Selective Electrochemical Oxygen Reduction to Hydrogen Peroxide by Confinement of Cobalt Porphyrins in a Metal-Organic Framework.

Sustainable alternatives for the energy intensive synthesis of H2O2 are necessary. Molecular cobalt catalysts show potential but are typically restricted by undesired bimolecular pathways leading to the breakdown of both H2O2 and the catalyst. The confinement of cobalt porphyrins in the PCN-224 metal-organic framework leads to an enhanced selectivity towards H2O2 and stability of the catalyst. Consequently, oxygen can now be selectively reduced to hydrogen peroxide with a stable conversion for at least 5 h, illustrating the potential of catalysts confined in MOFs to increase the selectivity and stability of electrocatalytic conversions.

Sol-Gel-Derived Ordered Mesoporous High Entropy Spinel Ferrites and Assessment of Their Photoelectrochemical and Electrocatalytic Water Splitting Performance.

The novel material class of high entropy oxides with their unique and unexpected physicochemical properties is a candidate for energy applications. Herein, it is reported for the first time about the physico- and (photo-) electrochemical properties of ordered mesoporous (CoNiCuZnMg)Fe2 O4 thin films synthesized by a soft-templating and dip-coating approach. The A-site high entropy ferrites (HEF) are composed of periodically ordered mesopores building a highly accessible inorganic nanoarchitecture with large specific surface areas. The mesoporous spinel HEF thin films are found to be phase-pure and crack-free on the meso- and macroscale. The formation of the spinel structure hosting six distinct cations is verified by X-ray-based characterization techniques. Photoelectron spectroscopy gives insight into the chemical state of the implemented transition metals supporting the structural characterization data. Applied as photoanode for photoelectrochemical water splitting, the HEFs are photostable over several hours but show only low photoconductivity owing to fast surface recombination, as evidenced by intensity-modulated photocurrent spectroscopy. When applied as oxygen evolution reaction electrocatalyst, the HEF thin films possess overpotentials of 420 mV at 10 mA cm-2 in 1 m KOH. The results imply that the increase of the compositional disorder enhances the electronic transport properties, which are beneficial for both energy applications.

Mesoporous CuFe2 O4 Photoanodes for Solar Water Oxidation: Impact of Surface Morphology on the Photoelectrochemical Properties.

Metal oxide-based photoelectrodes for solar water splitting often utilize nanostructures to increase the solid-liquid interface area. This reduces charge transport distances and increases the photocurrent for materials with short minority charge carrier diffusion lengths. While the merits of nanostructuring are well established, the effect of surface order on the photocurrent and carrier recombination has not yet received much attention in the literature. To evaluate the impact of pore ordering on the photoelectrochemical properties, mesoporous CuFe2 O4 (CFO) thin film photoanodes were prepared by dip-coating and soft-templating. Here, the pore order and geometry can be controlled by addition of copolymer surfactants poly(ethylene oxide)-block-poly(propylene oxide)-block-poly(ethylene oxide) (Pluronic® F-127), polyisobutylene-block-poly(ethylene oxide) (PIB-PEO) and poly(ethylene-co-butylene)-block-poly(ethylene oxide) (Kraton liquid™-PEO, KLE). The non-ordered CFO showed the highest photocurrent density of 0.2 mA/cm2 at 1.3 V vs. RHE for sulfite oxidation, but the least photocurrent density for water oxidation. Conversely, the ordered CFO presented the best photoelectrochemical water oxidation performance. These differences can be understood on the basis of the high surface area, which promotes hole transfer to sulfite (a fast hole acceptor), but retards oxidation of water (a slow hole acceptor) due to electron-hole recombination at the defective surface. This interpretation is confirmed by intensity-modulated photocurrent (IMPS) and vibrating Kelvin probe surface photovoltage spectroscopy (VKP-SPS). The lowest surface recombination rate was observed for the ordered KLE-based mesoporous CFO, which retains spherical pore shapes at the surface resulting in fewer surface defects. Overall, this work shows that the photoelectrochemical energy conversion efficiency of copper ferrite thin films is not just controlled by the surface area, but also by surface order.

