Construct Highly-Dispersed and Efficient Charge-Separation

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Construct Highly-Dispersed and Efficient ChargeSeparation-Transferring Interface by OxidativelyBonding Rh in CuO Surface for Dye Photodegradation 2

Xiurong Liu, Ruonan Ma, Dan Zhao, Chao Chen, and Ning Zhang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b00447 • Publication Date (Web): 22 Apr 2019 Downloaded from http://pubs.acs.org on April 22, 2019

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The Journal of Physical Chemistry

Revised manuscript of jp-2019-00447

Construct Highly-Dispersed and Efficient Charge-Separation-Transferring Interface by Oxidatively-Bonding Rh in Cu2O Surface for Dye Photodegradation Xiu-Rong Liu, Ruo-nan Ma, Dan Zhao*, Chao Chen*, Ning Zhang

Key Laboratory of Jiangxi Province for Environment and Energy Catalysis, College of Chemistry, Nanchang University, Nanchang, Jiangxi, 330031, China

* Corresponding authors: Associate Professor Dan Zhao Professor Chao Chen E-mail: [email protected] (Dan Zhao) [email protected] (Chao Chen) Phone: +86-15879176996 (Dan Zhao) +86-15179167359 (Chao Chen)

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Abstract Deliberately modulating the surface structure of oxide semiconductor materials was crucial to improve their photo-electronic conversion performance. Here, surface engineering of Cu2O-based semiconductor (cubic particles in average size of 300 nm) by redox embedding Rh with RhCl63- was attempted to get a series of Rh-Cu2O catalysts, and their photocatalytic properties were typically surveyed from dye (methyl orange, MO) photodegradation. With comprehensive characterizations, it was demonstrated that the highly-dispersed and tightly-integrated Rh-Cu2O interface, featured as oxidatively-bonded Rh in surface layers of Cu2O substrate, was well constructed by adjusting Rh loading from 0.04 wt.% to 0.38 wt.%, exhibiting the obvious enhancement in photocatalytic performance for MO degradation compared to bare Cu2O. The measurements from photo-production of H2, photo-current, electrochemical impedance and scavengers-present tests further illuminated that the Rh species bonded on Cu2O surface in electron-deficiency state greatly contributed for not only improving separation efficiency of photo-generated electron/hole pairs, but also lowering the resistance to speed charge transferring along Rh-Cu2O interface, resulting in the enhanced photocatalytic performance of Rh-Cu2O catalysts. In addition, the photocatalytic principle of Rh-Cu2O catalysts for dye degradation was also discussed. These results indicated that oxidatively-bonding metal species in surface

layers

of

Cu2O

charge-separation-transferring

was

feasible

M-Cu2O

interface

for so

that

getting to

efficient

improve

the

photo-electronic conversion performance, which could be referred as a notable system to directly engineer semiconductor surface for advanced photo energy applications.

2


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1 Introduction Metal oxide semiconductor materials have been widely investigated as photocatalysts for many promising and sustainable development technologies, such as producing H2 and O2 from photo-splitting water, getting fuels or chemicals by photo-reduction of CO2, harvesting electricity from solar energy conversion and purifying

water

through

photo-degradation

of

pollutants.1-3

Among

these

semiconductors, as a typical p-type conductor with a direct band gap of 2.0-2.2 eV, Cu2O has attracted considerable attentions due to its range-expanding application potentials especially on photo-electricity conversion and environment remediation.4-5 However, these potentials were still suspended to become reality from several obstacles, the most formidable one was the poor separation efficiency of photogenerated electrons and holes on Cu2O based photocatalysts, as well as on other oxide semiconductor materials. Many efforts have been devoted to enhancing the electron-hole separation efficiency of oxide semiconductors, in which, introducing metal species to modify or decorate semiconductor surface was demonstrated to be a feasible solution. In general, the species could be roughly differentiated as metal particles and metal ions. By depositing Ag or Au metal nanoparticles on a semiconductor surface, wang et al. found that two effects, the surface plasmon resonance (SPR) effect from plasmonic metals and a built-in electric field on metal-semiconductor interface named as Schottky Junction (SJ) effect conjunctively contributed for accelerating the separation of photogenerated electron/hole pairs on such plasmonic photocatalysts.6 Yu et al. 3



