Synthesis and characterization of nanoscaled BiPO4 and BiPO4:Tb

BiPO4 has been claimed for several aspects of technical application, including catalysis [1, 2], orthophosphate ion sensing by means of a quartz-crystal microbalance [3], as well as coprecipitation and separation of radioactive actinides [4, 5]. Especially for catalysis BiPO4 is a promising candidate and has been used, e.g., for a reduction of butyraldehyde to n-butanol [6]. Moreover, BiPO4 has been denoted as a host lattice for incorporation and luminescence of rare-earth ions [7, 8]. Structurally, BiPO4 has been described with three different modifications, whereof the monazite-type structure is thermodynamically most favored [9–11]. On the nanoscale, rodor wire-type BiPO4 has been recently synthesized via sonochemical [12] and CVD [13] methods. Interestingly, spherical shapes with uniform size and a low degree of agglomeration have not been addressed, yet. In this study, a polyol-mediated synthesis has applied to realize BiPO4 nanocrystals. The underlying concept of synthesis—a multidentate and high-boiling alcohol as the liquid phase (so-called polyol)—has been widely used already [14–17]. In a typical recipe, diethylene glycol (DEG, 100 mL, 99%, Acros) as the polyol was placed in a 250 mL beaker. BiI3 (1.2 g) was added and dissolved at 60 C (solution 1). In addition, 260 mg NH4H2PO4 were dissolved in 4 mL deionized H2O (solution 2). Solution 2 was added to solution 1 under vigorous stirring, resulting in an immediate nucleation. After 2 min the suspension was rapidly heated to 160 C and kept there for 1 h; thereafter the suspension was left to cool to room temperature. To separate the solid material, ethanol (100 mL) was added and the suspension centrifuged (15 min, 25000 rpm). Thereafter, the colorless fine powder was resuspended in ethanol and centrifuged twice in order to remove all DEG and remaining salts. Finally, the product was dried under reduced pressure (10 mbar, 1 h, 70 C). Nanoscale BiPO4:Tb (1 mol%) was realized by addition of 100 mg Tb(NO3)3 6 H2O to BiI3. Fractions of the final product were calcinated in a chamber furnace in air at 750 C (5 min) and 800 C (40 min) in order to study temperature-driven crystallinity, degree of agglomeration, as well as luminescence. As-prepared BiPO4 was characterized by dynamic light scattering (DLS, Malvern Instruments Nanosizer ZS), scanning electron microscopy (SEM, Zeiss Supra 40 VP, samples deposited on silicon and sputtered with Pt), X-ray powder diffraction (XRD, Stoe Stadi P system, Ge-monochromatized Cu-Ka radiation), Fourier-transform infrared spectroscopy (FT-IR, Bruker Vertex 70 FT-IR), and thermogravimetry (TG, Netzsch STA409C, nitrogen atmosphere). Photoluminescence (PL) was recorded with a Jobin Yvon Spex Fluorolog 3 equipped with a 450 W Xe-lamp and double grating excitation and emission monochromators. The quantum yield was measured by comparison to LaPO4:Ce,Tb (45 mol%, 15 mol%) as a standard lamp phosphor (Philips, particle size: 4–8 lm, quantum yield: 86% of Tb-related emission at kexcitation = 254 nm) [18]. Particle size, particle shape, and degree of agglomeration of as-prepared BiPO4 were validated for powder samples as well as for redispersed material. SEM (Fig. 1a) displays very uniform and non-agglomerated particles with a mean diameter of 33(12) nm. This mean diameter was gained based on a statistical evaluation of about 100 particles. DLS analysis of redispersed BiPO4 in DEG results in a M. Roming C. Feldmann (&) Institut fur Anorganische Chemie, Universitat Karlsruhe (TH), Engesserstrase 15, 76131 Karlsruhe, Germany e-mail: feldmann@aoc1.uni-karlsruhe.de