Electroreduction of CO2 on Au(310)@Cu High-index Facets.

The chemical selectivity and faradaic efficiency of high-index Cu facets for the CO2 reduction reaction (CO2 RR) is investigated. More specifically, shape-controlled nanoparticles enclosed by Cu {hk0} facets are fabricated using Cu multilayer deposition at three distinct layer thicknesses on the surface facets of Au truncated ditetragonal nanoprisms (Au DTPs). Au DTPs are shapes enclosed by 12 high-index {310} facets. Facet angle analysis confirms DTP geometry. Elemental mapping analysis shows Cu surface layers are uniformly distributed on the Au {310} facets of the DTPs. The 7 nm Au@Cu DTPs high-index {hk0} facets exhibit a CH4 : CO product ratio of almost 10 : 1 compared to a 1 : 1 ratio for the reference 7 nm Au@Cu nanoparticles (NPs). Operando Fourier transform infrared spectroscopy spectra disclose reactive adsorbed *CO as the main intermediate, whereas CO stripping experiments reveal the high-index facets enhance the *CO formation followed by rapid desorption or hydrogenation.

Electrochemical Generation of Catalytically Active Edge Sites in C2 N-Type Carbon Materials for Artificial Nitrogen Fixation.

The electrochemical nitrogen reduction reaction (NRR) to ammonia (NH3 ) is a potentially carbon-neutral and decentralized supplement to the established Haber-Bosch process. Catalytic activation of the highly stable dinitrogen molecules remains a great challenge. Especially metal-free nitrogen-doped carbon catalysts do not often reach the desired selectivity and ammonia production rates due to their low concentration of NRR active sites and possible instability of heteroatoms under electrochemical potential, which can even contribute to false positive results. In this context, the electrochemical activation of nitrogen-doped carbon electrocatalysts is an attractive, but not yet established method to create NRR catalytic sites. Herein, a metal-free C2 N material (HAT-700) is electrochemically etched prior to application in NRR to form active edge-sites originating from the removal of terminal nitrile groups. Resulting activated metal-free HAT-700-A shows remarkable catalytic activity in electrochemical nitrogen fixation with a maximum Faradaic efficiency of 11.4% and NH3 yield of 5.86 µg mg-1 cat h-1 . Experimental results and theoretical calculations are combined, and it is proposed that carbon radicals formed during activation together with adjacent pyridinic nitrogen atoms play a crucial role in nitrogen adsorption and activation. The results demonstrate the possibility to create catalytically active sites on purpose by etching labile functional groups prior to NRR.

Facet-Dependent Strain Determination in Electrochemically Synthetized Platinum Model Catalytic Nanoparticles.

Studying model nanoparticles is one approach to better understand the structural evolution of a catalyst during reactions. These nanoparticles feature well-defined faceting, offering the possibility to extract structural information as a function of facet orientation and compare it to theoretical simulations. Using Bragg Coherent X-ray Diffraction Imaging, the uniformity of electrochemically synthesized model catalysts is studied, here high-index faceted tetrahexahedral (THH) platinum nanoparticles at ambient conditions. 3D images of an individual nanoparticle are obtained, assessing not only its shape but also the specific components of the displacement and strain fields both at the surface of the nanocrystal and inside. The study reveals structural diversity of shapes and defects, and shows that the THH platinum nanoparticles present strain build-up close to facets and edges. A facet recognition algorithm is further applied to the imaged nanoparticles and provides facet-dependent structural information for all measured nanoparticles. In the context of strain engineering for model catalysts, this study provides insight into the shape-controlled synthesis of platinum nanoparticles with high-index facets.

Nanoscale Hybrid Amorphous/Graphitic Carbon as Key Towards Next-Generation Carbon-Based Oxidative Dehydrogenation Catalysts.

