Visible light-driven photocatalytic reduction of protons to H 2 is considered a promising way of solar-to-chemical energy conversion. Effective transfer of the photogenerated electrons and holes to the surface of the photocatalyst by minimizing their recombination is essential for achieving a high photo- catalytic activity. In general, a sacrificial electron donor is used as a hole scavenger to remove photogenerated holes from the valence band for the continuation of the photocatalytic hydro- gen (H 2 ) evolution process. Here, for the first time, the hole- transfer dynamics from Pt-loaded sol gel-prepared graphiticcarbonnitride (Pt-sg-CN) photocatalyst were investigated using different adsorbed hole acceptors along with a sacrificial agent (ascorbic acid). A significant increment (4.84 times) in H 2 production was achieved by employing phenothiazine (PTZ) as the hole acceptor with continuous H 2 production for 3 days. A detailed charge-transfer dynamic of the photocatalytic process in the presence of the hole acceptors was examined by time- resolved photoluminescence and in situ electron paramagnetic resonance studies.
metal-free nickel-containing sol–gel prepared mesoporous graphiticcarbonnitride (sg-CN) system, for the photocatalytic hydrogen production in the presence of sacrificial agent, has been uncovered for the first time by in situ EPR spectroscopy. In addition, the formed catalytic species were also investigated by ex situ EPR, XPS, TEM, HRTEM. In situ EPR studies clearly proved that Ni 2+ has been reduced to Ni 0 NPs during the photocatalytic process and acts as a co-catalyst for the reduction of protons to dihydrogen. Quick transportation of the photogenerated electrons to the Ni 0 co-catalyst could also be observed by EPR spectroscopy after Ni loading on sg-CN. Photodeposition of Ni 2+ onto sg-CN leads to a photocatalytic system for the long-term photochemical proton reduction with activities comparable to other non-noble co-catalyst systems on sg-CN. Finally, the sg-CN system investigated here clearly provides insight into the co-catalyst structure during photo- chemical H 2 evolution and will help in the understanding of the electron–hole separation after the co-catalyst loading.
The conversion of solar energy to chemical energy is one of the principal routes to establish a green global economy. 1 Particu- larly, hydrogen as a product of solar water splitting may assume a decisive role upon transition from fossil to renewable energy resources. 2 For realization of the solar-to-hydrogen conversion process by photoelectrochemical water splitting, suitable pho- toelectrodes still have to be developed and to meet important demands such as eﬃciency and durability. Most critical, elec- trode fabrication has to rely on cost-saving material consump- tion, and the use of earth-abundant materials appears inevitable for novel device architectures. In this respect, the combination of silicon and polymeric graphiticcarbonnitride (g-C 3 N 4 ) represents a highly attractive heterostructure, inte-
A thermally induced topotactic transformation of organic polymeric semiconductors is achieved using similarity of the chemical structures of two C,N,H-containing materials. Namely, the oligomer of 3-amino- 1,2,4-triazole (OATA) is transformed into an electronically modi ﬁed graphiticcarbonnitride (OATA-CN) upon heating at 550 C. During the transition, the ﬂat band potential of the organic semiconductor is only slightly shifted from 0.11 eV to 0.06 eV, while the optical band gap is signiﬁcantly expanded from 1.8 eV to 2.2 eV. The advantage of the suggested approach is the processability of the starting semiconductor combined with minor morphology changes during the heat-treatment that enable preservation of the original oligomer micro- and macrostructures in the resulting carbon nitrides. As an illustration, di ﬀerent OATA morphologies, including spherical nanoparticles, nanobarrels, nanowires and self-assembled macrospheres and composite sheets are synthesized and then transformed into OATA-CN with the retention of morphology. The surface area of the ﬁnal carbon nitrides reaches 66 m 2 g 1 , without using any template, auxiliary reagent or post treatment. As a consequence, the photocatalytic activity of the obtained carbon nitrides in visible light driven hydrogen evolution is up to 5 times higher than that measured for the reference bulk carbonnitride prepared by pyrolysis of melamine.
