Facile electrochemical deposition of Cu₇Te₄ thin films with visible-light driven photocatalytic activity and thermoelectric performance

Abstract

We report a facile strategy to fabricate thin films of Cu₇Te₄ via a two-electrode galvanic method and demonstrate its application as a recyclable photocatalyst with visible-light-driven photocatalytic activity and photostability. The organic dyes Methylene Blue (MB), and Rose Bengal (RB) and Cr(VI) as a toxic metal ion were chosen to explore the photocatalytic performance. MB was completely degraded (97%) under visible light irradiation in 60 min while RB was degraded up to 92% within 90 min irradiation. Furthermore, the Cu₇Te₄ thin film also showed superior photoactivity to reduce toxic Cr(VI) to Cr(III). The effective removal of Cr(VI) up to 99.8% at pH 2 was observed in 30 min. The related photocatalytic mechanism is discussed based on the active species trapping experiments. In addition, the Cu₇Te₄ thin film shows improved thermoelectric performance at room temperature, which may be attributed to the lower value of thermal conductivity obtained in the present case.

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1 Introduction

Copper telluride (Cu_xTe) is an I–VI compound semiconductor with different crystal structures depending on the value of x. Copper tellurides can exist as stoichiometric CuTe, Cu₂Te, stoichiometric Cu-rich Cu₃Te₄, Cu₇Te₄, Cu₇Te₅, nonstoichiometric Cu_2−xTe, and so on. According to the stoichiometry, the bandgap of the Cu_xTe has different values, ranging from 1.0 to 1.5 eV.^1,2 These compounds have attracted considerable research interest because of their potential applications in various fields, such as as back contact materials in CdS/CdTe solar cells, anodes in lithium ion batteries, memory devices, photodetectors, gas sensing, SERS probes and in photothermal therapy.^3–8 Previous studies on copper tellurides were mainly on their electrical properties. There are fewer reports about the thermoelectric property of copper tellurides. Applications of this group of materials in photocatalysis have not been explored earlier, although it is strongly desired from their band positions. Semiconductor photocatalysts have attracted considerable attention due to their wide applications such as in the degradation of organic dye pollutants, reduction of toxic metals.^9–11 TiO₂, ZnO have been proven to be promising photocatalyst for common environmental applications due to its interesting optical and electrical properties, low cost, and ease of availability.^12–14 However, their universal uses are limited to ultraviolet (UV) light due to their wide band gap. As compared to visible light, UV light is a minor component of sunlight which is only about 4% of total solar radiation. Many promising studies have been reported to develop photocatalyst that respond to visible light.^15–18 In this work, we have studied the photocatalytic property of Cu₇Te₄ thin films to degrade organic pollutant dyes under visible light irradiation. Apart from dye pollutants, toxic metal like chromium gets introduced into the environment by discharges from industries, such as leather tanning, steel production, electroplating, as well as from pigment and refractory industries.¹⁹ The increasing level of Cr(VI) metal ion in the environment is a great concern to the societies and regulation authorities around the world. Chromium occurs in two common oxidation states in nature, Cr(VI) and Cr(III). Hexavalent chromium ion is dreadfully toxic and carcinogenic and many times higher toxic than Cr(III).^20,21 In addition, Cr(VI) is mobile in nature because of its weak absorption to inorganic surfaces.²² On the other hand, less harmful Cr(III) can readily be precipitated as Cr(OH)₃ in neutral or alkaline solutions. U.S. Environmental Protection Agency (USEPA) have set the following limits for chromium discharges: 170 mg L⁻¹ of Cr(III) and 0.05 mg L⁻¹ of Cr(VI) and the current EPA maximum contaminant level for chromium in drinking water is 0.1 mg L⁻¹. The solubility of Cr(OH)₃ keeps chromium concentrations below the drinking water limit at pH 6–12. The methods for the removal of Cr(VI) include adsorption,²³ biosorption,²⁴ electrocoagulation,²⁵ ion exchange,²⁶ membrane filtration.²⁷ However, these technologies have some disadvantages, such as secondary pollution in cleaning step, high power consumption and expense for operation and maintenance. Therefore, the development of cost-effective and low energy consuming technology for the removal of Cr(VI) is highly desired. In particular, photocatalytic reduction of Cr(VI) to Cr(III) using the semiconductor photocatalysis as green technology has received considerable attention.^28,29 These materials are known to exhibit appreciable thermoelectric figure of merit (ZT) defined as:

