Transformation of Cuprous Oxide into Hollow Copper Sulfide Cubes

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Transformation of Cuprous Oxide into Hollow Copper Sulfide Cubes for Photocatalytic Hydrogen Generation Cui Ying Toe, Zhaoke Zheng, Hao Wu, Jason Scott, Rose Amal, and Yun Hau Ng J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b01169 • Publication Date (Web): 13 Apr 2018 Downloaded from http://pubs.acs.org on April 14, 2018

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The Journal of Physical Chemistry

Transformation of Cuprous Oxide into Hollow Copper Sulfide Cubes for Photocatalytic Hydrogen Generation Cui Ying Toe a, Zhaoke Zheng b, Hao Wu a, Jason Scott a, Rose Amal a, Yun Hau Ng a* a

Particles and Catalysis Research Group, School of Chemical Engineering, The University of

New South Wales, NSW 2052, Australia b

State Key Laboratory of Crystal Materials, Shandong University, Jinan 250100, China

1 ACS Paragon Plus Environment

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ABSTRACT. Copper sulfide (CuxS) with a hollow structure derived from the controlled transformation of cuprous oxide (Cu 2O) demonstrated high activity for photocatalytic hydrogen generation with good reproducibility. While the high photocatalytic activity was facilitated by the better crystallinity and lower copper defect density (thus suppressed charge recombination) yielded through the controlled and equilibrated transformation reaction, the stability of the hollow CuxS photocatalyst was attributed to its resistance to sintering induced by the chemical dissolution-redeposition of copper (physicochemical stability). Without the controlled synthesis environment, irregularly-shaped CuxS accompanied by a higher level of copper defects was obtained. During photocatalytic reaction, the irregularly-shaped CuxS was more susceptible to deactivation through an agglomeration process.


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INTRODUCTION Sulfide-based semiconductors have been considered as promising photocatalysts owing to their suitable optical band energies for energetic electron transfer. 1 A variety of sulfide photocatalysts (such as CdS, CuS, and ZnS) have demonstrated activities in the photocatalytic reduction of water and/or CO2. 2-4 Although these nanomaterials revealed excellent photocatalytic and photoelectrochemical (PEC) activities, in a few cases they suffered from photo-instability in aqueous solution (i.e. photocorrosion). The photocorrosion is induced by self-photooxidation, which correlates to the more susceptible valence bandedge energy of sulfide-containing orbitals. The photocorrosion issue can be suppressed by employing a redox electrolyte, such as a mixture of Na2S and Na2SO3,1 as the hole sacrificial agent. For instance, studies have shown that the photocorrosion of cadmium sulfide (CdS), the benchmark sulfide photocatalyst, was greatly suppressed in the presence of Na2S and Na2SO3 solution.5-8

Although the presence of the Na 2S and Na 2SO 3 redox couple has been demonstrated to enhance the photostability of sulfide-based semiconductors, the morphology transformation of metal sulfides during photocatalytic reaction is inevitable (non-photocorrosion). While Na2SO3 captures photoinduced holes for photocorrosion suppression, the instantaneous chemical dissolution-redeposition cycles of copper ions with S 2- from Na 2S often lead to morphology restructuring. 9-12 As a consequence of the dissolution-reprecipitation processes, the physically deformed catalyst (usually possessing a certain level of induced sintering) will lose in total surface area, which is detrimental to the overall performance. Given there is an enhanced photostability that commonly accompanies the simultaneous structural deterioration, morphology engineering is believed to be capable in reducing such a structural deformation.



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Copper sulfide (CuxS) is an attractive self-doped sulfide photocatalyst that has p-type semiconducting properties related to the presence of copper vacancies within its crystal lattice. In the presence of sulfur (e.g. S2-, S2O32- or thiourea) and copper precursors, CuxS can be conveniently obtained. Specifically, the mild conditions inherent in the ion-exchange route make it a feasible CuxS synthesis method. Interestingly, hollow Cu xS structures can be fabricated using Cu2O as the starting template. 13-15 Regardless of the initial Cu2O morphology, hollow CuxS structures with distinct morphologies were attained with the addition of sodium sulfide (Na2S).12,

16

The potential of this well-defined hollow structure in regard to its

physiochemical stability (i.e. suppression of structural deformation) should be further examined to enhance the overall performance of CuxS photocatalysts.

