Scalable Synthesis of Size-Tunable Small Cu2O Nanocubes and

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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 ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.9b00844 • Publication Date (Web): 15 May 2019 Downloaded from http://pubs.acs.org on May 16, 2019

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ACS Sustainable Chemistry & Engineering

Scalable Synthesis of Size-Tunable Small Cu2O Nanocubes and Octahedra for FacetDependent 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 E-mail: [email protected]

ABSTRACT:

Despite the wide spread 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 1 ACS Paragon Plus Environment

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500 mL in one reaction.

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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

INTRODUCTION 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 2 ACS Paragon Plus Environment

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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. 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 seed-mediated

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 3 ACS Paragon Plus Environment


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 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 4 ACS Paragon Plus Environment

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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. is presented.

A modified band diagram of Cu2O incorporating these effects

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 5

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nanocrystals originates from conditions used to make large copper cubes employing In this case, ascorbate (C6H7O6–) should

sodium ascorbate as the reducing agent.30

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 + C6H7O6– → Cu2O + C6H6O6 + 2H2O + OH–

(1)

2Cu(OH)42– + C6H7O6– → Cu2O + C6H6O6 + 2H2O + 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.

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. 6 ACS Paragon Plus Environment

The Cu2O nanocube

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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 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 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 7 ACS Paragon Plus Environment


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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 sec 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 sec 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 sec, all three samples showed light yellow color, yet the largest

63 nm cubes have developed a more intense yellow appearance till the end of the 8 ACS Paragon Plus Environment

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growth 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. provided in Figure S9.

The exact reagent amounts used are

We have found that keeping the amounts of all other reagents

constant, but increasing the NaOH solution volume from 50 to 170 L, Cu2O cube size can be increased progressively. Cu(OH)42– species are produced.

With more NaOH added, greater Cu(OH)2 and

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. particles appear to be highly uniform in size and possess sharp faces.

The

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 Cu2O nanocubes by adding different volumes of NaOH.

The smaller nanocubes started to

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form within 5 sec after adding the reducing agent, while the 200 nm cubes took about 15 sec 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 L for

making ultrasmall cubes to 200 L, and progressively decreasing the volume of sodium ascorbate solution introduced from 1.7 mL 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. S12.

Particle size distribution histograms are available in Figure

For all samples, the octahedra are super-uniform in size and shape.

These are

the smallest Cu2O octahedra ever reported achieving this high level of particle homogeneity. different hues. in Figure S5. cubes.

Figure S13 is a photograph of the three samples, showing slightly XPS spectrum of the 34 nm Cu2O octahedron sample is also included The peak positions are nearly identical to that of the 16 nm Cu2O

TEM characterization has been performed on the 34 and 41 nm octahedra

(Figure S14).

Again the octahedra are highly uniform under TEM observation. 10 ACS Paragon Plus Environment

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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 facetdependent optical properties.

Both UV–vis absorption and emission spectra of these

nanocrystals were taken and are plotted in Figure 5. 380 nm was used to obtain emission spectra.

An excitation wavelength of

With increasing nanocube size from 16

nm to 86 nm, their absorption band shows steady red-shifts from 442 nm 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 why people are focused on quantum nanostructures for emission properties and applications. However, the interest here is to check emission band shifts.

Expectedly, 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. report).34,35

The Bohr exciton radius of Cu2O is 1.4 nm (or 0.7 nm in another

The fact absorption and emission band shifts continuously for Cu2O 11 ACS Paragon Plus Environment


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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.

Emission

band of cubes is also more red-shifted than octahedra of similar sizes by about the same 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, 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 12 ACS Paragon Plus Environment

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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 of 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. homogeneity.

SEM images show very high size

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 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. octahedra.

Each preparation yields unprecedented 60 mg of ultrasmall Cu2O These particles with high surface areas are perfect for catalytic reactions,

and surface SDS should be easily removed with some washing steps.24 13 ACS Paragon Plus Environment


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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, 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 L of 0.2 M CuSO4 solution

were introduced, Cu2O cubes, {100}-truncated octahedra (types I and II), and octahedra were synthesized, respectively (see Figure 8). reagent amounts used.

Figure S16 offers the exact

The two types of {100}-truncated octahedra differ in the

degree of {100} face truncation. closer to perfect octahedra.

