Novel composites of graphitic carbon nitride and NiO nanosheet arrays as effective photocathodes with enhanced photocurrent performances

Abstract

Novel composites composed of graphitic carbon nitride (GCN) and p-type semiconductor of NiO nanosheet arrays were fabricated for the first time and demonstrated to be efficient photocathodes for enhancing charge separation and hole transfer. The NiO nanosheet arrays could effectively suppress the charge recombination of the GCN, and photo-generated holes could be effectively transferred to conductive substrates. The photocurrent density could be significantly improved up to −70 μA cm⁻² at 0.42 V versus a reversible hydrogen electrode (RHE) under visible light irradiation, which is nearly 60 times higher than that of pristine GCN under the same conditions. This work would open a new door for the design of highly efficient photocathode materials based on GCN.

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As the most stable allotrope in ambient conditions and a typical p-type metal-free polymeric semiconductor, graphitic-phase carbon nitride (GCN) has been widely applied in photocatalytic H₂ evolution,^1–6 the degradation of organic dyes,⁷ electrochemical sensors,^8–10 electrocatalytic oxidation,^11–14 and biomedical imaging^15,16 owing to its delocalized bandgap, thermal as well as chemical stability, and nontoxic properties.^17–19 However, its photoelectrochemical (PEC) performance for water splitting is far below that expected owing to the intrinsically sluggish kinetics of charge transfer between layers and weak adhesion on typical conductive substrates such as FTO and ITO.^20,22 To address these issues, several attempts have recently been performed to improve PEC performances. For example, Wang et al. proposed a sol processing way to obtain GCN/carbon nanotubes (CNT) nanocomposites and achieved photocurrent density improvement under visible light.²¹ Furthermore, Xie et al. demonstrated that the exfoliation of GCN into ultrathin nanosheets could promote the photoresponse.^15,16 Additionally, coupling with other semiconductor materials to develop hetero-junction is an effective way to promote charge separation and improve PEC performance. For GCN, many efforts have been devoted to suppression recombination of photo-electrons and holes by modification of various semiconductor materials.^23–26 For example, Dong et al. presented a novel GCN based photocathode with NiO electrode to facilitate PEC H₂ evolution.²⁷ Chen et al. reported the hybrid of NiO and GCN as photocatalyst for efficient methylene blue degradation.²⁸ More recently, we represented a scalable way to produce GCN nanosheets (GCNS) with efficient charge transport with respect to the bulk GCN samples.²² However, it should be noted that due to the intrinsic limitations, the PEC properties achieved by these approaches still remain unsatisfactory. Therefore, it is highly desirable to explore feasible and reliable strategies for substantially enhancing the GCN PEC performances.

The modification of p-type metal (Fe, Co, Ni) oxide or (oxy)hydroxide with reversible redox properties has been extensively employed to promote the surface charge separation as well as minimize the kinetic over potential of n-type photoanodes.^29–31 More specifically, the photoexcited holes could be effectively extracted from the semiconductor electrodes for suppressing photo-generated electron–hole recombination.^29,30,32,33 However, note that these p-type metal oxide cocatalysts were generally deposited on the surfaces of n-type semiconductors for hole transfer, their applications for facilitating the charge separation over p-type photocathode have been rarely reported. Herein, we demonstrate a simple impregnation method to fabricate a novel composite by uniformly coating GCNS on NiO nanosheet arrays. Under the visible light, the NiO nanosheets could effectively suppress the charge recombination of the GCNS, and photogenerated holes could be effectively transferred to conductive substrates through NiO nanosheet arrays. Furthermore, Mott–Schottky plots and Nyquist curves reveal that this NiO/GCNS hetero-structure could greatly increase the charge-carrier density and facilitate more efficient charge separation. As expected, their photocurrent density could be achieved up to −70 μA cm⁻² at 0.42 V vs. RHE under visible-light illumination, which is much higher than the PEC capabilities of pristine GCNS, NiO and bulk GCN under the same conditions.