ZT = (S²σ/κ)T = (P/κ)T

where S is the thermoelectric power, σ is the electrical conductivity, κ is the thermal conductivity, P is the power factor and T is the absolute temperature. ZT is enhanced in nanostructured materials due to increase in the selective scattering of phonons consequently decreasing the thermal conductivity (κ) maintaining the value of electrical conductivity (σ) and thermoelectric power (S). There are a few reports available on thermoelectric performance of copper telluride such as Cu_2−xTe,³⁰ direct annealed Cu₂Te,³¹ copper telluride nanosheets,³² Copper telluride pseudo binary system,³³ mechanically alloyed Cu₂Te.³⁴

In light of the above mentioned, the goal of the present work is to develop a copper telluride film which can show dual functional property. Band gap and band position of copper telluride compound make it suitable for dual functional properties. There are several reports of different semiconducting material based photocatalyst which can utilize natural solar light and show high photocatalytic performances. There are no reports of photocatalytic material that can also shows good thermoelectric property. There is no report in literature of copper telluride materials as photocatalyst. However, there are few reports of this material showing thermoelectric property^31–34 but the property is still not widely explored. In this paper, we have studied the photocatalytic and thermoelectric properties of electrochemically synthesized Cu₇Te₄ thin films. Copper telluride films can be grown by various methods like electrodeposition method,^35,36 microwave-assisted solvothermal method³⁷ and solvothermal reaction.^2,6 In this paper, we report a simple, inexpensive two electrode electrochemical route called the galvanic technique to deposit thin films of Cu₇Te₄. This technique has previously been successfully used in our laboratory to deposit various other thin film semiconductors.^38–40 Photocatalytic oxidation and reduction of toxic chemicals on the Cu₇Te₄ film surfaces have been studied and thermoelectric measurements were carried out to find the ZT value.

2 Experimental details

2.1 Chemicals

Cu(NO₃)₂·3H₂O, TeO₂ and C₁₀H₁₄N₂Na₂O₈·2H₂O (Na₂EDTA) were purchased from MERCK. 30% w/v H₂O₂, AgNO₃, p-benzoquinone (BQ), sodium oxalate (SO) were obtained from Fisher Scientific, tert-butanol (TBA) from SDFCL and potassium persulfate (K₂S₂O₈) from Sigma Aldrich. All the chemicals were used without further purification. Millipore water was used in all experiments.

2.2 Cell set up for the galvanic deposition of Cu₇Te₄ thin film

First of all, the FTO glass substrates (conducting layer: fluorine doped tin oxide) were cleaned with detergent and dipped into chromic acid solution for about 10 minutes and washed thoroughly with cold distilled water. Finally, they were boiled in water to remove any adhering impurities and successively in methanol for fast drying. A properly cleaned FTO glass substrate and a Sn foil were clamped vertically and dipped into 10⁻³ (M) Cu(NO₃)₂·3H₂O and 3 × 10⁻⁴ (M) TeO₂ solution containing 1 × 10⁻⁴ (M) solution of Na₂EDTA as a complexing agent. The total volume of the working solution was maintained at 100 ml with distilled water and the pH of the working solution was adjusted to 2.0 with sulphuric acid. Sn foil and FTO glass were short-circuited externally with Cu wire. The deposition was carried out at room temperature under continuous stirring, using a magnetic stirrer.

2.3 Materials characterizations

The structure, composition and morphology of the films were determined by X-ray diffraction (Seifert P3000, Cu Kα radiation λ = 1.54 Å), energy dispersive X-ray analysis (EDX, BRUKER) and field-emission SEM (FESEM, Carl-Zeiss-SIGMA). The optical properties of the film and photocatalytic measurements were carried out by a ‘JASCO V-530’ UV-VIS spectrophotometer. The thicknesses of the films were measured using a profilometer (BRUKER, DEKTAK-XT).