reported that a high photocatalytic activity under visible light observed on Cu(II)/Fe(III)-grafted TiO2 could be correlated with the direct interfacial charge transfer (IFCT) between TiO2 and the adhered Cu(II)/Fe(III) species.7 Tie et al. also demonstrated that the photo-degradation of methyl orange could be dramatically speeded from a two stage acceleration process that the first stage was introducing Ag+ as electron acceptor in Cu2O-contained solution to draw the photo-generated electron from Cu2O, and then the charge migration could be further improved upon Schottky junction interface which derived from Ag reductive deposition on Cu2O surface.8 Although these reports exhibited the advances on improving the photocatalytic performance of oxide semiconductors from surface engineering with metal particles and metal ions, respectively, some fundamental topics should be further surveyed. When use metal nanoparticles to decorate semiconductor surface, a well-constructed metal-semiconductor interface was crucial for the charge transferring,9-10 however, in most reports, it should be noted that the metal particles employed for photocatalysis were always bigger than 2.0 nm, the corresponding metal dispersion as low as not more than 50% suggested that at least half of metal atoms were not contacted with semiconductor surface, the confined metal-semiconductor interface could be a significant factor to limit charge separation efficiency.11-12 On the other hand, using metal ions to modify semiconductor, in spite of the electrostatic repulsion of same ion could offer the high dispersion of metal-semiconductor interface, the interactive structure might be not sufficiently firm in consideration of the good mobility of ions in the liquid solution involved in the most of photocatalysis systems. Therefore, 4


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constructing highly-dispersed and tightly-integrated metal-semiconductor interface should be an important pre-condition for obtaining such kind of advanced photocatalysts. As an attempt to get the interface, in this work, Rh was introduced on Cu2O substrate with the deliberately controlled low level of Rh loading (< 0.5 wt.%) through a surface redox embedding preparation without using any surfactants to acquire a series of Rh-Cu2O samples, and the photocatalysis performance of these samples were investigated by photodegradation of methyl orange. It was demonstrated that the introduced Rh could be anchored in the surface layer of Cu2O as oxidatively-bonded species, and the separation efficiency of electron/hole pairs could be obviously enhanced on such well-dispersed and firmly-integrated Rh-Cu2O interface, leading to the prominent photocatalytic improvement for dye degradation and photo-production of H2 with sacrificed agents. The findings could be expanded as reference for designing efficient charge transferring interface for oxide semiconductor materials, especially for Cu2O based photo application materials.

2 Experimental 2.1 Materials CuSO4·5H2O (99%), K3RhCl6, TiO2 (99.8%, 25 nm), Formic acid (FA, 99%) were purchased from Aladdin reagent Co., Ltd., Polyvinylpyrrolidone (PVP40000) was purchased from Sigma-Aldrich (product of USA). NaOH (≥96%), Methyl orange (MO), L(+)-Ascorbic acid (≥99.7%) were purchased from Xilong Scientific Co., Ltd. Congo red (CR) was purchased from China Yuanhang Reagent Factory., 5



Triethylamine (TEA), Anhydrous ethanol (CH3CH2OH) were purchased from Sinopharm Chemical Reagent Co., Ltd. All the chemicals were analytical grade reagents, they were used as received without further purification. Deionized water was used for all reactions and treatment. 2.2 Preparation of catalysts Cu2O cubic substrate was synthesized following our previous study,13 briefly, which was the reduction product of CuSO4 with ascorbic acid and PVP40000 as reductant and surfactant, respectively. A direct galvanic replacement process between K3RhCl6 and Cu2O in aqueous solution without any additives was employed to prepare Rhx-Cu2O (x presents weight loading of Rh on Cu2O) composites with following procedure: 100 mg of the as-prepared Cu2O cubic particles were ultrasonically dispersed in 150 mL deionized water, and the system was purged with nitrogen to remove the dissolved oxygen from the solution under magnetic stirring (800 rpm), then desired volumes of K3RhCl6 solution (0.5 mM) were dropped into the solution at 40 °C, the mixture was further stirred for 1 h. The obtained precipitates were removed out by centrifugation, washed with deionized water and absolute ethanol for several times, then dried in vacuum oven at 60 °C for 12 h to get Rh-Cu2O catalysts. For reference, a Rh/TiO2 catalyst was prepared from a conventional incipient-wetness impregnation method: the commercial TiO2 (anatase, 25 nm) powder was immersed in aqueous solution of K3RhCl6 for 6 h, then the impregnates were dried in air at 373 K for 2 h to obtain Rh/TiO2 catalyst, Rh weight loading on TiO2 was determined as 0.2 wt.% by ICP-OES measurement. 6


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2.3 Characterizations The actual loading of metal components in catalysts was quantified using an inductively coupled plasma-optical emission spectrometer (ICP-OES) on Agilent Technologies 5100 ICP-OES. X-ray diffraction (XRD) patterns of samples were measured on a Persee XD-3 X-ray diffractometer at a scan rate of 2°·min-1 in the angle (2θ) range of 20-90°. The wavelength of the incident radiation was 1.5406 Å (Cu Kα). The scanning electron microscopy (SEM) measurements were performed on a FEI Quanta 200 F microscope operating at 20.0 kV. The morphology of the samples was characterized by transmission electron microscopy (TEM) using a JEOL JEM-2100 microscope operating at 200 kV. The samples were prepared by placing a drop of the ethanol suspension containing sample powder on a carbon film coated Cu grid (3 mm, 400 mesh), followed by drying under ambient conditions. An Axis Ultra DLD apparatus with monochromatized Al Kα (hv=1486.6 eV) as the excitation source was employed to measure X-ray photoelectron spectroscopy (XPS) of samples. The adventitious carbon C 1s peak at 285.0 eV was used as reference to correct XPS data. Zeta potentials of samples were measured by a Nano-ZS90 analyzer from Malvern Company. The pH of the solution was adjusted by 0.05 M HCl or 0.05 M NaOH. The UV-visible (UV–vis.) absorption spectra were measured by an Agilent 7