Despite the potential application of CuxS in photocatalysis, interpretations of its band structures and band positions are inconsistent within the literature. While Liang et al. reported the energetic Cu2S conduction band for water reduction reaction; 17 Andronic et al. proposed that CuxS has a more positive conduction band when compared to TiO2, where occurrence of the water reduction reaction is kinetically unfavorable. 18 As semiconductor doping commonly modulates the materials electronic and optical properties, 19 the band structure inconsistency most likely originates from the self-doping ability of CuxS that allows for the formation of innumerable stoichiometries. This complicates the copper-sulfur system, whereby, the development of copper defects within the structure makes it highly disordered. Owing to the complexities involved in a copper-sulfur system, further investigations on this potential material are important to the research community.

Although the discrepancies relating to the CuxS electronic band structure have not be fully addressed, most CuxS-related photocatalytic systems that commonly employ CuxS as either a


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photosensitizer or co-catalyst have shown enhanced photocatalytic performances. For instance, the deposition of copper sulfide quantum dots with an average diameter of 10 nm on BiOBr was shown to improve the photocatalytic hydrogen evolution reaction.20

The

formations of CuxS heterostructures, by coupling with other photocatalysts such as CdS, 21 ZnS, 22 ZnO,23 TiO218, 24-25 and Cu2O14, 26, has also presented enhanced the photoreactivity of the semiconductors. Apart from improving the performance by forming heterojunctions with other semiconductors, selected reports have indicated there is a localized surface plasmon resonance (LSPR) in CuxS nanocrystals where the LSPR signal intensity is related to the level of Cu self-doping in the crystal (and thus the crystallinity of the photocatalyst).

19, 27-29

In this study, cubic Cu2O was used as the copper precursor as well as the initial template to synthesize well-defined hollow cubic CuxS particles. The beneficial role of the synthesis route (i.e. transforming Cu2O into CuxS) was examined by evaluating the photocatalytic and PEC performances of hollow cubic Cu xS, in comparison to irregularly shaped Cu xS. Our understanding is that little effort has been dedicated to investigating the ability of pure CuxS as a photocatalyst (i.e. instead of as a co-catalyst). Thus, in this work, Cu xS was used as a hydrogen evolving photocatalyst and proven to have catalytic ability for the photoreduction of water.



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EXPERIMENTAL SECTION Synthesis of cubic Cu2O Cubic Cu2O was synthesized following a facile solution-phase route adapted from literature.30 Typically 80 mL of ethanol (100% unsaturated, chem-supply) was added into 160 mL of 0.05 M CuSO 4•5H2O (> 99%, UNIVAR) solution under vigorous stirring. The solution was then heated to 60 °C followed by the addition of 40 mL 0.8 M aqueous sodium hydroxide (NaOH, > 98%, chem-supply) solution. After 5 min, 120 mL of 0.107 g/mL Dglucose (anhydrous, chem-supply) solution was introduced as the reducing agent into the heated solution. The brick-red powder was then washed (i.e. with ethanol and water alternately) after 3 h and recovered via centrifugation. Finally, the sample was dried and kept in a vacuum desiccator.

Synthesis of hollow cubic and irregularly-shaped CuxS Cubic Cu2O was used as the copper precursor and starting template to fabricate Cu xS. Hollow cubic CuxS was fabricated via a controlled ion-exchange synthesis route. Firstly, 200 mg of cubic Cu2O was dispersed in 20 mL water followed by continuous purging of N 2 gas. Meanwhile, 20 mL of 0.5 M Na 2S + 0.5 M Na2SO3 solution (0.36 M in excess) was added drop-wise at a constant rate of 0.65 mL/min into the Cu2O suspension under constant stirring and at room temperature. For CuxS with an irregular shape, 20mL of 0.5M Na 2S + 0.5M Na2SO3 solution was mixed directly into the Cu2O suspension without N 2 purging. An immediate color change from bright orange to black was observed, indicating the rapid transformation of Cu2O into CuxS. In both conditions, the solutions were continually stirred at room temperature for at least 30 min to allow complete transformation. Subsequently, the collected powders were washed alternately with water and ethanol and recovered via centrifugation. Lastly, the samples were vacuum-dried at 150 °C and stored in a vacuum 6 ACS Paragon Plus Environment

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desiccator. Here, hollow cubic and irregular shaped Cu xS are named as HC-CuxS and ISCuxS respectively.