2.4 Photocatalytic properties

The photocatalytic performances of the as-prepared Cu₇Te₄ samples were evaluated through the photocatalytic degradation of RB, MB dyes and photo reduction of aqueous solution of Cr(VI) individually. Photocatalytic degradation of RB and MB dyes were studied with the assistance of H₂O₂. The working solution was prepared by adding 10 ml of 10⁻⁴ (M) dye solution and 1 ml of H₂O₂ (30% W/W) to make the total volume 60 ml with deionized water. To study the photocatalytic reduction of Cr(VI), 10⁻⁵ (M) solution of the pollutant was taken initially. Four FTO substrates (surface area of 10 square cm each) covered with the as-prepared Cu₇Te₄ (thickness 100 nm) were placed in the pollutant solution. Prior to irradiation, the solution was magnetically stirred in the dark for 30 min to establish absorption–desorption equilibrium. Irradiation of light was carried out using a 200 W tungsten lamp (≥410 nm) and a 1 M solution of NaNO₂ was used as the UV cutoff filter.⁴¹ The tungsten lamp was placed vertically over the reaction vessel at a distance of 10 cm. The optical irradiance at the surface of pollutant solution was kept at 70 mW cm⁻². At regular time intervals, 3 ml of the aliquot solution was taken out for measurement by UV-Vis spectrophotometry.

2.5 Thermoelectric properties

DC electrical conductivity (σ) and thermoelectric power (S) of thin film of Cu₇Te₄ were measured within the temperature range 290–400 K using four probe method with a PID controlled oven (Scientific Equipments, Roorkee, India) and Hewlett-Packard data acquisition system 34970A respectively. Room temperature thermal conductivity (κ) has been taken in a Hot disk thermal constant analyzer (TPS 2500S, Sweden). The carrier concentration (n) of thin film of Cu₇Te₄ has been estimated using Hall Effect measurements.

3 Results and discussion

3.1 Growth mechanism

Sn being easily oxidisable, dissolves into the solution forming Sn²⁺ ions. The two electrons released moves through the externally short circuited path to the FTO electrode (acts as cathode). The Cu²⁺ and HTeO₂⁺ ions present in the solution are attracted to the cathode to take electrons and get deposited as Cu₇Te₄ on FTO.

At anode: Sn → Sn²⁺ + 2e⁻ (E⁰_ox = +0.136 V) (1)

At cathode: Cu²⁺ + 2e⁻ → Cu (E⁰_red = +0.337 V) (2)

HTeO₂⁺ + 3H⁺ + 4e⁻ → Te + 2H₂O (E⁰_red = +0.559 V) (3)

Overall: 7Cu + 4Te → Cu₇Te₄ (4)

E⁰_ox represents oxidation potential and E⁰_red represents reduction potential with respect to NHE. The potential values are the confirmations of spontaneity of the reaction shown in eqn (4). Composition of the working solution was optimized to get the desired stoichiometry of copper telluride as Cu₇Te₄. Na₂EDTA solution was added to control the reaction kinetics. pH of the solution was kept acidic due to prospective desire of deposition of HTeO₂⁺ to Te (Fig. 1).

Fig. 1 Schematic representation of ‘Galvanic Deposition’ set up.

3.2 Structural characterization

XRD analysis. The crystal structure of the products was investigated by the X-ray diffraction (XRD) analysis, as shown in Fig. 2. The XRD pattern indicates the formation of polycrystalline Cu₇Te₄. The diffractions were observed from (002), (102), (202), (212), (220), (302) and (400) planes which are readily indexed to the hexagonal phase and P3m1 space group of Cu₇Te₄ according to the JCPDS card no. 18-0456. No characteristic peaks of any impurities are detected in the pattern. The symbol ‘*’ represents diffractions from FTO substrate.

Fig. 2 XRD pattern of Cu₇Te₄ thin film.

3.3 EDX analysis & FESEM images

To further determine the composition of the products, elemental analysis was done by energy-dispersive X-ray analysis (EDX). The EDX spectrum (Fig. 3(a)) taken from FESEM indicates that the film is composed of only two elements, Cu and Te. The atom ratio of Cu and Te in the film is approximately 1.75 : 1, which is consistent with the observation from XRD. Other labeled peaks of O, F, Sn and Au arrive from FTO glass substrate and metal contact. It is evident from the spectrum that the film is completely pure and no impurity was detected. Fig. 3(b) and (c) represent the surface morphology of Cu₇Te₄ thin film. A compact surface morphology with overgrowth of clustered nanospherical crystallites creating mesopores among the agglomerates is visible from the images. The average grain size found to lay in the range 25–35 nm.

