Engineering Nanoscale Homo–Heterojunction for Robust Z‐Scheme CO2 Conversion through Synchronous Amalgamation of Oxygen‐Defective Ultrathin BiVO4 and Red/Black Phosphorus

Sunlight‐driven CO2 conversion into value‐added fuels emerges as an avant‐garde strategy to mitigate the imminent energy scarcity and global warming. Nevertheless, sluggish charge kinetics and rapid carrier recombination are deemed as the foremost bottlenecks of efficient CO2 photoreduction. In this contribution, the aforementioned shortcomings are addressed by assembling a homo–hetero architecture comprising oxygen‐defective ultrathin BiVO4 (BiVO4–Ov) and red/black phosphorus (RP/BP) homojunction with a built‐in nanoscale dual Z‐scheme electronic configuration. The development of BiVO4–Ov@RP/BP confers two electron relaying channels, that is, one from the RP/BP homojunction and another from the BiVO4–Ov/RP heterojunction, which warrant the efficient separation of electron–hole pairs. Besides, the ultrathin 2D and oxygen vacant nature of BiVO4–Ov exposes more active sites and renders electron trapping for efficient utilization of photogenerated charges. Amalgamating the desirable properties, the meticulously developed dual Z‐scheme homo–heterojunction system on the basis of surface defect‐engineered ultrathin BiVO4–Ov nanosheets integrated with RP/BP offers a new avenue for the exploration, design, and fabrication of new generation photocatalysts with exceptional photocatalytic activities. In particular, the BiVO4–Ov@RP/BP system demonstrates a remarkably high CH4 yield of 14.52 μmol g−1 after 6 h of visible light illumination, which resembles a leading‐edge photocatalytic performance among the existing BiVO4‐ and P‐based semiconductors.


Introduction
The fossil fuel energy system presents itself as the lifeblood of the modern economy as it powered the industrial revolution, lifted millions of people out of poverty, and molded today's cutting-edge society.The astronomical reliance on fossil fuels has not only inflicted worrisome energy crisis and fuelled global turmoil, their consumption also releases an abundance of anthropogenic greenhouse gases (GHGs), primarily carbon dioxide (CO 2 ).Since the industrial revolution in year 1750, the level of atmospheric CO 2 has reached a staggering concentration of 421 ppm in year 2022, [1] with its relentless increase being acknowledged as the dominant reason underlying the GHG effects and other associated environmental concerns, such as the El Niño phenomenon, the intensification of hurricanes and floods, as well as the reduction of marine plankton calcification. [2]he benefits of consuming fossil fuels are therefore accompanied with devastating downsides.One of the greatest responsibilities facing humanity today is to mitigate CO 2 -induced global warming and climate Sunlight-driven CO 2 conversion into value-added fuels emerges as an avantgarde strategy to mitigate the imminent energy scarcity and global warming.Nevertheless, sluggish charge kinetics and rapid carrier recombination are deemed as the foremost bottlenecks of efficient CO 2 photoreduction.In this contribution, the aforementioned shortcomings are addressed by assembling a homo-hetero architecture comprising oxygen-defective ultrathin BiVO 4 (BiVO 4 -Ov) and red/black phosphorus (RP/BP) homojunction with a built-in nanoscale dual Z-scheme electronic configuration.The development of BiVO 4 -Ov@RP/BP confers two electron relaying channels, that is, one from the RP/BP homojunction and another from the BiVO 4 -Ov/RP heterojunction, which warrant the efficient separation of electron-hole pairs.Besides, the ultrathin 2D and oxygen vacant nature of BiVO 4 -Ov exposes more active sites and renders electron trapping for efficient utilization of photogenerated charges.Amalgamating the desirable properties, the meticulously developed dual Z-scheme homo-heterojunction system on the basis of surface defect-engineered ultrathin BiVO 4 -Ov nanosheets integrated with RP/BP offers a new avenue for the exploration, design, and fabrication of new generation photocatalysts with exceptional photocatalytic activities.In particular, the BiVO 4 -Ov@RP/BP system demonstrates a remarkably high CH 4 yield of 14.52 μmol g À1 after 6 h of visible light illumination, which resembles a leading-edge photocatalytic performance among the existing BiVO 4 -and P-based semiconductors.
change and the core to all climate change solutions is to decrease the dependence on fossil fuel as well as minimize GHG emissions.The transformation of CO 2 into chemical building blocks and value-added fuels is garnering incessant attention ascribing to the growing sense of urgency to design, develop, and implement a carbon-neutral and sustainable economy.
The Earth receives plenty of solar energy for the society's modern energy-intensive lives.Hence, direct solar-to-fuel transformation via the photocatalysis technology accords a major advance in this direction as it harvests the inexhaustible, safe, and clean energy of the Sun in a nonhazardous, sustainable, and economically viable manner to convert CO 2 into fuels like methane (CH 4 ), formic acid (CH 2 O 2 ), and methanol (CH 3 OH). [3]Despite years of devotion in the research of CO 2 photocatalysis, CO 2 conversion performance is still wide of the target from meeting satisfactory industry requirement, which generally stems from aspects such as the intrinsic limitations of semiconductor photocatalysts and the thermodynamically stable electron configuration of the CO 2 molecules with linear symmetry.Traditional photocatalysts suffer from poor separation of charge carriers and inadequate surface active sites, resulting in inferior photocatalytic performances.This phenomenon is further exacerbated by the high overpotential needed to activate CO 2 molecules, which remains arduous for many photocatalytic materials. [4]To realize CO 2 transformation practically, the present research wave is steered by the construction of 2D structures that are exceptional for building efficient photocatalytic materials due to their distinctive 2D geometry, electronic anisotropy, enriched surface active sites, expanded surface area, reduced charge transportation routes, and improved conductivity owing to the increased surface defects. [5]he synergistic exploitation and optimization of 2D photocatalysts could impart outstanding activities in comparison to their bulk and 3D counterparts.Specifically, the advantages of a 2D photocatalyst are closely associated with three main aspects: 1) expanded specific surface area which proffers an increased density of highly-exposed active centers; 2) reduced transfer routes for the migration of photogenerated electron-hole pairs to the surface of the photocatalyst to participate in redox reactions; and 3) enhanced mechanical properties such as improved durability, especially when 2D structures are hybridized with other photocatalysts. [6]Moreover, 2D materials with ultrathin geometry proffer the opportunity to engineer and tune their electronic structures beyond the bulk realm, attributing to the obvious distinctions in local atomic structures. [7]The recent progress in 2D materials has therefore sparked a surge of interest in exploring metal oxide-based materials.Specifically, bismuth vanadate (BiVO 4 ), an n-type semiconductor offers vast potential in the photocatalysis scene as it is endowed with high photo-and chemical stability, nontoxic nature, low production cost, and a narrow bandgap of %2.4 eV with flexible optoelectronic features.The visible-light responsiveness of BiVO 4 renders it as a compet-itive candidate to harvest light energy from the solar spectrum.More importantly, BiVO 4 photocatalyst is spectacularly bestowed with a strong valence band (VB) potential of %2.7 V (vs.NHE), making it a befitting option to serve as a powerful oxidation counterpart for CO 2 -reduction process. [8][11] Typically, an oxygen vacancy is created when an oxygen atom escapes from the lattice of a metal oxide, resulting in enhancement of visible-light absorption attributed to the creation of isolated level below conduction band (CB) as well as ameliorating electrical conductivity. [12]The enhancement in optical properties and charge-carrier dynamics generally stems from the alteration of the intrinsic electronic properties and bandgap of the photocatalytic material upon the introduction of oxygen vacancy into its lattice structure