Material Characterizations Morphologies of both CuxS samples were examined using transmission electron microscopy (TEM, Philip CM200) and field-emission scanning electron microscopy (FESEM, FEI Nova NanoSEM 450). High-resolution transmission electron microscopic (HRTEM) images were also captured using a Philip CM200 to investigate the lattice spacing of the prepared samples. Crystallinities were analyzed using X-ray diffractometry (XRD, X'pert MPD with Cu Kα radiation at 45 kV and 40 mA) and crystallite sizes were inferred by the Scherrer formula. UV−Vis diffuse reflectance spectra and Vis-NIR absorbance spectra were obtained on a Shimadzu UV 3600 spectrophotometer. The Cu oxidation states on the surface of both samples were evaluated using X-ray photoelectron spectrometry (XPS, Thermo ESCALAB250Xi) probed with a monochromated Al Kα radiation source (hv = 1486.68 eV) in a vacuum chamber. The binding energies of the spectra were calibrated according to the C 1s peak at 284.4 eV. Brunauer-Emmet-Teller (BET) surface areas of both CuxS were assessed using a Micromeritics Tristar 3030 Analyzer at 77 K. Before the measurement, the samples were pre-treated at 150 °C for 3 h in a Micrometric VacPrep unit.

Photocatalytic Hydrogen Generation Generally, 50 mg of CuxS sample was dispersed in 200 mL 0.25 M Na2S + 0.25 M Na 2SO 3 solution in a Y-reactor covered with a quartz window. The system was purged with argon gas (100%, Coregas) for 30 min to de-aerate the solution before illumination. Top illumination was then performed using a 300 W xenon lamp (Oriel), with a measured light intensity of 3.86 mW/cm2 received by the suspension. The gas evolved was monitored periodically



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throughout the reaction using a gas chromatography (Shimadzu GC-8A, HayeSep DB column). The post-reaction catalysts were retrieved by centrifugation for further characterization.

Photoelectrochemical (PEC) measurement The working electrodes were fabricated by drop casting 2 mg/cm2 samples on cleaned FTO substrates. A standard three-electrode PEC setup comprising a of working electrode, Ag/AgCl reference electrode, Pt counter electrode and 0.1 M Na 2SO4 electrolyte, was used to perform PEC measurements. Photocurrent densities were evaluated from the amperometry curves recorded using a potentiostat (Model PGSTAT302N, Autolab) under on-off illumination by a 300 W Xenon lamp (Oriel). Conductance plots were computed from the FRA impedance measurements at 0 V vs. Ag/AgCl in the frequency range of 100 kHz to 0.01 Hz under the dark condition.

RESULTS AND DISCUSSION In the synthesis of CuxS, Cu2O was employed as both the initial sacrificial template and copper precursor. The scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images in Figure S1 show the uniformly grown Cu2O microcrystals with a rigid cubic structure. Ion-exchanges of the cubic Cu2O particles upon the addition of Na 2S allows for the formation of two distinct Cu xS structures: hollow cubic Cu xS (HC-CuxS) and irregularly-shaped CuxS (IS-CuxS). Examination of the captured SEM images clearly demonstrates the well-developed hollow cubic structure of HC-CuxS (Figure 1a-b). Distinct cavities can be observed for the partially broken particles, as illustrated in Figure 1a. By undertaking image analysis on 100 HC-CuxS particles, the size distribution histogram


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(Figure 1c) demonstrates an average edge length of 450 ± 72 nm for the sample. Void formation within the HC-CuxS particles can also be seen from the TEM images (Figure 1de). The hollow interiors (Figure 1d) indicate that all the HC-CuxS particles exhibit a hollow cubic structure. In addition, the high resolution-TEM (HRTEM) image of HC-CuxS (inset of Figure 1e) illustrates the crystalline region with characteristic lattice spacing of Cu 1.8S (3.2 Å), which again suggests the successful formation of Cu xS. With the wall thickness distribution illustrated in Figure 1f, the average wall thickness is calculated as 50 ± 9.7 nm (Figure 1e).

(a)

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15

10 5

0.5 μm

0

20 nm

30

40

50 60 70 Wall Thickness (nm)

80

Figure 1: (a, b) SEM images, (c) size distribution histogram, (d, e) TEM images (inset: HRTEM image with 2 nm scale bar) and (f) wall thickness distribution histogram of HCCuxS.



