Large dipole moment induced efficient bismuth chromate photocatalysts for wide-spectrum driven water oxidation and complete mineralization of pollutants

Abstract Herein, a wide-spectrum (∼678 nm) responsive Bi8(CrO4)O11 photocatalyst with a theoretical solar spectrum efficiency of 42.0% was successfully constructed. Bi8(CrO4)O11 showed highly efficient and stable photocatalytic water oxidation activity with a notable apparent quantum efficiency of 2.87% (420 nm), superior to many reported wide-spectrum driven photocatalysts. Most remarkably, its strong oxidation ability also enables the simultaneous degradation and complete mineralization for phenol, and its excellent performance is about 23.0 and 2.9 times higher than CdS and P25-TiO2, respectively. Its high activity is ascribed to the giant internal electric field induced by its large crystal dipole, which accelerates the rapid separation of photogenerated electron–hole pairs. Briefly, the discovery of wide-spectrum bismuth chromate and the mechanism of exponentially enhanced photocatalytic performance by increasing the crystal dipole throw light on improving solar energy conversion.


INTRODUCTION
The conversion and utilization of solar energy for chemical fuel production and environmental remediation through artificial photocatalysis have been recognized to be an ideal route to address critical energy and environmental concerns [1][2][3]. The full utilization of solar light is a great challenge for achieving sufficient efficiency in practical applications. In the early stages, UV light-activated materials, such as TiO 2 , SrTiO 3 , NaTaO 3 , etc., dominated photocatalysis study, due to the wide bandgap of conventional semiconductors and their strong redox capability of charge carriers for igniting chemical reactions [4][5][6][7]. Nonetheless, the extremely low ratio of UV photons in solar energy greatly hinders the ability to maximize the solar-to-chemical energy conversion efficiency. In recent years, a number of mixed-anion and non-oxide materials such as (oxy)nitrides and (oxy)sulfides have been developed as attractive broadband light-responsive photocatalysts [8][9][10][11]. The valence band maximums (VB) of the mixed-anion materials can be substantially regulated by hybridization of O 2p or other introduced anion orbitals, enabling both broadband light absorption and suitable band potentials for both reduction and oxidation of water [12,13]. For example, BaNbO 2 N, reported by Hisatomi et al., could broaden the light absorption up to 740 nm and simultaneously shows efficient water oxidation [14]. However, narrowing the bandgap of a photocatalyst weakens the driving force for redox reactions, especially water oxidation and pollutant degradation, because these reactions involve a complicated multi-electron process [15]. Therefore, the development of wide-spectrum responsive and highly efficient photocatalysts for water oxidation and pollutant degradation is a critical issue to be addressed at present.
Bi-based oxometallate materials, such as BiVO 4 , Bi 2 WO 6 , Bi 2 MoO 6 , etc . , have been widely studied as visible-light active photocatalysts, due to their high stability, abundant resources and low toxicity [16][17][18][19]. Moreover, they also exhibit excellent photocatalytic performance in water oxidation, which is mainly benefiting from their sufficiently deep VB C The Author(s) 2019. Published by Oxford University Press on behalf of China Science Publishing & Media Ltd. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted reuse, distribution, and reproduction in any medium, provided the original work is properly cited.  position as compared to the potential for water oxidation and pollutant degradation [20,21]. In particular, the BiVO 4 photocatalysts present highly efficient and stable water oxidation performance, and its highest solar-to-hydrogen energy conversion efficiency of 1.2% for Z-scheme pure-water splitting by coupling with SrTiO 3 : La, Rh has been reported [22]. Nevertheless, their relatively wide bandgaps (about 2.5 eV) greatly limit their further application. Recently, research into Cr-based layered double hydroxide photocatalysts revealed that the hybridization of Cr 3d orbitals with O 2p orbitals in [CrO 6 ] clusters shifts the conduction band minimum (CB) down and results in wide visible-light absorption [23,24]. Inspired by the above, the construction of bismuth chromate photocatalyst may be a desired route to achieve wide-spectrum driven, efficient, and stable photocatalytic performance. In this work, a wide-spectrum responsive Bi 8 (CrO 4 )O 11 photocatalyst was successfully constructed. Owing to the hybridization of Cr 3d with O 2p orbitals shifting the conduction band minimum down, Bi 8 (CrO 4 )O 11 allows its absorption up to the entire visible region (∼678 nm) with a theoretical solar spectrum efficiency of 42.0%. Moreover, attributed to the giant internal electric field (IEF) induced by its large dipole moment, Bi 8 (CrO 4 )O 11 realized evidently rapid separation of photogenerated electron-hole pairs, thus showed highly efficient photocatalytic water oxidation activity with a notable apparent quantum yield of 2.87% (420 nm), superior to many reported widespectrum driven photocatalysts. Most remarkably, its strong oxidation ability also enables simultaneous degradation and complete mineralization for phenol, and its excellent performance is about 23.0 and 2.9 times higher than CdS and P25-TiO 2 , respectively.