as the modification allows the generation of local energy defect levels below CB.As such, the introduction of oxygen vacancies has been demonstrated to improve a myriad of photocatalytic reactions [13][14][15] by virtue of these defect states serving as trapping points which lead to the suppression of exciton recombination process.Further survey reveals the ability of oxygen-vacancy defects to enrich the surface properties of BiVO 4 (water [H 2 O] adsorption sites and active reaction sites) in substrate binding for efficacious photocatalysis. [16]n recent years, elemental red phosphorus (RP) has been arousing extensive interests for multitudinous photocatalytic applications ascribed to its abundant availability, broad visiblelight responsivity, high stability, nontoxic, and low-cost nature. [17]onsiderable efforts have been devoted to developing RP as a functional material for photocatalytic CO 2 reduction with fascinating structural and optoelectronic properties.However, the photocatalytic potential of RP is restricted owing to the poor separation and transfer of electrons and holes due to charge-trapping centers being present on the semiconductor's surface.Therefore, continuous efforts toward engineering high-performing RP-based materials and modification strategies are the cruces in surmounting the limitations of RP.Hitherto, the fabrication of RP-based photocatalysts namely the preparation of nanostructured and/or crystalline RP, heterojunction construction, and cocatalyst loading to name a few is soaring within the research community.Among the strategies employed, designing a transitional phase of P-based photocatalyst that comprises red-black phosphorus (RP/BP) homojunction accords a desirable feat to curb the drawbacks of pure RP. [18] This is because the black counterpart possesses a tunable bandgap across a wide region, from a bulk value of 0.3 eV to a value of 2.0 eV for the monolayer structure, rendering its strong optical absorption in the visible and near-infrared (NIR) spectrum.BP's astounding charge-carrier mobility (%10 2 -%10 4 cm 2 V À1 s À1 ) makes the topotactic transformation of RP to BP to form a mixed-phase homojunction photocatalyst immensely attractive.Moreover, the development of the transitional phase RP/BP photocatalysts is ascertained to augment the light responsivity of bare RP photocatalyst, as evidenced by the powerful optical absorption properties of BP which extends up to the NIR region.In light of the intriguing physicochemical and semiconducting properties displayed by RP and BP, respectively, the development of RP/BP homojunction photocatalysts are highly propitious to heighten the efficiency of photocatalytic CO 2 reduction.
With that in mind, the opportune construction of an effective dual Z-scheme homo-heterojunction system (BiVO 4 -Ov@RP/BP) via the cooperative combination of oxygen-defective ultrathin 2D BiVO 4 (BiVO 4 -Ov) and a phosphorus (P)-based homojunction photocatalyst comprising RP/BP can provoke complementary synergism in accomplishing remarkable CO 2 -photoreduction performance.Attributing to the difference in Fermi levels between the semiconductors, depletion layers are generated at the interface of BiVO 4 -Ov and RP/BP homojunction, leading to the accelerated directional migration of electron-hole pairs powered by the built-in electric field.Working synergistically with the oxygen vacancies throughout the ultrathin 2D BiVO 4 nanosheets, the BiVO 4 -Ov@RP/BP composite photocatalytic system demonstrated an exceptional CH 4 yield of 14.52 μmol g À1 under visible-light irradiation.Merging all the aforementioned merits, we believe that this pioneering study will provide compelling insights into the significance of combining ultrathin 2D material design, defect engineering, and dual Z-scheme vectorial charge transfer to regulate nanoscale-level charge steering and modulate the density of surface active centers to promote superior photoreduction of CO 2 .

Morphology and Structural Characterization
X-Ray diffraction (XRD) was first utilized to assess the phase structure of the samples and the results are depicted in Figure 1a.As can be seen from the figure, the XRD diffraction peaks of BiVO 4 -Ov at 2θ = 19.0°,28.9°, 30.5°, 34.5°, 39.8°, 42.5°, 50.3°, and 53.2°correspond to the (110), (À121), (040), ( 200), (211), (051), (202), and (À161) planes of monoclinic scheelite type BiVO 4 (JCPDS No. 00-014-0688), respectively.The exposure of the (040) crystal plane is immensely conducive for higher photocatalytic oxidation reactions.Typically, active catalytic centers are located in the edge sites of metal oxides whereas the basal planes remain catalytically inert which restricts photocatalytic activity. [19]The introduction of oxygen-vacancy defects as well as the ultrathin dimension of the as-developed BiVO 4 -Ov nanosheets result in the activation of basal planes through the exposure of more catalytic reactive centers, thereby improving photocatalytic capacity.The (040) peak also reveals the orientation of the monoclinic BiVO 4 -Ov along the [010] zone axis. [20]oreover, preceding research have disclosed that the photoinduced charge carriers in monoclinic BiVO 4 would preferentially travel to {010} and {110} facets ascribed to the distinctive band energies between the two, thereby accelerating the separation of electrons and holes. [21]In contrast, the RP/BP homojunction exhibits the XRD patterns of pure BP (JCPDS card no.01-076-1967) as BP is highly crystalline in nature while RP possesses amorphous characteristics.That said RP/BP demonstrates XRD peaks at 2θ = 26.30°,35.20°, and 50.40°, which are well indexed to the (021), (040), and (060) planes of pure orthorhombic BP, respectively.Typical descriptive diffraction patterns of both BiVO 4 and mixed-phase RP/BP are present in the BiVO 4 -A@RP/BP and BiVO 4 -Ov@RP/BP composite photocatalysts.
The morphology, microstructure, and topography of the developed samples were then assessed using field-emission scanning electron microscope (FESEM), atomic force microscopy (AFM), transmission electron microscopy (TEM), and high-resolution TEM (HRTEM).From the FESEM image displayed in Figure S1a, Supporting Information, a porous morphology is clearly discerned for the RP/BP sample whereas TEM image featured in Figure 1b reveals the as-synthesized BiVO 4 -Ov as 2D nanosheets that possess quasi-square-sheet shapes with average lateral sizes ranging from 50 to 100 nm.To garner an overview of the thickness of BiVO 4 -Ov nanosheets, AFM was employed and the topography image with the corresponding height profiles are shown in Figure 1c.Based on the image, it can be evidently seen that BiVO 4 -Ov possesses an average thickness of 4 to 5 nm, corroborating its ultrathin nature.In stark comparison to the conventional irregular rod-like structure of bulk BiVO 4 (BiVO 4 -B) (Figure S1b, Supporting Information), BiVO 4 -Ov presents itself as a square-like ultrathin structure with less obvious bulk domains.The elemental composition of BiVO 4 -Ov@RP/BP was divulged using FESEM energy-dispersive X-Ray spectroscopy (FESEM-EDX) mapping (Figure S1c,d, Supporting Information), which show the homogeneous distribution of P, Bi, V, and O elements, affirming the successful hybridization between BiVO 4 -Ov and RP/BP.The successful construction of a BiVO 4 -Ov@RP/BP hybrid photocatalytic system is further verified by TEM imaging (Figure 1d).It can be visualized that BiVO 4 -Ov with a square-like geometry is successfully composited onto RP/BP with intimate contact.The examination of crystallographic structure and lattice fringe spacing measurements for the composite photocatalyst was subsequently undertaken using HRTEM (Figure 1e).Two interplanar d-spacings of 0.18 and 0.26 nm are attained from the HRTEM analysis, which could be indexed to the (202) and (200) crystallographic planes of monoclinic BiVO 4 -Ov, respectively. [22]These findings are in good agreement with the XRD analysis.The d-spacing of 0.42 nm, in contrast, corresponds to the (014) plane of the BP crystals. [23,24]The region which displays undistinctive lattice spacings with poor crystallinity (labeled with RP from inset of Figure 1e) is indicative of the amorphous nature of the RP allotrope.The coexistence of the lattice spacings of BiVO 4 -Ov, BP, and amorphous RP further affirms the propitious amalgamation of the three semiconductors to craft a BiVO 4 -Ov@RP/BP homoheterojunction.