Melam was ﬁrst prepared in 1834 by Liebig on heating an intimate mixture of potassium thiocyanate and an excess of ammonium chloride at temperatures around 580 – 640 K until all volatile products were given oﬀ. From the grayish-white fusion product a compound was isolated after treatment with moderately concentrated potassium hydroxide solution, which was arbitrarily given the name “melam” by Liebig. On boiling melam in “caustic potash” solution for several hours, two unknown hydrolysis products were obtained, which were named “melamine” and “ammeline” . Investigations of the thermal behavior of the above weak bases led Liebig to the discovery of two further insoluble compounds of the empirical formulas C 6 N 10 H 6 and C 6 N 9 H 3 , which were given the similarly unintuitive names “melem” and “melon”, respectively. They were conceived as condensation products of melamine, which form on successive elimination of ammonia from the starting material. In the following years, much eﬀort was devoted to the elucidation of the composition and structure of melamine and its pyrolysis products, which however was hampered by conﬂicting statements with respect to their elemental composition, ways of formation, and, above all, molecular structures [76,80,304,457]: Although general agreement had been achieved on their principal nature in that they may be considered as derivatives of cyanamide according to the general formula (NCNH 2 ) n · m NH 3 , suggestions for their structures were put forward at a much later date [76, 80]. Likewise, whereas the identity of melamine was resolved by the elucidation of its crystal structure in 1941 , chemical inertness and low solubility of its condensation products has impeded the characterization apart from elemental analysis ever since their early discovery and despite the comparatively easy availability. Only recently the crystal structure of melem was solved from X-ray powder data , thereby putting an end to the long-standing debate about its heptazine-type structure [76, 77, 80, 459–461], and at the same time giving valuable incentives to the ongoing quest for graphiticcarbonnitride (cf. Chapter 7 on page 280) [42, 82, 85, 124, 127].
Converting solar energy into the storable chemical energy of hydrogen by photocatalytic water splitting is of major interest for a future sustainable energy supply. A variety of semiconducting materials has been proposed as more or less efficient photocatalysts for this reaction. Beside the chemical composition, which in the first place determines that the material has a suitable band structure for the splitting of water, it has been shown that the introduction of a nanometre sized structure can have crucial influence on the photocatalytic activity. For example the accompanied increase in the surface area increases the number of catalytically active sites for the splitting of water. The same is true for a so-called graphiticcarbonnitride, a metal free, polymeric semiconductor, which has been reasonably shown to be a cheap and abundant alternative as water splitting photocatalyst. The initially low activity of the bulk material could be considerably enhanced by introduction of small pores into this material. In this work we introduce a facile synthesis of mesoporous carbon nitrides, which yield a material with further increased photocatalytic activity.
