Modified Semiconductor Band Diagrams Constructed from Optical

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C: Plasmonics; Optical, Magnetic, and Hybrid Materials

Modified Semiconductor Band Diagrams Constructed from Optical Characterization of Size-Tunable CuO Cubes, Octahedra, and Rhombic Dodecahedra 2

Jing-Yi Huang, Mahesh Madasu, and Michael H. Huang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b02169 • Publication Date (Web): 04 Jun 2018 Downloaded from http://pubs.acs.org on June 4, 2018

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The Journal of Physical Chemistry

Modified Semiconductor Band Diagrams Constructed from Optical Characterization of Size-Tunable Cu2O Cubes, Octahedra, and Rhombic Dodecahedra Jing-Yi Huang, Mahesh Madasu, and Michael H. Huang* Department of Chemistry and Frontier Research Center on Fundamental and Applied Sciences of Matters, National Tsing Hua University, Hsinchu 30013, Taiwan

ABSTRACT: By making Cu2O nanocubes, octahedra, and rhombic dodecahedra with tunable sizes and recording their light absorption and emission spectra, their absorption and emission bands shift steadily to longer wavelengths with increasing particle sizes from 10 nm to beyond 250 nm. smallest nanocubes. range.

Emission intensities are highest for the

Photoluminescence band shifts exceed 130 nm over this size

For particles having the same volume, rhombic dodecahedra absorb light of

shortest wavelength, while cubes show most red-shifted absorption with their band gaps differ by 0.17 eV (or 51.5 nm).

They show obviously different colors.

The

presence of optical size and facet effects in semiconductors means their emission wavelengths are tunable through facet control and use of nanocrystals much larger than quantum dots.

A modified and general band diagram for Cu2O crystals has

been constructed incorporating their optical size and facet effects with surface band bending.

In addition, a more complete understanding of the different orders of 1

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surface band bending for the {100}, {111}, and {110} facets used in explaining the facet-dependent photocatalytic activity, electrical conductivity, and light absorption properties of Cu2O crystals is presented.

INTRODUCTION Recent studies on semiconductor nanocrystals with well-defined shapes and thus exposed facets have revealed their strongly facet-dependent photocatalytic activity,1‒10 electrical conductivity,11‒15 light absorption,9,16‒20 and possibly heat transmission properties.17,18

These facet-dependent phenomena can be explained in terms of the

presence of an ultrathin surface layer having varying degrees of band bending and band structures for different lattice planes.

Among these properties, the statement

that light absorption property is facet-dependent seems most difficult to demonstrate and hard to accept, but this actually results from our ignorance to observe and think about the obvious question of the displayed colors of synthesized semiconductor nanocrystals with sizes well above the quantum dot regime.

For semiconductor

nanostructures of the same composition and crystalline phase but exposing different facets due to their varied morphologies, as well as particles of the same shape but have tunable sizes, they actually exhibit slight to easily recognizable color changes and hence give shifted absorption bands.21 Previously we have shown that 2


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ultrasmall to small Cu2O nanocubes consistently show a more red-shifted absorption band than octahedra of similar sizes by about 15 nm.16

These results reveal that

semiconductor band gap should have size and facet components.

This important

insight warrants further investigation by making {110}-bound Cu2O rhombic dodecahedra of tunable sizes for a more complete comparison of size and facet effects on semiconductor light absorption.

The fact band gap for semiconductor

nanocrystals is tunable by adjusting their sizes and exposed facets implies that their light emission band should behave the same way.

Emission spectra of Cu2O films,

wires, and spheres are known, but are not available for Cu2O polyhedra.22‒24

It is

therefore possible to capitalize on these discoveries to make sufficiently bright semiconductor nanocrystals to increase the photoluminescence range, so it is not always necessary to rely on quantum dots for tunable emission applications. Actually examples of facet-dependent light absorption and emission properties can be found in various materials, but quantum confinement effects are frequently used to explain these observations.5,25‒28 We think that even quantum nanostructures have facet effects using an argument presented in this work. Here we have synthesized small Cu2O nanocubes, octahedra, and rhombic dodecahedra with tunable sizes and recorded their absorption or reflectance spectra to show all samples exhibit size- and facet-dependent light absorption properties. 3


Their


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photoluminescence spectra indicate emission wavelengths are also tunable and correlated with their absorption wavelengths.

