3.4 Nanostructure engineering
Nanostructure engineering is an effective method for enhancing the performance of photocathodes by manipulating the dimensions and morphology of photocathode materials at the nanometer scale.[73,74] Notable examples of these nanostructures include nanorods, nanowires, dendritic structures, core/shell structures, as well as highly nanoporous and hollow structures. The benefits of employing such nanostructures are manifold. They facilitate enhanced light absorption through scattering and reduce bulk recombination. Additionally, these structures increase the specific surface area, leading to a corresponding increase in active sites.[75]
Furthermore, nanostructure engineering involves tuning the crystallographic orientations. Semiconductor nanocrystals typically exhibit anisotropic characteristics, composed of face-dependent electronic structures, adsorption energy/reactive sites, and photocorrosion resistance, attributable to different atomic configurations and coordination on various crystal facets.[76-81] Overall, synthesizing nanostructures with a high aspect ratio improves light harvesting and charge separation in the semiconductor bulk.[43]Additionally, nanostructures that expose selective facets, favorable for CO2 reduction kinetics, enhance charge separation at the surface.
For example, Liu et al. fabricated a photocathode for PEC CO2RR to CO by combining InP nanopillar arrays with Au-TiO2 interfaces (Au-TiO2/InP).[82] In the fabrication process, they used inductively coupled plasma reactive ion etching (ICP-RIE) on Au/SiOx-masked InP wafers to synthesize InP NPs. However, ICP-RIE led to plasma-induced surface defects on the InP NPs, resulting in a reduced minority carrier lifetime compared to planar InP. To remedy this, a dilute HCl solution was used to remove the plasma-damaged layer, leading to fewer surface defects in the InP NPs and improved minority carrier lifetime, as shown in Figure 6A. Thus, these treatments, aimed at eliminating surface defects, contributed to an increased surface area, reduced light reflection, and minimized carrier recombination losses, ultimately enhancing light-harvesting efficiency. The PEC performance of the nanostructured Au-TiO2/InP photocathodes demonstrated an onset potential of +0.3 V vs . RHE and an FECO of 84.2% at −0.11 V vs . RHE in a CO2-purged 0.1M KHCO3 solution under simulated 1 sun illumination, as illustrated in Figure 6B,C. Cai et al. constructed a core/shell structured nanosheet array of porous ZnO@ZnSe photocathode on FTO glass.[83] The synthesis involved a dissolution-recrystallization process, serving as a template for ZnSe growth through an anion exchange reaction with a Se2-source after cultivating the ZnO Ns array on the FTO glass coated with a ZnO seed layer. The optimal ZnSe loading demonstrated that while higher loading increased light absorption intensity and active site density, excessive ZnSe created additional surface defect states. Consequently, ZnO-10@ZnSe-3, with an appropriate loading amount, exhibited an onset potential of +0.39 V vs . RHE in a CO2-saturated 0.5M NaHCO3 electrolyte, a photocurrent of 1.17 mA cm–2 at +0.11 V vs . RHE, and an FECO of 52.9% at −0.4V vs . RHE under visible light irradiation. Hu et al. developed a photocathode for PEC CO2 conversion into CO featuring uniformly dispersed Au NPs with Au (111)/Au (200) boundaries on the p-Si surface (b-Au1/Si).[84] Initially, small, consistently sized Au seeds were distributed on the p-Si surface using a chemical deposition (CD) method. Subsequent electrodeposition ensured continuous growth of the Au above the CD seeds, maintaining a homogeneous distribution, as depicted in Figure 6D. DFT calculations suggested that the presence of Au (111)/Au (200) boundaries substantially decreased the energy barrier for forming the*COOH intermediate during CO2reduction. As a result, the b-Au1/Si photocathode achieved an impressive photocurrent density of −13.1 mA cm–2 at −1.0V vs. RHE with an FECO of 82.2% at −0.4 V vs. RHE, and it maintained remarkable operational stability for over a week, as shown in Figure 6E,F.
Mubarak et al. prepared a 3D nanoporous structured TiO2NPs on a thin Ti foil photocathode for CO2 reduction to HCOOH; the process involved a chemical treatment with H2O2, followed by calcination at elevated temperatures ranging from 400 to 800 ℃.[85] The resulting 300 to 500 nm thick 3D nanoporous layer on the Ti-foil surface displayed significant porosity. This structure enhanced the photon conversion efficiency of TiO2 NPs, increasing photon absorption per unit surface area and strengthening the electrochemical reaction capabilities due to the large specific surface area. The photocathode produced HCOOH as the primary product in PEC CO2RR after more than 25 h of chronoamperometric electrolysis and achieved an FEHCOOHof 64% with an HCOOH yield of 165 μmol cm–2h–1 at −1.3 V vs . Ag/AgCl. Paul et al.designed a photocathode using a morphology-controlled synthesis of Ag NPs distributed onto WO3 nanorods (Ag/WO3-NR) for PEC CO2RR to HCOO.[86] The one-pot fabrication process employed cetyltrimethylammonium bromide (CTAB) as a structure-directing agent for forming WO3 nanorods. Critical parameters such as CTAB concentration, reflux time, and temperature were pivotal in determining the nanorod morphology. The Ag/WO3-NR photocathode exhibited a significant current density of 0.4 mA cm–2 for HCOOproduction and achieved a rate of 31.7 mmol h–1.
Gurudayal et al. constructed a photocathode for PEC CO2RR to C2+ products, utilizing a back-illuminated n-type Si absorber covered with an Ag-supported dendritic Cu catalyst.[87] The synthesis of the Ag-supported dendritic Cu catalyst involved the evaporation of Ag, followed by high-rate electrodeposition of Cu, resulting in a highly porous structure with nanocactus-like morphology and featuring a dendritic Cu structure on the pyramid-shaped Ag. The Ag-supported dendritic Cu catalyst exhibited a high electrochemically active surface area, enabling the photocathode to operate at a high current density. Therefore, the photocathode produced C2+ products, including C2H4, C2H5OH, and C3H7OH, in CO2-saturated 0.1M CsHCO3 under simulated one sun illumination. Additionally, the photocathode maintained over 60% FE for hydrocarbon and oxygenated products, primarily C2H4, C2H5OH, and C3H7OH, for several days under simulated diurnal illumination. Kempler et al. utilized high loadings of Cu integrated onto Si microwire arrays (n+pSi μW/Cu) for PEC CO2RR to C2H4.[88] The Si microwire array structure could diminish trade-offs between catalyst loading and light absorption intensity. A Si photocathode with Cu electrodeposited onto the vertical sidewalls of high-aspect-ratio microwires was designed to minimize parasitic absorption by the catalyst, as revealed in Figure 6G, demonstrating a |Jph| exceeding 25 mA cm–2 before and after 48 h of PEC CO2RR, resulting in the production of C2H4 at more positive potentials under 1-sun illumination. Thus, the n+pSi μW/Cu photocathode revealed a maximum |JC2H4| of 2.1 \(\pm\) 0.2 mA cm–2 at −0.44 V vs . RHE and a maximum |JCH4| of 2.9 \(\pm\) 0.7 mA cm–2 at −0.62 V vs . RHE, as shown in Figure 6H,I.