Scheme 1 illustrates the synthetic process for GCNS coated on NiO nanosheet photoelectrode. Briefly, NiO nanosheet arrays grown on FTO glass were firstly fabricated through a modified hydrothermal method.³⁴ The GCNS were prepared by an acidification exfoliation process using bulk GCN as precursor, and finally dispersed on to NiO nanosheet arrays by a dip coating process. Fig. 1A, B and S1† show the typical scanning electron microscopy (SEM) images of the as-prepared NiO nanosheets and NiO/GCNS, clearly revealing that after GCNS coating, NiO nanosheet arrays have no evident structure or morphology change, which should be due to the uniform dispersion and thin thickness of GCNS. Furthermore, the transmission electron microscopy (TEM) was also performed to investigate the morphology and structure of NiO/GCNS. As shown in Fig. 1C, S2 and S3,† the sample with main sizes of hundreds of nanometers has been observed. Compared with the pure NiO (Fig. S3B†), the texture or morphology of NiO was retained after GCNS dip-coated (Fig. 1C and S2†), which is consistent with the SEM results. The HR-TEM image of NiO/GCNS sample shown in inset of Fig. 1C and S2† reveal that GCNS have been compactly attached on the surfaces of NiO nanosheets. Moreover, the calculated d value of 0.24 and 0.21 nm in the NiO region could be indexed to its (111) and (200) crystallographic planes. The energy-dispersive spectroscopy (EDS-TEM) was also performed to reveal the nature of the as-prepared sample. As shown in Fig. 1a–d, the nickel, oxygen, carbon and nitrogen elements were detected in the whole regions, further confirm the uniform dispersion of GCNS on the surfaces of NiO nanosheets. Based on the above results, it can be confirmed that GCNS have been successfully and uniformly coated on the NiO nanosheet arrays by this simple dip-coating process.

Scheme 1 Schematic illustration of the synthesis approach of the frame structure NiO/GCNS.

Fig. 1 (A) and (B) SEM images of NiO and NiO/GCNS, the inset SEM is the corresponding cross-sectional images; (C) typical scanning transmission electron microscopy (STEM) image (inset is the HRTEM image) and the corresponding elemental mapping images of (a) nickel, (b) oxygen, (c) carbon and (d) nitrogen in the selected area (red rectangle in (C)).

X-ray diffraction (XRD) analysis has also been performed to investigate the crystal phase of NiO and NiO/GCNS. The XRD patterns of NiO and NiO/GCNS are shown in Fig. S4.† As for pure NiO on FTO sample, the main diffraction peaks at 37.8° and 43.3° could be indexed to the (111) and (200) crystal planes of NiO (JCPDS 47-1049), which is in accordance with the above HR-TEM results. Compared with the XRD patterns of NiO, no evident diffraction peaks for GCNS can be observed in the NiO/GCNS, which may be due to the uniform dispersion and the thin thickness of GCNS onto the surface of NiO. The chemical state of elements and bonding environment of NiO/GCNS composites were further investigated by X-ray photoelectron spectroscopy (XPS). The survey XPS spectrum (Fig. 2A) clearly demonstrates the presence of Ni, O, C and N elements in the as-prepared samples. To gain insight into the chemical bonding between the carbon and nitrogen atoms in the sample, the high resolution C1s (Fig. 2B) could be decomposed into three Gaussian–Lorezian peaks. The peak centered at 288.7 eV (denoted as C1 in Fig. 2B) is assigned to the sp² hybridized carbon in the triazine ring bonded to the –NH₂ group, while the peak at 287.9 eV (C2) is attributed to the sp² hybridized carbon bonded to N atoms inside the triazine rings.^22,35 C3 peak at 284.8 eV is typically ascribed to the signal of standard reference carbon.²² The high resolution XPS spectrum of N1s could also be fitted with four different peaks. The dominant peak at 398.8 eV (denoted as N1 in Fig. 2C) is commonly ascribed to sp² hybridized N atoms involved in triazine rings,^2,4,22 while N2 peak (at 399.6 eV) and N3 peak (at 401 eV) are assigned to bridging N atoms in N–(C)₃ (ref. 2 and 4) and N atoms bonded with H atoms,^4,36 respectively. The weak N4 peak (at 405.3 eV) can be attributed to the charging effects or positive charge localization in heterocycles and the cyano-group.^2,4 These results are in fairly agreement with those reports about bulk GCN and GCNS, suggesting that the chemical states and the coordination of carbon and nitrogen in the GCN are retained during the fabrication process of NiO/GCNS.

