Photocatalytic hydrogen production (H2) is the core technology to solve many problems in future carbon based energy. Colloidal photocatalysts composed of plasma and catalytic nanoparticles are expected to efficiently catalyze the generation of H2 under sunlight, but their performance is severely hindered by light absorption and multiple scattering problems. In this article, the author designs a two-dimensional bimetallic catalyst by doping platinum nanoparticles into well-defined gold nanoparticle nanocrystals. Under visible light and solar irradiation conditions, the bimetallic nanocrystal catalyst exhibits a H2 yield of up to 139 mmol g-1cath-1 in the formic acid dehydrogenation process. Meanwhile, this configuration makes it possible to study the interaction between two metal materials and their impact on catalytic performance. In addition, the study found a correlation between the electric field strength in hotspots and the enhanced catalytic activity of platinum nanoparticles, and determined the secondary role of heat and gold to platinum charge transfer in performance enhancement. This study demonstrates the enormous potential of two-dimensional configuration catalysts with optimized structures in the field of liquid-phase photocatalysis.
Hydrogen energy will inevitably become a key player in the development of clean and sustainable energy in the future, serving as an efficient alternative to carbon based fuels and potentially reducing carbon footprint. Similar to what happens in nature, capturing energy from sunlight into H2 bonds has become a highly promising strategy for storing and converting solar energy, and materials that promote this energy conversion process have received widespread attention from researchers. Given this, sub wavelength metal nanoparticles (NPs) offer unique possibilities for photocatalytic reactions driven by local surface plasmon resonance. Plasma metals such as gold, aluminum, silver, magnesium, and copper exhibit resonance phenomena in the visible range of the electromagnetic spectrum, enabling them to effectively absorb light in the spectral region of maximum solar radiation. The interaction between light and matter can enhance the electric field on the surface of NPs, generating high-energy holes and electrons (hot carriers) as well as local heat. Despite the excellent optical properties mentioned above, plasma metal catalysts still exhibit relatively low activity to date. Therefore, in order to utilize these optical properties in chemical conversion reactions, researchers have integrated plasma NPs into multi-component nanostructures to form photocatalysts with better performance. Specifically, the bimetallic structure composed of catalytic metal and plasmonic metal can effectively combine the strong absorption and high reactivity of visible light.
Despite the enhanced activity of these bimetallic photocatalysts, their performance in liquid environments still faces a series of challenges. For example, high concentration solutions can undergo multiple scattering and absorption, hindering the activation of nanostructures by reducing the depth of light penetration through the reactor to several hundred micrometers (~500 μ m). This issue limits the working range of catalyst concentration in the solution and increases the challenge of designing photocatalytic reactors. In addition, providing stable ligands for colloidal NPs can lead to photochemical desorption, causing uncontrolled aggregation of nanostructures, resulting in changes in optical properties, aggregation deposition, and a reduction in the active surface of the catalyst. Transferring from colloidal suspension to membrane structure is a highly attractive strategy that can avoid the aforementioned limitations caused by light penetration and aggregation. In this regard, plasma two-dimensional (2D) nanocrystals can effectively promote the transition from 3D materials to quasi-2D materials, and can better utilize the advantages of bimetallic photocatalysts. It is worth noting that 2D represents the expansion of a crystal in two dimensions, which is not limited to a single layer, but means defining a certain number of layers in a given domain.
Usually, the proximity of a single NP in a 2D periodic array leads to the formation of plasma hotspots in particle gaps, thereby enhancing spatial regions characterized by electric fields. The sustained strong electric field in these hotspots can promote the weak absorption of catalytic centers in the visible light range and provide an important factor that needs to be adjusted between the thermal and electric field driven photoactivity of plasma bimetallic catalysts. Therefore, generating a large number of hotspots in densely packed nanocrystals is expected to maximize the utilization of active metals for catalytic applications.
Figure 1. (a, b) TEM images of Au nanocrystals composed of 22 nm AuNPs at different magnifications, with gaps of approximately 2 nm between the NPs. (c) Representative transmission microscope images of the nanocrystals, with different blue tones representing different numbers of particle layers and 1L representing a single layer; 2L is double layered; 3L is three layers; 3L+represents multiple layers. TEM images of (d, e) bimetallic 2D AuPt nanocrystals at different magnifications show that PtNPs (3 nm) are located at the particle gap (3.5 nm) between 22 nm AuNPs, and no interface is formed between different metals.
Figure 2. Experimental layer related reflectance, transmittance, and absorption spectra of pure Au nanocrystals (a) and bimetallic AuPt nanocrystals (b). By analyzing the components of different layers, the weighted reflectance, transmittance, and absorption spectra of (c) pure Au nanocrystals and (d) bimetallic AuPt nanocrystals were obtained.
Figure 3. (a) H2 yield of formic acid dehydrogenation reaction in Au and AuPt nanocrystals under dark and light conditions. (b) Arrhenius curve of AuPt nanocrystals.
Figure 4. (a) shows the wavelength dependent H2 yield (blue), electric field intensity (E) in the hot spot (black), and weighted absorbance (gray) in the visible light range. The photoactivity of the film reaches its peak at 650 nm. (b) The weighted electric field strength between two AuNPs, with PtNP located at the center of the gap. (c) Weighted electric field intensity diagram of bimetallic nanocrystals. (d) The transient absorption (A) spectra of Au and AuPt nanocrystals under the condition of λ=650 nm, where the orange and red dashed lines represent the gold detection wavelength and the purple dashed line represents the platinum detection wavelength. (e) Under the detection wavelength of λ pump=650 nm, no difference was observed in the dynamics of AuNPs on Au and AuPt nanocrystals during plasma decay, indicating that the thermal electron dynamics on platinum inclusions have not changed. (f) The kinetics under the conditions of λ probe=420 nm and λ pump=650 nm indicate platinum activation.
Overall, this article develops a plasma bimetallic nanocrystal that obtains the desired "antenna reactor" configuration from colloidal suspensions. While maintaining the configuration, the size of the superlattice can be expanded to several square millimeters and achieve single-layer, double-layer, and multi-layer domains. Research has found that the H2 yield of formic acid dehydrogenation reaction increases by two times under solar irradiation conditions, confirming the synergistic effect of plasma catalytic components in nanocrystals. By using structurally defined nanocrystals, it is possible to distinguish the contribution of plasma to the enhancement of catalytic performance. The results indicate that the performance of PtNPs is mainly determined by the electromagnetic field strength at the hotspot, while the thermal contribution and charge injection from AuNPs to the catalytic center have a relatively small impact on the enhancement of reaction performance.
Editor: Sichuan Jinzhongde Science and Technology Research Institute
Source: Today's New Materials
