Bulk Charge Separation Research Articles

Ferroelectric polarization effect promoting the bulk charge separation for enhance the efficiency of photocatalytic degradation

Focused on the separation of photoinduced carriers to improve the photocatalytic performance of semiconductors, polarized C/Bi/Bi2MoO6 were synthesized by electrical polarizing method in presence of organic–inorganic composite films. Significantly, this poling process leads to the better photoactivity for degrade bisphenol A (BPA) and ciprofloxacin (CIP), catalyzing the generation of superoxide radicals (•O2–) and holes (h+) to removal pollutants either through the ferroelectric polarization effect. Meanwhile, the non-destructiveness during the entire polarization process helps maintain the crystal structure and morphology of the catalyst to achieve better photocatalytic performance. This present work not only provides a great route for preparation of various polarized ferroelectric semiconductors, but also indicates the potential of ferroelectric polarization for modification of semiconductors.

A significant enhancement of bulk charge separation in photoelectrocatalysis by ferroelectric polarization induced in CdS/BaTiO3 nanowires.

Efficient charge separation, in particular bulk charge separation (BCS), is one of the most critical factors in determining the performance of photoelectrochemical (PEC) water-splitting. The BCS enhancement of CdS/BaTiO3 (CdS/BTO) nanowires (NWs) in photoelectrocatalysis has rarely been reported. This paper describes a remarkable PEC properties promotion of the CdS/BTO NWs, which is confirmed to be a result of the enhanced BCS efficiency induced by the ferroelectric polarization. The vertical arrays of BTO NWs endow fast transfer of carriers. Meanwhile, CdS is decorated uniformly on the surface of BTO NWs, which ensures a wide range of light absorption. After two negative polarizations, the CdS/BTO NWs have successfully obtained a remarkable photocurrent density, achieving 459.53 μA cm−2 at 1.2 V(vs.RHE), which is 2.86 times that of the unpolarized sample. However, after two positive polarizations, the photocurrent density dramatically decreases to 40.18 μA cm−2 at 1.2 V(vs.RHE), which is merely 0.25 times the original value. More importantly, the photocurrent density reaches up to a prominent value of −71.09 mA cm−2 at −0.8 V(vs.RHE) after two successive negative polarizations, which is a 40.87 mA cm−2 enhancement with respect to the sample without poling. Significantly, at −0.8 V(vs.RHE), the BCS efficiency of the CdS/BTO NWs is as high as 91.87% after two negative polarizations. The effects of ferroelectric polarization on the PEC performance of CdS/BTO NWs have been systematically studied. The results demonstrate that ferroelectric polarization, especially negative polarization, results in an internal electric field to tune band bending of CdS/BTO NWs, thus prominently enhancing the PEC performance.

Photoassisted chemical energy conversion into electricity using a sulfite‑iron photocatalytic fuel cell

Abstract Photocatalytic fuel cells (PFC) are light-assisted devices that convert chemical energy into electricity. However, conventional PFC produces extremely low photocurrents due to the slow kinetics of the photoanodes to oxidized organic fuels, thus limiting the power generation. Here, an all-inorganic PFC manufactured with heterostructured W-BiVO4/V2O5 photoanode separated from a platinum cathode by a porous membrane is described. Sulfite ions are used as a fuel anolyte and Fe3+ is the catholyte. Under lighting, the W-BiVO4/V2O5 photoanode is exceptionally efficient for oxidizing sulfite to sulfate, which results in a high photocurrent output. Under optimized conditions, the W-BiVO4/V2O5-Pt cell produces a high short-circuit current of 8.79 mA cm−2, an open-circuit potential 0.85 V, and a power density of 1.89 mW cm−2 at 4.30 mA cm−2. The excellent PFC performance is attributed to the photovoltage generated by W-BiVO4/sulfite-sulfate liquid-junctions and efficient charge separation and hole transport in the photoanode bulk promoted by the W-BiVO4/V2O5 junctions. Also, the kinetics of redox reactions are fast, eliminating the use of cocatalysts. Thus, this simple PFC concept can be a viable alternative for electricity generation.

