Negative Conduction Band Potential Research Articles

Unveiling the effects of facet-dependent oxygen vacancy on CeO2 for electron structure and surface intermediates in CO2 photoreduction reaction

CO2 photoreduction over CeO2 surfaces has been studied for a few years, and many efforts have been devoted to understanding the effect of exposed crystal facets and oxygen vacancies (OVs) on the catalytic activity. In this work, the formation of OVs over different CeO2 surfaces is investigated through X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, and electron paramagnetic resonance spectroscopy (EPR), revealing the facet-dependent OVsin CeO2. The experimental results and DFT calculations show that the OVs formed in (111)CeO2 and (100)CeO2 possess faster charge transfer and a more negative conduction band potential than (110)CeO2. The in-situ Fourier transform infrared spectroscopy (FTIR) and DFT results revealed the CO2 absorption, activation, and reduction processes over the three samples. Although (110)CeO2 surface shows the best adsorption capability for CO2, its catalytic activity is still limited by a high barrier (1.95 eV) in the formation of CO species from COOH* intermediate. Through the H2O-assisted CO2 activation process, (111)CeO2 and (100)CeO2 show better reduction activity for CO2, resulting in the better activity for CO2 photoreduction. This study demonstrates that the band structure and CO2 activation influenced by OVs show a strong crystal facet dependence in CeO2.

Metal oxide nanocrystals embedded polypyrrole nanohybrid: Exploring role of interface in photocatalytic hydrogen generation

Photocatalytic water splitting is considered as a sustainable way to produce hydrogen in the water−based energy cycle. Inorganic semiconductors, specifically metal oxides are extensively used as efficient photocatalysts, however, intrinsically poor charge-transfer and wide band gap with lower negative conduction band potential than the hydrogen evolution potential affect the water-splitting efficiency. In view of superior charge-transfer, and tunable energy levels, polymer-based nanohybrids (NHs) have been fabricated using polypyrrole (PPy) nanofibers with a series of transition metal oxides (MO) nanoparticles, namely TiO2, ZnO, CeO2, Fe2O3, NiO, and SnO2. A series of photoanodes have been tested for photoelectrochemical water splitting to elucidate the effect of interfacial charge transfer at the interfaces of NHs. The high photocurrent response of 44 μA cm−2 at 0.9 V vs. Ag/AgCl has been achieved for Fe2O3/PPy. While, ZnO/PPy NHs demonstrated a superior photocatalytic H2 generation rate of ∼162 mmol/g/h, which is ∼27 times higher than pure PPy. Based on energy level calculation, the charge transfer mechanism occurs through p−n junction in TiO2/PPy and SnO2/PPy NHs while the other NHs structures follow a Z−Scheme of charge transfer pathway. This finding opens an avenue for designing highly efficient polymer-metal oxide heterojunctions for water splitting and solar fuel applications.

Nb2O5-graphene heterojunction composite with ultrahigh photocatalytic activity for solar light driven photodegradation of ciprofloxacin

Poor visible light absorption capacity and intrinsic conductivity restrict many applications of Nb2O5 in photocatalysis. The paper describes one solution for synthesizing Nb2O5-graphene photocatalyst with functional graphene quantum dot (GQD). Firstly, Nb(V) was coordinated with GQD to form water-soluble complex. Then, it was bound to graphene sheet via π-π stacking, reduced with ascorbic acid to produce graphene oxide gel and annealed in N2 at 900 °C. The resultant Nb2O5 nanocrystals offer ultrasmall size of 8.8 ± 0.6 nm and good crystallinity. The introduction of GQD realizes the construction of heterojunction. This narrows bandgap energy of Nb2O5 and optimizes the band structure. The narrowed bandgap energy leads to an enhanced visible light absorption, and more negative conduction band potential and more positive valence band potential promote production of superoxide (•O2–) and hydroxyl radicals (•OH). Combination of graphene with GQD improves adsorption of ciprofloxacin, separation of generated electrons and holes and light absorption capacity. G-Nb2O5-900 exhibits an excellent photocatalytic performance for solar light driven photodegradation of ciprofloxacin. The photodegradation efficiency is more than 3.58 times under ultraviolet light radiation and 12.4 times under visible light radiation over pristine Nb2O5. G-Nb2O5-900 shows a broad application prospect in antibiotic treatment in environmental water.

