Shallow donor states induced by in-diffused Cu in ZnO: a combined HREELS and hybrid DFT study

Catalytic Hydrogenation of CO2 to Methanol: A Review

High-efficiency utilization of CO2 facilitates the reduction of CO2 concentration in the global atmosphere and hence the alleviation of the greenhouse effect. The catalytic hydrogenation of CO2 to produce value-added chemicals exhibits attractive prospects by potentially building energy recycling loops. Particularly, methanol is one of the practically important objective products, and the catalytic hydrogenation of CO2 to synthesize methanol has been extensively studied. In this review, we focus on some basic concepts on CO2 activation, the recent research advances in the catalytic hydrogenation of CO2 to methanol, the development of high-performance catalysts, and microscopic insight into the reaction mechanisms. Finally, some thinking on the present research and possible future trend is presented.

Chemical Bath Deposition of ZnO Nanowires Using Copper Nitrate as an Additive for Compensating Doping

The controlled incorporation of dopants like copper into ZnO nanowires (NWs) grown by chemical bath deposition (CBD) is still challenging despite its critical importance for the development of piezoelectric devices. In this context, the effects of the addition of copper nitrate during the CBD of ZnO NWs grown on Au seed layers are investigated in detail, where zinc nitrate and hexamethylenetetramine are used as standard chemical precursors and ammonia as an additive to tune the pH. By combining thermodynamic simulations with chemical and structural analyses, we show that copper oxide nanocrystals simultaneously form with ZnO NWs during the CBD process in the low-pH region associated with large supersaturation of Cu species. The Cu(II) and Zn(II) speciation diagrams reveal that both species show very similar behaviors, as they predominantly form either X²⁺ ions (with X = Cu or Zn) or X(NH₃)₄²⁺ ion complexes, depending on the pH value. Owing to their similar ionic structures, Cu²⁺ and Cu(NH₃)₄²⁺ ions preferentially formed in the low- and high-pH regions, respectively, are able to compete with the corresponding Zn²⁺ and Zn(NH₃)₄²⁺ ions to adsorb on the c-plane top facets of ZnO NWs despite repulsive electrostatic interactions, yielding the significant incorporation of Cu. At the highest pH value, additional attractive electrostatic interactions between the Cu(NH₃)₄²⁺ ion complexes and negatively charged c-plane top facets further enhance the incorporation of Cu into ZnO NWs. The present findings provide a deep insight into the physicochemical processes at work during the CBD of ZnO NWs following the addition of copper nitrate, as well as a detailed analysis of the incorporation mechanisms of Cu into ZnO NWs, which are considered beyond the only electrostatic forces usually driving the incorporation of dopants such as Al and Ga.

Excitation Dependent Phosphorous Property and New Model of the Structured Green Luminescence in ZnO

The copper induced green luminescence (GL) with two sets of fine structures in ZnO crystal has been found for several decades (i.e., R. Dingle, Phys. Rev. Lett. 23, 579 (1969)), but the physical origin of the doublet still remains as an open question up to now. In this paper, we provide new insight into the mechanism of the structured GL band in terms of new experimental findings and theoretical calculations. It is found, for the first time, that the GL signal exhibits persistent afterglow for tens of minutes after the switch-off of below-band-gap excitation light but it cannot occur under above-band-gap excitation. Such a phosphorous property may be interpreted as de-trapping and feeding of electrons from a shallow trapping level via the conduction band to the Cu-related luminescence centers where the Cu³⁺ ion is proposed to work as the final state of the GL emission. From first-principles calculation, such a Cu³⁺ ion in wurtzite ZnO prefers a high spin 3d⁸ state with two non-degenerated half-filled orbitals due to the Jahn-Teller effect, probably leading to the double structures in photoluminescence spectrum. Therefore, this model gives a comprehensively new understanding on the mechanism of the structured GL band in ZnO.

Enhanced wetting of Cu on ZnO by migration of subsurface oxygen vacancies

Metal adhesion on metal oxides is strongly controlled by the oxide surface structure and composition, but lack of control over the surface conditions often limits the possibilities to exploit this in opto- and micro-electronics applications and heterogeneous catalysis where nanostructural control is of utmost importance. The Cu/ZnO system is among the most investigated of such systems in model studies, but the presence of subsurface ZnO defects and their important role for adhesion on ZnO have been unappreciated so far. Here we reveal that the surface-directed migration of subsurface defects affects the Cu adhesion on polar ZnO(0001) in the technologically interesting temperature range up to 550 K. This leads to enhanced adhesion and ultimately complete wetting of ZnO(0001) by a Cu overlayer. On the basis of our experimental and computational results we demonstrate a mechanism which implies that defect concentrations in the bulk are an important, and possibly controllable, parameter for the metal-on-oxide growth.

Comprehensive elucidation of metal-support interactions is important for controlling and improving their performances in a range of pertinent technologies. Here, the authors reveal how subsurface defects influence the adhesion and wetting of a metal on the surface of a metal oxide.

Single-Layer Graphene as an Effective Mediator of the Metal-Support Interaction

Single-layer chemical vapor deposition (CVD)-grown graphene was transferred onto a ZnO (0001) substrate forming a large-area, low-defect density, protective layer. The quality of the graphene layer and its effect on the interaction between the ZnO support and vapor-deposited cobalt particles was investigated by spectroscopic and microscopic techniques. We demonstrate that the in-between graphene layer influences both the oxidation state and the morphology of cobalt upon annealing in vacuum. In particular, cobalt strongly interacts with the bare ZnO substrate forming flat particles, which are readily oxidized and redispersed upon annealing in ultrahigh vacuum conditions. In contrast, in the presence of the graphene interlayer, cobalt forms highly dispersed nanoparticles, which are resistant to oxidation, but prone to surface diffusion and agglomeration. The graphene layer exhibits remarkable stability upon cobalt deposition and vacuum annealing, while interaction with reactive gases can facilitate the formation of defects.

Dual-donor (Zn(i) and V(O)) mediated ferromagnetism in copper-doped ZnO micron-scale polycrystalline films: a thermally driven defect modulation process

The paper reports robust ferromagnetic Cu-doped ZnO micron-scale polycrystalline films via spin-coating using high-quality doped nanocrystals. A reliable magnetic response is observed in the 900 °C vacuum annealed film without any ferromagnetic contribution from other sources. Post-annealing treatment in terms of atmosphere and temperature can control the proportion of oxygen vacancies (V(O)) and zinc interstitials (Zn(i)) defects and further help to precisely regulate defect-related ferromagnetic behavior. Complex charge transfer processes derived from dual-donor (Zn(i) and V(O)) to Cu acceptor are revealed by photoluminescence (PL) and electron paramagnetic resonance (EPR) spectra. Based on the above, specific charge transfer (CT)-type Stoner splitting and indirect double-exchange mechanisms are proposed to understand the ferromagnetic origin. The improvable FM performance and annealing-specific modulation further indicate that a thermal driven process can delicately tailor the magnetic property of the transition metal ion-doped ZnO system.

Unambiguous identification of the role of a single Cu atom in the ZnO structured green band

High quality and purity single crystal ZnO samples doped with single isotopes of (63)Cu and (65)Cu, with equal concentrations of both these isotopes, and with natural Cu using a wet chemical atomic substitution reaction and anneal were studied using low temperature optical spectroscopy. Our data on the zero phonon line of the structured green band in ZnO confirm unambiguously the involvement of a single Cu atom in this defect emission. These data allow us to confirm the main features of the assignment proposed by Dingle in 1969 and to comment further on the defect structure.