Coupling ferroelectric polarization and anisotropic charge migration for enhanced CO2 photoreduction

CO2 photoreduction into solar fuels is promising for generating renewable energy. Herein, SrBi2Nb2O9 nanosheets are prepared as high-performance photocatalysts for CO2 reduction, highlighting superiority of ferroelectric polarization and anisotropic charge migration. Ferroelectric polarization within SrBi2Nb2O9 nanosheets provides an in-built electric field, which greatly facilitates the bulk charge separation. Also, the photogenerated electrons and holes migrate separately to the NbO6 octahedral layers and within the ab-plane in the Bi2O2 layers, achieving efficient anisotropic charge migration. Without co-catalyst or sacrificial agent, SrBi2Nb2O9 nanosheets show outstanding CO2 reduction activity in producing CH4. The ferroelectric polarization is further enhanced by electric poling and annealing post-treatments. The electrically poled SrBi2Nb2O9 shows a high CH4 evolution rate of 25.91 μmol g−1 h−1 with an AQE of 1.96 % at 365 nm, exceeding most of state-of-the-art photocatalysts reported to date. This work paves an avenue for development of highly efficient photocatalysts and beyond by tuning the ferroelectricity and electronic structure.

Role of Alkali Metal in BiVO4 Crystal Structure for Enhancing Charge Separation and Diffusion Length for Photoelectrochemical Water Splitting.

Alkali metal (Na or K) doping in BiVO4 was examined systematically for enhancing bulk charge separation and transport in addition to improving charge transfer from the surface. The alkali metal-doped BiVO4 thin film photoanodes having nanostructured porous grain surface morphology exhibited better photocurrent density than pristine BiVO4. In particular, Na:BiVO4/Fe:Ni/Co-Pi photoanode showed a significantly improved photocurrent of 3.2 ± 0.15 mA·cm-2 in 0.1 M K2HPO4 electrolyte at 1.23 VRHE under 1 sun illumination. The depth-dependent Doppler broadening spectroscopy measurements confirmed the significant reduction in Bi- and V-based defect density with Na metal doping, and this led to a higher bulk diffusion length of charge pairs (four times that of the pristine one). Na doping led to reduced surface defects resulting in improved surface charge transfer based on cyclic voltammetry experiments. The density functional theory calculations confirmed the improved performance in Na-doped BiVO4 photoanodes achieved through interband formation and reduction in the band gap.

Enhanced Bulk Transport in Copper Vanadate Photoanodes Identified by Combinatorial Alloying

<h2>Summary</h2> The impact of alloying on the performance of β-Cu₂V₂O₇ photoanodes was investigated using inkjet printing of composition libraries containing 1,809 Cu₂V₂O₇-based photoanodes. Six elements (Zr, Ca, Hf, Gd, La, and Lu) were alloyed and pairwise co-alloyed at concentrations up to 7 at % into Cu-rich, stoichiometric, and Cu-deficient host Cu₂V₂O₇. A 1.7-fold increase in oxygen evolution photocurrent in pH 9.2 electrolyte was obtained by alloying Ca into β-Cu₂V₂O₇. Experiments employing a hole scavenger to better characterize bulk charge separation and transport revealed a 2.2-fold increase in photoactivity via alloying with Hf, Zr, and La, which increased to 2.7-fold upon co-alloying these elements with Ca. Concurrent with increased photoactivity is substantially decreased photon absorption between 1.5 and 2 eV, a range reported to coincide with high exciton absorption in β-Cu₂V₂O₇, motivating further exploration of whether these co-alloy compositions may destabilize the excitonic state that appears to have limited performance to date.

Emerging Nanomaterials for Light‐Driven Reactions: Past, Present, and Future

Emerging Nanomaterials for Light‐Driven Reactions: Past, Present, and Future

Effect of Lactic Acid on the Photoelectrocatalytic Water Splitting of Hematite Prepared by Hydrothermal Method

The photoelectrocatalytic water splitting performance of the pristine hematite is limited because of its poor bulk charge separation efficiency. In this study, the lactic acid modified hematite films were grown in situ on Fluorine-doped tin oxide substrates by hydrothermal method and then annealed in nitrogen atmosphere. At 1.5 V versus RHE, the photocurrent density of the optimal lactic acid modified hematite photoanode (0.075LA-Fe2O3) was 1.5 mA cm− 2, which was three times that of the based hematite (0.5 mA cm− 2). The enhanced photoelectrocatalytic performance of the lactic acid modified hematite was attributed to its increased bulk charge separation efficiency caused by the oxygen vacancies and island-like pattern, as well as its improved surface charge injection efficiency. The effect of lactic acid on the morphology, crystalline structure and photoelectrocatalytic performance of the hematite photoanode are studied by systematical characterization.

