Scalable Synthesis of Size-Tunable Small Cu2O Nanocubes and

May 15, 2019 - Despite the widespread interest in the examinations of catalytic and facet-dependent properties of Cu2O crystals, it was still difficul...
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Research Article Cite This: ACS Sustainable Chem. Eng. 2019, 7, 10467−10476

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Scalable Synthesis of Size-Tunable Small Cu2O Nanocubes and Octahedra for Facet-Dependent Optical Characterization and Pseudomorphic Conversion to Cu Nanocrystals Subashchandrabose Thoka, An-Ting Lee, and Michael H. Huang* Department of Chemistry and Frontier Research Center on Fundamental and Applied Sciences of Matters, National Tsing Hua University, 101, Section 2, Kuang Fu Road, Hsinchu 30013, Taiwan Downloaded via BUFFALO STATE on July 23, 2019 at 05:25:28 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

S Supporting Information *

ABSTRACT: Despite the widespread interest in the examinations of catalytic and facet-dependent properties of Cu2O crystals, it was still difficult to grow ultrasmall Cu2O cubes and octahedra with tunable sizes at a large scale. In this work, CuSO4, NaOH, and sodium ascorbate of varying volumes were added to an aqueous sodium dodecyl sulfate (SDS) solution to generate Cu2O nanocubes with average edge lengths of 16, 25, 29, 36, 51, 63, 72, and 86 nm in just 10 min. Another series of Cu2O cubes with wide size tunability in the range of 27−200 nm is accomplished by simply adjusting the NaOH volume. Similar reaction conditions can also be used to make a large quantity of Cu2O octahedra with opposite corner distances of just 34, 41, and 49 nm. Remarkably, production of these small Cu2O cubes and octahedra is scalable to 500 mL in one reaction. UV−vis absorption and photoluminescence spectra establish their size and facet-dependent optical properties, and a modified band diagram of Cu2O is presented. Recognizing Cu2O nanocrystal shape evolution is possible by changing the cell potential, we have proven this concept to yield cubic to truncated octahedral and octahedral structures by varying the CuSO4 volume. Finally, the tiny Cu2O cubes and octahedra were pseudomorphically converted to Cu cubes and octahedra via the introduction of ammonia borane, so these small copper polyhedra become readily accessible for diverse catalytic and plasmonic applications. KEYWORDS: Cuprous oxide, Facet-dependent properties, Pseudomorphic conversion, Systematic shape evolution, Ultrasmall nanocrystals

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INTRODUCTION

Previous synthetic conditions for growing Cu2O cubes, octahedra, rhombic dodecahedra, and other intermediate structures have generally yielded particles with sizes of a few hundreds of nanometers or microcrystals.12−19 A seedmediated stepwise growth approach to making Cu2O nanocubes with tunable sizes is more complicated, and particles are not perfectly cubic.20 For facet-dependent optical property characterization, the synthesized Cu2O polyhedra must be as small as possible, but have tunable sizes, to avoid light scattering interference. An earlier attempt to prepare small Cu2O nanocubes and octahedra with respective edge lengths of 37 and 67 nm involved the use of hydrazine (N2H4) as a reducing agent.21 Another procedure to make Cu2O octahedra with opposite corner distances of 52 to 157 nm (or 85 to 161 nm in another study) from an aqueous mixture of NaOH, N2H4 and Cu(NO3)2, and nanocubes from 9 to 87 nm from a mixture of CuSO4, NaOH and ascorbic acid, gave very small product yields, and suffered from coexistence of large

Cu2O, Ag2O, TiO2, Ag3PO4, and PbS crystals have been shown to display strongly facet-dependent electronic, photocatalytic, and optical properties.1−6 The emergence of these novel physical and chemical behaviors can be understood from the presence of an ultrathin surface layer with plane-specific band structures and hence tunable degrees of band edge bending to present different levels of difficulty to charge transport. For optical properties, this surface layer can be considered as having facet-specific refractive indices to tune the absorption band positions of nanocrystals.7 Furthermore, recent density functional theory (DFT) calculations on tunable numbers of various Si and Ge planes have revealed metal-like band structures for the (111) and (211) planes and a semiconductor band structure for the (100) and (110) planes with notable deviations in Si−Si (or Ge−Ge) bond length and bond geometry, as well as frontier orbital electron counts, from those of bulk Si and Ge.8,9 Electrical conductivity measurements on Si and Ge wafers match with DFT predictions.10,11 Thus, the observed semiconductor facet phenomena have a quantum mechanical origin. © 2019 American Chemical Society