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In contrast, direct mixing of the Na 2S solution into the Cu2O suspension resulted in an ISCuxS sample comprising fragmented particles with irregular morphologies, as illustrated in its SEM and TEM images (Figure 2a-c). Unlike HC-CuxS, the instantaneous existence of excessive Na2S surrounding the Cu2O particles leads to immediate non-equilibrated transformation of Cu2O into small CuxS particles with random morphologies. The rapid reaction was visualized by observing the immediate color changes from bright orange (i.e. color of cubic Cu2O) to black.

(a)

(b)

(c)

200nm

50nm

20nm

Figure 2: (a) SEM images and (b, c) TEM images of IS-CuxS.

Successful growth of the hollow cubic HC-CuxS structure was achieved by a controlled drop-wise approach, whereby the gradual addition of Na 2S into the cubic Cu2O suspension in an inert environment gave sufficient time for Kirkendall effect to take place (see Figure S2). According to the Kirkendall effect, the migration of atoms at different rates through the interface of a coupled material allows for the formation of voids at the boundaries. 31-33 Arising from the differences in diffusion rate of O and S atoms, 12 HC-CuxS is formed through the Kirkendall effect. To form internal cavities within the HC-CuxS particles, it is postulated that the outward diffusion rate of O is higher than the inward diffusion rate of S as illustrated in Figure S2.34 Arising from the gradual addition of Na2S solution, the O and S atoms can 10 ACS Paragon Plus Environment

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diffuse accordingly; hence, the reaction will be equilibrated according to the O and S diffusion coefficients. The higher diffusion rate of O from the Cu 2O core along with formation of the CuxS outer layer (due to the slower diffusion rate of S) create Kirkendall voids within HC-CuxS particles such that hollow CuxS structures are eventually obtained.35 Additionally, an inert atmosphere (not limited to the use of N2) is another important parameter to minimize the formation of copper defects in Cu xS during the chemical transformation. The lower copper defect density in Cu xS afforded through this synthesis method, which constructively affects charge transport, can be illustrated by the presence of lower and red-shifted LSPR peak in Vis-NIR absorbance spectroscopy (as is discussed below).

Crystal phases of both the HC-CuxS and IS-CuxS samples were analyzed using X-ray diffraction (XRD) spectroscopy while crystallite sizes were derived using the Scherrer equation. The XRD patterns in Figure 3a show that HC-CuxS and IS-CuxS exhibit similar peak positions and peak ratios. Given that copper sulfide phases with different stoichiometries have very similar XRD patterns (i.e. the reference patterns for Cu1.8S and Cu1.765S are shown in Figure 3a),27 identifying the exact CuxS chemical composition from XRD is always challenging. Herein, both the HC-CuxS and IS-CuxS samples display relatively stronger peak intensities at 27.9, 32.3, 46.3 and 54.8°, signifying that they consist of mainly the Cu1.8S phase, whereas the smaller peaks formed at 29.6, 42.0 and 67.4°indicate that significant amount of Cu1.765S is also incorporated within the lattices. With a higher Cu1.8S composition, the main peaks at 2θ of 27.9, 32.3, 46.3 and 54.8° correspond to the (111), (200), (220) and (311) crystal planes, respectively. Although the XRD patterns of both samples are dominantly fitted to Cu 1.8S and Cu1.765S, the difference in compositions and possible existence of other copper-sulfur stoichiometries can’t be ruled out. Therefore, in this



study, both as-prepared samples are denoted as Cu xS which accommodates the slight differences in the copper-sulfur stoichiometries between the two samples.

(3 1 1)

HC-CuxS

HC-CuxS 520 nm

Abs. (a.u.)

(2 0 0)

(2 2 0)

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IS-CuxS

Cu1.8S 495 nm Cu1.765S

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IS-CuxS IS-CuxS

Abs. (a.u.)

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HC-CuxS

Cu(II)

IS-CuxS

600

1000 1400 Wavelength (nm)

1800

938

936 934 932 Binding Energy (eV)

930

Figure 3: (a) XRD patterns of HC-CuxS and IS-CuxS with the reference peaks of Cu1.8S and Cu1.765S, (b) UV-Vis diffuse reflectance, (c) Vis-NIR absorbance and (d) XPS Cu 2p3/2 spectra of HC-CuxS and IS-CuxS.