For visual comparison of optical facet

effects, Cu2O nanocubes, octahedra, and rhombic dodecahedra having the same particle volume were prepared and their band gaps determined.

A modified band

diagram of Cu2O with consideration of size and facet effects has been constructed. Finally, we also explain why the relative degrees of conduction band bending drawn for electrical conductivity, photocatalytic activity, and light absorption can be different for the {100}, {111}, and {110} surfaces of Cu2O.

EXPERIMENTAL METHODS Chemical.

Anhydrous copper (II) chloride (CuCl2, 97%, Sigma-Aldrich),

copper(II) sulfate pentahydrate (CuSO4·5H2O, 99%, Riedel-de Haën), copper(II) nitrate hemi(pentahydrate) (Cu(NO3)2·2.5H2O, 99%, Sigma-Aldrich), sodium hydroxide (NaOH, 98%, Aldrich), L-(+)-ascorbic acid (AA, 99.7%, Sigma-Aldrich), hydrazine hydrate (N2H4·H2O, 99.8%, Alfa Aesar), and hydroxylamine hydrochloride (NH2OH·HCl, 99%, Alfa Aesar) were used without purification.

Ultrapure distilled

and deionized water (18.2 MΩ) was used for all solution preparations. Synthesis of Small Cu2O Nanocubes with Tunable Sizes.

For the synthesis of

Cu2O nanocubes with average edge lengths of 10, 18, 27, 42, 53, 66, and 87 nm, 4


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9.050, 9.000, 8.945, 8.850, 8.815, 8.775, and 8.700 mL of deionized water were Subsequently 100 μL of 0.1 M CuSO4

respectively introduced into sample vials.

solution was added to each vial. The vials were kept in a 35 ºC water bath throughout particle synthesis. Next, 350, 400, 455, 550, 585, 625, and 700 μL of 1.0 M NaOH solution were introduced into respective sample vials with vigorous stirring. The resulting solution turned light blue, indicating the formation of Cu(OH)2 precipitate. Upon the addition of 500 μL of 0.2 M ascorbic acid solution, the solution turned bright yellow immediately. 10 mL.

The total solution volume in each vial is

The solution was stirred for 10 min for crystal growth in the water bath and

centrifuged at 8500 rpm for 10 min.

After the top solution was decanted, the

precipitate was centrifuged and washed three more times to remove unreacted chemical with 40 mL of 1:1 volume ratio of water and ethanol.

The final washing

step used 30 mL of ethanol, and the precipitate was dispersed in 1 mL of 99.8 % ethanol for storage and analysis. Synthesis of Small Cu2O Octahedra with Tunable Sizes.

For the synthesis of

Cu2O octahedra with average opposite corner distances of 85, 117, and 161 nm, 9.28, 9.23, and 9.18 mL of deionized water were respectively introduced into sample vials. Subsequently 100 μL of 0.1 M Cu(NO3)2 solution was added to each vial and stirred for 1 min.

Next, 20 μL of 1.0 M NaOH solution was introduced into each sample 5



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vial with vigorous stirring. The resulting solution turned light blue, indicating the formation of Cu(OH)2 precipitate. Upon the addition of 600, 650, and 700 μL of 0.2 M N2H4 solution, the solution turned yellow-orange in 10 min. were collected by centrifugation at 8500 rpm for 10 min.

Finally, the particles

After the top solution was

decanted, the precipitate was centrifuged three times with 40 mL of 1:1 volume ratio of water and ethanol.

The final washing step used 30 mL of ethanol, and the

precipitate was dispersed in 1 mL of 99.8 % ethanol for storage and analysis. Synthesis of Cu2O Rhombic Dodecahedra with Tunable Sizes.

To synthesize

150 nm Cu2O rhombic dodecahedra, 8.3 mL of deionized water was added to a sample vial. After adding 0.087 g of SDS powder with sonication to dissolve the powder, 0.5 mL of 0.1 M CuCl2 solution was injected under vigorous stirring.