Over the last few decades, main-group nitrides have significantly gained importance in the field of high-performance functional materials due to their exceptional chemical stability and properties. [1–3] With the spotlight focused on light element-based nitrides qualified for a multitude of technological applications by their structural variety and strong covalence, carbon nitrides have boomed owing to their specific chemical properties. Since the “harder than diamond” fever has been evoked by the work of Liu and Cohen, much effort has been made to synthesize dense 3D phases of binary carbonnitride, C 3 N 4 , which was predicted to show very low compressibility and superhardness. [4–8] Carbonnitride imide (C 2 N 2 (NH)), presented in 2007, was the first described crystalline 3D carbonnitride network and showed a defect wurtzite-type structure.  Recently, low-density 2D carbon nitrides have also been attracting interest owing to their manifold optical and electronic properties. Not only graphiticcarbonnitride (g-C 3 N 4 ), which is considered to be a precursor for high-pressure conversion into 3D C 3 N 4 and computed as the most stable modification under ambient conditions,  but also hydrogen-richer samples seem to be promising new materials for organic semiconductor science, catalytic applications, and as photoactive materials for converting solar light into electricity. [10–17]
6.2. Selective Etching of p-GaN
Completely removing the p-GaN without attacking the underlying AlGaN barrier re- quires highly selective etch process, i.e. a large etch rate for p-GaN is desired, whereas the etch rate of AlGaN should ideally be zero. Achieving different etch rates for mate- rials is primarily based on chemical reactions in contrast to physical etching by sputter- ing. Therefore, a dry-etch process in an inductively coupled plasma reactive ion etching (ICP-RIE) tool is preferred, in which the plasma density can be independently con- trolled through the ICP power. The dry-etching agent of choice for gallium nitride is chlorine, which shows a high etch rate for GaN and, slightly weaker, for AlGaN . Oxygen acts as a retarding agent in the dry-etching process with higher effectiveness for AlGaN compared to GaN  and can be used to adjust the selectivity. Another possi- bility is the use of a fluorine chemistry as retarding agent . For both, a competing process between the passivation of the surface by an oxide/fluoride due to chemical reactions, and etching by sputtering this passivation through ion bombardment and at- tacking the underlying material is established. These two processes need to be balanced to achieve high selectivity. This type of process was first introduced by Lee et al. in 2000  with a Cl 2 /O 2 /Ar chemistry leading to a selectivity of 24 and improved by Han et al.  with a Cl 2 /O 2 /N 2 chemistry and a selectivity of 60. Particularly the process from Han et al. has seen growing interest recently and shows very good re- sults [117, 125–127].
After the ion induced stress relaxation of a c-BN film, the next step is to transfer the sample back into the deposition chamber, to anneal the ion bombarded film at the deposition temperature 420°C for two hours [Widmayer 1999]. Before depositing a new c-BN film on top of the already relaxed layer sputter cleaning its surface should be applied. In our case, this is achieved by operating the assisting ion source as a sputter gun using the same mixture of argon and nitrogen ions with the energy of 280 eV as during deposition. The effectiveness of the cleaning step in removing carbon and oxygen contaminants from the surface can be seen from the Auger data given in Fig. 3.25, which shows typical AES spectra for the as-prepared state (Fig. 3.25 a), after (ex situ) ion bombardment to release strain (Fig. 3.25 b), and after in situ sputter cleaning prior to the deposition of an additional BN layer (Fig. 3.25 c).It is also worth noting that after such a sputter cleaning a plasmon energy of 31.3 eV is observed using 1.5 keV primary electrons. This value, indicating a high quality c-BN film, is practically identical to the plasmon energy determined in situ on the as-prepared film prior to exposing to ambient conditions to perform the ion induced stress relaxation. This result confirms that sputter cleaning under deposition conditions of a c-BN film, which bad been exposed to the ambient for additional ion treatment, restores the surface properties of the as-prepared film [Boyen 2000]. Once this condition is established, the deposition is restarted and a second c-BN film is prepared. If this is possible, the whole sequence will be repeated periodically to grow a thick, stress relieved c-BN film [Boyen 2000].
The carbon content was determined in a carbon ana- lyzer CS-800 (ELTRA GmbH, Haan, Germany), the nitrogen and oxygen content in an N/O analyzer Leco TC-436 (Leco Corporation, St. Joseph, MI). The hydrogen content was determined by the Mikroanalytisches Labor Pascher (Remagen, Germany) using the coupled plasma atomic emission spectroscopy (Thermo Instru- ments, iCAP 6500, Waltham, MA) and element analyzer (Pascher). The silicon content was calculated from the sum of constituting elements.
