Photovoltaic performance enhancement of Cu₂O photocathodes by electrostatic adsorption of polyoxometalate on Cu₂O crystal faces

Abstract

A novel incorporation of polyoxometalate (POM) into Cu₂O photocathodes was achieved by electrostatic adsorption of polyoxometalate on Cu₂O crystal faces, so that the Cu₂O/POM composite films exhibit significant photovoltaic enhancement because the POM could act as electron-acceptor to retard the electron–hole recombination and facilitate the photoexcited electron transfer.

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Cuprous oxide (Cu₂O), an important low-cost and environment-friendly semiconductor with a direct band gap 2.1 eV and relatively high absorption coefficient in the visible region, has attracted much attention because of its extensive applications in fields such as gas sensors, photocatalysis, water splitting, lithium ion batteries, optical devices, and solar cells.¹ However, the dissatisfactory power conversion efficiency really hinders its progress in practical applications. Generally, the efficiency is greatly limited by both the high carrier (electron and hole) recombination in the bulk and the low electron mobility of Cu₂O. In order to solve these problems, many attempts have been carried out such as controlling morphology, doping, combination with other materials, etc.² In particular, Cu₂O modified with other materials for improving power conversion efficiency has attracted much attention.³ Huang et al. developed the Au–Cu₂O core–shell heterostructures and found that the incorporation of Au nanocrystal cores into Cu₂O could evidently enhance the photocatalytic activity of Cu₂O nanocrystals.⁴ Ogale et al. reported the use of the Pulsed Laser Deposition (PLD) technique to deposit ZnO layer on Cu₂O film, so that an enhancement of photocatalytic activity was achieved.⁵ Recently, Huang's group reported on the synthesis of sharp-faced Cu₂O crystals with positively charged {111} and {110} faces, and then the positively charged Cu₂O crystal faces were employed to enhance the photocatalytic activity of photodegradation of methyl orange.⁶

Polyoxometalates (POMs), a class of molecular metal–oxo cluster compounds based mainly on Mo, W, V, Nb, and Ta elements, have shown fine physicochemical properties and various applications.⁷ In particular, POMs have an intrinsic characteristic of electron acceptor, which is capable of capturing the photogenerated electrons from the conduction band (CB) of semiconductors and thus promote power conversion efficiency of semiconductor. Ozer and Ferry investigated the use of POMs in TiO₂ suspensions to increase the photocatalytic efficiency on the degradation of 1,2-dichlorobenzene.⁸ Yoon et al. reported that phosphotungstic acid played a vital role in improving energy conversion efficiency of TiO₂ photoanode.⁹ In recent five years, our group carried out several researches on the incorporation of POMs into semiconductor films to enhance the photoelectrochemical performance of semiconductors.¹⁰ These above-mentioned results indicated that the modification of semiconductor materials with POMs is an effective strategy to promote their photoelectrochemical performance. Since we knew the current problem that Cu₂O as a semiconductor generally possesses both the high electron–hole recombination and the low electron mobility, we have certainly considered that POMs should also be possible to modify Cu₂O photocathodes so as to enhance the photovoltaic response. However, no demonstration of modifying Cu₂O with POMs has been reported up to now.

Herein, we report for the first time on the incorporation of polyoxometalate [(CH₃)₄N]₅PW₁₀Mo₂O₄₀·4H₂O]¹¹ (denoted as PW₁₀Mo₂) into Cu₂O film to improve the photovoltaic performance of the PW₁₀Mo₂/Cu₂O composite film photocathode. By means of the Cu₂O crystals prepared according to the literature method,⁶ a novel approach of electrostatic adsorption was used in the preparation of the PW₁₀Mo₂/Cu₂O composite in which the PW₁₀Mo₂/Cu₂O composite particles were made firstly by electrostatic interactions between the [PW₁₀Mo₂O₄₀]⁵⁻ anions of negative charge and the Cu₂O crystal faces of positive charge. Subsequently, the PW₁₀Mo₂/Cu₂O composite particles were made into the thin film on FTO glass by using a facile dropping-liquid-casting method.

