Zero Threshold for Water Adsorption on MAPbBr3

Hybrid organic-inorganic perovskites (HOIPs) have shown great promise in a wide range of optoelectronic applications. However, this performance is inhibited by the sensitivity of HOIPs to various environmental factors, particularly high levels of relative humidity. This study uses X-ray photoelectron spectroscopy (XPS) to determine that there is essentially no threshold to water adsorption on the in situ cleaved MAPbBr3 (001) single crystal surface. Using scanning tunneling microscopy (STM), it shows that the initial surface restructuring upon exposure to water vapor occurs in isolated regions, which grow in area with increasing exposure, providing insight into the initial degradation mechanism of HOIPs. The electronic structure evolution of the surface was also monitored via ultraviolet photoemission spectroscopy (UPS), evidencing an increased bandgap state density following water vapor exposure, which is attributed to surface defect formation due to lattice swelling. This study will help to inform the surface engineering and designs of future perovskite-based optoelectronic devices.

TiO2 Polarons in the Time Domain: Implications for Photocatalysis

Exploiting the availability of solar energy to produce valuable chemicals is imperative in our quest for a sustainable energy cycle. TiO₂ has emerged as an efficient photocatalyst, and as such its photochemistry has been studied extensively. It is well-known that polaronic defect states impact the activity of this chemistry. As such, understanding the fundamental excitation mechanisms deserves the attention of the scientific community. However, isolating the contribution of polarons to these processes has required increasingly creative experimental techniques and expensive theory. In this Perspective, we discuss recent advances in this field, with a particular focus on two-photon photoemission spectroscopy (2PPE) and density functional theory (DFT), and discuss the implications for photocatalysis.

Chemical Modification of Polaronic States in Anatase TiO2(101)

Two polymorphs of TiO₂, anatase and rutile, are employed in photocatalytic applications. It is broadly accepted that anatase is the more catalytically active and subsequently finds wider commercial use. In this work, we focus on the Ti³⁺ polaronic states of anatase TiO₂(101), which lie at ∼1.0 eV binding energy and are known to increase catalytic performance. Using UV-photoemission and two-photon photoemission spectroscopies, we demonstrate the capability to tune the excited state resonance of polarons by controlling the chemical environment. Anatase TiO₂(101) contains subsurface polarons which undergo sub-band-gap photoexcitation to states ∼2.0 eV above the Fermi level. Formic acid adsorption dramatically influences the polaronic states, increasing the binding energy by ∼0.3 eV. Moreover, the photoexcitation oscillator strength changes significantly, resonating with states ∼3.0 eV above the Fermi level. We show that this behavior is likely due to the surface migration of subsurface oxygen vacancies.

Polaron-Adsorbate Coupling at the TiO2(110)-Carboxylate Interface

Understanding how adsorbates influence polaron behavior is of fundamental importance in describing the catalytic properties of TiO₂. Carboxylic acids adsorb readily at TiO₂ surfaces, yet their influence on polaronic states is unknown. Using UV photoemission spectroscopy (UPS), two-photon photoemission spectroscopy (2PPE), and density functional theory (DFT) we show that dissociative adsorption of formic and acetic acids has profound, yet different, effects on the surface density, crystal field, and photoexcitation of polarons in rutile TiO₂(110). We also show that these variations are governed by the contrasting electrostatic properties of the acids, which impacts the extent of polaron-adsorbate coupling. The density of polarons in the surface region increases more in formate-terminated TiO₂(110) relative to acetate. Consequently, increased coupling gives rise to new photoexcitation channels via states 3.83 eV above the Fermi level. The onset of this process is 3.45 eV, likely adding to the catalytic photoyield.

Batten disease and mitochondrial pathways of proteolysis

Most forms of Batten Disease (BD), a group of neurodegenerative diseases, are characterized by the accumulation within lysosomes of the very hydrophobic protein subunit 9 of the mitochondrial F1F0-ATP synthase (F-ATPase). It is now known that the cause of the accumulation of this protein in BD is a reduction in its rate of degradation. Because the F-ATPase subunit 9 accumulates within lysosomes of BD tissues, the degradative defect seemed likely to be within lysosomes. However, a recent report showed that delayed degradation of F-ATPase subunit 9 was evident in fibroblasts from BD patients long before any of the protein could be found within lysosomes. Therefore, the defective degradation pathway in BD appears likely to be intramitochondrial. We review the rather limited information about pathways of degradation of mitochondrial proteins. Mitochondria can be taken up and degraded by lysosomes through a process called macroautophagy. However, substantial proteolysis also occurs within mitochondria. Several different proteases are present within mitochondria, but their normal protein substrates are largely unknown. Like proteases from bacteria, many of these proteases operate in concert with molecular chaperones. We hypothesize that a mutation in a gene encoding a mitochondrial protease or a mitochondrial molecular chaperone leads to impaired degradation of F-ATPase subunit 9 in BD. This proteolipid may then form intracellular aggregates that are eventually sequestered into lysosomes.

Antiserum raised against a synthetic phosphotyrosine-containing peptide selectively recognizes p185neu/erbB-2 and the epidermal growth factor receptor

Rabbits were immunized with a synthetic phosphopeptide corresponding to a major autophosphorylation site of p185neu/erbB2 to determine the feasibility of producing tyrosine-phosphopeptide-specific antibodies. A series of adsorption and affinity chromatography steps were used to select antibodies with the desired reactivity. Immunoblot experiments showed that the resulting serum is highly specific for tyrosine-phosphorylated forms of p185 and the related epidermal growth factor receptor. The serum recognized these two receptors selectively when compared to five other receptor tyrosine kinases and several phosphorylated substrates. The serum is compatible with tissue-based assays since it detected tyrosine phosphorylation of the epidermal growth factor receptor in immunofluorescence experiments on permeabilized cells. The generality of the procedures used means that similar anti-tyrosine phosphopeptide sera can be produced that recognize other tyrosine kinases and substrates. Such sera will have numerous applications in research and clinical settings.