Received: February 12, 2019 Revised: April 14, 2019 Published: May 15, 2019 10467

DOI: 10.1021/acssuschemeng.9b00844 ACS Sustainable Chem. Eng. 2019, 7, 10467−10476

Research Article

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Figure 1. SEM images of the synthesized Cu2O nanocubes with average edge lengths of (a) 16, (b) 25, (c) 29, (d) 36, (e) 51, (f) 63, (g) 72, and (f) 86 nm by adjusting the sodium ascorbate amount.

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particles.22,23 Consequently, separation of small from large particles was needed for spectral measurements. Although spectral measurements using a small amount of particles may be sufficient, it is most desirable to develop a scalable synthesis method for ultrasmall and highly uniform Cu2O polyhedra for accurate optical characterization, and the particles can also be employed as catalysts for diverse organic transformations such as the click reactions.21,24,25 While Cu2O cubes with sizes of ∼40 nm have been reported using an aqueous mixture of CuSO4, trisodium citrate, and NaOH, the particle sizes are still not sufficiently small for best optical characterization.26 Another procedure making 20 nm Cu2O cubes lacks additional characterization, and size tunability of ultrasmall cubes was not demonstrated.27 Recently, we have discovered pseudomorphic conversion of large Cu2O polyhedral particles to Cu crystals of the corresponding morphologies through the addition of ammonia borane.28,29 It is anticipated that the synthesized Cu2O nanocrystals can be converted to ultrasmall polyhedral Cu nanocrystals that may be difficult to make directly.30,31 In this study, we have developed a simple method to produce ultrasmall to small Cu2O nanocubes and octahedra at room temperature by adding CuSO4, NaOH, and sodium ascorbate to an aqueous solution containing SDS surfactant. Varying the amount of sodium ascorbate enables size tunability of Cu2O nanocubes and octahedra, while adjusting the NaOH amount produces another series of nanocubes with a greater size range from 27 to 200 nm. Remarkably, the synthetic conditions can be easily scaled up for mass production of the highly uniform particles. Optical characterization revealed clear presence of size- and facet-dependent absorption and emission band shifts. A modified band diagram of Cu2O incorporating these effects is presented. To test our understanding of particle shape control through tuning the reaction cell potential, the CuSO4 amount used was also varied to achieve systematic particle shape evolution from cubes to truncated octahedra and octahedra. Finally, the synthesized ultrasmall Cu2O nanocubes and octahedra were pseudomorphically converted to Cu particles of the corresponding shapes via the addition of ammonia borane, showing the additional benefit of producing ultrasmall Cu2O polyhedral particles.

RESULTS AND DISCUSSION It has been a great challenge for many years to synthesize ultrasmall Cu2O polyhedra with tunable sizes and shapes. This synthetic development for ultrasmall Cu2O nanocrystals originates from conditions used to make large copper cubes employing sodium ascorbate as the reducing agent.30 In this case, ascorbate (C6H7O6−) should react with Cu(OH)2 or Cu(OH)42− to produce Cu2O and dehydroascorbic acid (C6H6O6) as shown in the following equations. Cu(OH)2 and Cu(OH)42− are formed from the reaction of CuSO4 with NaOH. 2Cu(OH)2 + C6H 7O6− → Cu 2O + C6H6O6 + 2H 2O + OH−

(1)

2Cu(OH)4 2 − + C6H 7O6− → Cu 2O + C6H6O6 + 2H 2O + 5OH−

(2)