Ag(I)-grafted UiO-67 to enhance the photochemical efficiency under visible light for highly efficient photocatalytic reduction of Cr(VI) and degradation of reactive blue 13

As a new kind of organic-inorganic hybrid semiconductor materials, MOFs have come into focus as photocatalysts for decontamination of Cr(VI) oxates and reactive dyes from contaminated water. However, although most Zr-MOFs exhibit hydrolytic stability, they absorb almost only ultraviolet light. Assembly of MOFs with both hydrolytic stability and high visible light responsiveness thus has become a most feasible avenue recently. Here, composite of Ag@UiO-67 was intentionally designed by modifying the classical Zr-MOF of UiO-67 with highly photosensitive Ag+ ions. Results suggested that compared with pristine UiO-67, Ag@UiO-67 displays significantly narrowed band gap (Eg) value, sufficiently negative conduction band (CB) potential, suitable valence band (VB) potential and significantly improved charge transfer efficiency. Notably, the Eg value of 2.09 eV gives Ag@UiO-67 excellent ability of visible photocatalytic reduction of Cr(VI) and degradation of Reactive dark blue K-R (RB13), with reduction and degradation efficiencies of 91.37 % within 140 min (k = 0.024 min−1) and 92.63 % within 160 min (k = 0.016 min−1), respectively, which are 5.6 and 6.9 times of as synthesized UiO-67. Moreover, photocatalytic mechanism studies reveal that ·O2– plays key roles in bleaching RB13 and photogenerated species with sufficient negative potential can effectively reduce Cr(VI). And Ag@UiO-67 can utilize visible light more efficiently to accelerate the charge transfer rate. This work provides a feasible avenue to tailor the visible light sensitivity and band structure of classic Zr-MOFs, for eliminating inorganic and organic contaminants such as Cr(VI) ions and dye of RB13.

Charge localization induced by Fe doping in porous Bi5O7I Micro-flower for enhanced photoreduction of CO2 to CO

The bismuth-rich bismuth oxyhalide materials exhibit prominent photocatalytic CO2 reduction (PCR) performance owing to the considerable negative conduction band potential and unique intrinsic internal electric field. However, it possesses poor capture ability of CO2 molecule due to the intrinsically weak adsorption, greatly limiting its further PCR improvement. Herein, we modulate the spatial charge redistribution over hierarchically Bi5O7I micro-flower through a facile and cost-efficient iron doping strategy, achieving significantly enhanced CO2 capture capacity. Mechanistic studies indicate that the promoted adsorption ability originates from the localized electrons around the doped Fe sites. Benefiting from the resultant unique electronic structures, the Fe doped Bi5O7I micro-flowers also exhibit remarkably improved harvest of visible light and promoted charge separation and transfer capability, thus finally achieving a notably boosted photocatalytic CO evolution rate of 12.02 μmol·g−1·h−1 without any sacrifice agents.

Synthesis of Phase Junction Cadmium Sulfide Photocatalyst under Sulfur-Rich Solution System for Efficient Photocatalytic Hydrogen Evolution.

Photocatalyst with excellent semiconductor properties is the key point to realize the efficient photocatalytic hydrogen evolution (PHE). As a representative binary metal sulfide (BMS) semiconductor, cadmium sulfide (CdS) possesses suitable bandgap of 2.4eV and negative conduction band potential, which has a great potential to realize efficient visible-light PHE performance. In this work, CdS with unique cubic/hexagonal phase junction is facilely synthesized through a sulfur-rich butyldithiocarbamate acid (BDCA) solution process. The results illustrate that the phase junction can efficiently enhance the separation and transfer of photogenerated electron-hole pairs, resulting in an excellent PHE performance. In addition, the sulfur-rich property of BDCA solution leads to the absence of additional sulfur sources during the synthesis of CdS photocatalyst, which greatly simplifies the fabrication process. The optimal PHE rate of the BDCA-synthesized phase junction CdS photocatalyst is 7.294mmol g-1 h-1 and exhibits a favorable photostability. Moreover, density function theory calculations indicated that the apparent redistribution of charge density in the cubic/hexagonal phase junction regions gives a suitable hydrogen adsorption capacity, which is responsible for the enhanced PHE activity.