Fig. 3 (a) EDX spectrum (b) & (c) FESEM images of Cu₇Te₄ thin film at two different magnifications.

3.4 Optical characterization

Copper telluride is a well-known p-type semiconductor possessing a direct band gap; its band gap energy varies with the change of stoichiometry and phase. UV-Vis-NIR absorption spectrum of the Cu₇Te₄ thin film was taken to determine the band gap energy of the film. Fig. 4 represents the UV-Vis-NIR spectrum of the as-deposited Cu₇Te₄ thin film. The band gap value was calculated from the classical Tauc plot method. Fig. 4 (inset) shows the corresponding plot of (αhν)² vs. hν based on direct transition. The estimated value of hν at α = 0 gives direct band-gap of 1.1 eV for Cu₇Te₄. This value is closed to the reported band gap of 1.0–1.5 eV of the copper telluride group (Cu_xTe) compounds.^1,2

Fig. 4 UV-Vis-NIR spectrum of Cu₇Te₄ film (inset shows corresponding Tauc plot showing direct bandgap of 1.1 eV).

3.5 Photocatalytic degradation of organic dyes

Photocatalytic degradation of methylene blue (MB) and Rose Bengal (RB) pollutants were investigated under visible light irradiation. Fig. 5(a) and 6(a) show the photocatalytic activities of Cu₇Te₄ thin film catalysts under various conditions. When only the catalyst films were used in the system, only about 56% MB and 46% RB were degraded after 60 min and 90 min respectively. The photodegradation efficiencies of MB and RB reach about 97% and 92% after 60 min and 90 min, respectively, when H₂O₂ was used in the reaction medium. The kinetics of MB and RB degradation under visible light irradiation were deduced based on the spectral changes and are presented in Fig. 5(b) and 6(b), which depict first-order nature of the photocatalytic degradation kinetics with the Cu₇Te₄ thin films. The first-order rate constant k were estimated as 5.7 × 10⁻² min⁻¹ with MB and 2.8 × 10⁻² min⁻¹ with RB in presence of H₂O₂, indicating that Cu₇Te₄ thin film is more suitable for the photocatalytic degradation of MB than RB under visible light irradiation. Electrons in CB and holes in VB are generated in the catalyst film by the photo excitation. MB and RB are also excited under visible light irradiation to MB* and RB*, followed by photo-induced electron transfer from MB* and RB* to CB of Cu₇Te₄ thin film, which react with adsorbed species, usually O₂, to produce reactive superoxide radical anion (˙O₂⁻). CB of Cu₇Te₄ lies higher than the reduction potential of O₂/˙O₂⁻ (+0.07 V).^42,43 O₂ can easily take up electrons from the CB of Cu₇Te₄ and forms superoxide radical anion (˙O₂⁻). Superoxide radical anion is a very reactive species and readily reacts with water to give H₂O₂.⁴⁰ Photo oxidation and photo reduction of H₂O₂ take place with photo generated electrons and holes at the catalyst surfaces, resulting in the formation of oxidant species ˙OH which degrade the organic dye molecules into small colourless degraded products. Addition of H₂O₂ enhances the degradation rate which also supports the proposed degradation mechanism. The proposed photocatalytic degradation mechanism follows the steps:^39,40

Dye* → dye⁺ + (e_CB⁻)Cu₇Te₄ (7)

(e_CB⁻)Cu₇Te₄ + O₂ → ˙O₂⁻ (8)

H₂O + ˙O₂⁻ → ˙OOH + OH⁻ (9)

˙OOH + H₂O → ˙OH + H₂O₂ (10)

H₂O₂ + e⁻ → ˙OH + OH⁻ (11)

H₂O₂ + h⁺ → ˙OOH + H⁺ (12)

H₂O₂ + ˙OOH → ˙OH + H₂O + O₂ (13)

˙OH + dye⁺ → colourless degraded products (14)

Fig. 5 (a) Reaction profile of MB degradation in an aqueous solution against specific time intervals under various conditions. (b) First-order kinetic plot of ln(C₀/C_t) vs. time of MB degradation in presence of Cu₇Te₄ thin films with and without H₂O₂.