To verify the presence of oxygen vacancies, the BiVO 4 -Ov nanosheets were annealed in air at 500 °C to eliminate oxygen-vacancy defects and the validation was conducted by electron spin resonance (ESR).As one of the most efficacious methods to probe defective structures, ESR analysis evinces a symmetrical resonance line centered at g = 2.003 as can be seen from Figure 1f for the BiVO 4 -Ov sample due to the accumulation of electrons on the oxygen-vacancy defects. [25]As anticipated, no ESR signal was detected for the annealed sample (BiVO 4 -A).In monoclinic BiVO 4 , ESR signals are not contributed by both high (Bi 3þ and V 5þ ) and low valent ions (Bi þ , V 3þ , and O 2-), owing to the absence of unpaired electrons.Similarly, the ESR signal would not be caused by the oxygen vacancy that is doubly positively charged (V o 2þ ) as there is no diamagnetic electron.Therefore, the distinct ESR signal would be sourced from the oxygen vacancy that is singly negatively charged (V o -) in which the trap-free electron is provided by V 4þ and/or Bi 2þ . [26]A dramatic decrease of the ESR signal is witnessed for the BiVO 4 -A sample, suggesting the elimination of oxygen vacancies throughout the 2D BiVO 4 nanosheets upon the introduction of atmospheric oxygen.
To expound the elemental composition and chemical valency of the samples, X-Ray photoelectron spectroscopy (XPS) analysis was undertaken.The complete XPS survey spectra of BiVO 4 -Ov@RP/BP reveal the presence of P, Bi, V, and O as the main elements as evidenced from Figure S2, Supporting Information.In accordance with Figure 2a which unveils the P 2p spectra of the RP/BP sample, two narrow peaks were observed at 130.42 and 131.29 eV, which corresponds to elemental P 2p 3/2 and P 2p 1/2 of the P-P bonds, respectively.In contrast, a broader and wider sub-band can be found at 135.02 eV, suggesting that oxidized P species (P x O y ) is present. [27]Contrastingly, the P 2p 3/2 and 2p 1/2 characteristic peaks of the BiVO 4 -Ov@RP/BP sample located at 129.78 and 130.65 eV experience diminished intensities with the formation of two new peaks situated at 133.18 and 134.25 eV, attributing to the generation of O-P=O and P-O, respectively. [28,29]Notably, the P 2p 3/2 and 2p 1/2 XPS peaks for BiVO 4 -Ov@RP/BP are shifted toward lower binding energies with a difference of 0.64 eV in comparison to the RP/BP sample.Conversely, deconvolution of the Bi 4f spectra (Figure 2b) for the bare BiVO 4 -Ov sample features a classic spin-orbit doublet splitting centered at 159.20 and 164.50 eV with a difference of 5.30 eV, implying the presence of the Bi 3þ species whereas the V 2p spectra (Figure 2c) illustrates two characteristic peaks at 516.73 and 524.26 eV with a peak splitting of 7.53 eV, which represents V 5þ . [30,31]As opposed to bare BiVO 4 -Ov, the Bi 4f and V 2p characteristic peaks for the BiVO 4 -Ov@RP/BP composite photocatalyst experience an increment in binding energies, indicating the change in the chemical environment upon the integration with RP/BP.Interestingly, the positive shift in binding energies of the Bi 4f and V 2p spectra accompanied by the negative shift in binding energies of the P 2p spectra suggest the intimate interfacial contact and electronic coupling between BiVO 4 -Ov and P-based photocatalysts.In contrast, the asymmetric O 1s spectra for the BiVO 4 -Ov sample as illustrated in Figure 2d are deconvoluted into three peaks centered at 530.60, 531.40, and 532.60 eV, which reflects the lattice oxygen (O L ), deficient oxygen (O V ), and chemisorbed oxygen (O C ), respectively. [25,26,32]Accordingly, a notable amount of oxygen vacancies can be detected in the as-developed ultrathin BiVO 4 -Ov nanosheets.As for the hybrid photocatalyst, the O 1s spectra discloses three peaks situated at 530.55, 531.27, and 532.14 eV, which arise from the lattice O 2À atoms, Ov, and P-O-P, respectively. [33,34]Based on the XPS analysis, the shift in binding energies provides direct evidence for the successful formation of BiVO 4 -Ov@RP/BP homo-heterojunction with interfacial depletion layer that resembles the cascade flow of electrons. [35]

Optoelectronic Properties and Charge-Transfer Mechanisms
The insights on the light-harvesting kinetics of the as-prepared samples were assessed via ultraviolet-visible diffuse reflectance spectroscopy (UV-Vis DRS) as demonstrated in Figure 3a.The topotactic conversion of RP to mixed-phase RP/BP homojunction witnesses the enhancement of the absorption edge of RP to the NIR region due to the presence of BP with broadband absorption in the UV to NIR region.Specifically, the absorption edge of RP/BP sample is observed at 760 nm with an extension to the spectral range beyond 800 nm while exhibiting NIR absorption tail.On another note, the absorption edge of BiVO 4 -Ov is discerned at 530 nm with the display of a much more intense visible-light absorption capacity as opposed to the BiVO 4 -A counterpart.Additionally, the long absorption tail of BiVO 4 -Ov presents indisputable corroboration for the existence of large amount of surface defects throughout the sample such that the band tails of the defect states can be determined from the Urbach rule which is expressed by Equation (1). [36]¼ α 0 exp hv Eu (1)   α resembles the absorption coefficient whereas α 0 denotes a constant.The photon energy of the incident light is represented by hv, and the Urbach energy which is also the energy of the band tail is represented by Eu.By presuming the inverse gradient of the plot of ln α vs. hv as shown in Figure S3, Supporting Information, correlation between the energy of transition between