A prominent feature of nitride-based nanostructures are the large built-in electrostatic fields, which occur due to piezo- and pyroelectric polarization effects. Piezoelectric polarizations are induced by non-hydrostatic strain along polar axes 49 of the crystal (here the  axis). Thus, it only occurs in strained structures, such as the non-lattice-matched nanostructures considered in this work. Pyroelectric, or spontaneous, polarization occurs without any external stress applied. Due to the deviation of the bonds from the ideal tetrahedral structure in wurtzite, the dipoles of the polar bonds in each unit cell do not completely cancel, resulting in a residual dipole moment of the hole unit cell. The effect increases if the crystal structure deviates from ideal wurtzite, i.e. if the a 0 /c 0 ratio and/or the internal lattice parameter u deviate from their ideal values. 50
Nanomaterials 2020, 10, 351 12 of 16
3.3. Electrical Conductivity of Pristine and GNPs Doped PAN Nanofiber Mats
The results of electrical conductivity measurements of PAN and PAN/GNPs are shown in Figure 7 a,b. The transport was measured along and across the nanofiber axis direction for both types of samples. It can be seen that for all temperatures, doping with GNPs enhances the conductivity along the fiber direction. At 1000 ◦ C, the electrical conductivity is increased by almost 60% along the fiber axis. Similarly, a 24% increase in electrical conductivity was observed at 1700 ◦ C, suggesting that microstructure becomes more graphitic as supported by Raman and XRD results. A greater increment in conductivity at lower temperatures as compared to higher temperatures upon doping with GNPs can be attributed to the fact that the templating effect of nano-carbons is more pronounced at lower carbonization temperatures where a large portion of carbon is more amorphous [ 31 ]. Conductivity across the fiber directions is lower as compared to in fiber direction (e.g., at 1000 ◦ C from average of 77 S/cm in fiber direction to 8 S/cm across the fiber direction). Such a large discrepancy in electrical conductivities across and along fiber direction is obvious as there are fewer interconnects available for electrons. Fibers make occasional contacts even when they are not stretched however they are aligned in one direction due to use of rotating collector after spinning. Interestingly for PAN/GNPs, the anisotropy in electrical conductivity is decreased as compared to pristine CNF. In the case of pristine CNFs, there are fewer paths for electron flowing in the perpendicular direction, however with incorporation of GNPs into CNFs, there are more conductive network like pathways across nanofibers. Uniformly distributed GNPs in fiber matrix at different locations provide a path due to which the anisotropy in conductivity is reduced as compared to pristine CNF (Figure 8 ). The anisotropy of the electrical conductivity is much higher for 1700 ◦ C–T as compared to 1700 ◦ C samples (Figure 7 d). The average electrical conductivity parallel to fiber direction is 590.8 S/cm, however, across the fiber direction is 40.5 S/cm for PAN–1700 ◦
Diese Arbeit befasst sich mit der Herstellung und Charakterisierung von Nanodr¨ahten des Indiumgalliumnitrid (InGaN) Materialsystems. Die Anwendung der Gruppe III- Nitride in optoelektronischen Bauelementen hat sich in den letzten Jahren fest etabliert. Sie sind daher von großem Interesse f¨ ur Forschung und Industrie. Insbesondere ist die Verwendung von Galliumnitrid (GaN) - InGaN Heterostrukturschichten als Basis der Licht-emittierenden Dioden (LEDs) zu erw¨ahnen. Diese werden nicht nur als energieef- fiziente Lichtquellen verwendet, sondern z.B. auch als blaue Laser in Blu-Ray-Playern oder als Hintergrundbeleuchtung in Displays. Durch Verwendung dieses Materia- lystems in Form von Nanostrukturen, insbesondere Nanodr¨ahten, soll eine gesteigerte Effizienz der Bauelemente erzielt werden. Durch ihre einzigartige Morphologie und das hohe Oberfl¨achen-zu-Volumen Verh¨altnis kann beispielsweise bei Anwendung in der Photovoltaik die Reflektion des einfallenden Lichts verringert und der Anteil an absorbiertem Licht erh¨oht werden. Zus¨atzlich bieten Nanodr¨ahte, im Gegensatz zu planaren Schichten, die M¨oglichkeit, den Einbau von Gitterfehlern beim Kristallwach- stum auf Fremdsubstraten, wie Silizium oder Saphir, zu reduzieren.