RESULTS AND DISCUSSION
Herein, monoclinic Bi 8 (CrO 4 )O 11 nanorods (Figs S1-S3), a novel bismuth chromate photocatalyst, were successfully synthesized via a facile hydrothermal reaction. Then, density functional theory (DFT) was applied to calculate the electronic structure of this bismuth chromate. As shown in Fig. 1a  provided by the hybridization of Cr 3d orbitals with O 2p orbitals, demonstrating that the introduction of the [CrO 4 ] cluster plays a crucial role in extending absorption into the entire visible region. Moreover, the indirect band structure of Bi 8 (CrO 4 )O 11 is also confirmed by its electronic band diagram ( Fig. S4a), which is in favor of confining the recombination of photogenerated electron-hole pairs. In the diffuse reflectance spectrum (DRS) (Fig. 1b Considering that the photocatalytic redox ability mainly depends on the energy band potential, the redox potentials of the Bi 8 (CrO 4 )O 11 nanorod photocatalyst were calculated according to the DRS and Mott-Schottky plots (Fig. S5a) [27,28]. As shown in Fig. 1d, the CB of Bi 8 (CrO 4 )O 11 is located at 0.12 eV vs. NHE (pH = 0), a little deeper than the reduction potential of H + /H 2 . Also, its VB of 1.95 eV is more positive than the oxidation potential of OH − /O 2 , which indicates that the photogenerated holes of Bi 8 (CrO 4 )O 11 nanorod photocatalyst possess extremely strong oxidation capability, and can split water to release O 2 , and even completely mineralize organic pollutants under visible light.
We first evaluate the photocatalytic water oxidation performance over Bi 8 (CrO 4 )O 11 nanorods. Figure 2a shows a comparison of the photocatalytic O 2 evolution rate over different samples. It can be seen that Bi 8 (CrO 4 )O 11 exhibited far superior photocatalytic water oxidation performance, and its average O 2 evolution rate reached 14.94 μmol h −1 , about 11.5 and 4.0 times higher than that of Bi 2 WO 6 nanosheets [29] and commercial WO 3 nanoparticles. Besides, Bi 8 (CrO 4 )O 11 consequently achieved a considerable apparent quantum efficiency (AQE) 2.87% at 420 nm, even 0.65% at 650 nm (Fig. 2b), higher than many reported wide-spectrum driven photocatalysts (Table S2). In addition, the trend of AQE values for water oxidation over Bi 8 (CrO 4 )O 11 is also consistent with its UV-vis DRS, further confirming that the photocatalytic water oxidation reaction is driven by its absorbed incident photons. Furthermore, after loading Co(OH) 2 as co-catalyst, its photocatalytic water oxidation performance was improved by 2.1 times (Fig. S6). Just as importantly, no notable deactivation emerged over Bi 8 (CrO 4 )O 11 during a continuous photocatalytic water oxidation reaction for 72 h (Fig. S7a). By comparing its XRD patterns and XPS results before and after reaction (Figs S7b and S8), it could be found that the crystal structure and composition of Bi 8 (CrO 4 )O 11 after reaction show no marked change, further indicating its robust resistance to water and light corrosion.
Most noticeably, the excellent activity of Bi 8 (CrO 4 )O 11 is also manifested in photocatalytic degradation of phenol. As shown in Fig. 2c, Bi 8 (CrO 4 )O 11 showed a superior photocatalytic degradation performance for phenol under visible light, and its degradation reaction constant could reach 0.119 min −1 , about 22.5 and 8.8 times higher than CdS nanowires [30] and N,N -di(propanoic acid)-perylene-3,4,9,10-tetracarboxylic diimide (PDI) supramolecular [31,32] photocatalysts, respectively. Even its degradation activity is not inferior to P25 