the defect state and CB edge can be made using the Urbach energy.The Eu value was subsequently determined to be 0.18 eV, signifying the gap between the CB edge and the oxygen-vacancy-induced defect state.Evidently, it can be affirmed that the defect states are judiciously positioned below the CB.In addition, a redshift in the absorption edge of the BiVO 4 -Ov sample is identified, reflecting the narrowing of bandgap.In addition, the hybrid photocatalysts were found to be photoresponsive over a broad spectrum of solar energy.Further investigation unveils the formation of two absorption shoulders, as manifested in both BiVO 4 -Ov@RP/BP and BiVO 4 -A@RP/BP.In detail, the BiVO 4 photocatalysts contribute to the shoulder that is located at an absorption edge of 580 nm whereas the integration of RP/BP gives rise to the absorption edge beyond 600 nm; thus, this phenomenon validates the intimate contact between BiVO 4 -Ov and RP/BP.Integrating the favorable features of both RP/BP and BiVO 4 -Ov, the photoresponsivity of BiVO 4 -Ov@RP/BP covers a wide solar spectrum from the UV up to the NIR region, implying the capability of the composite system to generate greater number of charge carriers for participation in photocatalytic reactions, resulting in improved performance.In accordance with the transformed Kubelka-Munk (KM) plots as delineated in Figure S4a,b, Supporting Information, the bandgap values of RP/BP and BiVO 4 -Ov were found to be 1.80 and 2.56 eV, respectively.To investigate the electronic structure of RP/ BP and BiVO 4 -Ov as well as to glean mechanistic insights into the plausible charge-transfer pathway within the composite system, Mott-Schottky analysis was conducted as it provides critical information regarding the flat-band potential (E FB ) of the samples.Accordingly, the E FB of RP/BP was measured to be À1.04 V via the extrapolation of the Mott-Schottky plot (Figure S4c, Supporting Information) whereas BiVO 4 -Ov possesses an E FB of 0.01 V (Figure S4d, Supporting Information).The display of an apparent upward slope for both photocatalysts based on the Mott-Schottky graphs is consistent with the n-type behavior of semiconductors, and as a rule of thumb, the CB potential of n-type material is 0.20 V more negative than the E FB .With that in mind, the CB position of RP/BP and BiVO 4 were then predicted to be À1.24 and À0.19 V (vs.Ag/AgCl).Since the conversion of E FB with respect to NHE obeys the relationship [37] the CB potential of RP/BP was calculated to be À1.04 V whereas its VB potential was proposed to be 0.76 V (vs.NHE).By adopting a similar approach, the CB and VB potentials of BiVO 4 -Ov were found to be 0.01 V (vs.NHE) and 2.55 V (vs.NHE), respectively.
In accordance with the bandgap values, CB and VB potentials attained for BiVO 4 -Ov and RP/BP, dual Z-scheme mechanism and double-charge transportation are two probable routes involved in the photocatalytic reduction of CO 2 using RP/BP@BiVO 4 -Ov.First, in the dual Z-scheme system, by the recombination of CB electrons of BP and BiVO 4 -Ov with the VB holes of RP, the transfer of photoinduced electrons in the CB of RP to the CB of both BP and BiVO 4 -Ov is brought to a halt, which is beneficial for the spatial isolation of electrons in the CB of RP and holes in the VB of BP and BiVO 4 .Thus, the dual Z-scheme mechanism preserves the strong reduction and oxidation capacity of the photocatalyst owing to the seamless separation and maintenance of electrons and holes in the CB and VB, respectively. [38]To corroborate this claim, the Fermi energy level and work function of each component were studied using first principles calculations.According to density-functional theory (DFT) calculations (Figure 3b-d In other words, the dual Z-scheme mode propounds a much more superior reduction and oxidation potential for photocatalytic surface redox reactions accompanied by the improved separation, migration, and isolation of electron-hole pairs.That said, the dual Z-scheme system is highly likely to be generated.It is worth noting that the surface vacancy defects present throughout BiVO 4 -Ov nanosheets could act as potent electrontrapping centers to hinder electron-hole pairs recombination by extending the lifespan of photoexcited charges.The integration of dual Z-scheme mechanism with defect engineering via the formation of oxygen vacancies could govern the efficient photoreduction performance of BiVO 4 -Ov@RP/BP. Contrastingly, double-charge migration indicates the transfer of electrons from the CB of RP to the CB of BP and BiVO 4 -Ov (Figure S7, Supporting Information).On top of that, the doublecharge-transfer mechanism features the transportation of photogenerated holes in the VB of BP and BiVO 4 -Ov to the VB of RP.Despite the adequate electron-hole separation induced by the double-charge-transfer mode, such a homo-heterojunction system reduces the redox capability of light-excited charge carriers.On the one hand, a minimum redox potential of 0.82 V (vs.NHE) is essential for the photooxidation of H 2 O. Nevertheless, the oxidation potential of VB in RP (0.76 V vs. NHE) is less positive than 0.82 V. Hence, the double-charge-transfer system deprives the photogenerated holes of engaging in the oxidation of H 2 O. On the other hand, the CBM of BiVO 4 -Ov (0.01 V vs. NHE) is less negative than the redox potential required for the generation of CH 4 (À0.24V vs. NHE), thereby diminishing the driving force of CO 2 transformation into CH 4 .Consequently, the domination of the double-charge-transfer mechanism to justify the photocatalytic reduction of CO 2 over RP/BP@BiVO 4 -Ov is implausible.