The

vial was placed a water bath set at 31ºC. Next, 0.2 mL of 1.0 M NaOH solution was added, and the solution became light blue immediately, indicating the formation of Cu(OH)2. vial.

After 5 s, 1 mL of 0.2 M NH2OH·HCl solution was quickly added to the

The total volume is 10 mL. After stirring for 5 s, the vial remained in the

water bath for 50 min. The solution color changed from green to yellow and then orange immediately after NH2OH·HCl solution was added.

The solution was

centrifuged at 5000 rpm for 3 min, then 40 mL of water and ethanol in 1:1 volume ratio was used to wash the particles twice and finally preserved them in 1 mL of 99.8 6


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% ethanol for storage and analysis. To synthesize 248 and 435 nm Cu2O rhombic dodecahedra, 6.92 mL of deionized water was added to sample vials.

After adding 0.087 g of SDS powder with

sonication to dissolve SDS, 0.5 mL of 0.1 M CuCl2 solution was injected under vigorous stirring. The vials were placed in a water bath set at 31 ºC.

Next, 0.18

mL of 1.0 M NaOH solution was added and stirred for 5 and 7 sec respectively for making 248 and 435 nm rhombic dodecahedral particles. particle size control.

Precise time is needed for

Finally, 2.4 mL of 0.1 M NH2OH·HCl solution was injected at

the exact moment and stirred for 5 s.

The vials stayed in the water bath for 50 min.

The solution color changed from green to yellow and then orange immediately after NH2OH·HCl solution was added. After the reaction, the particles were washed as depicted above. Synthesis of 220 nm Cu2O Cubes and 400 nm Cu2O Octahedra.

To synthesize

220 nm nanocubes, 8.92 mL of deionized water was added to a sample vial.

After

adding 0.087 g of SDS powder with sonication to dissolve SDS, 0.5 mL of 0.1 M CuCl2 solution was injected under vigorous stirring. The vial was placed in a water bath set at 31 ºC.

Next, 0.18 mL of 1.0 M NaOH solution was added and stirred for

5 s. Finally, 0.8 mL of 0.1 M NH2OH·HCl solution was introduced and stirred for 20 s.

The vial stayed in the water bath for 1 h.

The solution was centrifuged at

7



5000 rpm for 3 min.

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Then the particles were washed with 40 mL of 1:1 volume ratio

of water and ethanol twice and preserved them in 1 mL of 99.8 % ethanol for storage and analysis. For the synthesis of 400 nm Cu2O octahedra, 9.05 mL of deionized water and 0.087 g of SDS powder were added to a vial.

After sonication to dissolve SDS

powder, 0.1 mL of 0.1 M CuCl2 solution was injected into the vial under vigorous stirring.

Next, 0.2 mL of 1.0 M NaOH was added and stirred for 5 s.

Finally, 0.65

mL of 0.2 M NH2OH·HCl solution was quickly added and stirred for 5 s. sample vial was left at room temperature for 30 min for octahedra growth.

The After the

reaction, the particles were washed as depicted above. Instrumentation.

SEM images of samples were obtained using a JEOL

JSM-700F electron microscope. TEM characterization was performed on a JEOL JEM-2100 electron microscope operating at 200 kV.

UV‒vis absorption spectra

were obtained using a JASCO V-670 spectrophotometer.

Diffuse reflectance spectra

were taken on the same spectrophotometer equipped with an integrating sphere. Photoluminescence spectra were acquired with the use of a HORIBA Jobin Yvon Fluorolog-3 spectrofluorometer.

XRD patterns were recorded on a Shimadzu

XRD-6000 diffractometer with Cu Kα radiation.

8


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RESULTS AND DISCUSSION To study the optical size and facet effects, the synthesized Cu2O nanocrystals should preferably have ultrasmall to small dimensions to avoid light scattering interference associated with large particles over 200 nm. adopt our reported methods.16,29

The synthetic procedures used here

By preparing an aqueous mixture of CuSO4, NaOH,

and ascorbic acid, small Cu2O nanocubes with average edge lengths of 10, 18, 27, 42, 53, 66, and 87 nm can be synthesized by simply adjusting the volume of 1.0 M NaOH solution introduced.