6.2.4 demonstrates the large impact of a small contaminated area on the speciﬁc resistance indicating a silicon surface related issue. All these experiments point to the depletion region of the Schottky-Mott contact but an explicit identiﬁcation of this cause is not presented in this thesis. An analysis of the lift-oﬀ steps is necessary to clarify the circumstance of any residual materials or contaminants causing a rise of the contact resistance. Here, a contact resistivity study by transmission line method is an appropriate approach to analyze in detail the junction resistance [ 238 ]. The results of the vanadium based nitride MOSFET indicate that a clean and pure metal evaporation fabrication process is crucial for a low subthreshold swing. For instance, the grain size of the evaporated material determines the speciﬁc resistance [ 239 ]. SEM images of in-house evaporated aluminum and vanadium and externally deposited aluminum layer show diﬀerent grain sizes. While vanadium exhibits an excellent smoothness and no small grain formation (see Fig. 6.5 a), the aluminum layers have much smaller grain sizes (see Fig. 6.5 b and c). Especially the in-house evaporated aluminum has even smaller fragments, which is conﬁrmed by the huge surface roughness. This might be an indication for various work functions and a detailed study of the evaporated metal should take this into account.
In this investigation the initial co-catalyst concentration was varied in a range from 0 to 5 wt% Pt (0–11 wt% H 2 PtCl 6 ) and the TEOA concentration was varied in a range from 2.5 to 60 vol%. 2.4.3. Large-scale hydrogen evolution. For large-scale hydrogen evolution a new setup was constructed which is shown in Fig. 6. It consists of a reservoir (Fig. 6a) with a maximum volume of 11 L for the photocatalyst dispersion placed on a stirring plate (IKA, RH basic 2) to keep the catalyst dispersed, a peristaltic pump (Ismatec, ISM1079B) to circulate the catalyst dispersion (Fig. 6b), and the photoreactor (Fig. 6c), which was designed similar to a solar panel. The window of the photoreactor is made of extruded polycarbonate (Quinn Plas- tics) with a thickness of 8 mm and an irradiation area of about 1 m 2 , which corresponds to a scale-up factor of approximately 800 compared to the lab-scale photoreactor. A further valve allows the connection of an argon bottle to purge the free gas volume prior to the photocatalytic experiments. To test the carbonnitride photocatalyst in this new setup for hydrogen evolution under real sunlight, 6.8 g Pt@mp-CN prepared using the ex situ method as described above was placed into the reservoir together with water and the sacricial agent TEOA (10 vol%). The solution was stirred and circulated between the
Even though the growth of III-nitride NWs is well-studied and complex axial hetero- structures can be realized [27, 28, 29, 30], a systematic study of doping is necessary for a fundamental understanding and the development of different types of devices, since low resistive current injection is needed for many applications. At present, Si is used as a donor in n-type GaN NWs grown by plasma-assisted molecular beam epitaxy (PAMBE) and the influence of Si-doping on the optical and electrical properties has been studied by different techniques [9, 31, 32, 33, 34]. Quantitative analysis of the resistance in four-point probing geometry or determination of carrier concentration by thermoelectric characterization is only reported for typically longer and thicker Si-doped microwires grown by MOCVD revealing a conductivity and carrier concentration up to 2700 S/cm and 2.6 × 10 20 cm −3 , respectively [35, 36].
Carbon Pricing for Low-Carbon Investment January 2011
Executive Summary 4
2 ETS Effectiveness
One objective of the European Emissions Trading Scheme, to reduce the investment in carbon- intensive assets, has been successfully achieved. Of the allowances that were reserved in 2006/2007 for new or expanded carbon-intensive installations, only 16 percent had actually been requested by 2009. It is however difficult to assess to what extent this result should not also be attributed to the more attractive investment opportunities in renewable energy sources, and the economic constraints imposed with the financial crisis.