TiO 2 under simulated sunlight, being about 2.9 times higher than the latter. Remarkably, Bi 8 (CrO 4 )O 11 also presented extremely strong mineralization ability, which almost enables simultaneous degradation and complete mineralization for phenol. The total organic carbon (TOC) removal rates of phenol over Bi 8 (CrO 4 )O 11 under visible light and simulated sunlight are 94.8% (degradation rate: 95.5%) and 97.3% (degradation rate: 98.1%) in 0.5 h, respectively, while that of CdS, PDI and P25 are significantly lower than their corresponding degradation rates. In particular, even under 650 nm red light irradiation, Bi 8 (CrO 4 )O 11 is still able to simultaneously degrade and completely mineralize phenol (Fig. 2d), and few wide-spectrum driven photocatalysts can achieve that [33]. Besides, Bi 8 (CrO 4 )O 11 also exhibited highly efficient photocatalytic formaldehyde degradation activity under visible light in a continuous-flow system, and the removal rate could be maintained at about 95% (Fig. S10c). No notable deactivation emerges during continuous measurement for 76 h.
It is well known that photocatalytic activity is closely related to the separation efficiency of photogenerated electron-hole pairs [34][35][36]. Previous studies have demonstrated that the IEF induced by the crystal dipole is considered to effectively boost the separation of photogenerated electronhole pairs and enhance the photocatalytic performance exponentially, such as in Bi 2 MoO 6 , BiPO 4 , and BiOCl [37][38][39]. Therefore, to reveal the high activity mechanism of Bi 8 (CrO 4 11 shows an obvious difference in the contact potential difference (CPD) between the edge and the bulk, about 202 mV (Fig. 3c), but the CPD difference over Bi 14 CrO 24 is virtually invisible, only about 39 mV. According to the literature, the relatively large CPD difference between the two regions reflects that a relatively strong IEF is formed in the crystal [40][41][42], consequently demonstrating the existence of a greater IEF in Bi 8 (CrO 4 )O 11 .
Furthermore, the intensity of their IEF was measured via the model developed by Kanata-Kito et al. (details are given in the supplementary data online) [43,44]. It can be found that the IEF of Bi 8 (CrO 4 )O 11 is 8.4 times as high as that of Bi 14 CrO 24 (Fig. S13), well consistent with the above results. Benefiting from its greater IEF, Bi 8 (CrO 4 )O 11 presented an evidently stronger surface photovoltage response and photocurrent density (Fig. S16), about 22.7 and 4.0 times higher than Bi 14 CrO 24 , respectively, revealing that a faster charge carrier transfer kinetics emerges in Bi 8 (CrO 4 )O 11 . As expected, Bi 8 (CrO 4 )O 11 exhibited 17.2 and 153.0 times higher photocatalytic water oxidation and degradation performance than Bi 14 CrO 24 , respectively. Then, after summarizing the above results into Fig. 3d, it can be found that the IEF, charge separation efficiency and photocatalytic activity of bismuth chromate are positively correlated with their dipole moments; thus Bi 8 (CrO 4 )O 11 with a greater dipole showed a significantly higher IEF, charge separation efficiency and photocatalytic performance. Therefore, as illustrated in Scheme 1, the large crystal dipole of Bi 8 (CrO 4 )O 11 induces a giant IEF, which accelerates the rapid separation of photogenerated electron-hole pairs and exponentially enhances its photocatalytic performance. Most importantly, based on the above mechanism, many more efficient photocatalysts can be designed successfully by regulating the crystal dipole.