A new strategy affords "non-nano" carbon materials as dehydrogenation catalysts that perform similarly to nanocarbons. Polymer-based carbon precursors that combine a soft-template approach with ion adsorption and catalytic graphitization are key to this synthesis strategy, thus offering control over macroscopic shape, texture, and crystallinity and resulting in a hybrid amorphous/graphitic carbon after pyrolysis. From this intermediate the active carbon catalyst is prepared by removing the amorphous parts of the hybrid carbon materials via selective oxidation. The oxidative dehydrogenation of ethanol was chosen as test reaction, which shows that fine-tuning the synthesis of the new carbon catalysts allows to obtain a catalytic material with an attractive high selectivity (82 %) similar to a carbon nanotube reference, while achieving 10 times higher space-time yields at 330 °C. This new class of carbon materials is accessible via a technically scalable, reproducible synthetic pathway and exhibits spherical particles with diameters around 100 µm, allowing unproblematic handling similar to classic non-nano catalysts.

Nanoweb Surface-Mounted Metal-Organic Framework Films with Tunable Amounts of Acid Sites as Tailored Catalysts.

Metal-organic frameworks (MOFs) are a promising class of materials for many applications, due to their high chemical tunability and superb porosity. By growing MOFs as (thin-)films, additional properties and potential applications become available. Here, copper (II) 1,3,5-benzenetricarboxylate (Cu-BTC) metal-organic framework (MOF) thin-films are reported, which were synthesized by spin-coating, resulting in "nanowebs", that is, fiber-like structures. These surface-mounted MOFs (SURMOFs) were studied by using photoinduced force microscopy (PiFM) and time-of-flight secondary ion mass spectrometry (ToF-SIMS). The optimal concentration of precursors (10 mm) was determined that resulted in chemically homogeneous, pure nanowebs. Furthermore, the morphology and (un)coordinated Cu sites in the web were tuned by varying the rotation speed of the spin-coating process. X-ray diffraction (XRD) analysis showed that rotation speeds ≥2000 rpm (with precursors in a water/ethanol solution) generate the catena-triaqua-µ-(1,3,5-benzenetricarboxylate)-copper(II), or Cu(BTC)(H2 O)3 coordination polymer. X-ray photoelectron spectroscopy (XPS) highlighted the strong decrease in number of (defective) Cu+ sites, as the nanowebs mainly consist of coordinated Cu2+ Lewis acid sites (LAS) and organic linker-linker, for example, hydrogen-bonding, interactions. Finally, the Lewis-acidic character of the Cu sites is illustrated by testing the films as catalysts in the isomerization of α-pinene oxide. The higher number of LAS (≥3000 rpm), result in higher campholenic aldehyde selectivity reaching up to 87.7 %. Furthermore, the strength of a combined micro- and spectroscopic approach in understanding the nature of MOF thin-films in a spatially resolved manner is highlighted.

Large area, patterned growth of 2D MoS2 and lateral MoS2-WS2 heterostructures for nano- and opto-electronic applications.

The patterned growth of transition metal dichalcogenides (TMDs) and their lateral heterostructures is paramount for the fabrication of application-oriented electronics and optoelectronics devices. However, the large scale patterned growth of TMDs remains challenging. Here, we demonstrate the synthesis of patterned polycrystalline 2D MoS2 thin films on device ready SiO2/Si substrates, eliminating any etching and transfer steps using a combination of plasma enhanced atomic layer deposition (PEALD) and thermal sulfurization. As an inherent advantage of ALD, precise thickness control ranging from a monolayer to few-layered MoS2 has been achieved. Furthermore, uniform films with exceptional conformality over 3D structures are obtained. Finally, the approach has been leveraged to obtain in-plane lateral heterostructures of 2D MoS2 and WS2 thin films over a large area which opens up an avenue for their direct integration in future nano- and opto-electronic device applications.

