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abstract: The formation of semiconductor composites comprising multicomponent or multiphase heterojunctions is a very effective strategy to design highly active photocatalyst systems. This review summarizes the recent strategies to develop such composites, and highlights the most recent developments in the fi eld. After a general introduction into the different strategies to improve photocatalytic activity through formation of heterojunctions, the three different types of heterojunctions are introduced in detail, followed by a historical introduction to semiconductor heterojunction systems and a thorough literature overview. Special chapters describe the highly investigated carbon nitride heterojunctions as well as very recent developments in terms of multiphase heterojunction formation, including the latest insights into the anatase rutile system. When carefully designed, semiconductor composites comprising two or three different materials or phases very effectively facilitate charge separation and charge carrier transfer, substantially improving photocatalytic and photoelectrochemical effi ciency.
author_names: Roland Marschall
title: Semiconductor Composites: Strategies for Enhancing Charge Carrier Separation to Improve Photocatalytic Activity
year: 2014

author_names: Roland Marschall
title: Photocatalysis: Semiconductor Composites: Strategies for Enhancing Charge Carrier Separation to Improve Photocatalytic Activity (Adv. Funct. Mater. 17/2014)
year: 2014

abstract: Water contamination by dyes is a matter of concern for human health and the environment. Various methods (membrane separation, coagulation and adsorption) have been explored to remove/degrade dyes. However, now the exploitation of semiconductor assisted materials using renewable solar energy has emerged as a potential candidate to resolve the issue. Although, single component photocatalysts (ZnO, TiO2, ZrO2) were experimented, due to their low efficiency and stability due to the high recombination rate electron hole pair and inefficient visible light absorption, composites of semiconductor materials are being used. Semiconductor heterojunction systems are developed by coupling two or more semiconductor components. The synergistic effect of their properties, such as adsorption and improved charge carrier migration, is observed to increase overall stability. This review covers recent progress in advanced nanocomposite materials based on g C3N4, TiO2 and ZnO used as photocatalysts with details of enhancing the photocatalytic properties by heterojunctions, crystallinity and doping. The conclusion at the end displays a summary, research gaps and future outlook. A holistic analysis of recent progress to demonstrate the efficient heterojunctions for photodegradation with optimal conditions, this review will be helpful for the development of efficient heterostructured systems for photodegradation. This review covers references from the year 2017 2020.
author_names: Meena Nemiwal

Tian Cheng Zhang

Dinesh Kumar
title: Recent progress in g C3N4, TiO2 and ZnO based photocatalysts for dye degradation: Strategies to improve photocatalytic activity.
venue: The Science of the total environment
year: 2021

abstract: Abstract Tantalum nitride (Ta3N5) is one of the most promising photocatalysts for overall water splitting owing to its suitable band structure and high solar to hydrogen conversion efficiency. However, photocatalytic performance of pristine Ta3N5 is still restricted by its high recombination rate of photo induced carriers. Building up a heterojunction at the semiconductor/semiconductor interface is an effective way to reduce the recombination rate of photoexcited carriers. In this work, a novel type II hetero structured photocatalyst (denoted as Ta3N5/STON) composed of one dimensional (1D) Ta3N5 nanorods and zero dimensional (0D) SrTaO2N (STON) nanoparticles has been developed via a simple one step nitridation of 1D Sr2KTa5O15 (SKTO) nanorods. Compared with pristine Ta3N5, STON and Ta3N5/STON (mix) composite, Ta3N5/STON (1D/0D) heterojunctions exhibit a remarkably enhanced hydrogen production rate (77.31 umol h 1 g 1) under visible light irradiation, which is 386.6, 11.9 and 8.8 times higher than that of Ta3N5, STON and Ta3N5/STON (mix) composite, respectively. The unique 1D/0D structure in Ta3N5/STON heterojunction, as well as the intimate interface contact between Ta3N5 and SrTaO2N, fosters the spatial charge separation and transfer efficiency to a large extent, leading to an improved hydrogen evolution rate. Our work suggests a new strategy toward the rational design of 1D/0D heterojunction photocatalyst for efficient solar to hydrogen conversion.
author_names: Xiaowei Jia

Wenjing Chen

Yunfeng Li

Xuanbo Zhou

Xiaodan Yu

Yan Xing
title: Enhanced photoexcited carrier separation in Ta3N5/SrTaO2N (1D/0D) heterojunctions for highly efficient visible light driven hydrogen evolution
year: 2020

