Transition-metal oxides

When research on transition-metal oxide systems began in the 1960s, many metallic oxides were discovered (Figure 1a).^{Reference Rao11–Reference Rao and Raveau13} For example, ReO₃ had the appearance of and conducted like copper. Oxides exhibiting metal–insulator (M–I) transitions (Ti₂O₃, VO₂, V₂O₃) were soon discovered (Figure 1b). V₂O₃ undergoes a transition with a 10-million-fold jump in resistivity at 150 K, when there is also a transition to the antiferromagnetic state. Work was conducted on metallic oxides as well as oxides showing M–I transitions.^{Reference Rao11,Reference Edwards, Ramakrishnan and Rao12}

Figure 1.

(a) Electrical resistivities of highly conducting (metallic) oxides. (b) The well-known metal–insulator transition in metal oxides. Note: ρ, resistivity; T, temperature.

Around 1965, work on properties and phenomena exhibited by perovskite oxides began, in particular, on perovskites of the type LnBO₃ (Ln = La–Lu or Y and B = Ti-Cu), which exhibit a fantastic variety of properties.^{Reference Goodenough9–Reference Rao and Raveau13} LaCrO₃ and other chromites are antiferromagnetic insulators. LaCoO₃ shows an interesting spin-state transition involving low and intermediate spin states of Co³⁺, whereas LaNiO₃ is a metal. We worked on the effect of various types of substitutions (e.g., Sr²⁺ in LaCoO₃, as well as solid solutions of different perovskites) on the magnetic and electronic properties. Particularly noteworthy is the La_1–xSr_xCoO₃ system, which shows a decrease in electrical resistivity with increase in x, the x = 0.5 composition being a ferromagnetic metal (Figure 2).^{Reference Wu and Leighton14} Mössbauer studies showed that rapid electron transfer occurs between the 3+ and 4+ states of cobalt.^{Reference Bhide, Rajoria, Rao, Rao and Jadhao15} My resolve to pursue research in materials chemistry got strengthened by my one-year stay at the University of Oxford as a visiting professor in 1974. It was in Oxford that I had for the first time all of the facilities I needed, and I could carry out many studies, including high-resolution electron microscopy and photoemission spectroscopy. I then decided to build a laboratory with similar facilities in India.

Figure 2.

(a) Magnetic and (b) electrical resistivity behavior of La_1–xSr_xCoO₃. LaCoO₃ is a semiconductor, but with the substitution of Sr in place of La with x > 0.2, La_1–xSr_xCoO₃ exhibits a metal-like temperature dependence. Note: M, magnetization; T, temperature; ρ, resistivity. Reproduced with permission from Reference Wu and LeightonReference 14. © 2003 American Physical Society.

High-temperature superconductivity

The discovery of high-temperature superconductivity in 1986 created a sensation, having broken the apparent 23 K barrier. This was discussed at an international conference at Bangalore. Soon after I gave my lecture, P. Anderson, from Princeton University, asked “Have you heard of the high T _c (35 K; T _c is critical temperature) superconductor?” When I asked him what the material was, he said that it is an oxide with lanthanum and copper. I got curious and asked him “Is it La₂CuO₄?” When he nodded yes, I got truly excited since I had worked on La₂CuO₄ as early as 1970 and shown it to be an antiferromagnetic insulator.^{Reference Ganguly and Rao16,Reference Ganguly and Rao17} Upon doping with holes (Sr, Ba, or O) it becomes a 35 K superconductor.

I started working on a number of Ln₂CuO₄ compounds with the 2D K₂NiF₄ structure as well as related structures. While this yielded interesting results, I worked with other researchers around the world looking for a liquid-nitrogen superconductor (T _c > 77 K). Wu et al. reported a cuprate that showed high T _c close to 90 K, however, the composition and structure were not known. In February 1987, we successfully characterized YBa₂Cu₃O₇ (123) in all respects (Figure 3).^{Reference Rao, Ganguly, Raychaudhuri, Ram and Sreedhar18} We made magnetic measurements using Raychaudhuri’s home-built apparatus.