Technologies Cary 60 UV-vis spectrophotometer. 2.4 Photocatalysis measurements The photocatalytic performance of catalysts was mainly evaluated from methyl orange (MO) degradation in aqueous solution, as supplementary experiments, photo-degradation of Congo red (CR) and photo-prodcution-H2 with formic acid (FA) or triethylamine (TEA) as sacrificial reagents over typical samples were also performed. All photocatalytic reactions were irradiated under a 300 W Xenon lamp equipped with a 420 nm cutoff filter with light intensity (reaching the cell) at 133 mW/cm2. To exclude heat effect, the reaction cell was maintained at room temperature (23 °C-25 °C) by circulating water through the outer jacket. For MO degradation on TiO2 and Rh0.2wt.%/TiO2 reference samples, reaction solutions were irradiated under Xenon lamp without any filter. For dyes degradation, 10 mg of the as-prepared photocatalyst was added to 100 mL of aqueous solution of MO or CR (4 × 10-5 mol·L-1). The mixed suspension was stirred under dark condition for 30 min to reach adsorption-desorption equilibrium. Subsequently, the photocatalyst-suspended solution was irradiated under Xenon lamp to initiate reaction. During light illumination, 2 mL of reaction solution were removed out every 10 min, and the composition of suspension after removing catalyst was analyzed by UV-vis spectrophotometer. For photo-prodcution-H2, 100 mg of the as-prepared photocatalyst was added to 100 mL of aqueous solution containing 0.125 mol·L-1 formic acid or triethylamine. After the system was purged with nitrogen for 1 h to remove the dissolved oxygen 8


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from the solution, the photocatalyst-suspended solution was irradiated under Xenon lamp equipped with a 420 nm cutoff filter . The amount of H2 produced was measured by a gas chromatography (Agilent Technologies: 6890 N). 2.5 Electrochemical measurements The photoelectrochemical (PEC) tests were conducted on a LK 98 BII electrochemical workstation with a three-electrode system, in which, a platinum wire was used as counter electrode, a saturated Ag/AgCl as reference electrode and as-prepared catalyst as working electrode. The working electrode was prepared as follows. Firstly, an ITO conducting glass (1.0 cm×3.0 cm) was ultrasonically cleaned using acetone, ethanol and water by turns. Meanwhile, 10 mg of samples were ultrasonally dispersed into 0.50 mL of ethanol for 1 h. Then 20 μL of the suspension was dropped on the cleaned FTO glass (the covering area: 1.0 cm×1.0 cm), and dried at room temperature. The electrolyte was 0.5 M Na2SO4 aqueous solution. The photo irradiation condition was as same as that employed in photocatalytic measurements. Electrochemical impedance spectroscopy (EIS) was also obtained in a three-electrode

system

on

an

IVIUMSTAT

(Netherlands)

electrochemical

workstation, the signal was recorded in 104-0.01 Hz frequency range under bias of 10 mV and simulated with Zsimpwin 3.5 program. The electrolyte was 4×10-5 mol·L-1 MO aqueous solution, as same as the reaction solution for photo-degradation of MO. A platinum wire and a saturated Ag/AgCl were employed as the counter electrode and the reference electrode, respectively. The working electrode was prepared as following procedure, firstly, a homogeneous ink was acquired via ultrasonic 9



dispersion of catalyst prepared (2.5 mg) in a Nafion solution (50 μL in 1 mL of distilled water); Then, 5 μL of the ink was dropped on a glass carbon electrode (diameter 3 mm) and dried at room temperature to get working electrode.

3 Results and discussion 3.1 Composition and structure features of Rh-Cu2O catalysts According to ICP-OES measurements, three Rh-Cu2O samples with Rh loading at 0.04 wt.%, 0.15 wt.% and 0.38 wt.% were chosen as representative samples for characterizations and tests. X-ray diffraction patterns of Cu2O, Rh0.04w.t%-Cu2O, Rh0.15wt.%-Cu2O and Rh0.38wt.%-Cu2O were given in Figure 1. All characteristic diffraction peaks for the Rh-Cu2O photocatalysts were all perfectly indexed to pure cubic Cu2O substrate, these peaks matched well with the standard diffractions of Cu2O (JCPDS Cards No.65-3288#). The diffraction peaks for Rh species were not found, which could correlate with the low loading of Rh or Rh present as highly dispersed species on Cu2O substrate. To recognize the distribution state of Rh, the morphological features of samples were investigated with SEM and TEM measurements, as shown in Figure 2. Although Rh loading among three Rh-Cu2O catalysts was fold-up modulated from 0.04 wt.% to 0.38 wt.%, the appearance of these samples was basically not altered with increasing Rh loading in comparison with Cu2O substrate. Thus, Rh0.15wt%-Cu2O was chosen as representative sample and compared with Cu2O substrate. In HRTEM images, only the electronic diffraction of Cu2O displayed by streaks with d-spacing value around 0.215 nm pertinent to Cu2O-[200] plane (JCPDS Cards No.65-3288#) 10


