NANO LETTERS
Solution-Phase Synthesis of Cu2O Nanocubes
2003 Vol. 3, No. 2 231-234
Linfeng Gou and Catherine J. Murphy* Department of Chemistry and Biochemistry, Graduate Science Research Center, 631 Sumter Street, UniVersity of South Carolina, Columbia, South Carolina 29208 Received November 4, 2002; Revised Manuscript Received December 5, 2002
ABSTRACT We report here the solution-phase synthesis of highly uniform and monodisperse cubic Cu2O nano- and microcubes. Copper(II) salts in water are reduced with sodium ascorbate in air, in the presence of a surfactant. The average edge length of the cubes varies from 200 to 450 nm, as a function of surfactant concentration. Transmission electronic microscopy suggests that these cubes are composed of small nanoparticles and appear to be hollow.
Inorganic nanoparticles of uniform size and shape are of special interest from both theoretical and practical perspectives.1,2 Fields which would greatly benefit from advances in the synthesis of well-defined nanostructures include photonics, nanoelectronics, information storage, catalysis, and biosensors.3-12 Two general synthesis strategies have been employed for “bottom-up” chemical syntheses of nanomaterials: (i) the use of hard templates, which physically confine the size and shape of the growing nanoparticles;13,14 and (ii) the use of capping agents during nanoparticle growth to control its direction and dimension.15-21 Copper(I) oxide is a semiconductor that has potential applications in solar energy conversion and catalysis.22-26 Upon photoexcitation, Cu2O’s excitons are longlived (10 microseconds) and there is evidence that their motion through the solid can be coherent, in a manner analogous to photon coherence in lasers.27 Cu2O nanoparticles with different structures have been synthesized by various methods.28-30 Here, we report a synthesis of Cu2O nanocubes of high uniformity. Our method involves the use of sodium ascorbate to reduce Cu(II) salts in water, in the presence of a surfactant and NaOH. Control of cube dimension is correlated with the surfactant concentration. The cubic Cu2O particles prepared this way are very homogeneous in both shape and size, and as far as we know, we are the first to observe that these particles appear to be hollow. Typically, the cuprous oxide colloid was prepared by the following procedure. First, a set of solutions were prepared by adding 0.25 mL of a 0.01 M aqueous CuSO4 to 9.0 mL of an aqueous cetyltrimethylammonium (CTAB) solution, with the CTAB concentration varying from 0.01-0.10 M. Next, 0.50 mL of a 0.10 M sodium ascorbate solution was * Corresponding author. E-mail:
[email protected]. Phone: (803) 777-3628. Fax: (803) 777-9521. 10.1021/nl0258776 CCC: $25.00 Published on Web 12/24/2002
© 2003 American Chemical Society
Figure 1. Powder X-ray diffraction pattern of the Cu2O nanocubes. The peaks are labeled with the standard cuprite reflections.
added into the Cu(II)-CTAB solution. The solutions were heated in a water bath to 55 °C for 5 min. Then, 0.20 mL of a 0.5 M aqueous NaOH solution was added to the mixture, and a bright yellow color appeared immediately. The solution was kept at 55 °C for another 10 min and was removed from heat and allowed to cool to room temperature. Within 30 min the solutions turned to a purple red, a light yellow, or a dark yellow depending on the concentration of CTAB. No precipitate was observed. The particles were separated from the solutions by centrifugation at 6000 rpm for 15 min. They were then resuspended in water and the centrifugation was repeated twice so as to remove the surfactant. After removing the supernatant, the precipitant containing nanocubes was
Figure 3. Scanning electron microscopy image of cubic cuprous oxide nanocubes, coated with gold. Scale bar is 2 microns.
Figure 2. Transmission electron microscopy (TEM) images of cuprous oxide nanocubes: (a) at 9000×; (b) at 30000× magnifications. Scale bar is (a) 2 microns; (b) 500 nm.