Briefly, to an aqueous SDS surfactant solution was added CuSO4, NaOH, and sodium ascorbate with stirring. After aging for 10 min for crystal growth at room temperature, ultrasmall Cu2O cubes with average edge lengths of 16, 25, 29, 36, 51, 63, 72, and 86 nm were obtained by decreasing the volume of 0.2 M sodium ascorbate added from 1.7 mL for making 16 nm cubes to 0.25 mL for 86 nm cubes. The exact reagent amounts and the reaction conditions are available in Figure S1 in the Supporting Information (SI). Figure 1 gives scanning electron microscopy (SEM) images of the synthesized Cu2O nanocubes. Additional SEM images of the 16, 25, 29, and 36 nm cubes at higher magnifications are shown in Figure S2. Figure S3 provides the particle size distribution histograms of the small nanocubes and their standard deviations. The nanocubes are remarkably uniform in size and possess sharp {100} faces for accurate facet effect examinations. The Cu2O nanocube samples below 30 nm should be the smallest ever reported having such excellent particle shape control and high product yield. X-ray diffraction (XRD) pattern of the 63 nm Cu2O nanocube sample confirms the formation of Cu2O (see Figure S4). Figure 2 shows transmission electron microscopy (TEM) characterization on a cube from the 51 nm Cu2O sample. The recorded selected-area electron diffraction (SAED) pattern indicates single-crystalline nature of the observed cube. Lattice fringes matching with the (110) planes of Cu2O are aligned 10468

DOI: 10.1021/acssuschemeng.9b00844 ACS Sustainable Chem. Eng. 2019, 7, 10467−10476

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with a broader particle size distribution reaching 270 nm were produced. Furthermore, while the dark yellowish color was observed in about 1 min for the 63 nm cube sample, it took about 2.5 min for the dimpled cubes to give a final orange solution color. Clearly the addition of SDS is crucial to ultrasmall Cu2O cube formation. UV−vis spectra of the CuSO4 solution with and without adding SDS reveal notable differences in Cu2+ ion absorption feature (Figure S6c), suggesting the formation of complex species or ion pair between Cu2+ ions and dodecyl sulfate. This tunes the reduction potential of the Cu ions, and leads to crystal growth rate and particle shape changes. Around 70 s after adding sodium ascorbate, strong visible light absorption from Cu2O has appeared in the presence of SDS, but absorption in the visible light region is relatively weak without SDS (Figure S6d). The results support a longer crystal growth period if SDS is not present. It is interesting to see the solution color differences of these small Cu2O nanocube samples (Figure S7). The deep yellow solution color for 86 nm cubes gradually becomes light yellow for 16 nm cubes. This color transition suggests continuous absorption band shifts to be determined by UV−vis spectroscopy. In the past, tuning the amount of reducing agent introduced has led to particle shape evolution.33 To assist in understanding why variation in the sodium ascorbate volume enables particle size tunability, photographs of the solution in the growth of 16, 36, and 63 nm Cu2O cubes are offered in Figure S8. Within 5 s after the addition of sodium ascorbate, the solution color has turned slightly yellowish for the 16 and 36 nm samples, but the 63 nm sample remained barely light blue from Cu(OH)2. From 15 s, all three samples showed light yellow color, yet the largest 63 nm cubes have developed a more intense yellow appearance until the end of the growth

Figure 2. (a, b) TEM image of a Cu2O nanocubes taken from the 51 nm sample and its corresponding SAED pattern. (c) HR-TEM image of the red box region in panel a. (d) TEM image of Cu2O nanocubes from the 16 nm sample.

toward edges of the cube. TEM image of the 16 nm Cu2O cubes is also provided, showing they possess mostly a cubic shape. X-ray photoelectron spectroscopy (XPS) of the 16 nm Cu2O cubes are presented in Figure S5. The peak positions match well with the reported XPS spectra of Cu2O crystals.32 To evaluate if introduction of SDS is necessary to achieve good particle shape control, the reaction conditions used to make 63 nm Cu2O nanocubes were employed but without the addition of SDS. Figure S6 shows face-dimpled Cu2O cubes

Figure 3. SEM images of the synthesized Cu2O nanocubes with average edge lengths of (a) 27, (b) 36, (c) 52, (d) 80, (e) 103, (f) 129, (g) 142, (h) 181, and (i) 200 nm by varying the NaOH volume. 10469

DOI: 10.1021/acssuschemeng.9b00844 ACS Sustainable Chem. Eng. 2019, 7, 10467−10476

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Figure 4. SEM images of the synthesized Cu2O octahedra with opposite corner distances of (a1, a2) 34, (b1, b2) 41, and (c1, c2) 49 nm. Inset of panel c2 gives a magnified view.