Metal-Organic Framework-Derived p-Cu2O/n-Ce-Fe2O3 Heterojunction Nanorod Photoanode Coupling with a FeOOH Cocatalyst for High-Performance Photoelectrochemical Water Oxidation.

Rapid charge recombination and slow water oxidation kinetics are key drawbacks that limit the photoelectrochemical water splitting efficiency of Fe2O3. In this work, we designed and fabricated for the first time that a metal-organic framework (MOF)-derived p-Cu2O/n-Ce-Fe2O3 nanorod array photoanode for the photogenerated charge effectively separated and transported at the Cu2O/Ce-Fe2O3 p-n heterojunction interface through a built-in electric field. In addition, the MOF-derived porous Cu2O nanoparticles have a large surface area, and thus, can offer more surface active sites for water oxidation. As anticipated, the novel structure Cu2O/Ce-Fe2O3 photoanode showed superior photocurrent density (3.2 mA cm-2), excellent bulk charge separation efficiency (38.4%), and surface charge separation efficiency (77.2%). After further modification with the FeOOH cocatalyst, the photocurrent density of the FeOOH/Cu2O/Ce-Fe2O3 photoanode reached 4.2 mA cm-2 at 1.23 VRHE (V vs reversible hydrogen electrode), having a low onset potential of 0.63 VRHE.

Research Progress and Challenges of&amp;alpha;-Fe2O3 Photoanode for Photoelectrochemical Water Splitting

In recent years,photoelectrochemical(PEC)water splitting to produce hydrogen provides a clean and renewable pathway for future energy demands.hematite(α-Fe2O3)is a promising photoanode material owing to its narrow band gap(~2.1 eV),non-toxicity,natural abundance,and photo-stability.However,some drawbacks greatly limited the photoconversion efficiencies of Fe2O3-based photoanodes,such as poor electrical conductivity,short hole diffusion length(2~4 nm),sluggish surface oxygen evolution kinetics,and short excited-state lifetime(<10×10^-12 sec).Here,we review the recent progressof hematite photoanodes for PEC water oxidation,focused on enhancing surface water oxidation reaction,accelerating charge separation and transport in bulk,improving light absorption.Finally,perspectives on the key challenges and future development of hematite photoelectrodes for PEC water splitting are given.

Efficient photocatalytic overall water splitting by synergistically enhancing bulk charge separation and surface reaction kinetics in Co3O4–decorated ZnO@ZnS core-shell structures

Due to hole capture effect of Co 3 O 4 and the existence of inner electric field built at the p–n junction interface as well as the presence of the core–shell heterojunction, the surface reaction kinetics and bulk charge separation can be greatly improved, resulting in an outstanding H 2 and O 2 production rates in the absence of sacrificial reagents. • The Co 3 O 4 /ZnO@ZnS composites were prepared from the Co-Zn bimetallic MOFs. • The p-n junction and core–shell structure can boost the bulk charge separation. • Co 3 O 4 was used as cocatalysts to facilitate the surface water oxidation. • The optimal H 2 and O 2 production rates reached up to 3853 and 1927 μmol g −1 h −1 . Spontaneously photocatalytic water splitting to generate H 2 and O 2 represents a promising strategy for converting solar energy to chemical energy. However, photocatalytic water splitting without sacrificial agents is difficult due to the poor bulk charge separation and retarded surface reaction kinetics. Herein, we first combined the p–type Co 3 O 4 cocatalysts with n–type ZnO@ZnS core–shell heterojunction to create a Co 3 O 4 –decorated ZnO@ZnS p–n junction by using bimetallic ZnCo–MOFs as template. In this hybrid, both the heterojunction and p–n junction can accelerate the bulk charge separation, while the highly dispersed Co 3 O 4 cocatalysts can enhance the surface reaction kinetics. As a result, the outstanding H 2 and O 2 production rates of 3853 and 1927 μmol g −1 h −1 were obtained for the optimal sample, approximately 1750, 550 and 110 times higher than those of ZnO, ZnO@ZnS and ZnO/Co 3 O 4 . Such synergistic enhancement can be evidenced by charge separation efficiency calculation. XAFS, XPS and DFT calculation indicated that the electrons and holes can be effectively assembled on ZnO and Co 3 O 4 , respectively. This work provides a new strategy for combining the advantages of cocatalyst and p–n junction as well as the core–shell heterojunction to enhance the photocatalytic water splitting without sacrificial agent.