Crystallite sizes of the samples were calculated according to the highest intensity (220) peaks using the Scherrer equation as described in the experimental section. As can be seen in


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Table 1, the calculated crystallite sizes of HC-CuxS and IS-CuxS are 26 and 20 nm, respectively. The formation of larger HC-CuxS crystals suggests it has better crystallinity when compared to IS-CuxS, which is attributed to the slow crystal growth during the controlled synthesis. Moreover, owing to the variation in particle sizes (i.e. HC-CuxS is shown to have an average particle size of 450 ± 72 nm while IS-CuxS is comprised of small and irregularly-shaped particles), the tabulated BET surface area for HC-CuxS (i.e. 8.1 ± 0.3 m2/g) is approximately half of that of IS-CuxS (i.e. 15.0 ± 0.7 m2/g) (Table 1). With the estimated surface area of HC-CuxS calculated by using the average edge length and a wall thickness of 450 and 50 nm (details can be found in supporting information, Table S1), the total surface area considering both the external and internal surfaces is estimated as 7.5 m2/g. Comparison of the estimated value to the measured BET surface area indicates that the internal walls of HC-CuxS are accessible through small pores located within the walls (pore size distribution of HC-CuxS is shown in Figure S3). Apart from the exterior and interior surfaces, the total surface area measured from BET also takes into account the surface roughness (i.e. rough surfaces are observed from SEM images) and porosity. Nevertheless, the specific surface area of IS-CuxS remains roughly twice that of than HC-CuxS due to the presence of the small and irregular IS-CuxS particles.

Table 1: Crystallite size and BET surface area of HC-CuxS and IS-CuxS. Total Surface Area Samples

Synthesis

Crystallite Size (nm)

HC-CuxS

Drop wise

26

8.1 ±0.3

IS-CuxS

Direct mixing

20

15.0 ±0.7

(m2/g)



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The UV−Vis diffuse reflectance spectra in Figure 3b demonstrate that the light absorption of HC-CuxS and IS-CuxS begins at 520 nm and 495 nm, respectively. The onsets correspond to their bandgaps which are calculated to be 2.39 and 2.51 eV. Despite having similar XRD patterns, the CuxS samples exhibit different band gap energies. The observed blue-shift in the absorption band edge for the IS-CuxS sample can be attributed to two possible causes: (1) particle size reduction; and (2) copper vacancy increment (i.e. smaller value of x in CuxS). As a decrease in particle size is often considered to be the main cause of blue-shifting in an absorption band edge, 19 the former reason is validated by the smaller particle and crystallite size of IS-CuxS when compared to HC-CuxS, as previously discussed. On the basis of copper vacancies, it has been reported that CuxS with a higher copper deficiency exhibits a blueshifted absorption on-set, thereby widening the bandgap energy. 36 Adopting this correlation, the blue-shift implies there is a higher copper defect density present within the IS-CuxS. Additionally, following the reported formation of Cu xS localized surface plasmon resonance (LSPR), 19, 27,

29, 37

examination of the plasmonic absorption peaks in Vis-NIR absorbance

spectroscopy also confirm the lower copper vacancy in HC-CuxS (i.e. higher copper deficiency in IS-CuxS). As illustrated in Figure 3c, the red-shifted and lower LSPR signal exhibited by HC-CuxS is associated with its lower copper defect density, which is consistent with the reported literature.28

To better understand the difference in copper defect density, HC-CuxS and IS-CuxS were characterized using XPS. The Cu 2p 3/2 spectra illustrated in Figure 3d clearly demonstrate the existence of two distinct copper species in both Cu xS samples; Cu(I) species at lower binding energy (932.6 eV) and Cu(II) species at higher binding energy (934.4 or 934.8 eV). 3841

Deconvoluting these spectra show the larger peak area of Cu(II) oxidation state for IS-CuxS

in comparison to HC-CuxS, indicating the higher amount of Cu 2+ presents in IS-CuxS. In this


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instance, the higher amount of Cu 2+ in the IS-CuxS sample indicates of its higher copper vacancy density as non-stoichiometric CuxS often displays the co-existence of Cu(I) and Cu(II) oxidation states.42-43 In addition, the Cu(II) peak of IS-CuxS is observed to be shifted to a higher binding energy (+0.4 eV), signifying the higher valence state of copper in ISCuxS. 44 The evaluations from the XPS Cu 2p spectra have collectively illustrated the higher copper defect density of IS-CuxS when compared to HC-CuxS, which is consistent with the UV-Vis and Vis-NIR results. The difference in copper-sulfur compositions of both samples is not observable from their XRD spectra, owing to the relatively similar XRD patterns across CuxS with different stoichiometries. The higher copper defect concentration in IS-CuxS is attributed to the synthesis condition (i.e. in an air atmosphere and rapid reaction rate). Jiao et al. have demonstrated the detrimental influence of air towards the formation of copper defects during Cu2S synthesis.12