S-scheme AgIO4/CeO2 heterojunction nanocomposite photocatalyst for degradation of rhodamine B dye

Photocatalysis is auspicious technology for transferring inexhaustible solar energy into storable chemical energy. In this novel research, an auspicious S-scheme heterojunction is constructed by hybridizing CeO2 nanoparticles with band gap energy [2.9 eV], surface area [75 m2/g], and negative conduction band potential (ECB = -0.38 eV) with AgIO4 nanorods with 22 m2/g surface area, band gap energy of 1.98 eV and valence band potential of + 3.27 eV. The novel AgIO4/CeO2 heterojunctions containing different compositions of CeO2 and AgIO4 are fabricated through coupled sol–gel and sonochemical route. SEM, XRD, BET, XPS, DRS, PL and HRTEM investigate the morphological, structural, textural and optical features of the solid specimens. Combined PL and DRS analysis have pointed out that AgIO4 prohibit the as-synthesized heterojunctions with strong visible light absorbability and highly separation efficiency of photogenerated electrons and holes. CeAg15 optimal catalyst decompose RhB dye with pseudo-first order rate of 0.024 min−1 which is tenfold higher than that of pristine CeO2. In the light of scrubber experimental results and PL analysis of terephthalic acid, S-scheme is proposed for analyzing the route of charge transportation and generation of charge carriers with strong redox power. The construction of S-scheme mechanism reveal the preservation of the positive holes of AgIO4 oxidative photocatalyst and negative electrons of CeO2 reductive photocatalyst for consuming in destruction of RhB dye into eco-friendly species as manifested from TOC analysis. This novel research work provide a clear insight on fabrication of S-scheme heterojunction with strong oxidative and reductive power for eradicate water pollutants.

Fabrication of flower-like WO3/ZnIn2S4 composite with special electronic transmission channels to improve carrier separation for photoinduced cathodic protection and electron storage

In this paper, the WO3/ZnIn2S4 composite with special morphology of flower-like structures was prepared by hydrothermal method and its performance of photoinduced cathodic protection (CP) and continuous CP in the dark was studied. The construction of the WO3/ZnIn2S4 composite with special morphology is based on the following considerations. Firstly, the stacking connections of the nanoflakes of flower-like structured WO3 and ZnIn2S4 forms two-dimensional electron transport channels, which is beneficial to the directional transport of photogenerated electrons; Secondly, the heterojunction electric field formed at the WO3/ZnIn2S4 interface can accelerate the separation of photogenerated carriers; Finally, taking advantage of electron storage properties of WO3 and good photoinduced CP performance of ZnIn2S4 with a relatively negative conduction band potential, the WO3/ZnIn2S4 composite with excellent performance of photoinduced CP and continuous CP in the dark is obtained. The WO3/ZnIn2S4 system can negatively shift the potential of 304 SS by 332.8 mV, and can store 0.17 C of electricity when light illumination duration is 2 h, which is enough to provide 11.88 h continuous CP for 304 SS in dark state. This study realizes long-term CP in the dark, and provides the possibility for practical application of the photoinduced CP technology.

Research progress in metal sulfides for photocatalysis: From activity to stability.

Metal sulfides are a type of reduction semiconductor photocatalysts with narrow bandgap and negative conduction band potential, which make them have unique photocatalytic performance in solar-to-fuel conversion and environmental purification. However, metal sulfides also suffer from low quantum efficiency and photocorrosion. In this review, the strategies to improve the photocatalytic activity of metal sulfide photocatalysts by stimulating the charge separation and improving light-harvesting ability are introduced, including morphology control, semiconductor coupling and surface modification. In addition, the recent research progress aiming at improving their photostability is also illustrated, such as, construction of hole transfer heterojunctions and deposition of hole transfer cocatalysts. Based on the electronic band structures, the applications of metal sulfides in photocatalysis, namely, hydrogen production, degradation of organic pollutants and reduction of CO2, are summarized. Finally, the perspectives of the promising future of metal-sulfide based photocatalysts and the challenges remaining to overcome are also presented.

Precursor-modified strategy to synthesize thin porous amino-rich graphitic carbon nitride with enhanced photocatalytic degradation of RhB and hydrogen evolution performances

The photocatalytic activity of carbon nitride (CN) materials is mainly limited to small specific surface areas, limited solar absorption, and low separation and mobility of photoinduced carriers. In this study, we developed a precursor-modified strategy for the synthesis of graphitic CN with highly efficient photocatalytic performance. The precursor dicyandiamide reformed by different acids undergoes a basic structural change and transforms into diverse new precursors. The thin porous amino-rich HNO3-CN (5H-CN) was calcined by dicyandiamidine nitrate, formed by concentrated nitric acid modified dicyandiamide, and presented the best photocatalytic degradation rate of RhB, more than 34 times that of bulk graphitic CN. Moreover, the photocatalytic hydrogen evolution rate of 5H-CN significantly improved. The TG-DSC-FTIR analyses indicated that the distinguishing thermal polymerization process of 5H-CN led to its thin porous amino-rich structure, and the theoretical calculations revealed that the negative conduction band potential of 5H-CN was attributed to its amino-rich structure. It is anticipated that the thin porous structure and the negative conduction band position of 5H-CN play important roles in the improvement of the photocatalytic performance. This study demonstrates that precursor modification is a promising project to induce a new thermal polycondensation process for the synthesis of CN with enhanced photocatalytic performance.