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can be resolved on Rh0.15wt%-Cu2O catalysts in comparison with 0.212 nm present on Cu2O substrate; in contrast, the uniform distribution of Rh on the sample was testified by element-distribution mapping measurement, which was accompanied by the slight fluctuation for Rh composition but the steep increase for Cu composition from edge to center on an individual Rh0.15wt%-Cu2O particle disclosed by the line-scan profiles ( see Figure S1 in supporting information), these results further confirmed that Rh could be highly dispersed on Cu2O as the previous deduction from XRD measurements. The features could be correlated with the nature of the preparation process for Rh-Cu2O samples. Essentially, the preparation process to get these samples was a heterogeneous etching-replacing reaction (Cu2O was etched by RhCl63to produce Cu2+ and Rh (0)) happened on the surface of Cu2O driven by the redox potential disparity between two compounds, which would facilitate to embed Rh into the surface layer of Cu2O.14-15 As the redox potentials shown in Table S1 (see in supporting information), we have previously practiced to prepare M-Cu2O (M= Au, Pd, Pt) following the same principle, however, the nanoparticles of these metals would form on Cu2O (described by the SEM images of typical samples in Figure S2 of supporting information) under the controlled metal loading level as similar as the current preparation of Rh-Cu2O samples, such M-Cu2O interface with metal nanoparticles anchored have been intensively studied in literature works.16-18 In view of metal dispersion of these samples, most metal atoms would be sealed inside metal particles, thus lead to a poor contact of metal with Cu2O. Unlike them, the absence of Rh electronic diffractions and the slight deviation of Cu2O electronic diffraction on 11



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current Rh-Cu2O catalysts suggested that Rh would be highly dispersed into the surface of Cu2O substrate through the redox etching-replacing reaction process. It means that most loaded Rh would fully contact with Cu2O to form an efficiently integrated Rh-Cu2O interface, the character could be attributed to the nature of Rh/RhCl63- redox pair. From Table S1, it can be found that, in comparison with other M/MClxy- (M= Au, Pd, Pt) redox pairs, the redox potential of Rh/RhCl63- pair is in the lowest level, which would allow a slower reduction speed of RhCl63- by Cu2O to facilitate the embedding and dispersing of Rh on Cu2O surface rather than piling Rh atoms themselves to particles, since more reduced metal atoms from quick reaction appeared simultaneously, the higher possibility for aggregation of metal atoms with each

other

present.

Consequently,

the

unique

Rh-Cu2O

interface

with

highly-dispersed Rh was acquired on corresponding samples in this work. The surface states of samples were investigated by XPS measurements, which were given in Figure S3 and Figure 3. The identical Cu 2p photoemissions from Cu2O substrate reappeared among Rh-Cu2O catalysts with reference to that of pure Cu2O sample, as shown in Figure S3A (see in supporting information). The more careful analysis of surface Cu species were conducted with Cu-LMM Auger spectra in Figure 3A, it can be found that the main peaks for all samples are centered around 570.0 eV, the peak position was in accordance with that reported for Cu(I) species,19-20 with peak-fitting measurements, the high surface composition of Cu(I)/Cu in 0.78-0.87 among Rh-Cu2O catalysts compared to 0.89 on Cu2O was resolved in Table S2 (see in supporting information), indicating that Cu2O surface was well preserved among 12


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Rh-Cu2O catalysts. As far as the composition alternation of Cu(II) species, although a slight increase in Cu(II) composition with increasing Rh loading was observed among Rh-Cu2O catalysts, in general, the difference of surface Cu(II) composition on Rh-Cu2O catalysts to that on Cu2O was limited, which could be attributed to that most Cu(II) species produced from redox reaction between RhCl63- and Cu2O would remove as Cu2+ ions into aqueous solution, so Cu(II) residue fraction on Rh-Cu2O catalysts was more close to the small Cu(II) ratio on naturally oxidized surface of Cu2O substrate. In Figure 3B, the distinct Rh 3d photoemission signals displayed on Rh-Cu2O catalysts, and the molar ratio of Rh: Cu2O on surface resolved from these signals for Rh-Cu2O catalysts were all ten-folds higher than that calculated from Rh weight loading, further confirming that Rh was highly dispersed on Cu2O or the highly-dispersed Rh-Cu2O interface was formed on Rh-Cu2O catalyst. When focusing on the binding energy of Rh photoemissions, the Rh 3d5/2 and Rh 3d3/2 peaks on Rh-Cu2O catalysts were centered around 309.7 eV and 314.6 eV, respectively, which indicated that Rh mainly present as Rh(III) ion-like species on the surface of these catalysts.21-24 In Figure 4, particularly when manipulating pH value of solution around 7.0 (the condition employed to prepare Rh-Cu2O catalysts), the positively-charged surface of Rh-Cu2O catalysts in comparison with the negatively charged surface of Cu2O denoted by Zeta potential analysis also indicated that introduced Rh on Cu2O would present as ion-like species. One may question that such Rh(III) ion-like species could come from the adsorbed RhCl63- ions on Cu2O, but as mentioned above, a redox replacement reaction would appear once RhCl63- ions encountered with Cu2O, so it is 13



