3D graphene: synthesis, properties, and solar cell applications

Three-dimensional (3D) graphene is one of the most important nanomaterials. This feature article highlights the advancements, with an emphasis on contributions from our group, in the synthesis of 3D graphene-based materials and their utilization in solar cells. Chemistries of graphene oxides, hydrocarbons, and alkali metals are discussed for the synthesis of 3D graphene materials. Their performances in dye-sensitized solar cells and perovskite solar cells (as counter electrodes, photoelectrodes, and electron extracting layers) were correlatively analyzed with their properties/structures (accessible surface area, electrical conductivity, defects, and functional groups). The challenges and prospects for their applications in photovoltaic solar cells are outlined.

3D Graphene Foam by Chemical Vapor Deposition: Synthesis, Properties, and Energy-Related Applications

In this review, we highlight recent advancements in 3D graphene foam synthesis by template-assisted chemical vapor deposition, as well as their potential energy storage and conversion applications. This method offers good control of the number of graphene layers and porosity, as well as continuous connection of the graphene sheets. The review covers all the substrate types, catalysts, and precursors used to synthesize 3D graphene by the CVD method, as well as their most viable energy-related applications.

Cost Effective Synthesis of Graphene Nanomaterials for Non-Enzymatic Electrochemical Sensors for Glucose: A Comprehensive Review

The high conductivity of graphene material (or its derivatives) and its very large surface area enhance the direct electron transfer, improving non-enzymatic electrochemical sensors sensitivity and its other characteristics. The offered large pores facilitate analyte transport enabling glucose detection even at very low concentration values. In the current review paper we classified the enzymeless graphene-based glucose electrocatalysts' synthesis methods that have been followed into the last few years into four main categories: (i) direct growth of graphene (or oxides) on metallic substrates, (ii) in-situ growth of metallic nanoparticles into graphene (or oxides) matrix, (iii) laser-induced graphene electrodes and (iv) polymer functionalized graphene (or oxides) electrodes. The increment of the specific surface area and the high degree reduction of the electrode internal resistance were recognized as their common targets. Analyzing glucose electrooxidation mechanism over Cu- Co- and Ni-(oxide)/graphene (or derivative) electrocatalysts, we deduced that glucose electrochemical sensing properties, such as sensitivity, detection limit and linear detection limit, totally depend on the route of the mass and charge transport between metal(II)/metal(III); and so both (specific area and internal resistance) should have the optimum values.

β-Cyclodextrin functionalized 3D reduced graphene oxide composite-based electrochemical sensor for the sensitive detection of dopamine

A three-dimensional reduced graphene oxide nanomaterial with β-cyclodextrin modified glassy carbon electrode (3D-rGO/β-CD/GCE) was constructed and used to detect the electrochemical behavior of dopamine (DA). The nanocomposite materials were characterized by scanning electron microscopy (SEM), infrared spectrometry (FT-IR), Raman spectrogram and thermogravimetric analysis (TGA), which showed that β-CD was well modified on 3D graphene with a porous structure. The electrochemical properties of different modified electrodes were investigated by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS), proving the highest electron transfer rate of the 3D-rGO/β-CD modified electrode. The experimental conditions such as scan rate, pH, enrichment time and layer thickness were optimized. Under the best experimental conditions, DA was detected by differential pulse voltammetry (DPV) by 3D-rGO/β-CD/GCE with excellent electrocatalytic ability and satisfactory recognition ability, resulting in a wide linear range of 0.5-100 μM and a low detection limit (LOD) of 0.013 μM. The modified electrode based on 3D-rGO/β-CD nanocomposites is promising in the field of electrochemical sensors due to its high sensitivity and other excellent properties.

Three-dimensional graphene networks and RGO-based counter electrode for DSSCs

Graphene is considered to be a potential replacement for the traditional Pt counter electrode (CE) in dye-sensitized solar cells (DSSCs). Besides a high electron transport ability, a close contact between the CE and electrolyte is crucial to its outstanding catalytic activity for the I3 -/I redox reaction. In this study, reduced graphene oxide (RGO) and three-dimensional graphene networks (3DGNs) were used to fabricate the CE, and the graphene-based CE endowed the resulting DSSC with excellent photovoltaic performance features. The high quality and continuous structure of the 3DGNs provided a channel amenable to fast transport of electrons, while the RGO afforded a close contact at the interface between the graphene basal plane and electrolyte. The obtained energy conversion efficiency (η) was closely related to the mass fraction and reduction degree of the RGO that was used. Corresponding optimization yielded, for the DSSCs based on the 3DGN-RGO CE, a value of η as high as 9.79%, comparable to that of the device using a Pt CE and hence implying promising prospects for the as-prepared CE.

