possess higher values for solar-driven hydrogen evolution reactions (HER), and they include noble metals, such as pure platinum (Pt), iridium (Ir), and ruthenium (Ru), as well as noble-metal-free photocatalysts. Many reports are available for Pt-based H₂ production using photocatalytic HER due to its high redox activity and zero overpotential ^[6]. However, the latest report on the approach for avoiding mass transport limitations and achieving the highest turnover frequency (TOF) when using Pt nanoparticles compared with the commercial platinum/carbon (Pt/C) catalysts illustrates that some of the pitfalls for obtaining a high-value TOF relate to measurement issues, such as the need for potential scale calibration, the choice of an incorrect counter electrode, and a lack of H₂ saturation ^[7] during solar photocatalytic HER. Similarly, as reported by Koo et al., platinum nanocubes synthesized using an aqueous colloidal route exhibited a promising photocurrent density of 1.77 A/mg at −100 mV ^[8]. Heterogeneous photocatalysis using oxide-based nanomaterials is becoming a pioneering research area, leading to prominent H₂ evolution results both in combination with, and without, noble metals ^[9]^[10]^[11], with Pt and Pt-group members being used with other inexpensive metal oxides to form alloys ^[12]. Therefore, Pt is the best catalyst in the field of catalysis to date and has also been explored for H₂ production using wastewater compounds ^[13]; however, the production of large amounts of H₂ is limited due to the cost of Pt and Pt-based commercial catalysts, high agglomeration rates, poor stability, and low removable efficacy. Low-cost, noble-metal-free photocatalysts are explored by Thakur et.al for efficient H₂ evolution (2531 μmol/g) based on a phosphorus-doped graphitic carbon nitride-P25 (TiO₂) composite and TiO₂/g-C₃N₄/p-g-C₃N₄ nanocomposite ^[14]. The optical properties of titanium nitride were enhanced using red phosphor, meaning the resulting nanocomposite could evolve the 0.5 μmol/g/h ^[15] of H₂. Sergei Poskunov et al. designed a novel photocatalyst, for which a single atom of gold, silver, and copper was deposited on the surface of TiO₂, and analyzed its electronic properties using real-time, time-dependent density functional theory (RT-TD-DFT) ^[16]. Conclusively, the wider research community has explored new emerging magnetic and non-magnetic nanomaterials that are based on noble- and non-noble-metal-based photocatalysts for eco-friendly H₂ generation ^[16]^[17]^[18].

2. Role of Removable Photocatalysts

Considering the many innovations achieved in the field of solar photocatalytic H₂ evolution, much less attention has been given to the byproducts generated after completing the reaction, which can cause a hazard to the environment ^[19]. Therefore, the first step towards sustainable and eco-friendly H₂ generation using novel nanomaterials is to find their dissociation mechanism and removal efficacy. Magnetic nano-catalysts such as Fe₂O₃ (hematite) and Fe₃O₄ (maghemite) are prominent and well established, with inherent or non-inherent magnetic properties, which allow them to be easily separated from an aqueous solution. Furthermore, spinel ferrite nanomaterials are novel types of magnetic nanomaterials that can be removed easily after the overall completion of HER. They are composed of an AB₂O₄ formula, where A and B are the divalent and trivalent cations coordinated with negatively charged oxygens or anions. Many compositions of spinel ferrites are possible due to the Earth’s abundance of metals, non-metals, and metalloids. Because nano-sized ferrites are an efficient photocatalyst, they are robust, and are characterized by thermal, chemical, and photostability; ease of production; a small band gap; tunable size; and higher levels of visible light absorption with appropriate positioning of the conduction band (CB) and valence band (VB); therefore, they are considered for photocatalytic HER. However, according to the available literature, until the year 2022, research on spinel ferrites for H₂ production has been limited. In the year 2019, Pu et al. developed a 1D recyclable p-n junction of a nanocomposite based on CoFe₂O₄/Cd_0.9Zn_0.1S, which was separated multiple times from the solution by using a proposed H₂ evolution mechanism ^[20]. Some other experimental reports are also available for the removal and recovery of ferrite-based nanomaterials ^[21]^[22]; however, they are limited and do not meet efficiency criteria compared with well-established nanomaterials, such as porous metal–organic frameworks (MOFs) ^[23].