The photoreactivity of HC-CuxS and IS-CuxS was examined by performing the photocatalytic hydrogen evolution reaction in aqueous Na2S + Na2SO3 solution. Figure 4 depicts the time course of photocatalytic hydrogen evolution for both CuxS samples. Evidently, the photocatalytic performance of HC-CuxS surpasses that of IS-CuxS (Figure 4a). Approximately 136 µmol of hydrogen was generated by HC-CuxS which is 43% higher than that of IS-CuxS (i.e. 95 µmol). Given that the overall photocatalytic performance of a photocatalyst is reliant on its available active sites that are in turn dependent on its specific surface area, catalysts with a higher surface area often exhibit a better performance. Nonetheless, the smaller total surface area of HC-CuxS does not restrain it from performing better than IS-CuxS. Upon normalizing the amount of hydrogen gas generated with respect to surface area, a more distinct enhancement for HC-CuxS is observed when compared to IS-



CuxS, as illustrated in Figure 4b. This further confirms the greater propensity of HC-CuxS for generating hydrogen under illumination.

(a) 140

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HC-Cuxcube S Hollow Irregular IS-CuxS

120

Amount of Hydrogen (μmol/m2)

Amount of Hydrogen (µmol)

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

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100

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Series2 IS-Cu xS

250

200 150 100 50 0

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240 360 Time (mins)

480

0

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480

Figure 4: Time course of photocatalytic hydrogen generation under 8 h of illumination (a) with 50 mg of respective samples and (b) time course normalized in relation to per m2 with BET surface area of 8.1 and 15.0 m2/g for HC-CuxS and IS-CuxS, respectively.

Given that specific surface area is not the primary factor defining the enhanced performance by HC-CuxS, better crystallinity in conjunction with its hollow structure are proposed to be of greater importance. Good crystallinity in a photocatalyst is generally acknowledged to facilitate more efficient charge transfer due to the reduction of defects that act as electron-hole trapping sites. Consistent with the UV-Vis and Vis-NIR results, the redshifted and lower LSPR signal of HC-CuxS indicates its lower copper defect density when compared to IS-CuxS. With fewer copper defects, the electron-hole recombination rate is decreased, thereby a higher amount of photogenerated electrons are available for the hydrogen evolution reaction. Thus, the results have confirmed that the controlled formation


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of HC-CuxS with good crystallinity and lower a defect density is beneficial for enhanced photocatalytic performance.

Repeated photocatalytic hydrogen evolution reactions were performed to evaluate the stability of HC-CuxS and IS-CuxS (Figure 5). The photocatalytic activity of HC-CuxS was maintained at ~90% of its original productivity throughout the three illumination cycles (Figure 5a). In contrast, the activity of IS-CuxS deteriorated to less than 50% of the initial amount of hydrogen generated (Figure 5b) after three repeat cycles. The post-reacted HCCuxS and IS-CuxS were characterized using XRD to identify any changes in sample characteristics after illumination. Figure S4 indicates a broadening of the diffraction peaks for both samples, which implies a loss in crystallinity. Interestingly, the diffraction peak positions of post-reacted HC-CuxS sample (black line) remain unchanged when compared to its initial form, with only minor changes to the peak ratios. In the case of IS-CuxS (red line), an extra peak at 48.0°, corresponding to crystalline CuS, is detected after the photocatalytic hydrogen evolution reaction. Upon irradiating CuxS, photoexcited electrons are consumed for the water reduction reaction to generate hydrogen gas, while the photoinduced holes are scavenged by SO 32-. Unlike Cu2O, the chemical dissolution of Cu xS is inevitable, in spite of the presence of a strong hole scavenger to suppress photo-dissolution. Nonetheless, the dissolved copper ions are instantaneously redeposited into Cu xS by reacting with S 2- from Na2S. Therefore, it is postulated that during the dissolution and reprecipitation processes of IS-CuxS, the formation of copper-deficient CuS from Cu2+ and S 2- is more favorable than copper-rich CuxS.



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80

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900

Figure 5: Repeated time course of photocatalytic hydrogen evolution reactions for (a) HCCuxS and (b) IS-CuxS.

