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, rod- or 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 (10−3 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-Kα 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 μm, quantum yield: 86% of Tb3+-related emission at λexcitation = 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 hydrodynamic diameter of 37(7) nm (Fig. 2). This value confirms the presence of non-agglomerated particles as well as the particle size as given by SEM. Furthermore, the specific surface of as-prepared BiPO4 powders was verified via nitrogen adsorption. Based on the BET-method (Brunauer-Emmett-Teller) a value of 36 m2 g−1 was determined. Considering a bulk density of ρ = 6.323 g cm−3 [19], a particle diameter of about 26 nm was deduced, which again is in accordance with the above results.

Fig. 1
figure 1

SEM images of the as-prepared BiPO4 particles (a) as well as subsequent to calcination at 750 °C (b) and 800 °C (c)

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Fig. 2
figure 2

Dynamic light scattering of BiPO4 resuspended in DEG

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Chemical composition and phase purity of nanoscaled BiPO4 were investigated by XRD as well as by FT-IR. To this end, XRD analysis of washed and dried powder samples exhibits broadened reflections, which match with monazite-type BiPO4 (Fig. 3). Based on Scherrer’s equation, the particle diameter was calculated to 34 nm for the (120) reflection (2Θ = 29.1°). This crystallite size is in good agreement to the primary particle diameter stemming from SEM and DLS. FT-IR spectra are very comparable to reference data of bulk BiPO4 (Fig. 4: ν(PO4): 1150–850 cm−1, δ(PO4): 650–550 cm−1) [10]. Based on former investigations, additional vibrations at 3650–3350 cm−1 (ν(O–H)), 2950–2800 cm−1 (ν(C–H)), 1600–1550 cm−1 (δ(O–H)), and 1250 cm−1 (ν(C–O)) can be related to DEG and H2O adhered on the particle surfaces [20, 21]. TG of solid powders showed a weight loss of 1% in two steps. A first weight loss of 0.7% starts at 40 °C and is completed at about 120 °C. A second step occurs with a weight loss of 0.3% between 260 °C and 400 °C. The first step can be attributed to the evaporation of surface-bound water and ethanol; the second step is due to the loss of DEG on the particle surface. It is interesting to note that differential thermal analysis does not show any thermal effect up to 800 °C, although exothermal transformation to the monoclinic high-temperature form was described in literature to occur at about 600 °C in case of bulk BiPO4 [9–11]. This difference might be due to kinetic effects. Concordantly, the residue of TG analysis with XRD analysis shows the monazite-type modification, too (Fig. 3).

Fig. 3
figure 3

X-ray powder diffraction of BiPO4 nanoparticles (ICDD-No. 15–767 as a reference)

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Fig. 4
figure 4

FT-IR spectra of BiPO4 nanoparticles

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Aiming at luminescence, BiPO4 nanocrystals were codoped with 1 mol% Tb3+. Excitation and emission spectra show characteristic Tb-related transitions (Fig. 5) [22]. This is excitation via strong 4f85d0 → 4f75d1 transition as well as via weak transitions is within the 4f-manifold of Tb3+. Emission is dominated by 5D4 → 7FJ relaxation. By comparison to LaPO4:Ce,Tb as a reference exhibiting very similar luminescent processes, the quantum yield of as-prepared BiPO4:Tb was measured to 2% [18]. Note that the host lattice is inactive to luminescence at room temperature due to well-known concentration quenching of Bi3+ [23]. By calcination at 750 °C for 5 min the emission intensity as well as the quantum yield (12%) was significantly increased. XRD analysis still proves the presence of the monazite modification (Fig. 3); FT-IR spectra of the calcinated powder only show a decrease of DEG- and water-related vibrations (Fig. 4). After short-timed calcination, BET analysis with 16 m2 g−1 suggests a certain agglomeration, which is confirmed by first small agglomerates visible on SEM images (Fig. 1b). With a mean diameter of 39(14) nm (SEM) and 30(4) nm (DLS) BiPO4:Tb is nevertheless nanoscaled and can still be redispersed in DEG or ethanol. Increasing duration and temperature of calcination to 40 min at 800 °C results in an even higher quantum yield (23%). However, significant agglomeration of nanoscaled particles is now observed considering SEM images (Fig. 1c) and the values of specific surface (0.3 m2 g−1). As a consequence, redispersion of BiPO4:Tb is not possible any more.

Fig. 5
figure 5

Excitation and emission spectra of BiPO4:Tb (λexc. = 254 nm, λem. = 542 nm)

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In summary, nanoscale BiPO4 was firstly gained via a polyol-mediated synthesis, resulting in non-agglomerated and very uniform particles, about 30 nm in size. Tb-codoped BiPO4 nanocrystals show intense emission of green light. The underlying quantum yield was increased to 12% by short-timed calcination (700 °C, 5 min), which still allows for redispersion.