Probing CO2 Reduction Pathways for Copper Catalysis Using an Ionic Liquid as a Chemical Trapping Agent.

The key to fully leveraging the potential of the electrochemical CO2 reduction reaction (CO2RR) to achieve a sustainable solar-power-based economy is the development of high-performance electrocatalysts. The development process relies heavily on trial and error methods due to poor mechanistic understanding of the reaction. Demonstrated here is that ionic liquids (ILs) can be employed as a chemical trapping agent to probe CO2RR mechanistic pathways. This method is implemented by introducing a small amount of an IL ([BMIm][NTf2 ]) to a copper foam catalyst, on which a wide range of CO2RR products, including formate, CO, alcohols, and hydrocarbons, can be produced. The IL can selectively suppress the formation of ethylene, ethanol and n-propanol while having little impact on others. Thus, reaction networks leading to various products can be disentangled. The results shed new light on the mechanistic understanding of the CO2RR, and provide guidelines for modulating the CO2RR properties. Chemical trapping using an IL adds to the toolbox to deduce the mechanistic understanding of electrocatalysis and could be applied to other reactions as well.

Elucidation of the Structure of a Thiol Functionalized Cu-tmpa Complex Anchored to Gold via a Self-Assembled Monolayer.

The structure of the copper complex of the 6-((1-butanethiol)oxy)-tris(2-pyridylmethyl)amine ligand (Cu-tmpa-O(CH2)4SH) anchored to a gold surface has been investigated. To enable covalent attachment of the complex to the gold surface, a heteromolecular self-assembled monolayer (SAM) of butanethiol and a thiol-substituted tmpa ligand was used. Subsequent formation of the immobilized copper complex by cyclic voltammetry in the presence of Cu(OTf)2 resulted in the formation of the anchored Cu-tmpa-O(CH2)4SH system which, according to scanning electron microscopy and X-ray diffraction, did not contain any accumulated copper nanoparticles or crystalline copper material. Electrochemical investigation of the heterogenized system barely showed any redox activity and lacked the typical CuII/I redox couple in contrast to the homogeneous complex in solution. The difference between the heterogenized system and the homogeneous complex was confirmed by X-ray photoelectron spectroscopy; the XPS spectrum did not show any satellite features of a CuII species but instead showed the presence of a CuI ion in a ∼2:3 ratio to nitrogen and a ∼2:7 ratio to sulfur. The +I oxidation state of the copper species was confirmed by the edge position in the X-ray absorption near-edge structure (XANES) region of the X-ray absorption spectrum. These results show that upon immobilization of Cu-tmpa-O(CH2)4SH, the resulting structure is not identical to the homogeneous CuII-tmpa complex. Upon anchoring, a novel CuI species is formed instead. This illustrates the importance of a thorough characterization of heterogenized molecular systems before drawing any conclusions regarding the structure-function relationships.

Enhancing the electrocatalytic activity of 2H-WS2 for hydrogen evolution via defect engineering.

Transition metal dichalcogenides (TMDs), such as MoS2 and WS2, are promising alternative non-noble metal catalysts to drive the electrocatalytic H2 evolution reaction (HER). However, their catalytic performance is inherently limited by the small number of active sites as well as their poor electrical conductivity. Here, we grow vertically aligned 2H-WS2 on different substrates to expose their edge sites for the HER and introduce a scalable approach to tune these active sites via defect engineering. In a thermal hydrogen treatment procedure, sulfur vacancies and metallic tungsten nanoparticles are formed. The extent of desulfurization, and thus the HER activity, can be tuned via controlling the H2 annealing conditions. The obtained W/WS2-x electrocatalysts are evaluated experimentally and theoretically to arrive at a better understanding of how to modify the inherently inert 2H-WS2 for more efficient HER.

Metal-Organic Frameworks as Catalyst Supports: Influence of Lattice Disorder on Metal Nanoparticle Formation.