abstract: Extending the absorption to the visible region by tuning the optical band gap of semiconductors and preventing charge carrier recombination are important parameters to achieve a higher efficiency in the field of photocatalysis. The inclusion of reduced graphene oxide (rGO) support in photocatalysts is one of the key strategies to address the above mentioned issues. In this study, rGO supported AgI mesoTiO2 photocatalysts were synthesized using a sonochemical approach. The physical effects of ultrasound not only improved the crystallinity of AgI mesoTiO2 but also increased the surface area and loading of the AgI mesoTiO2 nanocomposite on rGO sheets. The low intense oxygen functionalities (C O C and COOH groups) peak observed in the high resolution C1s spectrum of a hybrid AgI mesoTiO2 rGO photocatalyst clearly confirmed the successful reduction of graphene oxide (GO) to rGO. The interfacial charge transfer between the rGO and the p n junction of heterostructured photocatalysts has decreased the band gap of the photocatalyst from 2.80 to 2.65 eV. Importantly, the integration of rGO into AgI mesoTiO2 composites serves as a carrier separation centre and provides further insight into the electron transfer pathways of heterostructured nanocomposites. The individual effects of photo generated electrons and holes over rGO on the photocatalytic degradation efficiency of rhodamine (RhB) and methyl orange (MO) using AgI mesoTiO2 rGO photocatalysts were also studied. Our experimental results revealed that photo generated superoxide (O2( radicals are the main reactive species for the degradation of MO, whereas photo generated holes (h( are responsible for the degradation of RhB. As a result, 60% enhancement in MO degradation was observed in the presence of rGO in comparison to that of the pure AgI mesoTiO2 photocatalyst. This is due to the good electron acceptor and the ultrafast electron transfer properties of rGO that can effectively reduce the molecular oxygen to produce a large amount of reactive O2( radicals. However, in the case of RhB degradation, h( is the main reactive species which showed a slightly increased photocatalytic activity (12% in the presence of rGO support where the role of rGO is almost negligible. This study suggests the effective roles of rGO for the degradation of organics, i.e. the rate of photocatalytic degradation also depends on the nature of compound rather than rGO support.
author_names: Ramalingam Vinoth

P Karthik

Chellamuthu Muthamizhchelvan

Bernaurdshaw Neppolian

Muthupandian Ashokkumar
title: Carrier separation and charge transport characteristics of reduced graphene oxide supported visible light active photocatalysts.
venue: Physical chemistry chemical physics PCCP
year: 2016

abstract: Tantalum nitride (Ta3N5) is one of the most promising photoresponsive semiconductor materials for efficient solar energy conversion, but its fast carrier recombination has hindered research progress severely. Fabricating a heterojunction structure is an effective strategy to promote the charge separation efficiency and thus enhance solar conversion efficiency. Herein, we designed a p n heterojunction photoanode consisting of n type Ta3N5 nanorod arrays (NRAs) that grew along the a axis (light electron effective mass) and p type Cu2O nanoparticles. This NRA heterojunction shortens the hole diffusion distance, transfers electrons along the a axis effectively, and enlarges the space charge region. The heterojunction improves the charge separation efficiency of Ta3N5 NRAs significantly, and Ta3N5 Cu2O exhibits a photocurrent density of 9.19 mA cm 2 at 1.6 V vs normal hydrogen electrode (VNHE, pH 0) an onset potential of 0.326 VNHE, and a maximum incident photon to current efficiency of 60% at 380 nm. Our results demonstrated a potential crystal oriented growth strategy of nanostructured heterojunctions for overcoming the short carrier diffusion distance and fast carrier recombination.
author_names: Hongxu Wang

Zhan Shi

Shicheng Yan

Zhi-gang Zou
title: Enhanced charge separation by oriented growth of Ta3N5 Cu2O n p array heterojunction
venue: Applied Physics Letters
year: 2019

abstract: Abstract La2Ti2O7, a layered perovskite material, has attracted much attention in photocatalytic hydrogen production due to its high stability and non toxic. Combing La2Ti2O7 with narrow band gap semiconductors to construct 2D/2D heterojunction is a facile strategy to improve photocatalytic activity. In this work, we first report a novel 2D/2D N La2Ti2O7/ZnIn2S4 heterojunction via in situ growth of abundant well dispersed ZnIn2S4 nanosheets on the N La2Ti2O7 nanosheets surface. Experimental results show the optimized 2D/2D N La2Ti2O7/ZnIn2S4 sample exhibits the best light driven H2 evolution activity, which is nearly 5.2 and 1.7 times higher than that of pristine ZnIn2S4 and 2D/2D La2Ti2O7/ZnIn2S4 samples, respectively. This is due to the unique 2D/2D heterojunction produces high speed charge transfer channels, thus significantly enhancing photo carriers separation and migration efficiency. Meanwhile, DFT calculations confirm nitrogen doping forms impurity energy states at the top of valence band, thus narrowing the band gap of La2Ti2O7, considerably inhibiting electron hole pair recombination. Further the microstructures and charge separation properties are analyzed by SEM, PL, and DFT calculations to reveal the structure activity relationship of hetero structure and the synergistic mechanism of each functional component. This report will hopefully provide a practical strategy for designing heterojunction composite for high light driven hydrogen evolution rate.
author_names: Mengyang Xia