difficult to directly estimate the adsorption of RhCl63- ions on Cu2O. To address the issue, we prepared a Rh/TiO2 reference sample by impregnating RhCl63- on commercial TiO2 since TiO2 surface was inert for RhCl63-, the Rh 3d XPS spectra of the sample was compared with that of Rh-TiO2 with similar Rh loading in Figure S3B. Interestingly, the Rh(III) signal peaks from the adhered RhCl63- on TiO2 was obviously shifted to lower binding energy range in comparison with those on Rh-Cu2O catalyst, suggesting that a more intensive metal-substrate interaction could present on Rh-Cu2O interface rather than the simple adsorption of Rh(III)-like ions on substrate. In view of that an oxidative-bonding metal composite structure denoted as M1-O-M2 were believed as the reasonable interactive structure on well-constructed metal-oxide interface (M1/M2Ox) particularly when metal was highly dispersed on oxide substrate,25-27 on present Rh-Cu2O catalysts, above results further suggested that the Rh-O-Cu oxidative-bonding interface could form on these samples. In addition, as disclosed by TEM measurements, Rh could embed or immerse in the surface layer of Cu2O, which also offered the possibility to form tightly integrated Rh-Cu2O interface such as the Rh-O-Cu oxidative-bonding interface on these samples. Based on above characterizations, it can be recognized that preparation of Rh-Cu2O catalysts as Cu2O with oxidatively-bonded Rh in surface layers was achieved from RhCl63etching-embedding on Cu2O, the simplified process was shown in Scheme 1. As illuminated by the reports concerning metal-semiconductor photocatalysts,28-30 such highly-dispersed and tightly-integrated metal-semiconductor interface would act as an efficient charge transferring interface to facilitate the separation of photo-generated 14


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electrons and holes, thus, the following investigations of using these samples as photocatalysts were performed. 3.2 Photocatalytic measurements for MO degradation over Rh-Cu2O catalysts The photocatalytic performance of Rh-Cu2O catalysts was displayed by employing methyl orange (MO) as dye source for degradation. Before the measurements, the concentration change in the blank MO solution (without catalysts) under photo and in catalyst-MO mixture under light-off condition were investigated, as shown in Figure 5A, there were not significant MO degradation in two solutions, and the MO-adsorption-equilibrium could be finished in 30 min for all catalyst contained solution under light-off condition; when light was introduced on catalyst-MO mixture, in comparison with the slow change to the final degradation percentage around 15% in 80 min on Cu2O, the accelerated MO degradations were observed among Rh-Cu2O catalysts, the total degradation percentage increased from 70% on Rh0.04 wt.%-Cu2O to 95% on Rh0.15wt.%-Cu2O, and then fell to 87% with further increasing Rh loading to 0.38 wt.%. The trend was also confirmed by the kinetic measurements shown in Figure 5B, the reaction rate constant (k) on Rh-Cu2O catalysts was obviously higher than that on Cu2O, and the highest value was obtained on Rh0.15wt.%-Cu2O. These results indicated that oxidatively-bonding Rh on Cu2O even in such low loading would obviously improve photocatalytic performance in comparison with using bare Cu2O, and properly manipulating the introducing amount of Rh could further modulate or optimize the photocatalytic performance. In comparison with the reported Cu2O-based photocatalysts with precious metals such as 15



Au and Ag,31-32 the amount of precious metal required for achieving similar degradation percentage was greatly lowered among current Rh-Cu2O catalysts, which could be attributed to that Rh was oxidatively-bonded and highly dispersed on Cu2O in our samples instead of the metal nanoparticle in low dispersion employed in literature works. Based on above results, Rh0.15wt.%-Cu2O was chosen for investigating catalytic stability by continuous cycle measurements, the result was given in Figure 6A. A slight decrease not more than 8% in total MO degradation percentage during 4 continuous photocatalytic cycles was observed, the decrease could be attributed to the weight loss of catalysts from separation process between cycles. In addition, the cycle use of Rh/TiO2 reference catalyst was also estimated in Figure 6B. Although the adhered Rh(III) ions on Rh/TiO2 could also enhance the photocatalytic degradation of MO compared to TiO2, the enhancement would quickly damp in following catalytic recycles, and an evident Rh loss in solution was found after reaction, indicating that the mobility of weakly-adhered Rh(III) ions would not support the constant catalytic performance for cycle use. In contrast, the almost un-changed Rh surface feature between fresh and used Rh-Cu2O sample, as disclosed by the XPS spectra comparison in Figure S4 (see in supporting information), together with the stable catalytic cycle performance, further confirmed that the oxidatively-bonding Rh on Cu2O surface would not only be benefit for enhancing photocatalytic performance, but also be positive for constructing robust Rh-Cu2O interface for durable photocatalytic application involved in liquid reaction environment. 3.3 Principle investigation on the photocatalytic feature of Rh-Cu2O catalysts 16