Because of their high tunability and surface area, metal-organic frameworks (MOFs) show great promise as supports for metal nanoparticles. Depending on the synthesis route, MOFs may contain defects. Here, we show that highly crystalline MIL-100(Fe) and disordered Basolite® F300, with identical iron 1,3,5-benzenetricarboxylate composition, exhibit very divergent properties when used as a support for Pd nanoparticle deposition. While MIL-100(Fe) shows a regular MTN-zeotype crystal structure with two types of cages, Basolite® F300 lacks long-range order beyond 8 Šand has a single-pore system. The medium-range configurational linker-node disorder in Basolite® F300 results in a reduced number of Lewis acid sites, yielding more hydrophobic surface properties compared to hydrophilic MIL-100(Fe). The hydrophilic/hydrophobic nature of MIL-100(Fe) and Basolite® F300 impacts the amount of Pd and particle size distribution of Pd nanoparticles deposited during colloidal synthesis and dry impregnation methods, respectively. It is suggested that polar (apolar) solvents/precursors attractively interact with hydrophilic (hydrophobic) MOF surfaces, allowing tools at hand to increase the level of control over, for example, the nanoparticle size distribution.

Pinpointing the active species of the Cu(DAT) catalyzed oxygen reduction reaction.

Dinuclear CuII complexes bearing two 3,5-diamino-1,2,4-triazole (DAT) ligands have gained considerable attention as a potential model system for laccase due to their low overpotential for the oxygen reduction reaction (ORR). In this study, the active species for the ORR was investigated. The water soluble dinuclear copper complex (Cu(DAT)) was obtained by mixing a 1 : 1 ratio of Cu(OTf)2 and DAT in water. The electron paramagnetic resonance (EPR) spectrum of Cu(DAT) showed a broad axial signal with a g factor of 2.16 as well as a low intensity Ms = ±2 absorption characteristic of the Cu2(µ-DAT)2 moiety. Monitoring the typical 380 nm peak with UV-Vis spectroscopy revealed that the Cu2(µ-DAT)2 core is extremely sensitive to changes in pH, copper to ligand ratios and the presence of anions. Electrochemical quartz crystal microbalance experiments displayed a large decrease in frequency below 0.5 V versus the reversible hydrogen electrode (RHE) in a Cu(DAT) solution implying the formation of deposition. Rotating ring disk electrode experiments showed that this deposition is an active ORR catalyst which reduces O2 all the way to water at pH 5. The activity increased significantly in the course of time. X-ray photoelectron spectroscopy was utilized to analyze the composition of the deposition. Significant shifts in the Cu 2p3/2 and N 1s spectra were observed with respect to Cu(DAT). After ORR catalysis at pH 5, mostly CuI and/or Cu0 species are present and the deposition corresponds to previously reported electrodepositions of copper. This leads us to conclude that the active species is of a heterogeneous nature and lacks any structural similarity with laccase.

On the origin of the photocurrent of electrochemically passivated p-InP(100) photoelectrodes.

III-V semiconductors such as InP are highly efficient light absorbers for photoelectrochemical (PEC) water splitting devices. Yet, their cathodic stability is limited due to photocorrosion and the measured photocurrents do not necessarily originate from H2 evolution only. We evaluated the PEC stability and activation of model p-InP(100) photocathodes upon photoelectrochemical passivation (i.e. repeated surface oxidation/reduction). The electrode was subjected to a sequence of linear potential scans with or without intermittent passivation steps (repeated passivation and continuous reduction, respectively). The evolution of H2 and PH3 gases was monitored by online electrochemical mass spectrometry (OLEMS) and the Faradaic efficiencies of these processes were determined. Repeated passivation led to an increase of the photocurrent in 0.5 M H2SO4, while continuous reduction did not affect the photocurrent of p-InP(100). Neither H2 nor PH3 formation increased to the same extent as the photocurrent during the repeated passivation treatment. Surface analysis of the spent electrodes revealed substantial roughening of the electrode surface by repeated passivation, while continuous reduction left the surface unaltered. On the other hand, photocathodic conditioning performed in 0.5 M HCl led to the expected correlation between photocurrent increase and H2 formation. Ultimately, the H2 evolution rates of the photoelectrodes in H2SO4 and HCl are comparable. The much higher photocurrent in H2SO4 is due to competing side-reactions. The results emphasize the need for a detailed evaluation of the Faradaic efficiencies of all the involved processes using a chemical-specific technique like OLEMS. Photo-OLEMS can be beneficial in the study of photoelectrochemical reactions enabling the instantaneous detection of small amounts of reaction by-products.