Nickel interlayer enables indirect corrosion protection of magnesium alloy by photoelectrochemical cathodic protection

As a green and sustainable corrosion protection technology, photoelectrochemical cathodic protection (PECCP) cannot be applied to protect metals having low corrosion potential (Ec) due to the insufficient negative conduction band (CB) potential of the present semiconductor materials. Herein, for the first time, PECCP is reported to indirectly protect metals with low Ec from corrosion by introduction of an interlayer with high Ec between the substrate and semiconductor. As an example, a nickel interlayer was firstly deposited on magnesium alloy substrate by chemical deposition and electrodeposition to provide the high Ec, followed by electrodeposition of Cu2O semiconductor layer on the nickel layer. A series of characterizations, including high-resolution transmission electron microscope and X-ray photoelectron spectroscopy, indicate that the resultant Cu2O semiconductor has an octahedral structure and an edge length of ca. 514 nm. The n-type property of the semiconductor is determined by the Mott-Schottky curve, and the photo-response behavior, such as obvious photo-induced potential drop and stable photocurrent density as high as 747 μA cm−2 under visible light illumination, is confirmed by electrochemical and photoelectrochemical measurements. The new proposed methodology described in this study is not limited to magnesium alloy but also can be possibly applied in other structural metals having low Ec.

Thiosulfate enhanced degradation of organic pollutants in aqueous solution with g-C3N4 under visible light irradiation

Developing new strategies to design more practicable and efficient g-C3N4 based photocatalysts is important to solve the environmental issues. Thiosulfate (STS) is a common residual product found in wastewater and removal of STS remains a matter of great environmental concern. In this work, however, STS is activated by g-C3N4 under visible light irradiation, resulting in a fast degradation of Rhodamine B (RhB) and other pollutants. The performance of g-C3N4 prepared from urea was much higher than that from melamine, due to the higher surface area and more negative conduction band potential of the former catalyst. In addition, comparison with other oxidants and reductants such as peroxymonosulfate, peroxydisulfate, hydrogen peroxide and sulfite, the use of STS in g-C3N4/Vis system showed the highest efficiency for RhB degradation. During ten successive cycles, the excellent reusability of the catalyst was also obtained. The effect of different concentrations of STS and g-C3N4, and initial solution pH on the performance of the system were also studied. The mechanism study suggests that STS is first oxidized to S2O3− radicals by photohole, which will be transformed to other oxysulfur radicals such as SO3− and finally to SO42− ions. At the same time, the rate of O2 reduction by photoelectrons to O2− radicals as well as RhB degradation increases. The finding of this study provides a promising advanced oxidation process for organic pollutants degradation via STS activation.

Ultrathin carbon-coated Zr3+-ZrO2 nanostructures for efficient visible light photocatalytic antibiotic elimination

Broadband solar light absorption and visible light photocatalysis of surface modified ultrawide bandgap semiconductors are receiving wide attentions because of their more negative conduction band potential and positive valence band potential for refractory organics degradation. Here, we reported a novel modified solvothermal strategy by the introduction of distilled water into small molecular alcohols to synthesize crystallized carbon-coated Zr3+-ZrO2 composites for enhanced visible light absorption and visible light photocatalytic tetracycline degradation, in which the distilled water favored for the crystallization of Zr3+-ZrO2 and the formed ultrathin carbon layers derived from the polymerization of small molecular alcohols were contributing to the generation of Zr3+ species and well dispersion of Zr3+ self-doped ZrO2 nanoparticles. The experimental results also showed that carbon-coated Zr3+-ZrO2 composites exhibited enhanced visible light absorption, long-lived charges, efficient visible light photocatalytic performance and good recyclable stability. Besides, possible photocatalytic mechanism and antibiotic degradation pathways over carbon-coated Zr3+-ZrO2 composites under visible light irradiation were also proposed in detail.