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As known, the photocatalytic performance was greatly dependent on the light absorption, charge separation and surface reaction feature of photocatalysts.33 To elucidate the photocatalytic principle of Rh-Cu2O catalysts, firstly, the UV-vis. spectra of Rh-Cu2O catalysts were compared with that of Cu2O in Figure S5. As disclosed, the light-adsorption range and intensity on Rh-Cu2O catalysts almost completely overlapped with those on Cu2O, indicating that the light adsorption feature of Cu2O surface was not evidently altered by introducing of Rh. Considering MO is a dye compound, a reviewer pointed out that the dye sensitization effect could be responsible for the photocatalytic performance of samples. To clarify the possibility, Congo red (CR, another dye with different light adsorption feature to MO) was also employed as degradation source, as shown in Figure 7, the performance on Cu2O and typical Rh-Cu2O sample (Rh0.15wt.%-Cu2O) was compared, and a clear enhancement in degradation efficiency for CR on Rh-Cu2O versus Cu2O was observed. In addition, two sacrificial reagents, formic acid (FA) and triethylamine(TEA) were employed for photo-producing H2 under absence of dye, the contrast results between Rh0.15wt.%-Cu2O and Cu2O were also given in Figure 7. It can be found that 40-fold and 2-fold of H2 production compared to Cu2O appeared on Rh-Cu2O catalyst when altering sacrificial reagents. These results further confirmed that the Rh-Cu2O interface constructed in this work was more efficient for photocatalytic reactions than Cu2O surface, and the feature was independent with the properties of reactants. It was believed that the photocurrent-time dependence obtained from photoelectrochemical (PEC) measurement could reveal the interfacial generation and 17



separation dynamics of photogenerated charges of semiconductor photocatalysts, and a larger photocurrent indicates higher electrons and holes separation efficiency.33-34 Following the consideration, the photoelectrochemical (PEC) measurements for Cu2O and Rh-Cu2O catalysts were performed in neutral aqueous solution with Na2SO4 as electrolyte, the curves were present in Figure 8A. In comparison with Cu2O substrate, Rh-Cu2O catalysts exhibited obviously higher photo-current density. Considering that the working electrode was made by pasting catalyst on a FTO substrate, as reported Cu2O-based FTO electrode for PEC tests,35 when Cu2O-based materials were excited by light irradiation, the photoinduced cathodic current was compatible with Cu2O p-type semiconductor characteristics, the electrode reaction could be that the photoinduced holes on the VB migrate from FTO to the counter electrode to react with H2O to produce O2, simultaneously, the photoinduced electrons leaved on Cu2O based electrodes immediately react with H+ to produce H2. As supported by previous photo-producing H2 measurements, the considerable H2 productions on samples particularly the higher H2 production on Rh-Cu2O compared to Cu2O were acquired when sacrificial reagents were added in neutral aqueous solution. Therefore, the observed contrast results from PEC measurements between Cu2O substrate and Rh-Cu2O catalysts suggested that introducing Rh to construct oxidatively-bonding Rh-Cu2O interface was beneficial to enhance the separation efficiency of photo-produced electron/hole charge pairs. The electrochemical impedance spectroscopy (EIS) of catalysts in reaction solution was further measured, as shown in Figure 8B. Following a reported method to estimate EIS of Cu2O-based materials,36 18


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the well fitted Nyquist plots with experimental data based on the similar equivalent circuit model were resolved for all samples, and corresponding parameter simulated from Zsimpwin 3.5 program were listed in Table S3. It can be found that the electrochemical impedance on samples were all mainly contributed by charge-transfer resistance (Rct), moreover, the diameters of the Nyquist plot semicircles and the fitted Rct values of the Rh-Cu2O catalysts were evidently decreased in comparison with pure Cu2O substrate. Beyond all doubt, the testified superior charge-transferring feature of Rh-Cu2O interface with oxidatively-bonded Rh compared to Cu2O surface was another positive factor for not only enhancing the separation efficiency of photo-produced electron/hole charge pairs, but also accelerating the whole charge transferring process along such well-composited Rh-Cu2O interface, thus it is not surprised that the improved photocatalytic performance was observed on Rh-Cu2O catalysts. To further understand the photocatalytic feature of Rh-Cu2O catalyst, and considering that holes (h+), superoxide anion radicals (·O2-) and hydroxyl radicals (·OH) could play important roles in the photodegradation process of MO, isopropyl alcohol (IPA), ethylenediamine tetra-acetic acid disodium (EDTA-2Na) and p-benzoquinone (pBQ) were deliberately introduced in the MO solutions containing Rh0.15wt%-Cu2O catalyst before irradiation as scavengers for ·OH, h+, and ·O2-, respectively.37-38 The MO concentration change for these solutions under irradiation was shown in Figure 9A. The photodegradation of MO was completely inhibited upon the addition of pBQ and EDTA-2Na, indicating that both ·O2- and h+ were responsible 19