3. The Mechanism for H₂ Evolution

The photocatalytic hydrogen evolution reaction (HER) is only possible when the photocatalyst can absorb the energy provided by a light source; this is necessary for the excitation of electrons from the valence band (VB) to the conduction band (CB), a process that leaves behind a hole. Similarly, in the case of spinel ferrites, the absorption of visible light leads to the excitation of electrons from the VB to the CB, which causes a hole formation in the VB, because of their narrow band gap energy values, which are capable of absorbing most of the visible light. Initially, an excited electron in the CB of spinel ferrite contributes to the breaking of bonds in adsorbed H₂O molecules and governs the classic theory called Volmer, Heyrovsky, and Tafel reactions.

The Volmer step contributes to the dissociation of adsorbed water molecules:

H₂O + e⁻ → H* + OH (1)

Heyrovsky step and Tafel step contribute to the production of molecular H₂

H* + e⁻ + H₂O → H₂ + OH (2)

2H* + 2e⁻ → H₂(3)

Similarly in the case of AB₂O₄ (ferrites) as photocatalysts, the reaction mechanism, it involves,

AB₂O₄ + 2H₂O → AB₂O₄–H + H₂O (4)

AB₂O₄–H + H₃O +e⁻ → AB₂O₄–H + H₃O (5)

H₃O⁺ + e⁻ +AB₂O₄–H → AB₂O₄ + H₂ + H₂O (6)

AB₂O₄–H + AB₂O₄–H → 2 AB₂O₄ + H₂(7)

4. Contributing Factors

Many factors affect H₂ evolution during photocatalysis, and they are discussed extensively with a focus on the development of effective photocatalysts. An illustration of the factors that contribute to H₂ evolution is given in Figure 1.

Figure 1. Illustration of different factors contributing to H₂ evolution.

4.1. Morphology

Nanomaterials have high functionality and diverse physicochemical properties that are comprehensively related to their photocatalytic properties ^[24]^[25]. Pure and hybrid oxide-based nanomaterials have been employed as efficient photocatalysts for water splitting and can produce and store hydrogen (H₂) following hydrogen evolution reactions (HER). Zero-dimensional (0D) nanomaterials, such as CdS, CdSe, and carbon quantum dots, are employed for visible light H₂ evolution; however, they are limited in efficiency due to their high-corrosion and charge-recombination rates. Addressing this, Li. et al. reported the photo deposition of metal oxides on quantum dots and alleviated the drawbacks ^[26]. Bimetallic plasmonic nanomaterials such as Ag@Au, i.e., a core–shell structure with an Au core and Ag shell, have emerged as next-generation photocatalysts for H₂ generation ^[27]. One-dimensional (1D) nanomaterials consist of nanorods (NRs) ^[28], nanowires (NWs) ^[29], nanotubes (NTs) ^[30], and nanofibers (NFs), contributing to their superior properties, which are suitable for photocatalysis for H₂ evolution. Their extremely large surface-to-volume ratio is favorable for photogenerated charge carriers and their ballistic transport. Zn₂GeO₄ is a type of 1D NRs that produces the highest rate of H₂, namely, 0.6 mmol/h in basic conditions ^[31]. The morphological characteristics of 1D porous ^[32] TiO₂ NTs are effectively utilized for photocatalytic activity because their inner diameter supports internal reflections of photons and results in a higher photocurrent density. TiO₂ NTs are broadly explored in pure and doped form for solar-based photocatalysis and H₂ generation, where doped TiO₂ NTs can cover the entire solar spectrum, hence increasing the H₂ production efficiency to 17.39 μmol h⁻¹ cm⁻². Carbon NTs and NFs have extraordinary mechanical and thermal stability.