Refining the band structure of BiOBr nanosheets through the synergetic effect of VO43− ions replacement and oxygen vacancies for promoted visible-light-driven photocatalysis

The band structure of BiOBr nanosheets is regulated by the oxygen vacancies and VO43− ions replacement. The BiOBr nanosheets possess defective states and a more negative conduction band potential, promoting visible-light photocatalytic activity.

Phenyl-Bridged Graphitic Carbon Nitride with a Porous and Hollow Sphere Structure to Enhance Dissociation of Photogenerated Charge Carriers and Visible-Light-Driven H2 Generation.

Graphitic carbon nitride (CN) suffers from rapid recombination of photoexcited charges due to the existing highly symmetrical tri-s-triazine ring and long charge diffusion path, resulting in moderate photocatalytic activity. The bridged phenyl embedded in the CN structure was used to reduce the symmetry of the tri-s-triazine ring. In addition, the CN material was constructed with a porous and hollow sphere structure to shorten the diffusion path of charge carriers. Herein, simple thermal polymerization of a trimesic acid-doped melamine-cyanuric acid (MCA) supramolecular was employed to construct phenyl-bridged graphitic carbon nitride (Ph-CN-MCA) with a hollow sphere structure composed of porous nanosheets for visible-light catalytic H2 evolution. The porous and hollow sphere-structured Ph-CN-MCA possessed increased degree of polymerization, more negative conduction band potential, enlarged Brunauer-Emmett-Teller (BET) surface area, and shortened charge diffusion path. In addition, bridged phenyl embedded in the Ph-CN-MCA structure not only accelerated the dissociation of photogenerated carriers but also narrowed the band gap and extended the visible-light absorption. Further, the separated highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of Ph-CN-MCA facilitated the spatial dissociation of photogenerated charges, which was also confirmed by theoretical calculations. As a consequence, compared with the reference CN-MA catalyst prepared from melamine, Ph-CN-MCA showed approximately 48.42 times the photocatalytic H2 evolution under visible-light irradiation. The developed synthetic method herein highlights that phenyl-bridged graphitic carbon nitride with a porous and hollow sphere structure could provide an efficient platform to boost the dissociation of photoexcited charge carriers and photocatalytic H2 evolution.

Fe-doped zirconia nanoparticles with highly negative conduction band potential for enhancing visible light photocatalytic performance

Maintaining a highly negative value of conduction band (CB) potential of photocatalysts through metal doping has been a great challenge. Herein, we investigated the effects of Fe-doping on the crystal structures and electronic structures of zirconia (ZrO2) in detail by combining experiments and Density Functional Theory (DFT). It was revealed that the Fe-doping obviously triggered the formation of tetragonal phase ZrO2 and induced a defect states in the bandgap of Fe-doped ZrO2. Due to the coherent effect between the electrons of the Fe 3d orbitals and ZrO2 intrinsic electronic states, the defect states expanded the light absorption edge of ZrO2 and Fe-doped ZrO2 showed a small bandgap with maintaining a highly negative value of the CB potential. Meanwhile, Fe-doping significantly enhanced photoelectrochemical properties, the efficiency separation of electron-hole pairs and photocatalytic activity in comparison with the pristine ZrO2. We also found that 0.5%Fe-ZrO2 exhibited the optimum photocatalytic performance, and reached 96% for degradation of Rhodamine B (RhB) within 150 min under visible light irradiation. This work elucidated the mechanism of the photocatalytic degradation of RhB over Fe-doped ZrO2 and provided a promising approach in designing highly negative CB potential of ZrO2 via Fe doping toward sustainable environmental remediation.

Selective CO2 reduction into formate using Ln–Ta oxynitrides combined with a binuclear Ru(II) complex under visible light

Ln–Ta oxynitrides with defect fluorite structure were found to serve as effective semiconductor components in visible-light-driven Z-scheme CO 2 reduction with a binuclear Ru(II) complex, giving high selectivity to formate (>99%). • Ln–Ta oxynitrides LnTaO x N y (Ln = Nd, Sm, Gd, Tb, Dy and Ho) were synthesized. • Some of LnTaO x N y loaded with Ag and a Ru(II) complex photocatalyzed CO 2 reduction. • The highest activity was obtained with TbTaO x N y . • High formate selectivity (>99%) was accomplished. Hybrid materials constructed from a visible-light-absorbing semiconductor and a functional metal complex have attracted attention as efficient photocatalysts for CO 2 reduction with high selectivity to a desired product. In this work, defect fluorite-type Ln–Ta oxynitrides LnTaO x N y (Ln = Nd, Sm, Gd, Tb, Dy and Ho) were examined as the semiconductor component in a hybrid photocatalyst system combined with known Ag nanoparticle promoter and binuclear ruthenium(II) complex ( RuRu′ ). Among the LnTaO x N y examined, TbTaO x N y gave the highest performance for CO 2 reduction under visible light ( λ > 400 nm), with a RuRu′ -based turnover number of 18 and high selectivity to formate (>99%). Physicochemical analyses indicated that high crystallinity and more negative conduction band potential of LnTaO x N y with the absence of Ln-4f states in the band gap structure contributed to higher activity of the hybrid photocatalyst.