Fig. 2 (A) XPS profile of survey of NiO/GCNS; (B) high resolution XPS spectra of C1s; (C) high resolution XPS spectra of N1s; (D) UV-Vis diffusive absorption spectra of NiO and NiO/GCNS.

Furthermore, Fourier transform infrared spectroscopy (FTIR) was also performed to further reveal the typical molecular structure of NiO/GCNS (Fig. S6†). In spite of the technical limitation of FTIR to test the composites on FTO, the absorption band at 806 cm⁻¹ (Fig. S6B†) can be clearly observed and attributed to triazine ring mode.⁴ The absorption bands in the 1200–1600 cm⁻¹ region reveal the typical stretching mode of aromatic GCN heterocycles.⁴ The FTIR results for NiO/GCNS composite are in good agreement with those of bulk GCN and GCNS, further confirm the existence of GCNS in the as-prepared samples. UV-Vis absorption spectra were collected to determine the physicochemical properties of NiO/GCNS, NiO, and GCNS. The absorption edge of NiO/GCNS is at 420 nm with respect to 375 nm of NiO (Fig. 2D), and the derived band gaps of the light absorbed are 2.86 eV and 3.5 eV, respectively, indicating the enhancement of optical absorption of the NiO after combination with GCNS. As shown in Fig. S7,† the GCNS and NiO–GCNS seem to have same absorption band except for the change of absorption intensity, revealing that the generation of photocurrent under visible light is mainly from GCNS. The larger band gap of NiO/GCNS compared with bulk GCN (∼2.7 eV) should be attributed to the well-known quantum confinement effect as a result of thin GCN nanosheet structures. The intrinsic electronic properties of GCNS and NiO/GCNS were carefully examined. The electrochemical impedance spectroscopy (EIS) Nyquist plots (Fig. 3A) clearly reveal that the GCNS shows a smaller semicircle at high frequencies compared with bulk GCN, suggesting that GCNS possesses more efficient charge transport mobility than bulk sample due to their ultrathin layered structure.^20,22,37 In addition, Fig. 3A clearly indicates that NiO/GCNS also exhibit a faster charge separation and transfer than pure GCNS and bulk GCN. Moreover, the flat-band potential of GCNS can be determined by electrochemical Mott–Schottky plots. The flat-band potential of GCNS was measured to be −0.3 V vs. RHE, almost the same as the value obtained for typical bulk GCN sample (Fig. 3B).

Fig. 3 (A) Nyquist plots measured under 0.42 V versus RHE bias potential; (B) Mott–Schottky plots.