The as-prepared Cu₂O pristine crystals exhibited rhombic dodecahedral and corner-truncated rhombic dodecahedral structures (Fig. 2B and ESI†). The structures of rhombic dodecahedral and corner-truncated rhombic dodecahedral Cu₂O have more active faces than other morphologies.⁶ The rhombic dodecahedral and corner-truncated rhombic dodecahedral Cu₂O oriented to show the {111} and {110} planes which were analyzed by high-resolution TEM (Fig. 2A) and XRD analysis (Fig. 3B). The {110} and {111} faces are likely more positively charged due to the high density of Cu atoms on the {110} and {111} faces; this brings an occasion towards electrostatic adsorption with negative charge [PW₁₀Mo₂O₄₀]⁵⁻ anions. As shown in Fig. 1, the formation of PW₁₀Mo₂/Cu₂O composite was realized by a self-assembly process in solution in that the negative charge [PW₁₀Mo₂O₄₀]⁵⁻ anions could be absorbed onto Cu₂O active faces by means of the electrostatic attractive force.

Fig. 1 Schematic to fabricate PW₁₀Mo₂/Cu₂O composite film by self-directed electrostatic adsorption of [PW₁₀Mo₂O₄₀]⁵⁻ on active positively charged Cu₂O faces. The composite material film was then prepared on FTO glass.

These particle morphologies and sizes were analyzed by TEM and SEM (Fig. 2 and S3 in ESI†). The observed sizes of the Cu₂O crystals appeared in the size of submicrometer or micrometer (approximately 1–2 μm). Fig. 2B shows the morphology and a smooth surface of pristine Cu₂O crystals. Fig. 2C and D exhibit the rough morphologies of the PW₁₀Mo₂/Cu₂O composite particles. Compared with Fig. 2B, both Fig. 2C and D appear in a rougher particle morphology of Cu₂O due to the surface deposition of PW₁₀Mo₂ on Cu₂O crystals. Also, the energy dispersive X-ray (EDX) analysis of the PW₁₀Mo₂/Cu₂O composite demonstrates the coexistence of PW₁₀Mo₂ and Cu₂O in the composite (Fig. 2E). In IR spectra (Fig. 3A), the peaks (curve b) at 1036, 943, 802, and 704 cm⁻¹ are assigned to ν_as(P–O), ν_as(W=O), ν_as(W–O–W) and ν_as(Mo–O–Mo) respectively, which are the characteristics of PW₁₀Mo₂. The peaks at 621 cm⁻¹ in curve a and curve b are assigned to the ν_as(Cu₂O–O). These above-mentioned results indicate that the PW₁₀Mo₂ was successfully absorbed on the active faces of Cu₂O, and the PW₁₀Mo₂ still remains its characteristic structure. The XRD (Fig. 3B) of the pristine Cu₂O particles (curve a) and PW₁₀Mo₂/Cu₂O composite particles (curve b) reveals that the diffraction peaks of PW₁₀Mo₂/Cu₂O composite particles highly match those of pristine Cu₂O particles. Because the amount of PW₁₀Mo₂ absorbed on Cu₂O crystals is rather less, the PW₁₀Mo₂/Cu₂O composite particles still retain the morphology of pristine Cu₂O crystals. Therefore, such an incorporation of PW₁₀Mo₂ with Cu₂O did not influence the crystal structure of Cu₂O.

Fig. 2 Typical HRTEM and SEM images of Cu₂O crystals (A and B) and PW₁₀Mo₂/Cu₂O composite crystals (C and D), and EDX diagram of the PW₁₀Mo₂/Cu₂O film.

Fig. 3 IR spectra (A), XRD patterns (B), current–time curves (C) and current–voltage curves (D). In IR spectra and XRD patterns, the pristine Cu₂O particles (a) and the PW₁₀Mo₂/Cu₂O composite (b). In current–time curves and current–voltage curves, the pristine Cu₂O film (a) and the PW₁₀Mo₂/Cu₂O film (b).

A three-electrode system was employed to measure the photovoltaic performance of the Cu₂O photocathodes (ESI, Fig. S4†). The photocurrent response measurements in Fig. 3C were carried out at a constant bias of 0 V (ESI†). As shown in Fig. 3C, the pristine Cu₂O film shows weak photocurrent response due to the influence of fast electron–hole recombination, while the PW₁₀Mo₂/Cu₂O film presents much higher photocurrent than the pristine Cu₂O film. Compared to the pristine Cu₂O film, the photocurrent of PW₁₀Mo₂/Cu₂O film has an increase of 179% owing to the incorporation of PW₁₀Mo₂. Since the photocurrent increase closely depends on the charge-transfer power, PW₁₀Mo₂ should be responsible for improving the photoelectrochemical performance of Cu₂O through facilitating photogenerated electron transfer and restraining electron–hole recombination.¹⁰