To gain further insights on the enhanced stability of HC-CuxS, the morphology of postreacted CuxS samples were examined using SEM and TEM microscopies. The constant photoreactivity of HC-CuxS after three cycles of illumination is assigned to its preserved hollow structure (Figure 6a-d). Although minor restructuration is observed on the surface of the particles (such morphological changes are expected due to the unavoidable occurrence of the chemical dissolution and redeposition processes), the hollow interior of the sample is retained as signified by the cubic cavities observed from the TEM images of post-reacted HC-CuxS (Figure S5). As such, the photocatalytic performance of HC-CuxS, reliant on the surface active sites, is retained due to the suppression of agglomeration such that the surface active sites are preserved. In contrast to HC-CuxS, deterioration in the photoreactivity of ISCuxS is attributed to a large degree of particle sintering, as illustrated in its post-reacted SEM images (Figure 6e-h). Agglomerated clusters with a size larger than 200 nm are observed after 15 h of illumination (Figure 6h). The progressive growth of the IS-CuxS particles is again credited to the dissolution and reprecipitation of Cu xS during the reaction. 18 ACS Paragon Plus Environment

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Consequently, the decline in IS-CuxS performance derives from the decrease in surface active sites of the sintered particles as well as its partial transformation into CuS. While the presence of a strong hole scavenger (Na 2S + Na2SO3) can suppress photodissolution, the undesirable sacrifice of CuxS structural stability due to chemical dissolution is, however, detrimental to the overall performance of IS-CuxS.

(a)

(b)

(c)

200nm

200nm

200nm

5 hours

After 0 hour (e)

200nm

10 hours

(f)

200nm

(d)

15 hours of illumination

(g)

200nm

(h)

200nm

200nm

Figure 6: SEM images after each cycle of 5-h illuminated photocatalytic hydrogen evolution reaction for (a-d) HC-CuxS and (e-h) IS-CuxS.

Although surface deformation was also observed for HC-CuxS, its hollow structure helps to retain the original shape of the particles. Arising from the ability of the rigid structure to minimize structural rearrangement, the well-defined hollow cube is beneficial for suppressing sintering of the HC-CuxS particles. The lower rate of dissolution and reprecipitation also reduces the tendency of CuS formation. Thus, the constructive role of the hollow structure in hindering particle agglomeration during the photocatalytic reaction strongly signifies the importance of well-defined morphologies for improved catalytic stability.



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With different working principles, the photoactivity of a semiconductor can be assessed on the basis of a suspension system (powdered photocatalytic system) or a photoelectrochemical (PEC) system. The desired properties of a photocatalyst can vary depending on the ultimate application (be it suspension or PEC systems). To compare the photoactivity of the powder form and as a photoelectrode, HC-CuxS and IS-CuxS were made into electrodes for PEC examination. The prepared thin films were used as the photocathode in a three-electrode PEC system, consisting of Ag/AgCl as the reference electrode and Pt foil as the counter electrode. Figure 7a displays the amperometric photocurrent generated by HC-CuxS and IS-CuxS under chopped illumination cycles. Interestingly, the enhanced photoactivity of HC-CuxS observed from photocatalytic hydrogen evolution (Figure 4) is not identifable from the PEC cathodic photocurrent measurement. The comparable photocurrent densities exhibited by both HCCuxS and IS-CuxS suggest the discrepency in regulating factors between powdered and electrode system. As CuxS serves as a photocathode, electrons that flow from the Pt counter electrode to the FTO substrate need to be effectively transported through the CuxS samples for the water reduction reaction. This indicates that the charge transfer efficiency from the loaded CuxS samples to the conducting substrate plays an important role in improving the overall PEC performance.


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(a) 20

Hollow HC-Cuxcube S Irregular IS-CuxS

μA/cm2

(b) 1400 HC-Cu HC xS 1200

IS xS IS-Cu

1000

Absolute Z (Ω)

Current Density

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


light on

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0 0.01

2500

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1 100 Frequency (Hz)

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(d)

void FTO substrate

FTO substrate HC-CuxS

200nm

IS-CuxS

200nm

Figure 7: (a) PEC amperometric photocurrent densities under chopped illumination, (b) Bode plots computed from electrochemical impedence spectroscopy (EIS) of HC-CuxS and ISCuxS, cross sectional SEM images of (c) HC-CuxS and (d) IS-CuxS.

