82 single-crystalline cubic MnO and tetragonal Mn3O4 nanowires was reported to be the ﬁnal product of a synthesis involving thermal evaporation of MnCl2 under argon ﬂow, and the nanowires showed unusual magnetic properties [8] (i.e., the MnO nanowires appeared ferromagnetic (FM) with TC = 12 K, though bulk MnO is antiferromagnetic, AFM). In this paper we present a hydrothermal method for the synthesis of single-crystalline b-MnO2 nanorods of excellent struc- tural quality. We examine phase purity of the nanorods and show that structurally well-ordered b-MnO2 material is covered by a thin (~2.5 nm) surface layer of a-Mn2O3 phase with a reduced Mn valence state. We also present magnetic properties of the b-MnO2 nanorods and demonstrate the important role of the surface layer in the rich temperature-dependent mag- netism of this material. Synthesis and characterization The synthesis of the b-MnO2 nanorods involved LiOH, Mn(NO3)2 · H2O and citric acid in a molar ratio 1.2:2:3.2 as the starting compounds, which were dis- solved in water and the polymer poly(ethylene glycol)– block–poly(propylene glycol)–(1100) was added. The mixture was stirred in a magnetic ﬁeld of 0.6 T strength and then 50 ml of the solution was sealed in an 80 ml teﬂon-lined autoclave. Hydrothermal synthesis was performed at 100 C for 48 h. The product was ﬁltered and rinsed with ethanol and dried at 100 C for 12 h. The polymer involved in the synthesis was ﬁred out by sintering at 500 C in air for 2 h, which resulted in a black solid as the ﬁnal product. The role of the polymer in the synthesis and the calcination of the polymeric precursor at high temperature in air are described elsewhere [9], whereas the chemistry of manganese citrate complexes is explained in [10]. Scanning elec- tron- (SEM) and transmission electron (TEM) micro- scope images of the product material (Fig. 1) show its morphology in the form of nanorods of dimensions 50–100 nm in diameter and 1–3 lm in length. SEM image (Fig. 1a) also reveals that the nanorods were synthesized with a high yield. The x-ray spectra revealed the presence of two phases (Fig. 2). The majority phase is the b-MnO2 pyrolusite tetragonal structure with a = 4.40410 A˚ , c = 2.87650 A˚ and space group P42/mnm, whereas the minority phase could be indexed to the a-Mn2O3 bixbyte orthorhombic structure with a = 9.41610 A˚ , b = 9.42370 A˚ , c = 9.40510 A˚ and space group Pcab. The relation of the two phases becomes evident from the high-resolution (HRTEM) image of a single Nanoscale Res Lett (2007) 2:81–86 Fig. 1 (a) SEM image of b-MnO2 nanorods and (b) TEM image of a single nanorod nanorod (Fig. 3a) that was acquired using JEOL JEM–ARM1300S high-voltage electron microscope (operated at 1250 kV) with 1.2 A˚ point–point resolu- tion. It is observed that the interior of the selected nanorod of 70 nm diameter consists of well-ordered crystalline material, which is at the surface covered by a 2.5-nm thin layer of another crystalline phase. The electron diffraction patterns in Fig. 3a conﬁrm that the interior of the nanorod is the b-MnO2 phase, whereas the surface layer is the a-Mn2O3 phase, in agreement with the x-ray analysis of Fig. 2. The manganese in the b-MnO2 phase is in a Mn4+ state, whereas its valence is reduced to Mn3+ in the a-Mn2O3 surface layer, so that the environment (very likely carbon coating during ﬁring procedure in air) obviously acts as a reducent to