Furthermore, I–V curves (Fig. 3D) have been measured upon illumination. It was seen from Fig. 3D that the Cu₂O film shows a weak photoelectrochemical performance with power conversion efficiency (η) of 0.0081%, fill factor of 0.19, short-circuit current (J_sc) of 36.49 μA cm⁻², and open-circuit voltage (V_oc) of 0.16 V.¹² However, the η of PW₁₀Mo₂/Cu₂O composite film is 0.018%, with J_sc = 67.76 μA cm⁻², V_oc = 0.22 V and fill factor (ff) = 0.17. As compared to the pristine Cu₂O film, an increase of 125% in the η value was gained for the PW₁₀Mo₂/Cu₂O composite film. Such data are well in agreement with the results of the photocurrent response measurements, suggesting that POMs could act as an electron transfer mediator to improve effectively the photovoltaic performance of Cu₂O photocathode.

The surface photovoltage spectra (SPS) can provide important information about the behavior of carrier separation and transfer at the surface or interface.¹³ The SPS arises from the photoexcitation of excess free carriers (electron and hole), which causes a redistribution of carrier charges. The surface photovoltage responses of the pristine Cu₂O film and the PW₁₀Mo₂/Cu₂O film are presented in Fig. S5.† The PW₁₀Mo₂/Cu₂O film displayed the stronger SPS response than the Cu₂O film. This is attributed to the fact that under the illumination the photoinduced electrons could be trapped by PW₁₀Mo₂, and then the charge separation in Cu₂O was facilitated further.

Fig. S6† shows the fluorescence emission spectra of the Cu₂O film and the PW₁₀Mo₂/Cu₂O film. Both films have a fluorescence emission band at 390 nm. The fluorescence is clearly quenched in the PW₁₀Mo₂/Cu₂O film, though the position and shape of the band remain the same. The photoinduced electron transfer from the Cu₂O to PW₁₀Mo₂ clusters should be responsible for the fluorescence quenching.¹⁴ Therefore, such fluorescence quenching in the PW₁₀Mo₂/Cu₂O film further proved POM as electron transfer mediator to decrease the charge recombination and facilitate the electron transfer.

From the above results, the mechanism for the photocurrent generation of PW₁₀Mo₂/Cu₂O composite photocathode is proposed as illustrated in Fig. S7:† under the illumination, Cu₂O absorbs light and generates electron–hole pairs. The photogenerated electrons transfer from the CB of Cu₂O¹⁵ into the CB of PW₁₀Mo₂ (Fig. S1 in ESI†), and then the electrons transfer from PW₁₀Mo₂ to the semiconductor–electrolyte interface leading to the electrolyte. Accordingly, the holes transfer from VB of Cu₂O to the external circuits and finally transfer to the counter electrode. Evidently, PW₁₀Mo₂ played an important role in the electrons transfer process, in which PW₁₀Mo₂ functioned as “electron shuttle” to catch the photoinduced electron and subsequently transfer to next shuttle.

In summary, we have developed a unique means of making POM/Cu₂O composites by electrostatic attractive interaction in which [PW₁₀Mo₂O₄₀]⁵⁻ anions were adsorbed on Cu₂O crystal faces of positive charge. The most interesting result is the first demonstration that the introduction of POM into Cu₂O could effectively improve the photovoltaic performance of Cu₂O, because POM could act as an electron transfer mediator to retard the electron–hole recombination and facilitate the photoexcited electron transfer. This opens a new way to advance the light-to-electricity conversion efficiency of Cu₂O-based solar cells. On the other hand, such an improvement of charge separation should also be advantageous to photocatalytic hydrogen evolution (by electron) and photocatalytic oxidation (by hole). Thus, the results obtained from this study are potentially useful in many applications including photocatalysis, solar photovoltaic cells, and water splitting devices. Further research on the assembly and optimization of POM/Cu₂O photovoltaic devices instead of the photoelectrode is underway.

This work is financially supported by the Natural Science Foundation of China (Grant no. 21001021, 21273031 and 21361027).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Details of the synthesis, measurements and additional figures, fluorescence spectra, TEM, SEM, SPS, DRS, and cyclic voltammogram reflectivity. See DOI: 10.1039/c3ra44506b

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