Of course, formation of some larger nanocrystals is

unavoidable, but these can be removed through centrifugation and collection of the upper solution containing the smaller nanoparticles.

This also means that X-ray

diffraction (XRD) patterns of the ultrasmall Cu2O nanocubes are more difficult to obtain due to lack of a sufficent particle amount.

Figure 1 presents scanning electron

microscopy (SEM) images of the synthesized Cu2O nanocubes with tunable sizes. Clearly cubic nanocrystals have been made with high size uniformity.

Figure S1 in

the Supporting Information gives the size distribution histograms of these nanocube samples. Transmission electron microscopy (TEM) images of 10 and 18 nm cubes were also taken, showing a d-spacing value matching with that of Cu2O (110) lattice planes.

Large-area SEM image of the 10 nm Cu2O cubes is available in Figure S2.

XRD patterns of the ultrasmall Cu2O nanocubes and octahedra prepared using the 9



same methods have been reported showing only reflection peaks from Cu2O.16 Nevertheless, Figure S2 also provides an XRD pattern of the synthesized 10 nm Cu2O cubes.

With these homogeneous nanocubes, their UV‒vis absorption spectra were

taken (see Figure 2a).

The nanocubes show a progressively red-shifted absorption

band centered at 457, 459, 465, 473, 482, 490, and 510 nm with increasing sizes. These absorption band positions match quite well with those of similar-sized Cu2O nanocubes reported previously.16

Since the absorption band is shifting continuously

with increasing particle size, the results indicate that optical band gap has size component not recognized before.

Remember that all these particles are quite large

even for the 10 nm nanocubes considering the Bohr exciton radius of Cu2O at just 1.4 nm.30

This means that spectral shifts of Cu2O do not end soon after the quantum

confinment distance, but continue to be observable in particles tens and even hunderds of nanometers in size. correct.

The notion of quantum confinement is not quite

Since light absorption shows steady shifts with particle size, we expect their

emission band to behave the same way. Figure 2b and 2c show photoluminescence spectra of these Cu2O nanocubes excited at 380 nm.

Remarkably, their emission

band also red-shifts continuously with increasing nanocube sizes from 459 nm to 492, 499, 508, 518, 525, and 532 nm.

The emission band for the 10 nm cubes is not

much shifted from their absorption band position presumably because different 10


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samples were used for the spectral characterization, and the band gap of the smallest particles is most sensititive to tiny size variation as their dimensions approach the quantum confiment regime.

Emission band width is large due to certain speard of

particle sizes in each sample. However, emission intensity drops significantly as particle size increases (see Figure 2d).

This makes sense because excited electrons

and holes are more likely to find each other in a smaller particle.

Clearly emission

band tunability is possible in much larger Cu2O nanocrystals (73 nm emission band shift from 10 nm to 87 nm cubes).

If smaller Cu2O cubes can be made (say 5 nm),

the range of emission wavelegnth tunability can be even wider.

The drastic decrease

in the emission intensity with increase in particle size shows why quantum dot emission should be brightest.

However, much larger Cu2O cubes with sizes below

30 nm may still have sufficiently high photoluminescence intensities.

The idea is

that much larger semiconductor particles beyond quantum dot dimensions may still be useful for light emission applications.

It is also important to note that the magnitude

of emission band shift is largest for the smallest nanocubes in the range of 10‒18 nm. As particle size increases, emission shifts become more modest but not ceased. For optical facet effect observation, small-sized Cu2O octahedra were also prepared.

Cu2O octahedra were synthesized from an aqueous mixture of Cu(NO3)2,

NaOH, and N2H4.

By adjusting the volume of 0.2 M N2H4 solution added, Cu2O 11



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octahedra with average opposite corner distances of 85, 117, and 161 nm were obtained.

Figures S3 and S4 present SEM images and size distribution histograms of

these Cu2O octahedra, showing good shape and size uniformity.

UV‒vis absorption

spectra of these Cu2O octahedra give an absorption band at 460, 476, and 500 nm with increasing particle sizes (Figure 3a).