Carbon quantum dots modified anatase/rutile TiO2 photoanode with dramatically enhanced photoelectrochemical performance

TiO2 is a promising photoanode material for photoelectrochemical (PEC) water splitting, but its severe bulk recombination of photogenerated carriers, sluggish oxygen evolution reaction (OER) kinetics and poor visible light response are the main bottleneck problems. Here, the carbon quantum dots (CQDs) modified anatase/rutile TiO2 photoanode (CQDs/A/R-TiO2) was designed by growth of anatase TiO2 nanothorns on rutile TiO2 nanorods and further surface modification of CQDs. The results revealed that the A/R-TiO2 heterojunction significantly suppressed the bulk recombination of photogenerated carriers. With further incorporation of CQDs into A/R-TiO2, dramatical improvement of OER kinetics and ultraviolet-visible (UV–vis) light harvesting were obtained. The bulk charge separation efficiency (ηbulk) and surface charge injection efficiency (ηsurface) of CQDs/A/R-TiO2 are 1.69 and 5.74 times higher than that of the pristine TiO2 at 0.6 V vs. RHE, respectively. Meanwhile, the photocurrent is increased by 11.72 times and the onset potential is negatively shifted by 240 mV for CQDs/A/R-TiO2.

Surface oxygen vacancy modified Bi2MoO6/MIL-88B(Fe) heterostructure with enhanced spatial charge separation at the bulk & interface

The development of efficient strategies to promote spatial charge separation is a promising way for the improvement of photocatalytic performance. Herein, a novel surface oxygen vacancy (OVs) modified Bi2MoO6/MIL-88B(Fe) heterostructure (BMO/M88) was exemplified by a facile two steps solvothermal method to demonstrate synergistic promotion of bulk & interface charge separation. Due to the existence of OVs and the construction of heterostructure, the Bi2MoO6/MIL-88B(Fe) composites exhibited significant enhancement of visible-light responsive range and possessed more efficient bulk & interface charge separation. This novel Bi2MoO6/MIL-88B(Fe) composites exhibited high visible light photocatalytic performance toward dye pollutant rhodamine B (RhB) and model colorless pollutant bisphenol A (BPA) photodegradation, and the possible Type II heterojunction mechanism of photocatalysis process was investigated. This work not only provides a novel pathway for design of MOFs-based heterojunction with surface OVs defect modification but also offers a strategy for significantly enhanced bulk & interface charge carriers separation.

A oxygen vacancy-modulated homojunction structural CuBi2O4 photocathodes for efficient solar water reduction.

The photoelectrochemical (PEC) water reduction performance of CuBi2O4 (CBO)-based photocathodes is still far from their theoretical values due to low bulk and surface charge separation efficiencies. Herein, we propose a regrowth strategy to prepare a photocathode with CBO coating on Zn-doped CBO (CBO/Zn-CBO). Furthermore, NaBH4 treatment of CBO/Zn-CBO introduced oxygen vacancies (Ov) on CBO/Zn-CBO. It was found that Zn-doping not only increases the charge carrier concentration of CBO, but also leads to appropriate band alignment to form homojunctions. This homojunction can effectively promote the separation of electron-hole pairs, thus obtaining excellent photocurrent density (0.5 mA cm-2 at 0.3 V vs. RHE) and charge separation efficiency (1.5 times than CBO). The following surface treatment induced Ov on CBO/Zn-CBO, which significantly increased the active area of the surface catalytic reaction and further enhanced the photocurrent density (0.6 mA cm-2). In the absence of cocatalysts, the electron injection efficiency of Ov/CBO/Zn-CBO was 1.47 times improved than that of CBO. This work demonstrates a homojunction photocathode with Ov modulation, which provides a new view for future photoelectrochemical water splitting.

Photoelectrochemical water splitting using lithium doped bismuth vanadate photoanode with near-complete bulk charge separation

Photoelectrochemical performance of BiVO4 photoanode is limited by poor light absorption, charge separation and transfer efficiencies. For the first time in the literature, Li doped nano-porous BiVO4 photoanode showed complete bulk charge separation efficiency (~100%) at 1.23 V vs RHE along with enhanced light absorption for water splitting. Li doping showed an increase (>20 times) in the photoelectrochemical water splitting compared to pristine BiVO4 photoanode. In particular, oxygen evolution catalyst was also employed for further improving the photoelectrochemical performance (4.2 ± 0.18 mA cm−2) of Li:BiVO4 photoanodes. The density functional theory calculations showing the formation of inter-band with band gap reduction due to interstitial Li doping in BiVO4 structure support enhancement in photoelectrochemical performance. In addition, Li doping in the BiVO4 lattice void positions led to a record photocurrent density of 7.3 ± 0.36 mA cm−2 at 1.23 V vs RHE in the presence of hole scavenger under one sun illumination. Further, present study systematically demonstrates role of Li in BiVO4 host for water oxidation through a detailed characterization and study of optical and charge transport properties.