Photoreduction of CO 2 and Mechanisms for Improved Performance
To examine the photocatalytic performances of the as-developed samples, visible-light-powered gas-phase CO 2 reduction was undertaken at ambient conditions.The total CH 4 yield generation over the photocatalysts is rendered in Figure 4a.As envisaged, BiVO 4 -Ov@RP/BP manifested the highest CH 4 yield of 14.52 μmol g À1 while negligible CH 4 production was detected for the solitary RP/BP, BiVO 4 -A, and BiVO 4 -Ov samples.The maximum CH 4 generation exhibited by the BiVO 4 -Ov@RP/BP composite provides direct evidence on the importance of Z-scheme homo-heterojunction formation as the coupling between BiVO 4 -Ov, which possesses powerful oxidation capacity and RP/BP with high reduction potential complements one another seamlessly by demonstrating a stark enhancement in photoactivity.To consolidate the pivotal role of oxygen-vacancy defects in ameliorating photocatalytic CO 2 -reduction reactions, comparisons between the CH 4 yield of BiVO 4 -A@RP/BP and BiVO 4 -Ov@RP/BP were made whereby the latter displayed a distinctive 1.5-fold improvement in photoactivity as opposed to BiVO 4 -A@RP/BP (9.41 μmol g À1 ) without oxygen vacancies.This phenomenon could be ascribed to the attraction of negative charge carriers to the vacancy centers as a result of the absence of oxygen atom from the BiVO 4 crystal lattice in the BiVO 4 -Ov@RP/BP sample, leading to a higher density of local charge carriers.Consequently, fortified visible-light harvesting with intense tail absorption in the NIR region is induced as proven by the UV-Vis spectra (Figure 3a) due to bandgap narrowing of BiVO 4 -Ov, this phenomenon is highly desirable as the strengthened light harvesting capability of BiVO 4 -Ov is envisaged to increase the photogeneration of electron-hole pairs, which is conducive for photocatalytic reactions.Moreover, oxygen-vacancy defects and the generation of sub-bands in BiVO 4 -Ov as featured in Figure 3f play indispensable roles in enhancing the separation of electron-hole pairs as an additional pathway is created for second-step electron transfer from the CB of BiVO 4 -Ov to the local defect states in lieu of the direct transition of electrons between CB and VB.When photogenerated electrons subside back to the VB from the CB, the electrons could reside temporarily in the sub-bands prior to recombination with the VB holes.This mechanism ultimately improves the spatial separation of charge carriers as well as photocatalytic performance as the faster-moving holes are now available in abundance to participate in photocatalytic oxidation reactions.Additionally, the presence of surface oxygen vacancies also strengthens the interaction between BiVO 4 -Ov and H 2 O, facilitating charge transfer and decreasing the energy required to dissociate H 2 O. [39] On top of that, the importance of ultrathin 2D material design for semiconductors was also investigated through assessing the yield of BiVO 4 -A@RP/BP with ultrathin 2D BiVO 4 nanosheets and BiVO 4 -B@RP/BP with bulk BiVO 4 .As discerned from Figure 4a, the CH 4 yield recorded for BiVO 4 -A@RP/BP is an astounding 1.5-time enhancement in comparison to BiVO 4 -B@RP/BP (6.15 μmol g À1 ), corroborating the significance of ultrathin 2D material construction in elevating photocatalytic activities.Specifically, the ultrathin 2D BiVO 4 could improve exciton spatial separation and migration, molecule adsorption, as well as the activation of photocatalytic processes owing to their thickness in the atomic scale. [10,40,41]Not only that, the ultrathin 2D could generate a higher density of surface atoms which function as active centers to boost photocatalytic processes as a result of the substantially enhanced specific surface area.The Brunauer-Emmett-Teller (BET) surface area of BiVO 4 -A nanosheets were determined to be 144.37 m 2 g À1 , which is a striking 4.4-fold enhancement in comparison to BiVO 4 -B (32.47 m 2 g À1 ). Figure S8, Supporting Information, features the N 2 adsorptiondesorption isotherms of BiVO 4 -A and BiVO 4 -B samples.The ultrathin structure also synchronically reduces the transportation distance of electron-hole pairs formed in the core of BiVO 4 to the photocatalytic surface, effectively hampering the recombination of light-excited charge carriers. [22]These findings indubitably reveal that the rational design and hybridization of BiVO 4 with RP/BP with the coexistence of Z-scheme homo-heterojunction, surface disorder defects, and ultrathin 2D design are key contributors in achieving excellent CH 4 production.
To further consolidate the eminency and advantages of the ternary homo-heterojunction with dual Z-scheme system in comparison to binary heterojunction with single Z-scheme mechanism, CO 2 photoreduction was conducted for BiVO 4 -Ov@RP, BiVO 4 -A@RP, and BiVO 4 -B@RP samples.In accordance with Figure 4a, the binary-component systems exhibited a much more inferior CH 4 production as opposed to the BiVO 4 -Ov@RP/BP photocatalyst (14.52 μmol g À1 ).The more superior photocatalytic performance demonstrated by the ternary dual Z-scheme system in comparison to the binary single Z-scheme mode could be justified by the presence of BP which endows a supplementary Ohmic junction for enhanced separation and transportation of charge carriers.The formation of two charge relay pathways at the interface of RP/BP and RP/BiVO 4 -Ov indubitably improves the charge-carrier dynamics through encouraging the facile transfer of electrons by virtue of the more intense electric fields generated at the homo-heterojunction.These findings substantiate the merits of crafting a ternary dual Z-scheme photocatalytic system over binary single Z-scheme composites for the photoconversion of CO 2 to CH 4 .To clarify the carbon source of CH 4 generated for the BiVO 4 -Ov@RP/BP sample, isotope tracer analysis was undertaken by using 13 CO 2 gas as the feedstock.Based on the inset of Figure 4a, 13 CH 4 was generated from the isotope experiment, verifying that the carbon source for CH 4 production is not carbon-containing impurities.[44][45][46][47][48][49] In this respect, BiVO 4 -Ov@RP/BP displays much better CO 2 -reduction performance than most BiVO 4 -based photocatalysts.More importantly, exceptional photostability is demonstrated by the BiVO 4 -Ov@RP/BP sample (Figure 4b), where the composite system maintained %91% of photocatalytic activity after three successive test cycles of CO 2 conversion.It is worth to note that the decline in photoactivity is a common occurrence for oxygen-defective materials as the dissociation of CO 2 molecules produces free oxygen atoms, that reoxidizes the surface with oxygen vacancies, resulting in the annihilation of oxygen defects. [36]The sample information and stability of the as-synthesized BiVO 4 -Ov@RP/BP photocatalyst before and after three successive test cycles of photocatalytic CO 2 reduction were also investigated via XRD analysis as shown in Figure S9, Supporting Information.The results indicate no discernible change in diffraction peaks, suggesting that the asdeveloped composite photocatalyst possesses a relatively stable structure.
To unravel the mechanistic insights and charge transportation behaviors which led to the superlative CO 2 -photoreduction performance of the BiVO 4 -Ov@RP/BP hybrid photocatalyst, electrochemical impedance spectroscopy (EIS) Nyquist analysis was undertaken, and the Nyquist plots are depicted in Figure 4c.Based on the figure, BiVO 4 -Ov@RP/BP exhibits a semicircle that is distinctly smaller and much more depressed than that of RP/BP, BiVO 4 -A, BiVO 4 -Ov, and BiVO 4 -A@RP/BP.This suggests that the process of relaying electrons in the BiVO 4 -Ov@RP/BP Z-scheme homo-heterojunction is a facile and smooth transportation process due to the intimate interfacial contact formed between RP and BiVO 4 -Ov as well as RP and BP.This fortifies the presence of an internal electric field which is responsible for the vectorial transfer of electrons alongside the retardation of electron-hole pair recombination.In addition, the smaller arc radius observed for the Nyquist plot of BiVO 4 -Ov@RP/BP entrenches that the sample possesses excellent electroconductivity due to the presence of surface oxygenvacancy defects. [50]On the contrary, bare RP/BP, BiVO 4 -A, and BiVO 4 -Ov exhibit enlarged arc radius due to the absence of a secondary material that acts as a charge relay platform.In accordance with the Nyquist plot (Figure 4c), all the photocatalysts show a similar trend to their CO 2 -reduction performances (Figure 4a).That said the charge-transfer impedance follows the descending order of RP/BP > BiVO 4 -A > BiVO 4 -Ov > BiVO 4 -A@RP/BP > BiVO 4 -Ov@RP/BP.Furthermore, results obtained from the EIS Nyquist analysis concur with the photoluminescence (PL) spectra in which the weaker emission peak of BiVO 4 -Ov@RP/BP (Figure 4d) represents an enhanced separation and migration efficiency of electron-hole pairs as well as a concomitant improvement in charge-carrier lifespan attributed to the presence of oxygen vacancies endowing the trapping of electrons for suppressed recombination.Moreover, the large lateral dimension to thickness ratio of ultrathin 2D BiVO 4 nanosheets are well capable of shortening the migration distance of charge carriers, hence promoting a higher density of charges to participate in photocatalytic redox reactions prior to recombination.To study the fluorescence decay lifetime as well as the charge-carrier dynamics of the photocatalysts, time-resolved PL (TRPL) was employed as disclosed in Figure S10, Supporting Information.The standard bi-exponential model best describes the fluorescence decay profile as delineated by Equation ( 2) whereby B and τ represent the relative amplitude and lifetime for the individual component, respectively.