Type II octahedra have smaller truncations and are

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 14 ACS Paragon Plus Environment

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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 sec, all solutions other than that for the growth of cubes have turned slightly yellow.

After 30 sec of reaction, the cubes also gave a lightly yellow hue.

Thus, cubes were formed at a slower rate than that 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 sec for the 200 nm cubes) than cubes prepared here (Figure S18, 30 sec 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 addition of NaOH.

The key difference is

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 15 ACS Paragon Plus Environment


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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 singlecrystalline (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-


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shift may result from some extent of cube aggregation.

Nevertheless, this approach

offers a simple solution to prepare ultrasmall copper cubes and octahedra for their various applications, and this again illustrates the value to pursue the synthesis of ultrasmall Cu2O polyhedral crystals.

CONCLUSIONS It has been a great challenge for many years to prepare a large quantity of ultrasmall Cu2O polyhedra for their facet-dependent 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 50-fold 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, 17 ACS Paragon Plus Environment


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and a modified band diagram of Cu2O 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.

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 (NaOH, 98%, Sigma-Aldrich), ammonia borane (H3N-BH3, 97%, Sigma-Aldrich) and anhydrous ethanol (99.5%) were used as received 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, 18

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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 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. volume in each vial is 10 mL.

The total solution

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 12000

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 19 ACS Paragon Plus Environment


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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 12000 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.

were aged for 55 min for crystal growth.

The samples

The solution color turned from light orange

to dark orange as octahedra increase in size. The particles were collected by centrifugation at 12000 rpm for 10 min. cubes.

The washing process is same as that for the

Final precipitate was dispersed in 1 mL of ethanol for storage and analysis.

Cu2O Shape Evolution from Cubic to Octahedral Structures by Varying the 20 ACS Paragon Plus Environment

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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.

for 15 min, but 40 min for the growth of octahedra. centrifugation at 6500 rpm for 5 min. above.

The solutions were aged

The particles were collected by

The washing process is same as described

The final precipitate was dispersed in 1 mL of ethanol for storage and

analysis. 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. particles were collected by centrifugation at 12000 rpm for 3 min.

After decanting

upper solution, the precipitate was washed three times with 10 mL of ethanol. Cu polyhedra were dispersed in 1 mL of ethanol for characterization. 21 ACS Paragon Plus Environment

The

The


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 JEM-3000F 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 Flurolog-3 spectrofluorometer.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: 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.

AUTHOR INFORMATION 22 ACS Paragon Plus Environment

Page 22 of 39

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Corresponding Author *E-mail: *[email protected]. ORCID Michael H. Huang: 0000-0002-5648-4345 Notes The authors declare no competing financial interest.

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 104-2119-M-007-013-MY3 and 107-3017-F-007-002).

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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.



Figure 2.

(a, b) TEM image of a Cu2O nanocubes taken from the 51 nm sample and

its corresponding SAED pattern. a.

(c) HR-TEM image of the red box region in panel

(d) TEM image of Cu2O nanocubes from the 16 nm sample.


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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.



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. magnified view.


Inset of panel c2 gives a

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b

a

c

d

e

f

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 35 ACS Paragon Plus Environment


respect to Cu2O cube and octahedron sizes expressed in particle volumes.

Particle

sizes are only labeled for the absorption curves.

Facet effect {111} {100}

Size effect

Octahedra 2.86 eV 2.83 2.79

Cubes

34 nm 41 nm 49 nm

16 nm 25 nm 29 nm 36 nm 51 nm 63 nm 72 nm 86 nm

2.80 eV 2.77 2.74 2.70 2.61 2.58 2.54

Size effect

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 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 36 of 39

CB

2.50 2.17 eV bulk Cu2O

Light absorption Figure 6.

VB

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.


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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.

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.



Figure 9. octahedra.

(a–c) SEM images of (a) 36 and (b) 63 nm Cu2O cubes and (c) 34 nm (d–f) SEM images of the pseudomorphically converted (d, e) Cu cubes

and (f) octahedra.


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Figure 10.

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

SDS + water + CuSO4 + NaOH + sodium sulfate → small Cu2O

For Table of Contents Use Only Synopsis Scalable quantities of size-tunable Cu2O nanocubes and octahedra can be prepared within 10 min for applications such as ultrasmall Cu polyhedra synthesis.


Scalable Synthesis of Size-Tunable Small Cu2O Nanocubes and

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