Again these positions match with those of

similar-sized Cu2O octahedra reported before.16 Their emission band wavelength red-shifts from 464 nm to 508 and 524 nm with increasing particle size (Figure 3b). For fair comparison, the absorption and emission band positions of Cu2O nanocubes and octahedra are plotted against their volumes to reveal the presence of optical facet effects (Figure 3c). Although both absorption and emission band positions are close to each other for both nanocubes and octahedra of smallest sizes, strangely the difference widens to 22‒36 nm for larger particles including the 18 nm cubes.

For

nanocubes and octahedra of similar volumes, their absorption band difference is consistently 10‒17 nm.

This demonstrates the presence of optical facet effect.

Using the limited octahedra sizes, one can still see that the emission band positions of nanocubes and octahedra also differ by similar amount (see the 117 and 161 nm octahedra samples). So emission properties also have a facet component. To complete the optical analysis, Cu2O rhombic dodecahedra with average face distances of 150, 248, and 435 nm were also prepared through modification of our 12


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reported procedure by making an aqueous mixture of sodium dodecyl sulfate (SDS), CuCl2, NaOH, and NH2OH·HCl (see Scheme S1 for the reagent amounts used).29 Smaller rhombic dodecahedra have not been synthesized, so they have much larger particle volumes than the cubes and octahedra.

Figures 4 and S5 offer SEM images

of the synthesized Cu2O rhombic dodecahedra and their size distribution histograms, showing high shape and size homogeneity. Diffuse reflectance spectra were collected on these particles for the determination of their band gap energies (Figure 4). The determined band gap values are 2.23 (150 nm), 2.11 (248 nm), and 2.06 eV (435 nm).

As expected, band gap diminishes gradually with increasing particle size even

for such large Cu2O rhombic dodecahedra.

Photoluminescence spectra of the 150,

248, and 435 nm rhombic dodecahedra give an emission band centered at 561, 591, and 604 nm, respectively (Figure 3d).

Tunable emission wavelength is still

observable for such large semiconductor crystals, but further emission shift is quite small for particles bigger than 250 nm.

Converting the particle sizes to their

volumes and band gap values to absorption wavelengths, Figure 3e shows absorption and emission wavelengths are very close for rhombic dodecahedra.

Comapring the

emission intensities of all Cu2O crystals, it is clear that emission intensity drops continuously with increasing particle size regardless of particle shape (Figure S6). Because all Cu2O rhombic dodecahedra are significantly larger than the biggest 13



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nanocubes and octahedra, it is highly desirable to make Cu2O cubes and octahedra having the same volume as rhombic dodecahedra for best confirmation of the optical facet effects present in semiconductor crystals.

We calculated the sizes of Cu2O

cubes and octahedra needed to have the same volume as that of 248 nm rhombic dodecahedra, and tuned the reaction conditions to make cubes with an edge length of 220 nm and octahedra with an opposite corner distance of 400 nm (see Figure S7 for the calculations).

SEM images of the 220 nm Cu2O cubes and 400 nm octahedra and

their size distribution histograms are shown in Figures S8 and S9, again revealing excellent size and shape monodispersity. XRD patterns of these particles and the 248 nm rhombic dodecahedra are available in Figure S10 confirming their Cu2O composition.

These Cu2O cubes, octahedra, and rhombic dodecahedra having the

same particle volume actually display obviously different colors. visual demonstration of semiconductor optical facet effect.

This is powerful

Because they have the

same size in terms of their volume, the color variation must come from their exposed facets as this is the only difference among these crystals.

Figure S8 also offer diffuse

reflectance spectra for the determination of band gap energies of 220 nm Cu2O cubes (1.94 eV) and 400 nm octahedra (2.0 eV).