The bi-exponential model converged and rendered a two-step fluorescence energy transfer in which the photocatalysts participated in both fast τ 1 (near band-edge emission) and slow τ 2 (trap state emission) energy-transfer processes.Intriguingly, The BiVO 4 -Ov sample was revealed to exhibit the longest decay lifetime of 1.0 ns, which is greater in comparison to the average decay lifetime of BiVO 4 -A (0.68 ns) and RP/BP (0.45 ns).The prolonged average decay lifetime of BiVO 4 -Ov as opposed to BiVO 4 -A corroborates the effective impediment of charge-carrier recombination attributing to the incorporation of oxygen vacancies in BiVO 4 that serve as electron reservoir.In contrast, the average decay lifetime of BiVO 4 -Ov@RP/BP is significantly reduced to 0.35 ns, implying the enhanced charge migration behavior and expedited electron transfer separating the emissive state across the internal electric field of the Z-scheme system.
First principles calculations were employed to further unravel the effects of oxygen-vacancy defects incorporation on the electronic properties and structure of ultrathin 2D BiVO 4 -Ov nanosheets.We determined the defect formation energies as summarized in Table S2, Supporting Information, for the BiVO 4 -lattice where four sites were investigated to dictate the most favorable site for oxygen atom removal; they are surface oxygen atoms coordinated to V (O1), surface oxygen atoms coordinated to Bi (O 2 ), bulk oxygen atoms coordinated to Bi (O 3 ), and bulk oxygen atoms coordinated to V (O 4 ).Intriguingly, the formation of oxygen-vacancy defect at O 2 (circled in blue as shown in Figure 5a) rendered the lowest formation energy, i.e., À3.37 eV, revealing it to be the most probable site for defect (energetically favorable) in BiVO 4 since a more negative energy of formation suggests the thermodynamic feasibility and stability of the configuration.Henceforth, O 2 configuration was adopted as the model of concept for BiVO 4 -Ov, and its optoelectronic behaviors and charge dynamics were explored to unearth the significance of defect engineering via oxygen-vacancy incorporation in improving photocatalytic performance.
The partial density of states (PDOS) of BiVO 4 integrated with and without oxygen vacancies can be observed in Figure 5b.It is noted that both pristine BiVO 4 and BiVO 4 -Ov nanosheets share similar PDOS profile in which their VB is greatly dominated by O 2p orbital whereas their CB is contributed by O 2p, Bi 6p, and V 3d orbitals.The integration of oxygen vacancies witnesses the downshift of CB energy level, indicating bandgap narrowing, and suggests that the oxygen vacancies can efficaciously augment the visible-light responsivity of BiVO 4 -Ov.These alterations favorably elevate the photogeneration and photoexcitation of electrons and holes for participation in reduction and oxidation reactions, respectively.Figure 5c delineates the charge density spatial distribution at the Fermi level.In accordance with the figure, majority of positive charge carriers (yellow regions) are concentrated around the oxygen-vacancy sites whereas negative charge carriers (dark blue regions) are localized around surrounding neighboring atoms.The electron-depleted oxygen-vacancy defect states in BiVO 4 -Ov allow the trapping of photogenerated electrons upon the illumination of visible light.This corroborates the claim that oxygen vacancies facilitate the accumulation of electrons, which lead to the creation of an inner electric zone that is beneficial for the formation of depletion layers at the interface of BiVO 4 -OV@RP/BP. [16]Hence, effective migration of electrons and holes can occur, leading to the construction of cascade electron flow, which represents the Z-scheme configuration.

Conclusion
In summary, we have successfully developed and coupled ultrathin 2D BiVO 4 with RP/BP with the coexistence of oxygen-vacancy defects on BiVO 4 and dual Z-scheme homo-heterointerface.The as-synthesized BiVO 4 nanosheets possess an average thickness of less than 5 nm and surface oxygen-vacancy defects.These exemplary features bestow BiVO 4 -Ov nanosheets with superior optoelectronic properties and photooxidation performance such that when it is amalgamated with RP/BP, the CH 4 evolution rate reached 14.52 μmol g À1 .In stark contrast, RP/BP, BiVO 4 -Ov, and BiVO 4 -A displayed unsatisfactory CO 2 -reduction performance as no CH 4 was detected for these samples.With the ingenious combination of defect engineering and ultrathin 2D material design in BiVO 4 and its subsequent hybridization with mixed-phase RP/BP for the formation of dual Z-scheme homoheterojunction, the isolation and life span of electron-hole pairs are substantially enhanced.In particular, surface oxygen-vacancy defects present in BiVO 4 temporarily occupy the electrons to delay exciton recombination whereas its ultrathin 2D structure effectively amplifies the contact surface area while minimizing the charge migration distance from the core to the surface of the photocatalyst.Cooperating synergistically with the intense internal electric field at the BiVO 4 -Ov@RP/BP homo-heterointerface which warrants a dominant steering force for Z-scheme electron transportation, the presence of dual space-charged regions and lower resistance to charge-transfer encourage the smooth transportation of electron-hole pairs to the surface of the photocatalysts to augment their participation in photocatalytic reactions.As a whole, the development of BiVO 4 -Ov@RP/BP in this work affords new inspiration for the rational steering of charge transfer and modulation of catalytic active sites to realize efficacious CO 2 -reduction activity.
Purification of RP: Purification of RP photocatalysts was first undertaken via a facile hydrothermal treatment route to eliminate the oxide layer on the surface of the material.Typically, 3.0 g of commercial bulk RP was added into 80 mL of deionized H 2 O and vigorously stirred.The dissolved mixture was subsequently loaded and sealed in a Teflon-lined stainless autoclave.The autoclave reactor was then heated to 200 °C in a muffle furnace and maintained for 12 h.After cooling down to room temperature, the products were collected and washed with ethanol and deionized H 2 O three times, respectively.Finally, the products were left to dry overnight at 60 °C in a vacuum oven.
Synthesis of Mixed-Phase RP/BP Homojunction Photocatalyst: By using EDA as the reaction solvent and purified RP as the feedstock, a simple one-step wet-chemistry approach was employed to fabricate the mixedphase RP/BP homojunction.In a typical procedure, 60 mL of EDA solution was used to dissolve 1.80 g of purified RP by vigorously stirring the mixture for 30 min.Thereafter, the solution consisting of RP uniformly dispersed in EDA solvent was poured into a Teflon-lined stainless autoclave.The mixture was then heated to 165 °C in a muffle furnace and maintained for 16 h.After cooling down to room temperature, the products were collected and washed with ethanol and deionized H 2 O three times, respectively.Finally, the products were left to dry overnight at 60 °C in a vacuum oven.
