As a fascinating conjugated polymer, graphitic carbon nitride (g-C3N4) has become a new research hotspot and drawn broad interdisciplinary attention as a metal-free and visible-light-responsive photocatalyst in the arena of solar energy conversion and environmental remediation. This is due to its appealing electronic band structure, high physicochemical stability, and “earth-abundant” nature. This critical review summarizes a panorama of the latest progress related to the design and construction of pristine g-C3N4 and g-C3N4-based nanocomposites, including (1) nanoarchitecture design of bare g-C3N4, such as hard and soft templating approaches, supramolecular preorganization assembly, exfoliation, and template-free synthesis routes, (2) functionalization of g-C3N4 at an atomic level (elemental doping) and molecular level (copolymerization), and (3) modification of g-C3N4 with well-matched energy levels of another semiconductor or a metal as a cocatalyst to form heterojunction nanostructures. The constructi...

Graphitic Carbon Nitride (g-C3N4)-Based Photocatalysts for Artificial Photosynthesis and Environmental Remediation: Are We a Step Closer To Achieving Sustainability?

Jenny Schneider,*,† Masaya Matsuoka,‡ Masato Takeuchi,‡ Jinlong Zhang, Yu Horiuchi,‡ Masakazu Anpo,‡ and Detlef W. Bahnemann*,† †Institut fur Technische Chemie, Leibniz Universitaẗ Hannover, Callinstrasse 3, D-30167 Hannover, Germany ‡Faculty of Engineering, Osaka Prefecture University, 1 Gakuen-cho, Sakai Osaka 599-8531, Japan Key Lab for Advanced Materials and Institute of Fine Chemicals, East China University of Science and Technology, Shanghai 200237, China

Understanding TiO2 photocatalysis: mechanisms and materials.

Recent years have seen a renewed interest in the harvesting and conversion of solar energy. Among various technologies, the direct conversion of solar to chemical energy using photocatalysts has received significant attention. Although heterogeneous photocatalysts are almost exclusively semiconductors, it has been demonstrated recently that plasmonic nanostructures of noble metals (mainly silver and gold) also show significant promise. Here we review recent progress in using plasmonic metallic nanostructures in the field of photocatalysis. We focus on plasmon-enhanced water splitting on composite photocatalysts containing semiconductor and plasmonic-metal building blocks, and recently reported plasmon-mediated photocatalytic reactions on plasmonic nanostructures of noble metals. We also discuss the areas where major advancements are needed to move the field of plasmon-mediated photocatalysis forward.

Plasmonic-metal nanostructures for efficient conversion of solar to chemical energy

A fundamental change has been achieved in understanding surface electrochemistry due to the profound knowledge of the nature of electrocatalytic processes accumulated over the past several decades and to the recent technological advances in spectroscopy and high resolution imaging. Nowadays one can preferably design electrocatalysts based on the deep theoretical knowledge of electronic structures, via computer-guided engineering of the surface and (electro)chemical properties of materials, followed by the synthesis of practical materials with high performance for specific reactions. This review provides insights into both theoretical and experimental electrochemistry toward a better understanding of a series of key clean energy conversion reactions including oxygen reduction reaction (ORR), oxygen evolution reaction (OER), and hydrogen evolution reaction (HER). The emphasis of this review is on the origin of the electrocatalytic activity of nanostructured catalysts toward the aforementioned reactions by correlating the apparent electrode performance with their intrinsic electrochemical properties. Also, a rational design of electrocatalysts is proposed starting from the most fundamental aspects of the electronic structure engineering to a more practical level of nanotechnological fabrication.

Design of electrocatalysts for oxygen- and hydrogen-involving energy conversion reactions

The pace of materials discovery for heterogeneous catalysts and electrocatalysts could, in principle, be accelerated by the development of efficient computational screening methods. This would require an integrated approach, where the catalytic activity and stability of new materials are evaluated and where predictions are benchmarked by careful synthesis and experimental tests. In this contribution, we present a density functional theory-based, high-throughput screening scheme that successfully uses these strategies to identify a new electrocatalyst for the hydrogen evolution reaction (HER). The activity of over 700 binary surface alloys is evaluated theoretically; the stability of each alloy in electrochemical environments is also estimated. BiPt is found to have a predicted activity comparable to, or even better than, pure Pt, the archetypical HER catalyst. This alloy is synthesized and tested experimentally and shows improved HER performance compared with pure Pt, in agreement with the computational screening results.