A novel nitrogen-deficient g-C3N4 photocatalyst fabricated via liquid phase reduction route and its high photocatalytic performance for hydrogen production and Cr(VI) reduction

A facile liquid phase chemical reduction method was developed to synthesize nitrogen-deficient graphitic carbon nitride (FH-g-C3N4) by application of the weak reducibility of formaldehyde in weak acidic solution. The nitrogen defects in g-C3N4 were confirmed by EA, EPR, XPS and EDS. The extra electrons in nitrogen-deficient sites can easily capture photogenerated holes and reduce the recombination of photogenerated electrons and holes. With appropriate amount of nitrogen defects, FH-g-C3N4 possesses the best photocatalytic activity which is 58% higher than that of H-g-C3N4 (without nitrogen defects) and P25 (TiO2) for Cr(VI) reduction, and its hydrogen production rate is twice of the H-g-C3N4. Compared with H-g-C3N4, FH-g-C3N4 has a stronger light absorption capacity, a more negative potential of conduction band, and a stronger reduction capacity. This work could provide significant information for the simple synthesis of nitrogen-deficient g-C3N4 photocatalyst with distinct reduction performance.

Enhanced photocatalytic activity of BiOCl with regulated morphology and band structure through controlling the adding amount of HCl

BiOCl photocatalysts with different morphologies were prepared by controlling the adding amount of HCl using a simple one-step solvothermal method, the hierarchical structure of BiOCl changes from nanoneedles to nanosheets and the space between the hierarchical structure increases with the increase of the adding amount of HCl. BiOCl-6 possesses the best photocatalytic rhodamine B (RhB) degradation performance under white light illumination. BiOCl-6 has the more negative conduction band potential, thus the electrons on the conduction band have stronger reduction ability. Free radical trapping experiments show that superoxide radicals play the major role in the photocatalytic RhB degradation of BiOCl-6.

What Is the Transfer Mechanism of Photoexcited Charge Carriers for g-C3N4/TiO2 Heterojunction Photocatalysts? Verification of the Relative p–n Junction Theory

Investigation of photoexcited charge transfer mechanism has always been one of the hotspots of the photocatalysis field. In our recent studies, the relative p–n junction was proposed as a new concept, and the built-in electric field formed in the heterojunction is the inner impetus for driving photoexcited charge transfer. Here, a series of g-C3N4/TiO2 samples with different mass percentage contents were synthesized and further characterized by physical and chemical techniques for the investigation of the charge transfer mechanism and internal natural law. The results state clearly that the migration of photoexcited charges belongs to Z-scheme mechanism, which is suitable for as-synthesized g-C3N4/TiO2 samples, whether the main part of the g-C3N4/TiO2 is TiO2 or g-C3N4. The photoexcited electrons enriched in g-C3N4 with a higher negative conduction band (CB) potential have reduction ability to convert O2 into superoxide radicals (•O2–). Meanwhile, the photoexcited holes in TiO2 with a higher positive valence band (VB) potential have oxidation ability to activate H2O or hydroxyl ions (OH–) to hydroxyl radicals (•OH). Furthermore, the g-C3N4/TiO2 photocatalyst exhibits better photocatalytic performance than TiO2 and g-C3N4. It is encouraging that the abovementioned Z-scheme mechanism of photoexcited charge transfer can also be explained and confirmed by the relative p–n junction theory. The built-in electric field promotes the migration of the photoexcited charges in the heterojunction, and its migration direction is opposite to that of the photoexcited charge in the CB and VB of g-C3N4 and TiO2. Therefore, the relative p–n junction theory not only is used to explicate the migration mechanism and internal natural law of the photoexcited charge in the heterojunction photocatalysts but also has crucial guiding significance for the theoretical design and practical construction of composite photocatalysts.