Synthesis of Oxygen-Defective Ultrathin BiVO4 Nanosheets: In a typical synthesis, 0.5 mmol of Bi(NO 3 ) 3 •5H 2 O was added to a solution comprising 1 mL of OA, 1 mL of OLA, and 10 mL of ODE, and the mixture was poured into a three-neck flask.The solution in the three-neck flask was degassed by purging nitrogen (N 2 ) for 30 min under vigorous stirring and then heated to 170 °C using a heating mantle equipped with a magnetic stirrer.In another beaker, a mixture of 10 mL of deionized H 2 O and 2 mL of HNO 3 was prepared and 1 mmol of NH 4 VO 3 was later added and dissolved in the H 2 O/HNO 3 mixture until a bright yellow solution was obtained.The aqueous NH 4 VO 3 solution was then carefully injected into the three-neck flask comprising dissolved Bi(NO 3 ) 3 •5H 2 O in OA/OLA/ODE mixture.Under inert N 2 environment, the reaction temperature was adjusted to 100 °C and maintained for 30 min.The mixture was left to cool naturally upon the termination of the heat source after 30 min.The products in the three-neck flask were then poured into a mixture of ethanol and hexane to stratify the solution and the subjacent aqueous solution was removed.The remaining solution which contained the ultrathin BiVO 4 -Ov nanosheets was centrifuged at 7500 rpm for 10 min.A mixture of ethanol and hexane was used to wash the precipitates five times.The final precipitates were collected and dried overnight.It must be emphasized that the addition of HNO 3 was indispensable in dissolving NH 4 VO 3 and introducing oxygen vacancies into the BiVO 4 nanosheets owing to the presence of hydroxyl groups.When heated to a temperature of 100 °C in the presence of OA, OLA, and ODE, VO 3 À or VO 4 3À was postulated to be present as VO 2 þ in HNO 3 aqueous solution, and the reduction of VO 2 þ by OA and OLA eventually led to the formation of oxygen vacancies (V 4þ ).Apart from that, HNO 3 also played a pivotal role in constructing the ultrathin 2D structure as the hydroxyl groups present established a highly acidic environment that decreased the surface energy of the BiVO 4 to encourage the formation of 2D geometry.It is therefore believed that the ultrathin BiVO 4 nanosheets that were exposed in a solution of highly concentrated HNO 3 possessed a much more stable structure and lowest surface free energy. [20]ynthesis of Ultrathin BiVO4 Nanosheets without Oxygen Vacancies and Bulk BiVO4: To corroborate the existence of oxygen vacancies in BiVO 4 -Ov nanosheets and study their effects on optoelectronic properties and photocatalytic reactions, BiVO 4 nanosheets without oxygen vacancies were also prepared.In detail, the ultrathin BiVO 4 -Ov samples underwent thermal treatment via annealing in air at 500 °C.By virtue of the passivation impacts of oxygen, the number of oxygen-vacancy defects would be substantially minimized.The annealed sample is referred to as BiVO 4 -A.Bulk BiVO 4 (BiVO 4 -B) was also prepared.Specifically, 20 mmol of Bi(NO 3 ) 3 •5H 2 O and 20 mmol of NH 4 VO 3 were dissolved in 80 mL of deionized H 2 O under 1 h of continuous stirring.Thereafter, the solution was poured into a Teflon-lined stainless autoclave.The mixture was then heated to 180 °C in a muffle furnace and maintained for 6 h.After cooling down to room temperature, the products were collected and washed with ethanol and deionized H 2 O three times, respectively.Finally, the products were left to dry overnight at 60 °C in a vacuum oven.
Preparation of Ultrathin BiVO 4 -Ov@RP/BP Homo-Heterojunction Photocatalyst: The BiVO 4 -Ov@RP/BP samples were fabricated by a facile sonochemical strategy.In detail, 0.1 g of RP/BP and 0.1 g of BiVO 4 -Ov were added and dispersed in 100 mL of DMSO solution.The RP/BP and BiVO 4 dispersion was stirred continuously for 2 h and subsequently ultrasonicated for another 2 h.High speed centrifugation (7500 rpm, 30 min) was employed to collect the products from the dispersion.The products were later washed with ethanol and deionized H 2 O three times, respectively.Finally, the BiVO 4 -Ov@RP/BP products were dried overnight at 60 °C in a vacuum oven.Similar technique was used to prepare BiVO 4 -A@RP/BP and BiVO 4 -B@RP/BP.
Materials Characterization: The structural characteristics, morphology, and elemental composition of the photocatalytic samples were observed and assessed using a Hitachi SU8010 FESEM integrated with EDX.In contrast, HRTEM images were acquired using TECNAI G2 F20 TEM.The topography and thickness of BiVO 4 -Ov nanosheets were obtained using Bruker Multimode 8 AFM.The XPS analysis was conducted using a Phoibos 100 spectrometer (SPECS, Germany), which employs a monochromatic Al-Kα X-Ray source.A charge correction before deconvolution with reference to the C-C peak possessing a binding energy of 284.60 eV from the C 1s spectra was applied to the binding energies obtained for all the samples.UPS analysis was undertaken using Nexsa G2 Surface Analysis System (Thermo Fisher Scientific) by using vacuum UV radiation for the induction of photoelectric effects.The emission of photons by helium gas possessed an energy of 21.2 eV (He I).The powder XRD patterns and crystallographic behaviors of the developed materials were analyzed using a Bruker D8 Discover X-Ray diffractometer at a scanning rate of 0.02 s À1 with nickel (Ni)-filtered copper (Cu) Kα radiation.To verify the presence of oxygen-vacancy defects in BiVO 4 -Ov nanosheets, ESR spectrometer (JEOL, JES-FA200) was employed.By utilizing a Micromeritics 3Flex instrument, the BET surface area of the photocatalysts was attained via the adsorption of N 2 at a temperature of 77 K.In addition, Agilent Cary 100 UV-Vis spectrophotometer equipped with an integrated sphere was utilized to measure and analyze the optical absorption spectra of all samples for the wavelength ranging from 200 to 800 nm, with BaSO 4 as the reflectance standard.PL) measurements were obtained with the assistance of a LS 55, Perkin Elmer fluorescence spectrophotometer, at an excitation wavelength of 300 nm.The TRPL properties of the photocatalysts were assessed using HORIBA DeltraPro-DD time-resolved spectrofluorometer at an excitation wavelength of 317 nm.
Electrochemical Analysis: The photoelectrochemical measurements of the developed photocatalysts namely Mott-Schottky and Nyquist analysis were undertaken using a CHI 6005 E electrochemical workstation equipped with a standard three-electrode cell.In the electrochemical system, Pt rod functioned as the counter electrode whereas Ag/AgCl saturated with 3 M KCl served as the reference electrode.The system also comprised a working electrode and an electrolyte (0.5 M Na 2 SO 4 ).To prepare the working electrode, photocatalyst powder was first suspended in ethanol/Nafion (4.90 mL: 0.10 mL) solution and subsequently spraycoated onto an fluorine-doped tin oxide (FTO) glass slide with 1 cm 2 of active area.Mott-Schottky analysis was conducted at a frequency of 100 Hz within a range of À2.0-0.50V and a potential step of 50.0 mV.Conversely, at an applied potential of 0.20 V (vs.Ag/AgCl), the EIS measurements were recorded at an AC sinusoidal perturbation signal of 10.0 mV and a frequency of 10 À2 -10 6 Hz.