Fig. 6 (a) Reaction profile of RB degradation in an aqueous solution against specific time intervals under various conditions. (b) First-order kinetic plot of ln(C₀/C_t) vs. time of RB degradation in presence of Cu₇Te₄ thin films with and without H₂O₂.

To evaluate the reusability of the Cu₇Te₄ film photocatalyst, four cycles have been performed, as shown in Fig. 7. No significant change in photocatalytic efficiency indicates its suitability as a reusable photocatalyst for the photodegradation reactions. Fig. 8 represents the schematic for the photocatalytic degradation mechanism. In the photocatalytic process, the main active species can be detected by adding tert-butanol (TBA) as OH˙ scavenger, Sodium Oxalate (SO) as h⁺ scavenger, AgNO₃ as e⁻ scavenger and p-benzoquinone (BQ) as ˙O₂⁻ scavenger. This is crucial for elucidating the photocatalytic mechanism. As shown in Fig. 9, the additions of different scavengers induce different extent of change in MB and RB photodegradation. The photocatalytic degradation of MB and RB were greatly suppressed by the addition of e⁻, h⁺, OH˙ and ˙O₂⁻ scavengers, indicating their important roles in the degradation process.

Fig. 7 Relative dye concentration versus light exposure time for four consecutive cycles of operation with the Cu₇Te₄ catalyst films.

Fig. 8 Schematic representation of photocatalytic mechanism.

Fig. 9 Reaction profile of photo catalytic degradation of (a) MB (b) RB as a function of irradiation time in presence of Cu₇Te₄ thin films and different scavengers without the aid of H₂O₂.

3.6 Photocatalytic reduction of Cr(VI)

Apart from dye degradation reactions, the as-prepared Cu₇Te₄ thin films were found to have effective application in the reduction of another water pollutant, Cr(VI). Cr₂O₇²⁻/Cr³⁺ reduction potential is less negative than the CB of Cu₇Te₄ (ref. 43) and thus the photoreduction of Cr₂O₇²⁻ to Cr³⁺ is thermodynamically feasible on the Cu₇Te₄ thin film surface. The effective removal of Cr(VI) up to 99.8% at pH 2 was observed in 30 minutes (Fig. 10(a)). The plot ln(C₀/C_t) vs. irradiation time expresses a linear behavior as shown in inset of Fig. 10(a). The reduction of Cr(VI) follows the pseudo first order reaction. From the slope of the linear plot, high rate constant value of 1.4 × 10⁻¹ min⁻¹ has been calculated. Cr(VI) is reduced to Cr(III) by the photo-generated electrons as follows,

Cr₂O₇²⁻ + 6e⁻ + 14H⁺ → 2Cr³⁺ + 7H₂O

Fig. 10 (a) Reaction profile for the photocatalytic reduction of Cr(VI) in presence of Cu₇Te₄ catalyst films under visible light irradiation, inset: corresponding linear plot of ln(C₀/C_t) vs. time evaluating the rate constant of the Cr(VI) reduction reaction in presence of catalyst films. (b) Relative dye concentration versus light exposure time for four consecutive cycles of operation of the catalyst films.

To support the role of the photogenerated electrons for the photoreduction of Cr(VI) over the Cu₇Te₄ thin films, the effect of adding electron scavenger K₂S₂O₈ on the photocatalytic activities were analyzed. As shown in Fig. 10(a), the addition of K₂S₂O₈ almost terminated the Cr(VI) reduction process.⁴⁴ The compared results evidently demonstrate that the photogenerated electron in the catalyst film indeed contributes to the photocatalytic reduction of Cr(VI). To evaluate the reusability of the Cu₇Te₄ film photocatalyst, four cycles of photocatalysis of Cr(VI) aqueous solutions have been performed, as shown in Fig. 10(b). No significant change in photocatalytic efficiency indicates its suitability as a reusable photocatalyst to reduce Cr(VI) to Cr(III).

