How infrared transmission reveals the molecular transformation of chemically reduced graphene oxide
Imagine a material so thin it's considered two-dimensional, yet so strong it's hundreds of times tougher than steel. This isn't science fiction—this is graphene oxide (GO), a derivative of the Nobel Prize-winning material graphene.
A single-atomic-layered material made by the oxidation of graphite, which is cheap and readily available. GO is an electrical insulator but can be chemically reduced to become conductive.
A technique that measures the interaction of infrared radiation with matter. It provides information about molecular vibrations and chemical functional groups present in a material.
For decades, the infrared spectrum of graphene oxide has been like a complex piece of music that scientists could only partially decipher. The first infrared spectrum of GO was recorded as early as 1955, yet surprisingly, these spectra have never been fully resolved 1 .
The region between 1600 and 800 cm⁻¹ where multiple vibration modes overlap, creating broad, poorly resolved patterns that are difficult to interpret accurately 1 .
FTIR spectroscopy is simple, cheap, and accessible, with instruments available in nearly every research lab, making it an attractive alternative if properly understood 1 .
Researchers compared two different reduction approaches using the same reducing agent—hydrazine hydrate—with dramatically different results 2 .
Chemical reduction of graphene oxide is essentially a molecular transformation that strips away oxygen atoms while restoring the graphene-like honeycomb structure 2 .
Two-dimensional carbon sheet decorated with oxygen-containing functional groups including hydroxyls, epoxies, carbonyls, and carboxylic acids. Highly disordered and electrically insulating .
Hydrazine hydrate attacks oxygen functional groups, removing them as water or other byproducts. This restores the conjugated sp² hybridized network characteristic of graphene 2 .
Interlayer distance shrinks from about 0.9 nm in GO toward 0.36 nm in rGO (closer to pristine graphene's 0.34 nm spacing) as bulky oxygen groups are removed 8 .
Electrical conductivity increases by orders of magnitude as reduction progresses. Optical properties transform with band gap narrowing from ~2.2 eV to less than 1.5 eV .
| Functional Group | Change | IR Evidence |
|---|---|---|
| O-H (hydroxyl) | Decreases | Weakening of ~3400 cm⁻¹ band |
| C=O (carbonyl) | Decreases | Diminishing ~1720 cm⁻¹ band |
| C-O (epoxy/alcohol) | Decreases | Reduced 1000-1300 cm⁻¹ vibrations |
| C=C (graphitic) | Increases | Strengthening skeletal vibrations |
One particularly insightful study systematically tracked the reduction process by creating a family of rGO samples with varying oxygen content through stepwise reduction using hydrazine hydrate .
| Peak (cm⁻¹) | Correct Assignment | Common Misassignment |
|---|---|---|
| ~1720 | C=O stretching | Correctly assigned |
| ~1620 | Water bending vibrations | C=C bond vibrations |
| ~1360 | OH bending in alcohols | Various functionalities |
| ~1223 | S=O bonds of sulfates | C-O-C epoxy groups |
| ~1040 | Unknown (no consensus) | C-O in alcohols/epoxies |
Based on data from 1
| Property | rGOV (Vapor) | rGOL (Liquid) |
|---|---|---|
| Thickness | 110 nm (16× thicker) | ~7 nm (similar to GO) |
| Resistivity | 90× higher than rGOL | Lowest resistivity |
| D″ Raman Peak | Reduced compared to GO | Most reduced |
| C-OH FTIR Peak | Almost disappears | Less intense than GO |
Based on data from 2
Based on data from showing orders of magnitude improvement in conductivity
The precise understanding and control of graphene oxide reduction opens doors to transformative technological applications across multiple fields.
Partially reduced GO offers a compromise between processability and performance for transistors, transparent conductors, and photodetectors .
Supercapacitors and batteries based on rGO electrodes leverage high surface area and tunable conductivity with specific capacitance up to 549.8 F g⁻¹ 3 .
rGO's wrinkled surface and oxygen functional groups make it an excellent adsorbent for removing organic pollutants from water .
The journey to understand the infrared transmission of chemically reduced graphene oxide has been filled with surprises, corrections, and incremental advances.
From early misinterpretations of spectral features to recent precise nanoscale mapping of functional groups, our ability to "listen" to the molecular vibrations of this remarkable material has matured dramatically. Scientists are now designing rGO materials with specific properties tailored for particular functions—whether as conductive inks for printed electronics, high-surface-area electrodes for supercapacitors, or reinforced composites for biomedical devices.
The story of graphene oxide reduction exemplifies how deep material understanding enables technological innovation.