Xiaoqin Yan

He Li

Nathan P Wells

Guidong Yang
title: Well designed efficient charge separation in 2D/2D N doped La2Ti2O7/ZnIn2S4 heterojunction through band structure/morphology regulation synergistic effect
year: 2020

abstract: As an alternative to the gold standard TiO2 photocatalyst, the use of zinc oxide (ZnO) as a robust candidate for wastewater treatment is widespread due to its similarity in charge carrier dynamics upon bandgap excitation and the generation of reactive oxygen species in aqueous suspensions with TiO2. However, the large bandgap of ZnO, the massive charge carrier recombination, and the photoinduced corrosion dissolution at extreme pH conditions, together with the formation of inert Zn(OH)2 during photocatalytic reactions act as barriers for its extensive applicability. To this end, research has been intensified to improve the performance of ZnO by tailoring its surface bulk structure and by altering its photogenerated charge transfer pathways with an intention to inhibit the surface bulk charge carrier recombination. For the first time, the several strategies, such as tailoring the intrinsic defects, surface modification with organic compounds, doping with foreign ions, noble metal deposition, heterostructuring with other semiconductors and modification with carbon nanostructures, which have been successfully employed to improve the photoactivity and stability of ZnO are critically reviewed. Such modifications enhance the charge separation and facilitate the generation of reactive oxygenated free radicals, and also the interaction with the pollutant molecules. The synthetic route to obtain hierarchical nanostructured morphologies and study their impact on the photocatalytic performance is explained by considering the morphological influence and the defect rich chemistry of ZnO. Finally, the crystal facet engineering of polar and non polar facets and their relevance in photocatalysis is outlined. It is with this intention that the present review directs the further design, tailoring and tuning of the physico chemical and optoelectronic properties of ZnO for better applications, ranging from photocatalysis to photovoltaics.
author_names: Siva G Kumar

K S R Koteswara Rao
title: Zinc oxide based photocatalysis: tailoring surface bulk structure and related interfacial charge carrier dynamics for better environmental applications
year: 2015

abstract: Designing an efficient hybrid structure photocatalyst for photocatalytic decomposition and hydrogen (H2) evolution has been considered a great choice intended to develop renewable technologies for clean energy and environmental remediation. Enhanced charge transfer (CT) based on the interaction between a noble metal and a semiconductor is a crucial factor influencing the movement of photogenerated electron hole pairs. Herein, we focus on the recent advances in related to plasmon enhanced noble metals and semiconductor nature to drive photocatalytic H2 production and the organic dye Rhodamine B's (RhB) photodegradation under UV and visible light irradiation. Specifically, the combination of concerted catalysis and green nano engineering strategies to design ZnO based composite photocatalysts and their decoration with metallic Ag has been realized by the RF sputtering technique at room temperature. This simultaneity enhances the interface coupling between Ag and ZnO and reduces the energy threshold. The creation of charge transfer in the heterojunction and Schottky barrier changes in the photo electronic properties of the as synthesized AZO; afterwards, these effects promote the migration, transportation and separation of photoinduced charge carriers and enhance the light harvesting efficiency. As a result, the as synthesized AZO 20 hybrid nanostructure exhibits a photocurrent density of 2.5 mA/cm2 vs. Ag/AgCl, which is improved by almost 12 times compared with that of bare ZnO (0.2 mA/cm2) The hydrogen evolution rates of AZO 20 were 38 and 24 mol.h 1 under UV and visible light exposure, which are almost 5 and 10 fold higher than those of pristine ZnO. Additionally, the RhB degradation efficacy of the obtained AZO 20 was greater than almost 97 and 82% under the illumination of UV and visible light, respectively. The achieved apparent rate constant for the photocatalytic RhB decomposition was 0.014 min 1, indicating that it is 14 fold as much as that in pristine ZnO (0.001 min 1) Heterostructure AZO photocatalysts possess excellent practical stability in the water splitting reaction and photocatalytic RhB decomposition, posing as promising candidates in practical works for pollution and energy challenges.
author_names: Ton Nu Quynh Trang

Thang Bach Phan

Nguyen Dang Nam

Vu Thi Hanh Thu
title: In situ charge transfer at the Ag@ZnO photoelectrochemical interface toward the high photocatalytic performance of H2 evolution and RhB degradation.
venue: ACS applied materials interfaces
year: 2020

abstract: The charge transfer from the main catalyst to the cocatalyst is a key factor to enhance catalytic activity for photocatalytic nanocomposite materials. In order to enhance the charge transfer between Bi2WO6 and graphene, we inlet MoS2 as a "stepping stone" into Bi2WO6 and graphene. Here, we report an effective strategy to synthesize ternary Bi2WO6@MoS2/graphene nanocomposite photocatalyst by a facile two step hydrothermal method, which is afforded by assembling two cocatalysts, graphene and MoS2, into the Bi2WO6 matrix with a nanoparticle morphology as a visible light harvester. Compared with Bi2WO6/graphene, Bi2WO6/MoS2 and pure Bi2WO6, the Bi2WO6@MoS2/graphene ternary composites exhibit superior photocatalytic activity owing to an enhanced charge carrier separation via gradual charge transferred pathway. This work indicates a promising cocatalyst strategy for designing a more efficient graphene based semiconductor photocatalyst toward degradation of organic pollutants.
author_names: Ming Liu

Xin Xue

Shansheng Yu

Xiaoyi Wang

Xiaoying Hu

Hongwei Tian

Hong Chen

Weitao Zheng
title: Improving Photocatalytic Performance from Bi2WO6@MoS2/graphene Hybrids via Gradual Charge Transferred Pathway
venue: Scientific Reports
year: 2017