Cu2O nanocubes by adding different volumes of NaOH. The smaller nanocubes started to form within 5 s after adding the reducing agent, while the 200 nm cubes took about 15 s to display a lightly yellow hue. Again the smallest samples are brightly yellow, while the solution turns somewhat orange with increasing particle sizes. This illustrates the optical size effects of semiconductor nanocrystals. Similar reaction conditions employed to make ultrasmall Cu2O cubes by varying the volume of sodium ascorbate added can also be used to grow ultrasmall Cu2O octahedra. By simply raising the volume of 0.1 M CuSO4 solution from 100 μL for making ultrasmall cubes to 200 μL, and progressively decreasing the volume of sodium ascorbate solution introduced from 1.7 to 0.9 mL, octahedral Cu2O particles with opposite corner distances of 34, 41, and 49 nm (or edge lengths of 24, 29, and 35 nm) were obtained (see Figure 4). The exact reagent amounts used are displayed in Figure S11. Particle size distribution histograms are available in Figure S12. For all samples, the octahedra are superuniform in size and shape. These are the smallest Cu2O octahedra ever reported achieving this high level of particle homogeneity. Figure S13 is a photograph of the three samples, showing slightly different hues. XPS spectrum of the 34 nm Cu2O octahedron sample is also included in Figure S5. The peak positions are nearly identical to that of the 16 nm Cu2O cubes. TEM character-

period at 15 min. The observations suggest the smallest nanocubes are formed at a much faster rate. Thus, the rate of nucleation particle formation, tuned through sodium ascorbate volume variation, may be related to the final crystal size. Before presenting optical characterization of these Cu2O nanocubes, formation of another series of Cu2O cubes from 27 to 200 nm by adjusting the volume of 1.0 M NaOH solution added is described first. The exact reagent amounts used are provided in Figure S9. We have found that keeping the amounts of all other reagents constant, but increasing the NaOH solution volume from 50 to 170 μL, Cu2O cube size can be increased progressively. With more NaOH added, greater Cu(OH)2 and Cu(OH)42− species are produced. It may be possible that they are good copper sources for facile formation of Cu2O, so tuning NaOH amount has a notable effect on particle size control. Figure 3 displays SEM images of the synthesized Cu2O cubes with average edge lengths of 27, 36, 52, 80, 103, 129, 142, 181, and 200 nm. The particles appear to be highly uniform in size and possess sharp faces. This wide range of size control from ultrasmall cubes to large cubes reaching 200 nm in a single reaction step is unprecedented. Depending on the research design, this procedure can easily offer Cu2O cubes having the desired sizes. Figure S10 shows the solution color changes as a function of reaction time in the preparation of some of these 10470

DOI: 10.1021/acssuschemeng.9b00844 ACS Sustainable Chem. Eng. 2019, 7, 10467−10476

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Figure 5. (a, b) UV−vis absorption spectra and photoluminescence spectra of the synthesized Cu2O nanocubes with tunable sizes. (c, d) UV−vis absorption spectra and photoluminescence spectra of the synthesized Cu2O octahedra with tunable sizes. (e, f) Plots showing the variation in absorption and emission band wavelengths with respect to Cu2O cube and octahedron sizes expressed in particle volumes. Particle sizes are only labeled for the absorption curves.

why people are focused on quantum nanostructures for emission properties and applications. However, the interest here is to check emission band shifts. As expected, emission band red-shifts progressively from 470 nm for 16 nm cubes to 518 nm for 86 nm cubes. For 34, 41, and 49 nm nanooctahedra, their emission band positions are at 461, 465, and 470 nm, respectively. Converting the particle sizes to their volumes, Figure 5 also compares absorption and emission band shifts of these Cu2O nanocubes and octahedra. The Bohr exciton radius of Cu2O is 1.4 nm (or 0.7 nm in another report).34,35 The fact absorption and emission band shifts continuously for Cu2O nanocubes beyond 86 nm reveals that our understanding of quantum confinement effects is not quite correct. Evidently, at any particle volume, absorption band of cubes is consistently more red-shifted than octahedra by around 10 nm. The emission band of cubes is also more redshifted than octahedra of similar sizes by about the same

ization has been performed on the 34 and 41 nm octahedra (Figure S14). Again the octahedra are highly uniform under TEM observation. Cu2O (111) and (110) lattice planes have been measured. SAED pattern also indicates single-crystallinity of the observed octahedral particle. The successful synthesis of these Cu2O nanocubes and nanooctahedra with tunable sizes provides an excellent opportunity to confirm their size- and facet-dependent optical properties. Both UV−vis absorption and emission spectra of these nanocrystals were taken and are plotted in Figure 5. An excitation wavelength of 380 nm was used to obtain emission spectra. With increasing nanocube size from 16 to 86 nm, their absorption band shows steady red-shifts from 442 to 495 nm. Similarly, 34 nm octahedra absorb at 434 nm, while 49 nm octahedra gives an absorption band centered at 444 nm. The smallest nanocubes and octahedra exhibit far stronger photoluminescence intensities than the larger ones, reflecting 10471