Computational high-throughput screening of electrocatalytic materials for hydrogen evolution

Quantifying Losses and Assessing the Photovoltage Limits in Metal–Insulator–Semiconductor Water Splitting Systems

While the high sensitivity of plasmonic metal nanoparticles in Surface-Enhanced Raman Spectroscopy (SERS) is well established, applications of SERS in the selective identification of trace quantities of targeted molecules in a mixture of species remain to be a challenge. In this contribution, we demonstrate the design of plasmonic substrates that selectively enhance specific Raman vibrational bands, thereby offering high molecular sensitivity. We show that by changing the density and shape of plasmonic Ag nanostructures in Ag aggregates we can selectively control their optical properties and the degree to which they enhance different vibrational bands. By correlating the optical extinction spectra of substrates and the enhancement factors for different Raman vibrational bands, we demonstrate that the observed difference in the enhancement of vibrational modes is due to the difference in the localized surface plasmon resonance intensity at the Stokes Raman shifted wavelengths. We also shed light on critica...

Design of Plasmonic Platforms for Selective Molecular Sensing Based on Surface-Enhanced Raman Spectroscopy

Protective insulating layers between a semiconductor and an electrocatalyst enable otherwise unstable semiconductors to be used in photocatalytic water splitting. It is generally argued that in these systems the metal electrocatalyst must have work function properties that set a high inherent barrier height between the semiconductor and electrocatalyst and that the insulating layer should be as thin as possible. In this study we show that, for systems which suffer from inherently low barrier heights, the photovoltage can be significantly improved by tuning the thickness of the insulating layer. We demonstrate this in a case study of a system consisting of n-type silicon, a hafnium oxide protective layer (thickness 0–3 nm), and a Ni electrocatalyst. By optimizing the protective layer thickness, we observe increased efficiencies for photocatalytic oxygen evolution with a thick Ni electrocatalyst supported on n-Si. Our findings open avenues for the use of inexpensive electrocatalysts with favorable electroca...

Maximizing Solar Water Splitting Performance by Nanoscopic Control of the Charge Carrier Fluxes across Semiconductor–Electrocatalyst Junctions

The inclusion of metal nanoparticle electrocatalysts on semiconductor photoelectrodes is often used to increase the efficiency of photocatalytic reactions on semiconductors. It is well established that metal electrocatalysts can limit overpotential losses associated with many important photocatalytic reactions, significantly increasing the efficiency of the conversion of solar into chemical energy. On the other hand, the introduction of metal nanoparticles can also impact the light-absorbing properties of semiconductors. We have studied the impact of the introduction of Pt nanoparticles on the optical and catalytic properties of flat Si photoelectrodes for the photoelectrocatalytic evolution of hydrogen. We demonstrate that the deposition of platinum nanoparticles onto planar Si photocathodes results in improved catalytic rates for the hydrogen evolution reaction, but also a diminished light-limited rate of photon to hydrogen conversion. We also show that by embedding Pt nanoparticles into Si, the light-l...

Engineering the Optical and Catalytic Properties of Co-Catalyst/Semiconductor Photocatalysts

We use experimental and computational studies of core–shell metal–semiconductor and metal–molecule systems to investigate the mechanism of energy flow and energetic charge carrier generation in multicomponent plasmonic systems. We demonstrate that the rates of plasmon decay through the formation of energetic charge carriers are governed by two factors: (1) the intensity of the local plasmon induced electric fields at a specific location in the multicomponent nanostructure, and (2) the availability of direct, momentum conserved electronic excitations in the material located in that specific location. We propose a unifying physical framework that describes the flow of energy in all multicomponent plasmonic systems and leads us towards molecular control of the energy flow and excited charge carrier generation in these systems.

Suljo Linic

Papers

Quantifying Losses and Assessing the Photovoltage Limits in Metal–Insulator–Semiconductor Water Splitting Systems

Design of Plasmonic Platforms for Selective Molecular Sensing Based on Surface-Enhanced Raman Spectroscopy

Maximizing Solar Water Splitting Performance by Nanoscopic Control of the Charge Carrier Fluxes across Semiconductor–Electrocatalyst Junctions

Engineering the Optical and Catalytic Properties of Co-Catalyst/Semiconductor Photocatalysts

Unearthing the factors governing site specific rates of electronic excitations in multicomponent plasmonic systems and catalysts