3.7 Thermoelectric property

Fig. 11(a) and (b) show the variation of electrical conductivity and thermoelectric power with temperature in the temperature range 290–400 K respectively. With the variation of temperature electrical conductivity increases and thermoelectric power decreases which denotes semiconducting type of nature of the thin film of Cu₇Te₄. At room temperature the film of Cu₇Te₄ shows p-type of conduction and here holes are the dominant charge carriers. After temperature 345 K, electrons play a dominant role in the conduction process. Fig. 11(c) shows the variation of power factor (P) with temperature. The room temperature value of power factor is 4.1 μW cm⁻¹ K⁻² which is comparable to the value obtained for spark plasma sintered (SPS) and annealed Cu₂Te.³¹ In the present work the value of thermal conductivity (κ) at room temperature is 0.94 W mK⁻¹, which is lower than the reported values (2.1 W mK⁻¹), (1.5 W mK⁻¹), (1.4 W mK⁻¹), (1.35 W mK⁻¹) (1.8 W mK⁻¹), (1.95 W mK⁻¹),³¹ (1.25 W mK⁻¹)³² and (5 W mK⁻¹).³³ So room temperature figure of merit (ZT = 0.13), is substantially enhanced compared to solvothermal synthesis (ZT ∼ 0.00012)³² and high energy SPS process (ZT ∼ 0.04).³¹ All the room temperature parameters (σ, S, κ, P, ZT and n) are presented in a tabular form in Table 1. The carrier concentration (n) obtained from Hall-effect measurement is shown in Fig. 11(d) with variation of temperature. The increase in the carrier concentration supports the variation of electrical conductivity and subsequently the thermoelectric power with temperature.

Fig. 11 Temperature variation of (a) electrical conductivity (σ), (b) thermoelectric power (S), (c) power factor (P) and (d) carrier concentration (n) of Cu₇Te₄ thin film.

Table 1 Comparison of room temperature transport properties (σ, S, κ, P, ZT and n) with other literatures

Synthesis procedure	σ (S cm^−1)	S (μV K^−1)	κ (W mK^−1)	P (μW cm^−1 K^−2)	ZT	n (/cm^3)	Reference
Cu7Te4 galvanic deposition	2550	+40	0.94	4.1	0.13	5.08 × 10^19	Our work
Cu2−xTe, 0 < x < 0.03	500	+25	—	—	—	∼10^20	30
	800	+22
	1200	+16
	5500	+5
Cu2Te, SPS (annealed at different temperature)	4200	+25	2.1	2.6	0.04 annealing temperature	∼10^20 to 10^22	31
	4000	+32	1.5	4.1	0.08 (753 K)
	4000	+30	1.4	3.6	0.08 (793 K)
	3400	+38	1.35	4.9	0.11 (833 K)
	2500	+42	1.8	4.4	0.07 (953 K)
	2000	+40	1.95	3.2	0.05 (1003 K)
Cu1.75Te nanosheets solvothermal	1250	+2	1.25	0.05	0.00012	—	32
Cu2Te based binary system	1000	+10	5	0.1	0.0006		33
Cu2Te mechanical alloying	—	+10–40	—	—	—	—	34

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4 Conclusions

High-efficiency visible-light-driven Cu₇Te₄ thin film photocatalyst has been synthesized by a facile electrochemical process. The as-deposited Cu₇Te₄ thin films exhibited excellent photocatalytic activity for the decolorization of MB over RB. H₂O₂ played an important role on the photocatalytic activity of Cu₇Te₄ thin films. Being a narrow band gap material, there is potential for the Cu₇Te₄ to be an effective natural sunlight-driven photocatalyst. Cu₇Te₄ thin films showed excellent photoactivity to reduce Cr(VI) to Cr(III). Thin films can reduce thermal conductivity while maintaining the electrical conductivity and hence an increase in the figure of merit, ZT (0.13) can be observed. With the variation of temperature the carrier concentration of the thin film is found to be enhanced leading to improvement in electrical conductivity and reduction in thermopower. On the basis of the results of this study, the Cu₇Te₄ thin films are expected to be effective as a useful visible-light photocatalyst for practical applications as well as an efficient thermoelectric material.

Acknowledgements

The authors A. Ghosh and M. Mitra are thankful to University Grants Commission, India (SRF-NET) and DST-INSPIRE (IF 130168) respectively for their research fellowships. Authors also acknowledge All India Council for Technical Education and U.G.C.-S.A.P. (India) for providing instrumental facilities to the Department of Chemistry, IIEST, Shibpur, India. A. M. acknowledges DST-SERI, India for providing XRD facility.

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