Intentional Extrinsic Doping into ZnFe2O4 Nanorod Photoanode for Enhanced Photoelectrochemical Water Splitting

Zinc ferrite (ZnFe2O4) is a promising candidate photoanode material for photoelectrochemical water splitting, but its poor electronic properties deter it from achieving high performance. To improve the electronic properties, Ti4+ and Sn4+ are incorporated into Fe3+ sites of ZnFe2O4 as electron donors under annealing conditions of 550, 800 °C, or hybrid microwave annealing (HMA). The intentional external doping mainly promotes charge transport with increased charge carrier density and thereby enhances the bulk charge separation efficiency (ηbulk). Effectiveness of the doping varies with annealing conditions. Both Ti‐doped and Sn‐doped ZnFe2O4 annealed at 550 °C show the most significant improvement in ηbulk with little effect on the surface charge separation efficiency (ηsurface). Doping under high‐temperature annealing (800 °C, HMA) improves ηbulk to a lesser degree but creates a passivation layer of TiOx or SnOx, resulting in a higher ηsurface as well. Finally, loading the NiFeOx oxygen evolution catalyst (OEC) further improves ηsurface values, and especially 2% Ti‐doped ZnFe2O4 annealed at 800 °C shows near 100% ηsurface at 1.23 VRHE, and its photocurrent generation under simulated 1 sun (0.312 mA cm−2 at 1.23 VRHE) represents a ≈18 times increment from that of unmodified ZnFe2O4 annealed at 550 °C.

Photoelectrochemical Solar Water Splitting: The Role of the Carbon Nanomaterials in Bismuth Vanadate Composite Photoanodes toward Efficient Charge Separation and Transport.

Photoelectrochemical performance of bismuth vanadate (BiVO4) photoanode is limited by poor charge separation and transport properties. The roles of carbon nanotube, reduced graphene oxide, or graphitic carbon nitride in BiVO4 composite photoanode were investigated toward enhancing light absorption and reducing overall impedance during photoelectrochemical water oxidation process. X-ray diffraction and Tauc analysis showed that BiVO4 retains its monoclinic phase, n-type semiconductor nature, and band gap in all carbon nanomaterials-incorporated composite photoanodes. It was observed that the carbon nanomaterials incorporation in BiVO4 film increases its surface porosity, ultimately leading to enhanced light absorption. The BiVO4 photoanode with reduced graphene oxide and graphitic carbon nitride showed same bulk charge separation efficiency, whereas the latter showed better charge transfer. It was found that the graphitic carbon nitride formed composite with BiVO4 to provide enhanced light absorption efficiency, i.e., 89% in 350-505 nm range. The BiVO4 with graphitic carbon nitride photoanode showed the best performance with a photocurrent of 2.2 mA cm-2, charge separation efficiency of 67%, and photocurrent of 4.0 mA cm-2 with cobalt-phosphate surface catalyst at 1.23 VRHE for water oxidation under 1 sun illumination. The Mott-Schottky and impedance measurements confirmed the shift of conduction band position toward hydrogen reduction potential and reduction in film resistance, respectively, with carbon nanomaterials addition, and the shift was most significant for graphitic carbon nitride. It is concluded that by concomitant formation of junction during photoanode fabrication between carbon nanomaterials, BiVO4, and fluorine-doped tin oxide glass substrate, better charge separation, transport, and light absorption can be achieved.

One-Pot-Architectured Au-Nanodot-Promoted MoS2/ZnIn2S4: A Novel p-n Heterojunction Photocatalyst for Enhanced Hydrogen Production and Phenol Degradation.