To further evaluate the PEC properties of the as-prepared NiO/GCNS sample, systematic PEC measurements were carried out under visible-light irradiation (λ > 420 nm). For comparison, the PEC properties of pure GCNS, bulk GCN, NiO, and NiO–GCN have also been investigated. As shown in Fig. 4A and S8,† a set of linear sweep voltammetry (LSV) in dark and visible light irradiation has been studied. Comparing with the NiO/GCNS in dark, the photocurrent of NiO/GCNS sample increased significantly under visible light irradiation. Interestingly, the current density of NiO/GCNS in the dark is not zero, this may be caused by some reaction happened such as NiOOH formed during the PEC measurements,²⁹ and corresponded to LSV scanning for pure NiO (Fig. S8A†). The dependence plots of photocurrent as a function of increasing potentials of NiO/GCNS under chopped light is demonstrated in Fig. S8B.† From the transient photocurrent–voltage curve under light-on situation, the NiO/GCNS photocathode has an obvious −60 μA cm⁻² photocurrent increased with respect to without light illumination at 0.42 V (vs. RHE). In addition, we also performed amperometric I–t curves under transient illumination at an applied voltage of 0.42 V (vs. RHE) to examine the photo-response vs. time. As shown in Fig. 4B, the photocurrent value of the NiO/GCNS sample could reach up to −70 μA cm⁻², which is much higher than that of bulk GCN, pure GCNS, NiO, and NiO–GCN (Fig. S9†). Moreover, this photocurrent value is also higher than the previous published work where GCN as photocathode.^21,27 Specially, the NiO–GCN sample also exhibits higher photocurrent density with respect to that of the pure NiO, GCNS, and GCN but with a higher dark current density, indicating the poor attached on the surface of NiO nanosheets.²¹ However, the I–t values are not in good agreement with LSV data, this phenomenon could be attributed to the rapid change of external bias voltage and the reaction happened aforementioned. Thinking of that the magnitude of the photocurrent can represent the charge collection efficiency of the electrode surface, the high value indicates that the novel NiO/GCNS sample can greatly enhance the PEC properties.

Fig. 4 (A) Linear sweep voltammograms from 0.6 to −0.2 V (vs. RHE) for NiO/GCNS with and without visible light illumination, scan rate: 50 mV s⁻¹; (B) amperometric I–t curves of GCNS and NiO/GCNS at an applied voltage of 0.42 V vs. RHE with 10 s light on/off cycles.

Based on the above results, we proposed a possible mechanism for the PEC performance improvement over NiO/GCNS composites under visible light irradiation. Since the valence and conduction band position of GCNS is lower than those of NiO (Fig. S10†), respectively, a well-matched hetero-junction was formed. As shown in Scheme 2, GCNS which owned a narrow band gap energy (2.86 eV) could be excited and induce the generation of photoelectrons and holes, while NiO could not be excited because of its wide energy gap (3.5 eV). Under visible light irradiation, the photo-generated electrons are excited from valence band (VB) of GCNS to its conduction band (CB) then transferred to electrolyte and consumed by H⁺ for H₂ generation. Correspondingly, holes transferred from VB of GCNS to VB of NiO. Finally, holes arrived at the counter electrode through the external circuit and consumed by the H₂O for generation of O₂. During the photoexcited process, the NiO nanosheets provide a conduction path for holes transfer to conductive substrate. Moreover, the formed heterojunction between GCNS and NiO could further prevent the recombination of photoelectrons and holes. Thereby, the photoinduced electron–hole pairs could be effectively separated for enhancing the photocurrent efficiency.

Scheme 2 Illustration for the charge separation and transport of the NiO/GCNS.

In conclusion, we have demonstrated a feasible strategy for rationally constructing GCN nanosheets on the surfaces of NiO nanosheet arrays, in which NiO nanosheets serve as an efficient cocatalyst for promoting the charge separation and holes migration, thus suppressing the interfacial charge recombination and improving the photoelectrochemical performances of GCN. The photocurrent density of ultrathin GCN nanosheets coated on NiO nanosheet arrays could be achieved up to −70 μA cm⁻² at 0.42 V vs. RHE under visible-light illumination, which is much higher than the PEC capabilities of pristine GCNS and bulk GCN under the same conditions. Moreover, the NiO nanosheet arrays can be replaced by other p-type semiconductor materials or be further modified other cocatalyst to form sandwich structure to enhance photocurrent density. This work may guide future designs for highly efficient photocathode materials based on GCN and suggest paths for further improvements to allow the technological development.

Acknowledgements

This work was supported by the “Hundred Talents Program” of the Chinese Academy of Sciences and the National Natural Science Foundation of China (21573264, 21273255, 21303232).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra18999g

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