Figure 3.

YBa₂Cu₃O₇, the first liquid-nitrogen superconductor. (a) Crystal structure.^{Reference Rao11} (b) Resistivity versus temperature plot.^{Reference Rao, Ganguly, Raychaudhuri, Ram and Sreedhar18} Cu1 and Cu2 represent Cu at different crystallographic sites and O1, O2, O3, and O4 represent O at different crystallographic sites. O5 represents O vacancy site. Note: T, temperature; ρ, resistivity.^{Reference Rao, Ganguly, Raychaudhuri, Ram and Sreedhar18}

We worked on the various properties of 123 compounds, including the nonresonant microwave absorption. Chemists prepared a number of novel cuprates during that time, and we contributed to this effort.^{Reference Rao and Ganguli19} Superconductors became chemicals and chemistry was recognized as an important part of materials science.^{Reference Rao20}

Colossal magnetoresistance

We studied magnetic and other properties of rare-earth manganites in the late 1970s and early 1980s. Substitution of La in LaMnO₃ by Sr makes it metallic and ferromagnetic, while substitution by Ca gives rise to a charge-ordered antiferromagnetic state. Observation of large magnetoresistance around the magnetic transition of La_1–xCa_xMnO₃ and related materials caused great interest (Figure 4a).^{Reference Rao and Raveau21} These materials exhibit interesting properties such as charge ordering and orbital ordering,^{Reference Rao22,Reference Shenoy and Rao23} aspects that Goodenough had discussed earlier.^{Reference Goodenough24}

Figure 4.

(a) Rare-earth manganites and CMR. The manganites exhibit a peak in resistivity indicating insulator–metal transition and colossal magentoresistance on application of a magnetic field. (b) Electronic phase separation in Nd_0.5Sr_0.5MnO₃. Volume fraction of phases as a function of T indicates the simultaneous existence of CE-AFM, A-AFM, and FMM. Here, CE and A represent different types of AFM phases.^{Reference Rao and Raveau21,Reference Woodward, Cox, Vogt, Rao and Cheetham25} Note: CMR, colossal magnetoresistance; T, temperature, FMM, ferromagnetic and metallic; AFM, antiferromagnetic.

While colossal magnetoresistance itself was of great interest, properties of the charge-ordered (CO) state revealed many fascinating features. The robust CO state can be melted into a metallic ferromagnetic state by magnetic fields, chemical substitution, and electric fields. The most interesting aspect is that many of the CO manganites exhibit electronic phase separation, wherein a seemingly pure single-phase material at room temperature, on cooling to lower temperatures exhibits more than one phase.^{Reference Rao22,Reference Shenoy and Rao23} This is a reversible phenomenon. The most vivid example is that of Nd_0.5Sr_0.5MnO₃ (Figure 4b), which is ferromagnetic and metallic (FMM) at 300 K and transforms to an antiferromagnetic state at 200 K, and to another antiferromagnetic CO state at 150 K. At low temperatures close to 0 K, the FMM state coexists along with the two antiferromagnetic states.^{Reference Woodward, Cox, Vogt, Rao and Cheetham25} When we first discovered this phenomenon, we had difficulty in getting the work accepted. Various aspects of electronic phase separation in manganites have since been discussed in the literature.^{Reference Rao and Raveau21–Reference Shenoy and Rao23}

Magnetoelectrics

Coexistence of ferroelectricity and ferromagnetism in a material was considered difficult or impossible because of the entirely different requirements of the two phenomena. Many oxides have since been found to be multiferroic or magnetoelectric because of the coupling between the two order parameters.^{Reference Rao, Sundaresan and Saha26} This can arise from different causes with an interesting case being ferroelectricity having a magnetic origin. An example of this is TbMnO₃.