DOI: 10.1021/acssuschemeng.9b00844 ACS Sustainable Chem. Eng. 2019, 7, 10467−10476

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Figure 6. Modified band diagram of Cu2O nanocrystals to account for the observed optical size and facet effects. Crystal sizes and their band gaps in eV are indicated.

magnitude. Because of the exceptional size uniformity of these particles, the absorption and emission band separations measured here between Cu2O nanocubes and octahedra of similar volumes should be more accurate. Previously absorption band separation of 13−16 nm was recorded for Cu2O nanocubes and octahedra, showing the value in pursuing the highest degree of crystal size control.22 On the basis of the spectral data, a modified band diagram of Cu2O nanocrystals accounting for the observed optical size and facet effects is constructed and shown in Figure 6. For simplicity, the valence band energy of Cu2O is kept constant, although one needs to realize that it is slightly tunable with particle shape.36 Because particles of different sizes absorb light of different wavelengths, there is the size component in the band gap with smaller particles having larger band gaps. In addition, there is the facet component due to the presence of an ultrathin surface layer having dissimilar band structures for various surface planes. Since octahedra absorb light of shorter wavelengths, their band edge bending is larger than that of cubes to signify absorption of light of higher energies. Therefore, an UV−vis spectrum of semiconductor nanocrystals is actually composed of bulk interior absorption and the surface layer absorption as the arrows in Figure 6 indicate. With synthetic advances of nanocrystals, the modified band diagram represents a more accurate picture of the true nature of semiconductor materials. The theory of quantum confinement without regard for the presence of facet effects results from our inability to synthesize semiconductor nanocrystals at this high level of shape control. Excited about the prospect to prepare ultrasmall Cu2O nanocubes and octahedra at a large scale to prove the utility of this new synthetic method, we have increased the amounts of reagents used by 50-fold using the conditions for making 16 nm cubes. Figure 7 presents a photograph of the large scale preparation of Cu2O nanocubes reaching a solution volume of 500 mL. SEM images show very high size homogeneity. The average nanocube edge length was found to be 23 nm. Total particle weight was 30 mg in each preparation, meaning several batches of the reaction can produce a large quantity of ultrasmall Cu2O nanocubes previously unattainable. When the

Figure 7. (a) Photograph of the large-scale preparation of Cu2O nanocubes with average edge length of 23 nm in a 600 mL beaker. (b, c) SEM images of the synthesized Cu2O nanocubes.

reagent amounts used to make 34 nm Cu2O octahedra were also increased by 50-folds, highly uniform 34 nm octahedra were produced as seen in Figure S15. Each preparation yields unprecedented 60 mg of ultrasmall Cu2O octahedra. These particles with high surface areas are perfect for catalytic reactions, and surface SDS should be easily removed with some washing steps.24 It has been known that systematic shape evolution of plasmonic metal and Cu2O crystals formed by redox reactions can be achieved by tuning the cell potential (E) through variation in the amount of reducing agent added.33,37 This works because the reaction quotient (Q) in the Nernst equation has been changed. Recently, it has been demonstrated that adjustment in the amount of HAuCl4 used also enables the growth of Au nanocrystals with systematic shape evolution because Q is similarly tuned.38 If this understanding of particle shape control is general and applicable to metal oxide crystals prepared via redox chemistry, then it should be possible to vary the volumes of CuSO4 solution, and thus the reaction quotient, to yield different particle morphologies. Indeed, when 75, 100, 150, and 175 μL of 0.2 M CuSO4 solution were introduced, Cu2O cubes, {100}-truncated octahedra (types I and II), and 10472

DOI: 10.1021/acssuschemeng.9b00844 ACS Sustainable Chem. Eng. 2019, 7, 10467−10476