The 248 nm Cu2O rhombic dodecahedra

have a band gap of 2.11 eV. That corresponds to a separation of 51.5 nm in absorption band positions between rhombic dodecahedra and cubes. 14


It is important

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to recognize that these Cu2O crystals were synthesized using the same chemicals and surfactant, so the possibility of adsorbate-induced band bending causing the facet-dependent spectral shifts is not valid here.31,32 Previously, infrared spectra taken on Cu2O cubes, octahedra, and rhombic dodecahedra prepared using the same synthetic method have revealed clean crystal surfaces without the presence of SDS surfactant after a couple cycles of washing,33 and they gave identical X-ray photoelectron spectra (XPS) without sulfur peaks from SDS.3,34 Consistent with previous observations on Au‒Cu2O core‒shell nanocrystals, Cu2O rhombic dodecahedra bound by the {110} faces absorb light of shortest wavelengths, while cubes bound by the {100} facets absorb light of longest wavelengths.

Octahedra

exposing the {111} surfaces have an intermediate absorption position.

The gold

plasmonic band positions in Au‒Cu2O core‒shell rhombic dodecahedra and cubes differ by 47 nm.35 To explain the observed facet-dependent optical properties of Cu2O, a modified band diagram is constructed and presented in Figure 5.

Keeping the valence band

energy fixed, the nanocrystals with tunable sizes must absorb light of different wavelengths as they have varying band gap values.

This is the optical size effect

with larger nanocrystals absorbing light of longer wavelengths extending to crystals of hundreds of nanometers in size.

On top of this of size effect, there is optical facet 15



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effect to explain the varying band gap values for Cu2O crystals having the same volume.

This means the thin surface layer of about 1 nm or less, determined from

density of states calculations, must have different degrees of band bending.11

Cubes

have the smallest band gap, so their band bending is to the least degree in this surface layer.

Octahedra should have a larger degree of upward band bending than cubes

since they have more blue-shifted light absorption.

The {110}-bound rhombic

dodecahedra have the largest band bending because they have the largest band gap and so absorb light of shortest wavelengths. This is the general case for most nanocrystals.

For very large crystals (for example, 435 nm rhombic dodecahedra),

downward band bending is possible to reflect the fact their band gap value (2.13 eV) is smaller than that of bulk Cu2O at 2.17 eV. Previously band gap bending to varying degrees has been used to explain the observed facet-dependent photocatalytic activity and electrical conductivity properties of Cu2O crystals.1‒3,11

Figure 6 show the relative degrees of band bending proposed

for the {100}, {111}, and {110} facets of Cu2O in these investigations.

Clearly the

order of the conduction band bending for the three facets is different in each case. Although the presented band diagrams explain the respective physical properties, it is puzzling why the surface band bending can be different.

We now understand that

optical properties are fundamentally different from other properties. 16


Consider the

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thin layer as composed of different materials because of varying band structures for the three facets, factors including refractive index affecting light absorption are possible within the layer (think as if the particles are dispersed in different solutions).11 Coating an ultrathin semiconductor film of tunable refractive indices or graphene on a transparent glass, for instance, has only slight effect on its optical transparency, but such film can have significant effects on the conductivity of the glass.

This analysis decouples optical facet effects from other properties involving

charge transport into and out of a crystal through an interface or particle surface, so their relative degrees of facet band bending can be different.

For photocatalysis,

using the {100} facet as an example, Cu2O are photocatalytically inactive because no radicals are produced upon light irradiation, so facet band bending is drawn to represent a very high barrier for photoexcited electrons to move past this surface.1 For electrical conductivity, electrical current flows into the crystal from an external conductor or metal electrode.

The {100} facet of Cu2O is only moderately

conductive, while the {111} facet is highly conductive and the {110} facet is not conductive.11

Although electrons moving out or into the crystal meets the same

{100} face, the metal band energy in the case of electrical conductivity and the bulk conduction band energy for photoexcited electron transport are different, so electrons inside the crystal and those outside see different barrier heights for the same crystal 17



face.

A similar argument has recently been adopted to explain the facet-dependent

electrical conductivity properties of silicon wafers.36,37

In addition, to photocatalysis

the surface layer is part of the crystal, but electrical conductivity generally involves current moving from a metal into a semiconductor.

This means the relative degrees

of facet band bending can also be different for photocatalytic and electrical conductivity processes.

Now a more complete understanding of the various facet

effects exhibited by Cu2O crystals is achieved.