Hierarchical TiO2 microspheres composed with nanoparticle-decorated nanorods for the enhanced photovoltaic performance in dye-sensitized solar cells

Hierarchical TiO₂ microspheres composed of nanoparticle-decorated nanorods (NP-MS) were successfully prepared with a two-step solvothermal method. There were three benefits associated with the use of NP-MS as a photoanode material. The decoration of nanoparticles improved the specific surface area and directly enhanced the dye loading ability. Rutile nanorods serving as electron transport paths resulted in fast electron transport and inhibited the charge recombination process. The three-dimensional hierarchical NP-MS structure supplied a strong light scattering capability and good connectivity. Thus, the hierarchical NP-MS combined the beneficial properties of improved scattering capability, dye loading ability, electron transport and inhibited charge recombination. Attributed to these advantages, a photoelectric conversion efficiency of up to 7.32% was obtained with the NP-MS film-based photoanode, resulting in a 43.5% enhancement compared to the efficiency of the P25 film-based photoanode (5.10%) at a similar thickness. Compared to traditional photoanodes with scattering layers or scattering centers, the fabrication process for single layered photoanodes with enhanced scattering capability was very simple. We believe the strategy would be beneficial for the easy fabrication of efficient dye-sensitized solar cells.

Hierarchical NP-MS combines the beneficial properties of improved scattering capability, dye loading ability, electron transport and inhibited charge recombination. The photoelectric conversion efficiency up to 7.32% has been obtained.

Selective adsorption and decomposition of pollutants using RGO-TiO2 with optimized surface functional groups

Reduced graphene oxide (RGO) samples with optimized types of surface functional groups were hybridized with TiO2 to achieve the selective adsorption and removal of various pollutants. A high ratio of hydroxyl groups was found to be remarkably advantageous for the adsorbtion and decomposition of rhodamine-B (and similar pollutants), while a high ratio of carboxyl groups was found to promote the ability to adsorb and decompose phenol. Moreover, the presence of carboxyl groups on the RGO edge provides a pre-condition to form a close chemical connection with TiO2, which has been proven by the obtained electron paramagnetic resonance (EPR) curve, infrared spectroscopy (IR) and electron lifetime. The resulting composite photocatalysts display excellent photocatalytic activities under both UV- and visible-light illumination, indicating that the well-designed surface micro-circumstances of the RGO are quite significant.

Graphene based photoanode for DSSCs with high performances

Graphene assisted photoanodes are promising because of the high performance of the resulting dye sensitized solar cells (DSSCs). A photoanode with a three-layer structure is prepared in this study and the synergy between each layer was found to play a vital role in its photovoltaic properties. The influence of interface contact between the transport layer and work layer is revealed. After ameliorating the interface contact level (enhancing the electron transport ability), the functions of the adopted reduced graphene oxide (RGO) and three-dimensional graphene networks (3DGNs) in the transport layer and work layer, respectively, can be made full use of. In order to further enhance the scattering ability for the incident light and improve the adsorption ability for dye molecules, a scattering layer based on the RGO-TiO2 is added in the photoanode. After a comprehensive optimization (including the types of functional groups and mass fractions of the RGO in the work layer and scattering layer), the resulting power conversion efficiency reaches 11.8%, which is much higher than that of previous reported graphene modified DSSCs.

Influence from defects of three-dimensional graphene networks on the interface condition between the graphene basal plane and various resins

Graphene assisted thermal interface materials (TIMs) attract more and more attention because of their high thermal conductivities. However, how to improve the phonon transport ability at the interface between the graphene basal plane and the matrix is still unclear. In this study, three-dimensional graphene networks (3DGNs) with varying defect densities are fabricated by adjusting the chemical vapor deposition processes, and these specimens are adopted as fillers to modify various resins with different functional groups to reveal the interface contact rule. By optimizing the defect density of the 3DGNs, a synergy between the thermal boundary resistances of the filler and matrix and the high intrinsic thermal conductivity of the filler can be achieved in the resulting TIMs, and the epoxy group from the matrix is found to be the best active group to form a close contact between the defects of the 3DGNs. After that, the thermal performance stability of the resulting TIMs under a high temperature for a long time is detected, and the influence from increased Umklapp scattering is partially offset by the weakened Kapitza scattering. Moreover, the corresponding mechanical properties have been measured to confirm the feasibility of the resulting TIMs for practical application areas.

Graphene Modified TiO₂ Composite Photocatalysts: Mechanism, Progress and Perspective

Graphene modified TiO₂ composite photocatalysts have drawn increasing attention because of their high performance. Some significant advancements have been achieved with the continuous research, such as the corresponding photocatalytic mechanism that has been revealed. Specific influencing factors have been discovered and potential optimizing methods are proposed. The latest developments in graphene assisted TiO₂ composite photocatalysts are abstracted and discussed. Based on the primary reasons behind the observed phenomena of these composite photocatalysts, probable development directions and further optimizing strategies are presented. Moreover, several novel detective technologies-beyond the decomposition test-which can be used to judge the photocatalytic performances of the resulting photocatalysts are listed and analyzed. Although some objectives have been achieved, new challenges still exist and hinder the widespread application of graphene-TiO₂ composite photocatalysts, which deserves further study.

Influence from the types of surface functional groups of RGO on the performances of thermal interface materials

RGO nanosheets-modified epoxy resin (ER) is a major research interest because of the high thermal performance of the resulting thermal interface materials (TIMs).