We have worked on TbMnO₃ as well as the better-known BiFeO₃. We had proposed that rare-earth chromites, LnCrO₃, were ferroelectric although they possess a centrosymmetric structure. We could establish the presence of local noncentrosymmetry by pair distribution function analysis of neutron diffraction data.^{Reference Ramesha, Llobet, Proffen, Serrao and Rao27} It was proposed that BiMnO₃ is both ferromagnetic and ferroelectric, the former due to orbital ordering. Again, the problem was the centrosymmetric structure. It was found that slight oxygen nonstoichiometry renders the material noncentrosymmetric.^{Reference Eggeman, Sundaresan, Rao and Midgley28} A serious problem with many of the studies of multiferroics has to do with the leaky nature of the materials. Stringent procedures have been proposed recently to ensure that a material is ferroelectric in spite of its leaky nature.

Hybrid frameworks and other novel materials

Organic–inorganic hybrids have been of interest to me for some time and have emerged to provide a variety of novel materials with interesting properties.^{Reference Cheetham and Rao29} In particular, we have synthesized, characterized, and studied properties of several metal–organic frameworks and also examined the mechanism of formation of these materials starting from zero-dimensional molecular species to three-dimensional networks.^{Reference Rao, Natarajan, Choudhury, Neeraj and Ayi30} Besides investigating frameworks based on phosphates,^{Reference Murugavel, Choudhury, Walawalkar, Pothiraja and Rao31} carboxylates,^{Reference Rao, Natarajan and Vaidhyanathan32} and sulfates,^{Reference Rao, Behera and Dan33} we have provided a method of classifying these materials.^{Reference Cheetham, Rao and Feller34}

One of the main objectives of the chemistry of materials is not only to synthesize new and novel materials, but also to explore new ways of doing so. One of the strategies that we have employed to design inorganic materials with significantly different electronic and other properties has been to carry out aliovalent anion substitution, rather than isovalent anion substitution or cation substitution (e.g., Zn₂NF³⁵). We can vary the bandgap of ZnO by this means, as illustrated in Figure 5. We replaced sulfur in CdS or ZnS by P and Cl to alter the bandgap and associated properties.^{Reference Rao36}

Figure 5.

N, F-doped ZnO and their properties. (a) Tauc plots of N, F-doped ZnO and (b) variation of bandgap with the N content (x _N). Bandgap of the material varies with the composition of N, F-doped ZnO. Inset represents photographs of the materials.^{Reference Lingampalli, Manjunath, Shenoy, Waghmare and Rao35} Note: x _N, fraction of N; h, Planck’s constant; ν, wave number; α, extinction coefficient.

Nanomaterials

My interest in nanomaterials was stimulated by the question, “How many atoms make a metal?” We started investigating this problem in the mid-1980s by employing photoemission spectroscopy along with electron microscopy.^{Reference Rao, Vijayakrishnan, Sarma and Chainani37,Reference Vijayakrishnan, Chainani, Sarma and Rao38} A study of gold clusters showed that particles close to 1 nm in diameter were nonmetallic. Such nonmetallic gold and nickel have active surfaces. We also examined thiol-covered metal particles and the lattices they formed.^{Reference Rao, Kulkarni, Thomas and Edwards39}

Kratchmer et al. reported the laboratory preparation of C₆₀ and C₇₀ in 1990, and we started examining several of their properties. Phase transitions of C₆₀ and C₇₀, especially under pressure, were investigated.^{Reference Rao and Seshadri40,Reference Chandrabhas, Sood, Muthu and Rao41} An aspect of this study, which included Raman spectroscopy, was to understand why C₆₀ became irreversibly amorphous under pressure while C₇₀ was reversibly amorphous. In 1991, soon after the discovery of carbon nanotubes (CNTs), we started making them by arc discharge and other methods and examined many chemical and physical properties.^{Reference Rao and Govindaraj42} Functionalization of CNTs and using them as templates were some aspects of these studies. Synthesis and characterization of inorganic fullerenes and nanotubes, especially the latter, were major efforts in the laboratory.^{Reference Rao and Govindaraj43}