CONCLUSIONS Cu2O nanocubes, octahedra, and rhombic dodecahedra with tunable sizes were synthesized to enable a complete investigation of the existence of size- and facet-dependent light absorption and emission properties in semiconductor nanocrystals for the first time. With increasing nanocrystal size, light absorption band shifts to longer wavelengths continuously from 10 nm particles to ones over 250 nm.

Their photoluminescence spectra also show similar red-shifts with tunability

beyond 130 nm as particle size increases.

For crystals having the same size in terms

of their particle volume, rhombic dodecahedra show most blue-shifted absorption, while cubes absorb light of longest wavelengths. separated by ~ 50 nm.

Their absorption bands can be

This work establishes that semiconductor fluorescence 18


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wavelengths can be greatly tuned through the use of larger nanocrystals beyond quantum dots and their facet control.

A more general band diagram for Cu2O

incorporating optical size and facet effects has been constructed.

Lastly,

explanations have been provided to understand the different orders of surface band bending for the {100}, {111}, and {110} facets used in elucidating the facet-dependent photocatalytic activity, electrical conductivity, and light absorption properties of Cu2O crystals.

ASSOCIATED CONTENT Supporting Information This Supporting Information is available free of charge on the ACS Publications website at DOI: Experimental procedures, size distribution histograms, SEM images of Cu2O nanocrystals, diffuse reflectance spectra, and XRD patterns of Cu2O crystals.

AUTHOR INFORMATION Corresponding Author *[email protected] ORCID 19



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Michael H. Huang: 0000-0002-5648-4345 Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was funded by Ministry of Science and Technology of Taiwan (MOST 104-2119-M-007-013-MY3 and 105-2633-M-007-003).

We thank Chi-Fu Hsia and

An-Ting Lee for assistance in the TEM characterization of ultrasmall Cu2O nanocubes.

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Figure 1.

SEM images of the synthesized ultrasmall to small Cu2O nanocubes with

average edge lengths of (a) 10, (c) 18, (d) 27, (e) 42, (f) 53, (g) 66, and (h) 87 nm. c) TEM images of the 10 nm and 18 nm (inset) Cu2O nanocubes. 27


(b,


Figure 2.

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(a) UV‒vis absorption spectra of the synthesized Cu2O nanocubes with

tunable sizes. The spectra are separated for clarity of presentation. Photoluminescence spectra of these Cu2O nanocubes. was 380 nm.

(b, c)

Excitation wavelength used

(d) PL intensity vs. particle size plot.

28


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Figure 3.

(a) UV‒vis absorption spectra of the synthesized Cu2O octahedra with

tunable sizes.

(b) Photoluminescence spectra of these Cu2O octahedra.

wavelength used was 380 nm.

Excitation

(c) Plot showing the variation in absorption and

emission band wavelengths with respect to Cu2O cube and octahedron sizes expressed in particle volumes.

(d) Photoluminescence spectra of Cu2O rhombic dodecahedra

with sizes of 150, 248, and 435 nm.

Excitation wavelength used was 380 nm. 29


(e)


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Plot of light absorption and emission band wavelengths of Cu2O rhombic dodecahedra with sizes of 150, 248, and 435 nm expressed in particle volumes.

(f)

Photograph showing aqueous solutions of Cu2O rhombic dodecahedra, octahedra, and cubes having the same particle volume but displaying different colors and hence band gaps to illustrate the visually observable optical facet effects.

Figure 4.

SEM, solid-state reflectance spectra, and Tauc plots for the determination

of band gaps of Cu2O rhombic dodecahedra with sizes of (a) 150, (b) 248, and (c) 435 nm.

30


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Figure 5.

Modified general band diagram of Cu2O nanocrystals accounting for their

observed optical size and facet effects. Crystal sizes and their band gap values in eV are indicated. Cubes are shown to the right side of the diagram for better presentation.

Figure 6.

Conduction band bending for different surface planes of Cu2O crystals in

the explanation of their facet-dependent photocatalytic activity, electrical conductivity, and light absorption properties. 31



TOC Graphic

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Modified Semiconductor Band Diagrams Constructed from Optical

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