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for octahedra. Also note that the Cu2O cubes seen in Figure 3 are far more uniform in size than cubes obtained here without adding NaOH. Because cubes in Figure 3 are formed at significant faster rates even for the largest cubes (Figure S10, 15 s for the 200 nm cubes) than cubes prepared here (Figure S18, 30 s for cubes), this should produce particles with more uniform sizes due to a shorter crystal formation time. Still all particle shapes are produced at much slower rates than those seen in the preparation of ultrasmall Cu2O cubes by varying the sodium ascorbate volume (Figure S8). The key difference is the addition of NaOH. Formation of Cu(OH)2 and Cu(OH)42− possibly facilitate their reduction to yield Cu2O, and this inhibits particle shape changes. This experiment also shows that NaOH is not always necessary in the synthesis of Cu2O polyhedra. Previously it has been quite difficult to make small copper polyhedra with sizes less than 70 nm that are free of a hydrophobic medium and chemicals like hexadecylamine.30,40 High-temperature heating is normally needed. Our recent discovery to use ammonia borane to pseudomorphically convert large Cu2O cubes, octahedra, and rhombic dodecahedra to Cu crystals of the corresponding shapes has opened up an opportunity to make ultrasmall Cu polyhedra directly using these tiny Cu2O cubes and octahedra.28,29 Simply adding ammonia borane to an ethanol solution of Cu2O nanocubes (36 and 63 nm selected) and octahedra (34 and 41 nm used), Cu nanocubes and octahedra of the same sizes and shapes have been obtained within 3 min at room temperature (see Figure 9). Figure 10 confirms the complete conversion of 63 nm Cu2O cubes to Cu crystals. TEM characterization on the Cu cubes shows a hollowing structure for some particles, but the cubes are largely single-crystalline (Figure S19). Cu (111) lattice planes were also measured. TEM analysis on the ultrasmall Cu octahedra establishes their octahedral particle shape. Again the particles can have somewhat hollow interior (Figure S20). UV−vis absorption spectra show a surface plasmon resonance (SPR) absorption band at 620 nm for the 34 nm Cu octahedra (Figure S21). The SPR band red-shifts to 670 and 760 nm for the 36 and 63 nm Cu cubes, respectively. The significant spectral red-shift may result from some extent of cube aggregation. Nevertheless, this approach offers a simple solution to prepare ultrasmall copper cubes and octahedra for

octahedra were synthesized, respectively (see Figure 8). Figure S16 offers the exact reagent amounts used. The two types of

Figure 8. SEM images of the synthesized Cu2O (a) cubes, (b) {100}truncated octahedra (type I), (c) {100}-truncated octahedra (type II), and (d) octahedra by adjusting the CuSO4 volume used.

{100}-truncated octahedra differ in the degree of {100} face truncation. Type II octahedra have smaller truncations and are closer to perfect octahedra. The synthesized Cu2O cubes, truncated octahedra (I), truncated octahedra (II), and octahedra have average sizes of 221, 315, 475, and 525 nm, respectively. The results demonstrate that tuning metal source concentration is also effective at particle shape evolution. Incidentally, variation in the concentration of CuCl2 has recently been shown to yield different Cu2O particle shapes in the presence of NaBH4.39 Please note that the ultrasmall Cu2O cubes and octahedra were synthesized by mainly changing the CuSO4 volume. Figure S17 gives XRD patterns of the synthesized Cu2O cubes, {100}-truncated octahedra, and octahedra. A noticeable transition in the relative intensities of the (111) and (200) peaks can be observed with morphology change. Photographs showing the solution color changes in the preparation of Cu2O crystals with tunable shapes are provided in Figure S18. Within 15 s, all solutions other than that for the growth of cubes have turned slightly yellow. After 30 s of reaction, the cubes also gave a lightly yellow hue. Thus, cubes were formed at a slower rate than that

Figure 9. (a−c) SEM images of (a) 36 and (b) 63 nm Cu2O cubes and (c) 34 nm octahedra. (d−f) SEM images of the pseudomorphically converted (d, e) Cu cubes and (f) octahedra. 10473