X-ray Excited Optical Fluorescence and Diffraction Imaging of Reactivity and Crystallinity in a Zeolite Crystal: Crystallography and Molecular Spectroscopy in One.

Structure-activity relationships in heterogeneous catalysis are challenging to be measured on a single-particle level. For the first time, one X-ray beam is used to determine the crystallographic structure and reactivity of a single zeolite crystal. The method generates µm-resolved X-ray diffraction (µ-XRD) and X-ray excited optical fluorescence (µ-XEOF) maps of the crystallinity and Brønsted reactivity of a zeolite crystal previously reacted with a styrene probe molecule. The local gradients in chemical reactivity (derived from µ-XEOF) were correlated with local crystallinity and framework Al content, determined by µ-XRD. Two distinctly different types of fluorescent species formed selectively, depending on the local zeolite crystallinity. The results illustrate the potential of this approach to resolve the crystallographic structure of a porous material and its reactivity in one experiment via X-ray induced fluorescence of organic molecules formed at the reactive centers.

Quantitative 3D Fluorescence Imaging of Single Catalytic Turnovers Reveals Spatiotemporal Gradients in Reactivity of Zeolite H-ZSM-5 Crystals upon Steaming.

Optimizing the number, distribution, and accessibility of Brønsted acid sites in zeolite-based catalysts is of a paramount importance to further improve their catalytic performance. However, it remains challenging to measure real-time changes in reactivity of single zeolite catalyst particles by ensemble-averaging characterization methods. In this work, a detailed 3D single molecule, single turnover sensitive fluorescence microscopy study is presented to quantify the reactivity of Brønsted acid sites in zeolite H-ZSM-5 crystals upon steaming. This approach, in combination with the oligomerization of furfuryl alcohol as a probe reaction, allowed the stochastic behavior of single catalytic turnovers and temporally resolved turnover frequencies of zeolite domains smaller than the diffraction limited resolution to be investigated with great precision. It was found that the single turnover kinetics of the parent zeolite crystal proceeds with significant spatial differences in turnover frequencies on the nanoscale and noncorrelated temporal fluctuations. Mild steaming of zeolite H-ZSM-5 crystals at 500 °C led to an enhanced surface reactivity, with up to 4 times higher local turnover rates than those of the parent H-ZSM-5 crystals, and revealed remarkable heterogeneities in surface reactivity. In strong contrast, severe steaming at 700 °C significantly dealuminated the zeolite H-ZSM-5 material, leading to a 460 times lower turnover rate. The differences in measured turnover activities are explained by changes in the 3D aluminum distribution due to migration of extraframework Al-species and their subsequent effect on pore accessibility, as corroborated by time-of-flight secondary ion mass spectrometry (TOF-SIMS) sputter depth profiling data.

On the formation of Cd-Zn sulfide photocatalysts from insoluble hydroxide precursors.