As nanomaterials gained great popularity, we started working on a variety of nanoparticles and nanowires of oxides, nitrides, and sulfides as well as nanotubes (specifically of chalcogenides).^{Reference Rao, Matte, Voggu and Govindaraj44,Reference Rao, Deepak, Gundiah and Govindaraj45} Some of the nanomaterials were novel, as exemplified by metallic ReO₃ nanoparticles.^{Reference Biswas and Rao46} Functionalization of inorganic nanoparticles was accomplished. Novel methods of synthesis were discovered, the use of the liquid–liquid interface being one of them.^{Reference Rao and Kalyanikutty47} By this method, we could make ultrathin films containing nanoparticles or 2D sheets, some of which were single crystalline (e.g., CuS). The mechanism of formation of these nanostructures was examined by the study of ultrathin films at the interface by x-ray and neutron techniques.

CNTs doped with B or N as well as Y-junction nanotubes were made using appropriate reactants.^{Reference Panchakarla, Subrahmanyam, Saha, Govindaraj, Krishnamurthy, Waghmare and Rao48,Reference Voggu, Govindaraj and Rao49} A simple method to separate semiconducting and metallic carbon nanotubes was discovered,^{Reference Voggu, Rao, George and Rao50} in addition to a method of making metallic nanotubes exclusively. Doping CNTs by molecular charge-transfer was also accomplished.^{Reference Rao and Voggu51} A major result from this laboratory is that all inorganic nanomaterials are weakly ferromagnetic, although they do not possess intrinsic magnetism.^{Reference Sundaresan and Rao52} Proper theory for this observation is still lacking.

Two-dimensional nanomaterials

Discovery of one-atom-thick 2D graphene with its amazing properties has prompted research on other 2D nanomaterials. We examined several aspects of graphene, such as functionalization, gas adsorption, and interactions with electron donors and acceptors.^{Reference Rao and Voggu51,Reference Rao, Sood, Subrahmanyam and Govindaraj53,Reference Rao and Sood54} Early in the game, we fabricated the first supercapacitor with graphene and initiated a major program on the inorganic analogues of graphene.^{Reference Rao and Waghmare55–Reference Late, Liu, Matte, Dravid and Rao59} Graphene, along with nanodiamond and CNTs, exhibits synergy in mechanical properties.

We have synthesized 2D nanosheets of various metal dichalcogenides, including MoS₂, by various methods. Unlike graphene, simple layers of many other inorganic layered compounds are not planar sheets (see Figure 6 showing a single layer of phosphorene).^{Reference Vishnoi, Rajesh, Manjunatha, Bandyopadhyay, Barua, Pati and Rao60} While single layers of MoS₂ prepared by mechanical exfoliation or ultrasonication are in the 2 H form (hexagonal unit cell), those obtained by lithium intercalation followed by exfoliation are in the 1 T form (trigonal unit cell), which is metallic. The coordination of Mo in the two forms is different, and Raman spectra show distinct differences (Figure 7). The single-layer 2 H nanosheets exhibit many novel properties,^{Reference Rao and Maitra56,Reference Rao, Maitra and Waghmare57} such as highly intense photoluminescence since it is a direct bandgap semiconductor. The 2 H form has been used for many applications, including gas sensors and supercapacitors. Batteries and field-effect transistors have been fabricated using 2 H sheets.^{Reference Rao, Maitra and Waghmare57–Reference Late, Liu, Matte, Dravid and Rao59} The 1 T forms of MoS₂ and MoSe₂ are excellent photochemical catalysts for generating hydrogen.^{Reference Rao and Maitra56,Reference Rao, Maitra and Waghmare57} MoS₂ and MoSe₂ sheets in the 1 T form can be functionalized through nucleophilic substitution.^{Reference Vishnoi, Sampath, Waghmare and Rao61}

Figure 6.

