Novel Materials for Photocatalysis

The exploration of efficient daylight-driven photocatalysts remains a most essential challenge. Here, anatas-TiO2 is most widely applied but due to it wide band gap (2.8 eV) only active under UV-light excitation. Moreover, BiVO4 has become the most examined material for visible-light driven photocatalysis so far. Owing to the complexity of the underlying processes, homogeneity, morphology, surface chemistry and crystallinity are all crucial and depend strongly on the preparation of the regarded phase.

As an alternative, we could recently present β-SnWO4 as a new photocatalyst. This phase, however, is as elusive as promising and has remained almost unstudied, yet. Thermodynamically favoured above 670 °C, but metastable at room temperature, the synthesis of crystalline and nanoscaled β-SnWO4 at room temperature is a challenge. Via precise controlling of the experimental conditions, we can prepare high-quality nanoparticles as well as facetted microcrystals of β-SnWO4 (Figure 1). β-SnWO4 turned out as an excellent photocatalyst and shows high photocatalytic activity regarding the degradation of organic contaminants under simulated daylight (e.g. methylene blue). β-SnWO4 microcrystals outperform faceted microcrystals of Ag3PO4 and m-BiVO4 (Figure 1).

 

Figure 1: SEM images of photocatalysts and their activities for degradation of methylene blue under simulated daylight (150 W halogen lamp) at room temperature: β-SnWO4 faceted microcrystals, β-SnWO4 nanoparticles and conventional bulk-β-SnWO4 (right); faceted microcrystals of β-SnWO4, m-BiVO4 and Ag3PO4 (left).

β-SnWO4 nanomaterials can become relevant for a wide range of applications, including photocatalytic degradation of organic molecules for water cleaning, photocatalytic killing of germs for disinfection, photocatalytic water splitting for hydrogen generation, or photodynamic therapy for tumor treatment. In-vitro studies with the β-SnWO4 nanoparticles transfected into human liver carcinoma cells (HepG2 cells) as a conceptual study based on CLSM studies and MTT assays validate a significant phototoxicity at low systemic cytotoxicity under artificial daylight illumination (Figure 15). Due to its high zeta-potential (-45 mV), β-SnWO4 can be prepared via simple nucleation in water with the nanoparticles showing high quality in terms of size (8±2 nm), size distribution and colloidal stability under physiological conditions (Figure 2). Upon excitation (lexc = 458 nm), the β-SnWO4 nanoparticles show emission of green light (λem = 530 nm) that can be used to validate cell uptake. Based on this portfolio of properties (i.e., high colloidal stability, intrinsic fluorescence, phototoxicity), the β-SnWO4 nanoparticles can be considered as a multifunctional material but at low materials complexity.


Figure 2: Suspensions and particle sie of β-SnWO4 nanoparticles as well as results of MTT assays of HepG2 cells after transfection with (0.5-10 µM): 2 h in the dark only (dark green); 2 h in the dark followed by 2 h of artificial daylight illumination (red); 72 h in the dark followed by 5 h of daylight illumination (light green).

For more information see:
P. Schmitt, N. Brem, S. Schunk, C. Feldmann*, Polyol-mediated Synthesis of Nanoscale Molybdates/Tungstates and Its Properties: Color, Luminescence, Catalysis, Adv. Funct. Mater. 2011, 21, 3037–3046.

J. Ungelenk, C. Feldmann*, Synthesis of Faceted β-SnWO4 Microcrystals and Enhanced Visible-light Photocatalytic Properties, Chem. Commun. 2012, 48, 7838–7840.

J. Ungelenk, C. Feldmann*, Adjustable Kinetics in Heterogeneous Photocatalysis Demonstrating the Relevance of Electrostatic Interactions, Appl. Catal. B 2012, 127, 11–17.

J. Ungelenk, C. Seidl, E. Zittel, S. Roming, U. Schepers*, C. Feldmann*, Cell Uptake and Phototoxicity of ß-SnWO4 Nanoparticles, Chem. Commun. 2014, in press.