for the photodegradation of MO on Rh0.15wt.%-Cu2O. In contrast, the decrease in the intensity of C/C0 with IPA addition was much lower than that with pBQ, implying that only small amounts of ·OH participated in the degradation process. In addition, considering that ·OH and ·O2- could be correlated with the O2 dissolved in solutions, the reactions under different atmospheres such as air-, O2- and N2-saturated conditions were investigated, and results were given in Figure 9B. The photo-induced electrons trapped by O2 dissolved in solution could form ·O2- superoxide radicals, and which could subsequently transform into ·OH radicals. As mentioned above, both ·O2and ·OH are highly reactive and capable to convert most organic waste materials into low toxicity inorganic small molecules. Therefore, in Figure 9B, when deliberately introducing O2 in solution, the performance of photodegradation considerably enhanced compared to air, however, after the O2 elimination by bubbling with high purity nitrogen gas, the efficiency of MO degradation was inhabited. These results indicated that the introduction of O2 into solution was also an important factor to facilitate photocatalytic conversion, in our case, air-saturated condition was employed to estimate photocatalytic performance in view of the condition was more convenient for application. Based on above investigations, the comprehensive photocatalytic screen of Rh-Cu2O catalyst prepared in this work was given in Scheme 2. The cubic Cu2O substrate (about 300 nm) employed in this work had a narrower band gap around 2.06 eV, typically, the valence band (VB) edge position and the conduction band (CB) edge position were located at 0.45 eV and -1.60 eV, respectively.39 The work 20


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functions of Rh and Cu2O were 4.98 and 5.27 eV, on similar metal-semiconductor interface,35,

40

electrons will transfer from metal to semiconductor until

thermodynamic equilibrium is established, as a result, an inner electric field would form along metal-semiconductor interface with metal in positively-charged or electron-deficiency state; meanwhile, the Femi level of metal will be descended, and the Femi level of semiconductor will be raised up; so the energy band at the interface is curved, and the conduction band (CB) edge of semiconductor will be higher than the

Femi

level

of

the

thermodynamic

equilibrium

state.

When

such

metal-semiconductor interface are excited by light irradiation, photoinduced electron-hole pairs are generated; the electrons in the CB of semiconductor would migrate to metal driven by the inner electric field, whereas the electrons on metal cannot be transferred to semiconductor due to the Schottky barrier, leading to the enhanced separation efficiency of photo-produced electron/hole charge pairs.31, 35 In our case, an intensive inner electric field could form along Rh-Cu2O interface, since the highly dispersed and tightly integrated Rh-Cu2O interface was evident by the oxidatively-bonded Rh species formed within Cu2O surface layer; along such interface, the Rh species in electron-deficiency state would facilitate the quick migration of photogenerated electrons (e) from Cu2O,7-8, 41 the transferred electrons could be captured by O2 in water to generate ·O2- radicals, which would act as vigorous oxidants to oxidize MO to its degradation products. In addition, the photogenerated holes (h+) left on Cu2O could also oxidize MO because the energy level of valence band of Cu2O (0.45) is more positive than that of the highest 21



occupied molecular orbital (HOMO) of MO (0.14);42 as far as ·OH radicals, instead of the low-possibility to produce ·OH by h+ and water due to the higher potential of H2O/·OH (2.73 V vs. NHE) required for the transformation,43 the ·OH radicals present on Rh-Cu2O could be more likely to be derived from the reaction of ·O2- with H+.44 Above charge carrier transfer and MO degradation processes on current Rh-Cu2O catalyst can be described as following network: Cu2O + hv  Cu2O (h+)+ Cu2O (e) e+ Rh  Rh (e) O2 + Rh (e)  ·O2- + Rh ·O2- + H+  ·HO2; 2·HO2  H2O2 + O2; H2O2 + e  ·OH + OH·O2- (·OH or h+)+ MO  CO2 + H2O In view of the ·OH way involved more steps in comparison with ·O2- or h+ way for oxidizing MO, thus only small amounts of ·OH participated in the degradation process as disclosed by scavenger-present measurements (Figure 9A) over Rh-Cu2O catalyst.