The formation of Cd-Zn sulfide solid solutions from mixed hydroxides under hydrothermal conditions is investigated in detail. The work specifically aims to understand the formation and the role of nanotwinned mixed sulfide particles that have been reported to show excellent performance in photocatalytic water splitting (Liu, M.; et al. Energy Environ. Sci. 2011, 4, 1372). The influence of additives, pH, autoclave tumbling, and the state of the mixed hydroxide precursor on the mixed sulfides was studied by XRD, XPS, TEM, DR UV-vis, and N2 physisorption. Cd-Zn sulfides are formed via a dissolution-precipitation mechanism. Agitation of the synthetic medium and the formation of soluble intermediate complexes during hydrothermal treatment suppress the formation of a hexagonal wurtzite crystal phase and improve the photocatalytic activity of the mixed sulfides. The role of additives can be understood in terms of complex formation, pH maintenance, and adsorption on the facets of growing crystallites. All Cd-Zn sulfide samples exhibit compositional inhomogeneities, resulting in XRD line broadening and decreased bandgaps as compared with the values predicted by Vegard's law. Detailed TEM analysis revealed that the samples with higher amounts of nanotwinned particles were significantly less active in water reduction. The influence of nanotwinned particles is discussed in terms of extended crystal defects and charge carrier recombination.

Recent advances in secondary ion mass spectrometry of solid acid catalysts: large zeolite crystals under bombardment.

This Perspective aims to inform the heterogeneous catalysis and materials science community about the recent advances in Time-of-Flight-Secondary Ion Mass Spectrometry (ToF-SIMS) to characterize catalytic solids by taking large model H-ZSM-5 zeolite crystals as a showcase system. SIMS-based techniques have been explored in the 1980-1990's to study porous catalyst materials but, due to their limited spectral and spatiotemporal resolution, there was no real major breakthrough at that time. The technical advancements in SIMS instruments, namely improved ion gun design and new mass analyser concepts, nowadays allow for a much more detailed analysis of surface species relevant to catalytic action. Imaging with high mass and lateral resolution, determination of fragment ion patterns, novel sputter ion concepts as well as new mass analysers (e.g. ToF, FTICR) are just a few novelties, which will lead to new fundamental insight from SIMS analysis of heterogeneous catalysts. The Perspective article ends with an outlook on instrumental innovations and their potential use for catalytic systems other than zeolite crystals.

On the structural evolution of nanoporous optically transparent CuO photocathodes upon calcination for photoelectrochemical applications.

Copper oxides are promising photocathode materials for solar hydrogen production due to their narrow optical band gap energy allowing broad visible light absorption. However, they suffer from severe photocorrosion upon illumination, mainly due to copper reduction. Nanostructuring has been proven to enhance the photoresponse of CuO photocathodes; however, there is a lack of precise structural control on the nanoscale upon sol-gel synthesis and calcination for achieving optically transparent CuO thin film photoabsorbers. In this study, nanoporous and nanocrystalline CuO networks were prepared by a soft-templating and dip-coating method utilizing poly(ethylene oxide)-block-poly(propylene oxide)-block-poly(ethylene oxide) (Pluronic® F-127) as a structure-directing agent, resulting for the first-time in uniformly structured, crack-free, and optically transparent CuO thin films. The photoelectrochemical properties of the nanoporous CuO frameworks were investigated as a function of the calcination temperature and film thickness, revealing important information about the photocurrent, photostability, and photovoltage. Based on surface photovoltage spectroscopy (SPV), the films are p-type and generate up to 60 mV photovoltage at 2.0 eV (0.050 mW cm-2) irradiation for the film annealed at 750 °C. For these high annealing temperatures, the nanocrystalline domains in the thin film structure are more developed, resulting in improved electronic quality. In aqueous electrolytes with or without methyl viologen (as a fast electron acceptor), CuO films show cathodic photocurrents of up to -2.4 mA cm-2 at 0.32 V vs. RHE (air mass (AM) 1.5). However, the photocurrents were found to be entirely due to photocorrosion of the films and decay to near zero over the course of 20 min under AM 1.5 illumination. These fundamental results on the structural and morphological development upon calcination provide a direction and show the necessity for further (surface) treatment of sol-gel derived CuO photocathodes for photoelectrochemical applications. The study demonstrates how to control the size of nanopores starting from mesopore formation at 400 °C to the evolution of macroporous frameworks at 750 °C.