Single layer of phosphorene. (a) Schematic of synthesis, (b) transmission electron microscope image (inset: Fourier transform), and (c) Raman spectrum (inset: blue arrows indicate optical phonon modes) of a single layer of phosporene. Note: NMP, N-methyl-2-pyrrolidone; DMF, N,N-dimethylformamide.^{Reference Vishnoi, Rajesh, Manjunatha, Bandyopadhyay, Barua, Pati and Rao60}

Figure 7.

Raman spectra of 1 T and 2 H phases of (a) MoS₂ and (b) MoSe₂ sheets. Due to the difference in structure, new Raman modes arise in the 1T form, named as J₁, J₂, and J₃. (c–d) Metal coordination in 1 T and 2 H phases, respectively. Red and yellow balls represent Mo and S atoms, respectively.^{Reference Rao, Maitra and Waghmare57} Note: 1 T, trigonal unit cell; 2 H, hexagonal unit cell.

Single and few layers of oxides such as MoO₃ and Bi₅Ti₃O₁₂ have been generated using ultrasonication or Li intercalation. Single layers of BN and many other layered materials (e.g., GaS, GaSe) have been investigated.^{Reference Rao and Waghmare55} An especially noteworthy case is that of the borocarbonitrides, B_xC_yN_z, which contain BN and graphene domains in addition to BCN rings.^{Reference Rao and Gopalakrishnan62} They exhibit impressive gas adsorption and catalytic properties and also have potential applications in energy devices.

A recent development has been to covalently link layers of the same material (graphene, MoS₂) or two different materials^{Reference Rao, Pramoda and Kumar63} (graphene-BN, graphene-MoS₂, MoS₂-C₃N₄) as shown in Figure 8. These nanocomposites possess unique properties that are distinctly different from those of van der Waals heterostructures. Thus, MoS₂-C₃N₄ is a good electrocatalyst for hydrogen generation.^{Reference Rao, Pramoda and Kumar63,Reference Pramoda, Gupta, Chhetri, Bandyopadhyay, Pati and Rao64} The best example of an electrocatalyst comparable to platinum is the nanocomposite, BC₇N-MoS₂.^{Reference Pramoda, Ayyub, Singh, Chhetri, Gupta, Soni and Rao65} Making sheets of the stable 2 H-MoS₂ is therefore useful for the purpose.^{Reference Pramoda, Gupta, Chhetri, Bandyopadhyay, Pati and Rao64}

Figure 8.

(a) Schematic of synthesis of covalently cross-linked C₃N₄-MoS₂ and C₃N₄-NRGO nanocomposites. EDC is used as a coupling reagent. (b) (Top) FESEM image showing that 3D assemblies are generated due to cross-linking and (bottom) elemental mapping of C₃N₄-MoS₂. Scale bars in the EDS images = 200 nm. Note: NRGO, nitrogenated reduced graphene oxide; EDC, N-(3-(dimethylamino)propyl)-N′-ethylcarbodiimide hydrochloride; FESEM, field-emission scanning electron microscope; EDS, energy-dispersive spectroscopy; g-C₃N₄, graphitic carbon nitride.^{Reference Pramoda, Gupta, Chhetri, Bandyopadhyay, Pati and Rao64}

Concluding remarks

Six decades is a long period and materials research has changed in many ways during this time, periodically providing many astounding discoveries. In trying to cover my long journey in a limited time and space, I could not cover many interesting aspects. For example, the entire area of thermal, electrochemical, and photochemical water splitting^{Reference Rao, Lingampalli, Dey and Roy66–Reference Chhetri, Sultan and Rao68} has not been included here.

Chemistry of materials has emerged to become a major stream of chemistry. It has been fascinating to see the subject grow over the decades and to grow with it. It has been pure pleasure working in this area with my young students. This article attempts to provide the flavor of the research work that I have carried out for 60 years.