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(NaOH, 98%, Sigma-Aldrich), ammonia borane (H3N-BH3, 97%, Sigma-Aldrich), and anhydrous ethanol (99.5%) were used asreceived without further purification. Ultrapure distilled and deionized water (18.3 MΩ) was used for all solution preparation. Synthesis of Ultrasmall to Small Cu2O Nanocubes by Tuning the Sodium Ascorbate Amount. For the synthesis of ultrasmall to small Cu2O nanocubes, 8.160 to 9.610 mL of deionized water were introduced into sample vials containing 0.0576 g (or 0.015 M final concentration) of SDS. After stirring for 5 min to dissolve SDS, 100 μL of 0.1 M CuSO4 was added to each vial. Next, 40 μL of 1.0 M NaOH solution was introduced into each vial with constant stirring. Within a few seconds the resultant solution became light bluish indicating the formation of Cu(OH)2 precipitate. Finally, 1.7, 1.45, 1.0, 0.8, 0.5, 0.35, 0.3, and 0.25 mL of 0.2 M sodium ascorbate solution was added into each sample vial to make Cu2O nanocubes with average edge lengths of 16, 25, 29, 36, 51, 63, 72, and 86 nm, respectively. The total solution volume in each vial is 10 mL. After stirring for 5 min and undisturbed aging for 10 min for crystal growth, the final solution color turned light yellow to bright yellow as the nanocube size increases. The particles were collected by centrifugation at 12 000 rpm for 10 min for 16, 25, 29, and 36 nm cubes and 9500 rpm for 10 min for 51, 63, 72, and 86 nm cubes. After decanting upper solution, the precipitate was washed three times with 10 mL of 1:1 volume ratio of ethanol and water. The final washing step used 10 mL of ethanol, and the precipitate was dispersed in 1 mL of ethanol for storage and analysis. Synthesis of Size-Tunable Cu2O Cubes by Varying the NaOH Amount. To make this series of Cu2O nanocubes with sizes ranging from 27 to 200 nm, 8.03−8.15 mL of deionized water were introduced into sample vials containing 0.0576 g of SDS. After stirring for 5 min to dissolve SDS, 100 μL of 0.1 M CuSO4 solution was added into each vial. Next, 50, 60, 70, 80, 90, 100, 120, 150, and 170 μL of 1.0 M NaOH were respectively added for the growth of 27, 36, 52, 80, 103, 129, 142, 181, and 200 nm Cu2O cubes, followed by the addition of 1.7 mL of 0.2 M sodium ascorbate solution with stirring for 5 min and aging for 10 min. The particles were collected by centrifugation at 12 000 rpm for 10 min for 27, 36, and 51 nm cubes, 9500 rpm for 10 min for 80, 103, and 129 nm cubes, and 7000 rpm for 10 min for 142, 181, and 200 nm cubes. The washing procedure is same as above-mentioned. Final precipitate was dispersed in 1 mL of ethanol for storage and analysis. Synthesis of Ultrasmall Cu2O Octahedra with Tunable Sizes. To make ultrasmall Cu2O octahedra, 8.06−8.86 mL of deionized water were added to each sample vial containing 0.0576 g of SDS. Next, 200 μL of 0.1 M CuSO4 solution was introduced, followed by the addition of 40 μL of 1.0 M NaOH solution. Then 1.7, 1.4, and 0.9 mL of 0.2 M sodium ascorbate solution were added respectively with stirring for 5 min to make Cu2O octahedra with average opposite corner distances of 34, 41, and 49 nm. The total solution volume in each vial is 10 mL. The samples were aged for 55 min for crystal growth. The solution color turned from light orange to dark orange as octahedra increase in size. The particles were collected by centrifugation at 12 000 rpm for 10 min. The washing process is same as that for the cubes. Final precipitate was dispersed in 1 mL of ethanol for storage and analysis. Cu2O Shape Evolution from Cubic to Octahedral Structures by Varying the CuSO4 Amount. For the syntheses of Cu2O crystals with various morphologies from cubes to octahedra, 9.675−9.725 mL of deionized water were added to four vials containing 0.0576 g of SDS. After constant stirring, 75, 100, 150, and 175 μL of 0.1 M CuSO4 solution was added to the vials for the growth of Cu2O cubes, {100}-truncated octahedra (type I), {100}-truncated octahedra (type II), and octahedra, respectively. Finally, 125 μL of 0.2 M sodium ascorbate was introduced to each vial and stirred for 5 min. The total solution volume is 10 mL. The solutions were aged for 15 min, but 40 min for the growth of octahedra. The particles were collected by centrifugation at 6500 rpm for 5 min. The washing process is same as described above. The final precipitate was dispersed in 1 mL of ethanol for storage and analysis.