Evaluation of Photocatalytic Activities: The photocatalytic performances of the developed photocatalysts were evaluated via the conversion of CO 2 to CH 4 .The CO 2 -photoreduction experiment was undertaken at ambient conditions in a photocatalysis rig which featured a custom-made, continuous gas flow reactor.A 500 W xenon arc lamp (CHF-XM-500 W) equipped with UV cutoff filter (λ > 400 nm) was used as the visible-light source.For the setup of the CO 2 conversion experiment, photocatalyst powder was coated onto two glass rods which were subsequently placed into their respective quartz columns prior to the start of the CO 2 -photoreduction experiment.The quartz columns were situated in an enclosed black box to eliminate external light source interferences.The columns were firmly hitched into the quartz column holder to ascertain that they remained affixed throughout the entire photoconversion experiment.Each end of the quartz column was then attached with individual piping.To provide CO 2 -H 2 O gas mixture, high purity CO 2 gas (99.999 %) was bubbled through liquid H 2 O and channeled into the photocatalysis rig as the reactant gas.Before starting the photocatalytic reduction experiment, the system was purged with CO 2 -H 2 O gas mixture at a flow rate of 50 mL min À1 for 30 min to eliminate contaminants from the quartz columns and to ensure the achievement of adsorption-desorption equilibrium on the photocatalyst surface.After purging, the CO 2 -H 2 O gas mixture flow rate was adjusted to 5 mL min À1 for 6 h under visible-light irradiation.A gas chromatography (Agilent 7820 A GC) using helium as a carrier gas and equipped with flame-ionized detector and thermal conductivity detector was employed to quantify and examine the product gas components at hourly intervals.The total CH 4 yield was calculated using Equation (3).
Yield ðμmol g À1 Þ ¼ Moles of product formed ðμmolÞ Mass of photocatalyst used ðgÞ To attest the carbon source of CH 4 gas generated, isotope experiment was undertaken for the BiVO 4 -Ov@RP/BP sample using 13 CO 2 gas as the feedstock and the products were analyzed using GC mass spectrometry (Agilent 7890 A and 5975 C).
DFT Calculation: To obtain the BiVO 4 structure in the present study, a vacuum of 15 Å was added in the z-direction of a bulk supercell (115-atom model). [51]The BiVO 4Àx model was generated by removing the surface O atoms coordinated to V (O 1 ) and Bi (O 2 ) and bulk O atoms coordinated to Bi (O 3 ) and V (O 4 ) with coordination number of two and three, resulting in a total of four BiVO 4Àx slabs models.The spin-polarized DFT calculations were carried out in the Vienna ab initio simulation package (VASP) suite of programs (v5.4.4). [52]The electron exchange-correlation interactions were described based on the Perdew-Burke-Ernzerhof of the generalized gradient approximation functional. [53]The projector-augmented-wave pseudopotentials for each atomic species specified in the calculation were determined by VASP. [54]The long-range van der Waals interactions were included based on Grimme's DFT-D3 method. [55]An energy cutoff of 500 eV was adopted for the plane-wave basis set.All ions were allowed to relax until the residual Hellmann-Feynman forces were less than 0.01 eV Å À1 and the electronic self-consistency was converged to below 10 À5 eV.Monkhorst-Pack scheme for the reciprocal space k-point meshes of 3 Â 3 Â 2 was used to sample the Brillouin zone. [56]
), the work functions of BiVO 4 -Ov, RP, and BP were determined to be 4.70, 4.23, and 4.93 eV, respectively.The work function values attained from DFT calculations are further ratified using ultraviolet photoelectron spectroscopy (UPS) analysis with He I as the excitation source.As depicted in FigureS5, Supporting Information, the secondary cutoff binding energy (E Cutoff ) of RP/BP and BiVO 4 -Ov were determined to be 17.64 and 17.31 eV, respectively, via the extrapolation of the linear region to the base line of the UPS spectra.By using the formula Φ = hv À E Cutoff , the work function (Φ) of RP/BP and BiVO 4 -Ov were calculated to be 4.36 and 4.69 eV, respectively, which is in good agreement with the work function values acquired using DFT calculations.Bader charge analysis was undertaken via DFT calculations to elucidate the transportation of charges and further substantiate the formation of Z-scheme at the BiVO 4 @RP/BP interface.In accordance with the Bader analysis, a positively charged atom typically resembles an electron-rich atom whereas a negatively charged atom implies a deficiency of electrons.In accordance with Figure3e, the negatively charged P atoms (À0.74 and À0.81 e) indicate depletion of electrons while the positively charged atoms of BiVO 4 -Ov (1.03 and 0.95 e) indicate accumulation of electrons.Hence, it can be deduced that free electrons were transferred from RP to BiVO 4 -Ov during the equilibration of Fermi levels.Working in unison with the bandgap values attained from the transformed KM plots, the respective electronic band structure of BiVO 4 -Ov, RP and BP is proposed as can be seen in FigureS6, Supporting Information.When all three components are in contact, the free electrons from RP possessing higher Fermi energy level diffuse into BiVO 4 -Ov and BP due to the imbalance in concentration of charges as a result of the difference in Fermi energy levels.Following the diffusion of charges at the interface of BiVO 4 -Ov/RP and RP/BP, the energy band of RP experiences a concomitant downward shift while the Fermi energy levels of BiVO 4 -Ov and BP shift upward until an equilibration in Fermi energy level is reached.This occurrence results in the generation of depletion layers at the interfaces of the homo-heterostructure as the interface of RP amasses positive charges while the interfaces of BiVO 4 -Ov and BP accumulate negative charges.The formation of two space-charged regions, also referred to as the dual Z-scheme homo-heterojunction, can create internal electric fields and Coulomb interactions near the interface of BiVO 4 -Ov/RP and RP/BP, which represents Ohmic contact for vectorial transfer of electrons.This powers the accelerated separation and migration of charge carriers which in turn depresses the recombination of electron and holes while maintaining strong redox performance.A clearer insight into the plausible charge transportation pathway of the nanocomposite is portrayed in Figure3f.The rate of CH 4 production is further enhanced as more reductive electrons and oxidative holes are formed in the CB and VB of RP and BiVO 4 , respectively.This is because a more negative redox potential of À1.04 V (vs.NHE) is proffered for the reduction of CO 2 to CH 4 .Similarly, more oxidative holes are formed in the VB of BiVO 4 -Ov for H 2 O oxidation.

Figure 5 .
Figure 5. a) Schematic view of the optimized structure for the four considered configurations of oxygen-vacancy defect sites (green ball-and-stick) for BiVO 4 .Formation of oxygen-vacancy defect at O 2 (circled in blue) is the most energetically favorable site.b) Partial density of states (PDOS) of BiVO 4 and BiVO 4 -Ov.Fermi level is resembled by the dashed line.c) Charge density differences for the most preferable oxygen-vacancy defect site for BiVO 4 -Ov.The yellow and dark blue regions represent depletion and accumulation of electrons, respectively.