CONCLUSION
In conclusion, a wide-spectrum (∼678 nm) responsive Bi 8 (CrO 4 )O 11 nanorod photocatalyst was constructed via the hybridization of Cr 3d with O 2p orbitals. Attributed to the giant IEF induced by its large dipole moment, Bi 8 (CrO 4 )O 11 realizes evidently rapid separation of photogenerated electronhole pairs, thus showing highly efficient photocatalytic water oxidation performance with a notable apparent quantum yield of 2.87% (420 nm), superior to many reported wide-spectrum driven photocatalysts. Most remarkably, its strong oxidation ability also enables simultaneous degradation and complete mineralization for phenol, and its excellent performance is about 23.0 and 2.9 times higher than CdS and P25-TiO 2 , respectively. Briefly, the discovery of wide-spectrum bismuth chromate and the mechanism of exponentially enhanced photocatalytic performance by increasing the crystal dipole throw light on designing efficient wide-spectrum photocatalysts.

Synthesis of samples
NaBiO 3 and other chemicals were purchased from Aladdin and Sigma-Aldrich, respectively, and used without further purification. For Bi 8 (CrO 4 )O 11 nanorods, 0.56 g NaBiO 3 was ultrasonically dispersed in 80 mL deionized water, followed by addition of 7.35 mL 25 mmol L −1 Cr(NO) 3 aqueous solution under vigorous stirring. Then the resulting solution was transferred into a 100 mL Teflon-lined stainless autoclave and maintained at 180 • C for 6 h. The brown-red Bi 8 (CrO 4 )O 11 was collected by centrifuge separation, rinsed thoroughly with ethanol and deionized water several times, and dried at 70 • C overnight. For Bi 14 CrO 24 nanosheets, 1 mmol Bi(NO 3 ) 3 and 0.084 mmol Cr(NO 3 ) 3 were ultrasonically dissolved in a certain concentration of mannitol aqueous solution (25 mL), followed by addition of 5 ml saturated Na 2 CO 3 solution under vigorous stirring. Then the resulting solution was transferred into a 50 mL Teflon-lined stainless autoclave and maintained at 150 • C for 12 h. The precursor was collected by centrifuge separation, rinsed thoroughly with ethanol and deionized water several times, and dried at 70 • C overnight. The precursor was then calcined in a crucible at a certain temperature for 10 min under air atmosphere to yield orange-red Bi 14 CrO 24 nanosheets.
For comparison, BiWO 6 nanoplates were synthesized as in [29], and WO 3 nanoparticles were purchased from Aladdin.

Characterization
XRD patterns of the samples were obtained on a Rigaku D/max-2400 X-ray diffractometer using Cu Kα1 (λ = 0.154 18 nm) at 40 kV and 200 mA, with a scan step of 0.02 • . The morphologies of the samples were measured by transmission electron microscopy (TEM) on a Hitachi HT 7700 at an accelerating voltage of 100 kV and high-resolution transmission electron microscopy (HRTEM) on a JEOL JEM-2100F operated at an accelerating voltage of 200 kV. Field emission scanning electron microscopy (FESEM) on a Hitachi SU-8010 was used to further investigate the morphology. XPS measurements were performed using an ES-CALAB 250Xi instrument (Thermo Scientific) with Al Kα radiation. DRS were obtained on a Cary 5000 (Varian) with BaSO 4 as a reference. The surface potential images of the samples were measured by Kelvin probe force microscopy (KPFM) in ambient atmosphere on a Cypher VRS (Oxford Instruments) and a Pt/Ir-coated Si tip was used as a Kelvin tip. The surface photovoltage measurements were conducted with a home-built instrument as previously reported [45]. Photoelectrochemical measurements were performed on a CHI660E electrochemical workstation, using a standard three-electrode cell with a working electrode, a Pt-wire counter electrode and a saturated calomel reference electrode. Na 2 SO 4 (0.1 mol L −1 ) was used the electrolyte solution. The working electrode was prepared by dip-coating photocatalyst slurry on ITO glass electrode (2 × 4 cm 2 ).

Photocatalytic performance evaluation
The photocatalytic water oxidation reaction under visible-light irradiation was performed in a Pyrex top-irradiation reaction vessel with a stationary temperature at 5 • C, which was connected to a glass closed gas system (Labsolar-6A, PerfectLight). 100 mg photocatalyst was suspended individually in 100 mL aqueous solution (pH = 2.5) containing 10 mmol L −1 Fe(NO 3 ) 3 as a sacrificial reagent. The suspension was then thoroughly degassed and irradiated using a 300 W Xe lamp with a cut-off filter (λ ≥ 420 nm, light intensity 250-260 mW cm −2 ). The evolved gases were analyzed at given time intervals by an online gas chromatograph (GC-2002 N/TFF, TCD detector, Ar carrier, 5Å molecular sieve column).
The AQE for water oxidation was measured using a 300 W Xe lamp (FX300, PerfectLight) with different band-pass filters of 420, 450, 500, 550, 600, and 650 nm (FWHM = 15 nm). The irradiation area was controlled as 1.2 × 1.2 cm 2 . The average intensity was determined by an optical power meter (S310C connected to a PM100D console, Thorlabs). The AQE was calculated as follows: 4 × the number of evolved O 2 molecules the number of incident photons × 100%.
The photodegradation reactions were carried in quartz tube reactors with a 50 mL 10 ppm phenol pollutant solution and 25 mg photocatalyst powders. The reaction solution was kept at 35 • C by a recirculating cooling water system. The visible-light source was obtained from a 300 W Xe lamp with a cut-off filter (λ ≥ 420 nm). Before light irradiation, the suspension solutions were first ultrasonically dispersed for 5 min and then magnetically stirred for 1 h in the dark to reach adsorption-desorption equilibrium. At certain time intervals, a suspension (2 mL) was extracted and centrifuged to remove the photocatalysts. The concentration of phenol pollutants was determined by a high-performance liquid chromatography (HPLC) system (Shimadzu LC-20AT) with a C18 reversed-phase column, and the total organic carbon (TOC) in the aqueous solution was analyzed using a TOC analyzer (Multi N/C 2100, Analytik Jena AG).