Figure 10. XRD patterns of the 63 nm Cu2O cubes and the converted Cu cubes.

their various applications, and this again illustrates the value to pursue the synthesis of ultrasmall Cu2O polyhedral crystals.

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CONCLUSIONS It has been a great challenge for many years to prepare a large quantity of ultrasmall Cu2O polyhedra for their facetdependent optical and catalytic investigations. Here methods to grow ultrasmall Cu2O nanocubes and octahedra with tunable sizes and high yields have been developed. Adding to an aqueous SDS solution a mixture of CuSO4, NaOH, and varying amounts of sodium ascorbate enables the preparation of Cu2O cubes with edge lengths of 16, 25, 29, 36, 51, 63, 72, and 86 nm in just 10 min. Varying NaOH volume can produce another series of cubes with tunable sizes in the range of 27− 200 nm. Doubling the amount of CuSO4 used and adjusting sodium ascorbate volume introduced allows the formation of exceptionally small Cu2O octahedra with opposite corner distances of only 34, 41, and 49 nm. Both the syntheses of small Cu2O cubes and octahedra can be easily increased 50fold to 500 mL in one reaction for large scale production. Facet-dependent light absorption and emission properties have been established using these uniform cubes and octahedra, and a modified band diagram of Cu 2 O was constructed incorporating size and facet components. We have further shown that variation in the amount of CuSO4 solution added enables particle shape evolution. This is a novel concept demonstrated on the understanding that particle shape control is governed by cell potential. Finally, the small Cu2O cubes and octahedra were pseudomorphically converted to Cu nanocrystals of the corresponding shapes, providing a facile approach to make copper cubes and octahedra over a wide range of sizes.

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EXPERIMENTAL SECTION

Chemicals. Copper(II) sulfate pentahydrate (CuSO4·5H2O,99%, Riedel-de Haën), L(+)-sodium ascorbate (SA, 98%, Sigma-Aldrich), sodium dodecyl sulfate (SDS, 99%, J.T. Baker), sodium hydroxide 10474

DOI: 10.1021/acssuschemeng.9b00844 ACS Sustainable Chem. Eng. 2019, 7, 10467−10476

Research Article

ACS Sustainable Chemistry & Engineering Pseudomorphic Conversion of Ultrasmall Cu2O Cubes and Octahedra to the Corresponding Cu Polyhedra. Initially 8.5 mL of ethanol was added to a vial, and 1 mL of 2 mg/mL Cu2O nanocrystals in ethanol was added and magnetically stirred. Then 0.5 mL of 0.2 M ammonia borane in ethanol was introduced. Within 3 min the solution color turned reddish brown indicating the formation of Cu crystals. The particles were collected by centrifugation at 12 000 rpm for 3 min. After decanting upper solution, the precipitate was washed three times with 10 mL of ethanol. The Cu polyhedra were dispersed in 1 mL of ethanol for characterization. Instrumentation. SEM images of the nanocrystals were obtained using a JEOL JSM-7000F electron microscope. XRD patterns were recorded on a Shimadzu XRD-6000 diffractometer with Cu Kα radiation. TEM characterization was performed on a JEOL JEM3000F microscope with an operating voltage of 300 kV. UV−vis absorption spectra were taken using a JASCO V-670 spectrophotometer. XPS spectra were collected using an ULVAC-PHI Quantera SXM spectrometer. Photoluminescence spectra were obtained with the use of a HORIBA Jobin Yvon Fluorolog-3 spectrofluorometer.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.9b00844. Synthesis conditions for Cu2O crystals, particle size distribution histograms, XRD patterns, XPS spectra, photographs of the crystal solutions, TEM characterization of Cu2O octahedra and Cu nanocubes and octahedra, and UV−vis spectra of polyhedral Cu nanoparticles (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Michael H. Huang: 0000-0002-5648-4345 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the Ministry of Science and Technology and the Ministry of Education of Taiwan under the Higher Education Sprout Project for the support of this research (MOST 1042119-M-007-013-MY3 and 107-3017-F-007-002).

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DOI: 10.1021/acssuschemeng.9b00844 ACS Sustainable Chem. Eng. 2019, 7, 10467−10476

Research Article

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DOI: 10.1021/acssuschemeng.9b00844 ACS Sustainable Chem. Eng. 2019, 7, 10467−10476