Developing effective and simple one-pot synthetic strategies regarding the formation of heterojunction photocatalytic semiconductors remains an intense challenge in research pursuits. Further scheming of the p-n heterojunction has sustained renewed interest in catalysis, photocatalysis, energy storage, and conversion because they easily accelerate the bulk charge separation efficiency. Thus we have successfully designed a Au-MoS2/ZnIn2S4 heterojunction photocatalyst for the first time by adopting a simple one-pot hydrothermal technique, followed by a deposition-precipitation method. By adjusting the mole ratio of Mo with that of Zn and In precursors, we have fabricated a MoS2/ZnIn2S4 p-n heterojunction photocatalyst, and the established p-n heterojunction between MoS2 and ZnIn2S4 is demonstrated by various physicochemical and morphological characterizations. An interfacial junction is created between MoS2 and ZnIn2S4 at the depletion region via an in situ formation mechanism, leading to the enhancement of the charge separation through the p-n heterojunction and thus improving the photocatalytic activity. Moreover, the photocatalytic activity is projected to further improve by the incorporation of Au nanodots on the surface of MoS2/ZnIn2S4 photocatalysts. The increase in activity is due to the generation and participation of a large number of direct-electron-transfer-induced hot electrons in the photochemical reaction. From the experimental results, Au-MoS2/ZnIn2S4 heterojunction photocatalysts with only 1% MoS2 and 1% Au loading content displayed a 561.25 μmol/h H2 evolution rate and 84% degradation of phenol, which are nearly 15 and 6 times higher than those neat ZnIn2S4. In addition Au-MoS2/ZnIn2S4 photocatalysts exhibit a photocurrent density of ∼2.56 mAcm-2, which is nearly 2.4 times higher than that of the MoS2/ZnIn2S4 heterojunction photocatalyst. This exertion represents the synergetic enhancement of photocatalytic activity through the p-n heterojunction as well as the hot-electron participation by the metal nanocatalyst, which is an inspiration for developing efficient photocatalysts.

Suppressed Charge Recombination in Hematite Photoanode via Protonation and Annealing

Hematite as promising photoanode for solar water splitting suffers from severe bulk and surface charge recombination. This work describes that a protonation-annealing treatment can effectively suppress both bulk and surface charge recombination in hematite. Protons/electrons are electrochemically incorporated into hematite under 0.2 VRHE followed by annealing at 120 oC. The photocurrent density increases from ~0.9 mA cm-2 to 1.8 mA cm-2 at 1.23 VRHE under 1 sun, and further to 2.7 mA cm-2 after loading cobalt phosphate, stabilizing at round 2.4 mA cm-2. A cathodic shift of the onset potential of photocurrent is also observed. H2O2 oxidation, impedance spectroscopy and Mott-Schottky measurements show that the protonation suppresses bulk recombination and enhances donor density, but introducing more surface recombination. The annealing reduces surface recombination, while preserving relatively high bulk charge separation efficiency. Different from previous reports on the electrochemically reduced hematite, this work demonstrates that the performance improvement should be ascribed to the proton incorporation instead of the formation of Fe3O4 or metal Fe. This facile treatment by protonation and annealing could be applied in other semiconductors to promote the development of high performing photoelectrodes.

Accelerating the charge separation of ZnFe2O4 nanorods by Cu-Sn ions gradient doping for efficient photoelectrochemical water splitting

Spinel zinc ferrite (ZnFe2O4) with an appropriate band gap (2.1 eV) is a promising photoanode in the photoelectrocatalysis field, however, the photoelectrochemical (PEC) performance of ZnFe2O4 is confined due to poor charge separation. Hence, improving charge separation efficiency is essential to modify the PEC performance of ZnFe2O4. Herein, the novel Cu-Sn ions doped ZnFe2O4 nanorods are fabricated for the first time. Cu2+ ions are doped into ZnFe2O4 nanorods from surface to inside with a degressive concentration, it is helpful to enhance the charge separation efficiency of ZnFe2O4 nanorods along horizontal direction. Sn4+ ions originated from fluorine-doped tin oxide (FTO) layer are doped into Cu-ZnFe2O4 nanorods with a decreasing concentration from bottom to top, it is conducive to reducing surface trapping states and accelerating charge separation along vertical direction. Therefore, Cu-Sn dual ions gradient doping synergistically boosts the PEC activity of ZnFe2O4. The Cu.Sn-ZnFe2O4 nanorods have an excellent photocurrent density of 0.46 mA⋅cm−2 at 1.23 V vs RHE, which is 4.18 times than that of ZnFe2O4. The bulk charge separation efficiency (ηbulk) and surface charge separation efficiency (ηsurface) of Cu.Sn-ZnFe2O4 are 17.52% and 55.22%, which are 5.84 and 3.78 times than that of ZnFe2O4, respectively. This work provides inspirations for promoting charge separation efficiency of photoanodes, thus achieving efficient PEC water splitting.