4 Conclusions The highly-dispersed and tightly-integrated Rh-Cu2O interface, present by the structure that oxidatively-bonded Rh within Cu2O surface layers, was feasibly constructed through redox embedding proper amount of Rh into Cu2O substrate. Along the interface, the Rh species in electron-deficiency state would facilitate the quick migration of photogenerated electrons (e) from Cu2O, which was also benefit to lower the charge transferring resistance of composited interface, these advances 22


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conjunctively contributed the great improvements for Cu2O substrate not only in enhancing separation efficiency of photo-generated electron/hole pairs but also in speeding charge transferring of whole interface, resulting in the prominent photocatalytic enhancements. Current work demonstrated that the catalytic feature of Cu2O could be feasibly adjusted from the surface engineering strategy as the construction of efficient charge-separation-transferring interface by redox embedding Rh, which could be referred as an important system for designing advanced Cu2O-based materials for corresponding heterogeneous catalysis as well as photo-electronic conversion displayed here.

Acknowledgements. This work was financially supported by the National Natural Science Foundation of China (NSFC, No.21003071, No.21563018 and No.21663016) and Doctoral Fund of Ministry of Education of China (No.20093601120007).

Supporting Information Available: Standard electrode potential of redox pairs for [MClxy-/M] (M=Au, Pt, Pd, Rh) and [Cu2+/Cu2O], surface composition of Cu species from Cu-LMM Auger spectra, EIS simulated parameters, Line scanning for an individual Rh0.15wt.%-Cu2O particle, SEM image of Pt/Pd-Cu2O, Cu 2p XPS spectra and UV-vis spectra of Rhx-Cu2O, Rh 3d XPS spectra of Rh0.15wt.%-Cu2O and Rh0.20w.t%/TiO2, and Rh 3d XPS spectra of Rh0.15wt%-Cu2O before and after photocatalytic cycle use. This material is available free of charge via the Internet at http://pubs.acs.org.

23



Page 24 of 40

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Scheme 1. Preparation of Cu2O with oxidatively-bonded Rh in surface layers from RhCl63- etching-embedding route.

29



Scheme 2. Photocatalytic screen of Rh-Cu2O (Cu2O with oxidatively-bonded Rh in surface layers) catalysts.

30


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Figure 1. XRD patterns of samples. a. Cu2O, b. Rh0.04wt.%-Cu2O, c. Rh0.15wt.%-Cu2O, d. Rh0.38wt.%-Cu2O.

31



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Figure 2. Morphological features of samples: (X-1) SEM image for general view; (X-2) TEM image for general view; (X-3) HRTEM image of particle edge; (X-4) Dark-field image of single particle; (X-5) Cu element-distribution-mapping; (X-6) Rh element-distribution-mapping (O-element-distribution-mapping for Cu2O). (X=A, B) A. Cu2O, B. Rh0.15wt.%-Cu2O

A-1

A-4

A-3

A-2

A-6

A-5

O

Cu B-1

B-4

B-3

B-2

B-5

B-6

Cu

32


Rh

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Figure 3. Surface state measurements of samples. (A) Cu-LMM Auger spectra; (B) Rh 3d XPS spectra. a. Cu2O, b. Rh0.04wt.%-Cu2O, c. Rh0.15wt.%-Cu2O, d. Rh0.38wt.%-Cu2O

A A

B A

33



Figure 4. Zeta potential alternation curves of samples. (A) Cu2O modified by different Rh loadings; (B) Influence of solution pH on surface potential of Cu2O and Rh0.15wt.%-Cu2O samples.

A A

B A

34


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Figure 5. Photo-degradation measurements of methyl orange (MO). (A) Dependence of degradation percentage ((1-C/C0)*100) on irradiation time over different catalysts; (B) Dependence of ln(C0/C) on reaction time over samples. o. Degradation without catalyst under light-on condition, c0. Light-off degradation equilibrium curve over example catalyst Rh0.15wt.%-Cu2O, a. Cu2O, b. Rh0.04wt.%-Cu2O, c. Rh0.15wt.%-Cu2O, d. Rh0.38wt.%-Cu2O

A A

B A

35



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Figure 6. Photo-degradation cycle measurements on Rh0.15wt.%-Cu2O (A) and Rh0.20wt.%/TiO2 (B).

A A

B A

36


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Figure 7. The general comparison of photocatalytic performance between Cu2O and Rh0.15wt.%-Cu2O for photo-degradation of methyl orange (MO) or Congo red (CR), and photo-production of H2 with formic acid (FA) or triethylamine (TEA) as sacrificial reagents.

37



Figure 8. (A) Photoelectrochemical properties of samples: amperometric I-t curves at an applied potential voltage of 0 V under visible-light (λ≥420nm) irradiation with 10 s light on/off cycles. (B) Nyquist plots of samples in reaction solution. a. Cu2O, b. Rh0.04wt.%-Cu2O, c. Rh0.15wt.%-Cu2O, d. Rh0.38wt.%-Cu2O

A A

B B AA

38


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Figure 9. Photocatalytic degradation of MO with Rh0.15wt.%-Cu2O under modulated conditions. (A) Adding different trapping agents in reaction solution: isopropanol (IPA) for ·OH, p-benzoquinone (pBQ) for ·O2- , EDTA-2Na for h+; (B) Saturating reaction solutions with O2, air and N2.

A A

B A

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TOC graphic

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Construct Highly-Dispersed and Efficient Charge-Separation

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