collected and redispersed in a small amount of deionized water for further characterization. At this point, all the nanocube solids appeared pale brick red. Powder X-ray diffraction was performed with a Rigaku D\Max-2200 powder X-ray diffractometer with BraggBrentano geometry using Cu KR radiation. The step-scan covered the angular range 0-95° in step of 0.02°. The diffraction data revealed that the material was crystalline cubic cuprous oxide, cuprite (Figure 1). The peak intensities in our samples follow those observed in the standard material (file PDF 05-0667), although our samples’ (200) and (220) reflections were a little more intense compared to the standard powder pattern. Figures 2 and 3 show the transmission electron microscopy (TEM) and scanning electron microscopy (SEM) images of the cuprous oxide particles 232
synthesized with 0.10 M CTAB. In Figure 2a, for the 646 cubic particles counted, we found that 91% of them had an edge length of 450(20 nm, indicating the particles were very monodisperse in size. Figure 2b suggests that some of these particles, if not all, are hollow inside. From the SEM (Figure 3; the material was coated with gold to help improve the contrast), it appears that these cubic particles have rough surfaces and may be composed of smaller particles. Thus, the SEM data is consistent with the idea that the nanocubes are made of smaller particles and have a partially hollow interior. Calculations for these nanocubes suggest that the surface area would only be ca. 2 m2/g for “filled” nanocubes and ca. 10 m2/g for “80% hollow” nanocubes. Given the uncertainties in the measurement of surface area for such small sample sizes, we did not attempt chemisorption measurements of the material. We also characterized the Cu2O nanocubes by infrared and ultraviolet-visible spectroscopies. After the washing procedure, no IR bands corresponding to organic species (e.g., C-H, C-C, CdO) were detected, implying that little or no CTAB or ascorbate was present on or in the nanocubes. The UV-visible spectra of the final washed Cu2O nanocube products showed broad absorption from 400 to 1200 nm, with broad maxima at ca. 500-600 nm. Varying the concentration of CTAB during the synthesis led to a dramatic change in the size and shape of these particles. Figure 4 shows the TEM of the particles obtained by using an increasing amount of CTAB as the protecting agent. We found that when the CCTAB (concentration of CTAB) was small (e0.02 M), the nanoparticles were smaller, but their shapes were irregular; when CCTAB g 0.06 M, only large nanocubes (edge length ∼450 nm) formed; in the range of 0.02-0.06 M CTAB, we found a clear transition from small cubic particles, a mixture of small ones and larger ones, to purely larger nanocubes. The UV-visible spectra of the solutions showed an increase in absorption at 500-600 nm as the concentration of CTAB increased, but no clear shifts in wavelength maxima were observed; the nanocubes are too large, and, at low CTAB, too heterogeneous, to observe quantum confinement effects that one might expect in a semiconductor nanoparticle.2 Nano Lett., Vol. 3, No. 2, 2003
Figure 4. TEM images of Cu2O nanoparticles prepared with different concentrations of CTAB. From a-f, CCTAB equals 0.01, 0.02, 0.04, 0.06, 0.08, and 0.10 M, respectively. Scale bars are 500 nm (a, b, c, e) or 2 microns (d, f).
The synthetic procedure we used to make the copper(I) oxide nanocubes is very similar to the procedure used to produce bulk copper(I) oxide, with the exception of the CTAB surfactant.31 In that procedure, copper(II) salts in alkaline solution are reduced with hydrazine to produce yellow powdery Cu2O, with ill-defined intermediates, although a yellow metastable “CuOH” is possible.31 Alternately, brick-red crystalline bulk Cu2O is produced by the thermal decomposition of CuO.31 In our case, we propose that the bulk Cu2O reaction in basic solution is what occurs in our system, except that the CTAB directs the final product’s dimensions and crystallinity to produce the final, pale brick red product. We also propose the following mechanism for the nanocube growth: when the concentration of the surfactant is too low, the particles are ineffectively capped, and they grow randomly. Increasing the amount of surfactant leads to adequate surface capping and uniform cubic nanoparticles that mirror the fundamental cubic crystal structure of the cuprite unit cell. Evidently, CTAB does not preferentially adsorb to different crystal faces of Cu2O to produce anisotropic nanoparticles. Further increases in CTAB concentration, however, aggregate the cubic particles, and here CTAB actually serves as a cohesive agent. We have observed concentration-dependent ordering in CTAB-coated gold nanorods,32 and we hypothesize that interchain tangling of the 16-carbon cetyl chains on adjacent cubes may assist in the assembly process. The scheme below depicts nanocube assembly as a function of increasing CTAB concentration: Nano Lett., Vol. 3, No. 2, 2003
Other factors that influence the morphology of these particles are the pH of the solution and temperature. We found that the Cu2O nanocubes can be formed over a range of temperatures (30-85 °C) and pH (10-12) without a significant change in product morphology. Our best results were obtained with mild heating (40-60 °C) and a pH value of 10. We have also repeated the synthesis by replacing sodium ascorbate with ascorbic acid as the reducing agent and find that under low concentrations of CTAB, (CCTAB < 0.06 M), the particles are similar to those formed with sodium ascorbate. At high surfactant concentration, CCTAB g 0.06 M, the particles appeared more flowery, and their growth was not as uniform as using the sodium salt of ascorbic acid 233
Acknowledgment. Wethank the National Science Foundation for funding. Also, we thank Professor H. C. zur Loye and his group for assistance with the powder X-ray diffraction. References (1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11) (12) (13) (14) (15) (16) (17) (18) (19) (20) (21) (22) (23) (24) (25) (26)
Figure 5. TEM images of cuprous oxide nanoparticles synthesized by using ascorbic acid as the reductant. Scale bar is (a) 500 nm, (b) 100 nm.
(Figure 5). These micrographs also confirm that the larger nanoparticles are built up by smaller nanocubes. In summary, we have succeeded in making cubic cuprous oxide nanocubes by reducing Cu2+ with sodium ascorbate and choosing CTAB as the protecting agent. The size of these particles can be tuned by varying the concentration of surfactant.
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NL0258776
Nano Lett., Vol. 3, No. 2, 2003