Associated to the similar PEC performance of HC-CuxS and IS-CuxS, electrochemical impedance spectroscopy (EIS) was employed to study the charge transfer behaviors between the FTO substrate, the CuxS samples and the electrolyte. The Bode plots in Figure 7b illustrate the unique impedance (Z) as a function of frequency for HC-CuxS and IS-CuxS samples that were deposited separately on FTO substrates. Given that impedance is correlated to the resistance and inversely proportional to the admittance (conductance), Bode plots of 21 ACS Paragon Plus Environment


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HC-CuxS and IS-CuxS can be used to compare their interfacial charge transfer properties. 45-46 The responses in the high frequency region (> 1 kHz) represent the impedance at the interface between the electrolyte and the working electrode (i.e. HC-CuxS or IS-CuxS samples).47 As EIS measurements for both samples were performed in the same electrolyte (i.e. 0.1 M Na2SO4 solution); comparable impedances are observed from this region (Figure 7b). On the contrary, the lower frequency signals (< 1 Hz) represent the impedances between the samples and the conducting substrate (i.e. FTO). Herein, the higher impedance of HC-CuxS in this region suggests its poorer conductivity towards the FTO substrate in comparison to IS-CuxS, thus, contributing to the comparable PEC performances for both CuxS samples.

The inferior conductance of HC-CuxS in the low frequency region was clarified by performing cross sectional imaging of the drop-casted electrodes. With larger HC-CuxS particles, limited contact between the Cu xS sample and the FTO substrate is observed (Figure 7c) due to particle stacking, creating large void spaces within the electrode. In contrast, a compact and uniformly distributed layer of IS-CuxS is revealed, as illustrated in Figure 7d. This suggests intimate contact between the smaller IS-CuxS particles and the FTO substrate is responsible for the improved charge transfer of this electrode. In relation to the crosssectional images, the distinct sample-substrate interfaces of HC-CuxS and IS-CuxS (also verified by the conductance plots) justify the comparable photocurrent densities attained for the PEC measurements. The lower charge transfer efficiency of the HC-CuxS electrode arises from void space formation within the electrode which restricts photocurrent generation, thus, off-setting the photoactivity enhancement achieved in the powdered photocatalytic system.


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CONCLUSIONS Morphological control has been shown to affect the performance of CuxS for the photocatalytic hydrogen evolution reaction, attributed to the enhanced structural stability of hollow CuxS structures (HC-CuxS) when compared to irregularly-shaped CuxS (IS-CuxS). The gradual addition of Na2S precursor, when transforming Cu2O into CuxS, offers better crystallinity in the final product due to improved crystal growth facilitated by a sufficient (equilibrated) reaction time between the sacrificial Cu 2O and Na 2S. A HC-CuxS possessing a higher crystallinity and lower copper defect density was obtained as a result. Photocatalytic hydrogen generation studies revealed that the HC-CuxS sample possessed enhanced photoreactivity, as well as better structural stability under prolonged photocatalytic reaction conditions. The improved photoactivity (i.e. larger amount of H 2 produced) of HC-CuxS was attributed to its higher crystallinity and lower copper defect density. Additionally, the rigid hollow structure acts to stabilize the particles by hindering agglomeration during the photocatalytic reaction, signifying the enhanced sintering resistance invoked by the hollow morphology. Apart from demonstrating the ability of Cu xS in generating hydrogen, the findings provide a greater understanding of the effects of CuxS synthesis on its self-doping ability and the beneficial attributes of hollow structures for photocatalytic reactions.



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ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publication website at DOI: SEM and TEM images of the cubic Cu2O template, schematic illustration of the formation of HC-CuxS via the Kirkendall effect, post-reaction TEM image of HC-CuxS and post-reaction XRD patterns of both samples.

AUTHOR INFORMATION Corresponding Author * Tel: +612-93854340, Email: [email protected] Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was financially supported by the Australian Research Council Discovery Project (DP180102540). The authors appreciate the facilities and technical support provided by the UNSW Mark Wainwright Analytical Centre.


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TOC GRAPHIC

Na2S

Cu2O

200nm

HC-CuxS

100nm

Hydrogen evolution (mmol/m2)

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0.35 Series3xS HC-Cu Series2 IS-Cu xS

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0.25 0.20 0.15

0.10 0.05 0.00 0

120 240 360 Time (mins)

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Transformation of Cuprous Oxide into Hollow Copper Sulfide Cubes

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