4.2. pH and Sacrificial Agents

The acidic pH of the solution contributes to HER, where the H⁺ ion concentration is high. Diluted acid such as hydrogen sulfate (H₂SO₄) is used for providing acidic conditions. However, hole scavengers, such as ethanol and methanol/glycerol reaction media, have a basic pH. Different types of scavengers such as methanol, glycerol, formic acid, lactic acid, ethylenediamine (EDTA), triethanolamine, and sodium sulfate (Na₂SO₄) assist in controlling the charge recombination rate. Likuta et al. have undertaken a kinetic study of pH dependent H₂ production ^[33]. The hydrogen ion concentration or protons are known as the pH of the solution, which also affects the other chemical interactions among the catalyst and substrate during photocatalysis, such as adsorption and the agglomeration of particulates.

4.3. Temperature

Temperature is a key factor that contributes to an increased rate of H₂ evolution. Recently, an increased rate of about 38.0 mmol·g⁻¹ h⁻¹ at 60 °C was achieved by Núñez et al. via HER, two times greater than room temperature ^[34]. The catalytic performance of TiO₂ nanoparticles can also be enhanced by deposition on the SiO₂ substrate, thereby increasing the temperature of the water-splitting reaction and lowering the overpotential and reaction time ^[35]. The temperature of the photocatalytic reaction setup can also be increased automatically via a high-energy sunlight source. Collectively, the elevated temperature causes an increased rate of carrier mobility and effective charge transfer during visible light photocatalysis. After a certain saturation temperature, the activity can be decreased depending on the stability of the photocatalyst at higher temperatures and carrier recombination. Therefore, the temperature range is optimized for each different nanomaterial. Achieving high photocatalytic activity is a crucial challenge and using transition-metal-based oxide materials is favorable.

4.4. Concentration of Photocatalyst

An aqueous solution of a particulate-form photocatalyst should be prepared such that an optimal surface will be available to water molecules for an adsorption phenomenon called chemisorption. The higher the concentration of the photocatalyst, the more the adsorption of water molecules takes place, and the higher the rate of H₂ evolution. The effect of varied concentrations of copper/zinc sulfide/CoFe₂O₄ (Cu/ZnS/COF) core–shell photocatalysts with 0.1 g/L to 0.6 g/L was studied by Wu et al. They observed that 0.3 g/L was the optimal concentration, where the maximum H₂ evolution was observed and then decreased ^[35].

5. Synthesis Approaches

The process of H₂ evolution is generally carried out with a particulate or thin films (anode/cathode) as photocatalysts. In particulate-form H₂ production, an amount of the powder-form photocatalyst is subjected to solar power while being continuously stirred; the thin-film forms of the photocatalysts are prepared by depositing them on the conducting substrate before they are placed in a hanging condition in the reactor vessel/tube. Particulate-form H₂ production limits the solar-to-hydrogen (STH) efficiency due to the scattering effects being larger than absorption, the small surface area, and the increased charge recombination, although factors limiting the efficiency of oxide-based thin films include the lack of composition of different types of oxide materials, enhancing charge diffusion and separation to actual redox sites. Cobalt and nickel ferrite-based graphene nanocomposites are well explored, and their electrochemical performance showed their aptness for HER ^[36]. Thin-film deposition methods for spinel ferrites as solar-based photocatalysts include spin coating, spray coating, vacuum deposition, laser deposition, and sputtering, where the particulate from spinel ferrites can be synthesized using the sol–gel method, hydrothermal method, ball milling, etc. Many other researchers reported the synthesis of thin films using chemical methods of synthesis. These methods also limit the uniform deposition and adherent thin films compared with physical methods of deposition. The properties of photocatalysts also depend on the synthesis condition or deposition technique, and in the case of TiO₂, the phase varies with the chemical and physical deposition technique of the thin film and holds different strengths and weaknesses. Chemical deposition methods for thin films include successive ionic layer adsorption and reaction (SILAR), hydrothermal coating, electrodeposition, electrospinning, etc. These methods can deposit thin films with different morphologies with proper optimization steps at a low cost. Spinel ferrite Co-ZnFe₂O₄ thin-film nanostructures developed using hydrothermal reactions possess good photocatalytic activity and could produce H₂ with 0.0088 μmol/cm² min.

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1. Introduction

2. Role of Removable Photocatalysts

3. The Mechanism for H₂ Evolution